Optimizing Early Compressive Strength of Ultrafine POFA-Based Green HSC: An Experimental and RSM Approach with Silica Fume and Hybrid Fibers

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Al-obaidi" }, { "@type": "Person", "name": "Nahla Hilal" }, { "@type": "Person", "name": "Megat Azmi Megat Johari" }, { "@type": "Person", "name": "Hayder Sadeq Al-Aasam" } ], "publisher": { "@type": "Organization", "name": "F1000Research", "logo": { "@type": "ImageObject", "url": "https://f1000research.com/img/AMP/F1000Research_image.png", "height": 480, "width": 60 } }, "image": { "@type": "ImageObject", "url": "https://f1000research.com/img/AMP/F1000Research_image.png", "height": 1200, "width": 150 }, "description": "Recently, studies have focused on producing green, sustainable concrete with improved environmental performance, utilizing recycled waste materials. The early compressive strength of Green High Strength Concrete (GHSC) containing up to 60% ultrafine palm oil fuel ash (UPOFA) has been optimized utilizing hybrid materials, Silica Fume (SF), steel fiber (ST), and Polyethylene Terephthalate (PET) fiber. UPOFA was used as a substitute binder for cement in GHSC manufacture at 0%, 30%, and 60% replacement levels. SF was substituted for 0%, 10%, and 20% of the residual cement mass. Steel fiber and PET fiber were included at a rate of 1% of the total binder mass. The mix design parameters were optimized using the Response Surface Method (RSM) with a central composite design (CCD) approach. The experimental results showed that the GHSC containing 30% UPOFA, 20% SF, 1% ST, and 1% PET fiber reached greater strength at 3, 7, and 28 days. The incorporation of 20% SF, 1% ST, and 1% PET fibers enhanced the early ages strength reduction of 60% UPOFA-GHSC by 17%, 25%, and 18% at 3, 7, and 28 days, respectively, relative to the control mix. The constructed models showed significant correlation values (R2) of 0.9697, 0.9546, and 0.9674 for compressive strength at 3, 7, and 28 days, respectively. Hence, RSM may be an efficient technique for improving mix design while lowering environmental impact by limiting waste volume and energy usage in cement manufacture, which contributes to the sustainability of concrete." } { "@context": "http://schema.org", "@type": "BreadcrumbList", "itemListElement": [ { "@type": "ListItem", "position": "1", "item": { "@id": "https://f1000research.com/", "name": "Home" } }, { "@type": "ListItem", "position": "2", "item": { "@id": "https://f1000research.com/browse/articles", "name": "Browse" } }, { "@type": "ListItem", "position": "3", "item": { "@id": "https://f1000research.com/articles/15-103/v3", "name": "Optimizing Early Compressive Strength of Ultrafine POFA-Based Green..." } } ] } Home Browse Optimizing Early Compressive Strength of Ultrafine POFA-Based Green... ALL Metrics - Views Downloads Get PDF Get XML Cite How to cite this article Alani AH, Ahmed D, Al-obaidi QA et al. Optimizing Early Compressive Strength of Ultrafine POFA-Based Green HSC: An Experimental and RSM Approach with Silica Fume and Hybrid Fibers [version 3; peer review: 1 approved, 1 approved with reservations] . F1000Research 2026, 15 :103 ( https://doi.org/10.12688/f1000research.175237.3 ) NOTE: If applicable, it is important to ensure the information in square brackets after the title is included in all citations of this article. Close Copy Citation Details Export Export Citation Sciwheel EndNote Ref. Manager Bibtex ProCite Sente EXPORT Select a format first Track Share ▬ ✚ Research Article Revised Optimizing Early Compressive Strength of Ultrafine POFA-Based Green HSC: An Experimental and RSM Approach with Silica Fume and Hybrid Fibers [version 3; peer review: 1 approved, 1 approved with reservations] Aktham H Alani https://orcid.org/0000-0001-8533-4859 1 , Doaa Ahmed 1 , Qasim A. Al-obaidi 2 , Nahla Hilal https://orcid.org/0000-0001-9403-9982 3 , Megat Azmi Megat Johari 4 , Hayder Sadeq Al-Aasam https://orcid.org/0000-0003-1418-3234 5 Aktham H Alani https://orcid.org/0000-0001-8533-4859 1 , Doaa Ahmed 1 , [...] Qasim A. Al-obaidi 2 , Nahla Hilal https://orcid.org/0000-0001-9403-9982 3 , Megat Azmi Megat Johari 4 , Hayder Sadeq Al-Aasam https://orcid.org/0000-0003-1418-3234 5 PUBLISHED 05 May 2026 Author details Author details 1 Department of Engineering Affairs, University of Fallujah, Al-Fallujah, Al Anbar Governorate, Iraq 2 University of Bilad AL-Rafidain, Baqubah, Iraq 3 University of Fallujah, Scientific Affairs Department, Fallujah, Iraq 4 Universiti Sains Malaysia School of Civil Engineering, Nibong Tebal, Penang, Malaysia 5 Technical Engineering College, Al-Esraa University, Baghdad, Iraq Aktham H Alani Roles: Conceptualization, Data Curation, Formal Analysis, Methodology, Supervision, Writing – Original Draft Preparation, Writing – Review & Editing Doaa Ahmed Roles: Methodology, Resources, Validation Qasim A. Al-obaidi Roles: Methodology, Resources, Software Nahla Hilal Roles: Data Curation, Resources, Software, Writing – Original Draft Preparation Megat Azmi Megat Johari Roles: Data Curation, Methodology, Resources, Validation, Writing – Original Draft Preparation Hayder Sadeq Al-Aasam Roles: Data Curation, Formal Analysis, Methodology, Resources OPEN PEER REVIEW DETAILS REVIEWER STATUS This article is included in the Fallujah Multidisciplinary Science and Innovation gateway. Abstract Recently, studies have focused on producing green, sustainable concrete with improved environmental performance, utilizing recycled waste materials. The early compressive strength of Green High Strength Concrete (GHSC) containing up to 60% ultrafine palm oil fuel ash (UPOFA) has been optimized utilizing hybrid materials, Silica Fume (SF), steel fiber (ST), and Polyethylene Terephthalate (PET) fiber. UPOFA was used as a substitute binder for cement in GHSC manufacture at 0%, 30%, and 60% replacement levels. SF was substituted for 0%, 10%, and 20% of the residual cement mass. Steel fiber and PET fiber were included at a rate of 1% of the total binder mass. The mix design parameters were optimized using the Response Surface Method (RSM) with a central composite design (CCD) approach. The experimental results showed that the GHSC containing 30% UPOFA, 20% SF, 1% ST, and 1% PET fiber reached greater strength at 3, 7, and 28 days. The incorporation of 20% SF, 1% ST, and 1% PET fibers enhanced the early ages strength reduction of 60% UPOFA-GHSC by 17%, 25%, and 18% at 3, 7, and 28 days, respectively, relative to the control mix. The constructed models showed significant correlation values (R 2 ) of 0.9697, 0.9546, and 0.9674 for compressive strength at 3, 7, and 28 days, respectively. Hence, RSM may be an efficient technique for improving mix design while lowering environmental impact by limiting waste volume and energy usage in cement manufacture, which contributes to the sustainability of concrete. READ ALL READ LESS Keywords Green concrete; Palm Oil Fuel Ash; Response surface method; Early compressive strength; Hybrid fiber; Steel fiber; PET fiber; Silica fume; Sustainability Corresponding Author(s) Aktham H Alani ( [email protected] ) Close Corresponding author: Aktham H Alani Competing interests: No competing interests were disclosed. Grant information: The author(s) declared that no grants were involved in supporting this work. Copyright: © 2026 Alani AH et al . This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. How to cite: Alani AH, Ahmed D, Al-obaidi QA et al. Optimizing Early Compressive Strength of Ultrafine POFA-Based Green HSC: An Experimental and RSM Approach with Silica Fume and Hybrid Fibers [version 3; peer review: 1 approved, 1 approved with reservations] . F1000Research 2026, 15 :103 ( https://doi.org/10.12688/f1000research.175237.3 ) First published: 22 Jan 2026, 15 :103 ( https://doi.org/10.12688/f1000research.175237.1 ) Latest published: 05 May 2026, 15 :103 ( https://doi.org/10.12688/f1000research.175237.3 ) Revised Amendments from Version 2 No changes have been made to the title, author list, figures, tables, or data in this revised version. The scientific content, results, and conclusions of the manuscript remain unchanged. Only minor editorial revisions have been carried out to improve the clarity and readability of the manuscript. These revisions were implemented in response to reviewer comments throughout the text. No changes have been made to the title, author list, figures, tables, or data in this revised version. The scientific content, results, and conclusions of the manuscript remain unchanged. Only minor editorial revisions have been carried out to improve the clarity and readability of the manuscript. These revisions were implemented in response to reviewer comments throughout the text. See the authors' detailed response to the review by Farid Debieb See the authors' detailed response to the review by Sattawat Haruehansapong READ REVIEWER RESPONSES 1. Introduction The construction industry currently heavily relies on concrete, consuming around 10 billion tons annually. This demand is projected to rise significantly by 2050, potentially reaching 18 billion tons per year due to global population growth. However, this substantial production of concrete also results in a significant generation and emission of CO 2 , posing a major environmental concern. 1 , 2 This issue requires further concern for developing eco-efficient and durable concrete alternatives. Numerous studies have shown that the cement sector alone contributes to about 8% of global CO 2 emissions, underscoring the importance of collective responsibility. 3 – 6 Researchers have played a pivotal role in examining the viability of using waste materials with significant pozzolanic potential as supplementary cementitious components in concrete formulations. 7 , 8 Their efforts have been part of a larger initiative to identify industrial and agricultural wastes that might serve as substitutes for cement and aggregates, thereby mitigating the harmful effects associated with cement production. 9 , 10 A common solution to this issue is replacing cement with recycled waste materials that have similar cementitious properties or pozzolanic attributes. Recent studies have identified palm oil fuel ash (POFA) as an effective pozzolanic material for cement replacement, highlighting its cost-effectiveness and environmental sustainability. 11 , 12 POFA is a by-product derived from the combustion of wastes generated from palm oil extraction, including empty fruit bunches, palm kernel shells, and fibers. So, POFA is the final residue after the combustion of these wastes to heat up the boiler to generate electricity in the palm oil manufacturing process. 13 , 14 Grinding–burning–regrinding is a novel treatment procedure that has been recently implemented to enhance the pozzolanic reactivity of the original POFA material and produce ultrafine POFA (UPOFA). 8 Several studies have suggested that UPOFA can be used as an effective pozzolanic supplement binder replacing significant volume of Portland cement, leading to a significant improvement in the engineering properties of concrete and ensuring superior resistance to aggressive ions from the exposure environment. 15 , 16 Nonetheless, several drawbacks have been reported from the substantial use of UPOFA as a supplementary cementitious ingredient in green concrete. These include significant reduction in early strength of concrete, where up to a 40% reduction in early concrete strength has been observed. Besides, the tensile strength reduced with UPOFA inclusion up to 20% content. This decrease is primarily due to the reduction in the cement binder content that the UPOFA replaced. These findings have major and crucial practical ramifications for the building sector, as demonstrated by numerous previous studies. 15 , 17 , 18 Moreover, the replacement rates of HSGC are probably high in POFA, which is constrained in the areas of the application where the removal of the formwork or pre-stressing of the structures is required in a very short period of time, and the high early-age strengths are required. 16 As a result, studies revealed significant findings about the use of silica fume (SF), a common pozzolanic additive, in high-strength concrete mixes. It was found that silica fume can effectively accelerate the development of early concrete strength by increasing cement hydration via the huge surface area of the much finer silica fume particles relative to cement. The very fine particle of silica fume, which translates into an enormous surface area, not only enhances its filler effect but also accelerates the cement hydration as well as hastens the onset of pozzolanic reaction, leading to a substantial increase in the compressive strength of concrete at a young age. 19 – 21 However, pozzolanic cementitious composite materials are prone to cracking due to their brittle characteristics. 15 , 22 , 23 Therefore, the incorporation of fibers to produce what is known as fiber-reinforced concrete was considered to address the issues of brittleness and reduction in compressive and tensile strength with UPOFA inclusion. There have been several attempts to increase concrete’s strength and durability performance. 24 , 25 Researchers discovered that by incorporating appropriate fibers into the concrete matrix, they could increase the mechanical characteristics of the material and improve its strength properties. These properties prevented micro-cracks from expanding into macro-cracks, which improved the material’s overall durability. There were reports of effective incorporation of various fibers, such as steel fiber, polyvinyl alcohol, polypropylene, basalt, glass, and carbon fiber, etc. 26 – 28 Steel fiber is the most common reinforcement used in concrete matrices, efficiently bridging micro-cracks caused by load conditions across crack surfaces, preventing macro-fracture growth. Steel fiber concentrations of less than 1.0% significantly increase concrete impact resistance. The studies attributed that to the steel fibers, which inhibit micro-crack propagation and their transition to macro-cracks during the impact compression process. 29 Mezzal et al. 30 investigated the effect of adding steel fiber at a volumetric percentage of 1% on concrete’s impact resistance, flexural, and tensile strength. The fracturing of concrete is a progressive, multi-scale phenomenon that has led to significant interest in incorporating steel fibers alongside other fibers. This attraction is due to their potential advantages at various loading stages and scales compared to single-fiber reinforcements. 31 , 32 In addition, the use of steel fibers in concrete elevates the structural weight of the material. The workability of the mix diminishes owing to the balling effect as the percentage of steel fibers increases. Steel fiber-reinforced concrete contains electric and magnetic fields, and the steel fibers inside the concrete are susceptible to corrosion. Nevertheless, fiber-reinforced concrete using a single fiber type offers reinforcement only at one level and within a restricted range of crack apertures. The appropriate mixture ratio of steel and synthetic fibers may provide excellent concrete performance, beyond the cumulative efficacy of the individual fibers. 32 – 34 Recently, the recycling of single-use plastic bottles and containers has emerged as a worldwide concern due to the proliferation of plastic products. Polyethylene terephthalate (PET) is a key material in a variety of drinking containers and other consumer products. Its disposal is challenging and detrimental to the environment. However, there is reassurance in the ongoing efforts of researchers investigating PET waste as reinforcement in concrete, offering a potential method for recycling. 35 , 36 Recent research found that inserting 0.25%-2% PET fibers into a concrete mix increased its strength by 43.4% at 28 days. 37 The addition of 1% plastic fibers increased the flexural and tensile strengths of concrete approximately by 17 % and 15 %, respectively. 38 – 40 Alani et al. 41 discovered that adding 1% PET to ultra-high-strength concrete containing 20% SF and 25% UPOFA increased compressive strength by 18%, 21%, and 27% at 3, 7, and 28 days, respectively, as compared to the plain concrete mix. Despite these positive results for UPOFA in green concrete manufacturing, the majority of current research demonstrates that its use in concrete remains limited by reducing concrete strength at early ages and requires further investigation. However, somehow these studies suggested the limitation of the POFA used because the highest was limited. The compressive strength was determined at a POFA replacement level of 20%, and the ultimate highest compressive strength of 28-days compressive strength achieved with the concretes that used POFA ranged between 60 and 86 MPa. 17 , 18 Hereby, the effect of the combination of SF, steel, and recycled PET fibers on GHSC strength containing up to 60% UPOFA has been investigated. The research also aimed to reduce the use of cement, which is linked to the reduction of building costs and the mitigation of the negative environmental impacts of CO 2 emissions. 1.1 Research significance Researchers are concerned about the early strength loss of green high-strength concrete with a high volume of UPOFA replacement. The majority of the studies have looked at how steel or PET fiber affects the compressive and tensile strengths of GHSC. Recent innovative research has employed fibers to investigate early GHSC strength behavior at UPOFA concentrations as low as 40% of the total binder. This study aimed to investigate the coupling effects of steel, PET fibers, and SF on the compressive and tensile strength of HSGC with low cement content. This includes up to 60% of UPOFA as a replacement binder with cement at the early ages of 3, 7, and 28 days. While UPOFA was employed as a replacement binder at a rate of 30% and 60%, SF was substituted at 10% and 20% of the total mass of the cement binder. Steel fiber was added at 1%, while PET fibers were added at 1%. The compressive and tensile strengths were analyzed and tested. Meanwhile, the response surface methodology (RSM) approach was employed to empirically determine the optimal hybrid fiber and pozzolanic secondary binders to achieve superior strength behavior for GHSC. Thus, the optimum values of UPOFA, SF, steel, and PET fibers for achieving GHSC’s superior compressive strength at early ages have been identified. 2. Materials and methods 2.1 Cement, UPOFA, and SF The main binder is standard Portland cement (OPC) [Type 1, 42.5 R], the specific surface area is 324 m 2/kg, and the specific gravity is 3.15. It is in accordance with the ASTM C150 standards. 42 UPOFA ( Figure 1 ) was selected as a partial replacement binder for its excellent pozzolanic properties and used in concentrations of 30% and 60% of the total mass of the OPC. Similarly, SF was chosen because of its peculiarities, accelerating influence on OPC hydration, and its remarkable pozzolanic activity. SF were substituted by 10% and 20% of the remaining of OPC. Table 1 presents the chemical and physical properties of the binders. Besides these properties, the role and anticipated impact of each material in the performance of the concrete are summarized in Table 2 . Figure 1. UPOFA. Table 1. Chemical and physical properties of binder materials. Property OPC UPOFA SF SiO 2 (%) 21.25 64.81 92.25 Al 2 O 3 (%) 5.11 5.66 0.88 Fe 2 O 3 (%) 3.35 4.73 1.86 CaO (%) 61.82 8.24 0.94 MgO (%) 2.91 4.63 0.96 K 2 O (%) 0.38 6.37 1.37 SO 3 (%) 2.42 0.36 0.35 Na 2 O (%) 0.26 0.063 - Loss on ignition (LOI) 2.51 2.55 4.96 Specific gravity 3.15 2.56 2.53 Particle size (μm) 15 2.11 0.1 to 1 Specific surface area (m 2 /g) 0.324 1.8 2.16 2.2 Aggregates The selected fine aggregate, river sand, has a maximal particle size of 4.75 mm, a specific gravity of 2.67, and a fineness modulus of 2.88. The crushed gravel, which was chosen for its potential as a coarse aggregate, has a specific gravity of 2.71 and a maximum size of 12.5 mm, to ensure that the compressive force may be imposed upon the matrix rather than on a rigid skeleton of aggregates, which decreases the stresses developed at the paste-aggregate interface. The more stress being transmitted by the aggregates and the surrounding matrix in HSC increased the opportunity for homogenous stress distribution. 43 2.3 Superplasticizer and water In order to achieve uniform dispersion and improve flowability with a low water-to-binder (w/b) ratio while maintaining high strength, a polycarboxylic ether-based high-range water-reducing admixture was utilized as a superplasticizer, which has a specific gravity of 1.080 and conforms to ASTM C 494 44 specifications for chemical admixtures used in concrete. The concrete mixes and specimen curing in this study were performed with ordinary tap water. 2.4 Steel fiber and PET fiber Short brass-coated micro-steel fibers used in this study, as shown in Figure 2(a) have a length of 13 mm, a diameter of 0.2 mm, a tensile strength of up to 2600 MPa, and a density of approximately 7.80 g/cm. Plastic waste from polyethylene terephthalate (PET), as shown in Figure 2(b) , with the following specifications: average length of 30 mm, breadth of 3 mm, thickness of 0.3 mm, specific gravity of 1.36, and water absorption of 0.17% was used as fiber reinforcement in this study. The contents of the steel and PET fibers were set to 1% volume, which is consistent with previous studies reporting that this amount has improved mechanical performance, crack control, and ductility whilst maintaining workability. 21 , 36 Figure 2. (a) Steel fiber and (b) PET fiber. 2.5 Experimental design based on RSM approach The Design-Expert v13 program was employed for modeling, statistical analysis, and response optimization in this study. The central composite design (CCD) is a common method used in RSM. CCD technique is favored to the full factorial and three-level design as it estimates the linear, quadratic and interaction effects with minimum tests, which will lead to a good model fitting, greater accuracy in prediction, reduced amount of residual error, and a high value of R 2 , that could permit the creation of three-dimensional response surfaces with great reliability. Consequently, it has commonly been considered as the best experimental design to investigate the second-order response models. 4 , 15 This approach was utilized to establish the relationship between the compressive strength responses at 3, 7, and 28 days and the parameters of UPOFA, SF, steel fiber, and PET fiber. The factorial experimental design for UPOFA and SF included three levels, while the designs for steel and PET fibers comprised two levels. As a result, the RSM technique was employed to optimize the responses of GHSC by varying the levels of these components. The independent variables and their corresponding coding values are shown in Table 3 . Besides, Tables 4 and 5 present the results of the CCD approach in variables (%) and mixtures in (kg/m 3 ), respectively, which were implemented in the design of forty-one experimental runs. The CCD design incorporated the center point of the experiment, which was replicated in five runs for M16, in addition to the initial thirty-six trials. This approach aimed to improve the precision of both the experimental design and the analysis. The optimum predictor quadratic model, as described in Equation (1) , was used to ascertain the optimal parameters for compressive strength responses at 3, 7, and 28 days. (1) Y = β 0 + ∑ i = 1 m β ii X i + ∑ i = 1 m β ii X i 2 + ∑ i = 1 m − 1 ∑ j m β ij X i X j + e Table 2. Binders role in concrete, proportion%, and expected performance. Material Role in concrete % Proportion Predictable effect on performance UPOFA Partial replacement with cement 30 and 60 Enhances long-term strength, reduces heat of hydration, and reduces early strength. SF Partial replacement with cement 10 and 20 Enhance compressive strength at early ages and durability. Refines pore structure and increases brittleness. Steel fiber Reinforcement 1 Improve the mechanical properties of concrete at early ages via crack resistance and improve concrete ductility. PET fiber Waste fiber reinforcement 1 Improves mechanical properties of concrete and reduce micro crack generation. Table 3. Independent variables, coding, and units. Factors Unit Level UPOFA (A) % -1 0 1 % 0 30 60 SF (B) % -1 0 1 % 0 10 20 Steel fiber (C) % -1 1 % 0 1 PET fiber (D) % -1 1 % 0 1 Table 4. CCD mix variables (%) using RSM. Mix no. Mix ID Variables (%) UPOFA SF Steel fiber PET fiber GI M0* HSC 0 0 0 0 M1 HSC-ST1 0 0 1 0 M2 HSC-PET1 0 0 0 1 M3 HSC-ST1-PET1 0 0 1 1 M4 HSC-SF10 0 10 0 0 M 5 HSC-SF10-ST1 0 10 1 0 M 6 HSC-SF10-PET1 0 10 0 1 M7 HSC-SF10-ST1-PET1 0 10 1 1 M8 HSC-SF20 0 20 0 0 M9 HSC-SF20-ST1 0 20 1 0 M10 HSC-SF20-PET1 0 20 0 1 M11 HSC-SF20-ST1-PET1 0 20 1 1 GII M12 GHSC-U30 30 0 0 0 M13 GHSC-U30-ST1 30 0 1 0 M14 GHSC-U30-PET1 30 0 0 1 M15 GHSC-U30-ST1-PET1 30 0 1 1 M(16-20) GHSC-U30-SF10 30 10 0 0 M21 GHSC-U30-SF10-ST1 30 10 1 0 M22 GHSC-U30-SF10-PET1 30 10 0 1 M23 GHSC-U30-SF10-ST1-PET1 30 10 1 1 M24 GHSC-U30-SF20 30 20 0 0 M25 GHSC-U30-SF20-ST1 30 20 1 0 M26 GHSC-U30-SF20-PET1 30 20 0 1 M27 GHSC-U30-SF20-ST1-PET1 30 20 1 1 GIII M28 GHSC-U60 60 0 0 0 M29 GHSC-U60-ST1 60 0 1 0 M30 GHSC-U60-PET1 60 0 0 1 M31 GHSC-U60-ST1-PET1 60 0 1 1 M32 GHSC-U60-SF10 60 10 0 0 M33 GHSC-U60-SF10-ST1 60 10 1 0 M34 GHSC-U60-SF10-PET1 60 10 0 1 M35 GHSC-U60-SF10-ST1-PET1 60 10 1 1 M36 GHSC-U60-SF20 60 20 0 0 M37 GHSC-U60-SF20-ST1 60 20 1 0 M38 GHSC-U60-SF20-PET1 60 20 0 1 M39 GHSC-U60-SF20-ST1-PET1 60 20 1 1 Where Y represents the response (compressive strength), and β and i denote the regression coefficients and linear coefficients, respectively. j represents the quadratic coefficient, X i X j denotes the coded values for UPOFA, SF, steel fiber, and PET fiber. m signifies the number of components, and e indicates the random error. Except for the control mix, Table 5 showed that OPC weight changed with UPOFA and SF replacement percentages. All GHSC mixes contain the same weight of the GHSC components, which include river sand, crushed gravel, w/b ratio, and SP. This research investigated the effects of UPOFA and SF replacement levels, both with and without steel and PET fibers, on compressive strength responses at 3, 7, and 28 days using analysis of variance (ANOVA). The reliability of the quadratic prediction model was tested using this technique. The model consequently computed a 95% confidence level (P-value) to ascertain statistical significance. Furthermore, the R 2 coefficient was calculated after a t-test with a 0.05 threshold of significance. The required model was developed based on actual findings from this research. Diagnostic diagrams were created to evaluate the model’s accuracy. 2.6 Samples preparation HSC and GHSC with and without fibers were prepared using a pan-type concrete mixer. During casting, the samples were consolidated on a vibrating table to obtain maximum density. After a critical 24-hour period, the specimens were carefully removed from their molds and underwent a rigorous water-curing procedure at a constant temperature of 27 ± 2°C until the day of testing. HSC is common for its varying mixture compositions, which provide high-performance characteristics with compressive strength ranging from 62 to 138 MPa. 2.7 Test procedures To attain the primary aim of this research, further experiments were conducted for each mixture of HSC and GHSC as given in Table 5 . 2.7.1 Compressive strength test In line with BS EN 12390-3, 45 100 mm concrete cubes were tested using a 2000 kN concrete compression machine at a loading rate of 0.30 MPa/s. The compressive strength was next determined. At least three samples were tested for the different mixtures for each age of 3, 7, and 28 days. 3. Experimental results and discussions The CCD related to varying variable content divides the HSC and GHSC mixtures, both with and without fiber, into three groups, as shown in Tables 4 and 5 . The first group (G I) consists of HSC mixes numbered 0–11. The second group (G II) comprises GHSC-U30 mixes numbered 12 through 27. The third group (G III) is known as GHSC-U60 and includes mixes numbered 28 to 39. 3.1 Compressive strength Figures 3 , 4 , and 5 show that the incorporation of UPOFA reduces the compressive strength of GHSC at all ages (3, 7, and 28 days), with a more significant drop in strength at a higher UPOFA replacement level of U60%. The greater reduction in compressive strength was recorded by M28 (GHSC-U60), about 10%, 7% and 8% relative to the control mix, at 3, 7, and 28 days, respectively, as shown in Figures 6 , 7 , and 8 . This decrease is due to the dilution effect from the low OPC content, contributing towards delayed hydration and pozzolanic reaction of the U60-GHSC mixture, in particular at the early age, which is characterized by a high level of 60% UPOFA. 46 As a consequence, the reduced strength was observed from the delayed and lower C-S-H production. Furthermore, the interaction between calcium hydroxide (CH) and silica concentration in POFA influences the concrete’s compressive strength. 47 , 48 Further, the increased concentration of amorphous silica, including 60% UPOFA, accelerates the consumption rate (CH); nevertheless, the reduced cement content hampers its production in areas where CH is being exhausted. This impact results in the pozzolanic reaction attaining a CH-limited state, thereby leading to a reduced and delayed synthesis of C-S-H. Thereby, the interaction between the availability of calcium hydroxide and silica content in palm oil fuel ash significantly influences the evolution of compressive strength. 8 , 20 Thus, mixes with high UPOFA concentrations had lower compressive strength than concrete with a lower POFA fraction. Furthermore, the 75.2% concentration of pozzolanic characteristics (SiO 2 , Al 2 O 3 , and Fe 2 O 3 ) may result in high calcium hydroxide (CH) consumption during hydration, as given in Table 1 . These factors may result in a CH deficiency, limiting strength development. 8 , 49 , 50 In contrast, the presence of SF, steel fiber, and PET fiber develops compressive strength at all ages of 3, 7, and 28 days. Figure 3. Experimental compressive strength results of GHSC at 3 days. Figure 4. Experimental compressive strength results of GHSC at 7 days. Figure 5. Experimental compressive strength results of GHSC at 28 days. Figure 6. Effect of hybrid materials on the compressive strength of GHSC at 3 days (HSC as reference). Figure 7. Effect of hybrid materials on the compressive strength of GHSC at 7 days (HSC as reference). Figure 8. Effect of hybrid materials on the compressive strength of GHSC at 28 days (HSC as reference). The compressive strength of HSC and GHSC is considerably improved upon the incorporation of SF at concentrations of 10% and 20% throughout all curing periods. Acceleration in OPC hydration as well as pozzolanic reactions between silica particulate and OPC are responsible for this enhancement. 51 , 52 The fine SF particles with huge surface area provide sites for the nucleation of OPC hydration products, hence promoting strength gain. In the case of pozzolanic reactions, a curing time of 3 to 4 days is typically required for these reactions (OPC-SF) to begin, according to previous research. 20 , 53 The compressive strength of GHSC is enhanced over a curing period of 7 to 28 days as a result of the accelerated hydration of conventional OPC and due to the pozzolanic effects of SF. The formation of secondary C-S-H is the consequence of the interaction between (Ca (OH) 2 ) produced during cement hydration and SiO 2 in SF. 54 , 55 This procedure increases the durability of the concrete by reducing the incidence of microcracks during periods of tension. In the same trend, adding steel and PET fibers improved the compressive strength of HSC and GHSC. The GHSC mixtures with a combination of (UPOFA-SF-ST-PET) registered the highest compressive strength of mixtures in GII and GIII at all curing periods. Hereby, the compressive strength of (M39) GHSC-U60 increased by 17%, 25% and 18% at ages of 3, 7, and 28 days, respectively, related to control mix. This strength development was attributed to the effectiveness of binary binders (UPOFA and SF) in increasing bond formation between steel and PET fibers and other concrete ingredients. The tiny particle size and pozzolanic reactivity of UPOFA and SF reduced void volume and produced extra C-S-H gels, strengthening the aggregate-paste interfacial connection and increasing bonding capabilities with fibers. Adding steel and PET fibers may improve the fracture resistance of fiber-reinforced GHSC composites by limiting micro-crack formation under load, which is accomplished by forming a bridge network within the concrete microstructure. 31 , 41 , 56 Further, particularly in the case of fiber-reinforced concrete (FRC) mixes, the incorporation of supplementary cementitious materials (SCMs) has the potential to enhance the generation of secondary hydration products and be of assistance in the process of filling the pores that are produced at the interface of the fibers. 57 , 58 A key contribution of SCMs such as SF to improving fiber-matrix interfacial characteristics is that the effectiveness of fibers within a concrete matrix relies heavily on the bond between the fibers and the matrix. Consequently, the load-bearing capacity was enhanced beyond the initial cracking threshold as the randomly dispersed fibers bonded effectively with the matrix, preventing crack propagation and specimen separation. 59 , 60 In this study, the combination of SF-steel and PET fibers enhanced the compressive strength of UPOFA-GHSC at all ages, even with a high UPOFA content of 60%. The highest strength was observed at M27 (GHSC-U30-SF20-ST1-PET1). 3.2 Mathematical modeling and statistical analysis results The correlation between the variables UPOFA, SF, and steel as well as PET fibers with the compressive strength responses at 3, 7, and 28 days for GHSC mixtures was assessed using the RSM. The model was constructed using actual data. Table 6 summarizes the results of the analysis of variance (ANOVA) conducted on the findings of the quadratic prediction model. Table 5. CCD mix proportion in (kg/m 3 ) using RSM. Mix ID Components (kg/m 3 ) OPC UPOFA SF Steel Fiber PET Fiber Fine Agg. Coarse Agg. Water SP GI M0* HSC 570 0 0 0 0 765 1057 155.2 14.25 M1 HSC-ST1 570 0 0 5.7 0 765 1057 155.2 14.25 M2 HSC-PET1 570 0 0 0 5.7 765 1057 155.2 14.25 M3 HSC-ST1-PET1 570 0 0 5.7 5.7 765 1057 155.2 14.25 M4 HSC-SF10 513 0 57 0 0 765 1057 155.2 14.25 M 5 HSC-SF10-ST1 513 0 57 5.7 0 765 1057 155.2 14.25 M 6 HSC-SF10-PET1 513 0 57 0 5.7 765 1057 155.2 14.25 M7 HSC-SF10-ST1-PET1 513 0 57 5.7 5.7 765 1057 155.2 14.25 M8 HSC-SF20 456 0 114 0 0 765 1057 155.2 14.25 M9 HSC-SF20-ST1 456 0 114 5.7 0 765 1057 155.2 14.25 M10 HSC-SF20-PET1 456 0 114 0 5.7 765 1057 155.2 14.25 M11 HSC-SF20-ST1-PET1 456 0 114 5.7 5.7 765 1057 155.2 14.25 GII M12 GHSC-U30 399 171 0 0 0 765 1057 155.2 14.25 M13 GHSC-U30-ST1 399 171 0 5.7 0 765 1057 155.2 14.25 M14 GHSC-U30-PET1 399 171 0 0 5.7 765 1057 155.2 14.25 M15 GHSC-U30-ST1-PET1 399 171 0 5.7 5.7 765 1057 155.2 14.25 M (16-20) GHSC-U30-SF10 342 171 57 0 0 765 1057 155.2 14.25 M21 GHSC-U30-SF10-ST1 342 171 57 5.7 0 765 1057 155.2 14.25 M22 GHSC-U30-SF10-PET1 342 171 57 0 5.7 765 1057 155.2 14.25 M23 GHSC-U30-SF10-ST1-PET1 342 171 57 5.7 5.7 765 1057 155.2 14.25 M24 GHSC-U30-SF20 285 171 114 0 0 765 1057 155.2 14.25 M25 GHSC-U30-SF20-ST1 285 171 114 5.7 0 765 1057 155.2 14.25 M26 GHSC-U30-SF20-PET1 285 171 114 0 5.7 765 1057 155.2 14.25 M27 GHSC-U30-SF20-ST1-PET1 285 171 114 5.7 5.7 765 1057 155.2 14.25 GIII M28 GHSC-U60 228 342 0 0 0 765 1057 155.2 14.25 M29 GHSC-U60-ST1 228 342 0 5.7 0 765 1057 155.2 14.25 M30 GHSC-U60-PET1 228 342 0 0 5.7 765 1057 155.2 14.25 M31 GHSC-U60-ST1-PET1 228 342 0 5.7 5.7 765 1057 155.2 14.25 M32 GHSC-U60-SF10 171 342 57 0 0 765 1057 155.2 14.25 M33 GHSC-U60-SF10-ST1 171 342 57 5.7 0 765 1057 155.2 14.25 M34 GHSC-U60-SF10-PET1 171 342 57 0 5.7 765 1057 155.2 14.25 M35 GHSC-U60-SF10-ST1-PET1 171 342 57 5.7 5.7 765 1057 155.2 14.25 M36 GHSC-U60-SF20 114 342 114 0 0 765 1057 155.2 14.25 M37 GHSC-U60-SF20-ST1 114 342 114 5.7 0 765 1057 155.2 14.25 M38 GHSC-U60-SF20-PET1 114 342 114 0 5.7 765 1057 155.2 14.25 M39 GHSC-U60-SF20-ST1-PET1 114 342 114 5.7 5.7 765 1057 155.2 14.25 Table 6 shows the ANOVA results for the compressive strength parameters that were measured at 3, 7, and 28 days. The models were statistically significant at the 95% confidence level, as the P-values were less than 0.05, as indicated by the results. Furthermore, the substantial P-values for lack of fit (greater than 0.05) throughout each response suggest that the F-value was not significant, indicating a meaningful relationship between the variables and the process responses. Based on empirical correlation, the second-order polynomial illustrates the relationship between compressive strength at 3, 7, and 28 days and the parameters of UPOFA, SF, steel and PET fibers, as shown in the equation below: (2) Compressive 3 days = 68.22 − 3.98 A + 2.52 B + 2.26 C + 0.9447 D − 0.1319 AB + 0.2342 AC + 0.2011 AD − 0.0261 BC − 0.0845 BD + 0.3018 CD − 0.6814 A 2 + 0.1798 B 2 (3) Compressive 7days = 78.67 − 3.92 A + 4.39 B + 3.09 C + 1.01 7D − 0.1069 AB + 0.2333 AC + 0.1392 AD − 0.0285 BC − 0.1190 BD + 0.8339 CD − 4.13 A 2 + 0.7453 B 2 (4) Compressive 28 days = 85.36 − 3.56 A + 4.83 B + 3.15 C + 1.29 D − 0.2513 AB + 0.4421 AC + 0.4196 AD − 0.0917 BC − 0.0558 BD + 0.3881 CD − 4.48 A 2 + 0.9556 B 2 Table 7 presents the model validation parameters for all responses. The ANOVA analysis demonstrates a high degree of confidence in the calculation of response efficiencies, represented as R 2 . The quadratic model demonstrates a commendable fit to the experimental data, as seen by a high R 2 value close to one and a suitable correlation with the modified R 2 . This finding has also been confirmed by Ghafari et al. and Alani et al. 61 , 62 Further, the coefficient of variation of a model is deemed repeatable if it is less than 10%. 63 The repeatability of all models was satisfactory, as demonstrated in Table 7 . To establish a favorable accord between the quadratic model and the actual data, the R 2 value must be consistent with the adjusted R 2 . The ANOVA results of this study indicate a highly reliable level of confidence in evaluating response efficacy. The R 2 and adjusted R 2 values for compressive strength are (R 2 = 0.9697 and adjusted R 2 = 0.9539) at 3 days, (R 2 = 0.9564 and adjusted R 2 = 0.9337) at 7 days, and (R 2 = 0.9674 and adjusted R 2 = 0.9504) at 28 days. Furthermore, the precision values are 32.1, 24.92, and 29.41 at 3, 7, and 28 days, respectively; these values exceed 4, which is considered favorable. Moreover, there exists a substantial level of confidence in the standard deviation, which was much smaller than the mean values obtained. At 3 days, the compressive strength exhibits a standard deviation of 1.19 and a mean of 65.91; at 7 days, the standard deviation is around 1.88 with a mean of 74.26; and at 28 days, the standard deviations are 1.63 and 81.1. This finding implies that the variance analysis was both suitable and statistically significant. A decreasing standard deviation of the created model relative to its mean indicates that the test data exhibit less variability. The experimental data will provide a model with reduced uncertainty. 10 , 64 Table 7. Model validation of compressive strength at 3, 7, and 28 days. Response Compressive Strength 3 days Compressive Strength 7 days Compressive Strength 28 days Standard deviation 1.19 1.88 1.63 Mean 65.91 74.26 81.1 R 2 0.9697 0.9564 0.9674 Adjusted R 2 0.9539 0.9337 0.9504 Predicted R 2 0.926 0.8957 0.92451 Precision 32.1 24.92 29.41 Table 6. ANOVA results for response surface quadratic model parameters. Responses Source SOS DF MS F-value p-value Remark Compressive strength 3 days Model 1045.15 12 87.1 61.40 <0.0001 Sign. A-UPOFA 276.29 1 276.29 194.79 <0.0001 Sign. B-SF 152.34 1 152.34 107.40 <0.0001 Sign. C-ST 134.17 1 134.17 94.59 <0.0001 Sign. D-PET 23.37 1 23.37 16.47 0.0005 Sign. AB 1.11 1 1.11 0.7846 0.3849 Not- Sign. AC 1.32 1 1.32 0.9284 0.3453 Not- Sign. AD 0.9703 1 0.9703 0.6841 0.4167 Not- Sign. BC 0.0654 1 0.0654 0.0461 0.8319 Not- Sign. BD 0.6853 1 0.6853 0.4831 0.4940 Not- Sign. CD 3.28 1 3.28 2.31 0.1420 Not- Sign. A 2 3.71 1 3.71 2.62 0.1193 Not- Sign. B 2 4.14 1 4.14 2.92 0.1011 Not- Sign. C 2 0.0000 0 D 2 0.0000 0 Residual 32.62 23 1.42 Cor. Total 1077.78 35 87.1 Compressive strength 7 days Model 1775.57 12 147.96 <0.0001 <0.0001 Sign. A-UPOFA 268.70 1 268.70 <0.0001 <0.0001 Sign. B-SF 462.53 1 462.53 <0.0001 <0.0001 Sign. C-ST 249.30 1 249.30 <0.0001 <0.0001 Sign. D-PET 26.85 1 26.85 0.0111 0.0111 Sign. AB 0.7310 1 0.7310 0.6527 0.6527 Not- Sign. AC 1.31 1 1.31 0.5481 0.5481 Not- Sign. AD 0.4648 1 0.4648 0.7195 0.7195 Not- Sign. BC 0.0782 1 0.0782 0.8828 0.8828 Not- Sign. BD 1.36 1 1.36 0.5404 0.5404 Not- Sign. CD 25.03 1 25.03 0.0137 0.0137 Sign. A 2 136.29 1 136.29 <0.0001 <0.0001 Sign. B 2 71.10 1 71.10 0.0002 0.0002 Sign. C 2 0.0000 0 D 2 0.0000 0 Residual 80.88 23 3.52 Cor. Total 1856.45 35 Compressive strength 28 days Model 1811.68 12 150.97 56.91 <0.0001 Sign. A-UPOFA 221.32 1 221.32 83.42 <0.0001 Sign. B-SF 559.60 1 559.60 210.93 <0.0001 Sign. C-ST 260.57 1 260.57 98.21 <0.0001 Sign. D-PET 43.87 1 43.87 16.54 0.0005 Sign. AB 4.04 1 4.04 1.52 0.2297 Not- Sign. AC 4.69 1 4.69 1.77 0.1967 Not- Sign. AD 4.23 1 4.23 1.59 0.2196 Not- Sign. BC 0.8067 1 0.8067 0.3041 0.5867 Not- Sign. BD 0.2993 1 0.2993 0.1128 0.7400 Not- Sign. CD 5.42 1 5.42 2.04 0.1663 Sign. A 2 160.65 1 160.65 60.55 <0.0001 Sign. B 2 116.89 1 116.89 44.06 <0.0001 Sign. C 2 0.0000 0 D 2 0.0000 0 Residual 61.02 23 2.65 Cor. Total 1872.70 35 Diagnostic charts are illustrated in Figures 9 and 10 , which include the normal plot of residuals and a plot comparing expected and actual values. Moreover, the remaining points of the equality line are evenly distributed. The data fell below the line of equality, showing the effectiveness of the regression models. Besides, the scheme of the normal probability plot of all replies is based on the distribution of response model data points. The graphic indicates that the remaining points are nearly linear, which means that the model of the invasion is normally distributed. Thereby, the residuals are randomly spread in a straight line, the distribution of residuals is nearly normal, and the model uniforms the data quite well. These diagnostic plots are instrumental in evaluating the models’ appropriateness and efficacy. 61 Thus, Figure 9 and Figure 10 , illustrate the plots of the predicted vs. actual values and the normal plots of residuals for compressive strength at 3, 7, and 28 days, respectively. The data indicate a vital correlation between the model-generated compressive strength responses and the real values. Figure 9. Diagnostics plots, predicted vs actual values for compressive strength at (a) 3 days, (b) 7 days, and (c) 28 days. Figure 10. Normal plot of residual for compressive strength at (a) 3 days, (b) 7 days, and (c) 28 days. 3.3 Process analysis The perturbation graphs shown in Figures 11 (a), (b), and (c) illustrate how the variables affect compressive strength at early ages of 3, 7, and 28 days. The perturbation plots in the RSM highlight the influence of each parameter by showing how changes in these variables are related to variations in compressive strength. As a result, the combined effect of SF, ST, and PET substantially mitigates the compressive strength declines associated with UPOFA content. Particularly, the curvature of SF-ST-PET (B, C, and D) was sharper than the convex shape for UPOFA (A). That may indicate that SF-ST-PET variables affected compressive strength at all ages more than UPOFA. Thereby, UPOFA (A) exhibits increasing compressive strength at intermediate values and decreasing it at high levels. In contrast, SF (B), ST (C), and PET (D) show a linear positive correlation, leading to a consistent increase in compressive strength with higher levels. Therefore, Post-hoc tests were not conducted as the principal analysis showed that the compressive strength increased significantly due to a combination of silica, steel fibers, and PET fibers, so the extra post-hoc comparisons were not necessary. Figure 11. Perturbation plot for compressive strength at (a) 3 days, (b) 7 days, and (c) 28 days. 3.4 Optimization of multiple responses In the numerical optimization process, the factors UPOFA, SF, ST, and PET fiber were assigned specific ranges. At the same time, the compressive strength responses at the ages of 3, 7, and 28 days were also defined within certain limits. The main objective of combining SF, ST, and PET is to achieve the highest compressive strength values at early ages for GHSC containing high amounts of UPOFA (30% and 60%). To identify the optimal conditions, factors based on the Response Surface Methodology (RSM) desirability criterion are utilized. The model equations were solved simultaneously to identify the process variables. At the optimal state, the compressive strengths achieved were 74.3 MPa, 88.65 MPa, and 95.95 MPa at the ages of 3, 7, and 28 days, respectively. This effect was achieved with 30% UPOFA, 20% SF, 1% ST, and 1% PET. As a result, the compressive strength of GHSC was optimized, leading to a desirability value of 0.98 based on the variable factors and responses, as illustrated in the graphical ramp from Figure 12 . There is indeed a need to mention that the optimized concrete mix includes silica, steel fiber, and PET fiber, has a much better compressive strength, ductility, and crack resistance. They increase its applicability especially in structural components with high load needs such as beams, columns, and slabs, and in areas where it is important that both durability, impact resistance, and long-term performance. Thus, related to current study the optimized GHSC is applicable in structural elements, which include beams, columns, and precast elements, especially in settings that demand high strength and durability. Consequently, The blending of fibers and pozzolanic substances will offer a sure way of minimizing the maintenance and the service life of the concrete structures. Figure 12. Ramp function graph for optimum compressive strength at 3, 7, and 28 days. 4. Conclusions This work used RSM to assess the feasibility of hybridizing different materials, including SF, steel, and PET fiber, to improve the significant decrease in compressive strength at early ages in GHSC containing substantial quantities of UPOFA. The following conclusions may be derived from the findings. (1) The compressive strength of GHSC decreased as the percentage of UPOFA replaced with OPC increased from 30% to 60%. The lowest strength values were recorded at the ages of 3, 7, and 28 days with a 60% UPOFA concentration. The incorporation of hybrid materials namely as SF, steel fiber, and PET fiber enhances the compressive strength of 30% and 60% UPOFA-GHSC. At the ages of 3, 7, and 28 days, the GHSC mix of 30% UPOFA, 20% SF, 1% ST, and 1% PET (M27) demonstrated superior strength of about 76.4 MPa, 89.21 MPa, and 95.35 MPa, respectively. Furthermore, adding hybrid materials to the GHSC with a higher UPOFA of 60% increased compressive strength (M39) by 17%, 25%, and 18% at 3, 7, and 28 days, respectively, when compared to the control mix. (2) For the GHSC combination M27, which consists of 30% UPOFA, 20% SF, 1% ST, and 1% PET, RSM predicts the optimum conditions. The results of the ANOVA show that SF ST-PET has a greater effect on compressive strength than UPOFA. The experimental findings were well predicted by the quadratic regression, with R 2 values of 0.9697, 0.9564, and 0.9674 for compressive strength at 3, 7 and 28 days, respectively. Hence, RSM might be a useful method for improving the mix design of green concrete with UPOFA, which reduces its strength at early ages. By lowering cement production energy consumption and waste volume, this method has the potential to increase the sustainability of concrete, make UPOFA more widely used in the industry, and lessen the negative environmental consequences. (3) Future works are required in the optimization of GHSC by RSM in terms of long-term durability performance in diverse environmental factors, such as chloride penetration, sulfate attack, carbonation, water absorption, and permeability. Moreover, machine learning in combination with RSM can make its predictions more accurate and create a more resilient multi-objective optimization model that will consider the mechanical and durability characteristics of sustainable concrete design. Data availability The datasets supporting the findings of this study are openly available in the Zenodo repository at https://doi.org/10.5281/zenodo.17991317 65 under a Creative Commons Attribution 4.0 International license (CC-BY 4.0). Acknowledgements The authors acknowledge the University of Fallujah, Iraq, and the Civil Engineering Department at Universiti Sains Malaysia for their support in providing the materials and equipment used in this study. References 1. Hamada H, Abed F, Alattar A, et al. : Influence of palm oil fuel ash on the high strength and ultra-high performance concrete: A comprehensive review. Eng. Sci. Technol. an Int. J. 2023; 45 : 101492. 2. Colangelo F, Farina I, Petrillo A, et al. : Innovative Materials in Italy for Eco-Friendly and Sustainable Buildings.2021. 3. 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Comments on this article Comments (0) Version 3 VERSION 3 PUBLISHED 22 Jan 2026 ADD YOUR COMMENT Comment Author details Author details 1 Department of Engineering Affairs, University of Fallujah, Al-Fallujah, Al Anbar Governorate, Iraq 2 University of Bilad AL-Rafidain, Baqubah, Iraq 3 University of Fallujah, Scientific Affairs Department, Fallujah, Iraq 4 Universiti Sains Malaysia School of Civil Engineering, Nibong Tebal, Penang, Malaysia 5 Technical Engineering College, Al-Esraa University, Baghdad, Iraq Aktham H Alani Roles: Conceptualization, Data Curation, Formal Analysis, Methodology, Supervision, Writing – Original Draft Preparation, Writing – Review & Editing Doaa Ahmed Roles: Methodology, Resources, Validation Qasim A. Al-obaidi Roles: Methodology, Resources, Software Nahla Hilal Roles: Data Curation, Resources, Software, Writing – Original Draft Preparation Megat Azmi Megat Johari Roles: Data Curation, Methodology, Resources, Validation, Writing – Original Draft Preparation Hayder Sadeq Al-Aasam Roles: Data Curation, Formal Analysis, Methodology, Resources Competing interests No competing interests were disclosed. Grant information The author(s) declared that no grants were involved in supporting this work. Article Versions (3) version 3 Revised Published: 05 May 2026, 15:103 https://doi.org/10.12688/f1000research.175237.3 version 2 Revised Published: 26 Feb 2026, 15:103 https://doi.org/10.12688/f1000research.175237.2 version 1 Published: 22 Jan 2026, 15:103 https://doi.org/10.12688/f1000research.175237.1 Copyright © 2026 Alani AH et al . This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Download Export To Sciwheel Bibtex EndNote ProCite Ref. Manager (RIS) Sente metrics Views Downloads F1000Research - - PubMed Central info_outline Data from PMC are received and updated monthly. - - Citations open_in_new 0 open_in_new 0 open_in_new SEE MORE DETAILS CITE how to cite this article Alani AH, Ahmed D, Al-obaidi QA et al. Optimizing Early Compressive Strength of Ultrafine POFA-Based Green HSC: An Experimental and RSM Approach with Silica Fume and Hybrid Fibers [version 3; peer review: 1 approved, 1 approved with reservations] . F1000Research 2026, 15 :103 ( https://doi.org/10.12688/f1000research.175237.3 ) NOTE: If applicable, it is important to ensure the information in square brackets after the title is included in all citations of this article. COPY CITATION DETAILS track receive updates on this article Track an article to receive email alerts on any updates to this article. TRACK THIS ARTICLE Share Open Peer Review Current Reviewer Status: ? Key to Reviewer Statuses VIEW HIDE Approved The paper is scientifically sound in its current form and only minor, if any, improvements are suggested Approved with reservations A number of small changes, sometimes more significant revisions are required to address specific details and improve the papers academic merit. Not approved Fundamental flaws in the paper seriously undermine the findings and conclusions Version 2 VERSION 2 PUBLISHED 26 Feb 2026 Revised Views 0 Cite How to cite this report: Debieb F. Reviewer Report For: Optimizing Early Compressive Strength of Ultrafine POFA-Based Green HSC: An Experimental and RSM Approach with Silica Fume and Hybrid Fibers [version 3; peer review: 1 approved, 1 approved with reservations] . F1000Research 2026, 15 :103 ( https://doi.org/10.5256/f1000research.196708.r469919 ) The direct URL for this report is: https://f1000research.com/articles/15-103/v2#referee-response-469919 NOTE: it is important to ensure the information in square brackets after the title is included in this citation. Close Copy Citation Details Reviewer Report 13 Apr 2026 Farid Debieb , University of Medea, Medea, Algeria Approved VIEWS 0 https://doi.org/10.5256/f1000research.196708.r469919 The manuscript presents a well-founded and effectively implemented study. The experiment plans, the use of statistics for data analysis, and the conclusions are all quite fitting. However, it fails in giving enough information for a complete duplication, especially regarding ... Continue reading READ ALL The manuscript presents a well-founded and effectively implemented study. The experiment plans, the use of statistics for data analysis, and the conclusions are all quite fitting. However, it fails in giving enough information for a complete duplication, especially regarding UPOFA processing, the mixing procedure, compaction, and workability data. Besides, there are minor problems with clarity, like repetition and references to figures. Here are some ways that might improve the paper's overall strength and clarity: 1- Why were steel fiber (ST) and PET fiber only at 0% or 1% levels and no intermediate levels (e.g., 0.5% or 1.5%) were considered, whereas UPOFA and SF each had three levels? 2 The article states that the compressive strength drops at 60% UPOFA are because of the dilution and low production of C-S-H. However, could you clarify the part of overconsumption of CH due to a high amount of amorphous silica? 3- According to the ANOVA results (Table 6), nearly all two-way interactions (AB, AC, AD, BC, BD) are not statistically significant (p > 0.05). So, is using a full quadratic RSM model necessary? 4- The study looks at only the early-age mechanical strength (3, 7, 28 days). What are the most important durability properties that need to be examined before this optimized GHSC can be used on an industrial scale? 5- Is it possible that the findings of this study apply to other pozzolanic wastes (e.g., rice husk ash) or other synthetic fibers (e.g., polypropylene, PVA)? Is the work clearly and accurately presented and does it cite the current literature? Partly Is the study design appropriate and is the work technically sound? Yes Are sufficient details of methods and analysis provided to allow replication by others? Partly If applicable, is the statistical analysis and its interpretation appropriate? Yes Are all the source data underlying the results available to ensure full reproducibility? Partly Are the conclusions drawn adequately supported by the results? Yes Competing Interests: No competing interests were disclosed. Reviewer Expertise: innovative materials for construction, concrete technology, recycled concrete I confirm that I have read this submission and believe that I have an appropriate level of expertise to confirm that it is of an acceptable scientific standard. Close READ LESS CITE CITE HOW TO CITE THIS REPORT Debieb F. Reviewer Report For: Optimizing Early Compressive Strength of Ultrafine POFA-Based Green HSC: An Experimental and RSM Approach with Silica Fume and Hybrid Fibers [version 3; peer review: 1 approved, 1 approved with reservations] . F1000Research 2026, 15 :103 ( https://doi.org/10.5256/f1000research.196708.r469919 ) The direct URL for this report is: https://f1000research.com/articles/15-103/v2#referee-response-469919 NOTE: it is important to ensure the information in square brackets after the title is included in all citations of this article. COPY CITATION DETAILS Report a concern Author Response 05 May 2026 Aktham Hatem Qasim , Department of Engineering Affairs, University of Fallujah, Al-Fallujah, Iraq 05 May 2026 Author Response Reviewer 2 Why were steel fiber (ST) and PET fiber only at 0% or 1% levels and no intermediate levels (e.g., 0.5% or 1.5%) were considered, whereas UPOFA ... Continue reading Reviewer 2 Why were steel fiber (ST) and PET fiber only at 0% or 1% levels and no intermediate levels (e.g., 0.5% or 1.5%) were considered, whereas UPOFA and SF each had three levels? Action Thanks for the reviewer’s important comment . The use of steel fiber (ST) and PET fiber was limited to only 0% and 1% without intermediate levels, as previous studies have consistently shown that the most significant effect of these types of fibers—especially when combined with pozzolanic materials—occurs around this percentage. Therefore, selecting 0% and 1% was sufficient to capture the primary influence of fiber addition, while avoiding unnecessary intermediate levels that may not produce substantially different results. In contrast, UPOFA and silica fume (SF) were studied at multiple levels because their influence on material properties is more gradual and requires a broader range to fully evaluate their effect. The article states that the compressive strength drops at 60% UPOFA are because of the dilution and low production of C-S-H. However, could you clarify the part of overconsumption of CH due to a high amount of amorphous silica? Action Thanks for the reviewer’s careful reading. (Done, added to manuscript see subsection 3.1). The increased concentration of amorphous silica, including 60% UPOFA, accelerates the consumption rate of Calcium Hydroxide (CH); nevertheless, the reduced cement content hampers its production in areas where CH is being exhausted. This impact results in the pozzolanic reaction attaining a CH-limited state, thereby leading to a reduced and delayed synthesis of Calcium Silicate Hydrate (C-S-H). Thereby, the interaction between the availability of calcium hydroxide and silica content in palm oil fuel ash significantly influences the evolution of compressive strength. According to the ANOVA results (Table 6), nearly all two-way interactions (AB, AC, AD, BC, BD) are not statistically significant (p > 0.05). So, is using a full quadratic RSM model necessary? Action Thanks for the reviewer’s careful reading. Although most of the two-way interactions were statistically insignificant (p > 0.05), the entire quadratic RSM model was retained to retain hierarchy and explain any curvature. The high statistical measures (R2> 0.93 and Precision> 24) also affirm the sufficiency of the model and a high predictive power and outstanding signal to noise. The study looks at only the early-age mechanical strength (3, 7, 28 days). What are the most important durability properties that need to be examined before this optimized GHSC can be used on an industrial scale? Action Thanks for the reviewer’s important comment. (Noted, done, the future work paragraph has been added at the end of the conclusion section). Future works are required in the optimization of GHSC by RSM in terms of long-term durability performance in diverse environmental factors, such as chloride penetration, sulfate attack, carbonation, water absorption, and permeability. Moreover, machine learning in combination with RSM can make its predictions more accurate and create a more resilient multi-objective optimization model that will consider the mechanical and durability characteristics of sustainable concrete design. Is it possible that the findings of this study apply to other pozzolanic wastes (e.g., rice husk ash) or other synthetic fibers (e.g., polypropylene, PVA)? Action Thanks for the reviewer’s important comment. Other pozzolanic fibers such as Rice Husk Ash and synthetic fibers, such as Polypropylene Fiber and Polyvinyl Alcohol Fiber can be qualitatively checked using the results since they are pozzolanically similar and react with the fiber-pozzolana mechanisms. It is however a system dependent utility since the variations in chemical compositions, curing, particle and interfacial properties can play a major role and thus should be assured on a case by case analysis. Reviewer 2 Why were steel fiber (ST) and PET fiber only at 0% or 1% levels and no intermediate levels (e.g., 0.5% or 1.5%) were considered, whereas UPOFA and SF each had three levels? Action Thanks for the reviewer’s important comment . The use of steel fiber (ST) and PET fiber was limited to only 0% and 1% without intermediate levels, as previous studies have consistently shown that the most significant effect of these types of fibers—especially when combined with pozzolanic materials—occurs around this percentage. Therefore, selecting 0% and 1% was sufficient to capture the primary influence of fiber addition, while avoiding unnecessary intermediate levels that may not produce substantially different results. In contrast, UPOFA and silica fume (SF) were studied at multiple levels because their influence on material properties is more gradual and requires a broader range to fully evaluate their effect. The article states that the compressive strength drops at 60% UPOFA are because of the dilution and low production of C-S-H. However, could you clarify the part of overconsumption of CH due to a high amount of amorphous silica? Action Thanks for the reviewer’s careful reading. (Done, added to manuscript see subsection 3.1). The increased concentration of amorphous silica, including 60% UPOFA, accelerates the consumption rate of Calcium Hydroxide (CH); nevertheless, the reduced cement content hampers its production in areas where CH is being exhausted. This impact results in the pozzolanic reaction attaining a CH-limited state, thereby leading to a reduced and delayed synthesis of Calcium Silicate Hydrate (C-S-H). Thereby, the interaction between the availability of calcium hydroxide and silica content in palm oil fuel ash significantly influences the evolution of compressive strength. According to the ANOVA results (Table 6), nearly all two-way interactions (AB, AC, AD, BC, BD) are not statistically significant (p > 0.05). So, is using a full quadratic RSM model necessary? Action Thanks for the reviewer’s careful reading. Although most of the two-way interactions were statistically insignificant (p > 0.05), the entire quadratic RSM model was retained to retain hierarchy and explain any curvature. The high statistical measures (R2> 0.93 and Precision> 24) also affirm the sufficiency of the model and a high predictive power and outstanding signal to noise. The study looks at only the early-age mechanical strength (3, 7, 28 days). What are the most important durability properties that need to be examined before this optimized GHSC can be used on an industrial scale? Action Thanks for the reviewer’s important comment. (Noted, done, the future work paragraph has been added at the end of the conclusion section). Future works are required in the optimization of GHSC by RSM in terms of long-term durability performance in diverse environmental factors, such as chloride penetration, sulfate attack, carbonation, water absorption, and permeability. Moreover, machine learning in combination with RSM can make its predictions more accurate and create a more resilient multi-objective optimization model that will consider the mechanical and durability characteristics of sustainable concrete design. Is it possible that the findings of this study apply to other pozzolanic wastes (e.g., rice husk ash) or other synthetic fibers (e.g., polypropylene, PVA)? Action Thanks for the reviewer’s important comment. Other pozzolanic fibers such as Rice Husk Ash and synthetic fibers, such as Polypropylene Fiber and Polyvinyl Alcohol Fiber can be qualitatively checked using the results since they are pozzolanically similar and react with the fiber-pozzolana mechanisms. It is however a system dependent utility since the variations in chemical compositions, curing, particle and interfacial properties can play a major role and thus should be assured on a case by case analysis. Competing Interests: No competing interests were disclosed. Close Report a concern Respond or Comment COMMENTS ON THIS REPORT Author Response 05 May 2026 Aktham Hatem Qasim , Department of Engineering Affairs, University of Fallujah, Al-Fallujah, Iraq 05 May 2026 Author Response Reviewer 2 Why were steel fiber (ST) and PET fiber only at 0% or 1% levels and no intermediate levels (e.g., 0.5% or 1.5%) were considered, whereas UPOFA ... Continue reading Reviewer 2 Why were steel fiber (ST) and PET fiber only at 0% or 1% levels and no intermediate levels (e.g., 0.5% or 1.5%) were considered, whereas UPOFA and SF each had three levels? Action Thanks for the reviewer’s important comment . The use of steel fiber (ST) and PET fiber was limited to only 0% and 1% without intermediate levels, as previous studies have consistently shown that the most significant effect of these types of fibers—especially when combined with pozzolanic materials—occurs around this percentage. Therefore, selecting 0% and 1% was sufficient to capture the primary influence of fiber addition, while avoiding unnecessary intermediate levels that may not produce substantially different results. In contrast, UPOFA and silica fume (SF) were studied at multiple levels because their influence on material properties is more gradual and requires a broader range to fully evaluate their effect. The article states that the compressive strength drops at 60% UPOFA are because of the dilution and low production of C-S-H. However, could you clarify the part of overconsumption of CH due to a high amount of amorphous silica? Action Thanks for the reviewer’s careful reading. (Done, added to manuscript see subsection 3.1). The increased concentration of amorphous silica, including 60% UPOFA, accelerates the consumption rate of Calcium Hydroxide (CH); nevertheless, the reduced cement content hampers its production in areas where CH is being exhausted. This impact results in the pozzolanic reaction attaining a CH-limited state, thereby leading to a reduced and delayed synthesis of Calcium Silicate Hydrate (C-S-H). Thereby, the interaction between the availability of calcium hydroxide and silica content in palm oil fuel ash significantly influences the evolution of compressive strength. According to the ANOVA results (Table 6), nearly all two-way interactions (AB, AC, AD, BC, BD) are not statistically significant (p > 0.05). So, is using a full quadratic RSM model necessary? Action Thanks for the reviewer’s careful reading. Although most of the two-way interactions were statistically insignificant (p > 0.05), the entire quadratic RSM model was retained to retain hierarchy and explain any curvature. The high statistical measures (R2> 0.93 and Precision> 24) also affirm the sufficiency of the model and a high predictive power and outstanding signal to noise. The study looks at only the early-age mechanical strength (3, 7, 28 days). What are the most important durability properties that need to be examined before this optimized GHSC can be used on an industrial scale? Action Thanks for the reviewer’s important comment. (Noted, done, the future work paragraph has been added at the end of the conclusion section). Future works are required in the optimization of GHSC by RSM in terms of long-term durability performance in diverse environmental factors, such as chloride penetration, sulfate attack, carbonation, water absorption, and permeability. Moreover, machine learning in combination with RSM can make its predictions more accurate and create a more resilient multi-objective optimization model that will consider the mechanical and durability characteristics of sustainable concrete design. Is it possible that the findings of this study apply to other pozzolanic wastes (e.g., rice husk ash) or other synthetic fibers (e.g., polypropylene, PVA)? Action Thanks for the reviewer’s important comment. Other pozzolanic fibers such as Rice Husk Ash and synthetic fibers, such as Polypropylene Fiber and Polyvinyl Alcohol Fiber can be qualitatively checked using the results since they are pozzolanically similar and react with the fiber-pozzolana mechanisms. It is however a system dependent utility since the variations in chemical compositions, curing, particle and interfacial properties can play a major role and thus should be assured on a case by case analysis. Reviewer 2 Why were steel fiber (ST) and PET fiber only at 0% or 1% levels and no intermediate levels (e.g., 0.5% or 1.5%) were considered, whereas UPOFA and SF each had three levels? Action Thanks for the reviewer’s important comment . The use of steel fiber (ST) and PET fiber was limited to only 0% and 1% without intermediate levels, as previous studies have consistently shown that the most significant effect of these types of fibers—especially when combined with pozzolanic materials—occurs around this percentage. Therefore, selecting 0% and 1% was sufficient to capture the primary influence of fiber addition, while avoiding unnecessary intermediate levels that may not produce substantially different results. In contrast, UPOFA and silica fume (SF) were studied at multiple levels because their influence on material properties is more gradual and requires a broader range to fully evaluate their effect. The article states that the compressive strength drops at 60% UPOFA are because of the dilution and low production of C-S-H. However, could you clarify the part of overconsumption of CH due to a high amount of amorphous silica? Action Thanks for the reviewer’s careful reading. (Done, added to manuscript see subsection 3.1). The increased concentration of amorphous silica, including 60% UPOFA, accelerates the consumption rate of Calcium Hydroxide (CH); nevertheless, the reduced cement content hampers its production in areas where CH is being exhausted. This impact results in the pozzolanic reaction attaining a CH-limited state, thereby leading to a reduced and delayed synthesis of Calcium Silicate Hydrate (C-S-H). Thereby, the interaction between the availability of calcium hydroxide and silica content in palm oil fuel ash significantly influences the evolution of compressive strength. According to the ANOVA results (Table 6), nearly all two-way interactions (AB, AC, AD, BC, BD) are not statistically significant (p > 0.05). So, is using a full quadratic RSM model necessary? Action Thanks for the reviewer’s careful reading. Although most of the two-way interactions were statistically insignificant (p > 0.05), the entire quadratic RSM model was retained to retain hierarchy and explain any curvature. The high statistical measures (R2> 0.93 and Precision> 24) also affirm the sufficiency of the model and a high predictive power and outstanding signal to noise. The study looks at only the early-age mechanical strength (3, 7, 28 days). What are the most important durability properties that need to be examined before this optimized GHSC can be used on an industrial scale? Action Thanks for the reviewer’s important comment. (Noted, done, the future work paragraph has been added at the end of the conclusion section). Future works are required in the optimization of GHSC by RSM in terms of long-term durability performance in diverse environmental factors, such as chloride penetration, sulfate attack, carbonation, water absorption, and permeability. Moreover, machine learning in combination with RSM can make its predictions more accurate and create a more resilient multi-objective optimization model that will consider the mechanical and durability characteristics of sustainable concrete design. Is it possible that the findings of this study apply to other pozzolanic wastes (e.g., rice husk ash) or other synthetic fibers (e.g., polypropylene, PVA)? Action Thanks for the reviewer’s important comment. Other pozzolanic fibers such as Rice Husk Ash and synthetic fibers, such as Polypropylene Fiber and Polyvinyl Alcohol Fiber can be qualitatively checked using the results since they are pozzolanically similar and react with the fiber-pozzolana mechanisms. It is however a system dependent utility since the variations in chemical compositions, curing, particle and interfacial properties can play a major role and thus should be assured on a case by case analysis. Competing Interests: No competing interests were disclosed. Close Report a concern COMMENT ON THIS REPORT Version 1 VERSION 1 PUBLISHED 22 Jan 2026 Views 0 Cite How to cite this report: Haruehansapong S. Reviewer Report For: Optimizing Early Compressive Strength of Ultrafine POFA-Based Green HSC: An Experimental and RSM Approach with Silica Fume and Hybrid Fibers [version 3; peer review: 1 approved, 1 approved with reservations] . F1000Research 2026, 15 :103 ( https://doi.org/10.5256/f1000research.193200.r456473 ) The direct URL for this report is: https://f1000research.com/articles/15-103/v1#referee-response-456473 NOTE: it is important to ensure the information in square brackets after the title is included in this citation. Close Copy Citation Details Reviewer Report 11 Feb 2026 Sattawat Haruehansapong , Rajamangala University of Technology Tawan-ok, Bangkok, Thailand Approved with Reservations VIEWS 0 https://doi.org/10.5256/f1000research.193200.r456473 The manuscript presents a well-structured experimental and modeling study on the optimization of early-age compressive strength of ultrafine POFA-based green high-strength concrete. The experimental program is sound, and the statistical modeling approach is appropriate. The conclusions are generally supported by ... Continue reading READ ALL The manuscript presents a well-structured experimental and modeling study on the optimization of early-age compressive strength of ultrafine POFA-based green high-strength concrete. The experimental program is sound, and the statistical modeling approach is appropriate. The conclusions are generally supported by the presented evidence. However, several minor but important revisions are recommended to improve clarity, conciseness, and engineering relevance before final approval. 1. Title The title is informative but could better reflect the use of statistical optimization techniques. Suggested action: Consider explicitly mentioning the use of response surface methodology or statistical optimization. 2. Abstract Clearly explain why early-age compressive strength is critical, particularly for UPOFA-based concrete. Explicitly state the application scope (non-structural or low-load applications). Separate experimental findings from modeling outcomes more clearly. Suggested action: Add one sentence clarifying application limitations. Revise for conciseness. 3. Introduction Reduce repetition related to general sustainability concepts. Strengthen the research gap, particularly regarding hybrid fiber systems in high-volume UPOFA concrete. Clearly distinguish this study from previous related works. 4.1 Materials Characterization Consider adding a summary table describing the role and expected effect of each material (UPOFA, silica fume, steel fiber, PET fiber). 4.2 Experimental Design and RSM Justify why steel fiber and PET fiber contents were fixed at specific values. Briefly justify the selection of CCD over other RSM designs. 5.1 Compressive Strength Results Reduce narrative repetition of numerical values already presented in figures. Strengthen microstructural explanations linking UPOFA dilution, silica fume nucleation, and fiber bridging effects. 5.2 Statistical Analysis and Modeling Briefly explain why post-hoc statistical tests were not conducted. Clearly distinguish statistical significance from engineering significance. 6. Optimization Results and Practical Implications Add a short subsection explicitly addressing engineering applications of the optimized mix. Suggested action: Clearly state where and how the optimized concrete can be practically used. 7. Conclusions Avoid repeating detailed numerical results. Explicitly acknowledge study limitations (laboratory scale, early-age focus). Provide a clear direction for future work. Is the work clearly and accurately presented and does it cite the current literature? Partly Is the study design appropriate and is the work technically sound? Yes Are sufficient details of methods and analysis provided to allow replication by others? Yes If applicable, is the statistical analysis and its interpretation appropriate? Partly Are all the source data underlying the results available to ensure full reproducibility? Partly Are the conclusions drawn adequately supported by the results? Yes Competing Interests: No competing interests were disclosed. Reviewer Expertise: concrete, materials, cement I confirm that I have read this submission and believe that I have an appropriate level of expertise to confirm that it is of an acceptable scientific standard, however I have significant reservations, as outlined above. Close READ LESS CITE CITE HOW TO CITE THIS REPORT Haruehansapong S. Reviewer Report For: Optimizing Early Compressive Strength of Ultrafine POFA-Based Green HSC: An Experimental and RSM Approach with Silica Fume and Hybrid Fibers [version 3; peer review: 1 approved, 1 approved with reservations] . F1000Research 2026, 15 :103 ( https://doi.org/10.5256/f1000research.193200.r456473 ) The direct URL for this report is: https://f1000research.com/articles/15-103/v1#referee-response-456473 NOTE: it is important to ensure the information in square brackets after the title is included in all citations of this article. COPY CITATION DETAILS Report a concern Author Response 05 May 2026 Aktham Hatem Qasim , Department of Engineering Affairs, University of Fallujah, Al-Fallujah, Iraq 05 May 2026 Author Response Response to Reviewer Comments The authors thank the reviewer for their careful reading and constructive comments below. We are very pleased to receive the reviewer’s recognition of the research work ... Continue reading Response to Reviewer Comments The authors thank the reviewer for their careful reading and constructive comments below. We are very pleased to receive the reviewer’s recognition of the research work in this paper. We believe that we have addressed all of the comments through our responses below, as well as in the text of the manuscript when appropriate. 1. Title The title is informative but could better reflect the use of statistical optimization techniques. Suggested action: Consider explicitly mentioning the use of response surface methodology or statistical optimization. Action Thanks for the reviewer’s important comment. (Noted, done) 2. Abstract Clearly explain why early-age compressive strength is critical, particularly for UPOFA-based concrete. Explicitly state the application scope (non-structural or low-load applications). Action Thanks for the reviewer’s important comment. (Noted, done) Moreover, the replacement rates of HSGC are probably high in POFA, which is constrained in the areas of the application where the removal of the formwork or pre-stressing of the structures is required in a very short period of time, and the high early-age strengths are required. 16 Separate experimental findings from modeling outcomes more clearly. Suggested action: Action Thanks for the reviewer’s suggestion, as shown, experimental findings and modeling outcomes are presented separately. Add one sentence clarifying application limitations. Action Thanks for the reviewer’s important comment (Noted, done). However, somehow these studies suggested the limitation of the POFA used because the highest was limited. The compressive strength was determined at a POFA replacement level of 20%, and the ultimate highest compressive strength of 28-days compressive strength achieved with the concretes that used POFA ranged between 60 and 86 MPa . 17, 18 3. Introduction Reduce repetition related to general sustainability concepts. Action Thanks for the reviewer’s comment (Noted, done, within introduction part). Strengthen the research gap, particularly regarding hybrid fiber systems in high-volume UPOFA concrete. Action Thanks for the reviewer’s comment (Noted, the research significance subsection already addresses the gap regarding hybrid fiber systems in high-volume UPOFA concrete. Clearly distinguish this study from previous related works. Action Thanks for the reviewer’s comment (Noted, the Introduction already highlights the study’s novelty and clearly distinguishes it from previous works. 4.1 Materials Characterization Consider adding a summary table describing the role and expected effect of each material (UPOFA, silica fume, steel fiber, PET fiber). Action Thanks for the reviewer’s excellent suggestion. (Noted, done, see Table 2). 4.2 Experimental Design and RSM Justify why the steel fiber and PET fiber contents were fixed at specific values. Action Thanks for the reviewer’s careful reading. (Noted, done). The contents of the steel and PET fibers were set to 1% volume, which is consistent with previous studies reporting that this amount has improved mechanical performance, crack control, and ductility whilst maintaining workability .21, 36 Briefly justify the selection of CCD over other RSM designs. Action Thanks for the reviewer’s comment. (Noted, done). CCD technique is favored to the full factorial and three-level design as it estimates the linear, quadratic and interaction effects with minimum tests, which will lead to a good model fitting, greater accuracy in prediction, reduced amount of residual error, and a high value of R2, that could permit the creation of three-dimensional response surfaces with great reliability. Consequently, it has commonly been considered as the best experimental design to investigate the second-order response models. 4, 15 5.1 Compressive Strength Results Reduce narrative repetition of numerical values already presented in figures. Thanks for the reviewer’s comment. Due to the high number of mixes and material changes, numerical values are supplied in the text in addition to tables and figures to assist the reader in understanding and following the patterns in the results. Strengthen microstructural explanations linking UPOFA dilution, silica fume nucleation, and fiber bridging effects. Action Thanks for the reviewer’s careful reading. (Noted, done) The compressive strength of GHSC is enhanced over a curing period of 7 to 28 days as a result of the accelerated hydration of conventional OPC and due to the pozzolanic effects of SF. The formation of secondary C-S-H is the consequence of the interaction between (Ca(OH)₂) produced during cement hydration and SiO₂ in SF 61,62 . This procedure increases the durability of the concrete by reducing the incidence of microcracks during periods of tension. In the same trend, adding steel and PET fibers improved the compressive strength of HSC and GHSC. The GHSC mixtures with a combination of (UPOFA-SF-ST-PET) registered the highest compressive strength of mixtures in GII and GIII at all curing periods. Hereby, the compressive strength of (M39) GHSC-U60 increased by 17%,25% and 18% at ages of 3, 7, and 28 days, respectively, related to control mix. This strength development was attributed to the effectiveness of binary binders (UPOFA and SF) in increasing bond formation between steel and PET fibers and other concrete ingredients. The tiny particle size and pozzolanic reactivity of UPOFA and SF reduced void volume and produced extra C-S-H gels, strengthening the aggregate-paste interfacial connection and increasing bonding capabilities with fibers. Adding steel and PET fibers may improve the fracture resistance of fiber-reinforced GHSC composites by limiting micro-crack formation under load, which is accomplished by forming a bridge network within the concrete microstructure 33,34,64 5.2 Statistical Analysis and Modeling Briefly explain why post-hoc statistical tests were not conducted. Action Thanks for the reviewer’s important comment. (Noted, done). Therefore, Post-hoc tests were not conducted as the principal analysis showed that the compressive strength increased significantly due to a combination of silica, steel fibers, and PET fibers, so the extra post-hoc comparisons were not necessary. .Clearly distinguish statistical significance from engineering significance. Action Thanks for the reviewer’s comment. (Noted). The practical relevance of these variations on material performance or structural design is reflected in engineering significance, whereas statistical significance shows that observed differences are unlikely to be the result of chance (e.g., p<0.05). Despite having little engineering significance, certain outcomes could be statistically significant. 6. Optimization Results and Practical Implications Add a short subsection explicitly addressing engineering applications of the optimized mix. Action Thanks for the reviewer’s important comment. (Noted, done). There is indeed a need to mention that the optimized concrete mix includes silica, steel fiber, and PET fiber, has a much better compressive strength, ductility, and crack resistance. They increase its applicability especially in structural components with high load needs such as beams, columns, and slabs, and in areas where it is important that both durability, impact resistance, and long-term performance. The blending of fibers and pozzolanic substances will offer a sure way of minimizing the maintenance and the service life of the concrete structures. Suggested action: Clearly state where and how the optimized concrete can be practically used. Thanks for the reviewer’s suggestion. 7. Conclusions Avoid repeating detailed numerical results. Explicitly acknowledge study limitations (laboratory scale, early-age focus). Provide a clear direction for future work Action Thanks for the reviewer’s important comment. (Noted, done). Further, this study primarily focused on early-age mechanical ability, conducted within a controlled civil engineering laboratory and environment. Consequently, additional future works are required to investigate durability, long-term performance, and comprehensive structural validation. Response to Reviewer Comments The authors thank the reviewer for their careful reading and constructive comments below. We are very pleased to receive the reviewer’s recognition of the research work in this paper. We believe that we have addressed all of the comments through our responses below, as well as in the text of the manuscript when appropriate. 1. Title The title is informative but could better reflect the use of statistical optimization techniques. Suggested action: Consider explicitly mentioning the use of response surface methodology or statistical optimization. Action Thanks for the reviewer’s important comment. (Noted, done) 2. Abstract Clearly explain why early-age compressive strength is critical, particularly for UPOFA-based concrete. Explicitly state the application scope (non-structural or low-load applications). Action Thanks for the reviewer’s important comment. (Noted, done) Moreover, the replacement rates of HSGC are probably high in POFA, which is constrained in the areas of the application where the removal of the formwork or pre-stressing of the structures is required in a very short period of time, and the high early-age strengths are required. 16 Separate experimental findings from modeling outcomes more clearly. Suggested action: Action Thanks for the reviewer’s suggestion, as shown, experimental findings and modeling outcomes are presented separately. Add one sentence clarifying application limitations. Action Thanks for the reviewer’s important comment (Noted, done). However, somehow these studies suggested the limitation of the POFA used because the highest was limited. The compressive strength was determined at a POFA replacement level of 20%, and the ultimate highest compressive strength of 28-days compressive strength achieved with the concretes that used POFA ranged between 60 and 86 MPa . 17, 18 3. Introduction Reduce repetition related to general sustainability concepts. Action Thanks for the reviewer’s comment (Noted, done, within introduction part). Strengthen the research gap, particularly regarding hybrid fiber systems in high-volume UPOFA concrete. Action Thanks for the reviewer’s comment (Noted, the research significance subsection already addresses the gap regarding hybrid fiber systems in high-volume UPOFA concrete. Clearly distinguish this study from previous related works. Action Thanks for the reviewer’s comment (Noted, the Introduction already highlights the study’s novelty and clearly distinguishes it from previous works. 4.1 Materials Characterization Consider adding a summary table describing the role and expected effect of each material (UPOFA, silica fume, steel fiber, PET fiber). Action Thanks for the reviewer’s excellent suggestion. (Noted, done, see Table 2). 4.2 Experimental Design and RSM Justify why the steel fiber and PET fiber contents were fixed at specific values. Action Thanks for the reviewer’s careful reading. (Noted, done). The contents of the steel and PET fibers were set to 1% volume, which is consistent with previous studies reporting that this amount has improved mechanical performance, crack control, and ductility whilst maintaining workability .21, 36 Briefly justify the selection of CCD over other RSM designs. Action Thanks for the reviewer’s comment. (Noted, done). CCD technique is favored to the full factorial and three-level design as it estimates the linear, quadratic and interaction effects with minimum tests, which will lead to a good model fitting, greater accuracy in prediction, reduced amount of residual error, and a high value of R2, that could permit the creation of three-dimensional response surfaces with great reliability. Consequently, it has commonly been considered as the best experimental design to investigate the second-order response models. 4, 15 5.1 Compressive Strength Results Reduce narrative repetition of numerical values already presented in figures. Thanks for the reviewer’s comment. Due to the high number of mixes and material changes, numerical values are supplied in the text in addition to tables and figures to assist the reader in understanding and following the patterns in the results. Strengthen microstructural explanations linking UPOFA dilution, silica fume nucleation, and fiber bridging effects. Action Thanks for the reviewer’s careful reading. (Noted, done) The compressive strength of GHSC is enhanced over a curing period of 7 to 28 days as a result of the accelerated hydration of conventional OPC and due to the pozzolanic effects of SF. The formation of secondary C-S-H is the consequence of the interaction between (Ca(OH)₂) produced during cement hydration and SiO₂ in SF 61,62 . This procedure increases the durability of the concrete by reducing the incidence of microcracks during periods of tension. In the same trend, adding steel and PET fibers improved the compressive strength of HSC and GHSC. The GHSC mixtures with a combination of (UPOFA-SF-ST-PET) registered the highest compressive strength of mixtures in GII and GIII at all curing periods. Hereby, the compressive strength of (M39) GHSC-U60 increased by 17%,25% and 18% at ages of 3, 7, and 28 days, respectively, related to control mix. This strength development was attributed to the effectiveness of binary binders (UPOFA and SF) in increasing bond formation between steel and PET fibers and other concrete ingredients. The tiny particle size and pozzolanic reactivity of UPOFA and SF reduced void volume and produced extra C-S-H gels, strengthening the aggregate-paste interfacial connection and increasing bonding capabilities with fibers. Adding steel and PET fibers may improve the fracture resistance of fiber-reinforced GHSC composites by limiting micro-crack formation under load, which is accomplished by forming a bridge network within the concrete microstructure 33,34,64 5.2 Statistical Analysis and Modeling Briefly explain why post-hoc statistical tests were not conducted. Action Thanks for the reviewer’s important comment. (Noted, done). Therefore, Post-hoc tests were not conducted as the principal analysis showed that the compressive strength increased significantly due to a combination of silica, steel fibers, and PET fibers, so the extra post-hoc comparisons were not necessary. .Clearly distinguish statistical significance from engineering significance. Action Thanks for the reviewer’s comment. (Noted). The practical relevance of these variations on material performance or structural design is reflected in engineering significance, whereas statistical significance shows that observed differences are unlikely to be the result of chance (e.g., p<0.05). Despite having little engineering significance, certain outcomes could be statistically significant. 6. Optimization Results and Practical Implications Add a short subsection explicitly addressing engineering applications of the optimized mix. Action Thanks for the reviewer’s important comment. (Noted, done). There is indeed a need to mention that the optimized concrete mix includes silica, steel fiber, and PET fiber, has a much better compressive strength, ductility, and crack resistance. They increase its applicability especially in structural components with high load needs such as beams, columns, and slabs, and in areas where it is important that both durability, impact resistance, and long-term performance. The blending of fibers and pozzolanic substances will offer a sure way of minimizing the maintenance and the service life of the concrete structures. Suggested action: Clearly state where and how the optimized concrete can be practically used. Thanks for the reviewer’s suggestion. 7. Conclusions Avoid repeating detailed numerical results. Explicitly acknowledge study limitations (laboratory scale, early-age focus). Provide a clear direction for future work Action Thanks for the reviewer’s important comment. (Noted, done). Further, this study primarily focused on early-age mechanical ability, conducted within a controlled civil engineering laboratory and environment. Consequently, additional future works are required to investigate durability, long-term performance, and comprehensive structural validation. Competing Interests: The authors declare that they have no competing interests. Close Report a concern Author Response 05 May 2026 Aktham Hatem Qasim , Department of Engineering Affairs, University of Fallujah, Al-Fallujah, Iraq 05 May 2026 Author Response Reviewer 1 1. Title The title is informative but could better reflect the use of statistical optimization techniques. Suggested action: Consider explicitly mentioning the use of response surface methodology or ... Continue reading Reviewer 1 1. Title The title is informative but could better reflect the use of statistical optimization techniques. Suggested action: Consider explicitly mentioning the use of response surface methodology or statistical optimization. Action Thanks for the reviewer’s important comment. (Noted, done). “ Optimizing Early Compressive Strength of Ultrafine POFA-Based Green HSC: An Experimental and RSM Approach with Silica Fume and Hybrid Fibers ” 2. Abstract Clearly explain why early-age compressive strength is critical, particularly for UPOFA-based concrete. Explicitly state the application scope (non-structural or low-load applications). Action Thanks for the reviewer’s important comment. (Noted, done) Moreover, the replacement rates of HSGC are probably high in POFA, which is constrained in the areas of the application where the removal of the formwork or pre-stressing of the structures is required in a very short period of time, and the high early-age strengths are required. 16 Separate experimental findings from modeling outcomes more clearly. Suggested action: Action Thanks for the reviewer’s suggestion, as shown, experimental findings and modeling outcomes are presented separately. (See, subsection 3.2: Mathematical modeling and statistical analysis results. Add one sentence clarifying application limitations. Action Thanks for the reviewer’s important comment (Noted, done). However, somehow these studies suggested the limitation of the POFA used because the highest was limited. The compressive strength was determined at a POFA replacement level of 20%, and the ultimate highest compressive strength of 28-days compressive strength achieved with the concretes that used POFA ranged between 60 and 86 MPa . 17, 18 3. Introduction Reduce repetition related to general sustainability concepts. Action Thanks for the reviewer’s comment (Noted, the manuscript has been revised to reduce repetition related to general sustainability concepts, particularly in the Introduction section.) Strengthen the research gap, particularly regarding hybrid fiber systems in high-volume UPOFA concrete. Action Thanks for the reviewer’s comment (Noted, done. See subsection 1.1: research significance which already addresses the gap regarding hybrid fiber systems in high-volume UPOFA concrete.) Clearly distinguish this study from previous related works. Action Thanks for the reviewer’s comment (Noted, the Introduction has been revised to more clearly highlight the novelty of this study and distinguish it from previous related work.) 4.1 Materials Characterization Consider adding a summary table describing the role and expected effect of each material (UPOFA, silica fume, steel fiber, PET fiber). Action Thanks for the reviewer’s excellent suggestion. (Noted, done. A summary table has been added (Table 2) to clearly describe the role and expected effect of each material, including UPOFA, silica fume, steel fiber, and PET fiber.). 4.2 Experimental Design and RSM Justify why the steel fiber and PET fiber contents were fixed at specific values. Action Thanks for the reviewer’s careful reading. (Noted, done). The contents of the steel and PET fibers were set to 1% volume, which is consistent with previous studies reporting that this amount has improved mechanical performance, crack control, and ductility whilst maintaining workability .21, 36. Further , studies done before have indicated that the strongest effect of these types of fibers, especially when combined with pozzolanic substances, is at this ratio. In turn, selecting 0% and 1% was sufficient to capture the main effect of the fiber addition and avoid irrelevant intermediate levels, which would not yield much different results. Briefly justify the selection of CCD over other RSM designs. Action Thanks for the reviewer’s comment. (Noted, done). CCD technique is favored to the full factorial and three-level design as it estimates the linear, quadratic and interaction effects with minimum tests, which will lead to a good model fitting, greater accuracy in prediction, reduced amount of residual error, and a high value of R 2 , that could permit the creation of three-dimensional response surfaces with great reliability. Consequently, it has commonly been considered as the best experimental design to investigate the second-order response models. 4, 15 5.1 Compressive Strength Results Reduce narrative repetition of numerical values already presented in figures. Thanks for the reviewer’s comment. Due to the high number of mixes and material changes, numerical values are supplied in the text in addition to tables and figures to assist the reader in understanding and following the patterns in the results. Strengthen microstructural explanations linking UPOFA dilution, silica fume nucleation, and fiber bridging effects. Action Thanks for the reviewer’s careful reading. (Noted, done) The compressive strength of GHSC is enhanced over a curing period of 7 to 28 days as a result of the accelerated hydration of conventional OPC and due to the pozzolanic effects of SF. The formation of secondary C-S-H is the consequence of the interaction between (Ca(OH)₂) produced during cement hydration and SiO₂ in SF 61,62 . This procedure increases the durability of the concrete by reducing the incidence of microcracks during periods of tension. In the same trend, adding steel and PET fibers improved the compressive strength of HSC and GHSC. The GHSC mixtures with a combination of (UPOFA-SF-ST-PET) registered the highest compressive strength of mixtures in GII and GIII at all curing periods. Hereby, the compressive strength of (M39) GHSC-U60 increased by 17%,25% and 18% at ages of 3, 7, and 28 days, respectively, related to control mix. This strength development was attributed to the effectiveness of binary binders (UPOFA and SF) in increasing bond formation between steel and PET fibers and other concrete ingredients. The tiny particle size and pozzolanic reactivity of UPOFA and SF reduced void volume and produced extra C-S-H gels, strengthening the aggregate-paste interfacial connection and increasing bonding capabilities with fibers. Adding steel and PET fibers may improve the fracture resistance of fiber-reinforced GHSC composites by limiting micro-crack formation under load, which is accomplished by forming a bridge network within the concrete microstructure 33,34,64 5.2 Statistical Analysis and Modeling Briefly explain why post-hoc statistical tests were not conducted. Action Thanks for the reviewer’s important comment. (Noted, done). Therefore, Post-hoc tests were not conducted as the principal analysis showed that the compressive strength increased significantly due to a combination of silica, steel fibers, and PET fibers, so the extra post-hoc comparisons were not necessary. .Clearly distinguish statistical significance from engineering significance. Action Thanks for the reviewer’s comment. (Noted). The practical relevance of these variations on material performance or structural design is reflected in engineering significance, whereas statistical significance shows that observed differences are unlikely to be the result of chance (e.g., p<0.05). Despite having little engineering significance, certain outcomes could be statistically significant. 6. Optimization Results and Practical Implications Add a short subsection explicitly addressing engineering applications of the optimized mix. Action Thanks for the reviewer’s important comment. (Noted, done). There is indeed a need to mention that the optimized concrete mix includes silica, steel fiber, and PET fiber, has a much better compressive strength, ductility, and crack resistance. They increase its applicability especially in structural components with high load needs such as beams, columns, and slabs, and in areas where it is important that both durability, impact resistance, and long-term performance. The blending of fibers and pozzolanic substances will offer a sure way of minimizing the maintenance and the service life of the concrete structures. Suggested action: Clearly state where and how the optimized concrete can be practically used. Action Thanks for the reviewer’s suggestion. ( See subsection 3.4) There is indeed a need to mention that the optimized concrete mix includes silica, steel fiber, and PET fiber, has a much better compressive strength, ductility, and crack resistance. They increase its applicability especially in structural components with high load needs such as beams, columns, and slabs, and in areas where it is important that both durability, impact resistance, and long-term performance. Thus, related to current study the optimized GHSC is applicable in structural elements, which include beams, columns, and precast elements, especially in settings that demand high strength and durability. 7. Conclusions Avoid repeating detailed numerical results. Explicitly acknowledge study limitations (laboratory scale, early-age focus). Provide a clear direction for future work Action Thanks for the reviewer’s important comment. (Noted, done, the future work paragraph has been added at the end of the Conclusion section). Future works are required in the optimization of GHSC by RSM in terms of long-term durability performance in diverse environmental factors, such as chloride penetration, sulfate attack, carbonation, water absorption, and permeability. Moreover, machine learning in combination with RSM can make its predictions more accurate and create a more resilient multi-objective optimization model that will consider the mechanical and durability characteristics of sustainable concrete design. Reviewer 1 1. Title The title is informative but could better reflect the use of statistical optimization techniques. Suggested action: Consider explicitly mentioning the use of response surface methodology or statistical optimization. Action Thanks for the reviewer’s important comment. (Noted, done). “ Optimizing Early Compressive Strength of Ultrafine POFA-Based Green HSC: An Experimental and RSM Approach with Silica Fume and Hybrid Fibers ” 2. Abstract Clearly explain why early-age compressive strength is critical, particularly for UPOFA-based concrete. Explicitly state the application scope (non-structural or low-load applications). Action Thanks for the reviewer’s important comment. (Noted, done) Moreover, the replacement rates of HSGC are probably high in POFA, which is constrained in the areas of the application where the removal of the formwork or pre-stressing of the structures is required in a very short period of time, and the high early-age strengths are required. 16 Separate experimental findings from modeling outcomes more clearly. Suggested action: Action Thanks for the reviewer’s suggestion, as shown, experimental findings and modeling outcomes are presented separately. (See, subsection 3.2: Mathematical modeling and statistical analysis results. Add one sentence clarifying application limitations. Action Thanks for the reviewer’s important comment (Noted, done). However, somehow these studies suggested the limitation of the POFA used because the highest was limited. The compressive strength was determined at a POFA replacement level of 20%, and the ultimate highest compressive strength of 28-days compressive strength achieved with the concretes that used POFA ranged between 60 and 86 MPa . 17, 18 3. Introduction Reduce repetition related to general sustainability concepts. Action Thanks for the reviewer’s comment (Noted, the manuscript has been revised to reduce repetition related to general sustainability concepts, particularly in the Introduction section.) Strengthen the research gap, particularly regarding hybrid fiber systems in high-volume UPOFA concrete. Action Thanks for the reviewer’s comment (Noted, done. See subsection 1.1: research significance which already addresses the gap regarding hybrid fiber systems in high-volume UPOFA concrete.) Clearly distinguish this study from previous related works. Action Thanks for the reviewer’s comment (Noted, the Introduction has been revised to more clearly highlight the novelty of this study and distinguish it from previous related work.) 4.1 Materials Characterization Consider adding a summary table describing the role and expected effect of each material (UPOFA, silica fume, steel fiber, PET fiber). Action Thanks for the reviewer’s excellent suggestion. (Noted, done. A summary table has been added (Table 2) to clearly describe the role and expected effect of each material, including UPOFA, silica fume, steel fiber, and PET fiber.). 4.2 Experimental Design and RSM Justify why the steel fiber and PET fiber contents were fixed at specific values. Action Thanks for the reviewer’s careful reading. (Noted, done). The contents of the steel and PET fibers were set to 1% volume, which is consistent with previous studies reporting that this amount has improved mechanical performance, crack control, and ductility whilst maintaining workability .21, 36. Further , studies done before have indicated that the strongest effect of these types of fibers, especially when combined with pozzolanic substances, is at this ratio. In turn, selecting 0% and 1% was sufficient to capture the main effect of the fiber addition and avoid irrelevant intermediate levels, which would not yield much different results. Briefly justify the selection of CCD over other RSM designs. Action Thanks for the reviewer’s comment. (Noted, done). CCD technique is favored to the full factorial and three-level design as it estimates the linear, quadratic and interaction effects with minimum tests, which will lead to a good model fitting, greater accuracy in prediction, reduced amount of residual error, and a high value of R 2 , that could permit the creation of three-dimensional response surfaces with great reliability. Consequently, it has commonly been considered as the best experimental design to investigate the second-order response models. 4, 15 5.1 Compressive Strength Results Reduce narrative repetition of numerical values already presented in figures. Thanks for the reviewer’s comment. Due to the high number of mixes and material changes, numerical values are supplied in the text in addition to tables and figures to assist the reader in understanding and following the patterns in the results. Strengthen microstructural explanations linking UPOFA dilution, silica fume nucleation, and fiber bridging effects. Action Thanks for the reviewer’s careful reading. (Noted, done) The compressive strength of GHSC is enhanced over a curing period of 7 to 28 days as a result of the accelerated hydration of conventional OPC and due to the pozzolanic effects of SF. The formation of secondary C-S-H is the consequence of the interaction between (Ca(OH)₂) produced during cement hydration and SiO₂ in SF 61,62 . This procedure increases the durability of the concrete by reducing the incidence of microcracks during periods of tension. In the same trend, adding steel and PET fibers improved the compressive strength of HSC and GHSC. The GHSC mixtures with a combination of (UPOFA-SF-ST-PET) registered the highest compressive strength of mixtures in GII and GIII at all curing periods. Hereby, the compressive strength of (M39) GHSC-U60 increased by 17%,25% and 18% at ages of 3, 7, and 28 days, respectively, related to control mix. This strength development was attributed to the effectiveness of binary binders (UPOFA and SF) in increasing bond formation between steel and PET fibers and other concrete ingredients. The tiny particle size and pozzolanic reactivity of UPOFA and SF reduced void volume and produced extra C-S-H gels, strengthening the aggregate-paste interfacial connection and increasing bonding capabilities with fibers. Adding steel and PET fibers may improve the fracture resistance of fiber-reinforced GHSC composites by limiting micro-crack formation under load, which is accomplished by forming a bridge network within the concrete microstructure 33,34,64 5.2 Statistical Analysis and Modeling Briefly explain why post-hoc statistical tests were not conducted. Action Thanks for the reviewer’s important comment. (Noted, done). Therefore, Post-hoc tests were not conducted as the principal analysis showed that the compressive strength increased significantly due to a combination of silica, steel fibers, and PET fibers, so the extra post-hoc comparisons were not necessary. .Clearly distinguish statistical significance from engineering significance. Action Thanks for the reviewer’s comment. (Noted). The practical relevance of these variations on material performance or structural design is reflected in engineering significance, whereas statistical significance shows that observed differences are unlikely to be the result of chance (e.g., p<0.05). Despite having little engineering significance, certain outcomes could be statistically significant. 6. Optimization Results and Practical Implications Add a short subsection explicitly addressing engineering applications of the optimized mix. Action Thanks for the reviewer’s important comment. (Noted, done). There is indeed a need to mention that the optimized concrete mix includes silica, steel fiber, and PET fiber, has a much better compressive strength, ductility, and crack resistance. They increase its applicability especially in structural components with high load needs such as beams, columns, and slabs, and in areas where it is important that both durability, impact resistance, and long-term performance. The blending of fibers and pozzolanic substances will offer a sure way of minimizing the maintenance and the service life of the concrete structures. Suggested action: Clearly state where and how the optimized concrete can be practically used. Action Thanks for the reviewer’s suggestion. ( See subsection 3.4) There is indeed a need to mention that the optimized concrete mix includes silica, steel fiber, and PET fiber, has a much better compressive strength, ductility, and crack resistance. They increase its applicability especially in structural components with high load needs such as beams, columns, and slabs, and in areas where it is important that both durability, impact resistance, and long-term performance. Thus, related to current study the optimized GHSC is applicable in structural elements, which include beams, columns, and precast elements, especially in settings that demand high strength and durability. 7. Conclusions Avoid repeating detailed numerical results. Explicitly acknowledge study limitations (laboratory scale, early-age focus). Provide a clear direction for future work Action Thanks for the reviewer’s important comment. (Noted, done, the future work paragraph has been added at the end of the Conclusion section). Future works are required in the optimization of GHSC by RSM in terms of long-term durability performance in diverse environmental factors, such as chloride penetration, sulfate attack, carbonation, water absorption, and permeability. Moreover, machine learning in combination with RSM can make its predictions more accurate and create a more resilient multi-objective optimization model that will consider the mechanical and durability characteristics of sustainable concrete design. Competing Interests: No competing interests were disclosed. Close Report a concern Respond or Comment COMMENTS ON THIS REPORT Author Response 05 May 2026 Aktham Hatem Qasim , Department of Engineering Affairs, University of Fallujah, Al-Fallujah, Iraq 05 May 2026 Author Response Response to Reviewer Comments The authors thank the reviewer for their careful reading and constructive comments below. We are very pleased to receive the reviewer’s recognition of the research work ... Continue reading Response to Reviewer Comments The authors thank the reviewer for their careful reading and constructive comments below. We are very pleased to receive the reviewer’s recognition of the research work in this paper. We believe that we have addressed all of the comments through our responses below, as well as in the text of the manuscript when appropriate. 1. Title The title is informative but could better reflect the use of statistical optimization techniques. Suggested action: Consider explicitly mentioning the use of response surface methodology or statistical optimization. Action Thanks for the reviewer’s important comment. (Noted, done) 2. Abstract Clearly explain why early-age compressive strength is critical, particularly for UPOFA-based concrete. Explicitly state the application scope (non-structural or low-load applications). Action Thanks for the reviewer’s important comment. (Noted, done) Moreover, the replacement rates of HSGC are probably high in POFA, which is constrained in the areas of the application where the removal of the formwork or pre-stressing of the structures is required in a very short period of time, and the high early-age strengths are required. 16 Separate experimental findings from modeling outcomes more clearly. Suggested action: Action Thanks for the reviewer’s suggestion, as shown, experimental findings and modeling outcomes are presented separately. Add one sentence clarifying application limitations. Action Thanks for the reviewer’s important comment (Noted, done). However, somehow these studies suggested the limitation of the POFA used because the highest was limited. The compressive strength was determined at a POFA replacement level of 20%, and the ultimate highest compressive strength of 28-days compressive strength achieved with the concretes that used POFA ranged between 60 and 86 MPa . 17, 18 3. Introduction Reduce repetition related to general sustainability concepts. Action Thanks for the reviewer’s comment (Noted, done, within introduction part). Strengthen the research gap, particularly regarding hybrid fiber systems in high-volume UPOFA concrete. Action Thanks for the reviewer’s comment (Noted, the research significance subsection already addresses the gap regarding hybrid fiber systems in high-volume UPOFA concrete. Clearly distinguish this study from previous related works. Action Thanks for the reviewer’s comment (Noted, the Introduction already highlights the study’s novelty and clearly distinguishes it from previous works. 4.1 Materials Characterization Consider adding a summary table describing the role and expected effect of each material (UPOFA, silica fume, steel fiber, PET fiber). Action Thanks for the reviewer’s excellent suggestion. (Noted, done, see Table 2). 4.2 Experimental Design and RSM Justify why the steel fiber and PET fiber contents were fixed at specific values. Action Thanks for the reviewer’s careful reading. (Noted, done). The contents of the steel and PET fibers were set to 1% volume, which is consistent with previous studies reporting that this amount has improved mechanical performance, crack control, and ductility whilst maintaining workability .21, 36 Briefly justify the selection of CCD over other RSM designs. Action Thanks for the reviewer’s comment. (Noted, done). CCD technique is favored to the full factorial and three-level design as it estimates the linear, quadratic and interaction effects with minimum tests, which will lead to a good model fitting, greater accuracy in prediction, reduced amount of residual error, and a high value of R2, that could permit the creation of three-dimensional response surfaces with great reliability. Consequently, it has commonly been considered as the best experimental design to investigate the second-order response models. 4, 15 5.1 Compressive Strength Results Reduce narrative repetition of numerical values already presented in figures. Thanks for the reviewer’s comment. Due to the high number of mixes and material changes, numerical values are supplied in the text in addition to tables and figures to assist the reader in understanding and following the patterns in the results. Strengthen microstructural explanations linking UPOFA dilution, silica fume nucleation, and fiber bridging effects. Action Thanks for the reviewer’s careful reading. (Noted, done) The compressive strength of GHSC is enhanced over a curing period of 7 to 28 days as a result of the accelerated hydration of conventional OPC and due to the pozzolanic effects of SF. The formation of secondary C-S-H is the consequence of the interaction between (Ca(OH)₂) produced during cement hydration and SiO₂ in SF 61,62 . This procedure increases the durability of the concrete by reducing the incidence of microcracks during periods of tension. In the same trend, adding steel and PET fibers improved the compressive strength of HSC and GHSC. The GHSC mixtures with a combination of (UPOFA-SF-ST-PET) registered the highest compressive strength of mixtures in GII and GIII at all curing periods. Hereby, the compressive strength of (M39) GHSC-U60 increased by 17%,25% and 18% at ages of 3, 7, and 28 days, respectively, related to control mix. This strength development was attributed to the effectiveness of binary binders (UPOFA and SF) in increasing bond formation between steel and PET fibers and other concrete ingredients. The tiny particle size and pozzolanic reactivity of UPOFA and SF reduced void volume and produced extra C-S-H gels, strengthening the aggregate-paste interfacial connection and increasing bonding capabilities with fibers. Adding steel and PET fibers may improve the fracture resistance of fiber-reinforced GHSC composites by limiting micro-crack formation under load, which is accomplished by forming a bridge network within the concrete microstructure 33,34,64 5.2 Statistical Analysis and Modeling Briefly explain why post-hoc statistical tests were not conducted. Action Thanks for the reviewer’s important comment. (Noted, done). Therefore, Post-hoc tests were not conducted as the principal analysis showed that the compressive strength increased significantly due to a combination of silica, steel fibers, and PET fibers, so the extra post-hoc comparisons were not necessary. .Clearly distinguish statistical significance from engineering significance. Action Thanks for the reviewer’s comment. (Noted). The practical relevance of these variations on material performance or structural design is reflected in engineering significance, whereas statistical significance shows that observed differences are unlikely to be the result of chance (e.g., p<0.05). Despite having little engineering significance, certain outcomes could be statistically significant. 6. Optimization Results and Practical Implications Add a short subsection explicitly addressing engineering applications of the optimized mix. Action Thanks for the reviewer’s important comment. (Noted, done). There is indeed a need to mention that the optimized concrete mix includes silica, steel fiber, and PET fiber, has a much better compressive strength, ductility, and crack resistance. They increase its applicability especially in structural components with high load needs such as beams, columns, and slabs, and in areas where it is important that both durability, impact resistance, and long-term performance. The blending of fibers and pozzolanic substances will offer a sure way of minimizing the maintenance and the service life of the concrete structures. Suggested action: Clearly state where and how the optimized concrete can be practically used. Thanks for the reviewer’s suggestion. 7. Conclusions Avoid repeating detailed numerical results. Explicitly acknowledge study limitations (laboratory scale, early-age focus). Provide a clear direction for future work Action Thanks for the reviewer’s important comment. (Noted, done). Further, this study primarily focused on early-age mechanical ability, conducted within a controlled civil engineering laboratory and environment. Consequently, additional future works are required to investigate durability, long-term performance, and comprehensive structural validation. Response to Reviewer Comments The authors thank the reviewer for their careful reading and constructive comments below. We are very pleased to receive the reviewer’s recognition of the research work in this paper. We believe that we have addressed all of the comments through our responses below, as well as in the text of the manuscript when appropriate. 1. Title The title is informative but could better reflect the use of statistical optimization techniques. Suggested action: Consider explicitly mentioning the use of response surface methodology or statistical optimization. Action Thanks for the reviewer’s important comment. (Noted, done) 2. Abstract Clearly explain why early-age compressive strength is critical, particularly for UPOFA-based concrete. Explicitly state the application scope (non-structural or low-load applications). Action Thanks for the reviewer’s important comment. (Noted, done) Moreover, the replacement rates of HSGC are probably high in POFA, which is constrained in the areas of the application where the removal of the formwork or pre-stressing of the structures is required in a very short period of time, and the high early-age strengths are required. 16 Separate experimental findings from modeling outcomes more clearly. Suggested action: Action Thanks for the reviewer’s suggestion, as shown, experimental findings and modeling outcomes are presented separately. Add one sentence clarifying application limitations. Action Thanks for the reviewer’s important comment (Noted, done). However, somehow these studies suggested the limitation of the POFA used because the highest was limited. The compressive strength was determined at a POFA replacement level of 20%, and the ultimate highest compressive strength of 28-days compressive strength achieved with the concretes that used POFA ranged between 60 and 86 MPa . 17, 18 3. Introduction Reduce repetition related to general sustainability concepts. Action Thanks for the reviewer’s comment (Noted, done, within introduction part). Strengthen the research gap, particularly regarding hybrid fiber systems in high-volume UPOFA concrete. Action Thanks for the reviewer’s comment (Noted, the research significance subsection already addresses the gap regarding hybrid fiber systems in high-volume UPOFA concrete. Clearly distinguish this study from previous related works. Action Thanks for the reviewer’s comment (Noted, the Introduction already highlights the study’s novelty and clearly distinguishes it from previous works. 4.1 Materials Characterization Consider adding a summary table describing the role and expected effect of each material (UPOFA, silica fume, steel fiber, PET fiber). Action Thanks for the reviewer’s excellent suggestion. (Noted, done, see Table 2). 4.2 Experimental Design and RSM Justify why the steel fiber and PET fiber contents were fixed at specific values. Action Thanks for the reviewer’s careful reading. (Noted, done). The contents of the steel and PET fibers were set to 1% volume, which is consistent with previous studies reporting that this amount has improved mechanical performance, crack control, and ductility whilst maintaining workability .21, 36 Briefly justify the selection of CCD over other RSM designs. Action Thanks for the reviewer’s comment. (Noted, done). CCD technique is favored to the full factorial and three-level design as it estimates the linear, quadratic and interaction effects with minimum tests, which will lead to a good model fitting, greater accuracy in prediction, reduced amount of residual error, and a high value of R2, that could permit the creation of three-dimensional response surfaces with great reliability. Consequently, it has commonly been considered as the best experimental design to investigate the second-order response models. 4, 15 5.1 Compressive Strength Results Reduce narrative repetition of numerical values already presented in figures. Thanks for the reviewer’s comment. Due to the high number of mixes and material changes, numerical values are supplied in the text in addition to tables and figures to assist the reader in understanding and following the patterns in the results. Strengthen microstructural explanations linking UPOFA dilution, silica fume nucleation, and fiber bridging effects. Action Thanks for the reviewer’s careful reading. (Noted, done) The compressive strength of GHSC is enhanced over a curing period of 7 to 28 days as a result of the accelerated hydration of conventional OPC and due to the pozzolanic effects of SF. The formation of secondary C-S-H is the consequence of the interaction between (Ca(OH)₂) produced during cement hydration and SiO₂ in SF 61,62 . This procedure increases the durability of the concrete by reducing the incidence of microcracks during periods of tension. In the same trend, adding steel and PET fibers improved the compressive strength of HSC and GHSC. The GHSC mixtures with a combination of (UPOFA-SF-ST-PET) registered the highest compressive strength of mixtures in GII and GIII at all curing periods. Hereby, the compressive strength of (M39) GHSC-U60 increased by 17%,25% and 18% at ages of 3, 7, and 28 days, respectively, related to control mix. This strength development was attributed to the effectiveness of binary binders (UPOFA and SF) in increasing bond formation between steel and PET fibers and other concrete ingredients. The tiny particle size and pozzolanic reactivity of UPOFA and SF reduced void volume and produced extra C-S-H gels, strengthening the aggregate-paste interfacial connection and increasing bonding capabilities with fibers. Adding steel and PET fibers may improve the fracture resistance of fiber-reinforced GHSC composites by limiting micro-crack formation under load, which is accomplished by forming a bridge network within the concrete microstructure 33,34,64 5.2 Statistical Analysis and Modeling Briefly explain why post-hoc statistical tests were not conducted. Action Thanks for the reviewer’s important comment. (Noted, done). Therefore, Post-hoc tests were not conducted as the principal analysis showed that the compressive strength increased significantly due to a combination of silica, steel fibers, and PET fibers, so the extra post-hoc comparisons were not necessary. .Clearly distinguish statistical significance from engineering significance. Action Thanks for the reviewer’s comment. (Noted). The practical relevance of these variations on material performance or structural design is reflected in engineering significance, whereas statistical significance shows that observed differences are unlikely to be the result of chance (e.g., p<0.05). Despite having little engineering significance, certain outcomes could be statistically significant. 6. Optimization Results and Practical Implications Add a short subsection explicitly addressing engineering applications of the optimized mix. Action Thanks for the reviewer’s important comment. (Noted, done). There is indeed a need to mention that the optimized concrete mix includes silica, steel fiber, and PET fiber, has a much better compressive strength, ductility, and crack resistance. They increase its applicability especially in structural components with high load needs such as beams, columns, and slabs, and in areas where it is important that both durability, impact resistance, and long-term performance. The blending of fibers and pozzolanic substances will offer a sure way of minimizing the maintenance and the service life of the concrete structures. Suggested action: Clearly state where and how the optimized concrete can be practically used. Thanks for the reviewer’s suggestion. 7. Conclusions Avoid repeating detailed numerical results. Explicitly acknowledge study limitations (laboratory scale, early-age focus). Provide a clear direction for future work Action Thanks for the reviewer’s important comment. (Noted, done). Further, this study primarily focused on early-age mechanical ability, conducted within a controlled civil engineering laboratory and environment. Consequently, additional future works are required to investigate durability, long-term performance, and comprehensive structural validation. Competing Interests: The authors declare that they have no competing interests. Close Report a concern Author Response 05 May 2026 Aktham Hatem Qasim , Department of Engineering Affairs, University of Fallujah, Al-Fallujah, Iraq 05 May 2026 Author Response Reviewer 1 1. Title The title is informative but could better reflect the use of statistical optimization techniques. Suggested action: Consider explicitly mentioning the use of response surface methodology or ... Continue reading Reviewer 1 1. Title The title is informative but could better reflect the use of statistical optimization techniques. Suggested action: Consider explicitly mentioning the use of response surface methodology or statistical optimization. Action Thanks for the reviewer’s important comment. (Noted, done). “ Optimizing Early Compressive Strength of Ultrafine POFA-Based Green HSC: An Experimental and RSM Approach with Silica Fume and Hybrid Fibers ” 2. Abstract Clearly explain why early-age compressive strength is critical, particularly for UPOFA-based concrete. Explicitly state the application scope (non-structural or low-load applications). Action Thanks for the reviewer’s important comment. (Noted, done) Moreover, the replacement rates of HSGC are probably high in POFA, which is constrained in the areas of the application where the removal of the formwork or pre-stressing of the structures is required in a very short period of time, and the high early-age strengths are required. 16 Separate experimental findings from modeling outcomes more clearly. Suggested action: Action Thanks for the reviewer’s suggestion, as shown, experimental findings and modeling outcomes are presented separately. (See, subsection 3.2: Mathematical modeling and statistical analysis results. Add one sentence clarifying application limitations. Action Thanks for the reviewer’s important comment (Noted, done). However, somehow these studies suggested the limitation of the POFA used because the highest was limited. The compressive strength was determined at a POFA replacement level of 20%, and the ultimate highest compressive strength of 28-days compressive strength achieved with the concretes that used POFA ranged between 60 and 86 MPa . 17, 18 3. Introduction Reduce repetition related to general sustainability concepts. Action Thanks for the reviewer’s comment (Noted, the manuscript has been revised to reduce repetition related to general sustainability concepts, particularly in the Introduction section.) Strengthen the research gap, particularly regarding hybrid fiber systems in high-volume UPOFA concrete. Action Thanks for the reviewer’s comment (Noted, done. See subsection 1.1: research significance which already addresses the gap regarding hybrid fiber systems in high-volume UPOFA concrete.) Clearly distinguish this study from previous related works. Action Thanks for the reviewer’s comment (Noted, the Introduction has been revised to more clearly highlight the novelty of this study and distinguish it from previous related work.) 4.1 Materials Characterization Consider adding a summary table describing the role and expected effect of each material (UPOFA, silica fume, steel fiber, PET fiber). Action Thanks for the reviewer’s excellent suggestion. (Noted, done. A summary table has been added (Table 2) to clearly describe the role and expected effect of each material, including UPOFA, silica fume, steel fiber, and PET fiber.). 4.2 Experimental Design and RSM Justify why the steel fiber and PET fiber contents were fixed at specific values. Action Thanks for the reviewer’s careful reading. (Noted, done). The contents of the steel and PET fibers were set to 1% volume, which is consistent with previous studies reporting that this amount has improved mechanical performance, crack control, and ductility whilst maintaining workability .21, 36. Further , studies done before have indicated that the strongest effect of these types of fibers, especially when combined with pozzolanic substances, is at this ratio. In turn, selecting 0% and 1% was sufficient to capture the main effect of the fiber addition and avoid irrelevant intermediate levels, which would not yield much different results. Briefly justify the selection of CCD over other RSM designs. Action Thanks for the reviewer’s comment. (Noted, done). CCD technique is favored to the full factorial and three-level design as it estimates the linear, quadratic and interaction effects with minimum tests, which will lead to a good model fitting, greater accuracy in prediction, reduced amount of residual error, and a high value of R 2 , that could permit the creation of three-dimensional response surfaces with great reliability. Consequently, it has commonly been considered as the best experimental design to investigate the second-order response models. 4, 15 5.1 Compressive Strength Results Reduce narrative repetition of numerical values already presented in figures. Thanks for the reviewer’s comment. Due to the high number of mixes and material changes, numerical values are supplied in the text in addition to tables and figures to assist the reader in understanding and following the patterns in the results. Strengthen microstructural explanations linking UPOFA dilution, silica fume nucleation, and fiber bridging effects. Action Thanks for the reviewer’s careful reading. (Noted, done) The compressive strength of GHSC is enhanced over a curing period of 7 to 28 days as a result of the accelerated hydration of conventional OPC and due to the pozzolanic effects of SF. The formation of secondary C-S-H is the consequence of the interaction between (Ca(OH)₂) produced during cement hydration and SiO₂ in SF 61,62 . This procedure increases the durability of the concrete by reducing the incidence of microcracks during periods of tension. In the same trend, adding steel and PET fibers improved the compressive strength of HSC and GHSC. The GHSC mixtures with a combination of (UPOFA-SF-ST-PET) registered the highest compressive strength of mixtures in GII and GIII at all curing periods. Hereby, the compressive strength of (M39) GHSC-U60 increased by 17%,25% and 18% at ages of 3, 7, and 28 days, respectively, related to control mix. This strength development was attributed to the effectiveness of binary binders (UPOFA and SF) in increasing bond formation between steel and PET fibers and other concrete ingredients. The tiny particle size and pozzolanic reactivity of UPOFA and SF reduced void volume and produced extra C-S-H gels, strengthening the aggregate-paste interfacial connection and increasing bonding capabilities with fibers. Adding steel and PET fibers may improve the fracture resistance of fiber-reinforced GHSC composites by limiting micro-crack formation under load, which is accomplished by forming a bridge network within the concrete microstructure 33,34,64 5.2 Statistical Analysis and Modeling Briefly explain why post-hoc statistical tests were not conducted. Action Thanks for the reviewer’s important comment. (Noted, done). Therefore, Post-hoc tests were not conducted as the principal analysis showed that the compressive strength increased significantly due to a combination of silica, steel fibers, and PET fibers, so the extra post-hoc comparisons were not necessary. .Clearly distinguish statistical significance from engineering significance. Action Thanks for the reviewer’s comment. (Noted). The practical relevance of these variations on material performance or structural design is reflected in engineering significance, whereas statistical significance shows that observed differences are unlikely to be the result of chance (e.g., p<0.05). Despite having little engineering significance, certain outcomes could be statistically significant. 6. Optimization Results and Practical Implications Add a short subsection explicitly addressing engineering applications of the optimized mix. Action Thanks for the reviewer’s important comment. (Noted, done). There is indeed a need to mention that the optimized concrete mix includes silica, steel fiber, and PET fiber, has a much better compressive strength, ductility, and crack resistance. They increase its applicability especially in structural components with high load needs such as beams, columns, and slabs, and in areas where it is important that both durability, impact resistance, and long-term performance. The blending of fibers and pozzolanic substances will offer a sure way of minimizing the maintenance and the service life of the concrete structures. Suggested action: Clearly state where and how the optimized concrete can be practically used. Action Thanks for the reviewer’s suggestion. ( See subsection 3.4) There is indeed a need to mention that the optimized concrete mix includes silica, steel fiber, and PET fiber, has a much better compressive strength, ductility, and crack resistance. They increase its applicability especially in structural components with high load needs such as beams, columns, and slabs, and in areas where it is important that both durability, impact resistance, and long-term performance. Thus, related to current study the optimized GHSC is applicable in structural elements, which include beams, columns, and precast elements, especially in settings that demand high strength and durability. 7. Conclusions Avoid repeating detailed numerical results. Explicitly acknowledge study limitations (laboratory scale, early-age focus). Provide a clear direction for future work Action Thanks for the reviewer’s important comment. (Noted, done, the future work paragraph has been added at the end of the Conclusion section). Future works are required in the optimization of GHSC by RSM in terms of long-term durability performance in diverse environmental factors, such as chloride penetration, sulfate attack, carbonation, water absorption, and permeability. Moreover, machine learning in combination with RSM can make its predictions more accurate and create a more resilient multi-objective optimization model that will consider the mechanical and durability characteristics of sustainable concrete design. Reviewer 1 1. Title The title is informative but could better reflect the use of statistical optimization techniques. Suggested action: Consider explicitly mentioning the use of response surface methodology or statistical optimization. Action Thanks for the reviewer’s important comment. (Noted, done). “ Optimizing Early Compressive Strength of Ultrafine POFA-Based Green HSC: An Experimental and RSM Approach with Silica Fume and Hybrid Fibers ” 2. Abstract Clearly explain why early-age compressive strength is critical, particularly for UPOFA-based concrete. Explicitly state the application scope (non-structural or low-load applications). Action Thanks for the reviewer’s important comment. (Noted, done) Moreover, the replacement rates of HSGC are probably high in POFA, which is constrained in the areas of the application where the removal of the formwork or pre-stressing of the structures is required in a very short period of time, and the high early-age strengths are required. 16 Separate experimental findings from modeling outcomes more clearly. Suggested action: Action Thanks for the reviewer’s suggestion, as shown, experimental findings and modeling outcomes are presented separately. (See, subsection 3.2: Mathematical modeling and statistical analysis results. Add one sentence clarifying application limitations. Action Thanks for the reviewer’s important comment (Noted, done). However, somehow these studies suggested the limitation of the POFA used because the highest was limited. The compressive strength was determined at a POFA replacement level of 20%, and the ultimate highest compressive strength of 28-days compressive strength achieved with the concretes that used POFA ranged between 60 and 86 MPa . 17, 18 3. Introduction Reduce repetition related to general sustainability concepts. Action Thanks for the reviewer’s comment (Noted, the manuscript has been revised to reduce repetition related to general sustainability concepts, particularly in the Introduction section.) Strengthen the research gap, particularly regarding hybrid fiber systems in high-volume UPOFA concrete. Action Thanks for the reviewer’s comment (Noted, done. See subsection 1.1: research significance which already addresses the gap regarding hybrid fiber systems in high-volume UPOFA concrete.) Clearly distinguish this study from previous related works. Action Thanks for the reviewer’s comment (Noted, the Introduction has been revised to more clearly highlight the novelty of this study and distinguish it from previous related work.) 4.1 Materials Characterization Consider adding a summary table describing the role and expected effect of each material (UPOFA, silica fume, steel fiber, PET fiber). Action Thanks for the reviewer’s excellent suggestion. (Noted, done. A summary table has been added (Table 2) to clearly describe the role and expected effect of each material, including UPOFA, silica fume, steel fiber, and PET fiber.). 4.2 Experimental Design and RSM Justify why the steel fiber and PET fiber contents were fixed at specific values. Action Thanks for the reviewer’s careful reading. (Noted, done). The contents of the steel and PET fibers were set to 1% volume, which is consistent with previous studies reporting that this amount has improved mechanical performance, crack control, and ductility whilst maintaining workability .21, 36. Further , studies done before have indicated that the strongest effect of these types of fibers, especially when combined with pozzolanic substances, is at this ratio. In turn, selecting 0% and 1% was sufficient to capture the main effect of the fiber addition and avoid irrelevant intermediate levels, which would not yield much different results. Briefly justify the selection of CCD over other RSM designs. Action Thanks for the reviewer’s comment. (Noted, done). CCD technique is favored to the full factorial and three-level design as it estimates the linear, quadratic and interaction effects with minimum tests, which will lead to a good model fitting, greater accuracy in prediction, reduced amount of residual error, and a high value of R 2 , that could permit the creation of three-dimensional response surfaces with great reliability. Consequently, it has commonly been considered as the best experimental design to investigate the second-order response models. 4, 15 5.1 Compressive Strength Results Reduce narrative repetition of numerical values already presented in figures. Thanks for the reviewer’s comment. Due to the high number of mixes and material changes, numerical values are supplied in the text in addition to tables and figures to assist the reader in understanding and following the patterns in the results. Strengthen microstructural explanations linking UPOFA dilution, silica fume nucleation, and fiber bridging effects. Action Thanks for the reviewer’s careful reading. (Noted, done) The compressive strength of GHSC is enhanced over a curing period of 7 to 28 days as a result of the accelerated hydration of conventional OPC and due to the pozzolanic effects of SF. The formation of secondary C-S-H is the consequence of the interaction between (Ca(OH)₂) produced during cement hydration and SiO₂ in SF 61,62 . This procedure increases the durability of the concrete by reducing the incidence of microcracks during periods of tension. In the same trend, adding steel and PET fibers improved the compressive strength of HSC and GHSC. The GHSC mixtures with a combination of (UPOFA-SF-ST-PET) registered the highest compressive strength of mixtures in GII and GIII at all curing periods. Hereby, the compressive strength of (M39) GHSC-U60 increased by 17%,25% and 18% at ages of 3, 7, and 28 days, respectively, related to control mix. This strength development was attributed to the effectiveness of binary binders (UPOFA and SF) in increasing bond formation between steel and PET fibers and other concrete ingredients. The tiny particle size and pozzolanic reactivity of UPOFA and SF reduced void volume and produced extra C-S-H gels, strengthening the aggregate-paste interfacial connection and increasing bonding capabilities with fibers. Adding steel and PET fibers may improve the fracture resistance of fiber-reinforced GHSC composites by limiting micro-crack formation under load, which is accomplished by forming a bridge network within the concrete microstructure 33,34,64 5.2 Statistical Analysis and Modeling Briefly explain why post-hoc statistical tests were not conducted. Action Thanks for the reviewer’s important comment. (Noted, done). Therefore, Post-hoc tests were not conducted as the principal analysis showed that the compressive strength increased significantly due to a combination of silica, steel fibers, and PET fibers, so the extra post-hoc comparisons were not necessary. .Clearly distinguish statistical significance from engineering significance. Action Thanks for the reviewer’s comment. (Noted). The practical relevance of these variations on material performance or structural design is reflected in engineering significance, whereas statistical significance shows that observed differences are unlikely to be the result of chance (e.g., p<0.05). Despite having little engineering significance, certain outcomes could be statistically significant. 6. Optimization Results and Practical Implications Add a short subsection explicitly addressing engineering applications of the optimized mix. Action Thanks for the reviewer’s important comment. (Noted, done). There is indeed a need to mention that the optimized concrete mix includes silica, steel fiber, and PET fiber, has a much better compressive strength, ductility, and crack resistance. They increase its applicability especially in structural components with high load needs such as beams, columns, and slabs, and in areas where it is important that both durability, impact resistance, and long-term performance. The blending of fibers and pozzolanic substances will offer a sure way of minimizing the maintenance and the service life of the concrete structures. Suggested action: Clearly state where and how the optimized concrete can be practically used. Action Thanks for the reviewer’s suggestion. ( See subsection 3.4) There is indeed a need to mention that the optimized concrete mix includes silica, steel fiber, and PET fiber, has a much better compressive strength, ductility, and crack resistance. They increase its applicability especially in structural components with high load needs such as beams, columns, and slabs, and in areas where it is important that both durability, impact resistance, and long-term performance. Thus, related to current study the optimized GHSC is applicable in structural elements, which include beams, columns, and precast elements, especially in settings that demand high strength and durability. 7. Conclusions Avoid repeating detailed numerical results. Explicitly acknowledge study limitations (laboratory scale, early-age focus). Provide a clear direction for future work Action Thanks for the reviewer’s important comment. (Noted, done, the future work paragraph has been added at the end of the Conclusion section). Future works are required in the optimization of GHSC by RSM in terms of long-term durability performance in diverse environmental factors, such as chloride penetration, sulfate attack, carbonation, water absorption, and permeability. Moreover, machine learning in combination with RSM can make its predictions more accurate and create a more resilient multi-objective optimization model that will consider the mechanical and durability characteristics of sustainable concrete design. Competing Interests: No competing interests were disclosed. Close Report a concern COMMENT ON THIS REPORT Comments on this article Comments (0) Version 3 VERSION 3 PUBLISHED 22 Jan 2026 ADD YOUR COMMENT Comment keyboard_arrow_left keyboard_arrow_right Open Peer Review Reviewer Status info_outline Alongside their report, reviewers assign a status to the article: Approved The paper is scientifically sound in its current form and only minor, if any, improvements are suggested Approved with reservations A number of small changes, sometimes more significant revisions are required to address specific details and improve the papers academic merit. Not approved Fundamental flaws in the paper seriously undermine the findings and conclusions Reviewer Reports Invited Reviewers 1 2 Version 3 (revision) 05 May 26 Version 2 (revision) 26 Feb 26 read Version 1 22 Jan 26 read Sattawat Haruehansapong , Rajamangala University of Technology Tawan-ok, Bangkok, Thailand Farid Debieb , University of Medea, Medea, Algeria Comments on this article All Comments (0) Add a comment Sign up for content alerts Sign Up You are now signed up to receive this alert Browse by related subjects keyboard_arrow_left Back to all reports Reviewer Report 0 Views copyright © 2026 Debieb F. This is an open access peer review report distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 13 Apr 2026 | for Version 2 Farid Debieb , University of Medea, Medea, Algeria 0 Views copyright © 2026 Debieb F. This is an open access peer review report distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. format_quote Cite this report speaker_notes Responses (1) Approved info_outline Alongside their report, reviewers assign a status to the article: Approved The paper is scientifically sound in its current form and only minor, if any, improvements are suggested Approved with reservations A number of small changes, sometimes more significant revisions are required to address specific details and improve the papers academic merit. Not approved Fundamental flaws in the paper seriously undermine the findings and conclusions The manuscript presents a well-founded and effectively implemented study. The experiment plans, the use of statistics for data analysis, and the conclusions are all quite fitting. However, it fails in giving enough information for a complete duplication, especially regarding UPOFA processing, the mixing procedure, compaction, and workability data. Besides, there are minor problems with clarity, like repetition and references to figures. Here are some ways that might improve the paper's overall strength and clarity: 1- Why were steel fiber (ST) and PET fiber only at 0% or 1% levels and no intermediate levels (e.g., 0.5% or 1.5%) were considered, whereas UPOFA and SF each had three levels? 2 The article states that the compressive strength drops at 60% UPOFA are because of the dilution and low production of C-S-H. However, could you clarify the part of overconsumption of CH due to a high amount of amorphous silica? 3- According to the ANOVA results (Table 6), nearly all two-way interactions (AB, AC, AD, BC, BD) are not statistically significant (p > 0.05). So, is using a full quadratic RSM model necessary? 4- The study looks at only the early-age mechanical strength (3, 7, 28 days). What are the most important durability properties that need to be examined before this optimized GHSC can be used on an industrial scale? 5- Is it possible that the findings of this study apply to other pozzolanic wastes (e.g., rice husk ash) or other synthetic fibers (e.g., polypropylene, PVA)? Is the work clearly and accurately presented and does it cite the current literature? Partly Is the study design appropriate and is the work technically sound? Yes Are sufficient details of methods and analysis provided to allow replication by others? Partly If applicable, is the statistical analysis and its interpretation appropriate? Yes Are all the source data underlying the results available to ensure full reproducibility? Partly Are the conclusions drawn adequately supported by the results? Yes Competing Interests No competing interests were disclosed. Reviewer Expertise innovative materials for construction, concrete technology, recycled concrete I confirm that I have read this submission and believe that I have an appropriate level of expertise to confirm that it is of an acceptable scientific standard. reply Respond to this report Responses (1) Author Response 05 May 2026 Aktham Hatem Qasim, Department of Engineering Affairs, University of Fallujah, Al-Fallujah, Iraq Reviewer 2 Why were steel fiber (ST) and PET fiber only at 0% or 1% levels and no intermediate levels (e.g., 0.5% or 1.5%) were considered, whereas UPOFA and SF each had three levels? Action Thanks for the reviewer’s important comment . The use of steel fiber (ST) and PET fiber was limited to only 0% and 1% without intermediate levels, as previous studies have consistently shown that the most significant effect of these types of fibers—especially when combined with pozzolanic materials—occurs around this percentage. Therefore, selecting 0% and 1% was sufficient to capture the primary influence of fiber addition, while avoiding unnecessary intermediate levels that may not produce substantially different results. In contrast, UPOFA and silica fume (SF) were studied at multiple levels because their influence on material properties is more gradual and requires a broader range to fully evaluate their effect. The article states that the compressive strength drops at 60% UPOFA are because of the dilution and low production of C-S-H. However, could you clarify the part of overconsumption of CH due to a high amount of amorphous silica? Action Thanks for the reviewer’s careful reading. (Done, added to manuscript see subsection 3.1). The increased concentration of amorphous silica, including 60% UPOFA, accelerates the consumption rate of Calcium Hydroxide (CH); nevertheless, the reduced cement content hampers its production in areas where CH is being exhausted. This impact results in the pozzolanic reaction attaining a CH-limited state, thereby leading to a reduced and delayed synthesis of Calcium Silicate Hydrate (C-S-H). Thereby, the interaction between the availability of calcium hydroxide and silica content in palm oil fuel ash significantly influences the evolution of compressive strength. According to the ANOVA results (Table 6), nearly all two-way interactions (AB, AC, AD, BC, BD) are not statistically significant (p > 0.05). So, is using a full quadratic RSM model necessary? Action Thanks for the reviewer’s careful reading. Although most of the two-way interactions were statistically insignificant (p > 0.05), the entire quadratic RSM model was retained to retain hierarchy and explain any curvature. The high statistical measures (R2> 0.93 and Precision> 24) also affirm the sufficiency of the model and a high predictive power and outstanding signal to noise. The study looks at only the early-age mechanical strength (3, 7, 28 days). What are the most important durability properties that need to be examined before this optimized GHSC can be used on an industrial scale? Action Thanks for the reviewer’s important comment. (Noted, done, the future work paragraph has been added at the end of the conclusion section). Future works are required in the optimization of GHSC by RSM in terms of long-term durability performance in diverse environmental factors, such as chloride penetration, sulfate attack, carbonation, water absorption, and permeability. Moreover, machine learning in combination with RSM can make its predictions more accurate and create a more resilient multi-objective optimization model that will consider the mechanical and durability characteristics of sustainable concrete design. Is it possible that the findings of this study apply to other pozzolanic wastes (e.g., rice husk ash) or other synthetic fibers (e.g., polypropylene, PVA)? Action Thanks for the reviewer’s important comment. Other pozzolanic fibers such as Rice Husk Ash and synthetic fibers, such as Polypropylene Fiber and Polyvinyl Alcohol Fiber can be qualitatively checked using the results since they are pozzolanically similar and react with the fiber-pozzolana mechanisms. It is however a system dependent utility since the variations in chemical compositions, curing, particle and interfacial properties can play a major role and thus should be assured on a case by case analysis. View more View less Competing Interests No competing interests were disclosed. reply Respond Report a concern Debieb F. Peer Review Report For: Optimizing Early Compressive Strength of Ultrafine POFA-Based Green HSC: An Experimental and RSM Approach with Silica Fume and Hybrid Fibers [version 3; peer review: 1 approved, 1 approved with reservations] . F1000Research 2026, 15 :103 ( https://doi.org/10.5256/f1000research.196708.r469919) NOTE: it is important to ensure the information in square brackets after the title is included in this citation. The direct URL for this report is: https://f1000research.com/articles/15-103/v2#referee-response-469919 keyboard_arrow_left Back to all reports Reviewer Report 0 Views copyright © 2026 Haruehansapong S. This is an open access peer review report distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 11 Feb 2026 | for Version 1 Sattawat Haruehansapong , Rajamangala University of Technology Tawan-ok, Bangkok, Thailand 0 Views copyright © 2026 Haruehansapong S. This is an open access peer review report distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. format_quote Cite this report speaker_notes Responses (2) Approved With Reservations info_outline Alongside their report, reviewers assign a status to the article: Approved The paper is scientifically sound in its current form and only minor, if any, improvements are suggested Approved with reservations A number of small changes, sometimes more significant revisions are required to address specific details and improve the papers academic merit. Not approved Fundamental flaws in the paper seriously undermine the findings and conclusions The manuscript presents a well-structured experimental and modeling study on the optimization of early-age compressive strength of ultrafine POFA-based green high-strength concrete. The experimental program is sound, and the statistical modeling approach is appropriate. The conclusions are generally supported by the presented evidence. However, several minor but important revisions are recommended to improve clarity, conciseness, and engineering relevance before final approval. 1. Title The title is informative but could better reflect the use of statistical optimization techniques. Suggested action: Consider explicitly mentioning the use of response surface methodology or statistical optimization. 2. Abstract Clearly explain why early-age compressive strength is critical, particularly for UPOFA-based concrete. Explicitly state the application scope (non-structural or low-load applications). Separate experimental findings from modeling outcomes more clearly. Suggested action: Add one sentence clarifying application limitations. Revise for conciseness. 3. Introduction Reduce repetition related to general sustainability concepts. Strengthen the research gap, particularly regarding hybrid fiber systems in high-volume UPOFA concrete. Clearly distinguish this study from previous related works. 4.1 Materials Characterization Consider adding a summary table describing the role and expected effect of each material (UPOFA, silica fume, steel fiber, PET fiber). 4.2 Experimental Design and RSM Justify why steel fiber and PET fiber contents were fixed at specific values. Briefly justify the selection of CCD over other RSM designs. 5.1 Compressive Strength Results Reduce narrative repetition of numerical values already presented in figures. Strengthen microstructural explanations linking UPOFA dilution, silica fume nucleation, and fiber bridging effects. 5.2 Statistical Analysis and Modeling Briefly explain why post-hoc statistical tests were not conducted. Clearly distinguish statistical significance from engineering significance. 6. Optimization Results and Practical Implications Add a short subsection explicitly addressing engineering applications of the optimized mix. Suggested action: Clearly state where and how the optimized concrete can be practically used. 7. Conclusions Avoid repeating detailed numerical results. Explicitly acknowledge study limitations (laboratory scale, early-age focus). Provide a clear direction for future work. Is the work clearly and accurately presented and does it cite the current literature? Partly Is the study design appropriate and is the work technically sound? Yes Are sufficient details of methods and analysis provided to allow replication by others? Yes If applicable, is the statistical analysis and its interpretation appropriate? Partly Are all the source data underlying the results available to ensure full reproducibility? Partly Are the conclusions drawn adequately supported by the results? Yes Competing Interests No competing interests were disclosed. Reviewer Expertise concrete, materials, cement I confirm that I have read this submission and believe that I have an appropriate level of expertise to confirm that it is of an acceptable scientific standard, however I have significant reservations, as outlined above. reply Respond to this report Responses (2) Author Response 05 May 2026 Aktham Hatem Qasim, Department of Engineering Affairs, University of Fallujah, Al-Fallujah, Iraq Response to Reviewer Comments The authors thank the reviewer for their careful reading and constructive comments below. We are very pleased to receive the reviewer’s recognition of the research work in this paper. We believe that we have addressed all of the comments through our responses below, as well as in the text of the manuscript when appropriate. 1. Title The title is informative but could better reflect the use of statistical optimization techniques. Suggested action: Consider explicitly mentioning the use of response surface methodology or statistical optimization. Action Thanks for the reviewer’s important comment. (Noted, done) 2. Abstract Clearly explain why early-age compressive strength is critical, particularly for UPOFA-based concrete. Explicitly state the application scope (non-structural or low-load applications). Action Thanks for the reviewer’s important comment. (Noted, done) Moreover, the replacement rates of HSGC are probably high in POFA, which is constrained in the areas of the application where the removal of the formwork or pre-stressing of the structures is required in a very short period of time, and the high early-age strengths are required. 16 Separate experimental findings from modeling outcomes more clearly. Suggested action: Action Thanks for the reviewer’s suggestion, as shown, experimental findings and modeling outcomes are presented separately. Add one sentence clarifying application limitations. Action Thanks for the reviewer’s important comment (Noted, done). However, somehow these studies suggested the limitation of the POFA used because the highest was limited. The compressive strength was determined at a POFA replacement level of 20%, and the ultimate highest compressive strength of 28-days compressive strength achieved with the concretes that used POFA ranged between 60 and 86 MPa . 17, 18 3. Introduction Reduce repetition related to general sustainability concepts. Action Thanks for the reviewer’s comment (Noted, done, within introduction part). Strengthen the research gap, particularly regarding hybrid fiber systems in high-volume UPOFA concrete. Action Thanks for the reviewer’s comment (Noted, the research significance subsection already addresses the gap regarding hybrid fiber systems in high-volume UPOFA concrete. Clearly distinguish this study from previous related works. Action Thanks for the reviewer’s comment (Noted, the Introduction already highlights the study’s novelty and clearly distinguishes it from previous works. 4.1 Materials Characterization Consider adding a summary table describing the role and expected effect of each material (UPOFA, silica fume, steel fiber, PET fiber). Action Thanks for the reviewer’s excellent suggestion. (Noted, done, see Table 2). 4.2 Experimental Design and RSM Justify why the steel fiber and PET fiber contents were fixed at specific values. Action Thanks for the reviewer’s careful reading. (Noted, done). The contents of the steel and PET fibers were set to 1% volume, which is consistent with previous studies reporting that this amount has improved mechanical performance, crack control, and ductility whilst maintaining workability .21, 36 Briefly justify the selection of CCD over other RSM designs. Action Thanks for the reviewer’s comment. (Noted, done). CCD technique is favored to the full factorial and three-level design as it estimates the linear, quadratic and interaction effects with minimum tests, which will lead to a good model fitting, greater accuracy in prediction, reduced amount of residual error, and a high value of R2, that could permit the creation of three-dimensional response surfaces with great reliability. Consequently, it has commonly been considered as the best experimental design to investigate the second-order response models. 4, 15 5.1 Compressive Strength Results Reduce narrative repetition of numerical values already presented in figures. Thanks for the reviewer’s comment. Due to the high number of mixes and material changes, numerical values are supplied in the text in addition to tables and figures to assist the reader in understanding and following the patterns in the results. Strengthen microstructural explanations linking UPOFA dilution, silica fume nucleation, and fiber bridging effects. Action Thanks for the reviewer’s careful reading. (Noted, done) The compressive strength of GHSC is enhanced over a curing period of 7 to 28 days as a result of the accelerated hydration of conventional OPC and due to the pozzolanic effects of SF. The formation of secondary C-S-H is the consequence of the interaction between (Ca(OH)₂) produced during cement hydration and SiO₂ in SF 61,62 . This procedure increases the durability of the concrete by reducing the incidence of microcracks during periods of tension. In the same trend, adding steel and PET fibers improved the compressive strength of HSC and GHSC. The GHSC mixtures with a combination of (UPOFA-SF-ST-PET) registered the highest compressive strength of mixtures in GII and GIII at all curing periods. Hereby, the compressive strength of (M39) GHSC-U60 increased by 17%,25% and 18% at ages of 3, 7, and 28 days, respectively, related to control mix. This strength development was attributed to the effectiveness of binary binders (UPOFA and SF) in increasing bond formation between steel and PET fibers and other concrete ingredients. The tiny particle size and pozzolanic reactivity of UPOFA and SF reduced void volume and produced extra C-S-H gels, strengthening the aggregate-paste interfacial connection and increasing bonding capabilities with fibers. Adding steel and PET fibers may improve the fracture resistance of fiber-reinforced GHSC composites by limiting micro-crack formation under load, which is accomplished by forming a bridge network within the concrete microstructure 33,34,64 5.2 Statistical Analysis and Modeling Briefly explain why post-hoc statistical tests were not conducted. Action Thanks for the reviewer’s important comment. (Noted, done). Therefore, Post-hoc tests were not conducted as the principal analysis showed that the compressive strength increased significantly due to a combination of silica, steel fibers, and PET fibers, so the extra post-hoc comparisons were not necessary. .Clearly distinguish statistical significance from engineering significance. Action Thanks for the reviewer’s comment. (Noted). The practical relevance of these variations on material performance or structural design is reflected in engineering significance, whereas statistical significance shows that observed differences are unlikely to be the result of chance (e.g., p<0.05). Despite having little engineering significance, certain outcomes could be statistically significant. 6. Optimization Results and Practical Implications Add a short subsection explicitly addressing engineering applications of the optimized mix. Action Thanks for the reviewer’s important comment. (Noted, done). There is indeed a need to mention that the optimized concrete mix includes silica, steel fiber, and PET fiber, has a much better compressive strength, ductility, and crack resistance. They increase its applicability especially in structural components with high load needs such as beams, columns, and slabs, and in areas where it is important that both durability, impact resistance, and long-term performance. The blending of fibers and pozzolanic substances will offer a sure way of minimizing the maintenance and the service life of the concrete structures. Suggested action: Clearly state where and how the optimized concrete can be practically used. Thanks for the reviewer’s suggestion. 7. Conclusions Avoid repeating detailed numerical results. Explicitly acknowledge study limitations (laboratory scale, early-age focus). Provide a clear direction for future work Action Thanks for the reviewer’s important comment. (Noted, done). Further, this study primarily focused on early-age mechanical ability, conducted within a controlled civil engineering laboratory and environment. Consequently, additional future works are required to investigate durability, long-term performance, and comprehensive structural validation. View more View less Competing Interests The authors declare that they have no competing interests. reply Respond Report a concern Author Response 05 May 2026 Aktham Hatem Qasim, Department of Engineering Affairs, University of Fallujah, Al-Fallujah, Iraq Reviewer 1 1. Title The title is informative but could better reflect the use of statistical optimization techniques. Suggested action: Consider explicitly mentioning the use of response surface methodology or statistical optimization. Action Thanks for the reviewer’s important comment. (Noted, done). “ Optimizing Early Compressive Strength of Ultrafine POFA-Based Green HSC: An Experimental and RSM Approach with Silica Fume and Hybrid Fibers ” 2. Abstract Clearly explain why early-age compressive strength is critical, particularly for UPOFA-based concrete. Explicitly state the application scope (non-structural or low-load applications). Action Thanks for the reviewer’s important comment. (Noted, done) Moreover, the replacement rates of HSGC are probably high in POFA, which is constrained in the areas of the application where the removal of the formwork or pre-stressing of the structures is required in a very short period of time, and the high early-age strengths are required. 16 Separate experimental findings from modeling outcomes more clearly. Suggested action: Action Thanks for the reviewer’s suggestion, as shown, experimental findings and modeling outcomes are presented separately. (See, subsection 3.2: Mathematical modeling and statistical analysis results. Add one sentence clarifying application limitations. Action Thanks for the reviewer’s important comment (Noted, done). However, somehow these studies suggested the limitation of the POFA used because the highest was limited. The compressive strength was determined at a POFA replacement level of 20%, and the ultimate highest compressive strength of 28-days compressive strength achieved with the concretes that used POFA ranged between 60 and 86 MPa . 17, 18 3. Introduction Reduce repetition related to general sustainability concepts. Action Thanks for the reviewer’s comment (Noted, the manuscript has been revised to reduce repetition related to general sustainability concepts, particularly in the Introduction section.) Strengthen the research gap, particularly regarding hybrid fiber systems in high-volume UPOFA concrete. Action Thanks for the reviewer’s comment (Noted, done. See subsection 1.1: research significance which already addresses the gap regarding hybrid fiber systems in high-volume UPOFA concrete.) Clearly distinguish this study from previous related works. Action Thanks for the reviewer’s comment (Noted, the Introduction has been revised to more clearly highlight the novelty of this study and distinguish it from previous related work.) 4.1 Materials Characterization Consider adding a summary table describing the role and expected effect of each material (UPOFA, silica fume, steel fiber, PET fiber). Action Thanks for the reviewer’s excellent suggestion. (Noted, done. A summary table has been added (Table 2) to clearly describe the role and expected effect of each material, including UPOFA, silica fume, steel fiber, and PET fiber.). 4.2 Experimental Design and RSM Justify why the steel fiber and PET fiber contents were fixed at specific values. Action Thanks for the reviewer’s careful reading. (Noted, done). The contents of the steel and PET fibers were set to 1% volume, which is consistent with previous studies reporting that this amount has improved mechanical performance, crack control, and ductility whilst maintaining workability .21, 36. Further , studies done before have indicated that the strongest effect of these types of fibers, especially when combined with pozzolanic substances, is at this ratio. In turn, selecting 0% and 1% was sufficient to capture the main effect of the fiber addition and avoid irrelevant intermediate levels, which would not yield much different results. Briefly justify the selection of CCD over other RSM designs. Action Thanks for the reviewer’s comment. (Noted, done). CCD technique is favored to the full factorial and three-level design as it estimates the linear, quadratic and interaction effects with minimum tests, which will lead to a good model fitting, greater accuracy in prediction, reduced amount of residual error, and a high value of R 2 , that could permit the creation of three-dimensional response surfaces with great reliability. Consequently, it has commonly been considered as the best experimental design to investigate the second-order response models. 4, 15 5.1 Compressive Strength Results Reduce narrative repetition of numerical values already presented in figures. Thanks for the reviewer’s comment. Due to the high number of mixes and material changes, numerical values are supplied in the text in addition to tables and figures to assist the reader in understanding and following the patterns in the results. Strengthen microstructural explanations linking UPOFA dilution, silica fume nucleation, and fiber bridging effects. Action Thanks for the reviewer’s careful reading. (Noted, done) The compressive strength of GHSC is enhanced over a curing period of 7 to 28 days as a result of the accelerated hydration of conventional OPC and due to the pozzolanic effects of SF. The formation of secondary C-S-H is the consequence of the interaction between (Ca(OH)₂) produced during cement hydration and SiO₂ in SF 61,62 . This procedure increases the durability of the concrete by reducing the incidence of microcracks during periods of tension. In the same trend, adding steel and PET fibers improved the compressive strength of HSC and GHSC. The GHSC mixtures with a combination of (UPOFA-SF-ST-PET) registered the highest compressive strength of mixtures in GII and GIII at all curing periods. Hereby, the compressive strength of (M39) GHSC-U60 increased by 17%,25% and 18% at ages of 3, 7, and 28 days, respectively, related to control mix. This strength development was attributed to the effectiveness of binary binders (UPOFA and SF) in increasing bond formation between steel and PET fibers and other concrete ingredients. The tiny particle size and pozzolanic reactivity of UPOFA and SF reduced void volume and produced extra C-S-H gels, strengthening the aggregate-paste interfacial connection and increasing bonding capabilities with fibers. Adding steel and PET fibers may improve the fracture resistance of fiber-reinforced GHSC composites by limiting micro-crack formation under load, which is accomplished by forming a bridge network within the concrete microstructure 33,34,64 5.2 Statistical Analysis and Modeling Briefly explain why post-hoc statistical tests were not conducted. Action Thanks for the reviewer’s important comment. (Noted, done). Therefore, Post-hoc tests were not conducted as the principal analysis showed that the compressive strength increased significantly due to a combination of silica, steel fibers, and PET fibers, so the extra post-hoc comparisons were not necessary. .Clearly distinguish statistical significance from engineering significance. Action Thanks for the reviewer’s comment. (Noted). The practical relevance of these variations on material performance or structural design is reflected in engineering significance, whereas statistical significance shows that observed differences are unlikely to be the result of chance (e.g., p<0.05). Despite having little engineering significance, certain outcomes could be statistically significant. 6. Optimization Results and Practical Implications Add a short subsection explicitly addressing engineering applications of the optimized mix. Action Thanks for the reviewer’s important comment. (Noted, done). There is indeed a need to mention that the optimized concrete mix includes silica, steel fiber, and PET fiber, has a much better compressive strength, ductility, and crack resistance. They increase its applicability especially in structural components with high load needs such as beams, columns, and slabs, and in areas where it is important that both durability, impact resistance, and long-term performance. The blending of fibers and pozzolanic substances will offer a sure way of minimizing the maintenance and the service life of the concrete structures. Suggested action: Clearly state where and how the optimized concrete can be practically used. Action Thanks for the reviewer’s suggestion. ( See subsection 3.4) There is indeed a need to mention that the optimized concrete mix includes silica, steel fiber, and PET fiber, has a much better compressive strength, ductility, and crack resistance. They increase its applicability especially in structural components with high load needs such as beams, columns, and slabs, and in areas where it is important that both durability, impact resistance, and long-term performance. Thus, related to current study the optimized GHSC is applicable in structural elements, which include beams, columns, and precast elements, especially in settings that demand high strength and durability. 7. Conclusions Avoid repeating detailed numerical results. Explicitly acknowledge study limitations (laboratory scale, early-age focus). Provide a clear direction for future work Action Thanks for the reviewer’s important comment. (Noted, done, the future work paragraph has been added at the end of the Conclusion section). Future works are required in the optimization of GHSC by RSM in terms of long-term durability performance in diverse environmental factors, such as chloride penetration, sulfate attack, carbonation, water absorption, and permeability. Moreover, machine learning in combination with RSM can make its predictions more accurate and create a more resilient multi-objective optimization model that will consider the mechanical and durability characteristics of sustainable concrete design. View more View less Competing Interests No competing interests were disclosed. reply Respond Report a concern Haruehansapong S. Peer Review Report For: Optimizing Early Compressive Strength of Ultrafine POFA-Based Green HSC: An Experimental and RSM Approach with Silica Fume and Hybrid Fibers [version 3; peer review: 1 approved, 1 approved with reservations] . F1000Research 2026, 15 :103 ( https://doi.org/10.5256/f1000research.193200.r456473) NOTE: it is important to ensure the information in square brackets after the title is included in this citation. 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