Development of a Low-Carbon Pozzolana–Lime Cement Using Sawdust Ash and Calcined Clay

Development of a Low-Carbon Pozzolana–Lime Cement Using Sawdust Ash and Calcined Clay | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Development of a Low-Carbon Pozzolana–Lime Cement Using Sawdust Ash and Calcined Clay Odiwuor Vincent Onyango, Enos W. Wambu, Ayabei Kiplagat, Samuel Lutta, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8782472/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 14 You are reading this latest preprint version Abstract The cement industry is a major contributor to global CO₂ emissions. This has driven the need for low-carbon, locally adaptable binder systems. This study develops and evaluates a fully clinker-free pozzolana–lime cement formulated using sawdust ash (SDA) and calcined kaolinitic clay (metakaolin, MK) as complementary supplementary materials. SDA and MK were thermally activated at 600°C to enhance pozzolanic reactivity and combined with hydrated lime to form ternary binders. Mortars were prepared at a constant binder-to-sand-to-water ratio of 1:3:0.8 by blending the powdered constituents, followed by controlled mechanical mixing and casting into 40 × 40 × 160 mm prisms. Specimens were demolded after 48 h and cured initially under high-humidity conditions and subsequently in ambient air to promote both pozzolanic reactions and natural carbonation. Chemical and mineralogical characterization was performed using X-ray fluorescence, X-ray diffraction, and loss on ignition, while pozzolanic activity was assessed through electrical conductivity measurements in saturated calcium hydroxide solutions. Hardened mortars were evaluated in terms of density, water absorption at 28 days, and flexural and compressive strength at 28 and 90 days. Results showed that both SDA and MK exhibit measurable pozzolanic activity, with MK providing stable aluminosilicate reactivity and SDA contributing high silica content and alkali-assisted dissolution. Blended mortars demonstrated significantly improved mechanical performance compared with pure lime, achieving compressive strengths up to 7.1 MPa at 90 days for an optimal 50:25:25 (lime:SDA:MK) formulation. Moderate pozzolan contents refined pore structure and reduced water absorption, while excessive replacement reduced density and strength due to calcium deficiency. The findings demonstrate a synergistic interaction between SDA and MK, enabling the formation of strength-contributing phases and supporting the development of a sustainable, low-emission, lime-based binder suitable for structural and non-structural applications. lime-based mortar sawdust ash metakaolin hydrated lime flexural strength compressive strength Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 1.Introduction The cement sector is widely recognized as one of the most carbon-intensive industries worldwide, accounting for approximately 7–8% of total human-generated CO₂ emissions [ 1 ]. Large volumes of carbon dioxide and other greenhouse gases are released throughout the cement production process. These emissions arise mainly from the thermal decomposition of limestone and the combustion of fossil fuels during clinker manufacture, both of which are inherent to the production of ordinary Portland cement (OPC) [ 2 ]. As a result, cement manufacturing is increasingly regarded as both economically demanding and environmentally burdensome. For every ton of Portland cement produced, an estimated 1.0–1.25 tons of CO₂ are emitted, while approximately 1.60 MWh of energy is consumed in the process [ 3 ]. Furthermore, the intensive extraction of raw materials required for cement production contributes to the depletion of natural resources [ 4 ]. As global construction activity continues to grow, emissions associated with cement production have become a major environmental challenge, particularly in developing regions where infrastructure development is closely linked to rising cement consumption [ 5 ]. Consequently, the design of sustainable, low-carbon binder systems has emerged as an urgent priority for both researchers and industry practitioners. In response, substantial efforts have focused on partially substituting OPC with supplementary cementitious materials (SCMs), including fly ash, ground granulated blast furnace slag, and silica fume [ 6 , 7 , 8 , 9 , 10 , 11 ]. Another alternative and increasingly studied strategy embraces the use of lime-based binders modified with SCMs, particularly naturally occurring aluminosilicate materials such as volcanic ash, zeolites, kaolinitic clays, and pumice [ 12 , 13 , 14 , 15 , 16 , 11 ]. These alternative materials have been shown to lower clinker demand while enhancing the long-term durability of cementitious systems [ 17 ]. However, their broader implementation is constrained by several practical challenges. The supply of some SCMs is diminishing as energy and metallurgical industries move away from coal-fired power generation and traditional smelting processes [ 18 ]. In addition, the performance of many SCMs depends strongly on regional availability, material consistency, and the need for industrial-scale processing, which restricts their use in low-income and resource-limited contexts [ 19 ]. As a result, conventional approaches based on widely used SCMs have not fully resolved the combined issues of material scarcity and economic feasibility in efforts to reduce cement-related emissions. To address these constraints, this study proposes the formulation of a low-carbon pozzolana–lime binder incorporating sawdust ash (SDA) and metakaolin (MK). Sawdust ash, a largely underexploited residue from the timber and biomass energy sectors, contains a high proportion of amorphous silica and can exhibit significant pozzolanic reactivity when appropriately processed. Likewise, thermally activated calcined clay, particularly metakaolin derived from low-grade kaolinitic sources, has demonstrated strong chemical reactivity and good compatibility with lime and blended binder systems due to its elevated alumina and silica contents [ 20 , 21 , 22 ]. The combined use of these two materials is intended to exploit their complementary chemical characteristics, enabling the production of a lime-based binder that achieves acceptable mechanical strength, reduced embodied carbon, and suitability for widespread local application. Pozzolanic ashes and calcined clays have attracted considerable interest because of their ability to react with calcium hydroxide in lime systems to generate secondary binding phases, primarily calcium silicate hydrates (C–S–H) and calcium aluminate hydrates (C–A–H). The formation of these phases contributes to strength development and microstructural refinement, thereby improving the overall performance of the binder [ 23 ]. Earlier investigations into the use of either SDA or MK have often reported limitations, including incomplete pozzolanic reactions, insufficient early-age strength, and variable long-term durability [ 20 ]. Although a substantial body of research exists on individual pozzolanic additives, much of the literature concentrates on binary systems, such as lime–SDA or lime–MK, as well as ternary blends based on OPC, rather than on fully clinker-free pozzolana–lime binders. Moreover, the interactive effects between silica-rich sawdust ash and alumina-rich MK within lime-based cementitious matrices remain inadequately understood. In particular, limited attention has been given to optimizing the relative proportions of calcium, silica, and alumina required to promote the formation of stable and mechanically efficient hydrate phases under ambient curing conditions [ 24 ]. While several studies have confirmed that metakaolin enhances the pozzolanic activity, strength, and durability of lime mortars [ 11 , 13 , 16 , 15 ], information regarding the specific influence of SDA in lime-based mortars remains scarce. In addition, standardized guidelines for proportioning SDA, MK, and lime are largely absent. In light of these research gaps, this work introduces a novel low-carbon pozzolana–lime binder developed through the combined application of sawdust ash and MK as mutually reinforcing reactive components. The originality of this approach lies in harnessing a synergistic pozzolanic interaction, whereby the silica-rich fraction of SDA complements the alumina-dominant phase of metakaolin, resulting in improved mechanical behavior. By focusing on a fully lime-based binder rather than partial replacement within OPC systems, this study extends existing knowledge on the joint use of two highly reactive and sustainable pozzolans as the primary binding phase, offering a pathway to enhanced performance alongside reduced environmental impact. Through an integrated evaluation of chemical reactivity and mechanical properties, this research provides new perspectives on the optimization of ternary pozzolana–lime systems for both structural and non-structural applications. Furthermore, it contributes to the field of sustainable construction materials by demonstrating the potential for transforming locally available biomass residues and natural clay resources into low-emission, environmentally responsible cementitious products. The proposed pozzolana–lime binder therefore represents a scientifically grounded and context-sensitive response to the persistent challenge of decarbonizing cement-based materials. 2 Materials and methods 2.1 Raw materials Sawdust was obtained from commercial sawmills located on the periphery of Eldoret City, Kenya. Kaolinitic clay was locally sourced through random sampling at two separate sites—Orembe and Namba Karabok in Homa Bay, kenya—areas well known for small-scale brick production. The collected samples collected were mechanically blended to produce a homogeneous composite, after which they were sealed in labeled polythene bags for storage. Commercial hydrated lime (Ca(OH)₂), employed as the chemical activator and compliant with KS EAS 2168-1:2020 [ 26 ], was supplied by Britchemicals Ltd. (Nairobi, Kenya). Naturally occurring river sand was used as the fine aggregate, while potable water was utilized for all mix preparations. 2.2 Methods 2.2.1 Sample preparation The collected sawdust was first air-dried and sieved, after which it was incinerated in a muffle furnace (Advantec KL-420, Tokyo, Japan) at 600°C for a duration of 2 h to eliminate residual carbon and maximize its pozzolanic potential [ 20 ]. The resulting ash was allowed to cool, then ground and passed through a 45 µm sieve in accordance with ASTM C618 [ 27 ] to remove oversized particles, improve material uniformity, and enhance reactivity. The kaolinitic clay samples were milled using a laboratory ball mill and subsequently calcined at 600°C for 2 h to obtain metakaolin, a process associated with lower energy demand and reduced CO₂ emissions compared to conventional cement manufacturing. After cooling, the produced MK was ground and sieved through a 45 µm mesh. This thermal treatment induces dehydroxylation and leads to the formation of a highly reactive amorphous aluminosilicate phase with pronounced pozzolanic activity [ 16 ]. Each pozzolanic material was further characterized using electrical conductivity measurements, X-ray diffraction (XRD), and X-ray fluorescence (XRF) analyses. The sand was washed with deionized water, sun-dried to a constant mass, and sieved through a 5 mm mesh to remove coarse particles and ensure uniform grading. 2.2.2 Material characterization The chemical composition of the pozzolanic materials and the reference hydrated lime was determined using an Epsilon 3XLE X-ray fluorescence (XRF) spectrometer (Malvern Panalytical, Almelo, Netherlands). The mineralogical structure, including both crystalline and amorphous phases within the pozzolans, was examined through X-ray diffraction (XRD) analysis conducted with a Bruker AXS diffractometer (Karlsruhe, Germany). Loss on ignition (LOI) testing was performed to quantify residual carbon and volatile constituents. Approximately 1 g of each raw pozzolan sample was first oven-dried at 110°C for 1 h, followed by heating at 1000°C for an additional 1 h. The samples were then cooled in a desiccator and reweighed. The percentage LOI was calculated in accordance with the method outlined by Marangu et al . [ 28 ] 2.2.3 Evaluation of Pozzolanic Activity The pozzolanic activity was assessed using a modified version of the method proposed by Luxán et al . [ 29 ], with minor procedural adjustments. A saturated calcium hydroxide solution was prepared by dissolving 0.8 g of Ca(OH)₂ in 200 mL of distilled water within a 250 mL beaker. The mixture was maintained at 38 ± 1°C on a thermostatically controlled magnetic hotplate and continuously stirred. The initial electrical conductivity of the solution was recorded using a conductivity meter (Oakton CON 510 Series model) once equilibrium was achieved. After stabilization of the lime-water system, 5 g of finely ground pozzolanic material was introduced into the system and stirred for two minutes. Electrical conductivity readings were then collected at 30-min intervals over a 4-h period. A parallel pozzolan–water suspension without the addition of lime was also tested, and its conductivity contribution was subtracted from the lime–pozzolan measurements to obtain corrected conductivity values. 2.2.4 Mix Proportioning The mortar mixtures were formulated using hydrated lime as the primary binder, modified through the incorporation of pozzolanic additives in varying proportions. Two supplementary materials, sawdust ash and metakaolin, were introduced in equal proportions across the different formulations. Three binder compositions were developed by adjusting the combined lime–SDA–MK content while keeping the sand-to-binder ratio constant at 3:1. The total pozzolanic component was evenly split between SDA and MK to assess their potential synergistic effects. A lime-only mortar was produced as a reference mix. The detailed mix proportions, expressed on a weight basis, are presented in Table 1 . Table 1 Compositions of mortars (weight proportions by %). Mix ID Lime (%) SDA (%) MK (%) Binder: sand: water L-C 100 - - 1: 3: 0.8 L-S-M-1 50 25 25 1: 3: 0.8 L-S-M-2 40 30 30 1: 3: 0.8 L-S-M-3 30 35 35 1: 3: 0.8 2.2.5 Mortar preparation The dry constituents; hydrated lime, sawdust ash, metakaolin, and sand, were combined in the proportions specified in Table 1 and mechanically blended for 2 minutes to achieve a uniform mixture. Water was then introduced gradually, followed by an additional 3 minutes of mixing to ensure thorough dispersion of all components. Final mixing was performed in an automatic mortar mixer (E095 Mortar Mixer, Matest, Brugherio, Italy) operating at rotational speeds of 140 and 285 rpm for 90 seconds. The fresh mortar was cast into greased, three-part molds measuring 40 × 40 × 160 mm. Each layer was compacted using a vibrating table to minimize entrapped air and promote proper consolidation. The specimens were removed from the molds after 48 hours and subsequently cured under controlled conditions at 20 ± 2°C and 95% relative humidity for the initial 7 days to facilitate pozzolanic reactions. After this period, the samples were transferred to laboratory ambient conditions (65 ± 5% relative humidity) to allow natural carbonation, in accordance with ASTM C109 [ 30 ]. Air curing was selected because strength development in lime-based mortars occurs predominantly through carbonation, which requires exposure to atmospheric CO₂ in the presence of adequate moisture. This curing regime enables the specimens to harden in a manner that closely reflects their natural setting behavior. Previous studies have also indicated that the use of non-carbonated lime in mortar systems may adversely affect the development of mechanical strength. 2.2.6 Water absorption and Density The water absorption of the mortar prisms (40 × 40 × 160 mm) was evaluated in accordance with BS EN 772 − 11:2011 [ 31 ]. Before testing, the specimens were oven-dried at 60 ± 5°C until a constant mass was reached and then allowed to cool in a desiccator. The initial dry mass of each specimen was recorded as m₁. Subsequently, the samples were completely submerged in a sealed water-filled container for a period of 48 h. To facilitate the release of entrapped air, the specimens were first positioned at an angle of approximately 45°, after which they were placed in an upright orientation for the duration of the immersion. At the end of the 48 h period, the specimens were removed, and excess surface water was carefully blotted off using a damp cloth. The saturated mass was then measured and recorded as m₂. Water absorption was calculated as a percentage using the standard expression presented in Eq. (1). W 48 = \(\:\frac{W2-W1}{W1}\:X\:100\) Eq. 1 The density of the hardened mortar specimens was measured in accordance with EN 1015-2 [ 32 ]. For determination in the dry condition, the samples were first dried until a constant mass was achieved. The volume of each specimen was calculated from its geometric dimensions, which were obtained using a dial caliper. The mass of each dried specimen was measured with a precision balance, and the corresponding density (ρ) of the mortar was subsequently computed. Both the density and water absorption evaluations were performed on specimens that had been cured for 28 days. 2.2.7 Determination of flexural and compressive strength The compressive strength of the cured mortar prisms was determined at 28 and 90 days in accordance with KS EAS 148-1:2017 [ 33 ]. For each curing period, three specimens measuring 40 × 40 × 160 mm were retrieved from the curing environment, and any surface residues were carefully removed using a dry woven cloth. After labeling, the samples were positioned in a compression testing machine (Matest S.p.A., Treviolo, Italy). An initial vertical load of 50 N/s was applied, followed by continuous loading at a rate of 2400 N/s until specimen failure. The compressive strength was calculated as the average of the three specimens and expressed in megapascals (MPa). Flexural strength testing was conducted following EN 196-1:2002 [ 34 ] standards, using an MTS 809 hydraulic press (MTS Systems Corporation, Eden Prairie, MN, USA). Mortar prisms measuring 40 × 40 × 160 mm were used, with three samples tested per mix. The final flexural strength was reported as the mean of the three measurements. 3. Results and Discussion 3.1 Elemental composition and LOI Table 2 presents the mean oxide composition and loss on ignition of MK, SDA, and hydrated lime, with values expressed as mass percentages. Table 2: Chemical composition and LOI (wt %) Material SiO₂ Al₂O₃ Fe₂O₃ CaO MgO K₂O Na₂O LOI Σ(SiO₂+Al₂O₃+Fe₂O₃) MK 51.28±0.05 35.87 ±0.08 1.25±0.03 0.44±0.02 0.24 + 0.01 0.53±0.02 0.95±0.03 9.14±0.02 88.38±0.03 SDA 67.24±0.01 4.69±0.04 2.42±0.03 12.35±0.02 0.94 + 0.02 2.12±0.02 1.71±0.01 13.23±0.03 74.35±0.04 Lime – 0.21 ±0.01 – 88.45±0.02 1.24 + 0.02 – – 27.14±0.02 – The chemical compositions of MK, SDA, and hydrated lime, as summarized in Table 2, highlight their distinct functional roles in a pozzolana–lime binder system. The MK sample is primarily composed of SiO₂ (51.28%) and Al₂O₃ (35.87%), with only minor Fe₂O₃ (1.25%) and trace amounts of CaO, MgO, and alkali oxides. These results align well with typical metakaolin derived from calcined kaolinite, which is widely reported to contain 50–60% SiO₂ and 30–40% Al₂O₃ and to exhibit strong pozzolanic reactivity due to its largely amorphous aluminosilicate structure [16, 35, 36]. The substantially higher Al₂O₃ content in MK, relative to SDA, stems from its origin as an aluminosilicate clay mineral (Al₂Si₂O₅(OH)₄) and the structural disorder introduced during calcination. This characteristic plays a critical role in improving pozzolanic reactivity, promoting strength gain, and refining the microstructure in low-carbon pozzolan–lime cement systems [16]. In contrast, SDA exhibits a silica-rich composition (67.24% SiO₂) with lower contents of Al₂O₃ (4.69%) and Fe₂O₃ (2.42%), yielding a total pozzolanic oxide content of 74.35%. This exceeds the 70% threshold commonly used to classify materials as pozzolans under ASTM C618 [27]. Such a profile is consistent with silica-dominant sawdust ashes reported in previous studies, where SiO₂ typically ranges between 60 and 80%, while alumina and iron oxides are minor constituents [20, 37, 38, 39]. However, some studies have reported SDA compositions that fall below the ASTM C618 [27] pozzolanic threshold, indicating limited reactivity [40, 41, 42]. In such cases, the material functions primarily as a filler or weak hydraulic component, as it cannot produce significant C–S–H or Calcium–Alumino–Silicate–Hydrate (C–A–S–H) gels without additional activators or blends [43]. When paired with MK, the complementary chemistry of alumina-rich MK alongside the silica- and calcium-rich SDA can drive synergistic hydration, fostering a more balanced formation of C–S–H and C–A–S–H phases and, in turn, enhancing overall binder performance. Hydrated lime differs markedly from both MK and SDA, with a dominant CaO content of 88.45% and negligible levels of silica, alumina, and iron oxides. This confirms its role as the main calcium source rather than as a pozzolanic material. Similar elemental compositions have been reported in prior studies [10, 36, 44, 45]. The combined CaO + MgO content exceeds 80%, meeting EN 459-1 [46] standards. In lime–pozzolan systems, the calcium provided by lime facilitates the carbonation reaction, which promotes the formation of calcium silicate hydrates and calcium aluminate hydrates. These hydration products are critical for developing strength and enhancing durability in lime-based mortars [47, 48]. Furthermore, CaO can act as an expanding agent to counteract shrinkage, and in some systems, it can substitute more expensive alkali activators. Its inclusion has been shown to improve compressive strength, reduce water absorption and porosity, and enhance overall durability [49, 50]. According to KS EAS 18-1:2017 [51], pozzolans and lime should contain less than 5% MgO. Both MK and SDA satisfied this criterion, while lime contained no MgO. Exceeding 5% MgO in cementitious materials can lead to harmful expansion, generating microcracks that serve as pathways for aggressive agents such as CO₂, sulfates, and chlorides, potentially compromising durability [48]. The standard also limits the alkali content (Na₂O and K₂O) of pozzolans to below 1.5%. MK met this requirement, whereas SDA exceeded the limit, consistent with reports by Raheem et al . [38]. While alkalis in cement can protect steel reinforcement and facilitate chemical reactions among cement phases, excessive levels may trigger deleterious alkali-aggregate reactions, causing expansion and cracking in hardened mortars or concrete [48, 52]. The LOI values of the materials further reflected their composition. SDA exhibited a LOI of 13.23%, within the range typical for biomass ashes, and attributable to unburnt carbon, bound water, and volatile organics [40, 41, 42]. This value is slightly higher than those reported by Udoeyo & Dashibil [20], Elinwa & Ejeh [37], Raheem et al. [38], and Olaiya et al . [39]. The LOI for MK was 9.14%, marginally higher than that of highly processed commercial metakaolins (often <5%) [10, 35], though comparable to laboratory-calcined clays where residual hydroxyl groups, unburnt organics, or incomplete dehydroxylation contribute to mass loss [45, 53]. Hydrated lime exhibited a very high LOI of 27.14%, reflecting thermal decomposition of residual carbonates and release of chemically bound water. This high LOI value is consistent with those reported by İsafça-Kaya et al . [10] and Shukla et al . [44]. Elevated LOI levels can adversely affect cement hydration, reduce workability, and ultimately influence concrete performance [54]. 3.2 Mineralogical Composition X-ray diffraction was used to determine both the crystalline and non-crystalline phases present in the metakaolin, sawdust ash, and hydrated lime samples. The corresponding diffraction patterns are presented in Figures 1–3. XRD results indicate that the metakaolin is primarily composed of a disordered aluminosilicate framework, with residual crystalline quartz and small amounts of accessory minerals, a phase assemblage typical of a highly reactive thermally treated clay. The breakdown of the kaolinite structure during dehydroxylation introduces significant structural irregularity, which increases the mobility and accessibility of reactive silicon and aluminum species. As a result, these species readily interact with calcium hydroxide released from hydrated lime, promoting the formation of calcium–aluminosilicate–hydrate gels and hydrotalcite-type phases. These reaction products are widely recognized as the main contributors to strength gain and densification of the microstructure in lime–metakaolin systems [55, 56]. The pronounced diffraction peaks observed at approximately 2θ values of 26.6°, 36.5°, 50.1°, 59.9°, and 68° are attributed to the presence of residual α-quartz (SiO₂), which is known to remain stable and largely unchanged during the calcination of kaolinitic clays. This observation confirms that the original kaolinite structure was effectively converted into a highly disordered and reactive aluminosilicate phase, while the thermodynamically stable quartz component persisted. This phase evolution is consistent with earlier studies on metakaolin used in both lime-based and cementitious binder systems [53]. In addition, the occurrence of weak and broadened features in the low- and mid-angle regions suggests the presence of poorly ordered or zeolitic aluminosilicate domains formed through partial structural rearrangement during thermal treatment. These disordered regions further enhance the release of reactive aluminum and silicon, thereby supporting the development of C–A–S–H phases in calcium-rich systems and contributing to improved mechanical performance [56, 57]. In contrast, the XRD pattern of the SDA sample reveals a predominantly crystalline material, characterized by several well-defined peaks corresponding to a range of mineral phases. The main reflections are associated with calcite (CaCO₃), diaspore (AlO(OH)), rosenhahnite (Ca₃H₂AlO₁₀Si₃), ferrosilite, and tilleyite (C₂Ca₅O₁₃Si₃). The identification of these phases indicates that the ash contains substantial amounts of calcium-, silica-, and alumina-bearing compounds, which are commonly linked to pozzolanic behavior. Comparable findings have been reported by Assiamah et al . [58] and Raheem et al . [38], who noted that SDA is largely composed of silica- and alumina-rich phases, together with discernible amorphous fractions. Likewise, Udoeyo and Dashibil [20] documented several crystalline constituents in SDA, with calcite as the dominant phase, followed by quartz. The presence of sharp and distinct diffraction peaks in the current SDA pattern (Fig. 2) indicates a relatively low proportion of amorphous material. Deviations from optimal combustion conditions, whether at insufficient or excessive temperatures, are known to favor the formation of crystalline phases, which can limit the suitability of the ash as a cement replacement material [59]. Nonetheless, the detection of reactive silicate and aluminate phases suggests that SDA may still function as a supplementary cementitious material, although its reduced amorphous silica content is likely to constrain its overall pozzolanic reactivity. The XRD pattern of the pure hydrated lime sample shown in Figure 3 is dominated by portlandite (Ca(OH)₂), as evidenced by strong, sharp diffraction peaks characteristic of this phase. The most intense peak, located at approximately 21° 2θ, is widely recognized as a diagnostic feature of hydrated lime. Additional high-angle reflections further confirm the material’s high degree of crystallinity and phase purity. Minor reflections attributable to calcite (CaCO₃) indicate partial carbonation, most likely resulting from exposure to atmospheric carbon dioxide during sample preparation or curing. Similar observations have been commonly reported even for freshly prepared lime-based materials [60], and comparable diffraction patterns have been documented by Abed et al . [61]. This phase assemblage provides a basis for understanding the typically low early-age strength and the slow long-term hardening behavior reported for pure lime mortars in the literature. The narrow peak widths and low background intensity point to a limited amorphous fraction and minimal occurrence of secondary reaction products. This interpretation is consistent with the work of Matschei et al . [62], who noted that hydrated lime generally remains highly crystalline unless modified with pozzolanic additions, which tend to introduce broader diffuse features associated with amorphous C–S–H or C–A–S–H phases. In contrast to lime–pozzolan systems, where the consumption of portlandite leads to a noticeable reduction in Ca(OH)₂ peak intensity, the persistence and prominence of portlandite reflections in this sample indicate negligible secondary reaction and confirm the non-hydraulic character of the binder [62, 63]. The absence of broad amorphous halos or diffraction features linked to hydraulic reaction products further supports the conclusion that the material remains largely unreacted and depends primarily on slow carbonation processes for long-term strength development. 3.3 Pozzolanic Activity Test Results Pozzolanic activity was assessed by tracking the decrease in electrical conductivity of a saturated calcium hydroxide solution following the introduction of MK and SDA that had been calcined under controlled conditions. The variation in conductivity loss with time is illustrated in Figure 4. The patterns of electrical conductivity (EC) reduction shown in Figure 4 clearly illustrate the comparative pozzolanic behavior of MK and SDA calcined at 600 °C for 2 h. For both materials, conductivity loss increases steadily with time, indicating the ongoing removal of Ca²⁺ and OH⁻ ions from the saturated calcium hydroxide solution as secondary hydration products, mainly calcium silicate hydrate and calcium aluminosilicate hydrate, are generated [29, 64]. The sharp rise in EC loss during the first 30–60 minutes for both the samples points to the rapid dissolution of reactive amorphous silica and alumina, a behavior typically associated with the highly disordered aluminosilicate structures formed through controlled thermal activation [28]. Despite this similar early response, MK consistently exhibits a lower overall magnitude of conductivity loss than SDA over the entire test duration, suggesting a comparatively steadier and more controlled reaction rate. The response of MK corresponds closely with well-documented findings for calcined kaolinitic materials. When kaolinite is dehydroxylated within the temperature range of approximately 500–700 °C, it transforms into a predominantly amorphous aluminosilicate phase that is highly reactive in the presence of calcium hydroxide [28]. According to Sabir et al . [16] and Siddique and Klaus [65], MK typically shows a strong early pozzolanic reaction, followed by a slower, diffusion-controlled stage as reaction products accumulate on particle surfaces and restrict further dissolution. The near-linear increase in EC loss observed for MK between 60 and 240 minutes in Figure 4 is consistent with this later stage, where ion transport through the developing C–A–S–H layer governs the overall reaction kinetics rather than the initial availability of reactive phases. By contrast, SDA displays a consistently higher and more rapid decline in conductivity throughout the testing period, reaching roughly 2.0 mS/cm at 240 minutes, compared with about 1.5 mS/cm for MK. This behavior likely reflects a higher presence of soluble alkalis, such as K₂O and Na₂O, along with readily available reactive silica in the SDA system (Table 2), which can accelerate the uptake of calcium ions from solution [66]. Comparable observations have been reported for biomass-derived ashes rich in amorphous silica, where alkali oxides increase the ionic strength of the pore solution and promote faster dissolution of siliceous components [67]. Nevertheless, previous studies also highlight that such materials often display less consistent long-term behavior due to their heterogeneous composition and the presence of relatively inert crystalline phases, including quartz. The elevated EC loss measured for SDA may therefore result from a combination of genuine pozzolanic reactions and the release of soluble salts, which can temporarily enhance ionic mobility in solution before becoming incorporated into hydration products (Figueiredo & Pavia, 2020). This interpretation aligns with the work of Chusilp et al . [54], who noted that agricultural waste ashes can exhibit pronounced early-age changes in conductivity that do not necessarily correspond directly to long-term strength development. In comparison, the more gradual and stable EC reduction observed for MK is commonly associated with the formation of well-structured C–A–S–H gels, which are known to improve microstructural densification and contribute to enhanced mechanical performance over time [16]. The trends presented in Figure 4 demonstrate that both MK and SDA possess measurable pozzolanic activity when calcined at 600 °C, although they differ markedly in their reaction rates and the likely stability of the hydration products formed. MK exhibits a more consistent and literature-supported reactivity profile, reinforcing its suitability as a dependable supplementary cementitious or lime–pozzolan material. In contrast, the higher apparent early reactivity of SDA underscores the need for careful chemical and mineralogical characterization to separate true pozzolanic contributions from the effects of soluble alkali release, as emphasized in prior research on biomass-based ashes. 3.4 Water absorption and Density Table 3 and Figure 5 present the 28-day saturated water-absorption percentages and density for the lime and lime-based composite mortars. Table 3: Physical and mechanical properties of lime and lime composite mortars Bulk density and water absorption are key indicators of a mortar’s durability and resistance to weathering. As shown in Figure 5, the bulk density values follow a clear and consistent downward trend from the control mix (L-C) to the modified mixes (L-S-M-1 through L-S-M-3), decreasing from 1742 kg/m³ for L-C to 1268 kg/m³ for L-S-M-3. This pattern indicates that the progressive incorporation of SDA and MK reduces the overall compactness of the solid framework within the mortar. The relatively high density of the lime-only mix reflects tighter particle packing and fewer internal voids. In contrast, the lower densities observed in the blended systems are associated with the finer particle size, lower specific gravity, and inherently higher porosity of the pozzolanic materials, as well as increased water demand and air entrainment, all of which tend to enlarge interparticle spaces and trapped air volumes [39]. Comparable trends have been reported in other lime–pozzolan systems, where the introduction of finely divided aluminosilicate materials produces a more open granular structure and reduced bulk density as a result of altered particle packing and surface characteristics [45, 68]. Among the modified mortars, mix L-S-M-1 showed the smallest reduction in density, at 12.58%, while L-S-M-3 exhibited the greatest decrease, at 27.21%. This suggests that L-S-M-1 represents an optimal proportioning of lime and pozzolanic materials, where the pozzolanic reactions contribute to localized densification and reduced porosity. At lower replacement levels (L-S-M-1), fine materials such as metakaolin and SDA can act as micro-fillers, improving particle packing and encouraging the formation of secondary calcium silicate and aluminate hydrates that enhance the internal structure. At higher replacement levels (L-S-M-2 and L-S-M-3), however, the influence of the more porous ash particles and the increase in capillary porosity become dominant, leading to progressively lower hardened densities [39]. Previous research has similarly shown that near-equal lime-to-pozzolan ratios (around 1:1) tend to promote balanced hydrate formation and denser microstructures, whereas pozzolan-rich mixes (2:1 to 3:1) initially refine pores but eventually become limited by available lime, resulting in lower density and higher permeability at elevated replacement levels [45, 69]. From a practical standpoint, the observed reduction in bulk density has implications for both mechanical performance and durability. Lower density is often associated with higher overall porosity, which can facilitate moisture movement and potentially reduce compressive strength if not offset by sufficient pozzolanic activity and binder development. In more reactive systems, however, the gradual formation of secondary calcium silicate hydrate and calcium aluminosilicate hydrate gels from metakaolin and reactive ash phases can partially refine the pore network over time, moderating some of the negative effects linked to early-stage density loss [70]. At the same time, reduced density may be advantageous in terms of structural loading, as lighter mortars can lower demands on formwork and contribute to reduced dead loads [45]. The data presented in Table 3 and Figure 5 show that water absorption by mass ranges from 4.31% to 7.12%. Overall, water uptake decreases from the control lime mortar (L-C) to the blended lime–SDA–metakaolin mixes before gradually increasing again as the proportion of supplementary materials rises. The control mix recorded the highest absorption value of 7.12%, reflecting the relatively open pore structure typical of air-lime mortars with limited hydraulic activity. During air curing and carbonation, fine surface pores form, and when the mortar is immersed, water readily penetrates these interconnected voids [60]. The addition of MK and SDA leads to a slight reduction in water absorption, with the effect most pronounced in mix L-S-M-1, which shows a 39.5% decrease. This behavior is attributed to the substantial formation of hydration products that make the hydrated lime mortar denser and less permeable [71]. The inclusion of pozzolanic materials generally refines the pore structure and limits water ingress. Among the blended mortars, L-S-M-1 exhibits the lowest absorption, consistent with its more balanced lime-to-pozzolan ratio. In this mix, reactive silica is more fully utilized, reducing the volume of permeable voids. The combined presence of SDA and MK promotes pore refinement through reactions with calcium hydroxide, producing secondary C–S–H and C–A–S–H gels that partially block capillary pathways and restrict moisture penetration [16, 72]. As the proportions of SDA and MK increase further in mixes L-S-M-2 and L-S-M-3, a modest rise in water absorption is observed. This reflects the interplay between chemical densification and physical packing. Higher contents of fine, irregular pozzolanic particles can increase total porosity or form interconnected pore networks when the available lime becomes insufficient to react with all added aluminosilicates [68]. In such cases, excess ash and less reactive filler phases can contribute to higher permeability and water uptake, which may compromise long-term durability [56]. These findings underscore the importance of optimizing the proportions of SDA and MK to achieve a balance between sustainability objectives and performance requirements in lime-based mortars. Although the incorporation of MK and SDA reduces permeability and water absorption, the mortars still retain a relatively high level of breathability. This characteristic is particularly important in the conservation of historic masonry, where maintaining moisture balance is essential. Enhanced vapor permeability allows moisture to move through the material rather than becoming trapped within walls, thereby reducing the risk of condensation, dampness, and associated deterioration. It also limits the buildup of harmful agents such as mold and persistent moisture within the masonry fabric [73]. By contrast, the use of impermeable, non-breathable materials can disrupt this balance and accelerate degradation processes. 3.5 Compressive and Flexural strength Figure 6 presents the mean flexural and compressive strength values of the specimens measured after 28 and 90 days of curing. The compressive and flexural strength data in Figure 6 reveal a clear and consistent effect of both binder composition and curing duration on the mechanical behavior of the lime–SDA–MK mortars. The control mixture (L-C), composed entirely of lime, displays very low compressive and flexural strengths at both testing ages when compared with the blended mortars. Its compressive strength increases slightly, from approximately 0.46 MPa at 28 days to about 0.97 MPa at 90 days, while flexural strength rises from 0.27 MPa to 0.52 MPa over the same period. These limited improvements reflect the inherently low strength, brittleness, and weak binding capacity of air-lime systems used without supplementary materials [25]. The gradual nature of strength development in L-C indicates that carbonation, rather than the formation of cementitious phases such as C–S–H or C–A–S–H, governs its hardening process. Comparable low strength levels for pure lime mortars at extended curing ages have been documented in experimental investigations of traditional lime-based materials [60, 69]. Introducing SDA and MK leads to a marked enhancement in both compressive and flexural performance, underscoring the role of pozzolanic reactions in improving matrix cohesion and resistance to cracking. Among the blended systems, mix L-S-M-1 (50% lime, 25% SDA, 25% MK) achieves the highest strengths, with compressive strength increasing from 4.96 MPa at 28 days to 7.12 MPa at 90 days, and flexural strength rising from 0.62 MPa to 1.34 MPa over the same period. This strong time-dependent gain is consistent with the progressive consumption of Ca(OH)₂ released by lime and the subsequent formation of secondary cementitious gels from the reactive silica and alumina provided by MK and SDA. These products refine the pore network and strengthen the interfacial transition zones between the binder and sand particles. Lime-based composite mortars develop strength through a combination of pozzolanic reactions, hydration, and carbonation [13, 74]. The superior performance of L-S-M-1 suggests that this formulation offers a favorable balance between available calcium and reactive pozzolanic oxides, promoting a dense microstructure and more efficient stress transfer. Metakaolin supplies a highly reactive source of alumina and silica, while SDA contributes additional silica and alkalis that can accelerate dissolution and support the formation of secondary hydrates. This combined effect results in a more uniform distribution of reaction products and a refined interfacial transition zone within the mortar matrix [16, 66]. Similar improvements in lime mortars containing pozzolans have been reported by Walker and Pavía [11], who linked strength enhancement to the high fineness and reactivity of pozzolanic additions that generate a denser and more continuous binding phase. The L-S-M-2 mixture (40% lime, 30% SDA, 30% MK) also demonstrated a relatively strong performance, with compressive strength increasing from 4.14 MPa at 28 days to 5.97 MPa at 90 days and flexural strength rising from 0.42 MPa to 1.24 MPa. Although these values remain slightly below those of L-S-M-1, the overall trend indicates sustained pozzolanic activity with time. The modest reduction in strength is likely associated with the lower lime content, which may limit the availability of calcium hydroxide needed to fully react with the higher proportion of pozzolanic material. This behavior is consistent with observations by Donatello et al. [75], who noted that beyond an optimal pozzolan dosage, both compressive and flexural strengths tend to level off or decrease as calcium becomes insufficient for complete hydrate formation. Mix L-S-M-3 (30% lime, 35% SDA, 35% MK) records the lowest strength among the blended mortars. Its compressive strength increases from 2.90 MPa at 28 days to 3.74 MPa at 90 days, while flexural strength rises from 0.36 MPa to 0.80 MPa. Although these values remain higher than those of the pure lime control, the decline relative to the other blended mixes indicates that excessive replacement of lime with pozzolanic materials results in a weaker and less cohesive binder network. At such high substitution levels, the system becomes rich in silica and alumina but deficient in calcium, leading to incomplete reactions and a more open, porous microstructure [45, 69, 75]. The resulting increase in porosity and reduced interparticle bonding ultimately diminishes both compressive and flexural performance. All mixtures exhibited continued strength development with extended curing. This behavior reflects the slow but ongoing reactions between lime, pozzolans, and water, as well as the gradual progress of carbonation [45]. The higher strength values recorded at 90 days for mixes containing SDA and MK confirm that prolonged curing supports further microstructural refinement and cementitious matrix development, leading to improved mechanical performance over time. 4. Conclusion In light of the results and analysis presented above, the following conclusions were made: 1. A completely clinker-free pozzolan–lime binder was successfully produced by combining SDA with MK as complementary supplementary materials. 2. Chemical and mineralogical characterization showed that MK supplies a highly reactive amorphous aluminosilicate phase, whereas SDA contributes a silica-rich matrix with demonstrable pozzolanic potential. 3. Electrical conductivity measurements confirmed that both SDA and MK are pozzolanic when calcined at 600 °C, with MK exhibiting steady and well-regulated reactivity, while SDA displayed more pronounced early-stage calcium uptake. 4. Mortars incorporating the blended binders developed substantially higher compressive and flexural strengths than the pure lime reference mix, underscoring the effectiveness of the lime–pozzolan interaction. 5. The formulation containing 50% lime, 25% SDA, and 25% MK delivered the best mechanical performance, reflecting an optimal balance between available calcium and reactive silica and alumina. 6. Controlled additions of SDA and MK lowered water absorption and promoted pore refinement, leading to improvements in durability-related characteristics. 7. At higher replacement levels, reductions in density and strength were observed, attributable to calcium limitation and incomplete pozzolanic reactions. 8. The SDA–MK–lime system shows considerable promise as a low-carbon, resource-efficient binder for both structural and non-structural construction, particularly in settings where material availability and sustainability are critical considerations. Declarations Data Availability The data used to support the findings of this study are available from the corresponding author upon request. Conflicts of Interest The authors declare that they have no conflicts of interest. Acknowledgments The authors wish to thank the University of Eldoret, the Ministry of Mining, Environment, and Natural Resources, and the Ministry of Roads for the provision of laboratory facilities used in this work. Funding This research did not receive any external funding. Ethics and declaration Not applicable Consent to participate declaration Not applicable Consent to publish declaration Not applicable References K. L. Scrivener, V. M. John, and E. M. Gartner, “Eco-efficient cements: Potential economically viable solutions for a low-CO₂ cement-based materials industry,” Cement and Concrete Research , vol. 114, pp. 2–26, Dec. 2018. M. A. Keerio, A. Saand, A. Kumar, N. Bheel, and K. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8782472","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":601468874,"identity":"46d1368e-9317-4110-8d4a-3a9d2be4579a","order_by":0,"name":"Odiwuor Vincent Onyango","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABEklEQVRIie3PMUvEMBTA8RcC7VLpmoCcXyFHoQgW/SAuLYVzsXCjQofAQVwqnQ/Ez1CXw80cDzr1A9TNw9XBzbvh0BS3g3Dn5pD/lpAfeQ/A5fqnaSIBPEqXb+zGHH0Qw63YS0Jf5eK0M0d6AIGB8KqL2a06gJzUuNKb5+RS9Klgr484CiltYF1CFFqI6Cdied9NiqZP0/F8gRGfeVNStRBzaSEsACQKB6JzvsCswUDAkYREaNtg3UC+Dckkbh8wezGEbA25sBDQ1wPRxbxCIrk0v9BAUPNLbNv/dxeVF7WvKLD2KmLoTfG4ZRGzDobvnxt1Xigafq1ZeTYK72ZPq48yGdeW9S3TArC/vHe5XC7XTj8kKmFmjWOCPgAAAABJRU5ErkJggg==","orcid":"","institution":"University of Eldoret","correspondingAuthor":true,"prefix":"","firstName":"Odiwuor","middleName":"Vincent","lastName":"Onyango","suffix":""},{"id":601468875,"identity":"ff26fb28-26dd-4146-b288-8736632d458d","order_by":1,"name":"Enos W. Wambu","email":"","orcid":"","institution":"University of Eldoret","correspondingAuthor":false,"prefix":"","firstName":"Enos","middleName":"W.","lastName":"Wambu","suffix":""},{"id":601468876,"identity":"52ca4972-15a7-42df-ab03-246977c2a250","order_by":2,"name":"Ayabei Kiplagat","email":"","orcid":"","institution":"University of Eldoret","correspondingAuthor":false,"prefix":"","firstName":"Ayabei","middleName":"","lastName":"Kiplagat","suffix":""},{"id":601468877,"identity":"f86f0d20-479c-42de-8730-5d7b88348bd3","order_by":3,"name":"Samuel Lutta","email":"","orcid":"","institution":"University of Eldoret","correspondingAuthor":false,"prefix":"","firstName":"Samuel","middleName":"","lastName":"Lutta","suffix":""},{"id":601468878,"identity":"b7797637-f10d-423d-bdfb-aa3451a60f7f","order_by":4,"name":"Onesmus Mulwa Munyao","email":"","orcid":"","institution":"Kenyatta University","correspondingAuthor":false,"prefix":"","firstName":"Onesmus","middleName":"Mulwa","lastName":"Munyao","suffix":""}],"badges":[],"createdAt":"2026-02-04 06:23:44","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8782472/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8782472/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":104403573,"identity":"f539cb5a-8dc5-4b94-b6c4-15aed33c4f69","added_by":"auto","created_at":"2026-03-11 12:18:36","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":168731,"visible":true,"origin":"","legend":"\u003cp\u003eXRD pattern of MK\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8782472/v1/5d86b6306668c9f298a1208f.png"},{"id":104403398,"identity":"18a4f74c-d200-4118-b284-f0ba459344a1","added_by":"auto","created_at":"2026-03-11 12:18:15","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":201872,"visible":true,"origin":"","legend":"\u003cp\u003eXRD pattern of SDA\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-8782472/v1/b1253f56d92023717b9ff3f4.png"},{"id":104102393,"identity":"e0e23e7a-d2de-4334-ab1f-19102c65390c","added_by":"auto","created_at":"2026-03-06 20:18:38","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":149248,"visible":true,"origin":"","legend":"\u003cp\u003eXRD pattern of hydrated lime\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8782472/v1/f66cd8df169bacb1139105c7.png"},{"id":104102397,"identity":"0fd25236-2a6d-4664-8a3d-fa60b452adcd","added_by":"auto","created_at":"2026-03-06 20:18:38","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":63854,"visible":true,"origin":"","legend":"\u003cp\u003ePozzolanic Activity of MK \u0026amp; SDA samples calcined for 2 h at 600 °C\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-8782472/v1/05368fe794f64251e83bc872.png"},{"id":104403673,"identity":"d7300b27-1d33-4ad5-8598-2039bee90301","added_by":"auto","created_at":"2026-03-11 12:18:49","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":42029,"visible":true,"origin":"","legend":"\u003cp\u003eWater absorption and density of lime and lime composite mortars\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-8782472/v1/ca35822b365d39aee80b61e4.png"},{"id":104102396,"identity":"a2f71b28-ff71-44d8-9937-9bc05713af8f","added_by":"auto","created_at":"2026-03-06 20:18:38","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":50458,"visible":true,"origin":"","legend":"\u003cp\u003eCompressive and Flexural strength of lime and lime composite mortars\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-8782472/v1/fb6c2a691e57081aa4ea9d04.png"},{"id":104409432,"identity":"8cf20064-3aee-410d-8d33-5079b77386f3","added_by":"auto","created_at":"2026-03-11 12:45:04","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1283707,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8782472/v1/cdb6a296-267d-4726-b308-094d05f1a827.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Development of a Low-Carbon Pozzolana–Lime Cement Using Sawdust Ash and Calcined Clay","fulltext":[{"header":"1.Introduction","content":"\u003cp\u003eThe cement sector is widely recognized as one of the most carbon-intensive industries worldwide, accounting for approximately 7\u0026ndash;8% of total human-generated CO₂ emissions [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. Large volumes of carbon dioxide and other greenhouse gases are released throughout the cement production process. These emissions arise mainly from the thermal decomposition of limestone and the combustion of fossil fuels during clinker manufacture, both of which are inherent to the production of ordinary Portland cement (OPC) [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. As a result, cement manufacturing is increasingly regarded as both economically demanding and environmentally burdensome. For every ton of Portland cement produced, an estimated 1.0\u0026ndash;1.25 tons of CO₂ are emitted, while approximately 1.60 MWh of energy is consumed in the process [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. Furthermore, the intensive extraction of raw materials required for cement production contributes to the depletion of natural resources [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eAs global construction activity continues to grow, emissions associated with cement production have become a major environmental challenge, particularly in developing regions where infrastructure development is closely linked to rising cement consumption [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Consequently, the design of sustainable, low-carbon binder systems has emerged as an urgent priority for both researchers and industry practitioners. In response, substantial efforts have focused on partially substituting OPC with supplementary cementitious materials (SCMs), including fly ash, ground granulated blast furnace slag, and silica fume [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. Another alternative and increasingly studied strategy embraces the use of lime-based binders modified with SCMs, particularly naturally occurring aluminosilicate materials such as volcanic ash, zeolites, kaolinitic clays, and pumice [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThese alternative materials have been shown to lower clinker demand while enhancing the long-term durability of cementitious systems [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. However, their broader implementation is constrained by several practical challenges. The supply of some SCMs is diminishing as energy and metallurgical industries move away from coal-fired power generation and traditional smelting processes [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. In addition, the performance of many SCMs depends strongly on regional availability, material consistency, and the need for industrial-scale processing, which restricts their use in low-income and resource-limited contexts [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. As a result, conventional approaches based on widely used SCMs have not fully resolved the combined issues of material scarcity and economic feasibility in efforts to reduce cement-related emissions.\u003c/p\u003e \u003cp\u003eTo address these constraints, this study proposes the formulation of a low-carbon pozzolana\u0026ndash;lime binder incorporating sawdust ash (SDA) and metakaolin (MK). Sawdust ash, a largely underexploited residue from the timber and biomass energy sectors, contains a high proportion of amorphous silica and can exhibit significant pozzolanic reactivity when appropriately processed. Likewise, thermally activated calcined clay, particularly metakaolin derived from low-grade kaolinitic sources, has demonstrated strong chemical reactivity and good compatibility with lime and blended binder systems due to its elevated alumina and silica contents [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. The combined use of these two materials is intended to exploit their complementary chemical characteristics, enabling the production of a lime-based binder that achieves acceptable mechanical strength, reduced embodied carbon, and suitability for widespread local application.\u003c/p\u003e \u003cp\u003ePozzolanic ashes and calcined clays have attracted considerable interest because of their ability to react with calcium hydroxide in lime systems to generate secondary binding phases, primarily calcium silicate hydrates (C\u0026ndash;S\u0026ndash;H) and calcium aluminate hydrates (C\u0026ndash;A\u0026ndash;H). The formation of these phases contributes to strength development and microstructural refinement, thereby improving the overall performance of the binder [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eEarlier investigations into the use of either SDA or MK have often reported limitations, including incomplete pozzolanic reactions, insufficient early-age strength, and variable long-term durability [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. Although a substantial body of research exists on individual pozzolanic additives, much of the literature concentrates on binary systems, such as lime\u0026ndash;SDA or lime\u0026ndash;MK, as well as ternary blends based on OPC, rather than on fully clinker-free pozzolana\u0026ndash;lime binders. Moreover, the interactive effects between silica-rich sawdust ash and alumina-rich MK within lime-based cementitious matrices remain inadequately understood. In particular, limited attention has been given to optimizing the relative proportions of calcium, silica, and alumina required to promote the formation of stable and mechanically efficient hydrate phases under ambient curing conditions [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. While several studies have confirmed that metakaolin enhances the pozzolanic activity, strength, and durability of lime mortars [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e], information regarding the specific influence of SDA in lime-based mortars remains scarce. In addition, standardized guidelines for proportioning SDA, MK, and lime are largely absent.\u003c/p\u003e \u003cp\u003eIn light of these research gaps, this work introduces a novel low-carbon pozzolana\u0026ndash;lime binder developed through the combined application of sawdust ash and MK as mutually reinforcing reactive components. The originality of this approach lies in harnessing a synergistic pozzolanic interaction, whereby the silica-rich fraction of SDA complements the alumina-dominant phase of metakaolin, resulting in improved mechanical behavior. By focusing on a fully lime-based binder rather than partial replacement within OPC systems, this study extends existing knowledge on the joint use of two highly reactive and sustainable pozzolans as the primary binding phase, offering a pathway to enhanced performance alongside reduced environmental impact.\u003c/p\u003e \u003cp\u003eThrough an integrated evaluation of chemical reactivity and mechanical properties, this research provides new perspectives on the optimization of ternary pozzolana\u0026ndash;lime systems for both structural and non-structural applications. Furthermore, it contributes to the field of sustainable construction materials by demonstrating the potential for transforming locally available biomass residues and natural clay resources into low-emission, environmentally responsible cementitious products. The proposed pozzolana\u0026ndash;lime binder therefore represents a scientifically grounded and context-sensitive response to the persistent challenge of decarbonizing cement-based materials.\u003c/p\u003e"},{"header":"2 Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Raw materials\u003c/h2\u003e \u003cp\u003eSawdust was obtained from commercial sawmills located on the periphery of Eldoret City, Kenya. Kaolinitic clay was locally sourced through random sampling at two separate sites\u0026mdash;Orembe and Namba Karabok in Homa Bay, kenya\u0026mdash;areas well known for small-scale brick production. The collected samples collected were mechanically blended to produce a homogeneous composite, after which they were sealed in labeled polythene bags for storage. Commercial hydrated lime (Ca(OH)₂), employed as the chemical activator and compliant with KS EAS 2168-1:2020 [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e], was supplied by Britchemicals Ltd. (Nairobi, Kenya). Naturally occurring river sand was used as the fine aggregate, while potable water was utilized for all mix preparations.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Methods\u003c/h2\u003e \u003cdiv id=\"Sec5\" class=\"Section3\"\u003e \u003ch2\u003e2.2.1 Sample preparation\u003c/h2\u003e \u003cp\u003eThe collected sawdust was first air-dried and sieved, after which it was incinerated in a muffle furnace (Advantec KL-420, Tokyo, Japan) at 600\u0026deg;C for a duration of 2 h to eliminate residual carbon and maximize its pozzolanic potential [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. The resulting ash was allowed to cool, then ground and passed through a 45 \u0026micro;m sieve in accordance with ASTM C618 [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e] to remove oversized particles, improve material uniformity, and enhance reactivity. The kaolinitic clay samples were milled using a laboratory ball mill and subsequently calcined at 600\u0026deg;C for 2 h to obtain metakaolin, a process associated with lower energy demand and reduced CO₂ emissions compared to conventional cement manufacturing. After cooling, the produced MK was ground and sieved through a 45 \u0026micro;m mesh. This thermal treatment induces dehydroxylation and leads to the formation of a highly reactive amorphous aluminosilicate phase with pronounced pozzolanic activity [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. Each pozzolanic material was further characterized using electrical conductivity measurements, X-ray diffraction (XRD), and X-ray fluorescence (XRF) analyses. The sand was washed with deionized water, sun-dried to a constant mass, and sieved through a 5 mm mesh to remove coarse particles and ensure uniform grading.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section3\"\u003e \u003ch2\u003e2.2.2 Material characterization\u003c/h2\u003e \u003cp\u003eThe chemical composition of the pozzolanic materials and the reference hydrated lime was determined using an Epsilon 3XLE X-ray fluorescence (XRF) spectrometer (Malvern Panalytical, Almelo, Netherlands). The mineralogical structure, including both crystalline and amorphous phases within the pozzolans, was examined through X-ray diffraction (XRD) analysis conducted with a Bruker AXS diffractometer (Karlsruhe, Germany). Loss on ignition (LOI) testing was performed to quantify residual carbon and volatile constituents. Approximately 1 g of each raw pozzolan sample was first oven-dried at 110\u0026deg;C for 1 h, followed by heating at 1000\u0026deg;C for an additional 1 h. The samples were then cooled in a desiccator and reweighed. The percentage LOI was calculated in accordance with the method outlined by Marangu \u003cem\u003eet al\u003c/em\u003e. [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section3\"\u003e \u003ch2\u003e2.2.3 Evaluation of Pozzolanic Activity\u003c/h2\u003e \u003cp\u003eThe pozzolanic activity was assessed using a modified version of the method proposed by Lux\u0026aacute;n \u003cem\u003eet al\u003c/em\u003e. [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e], with minor procedural adjustments. A saturated calcium hydroxide solution was prepared by dissolving 0.8 g of Ca(OH)₂ in 200 mL of distilled water within a 250 mL beaker. The mixture was maintained at 38\u0026thinsp;\u0026plusmn;\u0026thinsp;1\u0026deg;C on a thermostatically controlled magnetic hotplate and continuously stirred. The initial electrical conductivity of the solution was recorded using a conductivity meter (Oakton CON 510 Series model) once equilibrium was achieved. After stabilization of the lime-water system, 5 g of finely ground pozzolanic material was introduced into the system and stirred for two minutes. Electrical conductivity readings were then collected at 30-min intervals over a 4-h period. A parallel pozzolan\u0026ndash;water suspension without the addition of lime was also tested, and its conductivity contribution was subtracted from the lime\u0026ndash;pozzolan measurements to obtain corrected conductivity values.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section3\"\u003e \u003ch2\u003e2.2.4 Mix Proportioning\u003c/h2\u003e \u003cp\u003eThe mortar mixtures were formulated using hydrated lime as the primary binder, modified through the incorporation of pozzolanic additives in varying proportions. Two supplementary materials, sawdust ash and metakaolin, were introduced in equal proportions across the different formulations. Three binder compositions were developed by adjusting the combined lime\u0026ndash;SDA\u0026ndash;MK content while keeping the sand-to-binder ratio constant at 3:1. The total pozzolanic component was evenly split between SDA and MK to assess their potential synergistic effects. A lime-only mortar was produced as a reference mix. The detailed mix proportions, expressed on a weight basis, are presented in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eCompositions of mortars (weight proportions by %).\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMix ID\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eLime (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eSDA (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eMK (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eBinder: sand: water\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eL-C\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1: 3: 0.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eL-S-M-1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1: 3: 0.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eL-S-M-2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1: 3: 0.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eL-S-M-3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e35\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e35\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1: 3: 0.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section3\"\u003e \u003ch2\u003e2.2.5 Mortar preparation\u003c/h2\u003e \u003cp\u003eThe dry constituents; hydrated lime, sawdust ash, metakaolin, and sand, were combined in the proportions specified in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and mechanically blended for 2 minutes to achieve a uniform mixture. Water was then introduced gradually, followed by an additional 3 minutes of mixing to ensure thorough dispersion of all components. Final mixing was performed in an automatic mortar mixer (E095 Mortar Mixer, Matest, Brugherio, Italy) operating at rotational speeds of 140 and 285 rpm for 90 seconds. The fresh mortar was cast into greased, three-part molds measuring 40 \u0026times; 40 \u0026times; 160 mm. Each layer was compacted using a vibrating table to minimize entrapped air and promote proper consolidation. The specimens were removed from the molds after 48 hours and subsequently cured under controlled conditions at 20\u0026thinsp;\u0026plusmn;\u0026thinsp;2\u0026deg;C and 95% relative humidity for the initial 7 days to facilitate pozzolanic reactions. After this period, the samples were transferred to laboratory ambient conditions (65\u0026thinsp;\u0026plusmn;\u0026thinsp;5% relative humidity) to allow natural carbonation, in accordance with ASTM C109 [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. Air curing was selected because strength development in lime-based mortars occurs predominantly through carbonation, which requires exposure to atmospheric CO₂ in the presence of adequate moisture. This curing regime enables the specimens to harden in a manner that closely reflects their natural setting behavior. Previous studies have also indicated that the use of non-carbonated lime in mortar systems may adversely affect the development of mechanical strength.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section3\"\u003e \u003ch2\u003e2.2.6 Water absorption and Density\u003c/h2\u003e \u003cp\u003eThe water absorption of the mortar prisms (40 \u0026times; 40 \u0026times; 160 mm) was evaluated in accordance with BS EN 772\u0026thinsp;\u0026minus;\u0026thinsp;11:2011 [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. Before testing, the specimens were oven-dried at 60\u0026thinsp;\u0026plusmn;\u0026thinsp;5\u0026deg;C until a constant mass was reached and then allowed to cool in a desiccator. The initial dry mass of each specimen was recorded as m₁. Subsequently, the samples were completely submerged in a sealed water-filled container for a period of 48 h. To facilitate the release of entrapped air, the specimens were first positioned at an angle of approximately 45\u0026deg;, after which they were placed in an upright orientation for the duration of the immersion. At the end of the 48 h period, the specimens were removed, and excess surface water was carefully blotted off using a damp cloth. The saturated mass was then measured and recorded as m₂. Water absorption was calculated as a percentage using the standard expression presented in Eq.\u0026nbsp;(1).\u003c/p\u003e \u003cp\u003eW\u003csub\u003e48 =\u003c/sub\u003e \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\frac{W2-W1}{W1}\\:X\\:100\\)\u003c/span\u003e\u003c/span\u003e Eq.\u0026nbsp;1\u003c/p\u003e \u003cp\u003eThe density of the hardened mortar specimens was measured in accordance with EN 1015-2 [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. For determination in the dry condition, the samples were first dried until a constant mass was achieved. The volume of each specimen was calculated from its geometric dimensions, which were obtained using a dial caliper. The mass of each dried specimen was measured with a precision balance, and the corresponding density (ρ) of the mortar was subsequently computed.\u003c/p\u003e \u003cp\u003eBoth the density and water absorption evaluations were performed on specimens that had been cured for 28 days.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section3\"\u003e \u003ch2\u003e2.2.7 Determination of flexural and compressive strength\u003c/h2\u003e \u003cp\u003eThe compressive strength of the cured mortar prisms was determined at 28 and 90 days in accordance with KS EAS 148-1:2017 [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. For each curing period, three specimens measuring 40 \u0026times; 40 \u0026times; 160 mm were retrieved from the curing environment, and any surface residues were carefully removed using a dry woven cloth. After labeling, the samples were positioned in a compression testing machine (Matest S.p.A., Treviolo, Italy). An initial vertical load of 50 N/s was applied, followed by continuous loading at a rate of 2400 N/s until specimen failure. The compressive strength was calculated as the average of the three specimens and expressed in megapascals (MPa). Flexural strength testing was conducted following EN 196-1:2002 [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e] standards, using an MTS 809 hydraulic press (MTS Systems Corporation, Eden Prairie, MN, USA). Mortar prisms measuring 40 \u0026times; 40 \u0026times; 160 mm were used, with three samples tested per mix. The final flexural strength was reported as the mean of the three measurements.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"3. Results and Discussion","content":"\u003cp\u003e3.1 Elemental composition and LOI\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eTable 2 presents the mean oxide composition and loss on ignition of MK, SDA, and hydrated lime, with values expressed as mass percentages.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eTable 2: Chemical composition and LOI (wt %)\u003c/p\u003e\n\u003ctable border=\"0\" cellpadding=\"0\" align=\"\" width=\"624\"\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003e\u003cstrong\u003eMaterial\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u003cstrong\u003eSiO₂\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u003cstrong\u003e\u0026nbsp;Al₂O₃\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u003cstrong\u003eFe₂O₃\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u003cstrong\u003eCaO\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u003cstrong\u003eMgO\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u003cstrong\u003eK₂O\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u003cstrong\u003eNa₂O\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u003cstrong\u003eLOI\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003ctable border=\"0\" cellpadding=\"0\"\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003e\u003cstrong\u003e\u0026Sigma;(SiO₂+Al₂O₃+Fe₂O₃)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003c/table\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eMK\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e51.28\u0026plusmn;0.05\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp;35.87 \u0026plusmn;0.08\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp;1.25\u0026plusmn;0.03\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; 0.44\u0026plusmn;0.02\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp;0.24\u003cu\u003e+\u003c/u\u003e0.01\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e0.53\u0026plusmn;0.02\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp;0.95\u0026plusmn;0.03\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e9.14\u0026plusmn;0.02\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; 88.38\u0026plusmn;0.03\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eSDA\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e67.24\u0026plusmn;0.01\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp;4.69\u0026plusmn;0.04\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp;2.42\u0026plusmn;0.03\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; 12.35\u0026plusmn;0.02\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp;0.94\u003cu\u003e+\u003c/u\u003e0.02\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e2.12\u0026plusmn;0.02\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp;1.71\u0026plusmn;0.01\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e13.23\u0026plusmn;0.03\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; 74.35\u0026plusmn;0.04\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eLime\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026ndash;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e0.21 \u0026plusmn;0.01\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026ndash;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; 88.45\u0026plusmn;0.02\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp;1.24\u003cu\u003e+\u003c/u\u003e0.02\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026ndash;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026ndash;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e27.14\u0026plusmn;0.02\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026ndash;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003eThe chemical compositions of MK, SDA, and hydrated lime, as summarized in Table 2, highlight their distinct functional roles in a pozzolana\u0026ndash;lime binder system. The MK sample is primarily composed of SiO₂ (51.28%) and Al₂O₃ (35.87%), with only minor Fe₂O₃ (1.25%) and trace amounts of CaO, MgO, and alkali oxides. These results align well with typical metakaolin derived from calcined kaolinite, which is widely reported to contain 50\u0026ndash;60% SiO₂ and 30\u0026ndash;40% Al₂O₃ and to exhibit strong pozzolanic reactivity due to its largely amorphous aluminosilicate structure [16, 35, 36]. The substantially higher Al₂O₃ content in MK, relative to SDA, stems from its origin as an aluminosilicate clay mineral (Al₂Si₂O₅(OH)₄) and the structural disorder introduced during calcination. This characteristic plays a critical role in improving pozzolanic reactivity, promoting strength gain, and refining the microstructure in low-carbon pozzolan\u0026ndash;lime cement systems [16].\u003c/p\u003e\n\u003cp\u003eIn contrast, SDA exhibits a silica-rich composition (67.24% SiO₂) with lower contents of Al₂O₃ (4.69%) and Fe₂O₃ (2.42%), yielding a total pozzolanic oxide content of 74.35%. This exceeds the 70% threshold commonly used to classify materials as pozzolans under ASTM C618 [27]. Such a profile is consistent with silica-dominant sawdust ashes reported in previous studies, where SiO₂ typically ranges between 60 and 80%, while alumina and iron oxides are minor constituents [20, 37, 38, 39]. However, some studies have reported SDA compositions that fall below the ASTM C618 [27] pozzolanic threshold, indicating limited reactivity [40, 41, 42]. In such cases, the material functions primarily as a filler or weak hydraulic component, as it cannot produce significant C\u0026ndash;S\u0026ndash;H or \u003cstrong\u003eCalcium\u0026ndash;Alumino\u0026ndash;Silicate\u0026ndash;Hydrate\u003c/strong\u003e (C\u0026ndash;A\u0026ndash;S\u0026ndash;H) gels without additional activators or blends [43]. When paired with MK, the complementary chemistry of alumina-rich MK alongside the silica- and calcium-rich SDA can drive synergistic hydration, fostering a more balanced formation of C\u0026ndash;S\u0026ndash;H and C\u0026ndash;A\u0026ndash;S\u0026ndash;H phases and, in turn, enhancing overall binder performance.\u003c/p\u003e\n\u003cp\u003eHydrated lime differs markedly from both MK and SDA, with a dominant CaO content of 88.45% and negligible levels of silica, alumina, and iron oxides. This confirms its role as the main calcium source rather than as a pozzolanic material. Similar elemental compositions have been reported in prior studies [10, 36, 44, 45]. The combined CaO + MgO content exceeds 80%, meeting EN 459-1 [46] standards. In lime\u0026ndash;pozzolan systems, the calcium provided by lime facilitates the carbonation reaction, which promotes the formation of calcium silicate hydrates and calcium aluminate hydrates. These hydration products are critical for developing strength and enhancing durability in lime-based mortars [47, 48]. Furthermore, CaO can act as an expanding agent to counteract shrinkage, and in some systems, it can substitute more expensive alkali activators. Its inclusion has been shown to improve compressive strength, reduce water absorption and porosity, and enhance overall durability [49, 50].\u003c/p\u003e\n\u003cp\u003eAccording to KS EAS 18-1:2017 [51], pozzolans and lime should contain less than 5% MgO. Both MK and SDA satisfied this criterion, while lime contained no MgO. Exceeding 5% MgO in cementitious materials can lead to harmful expansion, generating microcracks that serve as pathways for aggressive agents such as CO₂, sulfates, and chlorides, potentially compromising durability [48]. The standard also limits the alkali content (Na₂O and K₂O) of pozzolans to below 1.5%. MK met this requirement, whereas SDA exceeded the limit, consistent with reports by Raheem \u003cem\u003eet al\u003c/em\u003e. [38]. While alkalis in cement can protect steel reinforcement and facilitate chemical reactions among cement phases, excessive levels may trigger deleterious alkali-aggregate reactions, causing expansion and cracking in hardened mortars or concrete [48, 52].\u003c/p\u003e\n\u003cp\u003eThe LOI values of the materials further reflected their composition. SDA exhibited a LOI of 13.23%, within the range typical for biomass ashes, and attributable to unburnt carbon, bound water, and volatile organics [40, 41, 42]. This value is slightly higher than those reported by Udoeyo \u0026amp; Dashibil [20], Elinwa \u0026amp; Ejeh [37], Raheem \u003cem\u003eet al.\u003c/em\u003e [38], and Olaiya \u003cem\u003eet al\u003c/em\u003e. [39]. The LOI for MK was 9.14%, marginally higher than that of highly processed commercial metakaolins (often \u0026lt;5%) [10, 35], though comparable to laboratory-calcined clays where residual hydroxyl groups, unburnt organics, or incomplete dehydroxylation contribute to mass loss [45, 53]. Hydrated lime exhibited a very high LOI of 27.14%, reflecting thermal decomposition of residual carbonates and release of chemically bound water. This high LOI value is consistent with those reported by İsaf\u0026ccedil;a-Kaya \u003cem\u003eet al\u003c/em\u003e. [10] and Shukla \u003cem\u003eet al\u003c/em\u003e. [44]. \u0026nbsp;Elevated LOI levels can adversely affect cement hydration, reduce workability, and ultimately influence concrete performance [54].\u003c/p\u003e\n\u003cp\u003e3.2 \u003cstrong\u003eMineralogical Composition\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eX-ray diffraction was used to determine both the crystalline and non-crystalline phases present in the metakaolin, sawdust ash, and hydrated lime samples. The corresponding diffraction patterns are presented in Figures 1\u0026ndash;3.\u003c/p\u003e\n\u003cp\u003eXRD results indicate that the metakaolin is primarily composed of a disordered aluminosilicate framework, with residual crystalline quartz and small amounts of accessory minerals, a phase assemblage typical of a highly reactive thermally treated clay. The breakdown of the kaolinite structure during dehydroxylation introduces significant structural irregularity, which increases the mobility and accessibility of reactive silicon and aluminum species. As a result, these species readily interact with calcium hydroxide released from hydrated lime, promoting the formation of calcium\u0026ndash;aluminosilicate\u0026ndash;hydrate gels and hydrotalcite-type phases. These reaction products are widely recognized as the main contributors to strength gain and densification of the microstructure in lime\u0026ndash;metakaolin systems [55, 56].\u003c/p\u003e\n\u003cp\u003eThe pronounced diffraction peaks observed at approximately 2\u0026theta; values of 26.6\u0026deg;, 36.5\u0026deg;, 50.1\u0026deg;, 59.9\u0026deg;, and 68\u0026deg; are attributed to the presence of residual \u0026alpha;-quartz (SiO₂), which is known to remain stable and largely unchanged during the calcination of kaolinitic clays. This observation confirms that the original kaolinite structure was effectively converted into a highly disordered and reactive aluminosilicate phase, while the thermodynamically stable quartz component persisted. This phase evolution is consistent with earlier studies on metakaolin used in both lime-based and cementitious binder systems [53]. In addition, the occurrence of weak and broadened features in the low- and mid-angle regions suggests the presence of poorly ordered or zeolitic aluminosilicate domains formed through partial structural rearrangement during thermal treatment. These disordered regions further enhance the release of reactive aluminum and silicon, thereby supporting the development of C\u0026ndash;A\u0026ndash;S\u0026ndash;H phases in calcium-rich systems and contributing to improved mechanical performance [56, 57].\u003c/p\u003e\n\u003cp\u003eIn contrast, the XRD pattern of the SDA sample reveals a predominantly crystalline material, characterized by several well-defined peaks corresponding to a range of mineral phases. The main reflections are associated with calcite (CaCO₃), diaspore (AlO(OH)), rosenhahnite (Ca₃H₂AlO₁₀Si₃), ferrosilite, and tilleyite (C₂Ca₅O₁₃Si₃). The identification of these phases indicates that the ash contains substantial amounts of calcium-, silica-, and alumina-bearing compounds, which are commonly linked to pozzolanic behavior. Comparable findings have been reported by Assiamah \u003cem\u003eet al\u003c/em\u003e. [58] and Raheem \u003cem\u003eet al\u003c/em\u003e. [38], who noted that SDA is largely composed of silica- and alumina-rich phases, together with discernible amorphous fractions. Likewise, Udoeyo and Dashibil [20] documented several crystalline constituents in SDA, with calcite as the dominant phase, followed by quartz. The presence of sharp and distinct diffraction peaks in the current SDA pattern (Fig. 2) indicates a relatively low proportion of amorphous material. Deviations from optimal combustion conditions, whether at insufficient or excessive temperatures, are known to favor the formation of crystalline phases, which can limit the suitability of the ash as a cement replacement material [59]. Nonetheless, the detection of reactive silicate and aluminate phases suggests that SDA may still function as a supplementary cementitious material, although its reduced amorphous silica content is likely to constrain its overall pozzolanic reactivity.\u003c/p\u003e\n\u003cp\u003eThe XRD pattern of the pure hydrated lime sample shown in Figure 3 is dominated by portlandite (Ca(OH)₂), as evidenced by strong, sharp diffraction peaks characteristic of this phase. The most intense peak, located at approximately 21\u0026deg; 2\u0026theta;, is widely recognized as a diagnostic feature of hydrated lime. Additional high-angle reflections further confirm the material\u0026rsquo;s high degree of crystallinity and phase purity. Minor reflections attributable to calcite (CaCO₃) indicate partial carbonation, most likely resulting from exposure to atmospheric carbon dioxide during sample preparation or curing. Similar observations have been commonly reported even for freshly prepared lime-based materials [60], and comparable diffraction patterns have been documented by Abed \u003cem\u003eet al\u003c/em\u003e. [61]. This phase assemblage provides a basis for understanding the typically low early-age strength and the slow long-term hardening behavior reported for pure lime mortars in the literature.\u003c/p\u003e\n\u003cp\u003eThe narrow peak widths and low background intensity point to a limited amorphous fraction and minimal occurrence of secondary reaction products. This interpretation is consistent with the work of Matschei \u003cem\u003eet al\u003c/em\u003e. [62], who noted that hydrated lime generally remains highly crystalline unless modified with pozzolanic additions, which tend to introduce broader diffuse features associated with amorphous C\u0026ndash;S\u0026ndash;H or C\u0026ndash;A\u0026ndash;S\u0026ndash;H phases. In contrast to lime\u0026ndash;pozzolan systems, where the consumption of portlandite leads to a noticeable reduction in Ca(OH)₂ peak intensity, the persistence and prominence of portlandite reflections in this sample indicate negligible secondary reaction and confirm the non-hydraulic character of the binder [62, 63]. The absence of broad amorphous halos or diffraction features linked to hydraulic reaction products further supports the conclusion that the material remains largely unreacted and depends primarily on slow carbonation processes for long-term strength development.\u003c/p\u003e\n\u003cp\u003e3.3 \u003cstrong\u003ePozzolanic Activity Test Results\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003ePozzolanic activity was assessed by tracking the decrease in electrical conductivity of a saturated calcium hydroxide solution following the introduction of MK and SDA that had been calcined under controlled conditions. The variation in conductivity loss with time is illustrated in Figure 4.\u003c/p\u003e\n\u003cp\u003eThe patterns of electrical conductivity (EC) reduction shown in Figure 4 clearly illustrate the comparative pozzolanic behavior of MK and SDA calcined at 600 \u0026deg;C for 2 h. For both materials, conductivity loss increases steadily with time, indicating the ongoing removal of Ca\u0026sup2;⁺ and OH⁻ ions from the saturated calcium hydroxide solution as secondary hydration products, mainly calcium silicate hydrate and calcium aluminosilicate hydrate, are generated [29, 64]. The sharp rise in EC loss during the first 30\u0026ndash;60 minutes for both the samples points to the rapid dissolution of reactive amorphous silica and alumina, a behavior typically associated with the highly disordered aluminosilicate structures formed through controlled thermal activation [28]. Despite this similar early response, MK consistently exhibits a lower overall magnitude of conductivity loss than SDA over the entire test duration, suggesting a comparatively steadier and more controlled reaction rate.\u003c/p\u003e\n\u003cp\u003eThe response of MK corresponds closely with well-documented findings for calcined kaolinitic materials. When kaolinite is dehydroxylated within the temperature range of approximately 500\u0026ndash;700 \u0026deg;C, it transforms into a predominantly amorphous aluminosilicate phase that is highly reactive in the presence of calcium hydroxide [28]. According to Sabir \u003cem\u003eet al\u003c/em\u003e. [16] and Siddique and Klaus [65], MK typically shows a strong early pozzolanic reaction, followed by a slower, diffusion-controlled stage as reaction products accumulate on particle surfaces and restrict further dissolution. The near-linear increase in EC loss observed for MK between 60 and 240 minutes in Figure 4 is consistent with this later stage, where ion transport through the developing C\u0026ndash;A\u0026ndash;S\u0026ndash;H layer governs the overall reaction kinetics rather than the initial availability of reactive phases.\u003c/p\u003e\n\u003cp\u003eBy contrast, SDA displays a consistently higher and more rapid decline in conductivity throughout the testing period, reaching roughly 2.0 mS/cm at 240 minutes, compared with about 1.5 mS/cm for MK. This behavior likely reflects a higher presence of soluble alkalis, such as K₂O and Na₂O, along with readily available reactive silica in the SDA system (Table 2), which can accelerate the uptake of calcium ions from solution [66]. Comparable observations have been reported for biomass-derived ashes rich in amorphous silica, where alkali oxides increase the ionic strength of the pore solution and promote faster dissolution of siliceous components [67]. Nevertheless, previous studies also highlight that such materials often display less consistent long-term behavior due to their heterogeneous composition and the presence of relatively inert crystalline phases, including quartz.\u003c/p\u003e\n\u003cp\u003eThe elevated EC loss measured for SDA may therefore result from a combination of genuine pozzolanic reactions and the release of soluble salts, which can temporarily enhance ionic mobility in solution before becoming incorporated into hydration products (Figueiredo \u0026amp; Pavia, 2020). This interpretation aligns with the work of Chusilp \u003cem\u003eet al\u003c/em\u003e. [54], who noted that agricultural waste ashes can exhibit pronounced early-age changes in conductivity that do not necessarily correspond directly to long-term strength development. In comparison, the more gradual and stable EC reduction observed for MK is commonly associated with the formation of well-structured C\u0026ndash;A\u0026ndash;S\u0026ndash;H gels, which are known to improve microstructural densification and contribute to enhanced mechanical performance over time [16].\u003c/p\u003e\n\u003cp\u003eThe trends presented in Figure 4 demonstrate that both MK and SDA possess measurable pozzolanic activity when calcined at 600 \u0026deg;C, although they differ markedly in their reaction rates and the likely stability of the hydration products formed. MK exhibits a more consistent and literature-supported reactivity profile, reinforcing its suitability as a dependable supplementary cementitious or lime\u0026ndash;pozzolan material. In contrast, the higher apparent early reactivity of SDA underscores the need for careful chemical and mineralogical characterization to separate true pozzolanic contributions from the effects of soluble alkali release, as emphasized in prior research on biomass-based ashes.\u003c/p\u003e\n\u003cp\u003e3.4 Water absorption and Density\u003c/p\u003e\n\u003cp\u003eTable 3 and Figure 5 present the 28-day saturated water-absorption percentages and density for the lime and lime-based composite mortars.\u003c/p\u003e\n\u003cp\u003eTable 3: Physical and mechanical properties of lime and lime composite mortars\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003cimg src=\"https://myfiles.space/user_files/58895_8739fc6c57c1c19a/58895_custom_files/img1772827903.png\" width=\"922\" height=\"414\"\u003e\u003c/p\u003e\n\u003cp\u003eBulk density and water absorption are key indicators of a mortar\u0026rsquo;s durability and resistance to weathering. As shown in Figure 5, the bulk density values follow a clear and consistent downward trend from the control mix (L-C) to the modified mixes (L-S-M-1 through L-S-M-3), decreasing from 1742 kg/m\u0026sup3; for L-C to 1268 kg/m\u0026sup3; for L-S-M-3. This pattern indicates that the progressive incorporation of SDA and MK reduces the overall compactness of the solid framework within the mortar. The relatively high density of the lime-only mix reflects tighter particle packing and fewer internal voids. In contrast, the lower densities observed in the blended systems are associated with the finer particle size, lower specific gravity, and inherently higher porosity of the pozzolanic materials, as well as increased water demand and air entrainment, all of which tend to enlarge interparticle spaces and trapped air volumes [39]. Comparable trends have been reported in other lime\u0026ndash;pozzolan systems, where the introduction of finely divided aluminosilicate materials produces a more open granular structure and reduced bulk density as a result of altered particle packing and surface characteristics [45, 68].\u003c/p\u003e\n\u003cp\u003eAmong the modified mortars, mix L-S-M-1 showed the smallest reduction in density, at 12.58%, while L-S-M-3 exhibited the greatest decrease, at 27.21%. This suggests that L-S-M-1 represents an optimal proportioning of lime and pozzolanic materials, where the pozzolanic reactions contribute to localized densification and reduced porosity. At lower replacement levels (L-S-M-1), fine materials such as metakaolin and SDA can act as micro-fillers, improving particle packing and encouraging the formation of secondary calcium silicate and aluminate hydrates that enhance the internal structure. At higher replacement levels (L-S-M-2 and L-S-M-3), however, the influence of the more porous ash particles and the increase in capillary porosity become dominant, leading to progressively lower hardened densities [39]. Previous research has similarly shown that near-equal lime-to-pozzolan ratios (around 1:1) tend to promote balanced hydrate formation and denser microstructures, whereas pozzolan-rich mixes (2:1 to 3:1) initially refine pores but eventually become limited by available lime, resulting in lower density and higher permeability at elevated replacement levels [45, 69].\u003c/p\u003e\n\u003cp\u003eFrom a practical standpoint, the observed reduction in bulk density has implications for both mechanical performance and durability. Lower density is often associated with higher overall porosity, which can facilitate moisture movement and potentially reduce compressive strength if not offset by sufficient pozzolanic activity and binder development. In more reactive systems, however, the gradual formation of secondary calcium silicate hydrate and calcium aluminosilicate hydrate gels from metakaolin and reactive ash phases can partially refine the pore network over time, moderating some of the negative effects linked to early-stage density loss [70]. At the same time, reduced density may be advantageous in terms of structural loading, as lighter mortars can lower demands on formwork and contribute to reduced dead loads [45].\u003c/p\u003e\n\u003cp\u003eThe data presented in Table 3 and Figure 5 show that water absorption by mass ranges from 4.31% to 7.12%. Overall, water uptake decreases from the control lime mortar (L-C) to the blended lime\u0026ndash;SDA\u0026ndash;metakaolin mixes before gradually increasing again as the proportion of supplementary materials rises. The control mix recorded the highest absorption value of 7.12%, reflecting the relatively open pore structure typical of air-lime mortars with limited hydraulic activity. During air curing and carbonation, fine surface pores form, and when the mortar is immersed, water readily penetrates these interconnected voids [60].\u003c/p\u003e\n\u003cp\u003eThe addition of MK and SDA leads to a slight reduction in water absorption, with the effect most pronounced in mix L-S-M-1, which shows a 39.5% decrease. This behavior is attributed to the substantial formation of hydration products that make the hydrated lime mortar denser and less permeable [71]. The inclusion of pozzolanic materials generally refines the pore structure and limits water ingress. Among the blended mortars, L-S-M-1 exhibits the lowest absorption, consistent with its more balanced lime-to-pozzolan ratio. In this mix, reactive silica is more fully utilized, reducing the volume of permeable voids. The combined presence of SDA and MK promotes pore refinement through reactions with calcium hydroxide, producing secondary C\u0026ndash;S\u0026ndash;H and C\u0026ndash;A\u0026ndash;S\u0026ndash;H gels that partially block capillary pathways and restrict moisture penetration [16, 72].\u003c/p\u003e\n\u003cp\u003eAs the proportions of SDA and MK increase further in mixes L-S-M-2 and L-S-M-3, a modest rise in water absorption is observed. This reflects the interplay between chemical densification and physical packing. Higher contents of fine, irregular pozzolanic particles can increase total porosity or form interconnected pore networks when the available lime becomes insufficient to react with all added aluminosilicates [68]. In such cases, excess ash and less reactive filler phases can contribute to higher permeability and water uptake, which may compromise long-term durability [56]. These findings underscore the importance of optimizing the proportions of SDA and MK to achieve a balance between sustainability objectives and performance requirements in lime-based mortars.\u003c/p\u003e\n\u003cp\u003eAlthough the incorporation of MK and SDA reduces permeability and water absorption, the mortars still retain a relatively high level of breathability. This characteristic is particularly important in the conservation of historic masonry, where maintaining moisture balance is essential. Enhanced vapor permeability allows moisture to move through the material rather than becoming trapped within walls, thereby reducing the risk of condensation, dampness, and associated deterioration. It also limits the buildup of harmful agents such as mold and persistent moisture within the masonry fabric [73]. By contrast, the use of impermeable, non-breathable materials can disrupt this balance and accelerate degradation processes.\u003c/p\u003e\n\u003cp\u003e3.5 Compressive and Flexural strength\u003c/p\u003e\n\u003cp\u003eFigure 6 presents the mean flexural and compressive strength values of the specimens measured after 28 and 90 days of curing.\u003c/p\u003e\n\u003cp\u003eThe compressive and flexural strength data in Figure 6 reveal a clear and consistent effect of both binder composition and curing duration on the mechanical behavior of the lime\u0026ndash;SDA\u0026ndash;MK mortars. The control mixture (L-C), composed entirely of lime, displays very low compressive and flexural strengths at both testing ages when compared with the blended mortars. Its compressive strength increases slightly, from approximately 0.46 MPa at 28 days to about 0.97 MPa at 90 days, while flexural strength rises from 0.27 MPa to 0.52 MPa over the same period. These limited improvements reflect the inherently low strength, brittleness, and weak binding capacity of air-lime systems used without supplementary materials [25]. The gradual nature of strength development in L-C indicates that carbonation, rather than the formation of cementitious phases such as C\u0026ndash;S\u0026ndash;H or C\u0026ndash;A\u0026ndash;S\u0026ndash;H, governs its hardening process. Comparable low strength levels for pure lime mortars at extended curing ages have been documented in experimental investigations of traditional lime-based materials [60, 69].\u003c/p\u003e\n\u003cp\u003eIntroducing SDA and MK leads to a marked enhancement in both compressive and flexural performance, underscoring the role of pozzolanic reactions in improving matrix cohesion and resistance to cracking. Among the blended systems, mix L-S-M-1 (50% lime, 25% SDA, 25% MK) achieves the highest strengths, with compressive strength increasing from 4.96 MPa at 28 days to 7.12 MPa at 90 days, and flexural strength rising from 0.62 MPa to 1.34 MPa over the same period. This strong time-dependent gain is consistent with the progressive consumption of Ca(OH)₂ released by lime and the subsequent formation of secondary cementitious gels from the reactive silica and alumina provided by MK and SDA. These products refine the pore network and strengthen the interfacial transition zones between the binder and sand particles. Lime-based composite mortars develop strength through a combination of pozzolanic reactions, hydration, and carbonation [13, 74].\u003c/p\u003e\n\u003cp\u003eThe superior performance of L-S-M-1 suggests that this formulation offers a favorable balance between available calcium and reactive pozzolanic oxides, promoting a dense microstructure and more efficient stress transfer. Metakaolin supplies a highly reactive source of alumina and silica, while SDA contributes additional silica and alkalis that can accelerate dissolution and support the formation of secondary hydrates. This combined effect results in a more uniform distribution of reaction products and a refined interfacial transition zone within the mortar matrix [16, 66]. Similar improvements in lime mortars containing pozzolans have been reported by Walker and Pav\u0026iacute;a [11], who linked strength enhancement to the high fineness and reactivity of pozzolanic additions that generate a denser and more continuous binding phase.\u003c/p\u003e\n\u003cp\u003eThe L-S-M-2 mixture (40% lime, 30% SDA, 30% MK) also demonstrated a relatively strong performance, with compressive strength increasing from 4.14 MPa at 28 days to 5.97 MPa at 90 days and flexural strength rising from 0.42 MPa to 1.24 MPa. Although these values remain slightly below those of L-S-M-1, the overall trend indicates sustained pozzolanic activity with time. The modest reduction in strength is likely associated with the lower lime content, which may limit the availability of calcium hydroxide needed to fully react with the higher proportion of pozzolanic material. This behavior is consistent with observations by Donatello \u003cem\u003eet al.\u003c/em\u003e [75], who noted that beyond an optimal pozzolan dosage, both compressive and flexural strengths tend to level off or decrease as calcium becomes insufficient for complete hydrate formation.\u003c/p\u003e\n\u003cp\u003eMix L-S-M-3 (30% lime, 35% SDA, 35% MK) records the lowest strength among the blended mortars. Its compressive strength increases from 2.90 MPa at 28 days to 3.74 MPa at 90 days, while flexural strength rises from 0.36 MPa to 0.80 MPa. Although these values remain higher than those of the pure lime control, the decline relative to the other blended mixes indicates that excessive replacement of lime with pozzolanic materials results in a weaker and less cohesive binder network. At such high substitution levels, the system becomes rich in silica and alumina but deficient in calcium, leading to incomplete reactions and a more open, porous microstructure [45, 69, 75]. The resulting increase in porosity and reduced interparticle bonding ultimately diminishes both compressive and flexural performance.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;All mixtures exhibited continued strength development with extended curing. This behavior reflects the slow but ongoing reactions between lime, pozzolans, and water, as well as the gradual progress of carbonation [45]. The higher strength values recorded at 90 days for mixes containing SDA and MK confirm that prolonged curing supports further microstructural refinement and cementitious matrix development, leading to improved mechanical performance over time.\u003c/p\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eIn light of the results and analysis presented above, the following conclusions were made:\u003c/p\u003e\n\u003cp\u003e1. A completely clinker-free pozzolan\u0026ndash;lime binder was successfully produced by combining SDA with MK as complementary supplementary materials.\u003c/p\u003e\n\u003cp\u003e2. Chemical and mineralogical characterization showed that MK supplies a highly reactive amorphous aluminosilicate phase, whereas SDA contributes a silica-rich matrix with demonstrable pozzolanic potential.\u003c/p\u003e\n\u003cp\u003e3. Electrical conductivity measurements confirmed that both SDA and MK are pozzolanic when calcined at 600 \u0026deg;C, with MK exhibiting steady and well-regulated reactivity, while SDA displayed more pronounced early-stage calcium uptake.\u003c/p\u003e\n\u003cp\u003e4. Mortars incorporating the blended binders developed substantially higher compressive and flexural strengths than the pure lime reference mix, underscoring the effectiveness of the lime\u0026ndash;pozzolan interaction.\u003c/p\u003e\n\u003cp\u003e5. The formulation containing 50% lime, 25% SDA, and 25% MK delivered the best mechanical performance, reflecting an optimal balance between available calcium and reactive silica and alumina.\u003c/p\u003e\n\u003cp\u003e6. Controlled additions of SDA and MK lowered water absorption and promoted pore refinement, leading to improvements in durability-related characteristics.\u003c/p\u003e\n\u003cp\u003e7. At higher replacement levels, reductions in density and strength were observed, attributable to calcium limitation and incomplete pozzolanic reactions.\u003c/p\u003e\n\u003cp\u003e8. The SDA\u0026ndash;MK\u0026ndash;lime system shows considerable promise as a low-carbon, resource-efficient binder for both structural and non-structural construction, particularly in settings where material availability and sustainability are critical considerations.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003eData Availability\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe data used to support the findings of this study are available from the corresponding author upon request.\u003c/p\u003e\n\u003cp\u003eConflicts of Interest\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no conflicts of interest.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAcknowledgments\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe authors wish to thank the University of Eldoret, the Ministry of Mining, Environment, and Natural Resources, and the Ministry of Roads for the provision of laboratory facilities used in this work.\u003c/p\u003e\n\u003cp\u003eFunding\u003c/p\u003e\n\u003cp\u003eThis research did not receive any external funding.\u003c/p\u003e\n\u003cp\u003eEthics and declaration\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eNot applicable\u003c/p\u003e\n\u003cp\u003eConsent to participate declaration\u003c/p\u003e\n\u003cp\u003eNot applicable\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eConsent to publish declaration\u003c/p\u003e\n\u003cp\u003eNot applicable\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eK. L. Scrivener, V. M. John, and E. M. Gartner, \u0026ldquo;Eco-efficient cements: Potential economically viable solutions for a low-CO₂ cement-based materials industry,\u0026rdquo; \u003cem\u003eCement and Concrete Research\u003c/em\u003e, vol. 114, pp. 2\u0026ndash;26, Dec. 2018. \u003c/li\u003e\n\u003cli\u003eM. A. Keerio, A. Saand, A. Kumar, N. Bheel, and K. Ali, \u0026ldquo;Effect of local metakaolin developed from natural material soorh and coal bottom ash on fresh, hardened properties and embodied carbon of self-compacting concrete,\u0026rdquo; \u003cem\u003eEnvironmental Science and Pollution Research\u003c/em\u003e, vol. 28, no. 42, pp. 60000\u0026ndash;60018, Jun. 2021.\u003c/li\u003e\n\u003cli\u003eNaraindas Bheel \u003cem\u003eet al.\u003c/em\u003e, \u0026ldquo;Effect of calcined clay and marble dust powder as cementitious material on the mechanical properties and embodied carbon of high strength concrete by using RSM-based modelling,\u0026rdquo; \u003cem\u003eEng. Technol. 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Maitra, \u0026ldquo;Some studies on the reaction between fly ash and lime,\u0026rdquo; \u003cem\u003eBulletin of Materials Science\u003c/em\u003e, vol. 28, no. 2, pp. 131\u0026ndash;136, 2005 \u003c/li\u003e\n\u003cli\u003eS. Donatello, M. Tyrer, and C. R. Cheeseman, \u0026ldquo;Comparison of test methods to assess pozzolanic activity,\u0026rdquo; \u003cem\u003eCement and Concrete Composites\u003c/em\u003e, vol. 32, no. 2, pp. 121\u0026ndash;127, 2010.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"discover-sustainability","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"disu","sideBox":"Learn more about [Discover Sustainability](https://www.springer.com/43621)","snPcode":"","submissionUrl":"","title":"Discover Sustainability","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Discover Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"lime-based mortar, sawdust ash, metakaolin, hydrated lime, flexural strength, compressive strength","lastPublishedDoi":"10.21203/rs.3.rs-8782472/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8782472/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe cement industry is a major contributor to global CO₂ emissions. This has driven the need for low-carbon, locally adaptable binder systems. This study develops and evaluates a fully clinker-free pozzolana\u0026ndash;lime cement formulated using sawdust ash (SDA) and calcined kaolinitic clay (metakaolin, MK) as complementary supplementary materials. SDA and MK were thermally activated at 600\u0026deg;C to enhance pozzolanic reactivity and combined with hydrated lime to form ternary binders. Mortars were prepared at a constant binder-to-sand-to-water ratio of 1:3:0.8 by blending the powdered constituents, followed by controlled mechanical mixing and casting into 40 \u0026times; 40 \u0026times; 160 mm prisms. Specimens were demolded after 48 h and cured initially under high-humidity conditions and subsequently in ambient air to promote both pozzolanic reactions and natural carbonation. Chemical and mineralogical characterization was performed using X-ray fluorescence, X-ray diffraction, and loss on ignition, while pozzolanic activity was assessed through electrical conductivity measurements in saturated calcium hydroxide solutions. Hardened mortars were evaluated in terms of density, water absorption at 28 days, and flexural and compressive strength at 28 and 90 days. Results showed that both SDA and MK exhibit measurable pozzolanic activity, with MK providing stable aluminosilicate reactivity and SDA contributing high silica content and alkali-assisted dissolution. Blended mortars demonstrated significantly improved mechanical performance compared with pure lime, achieving compressive strengths up to 7.1 MPa at 90 days for an optimal 50:25:25 (lime:SDA:MK) formulation. Moderate pozzolan contents refined pore structure and reduced water absorption, while excessive replacement reduced density and strength due to calcium deficiency. The findings demonstrate a synergistic interaction between SDA and MK, enabling the formation of strength-contributing phases and supporting the development of a sustainable, low-emission, lime-based binder suitable for structural and non-structural applications.\u003c/p\u003e","manuscriptTitle":"Development of a Low-Carbon Pozzolana–Lime Cement Using Sawdust Ash and Calcined Clay","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-03-06 20:18:33","doi":"10.21203/rs.3.rs-8782472/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-03-18T11:09:14+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-03-16T07:00:44+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"71927144181924037411945556477468576994","date":"2026-03-15T05:55:39+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-03-12T16:56:13+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"250546917864447618206588434005688590373","date":"2026-03-10T10:08:06+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-03-09T18:26:05+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"36534623365090894431825450684977676729","date":"2026-03-09T15:38:47+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"295430059737113857854024519656732304593","date":"2026-03-09T10:19:59+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-03-06T16:48:04+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"26562877550163950941924163654319837753","date":"2026-03-05T19:58:38+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-03-03T09:54:34+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-02-12T08:23:02+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-02-12T08:22:43+00:00","index":"","fulltext":""},{"type":"submitted","content":"Discover Sustainability","date":"2026-02-04T05:56:16+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"discover-sustainability","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"disu","sideBox":"Learn more about [Discover Sustainability](https://www.springer.com/43621)","snPcode":"","submissionUrl":"","title":"Discover Sustainability","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Discover Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"83fca48a-ce26-41fb-948c-0fdda1635516","owner":[],"postedDate":"March 6th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-03-24T15:09:11+00:00","versionOfRecord":[],"versionCreatedAt":"2026-03-06 20:18:33","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8782472","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8782472","identity":"rs-8782472","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}