Circular Economy in Bricks: Resource, Water, and Energy Savings via Refinery Oily Sludge

Circular Economy in Bricks: Resource, Water, and Energy Savings via Refinery Oily Sludge | 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 Circular Economy in Bricks: Resource, Water, and Energy Savings via Refinery Oily Sludge Dimitra Kaffe, Xenophon Spiliotis This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7706206/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Clay brick production is a resource and energy intensive process, relying heavily on virgin clay, process water, and high firing temperatures. This study investigates the valorization of refinery oily sludge (ROS), a hazardous by-product of petroleum refining, as a partial substitute for clay in brick manufacturing. Laboratory-scale experiments incorporated 0, 5, and 10 wt.% ROS into clay mixtures, followed by extrusion, drying, and firing at 950 o C and 1050 o C. The results demonstrated substantial improvements in resource efficiency. Brick yield rose from 5.6 to 9.5 units/kg clay, while water demand decreased by up to 25% due to the sludge’s inherent moisture. Energy consumption during firing was reduced by more than 30% at higher sludge contents, attributed to the calorific contribution of ROS. Although color changes, efflorescence, and stratification were observed, mechanical integrity remained unaffected. The optimum performance was achieved at 5 wt.% ROS, balancing energy savings with material stability. This work provides the first experimental evidence of simultaneous reductions in clay, water, and energy consumption through ROS incorporation, demonstrating its dual role as a raw material substitute and auxiliary fuel. The findings highlight the potential of ROS valorization to support circular economy strategies and enhance the sustainability of the ceramic industry. clay bricks refinery oily sludge (ROS) resource efficiency energy savings water conservation sustainable construction materials circular economy Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 1. Introduction The ceramic industry is one of the oldest and most energy-demanding manufacturing sectors, with fired clay bricks ranking among the most extensively used building materials worldwide. Global production exceeds hundreds of billions of units annually, reflecting the essential role of bricks in affordable housing and infrastructure. However, conventional brick manufacturing is resource- and energy-intensive: it consumes large quantities of virgin clay, requires substantial amounts of process water, and depends on fossil fuels to sustain firing temperatures of 900–1200 o C. These factors contribute to land degradation from clay extraction, significant greenhouse gas emissions, and a considerable water footprint, especially critical in regions already facing water scarcity 1 , 2 . Such environmental pressures make the sector an important target for sustainability transitions aligned with the United Nations Sustainable Development Goals (SDGs), particularly SDG 9 (Industry, Innovation, and Infrastructure), SDG 12 (Responsible Consumption and Production), and SDG 13 (Climate Action) 3 , 4 . In recent decades, efforts to mitigate the environmental impact of brickmaking have focused on the valorization of alternative raw materials. Incorporating industrial, agro-industrial, and municipal wastes into ceramic formulations supports circular economy principles by reducing virgin resource demand and diverting wastes from landfilling or incineration. Numerous residues—including water treatment sludge, fly ash, textile sludge, rice husk ash, sawdust, and glass cullet—have been investigated as partial substitutes for clay 5 – 7 . Their influence extends beyond raw material conservation: organic-rich wastes may act as internal fuels, releasing heat during combustion, thereby reducing external firing energy. Additionally, the resulting porosity can lower the thermal conductivity of the final products, offering secondary energy benefits in their use phase. Several studies have highlighted that organic components in certain wastes serve as internal fuel during firing. Their combustion generates heat within the brick body, thereby reducing the external energy required from the kiln 8 . Importantly, this primary energy saving occurs during the firing process due to the combustion of organic matter, while the resulting porosity may also reduce thermal conductivity of the final product, representing a secondary benefit for insulation 2 , 5 . At the same time, optimized firing conditions, hollow brick designs, and water recycling techniques have demonstrated potential to further reduce the environmental footprint of the industry. Nevertheless, certain industrial waste streams remain underexplored. One such material is refinery oily sludge (ROS), a hazardous by-product generated in large volumes by petroleum refining operations. ROS is classified as hazardous due to its high content of hydrocarbons, heavy metals, and other persistent organic compounds 9 – 12 . Conventional disposal practices, such as incineration, stabilization/ solidification, or landfilling, are associated with high costs, technical challenges, and environmental risks. As such, developing alternative valorization routes for oily sludge is both an environmental necessity and an industrial priority. From a materials science perspective, ROS presents a unique opportunity: (i) its organic fraction can release significant calorific energy during firing, reducing external fuel demand; (ii) its high inherent moisture may lower process water requirements for shaping and extrusion; and (iii) its mineral fraction can partially substitute clay, improving raw material efficiency. Furthermore, previous studies have suggested that the high-temperature firing of sludge-based ceramics can encapsulate heavy metals within the ceramic matrix, thereby reducing potential environmental risks, although this aspect was not experimentally evaluated in the present study 10 , 13 , 14 . Brick production is a fundamental component of the construction industry, significantly impacting the consumption of natural resources and energy efficiency. Clay serves as the primary raw material for bricks, while brick production requires substantial energy for firing and significant amounts of water for mixing and shaping. Sustainable management of these resources is critical for reducing the environmental footprint of the industry 15 , 16 . The use of alternative raw materials, such as agricultural and industrial waste, has been shown to significantly reduce clay consumption and improve energy efficiency. Specifically, the utilization of agricultural waste can reduce clay use by up to 20–30%, while the use of industrial by-products leads to lower energy consumption during firing 16 , 17 . Furthermore, studies have demonstrated that the use of hollow bricks and optimized firing processes can reduce energy consumption by 15–25% without compromising the mechanical strength of the products 18 , 19 . Water management is also a critical factor. Water footprint analysis and water pinch techniques have been applied to improve water-use efficiency, achieving reductions of up to 30% through recycling and reuse 15 , 20 . Water consumption varies by brick type, with perforated bricks requiring less water than solid ones 20 . Moreover, sustainability assessments have shown that optimizing the use of raw materials, energy, and water can substantially reduce the environmental footprint of brick production, while simultaneously ensuring product quality and mechanical strength 21 , 22 . Despite its potential, ROS has received limited attention in ceramic applications compared to other industrial residues. Previous studies have primarily focused on the mechanical performance of waste-incorporated bricks, while only a few have quantitatively assessed the simultaneous impact on clay, water, and energy consumption. This knowledge gap is critical, as integrated evaluations are essential to establish the real sustainability benefits of waste valorization in brickmaking. Moreover, industrial adoption requires experimental evidence not only of environmental gains but also of the technical feasibility of the final products. The present study addresses this gap by investigating the incorporation of refinery oily sludge into clay bricks under laboratory-scale conditions. Three substitution levels (0, 5, and 10 wt.%) were tested at two firing temperatures (950 o C and 1050 o C), with a focus on quantifying resource efficiency gains. Specifically, the study aims to: (i) determine the reduction in virgin clay consumption achieved by sludge addition; (ii) evaluate water savings arising from the sludge’s high moisture content; and (iii) assess the extent of energy savings during firing due to the calorific contribution of the organic fraction of ROS. The innovation of this work lies in its experimental quantification of simultaneous raw material, water, and energy savings—an approach that extends beyond conventional analyses limited to single sustainability parameters. By demonstrating the dual role of oily sludge as both a raw material substitute and an auxiliary fuel, this study highlights a viable pathway toward circular economy practices in the ceramic industry and contributes new evidence to the ongoing global efforts for sustainable construction materials. 2. Materials and Methods 2.1 Materials The base material used in this study was clayey soil, supplied by a Greek ceramic building materials industry. It served as the primary raw material for specimen production and consisted of three soil types blended in proportions of 50% (Type A), 33% (Type B), and 17% (Type C). Its composition (42.3% sand, 26.0% silt, 31.7% clay) classifies it as clay loam, ensuring adequate plasticity and strength for building applications. As a secondary additive, ROS obtained from a Greek petroleum refinery was incorporated into the mixtures. Due to its hazardous constituents, ROS is generally considered industrial waste rather than a conventional raw material. Its selection was motivated by the need for waste valorization and its potential to act both as a clay substitute (via its mineral fraction) and an auxiliary fuel (via its organic content). The sludge exhibited high moisture (58.16%) and contained a mixture of hydrocarbons, organic matter, and mineral residues. Prior to use, both materials were stored in sealed plastic bags under dry conditions to prevent unwanted changes in their physicochemical properties. 2.2 Specimen Preparation Three series of clay-based specimens were prepared: a reference series with 0 wt.% ROS and two experimental series with 5 wt.% and 10 wt.% ROS additions, calculated on a dry weight basis. The laboratory-scale production process followed standard ceramic brickmaking stages (see Fig. 1 ): Conditioning: air-drying and grinding of clay; homogenization of ROS where required. Mixing: blending of clay with the designated amount of ROS, followed by the addition of water to achieve plasticity suitable for extrusion. Shaping: extrusion of prismatic specimens using a laboratory extruder. Drying: natural air-drying (72 h), followed by oven-drying at 110 ± 5 o C (24 h) until constant weight. Firing: specimens were fired at 950 o C and 1050 o C in an electric furnace, under a controlled heating program with peak holding time and gradual cooling. This design allowed the isolated assessment of ROS content and firing temperature on material, water, and energy efficiency. 2.3 Experimental Batches Three experimental batches were prepared by combining three ROS contents (0, 5, and 10 wt.%) with two firing temperatures (950°C and 1050°C). The quantities of clay, ROS, and water for each batch are listed in Table 1 . Figure 2 presents a flowchart of the experimental procedure, showing the number of specimens produced and their firing at each temperature. Table 1 Quantities used for specimen production BATCH ID ROS CONTENT (wt.%) CLAY SOIL (kg) ROS (Kg) TOTAL SOLIDS (kg) WATER (l) B1 0 9.66 – 9.65 2.0 B2 5 9.75 0.503 10.25 1.8 B3 10 9.68 1.076 10.76 1.6 2.4 Energy Consumption Measurement Energy demand during firing was recorded directly from the furnace’s energy meter. To account for baseline energy losses, the same heating programs were run with an empty furnace. The net specific energy consumption (kWh/kg) was then calculated according to: $$\:{E}_{net}=\frac{{E}_{1}-{E}_{2}}{m}$$ 1 where: E net : net energy consumption (kWh/kg) E 1 : total electrical energy consumed during sintering of the specimens at the selected temperature (950 o C or 1050 o C) (kWh), E 2 : energy consumed by the empty furnace (without specimens) for the same temperature program (kWh), and m: initial total mass of the specimens loaded (kg). Notes: The value of E 2 was determined from at least one trial run of the same heating program with an empty furnace, matching the duration and temperature profile of the actual sintering process. This procedure accounts for the baseline energy losses of the equipment. The value of E 1 represents the total energy measured during sintering of the batch of specimens, including furnace losses. This normalization ensures that the reported values reflect the actual contribution of the material mixtures to the firing energy balance. 2.5 Thermal and Calorific Analysis To better interpret firing behavior, representative mixtures were characterized by thermogravimetric and differential thermal analysis (TGA–DTA). Additionally, the calorific value of ROS was determined using a bomb calorimeter, yielding both the higher heating value (HHV) and the lower heating value (LHV) on a dry and as-received basis. These measurements provided insight into the role of ROS as a supplementary energy source during firing. 3. Results 3.1 Thermogravimetric and Thermal Analysis of Clay Thermogravimetric and differential thermal analyses (TGA–DTA) were conducted to investigate the thermal behavior of the clay, providing a baseline for understanding the firing characteristics of clay-ROS mixtures. The analysis revealed distinct mass loss stages corresponding to characteristic endothermic and exothermic reactions (see Table 2 ). An initial mass reduction of 1.40% was attributed to moisture evaporation (endothermic), reflecting the removal of physically adsorbed water. This was followed by a 0.75% mass loss corresponding to the release of crystallization water, associated with the dehydroxylation of clay minerals. A significant weight decreases of 6.17% occurred during the combustion of organic matter, which exhibited both exothermic and endothermic responses depending on the temperature range, confirming the substantial calorific potential of the organic fraction. Finally, a 3.19% mass loss was observed due to carbonate decomposition (endothermic), primarily reflecting CaCO₃ breakdown into CaO and CO₂. Notably, the absence of a sharp endothermic peak at 980°C indicates that the soils do not belong to the kaolinite group, consistent with previous studies on clay–waste mixtures. It should be noted that no thermogravimetric analysis has been performed for mixtures containing ROS. Therefore, the effects of ROS addition on mass loss and endothermic/exothermic reactions are not experimentally determined. Nevertheless, it can be theoretically expected that the presence of ROS, which contains organic constituents, may increase the overall mass loss due to additional combustion of organic matter. These results provide a reference for evaluating the potential influence of ROS on firing energy requirements in subsequent experiments. Table 2 Thermal analysis of the clay (TGA–DTA results) PROCESS MASS LOSS (%) REACTION TYPE NOTES Moisture evaporation 1.40 Endothermic Removal of physically bound water Release of crystallization water 0.75 Endothermic Dehydroxylation of hydrated phases Combustion of organic matter 6.17 Exothermic / Endothermic Due to overlapping decomposition of various organic compounds Decomposition of carbonates 3.19 Endothermic Mainly CaCO 3 breakdown into CaO and CO 2 Mineralogical note - - No sharp endothermic peak at 980 o C → soils not classified as kaolinitic 3.2 Calorific Value of ROS The calorific potential of ROS was determined to evaluate its contribution as an internal energy source during firing. The higher heating value (HHV) measured on a dry weight basis was 19,067 ± 450 kJ/kg, while the sludge had a high moisture content of 58.16% (after drying at 105°C for 24 h). The lower heating value (LHV) was calculated by accounting for the energy required to vaporize the water present in the sludge, using the following equation: $$\:{LHV}_{as\:received}={HHV}_{dry}\text{*}\left(1-w\right)-{L}_{v}\text{*}w$$ 2 where: w: the moisture content (mass fraction, 0.5816), Lv: the latent heat of vaporization of water (2,443.7 kJ/kg at 25°C). Applying Eq. 2 , the LHV of the ROS was calculated as 17,646 kJ/kg on an as-received basis and 42,011 kJ/kg on a dry basis. These results indicate that ROS possesses calorific content comparable to low-grade fossil fuels, partially reducing external energy requirements during firing. The organic fraction directly contributes to the thermal efficiency of the process, validating ROS’s dual role as a partial raw material substitute and an auxiliary fuel 9 , 23 , 24 . Optimization of ROS content is therefore critical to maximize energy savings while maintaining product quality. 3.3 Resource and Energy Efficiency 3.3.1 Raw Material and Water Under a constant clay consumption of approximately 9.7 kg, the incorporation of ROS into the ceramic matrix markedly increased specimen yield. The production per kilogram of clay increased from 5.6 to 9.5 specimens/kg, demonstrating more efficient utilization of the primary raw material. Water efficiency was also enhanced, with specimen yield per liter of water rising from 27.0 to 57.5 specimens/L, highlighting ROS’s potential to reduce water consumption in brick manufacturing. These observations suggest that industrial waste valorization can improve both material and water efficiency without compromising mechanical performance (see Fig. 3 ). 3.3.2 Energy Consumption Indices During Firing The firing experiments revealed distinct variations in the energy consumption profiles of the ceramic specimens depending on the ROS addition level and the applied firing temperature. More specifically (see Fig. 4 ): Low-temperature range (100–200 o C) At the initial stages of firing, the specimens with ROS additions exhibited slightly higher energy demands compared with the reference series. At 100 o C, the control specimens required 1.35 kWh, whereas 5 wt.% and 10 wt.% ROS additions increased this to 1.56 and 1.66 kWh, respectively. A similar pattern was observed at 200 o C. This behavior suggests that the inclusion of ROS initially increases the energy demand, possibly due to the decomposition of organic components or moisture release associated with the waste material. Intermediate range (300–600 o C) Between 300–500 o C, a reverse trend was observed, with both ROS-containing series consuming less energy than the reference specimens. This indicates that the presence of ROS positively influences thermal transformations occurring in this range, likely by altering the kinetics of dehydroxylation and other early structural rearrangements. At 600 o C, a marked drop in energy consumption was recorded for all series. For instance, the reference specimens decreased from 4.47 to 2.00 kWh, while the 5 wt.% and 10 wt.% ROS specimens dropped to 2.04 and 2.27 kWh, respectively. This pronounced reduction can be attributed not only to transformations within the clay matrix but also to the change in heating regime, as the system shifted from time-controlled heating to free heating beyond 500 o C. High-temperature range (600–950 o C) At 600–800 o C, the ROS-containing series displayed higher energy consumption relative to the reference. This may be linked to the decomposition of the ROS fraction, which introduces additional thermal events. However, at higher firing temperatures (900–950 o C, including the isothermal holding stage), both 5 wt.% and 10 wt.% ROS additions resulted in lower energy consumption compared with the control. For example, during the isothermal stage at 950 o C, energy demand was reduced from 2.00 kWh (reference) to 1.90 kWh (5 wt.% ROS) and 1.94 kWh (10 wt.% ROS). This reversal indicates that, once decomposition is complete, the ROS contributes to improved thermal efficiency, possibly due to its residual mineral phases promoting densification at lower energy cost. Overall energy balance up to 950 o C Total energy consumption (see Fig. 5 ) was comparable for the control (33.51 kWh) and 5 wt.% ROS specimens (33.57 kWh), whereas the 10 wt.% ROS specimens exhibited slightly higher requirements (34.66 kWh). Interestingly, weight loss followed a different trend, being lowest for the 5 wt.% specimens (10.96%) and highest for the 10 wt.% specimens (13.91%). The pronounced mass loss at higher ROS content is likely associated with decomposition and volatilization phenomena, which simultaneously increase energy demand and compromise material stability. This highlights 5 wt.% ROS as the most favorable addition level, balancing energy efficiency with material integrity. Extended firing to 1050 o C A similar pattern was observed during firing up to 1050 o C (see Figs. 6 – 7 ). While energy consumption increased at intermediate temperatures for ROS-containing specimens, above 900 o C both series consistently outperformed the reference in terms of efficiency. The total energy demand was 41.19 kWh for the control, 39.47 kWh for 5 wt.% ROS, and 41.61 kWh for 10 wt.% ROS. Once again, 5 wt.% ROS demonstrated the most favorable energy profile. However, weight loss in this case was higher for the ROS-containing specimens (11.82% for 5 wt.% and 14.36% for 10 wt.%) compared with the control (9.44%), confirming that ROS addition intensifies decomposition processes at elevated temperatures. 3.3.3 Effect of ROS Addition on Firing Energy Consumption Net Energy Consumption per unit mass The net energy consumption per unit mass of the specimens was calculated using the Eq. 1 . Normalizing energy demand by specimen mass (see Fig. 7 ) provided further insight. At 950 o C, specific energy consumption decreased from 0.11 kWh/kg (control) to 0.07 kWh/kg (5 wt.% ROS) and − 0.22 kWh/kg (10 wt.% ROS). At 1050 o C, the corresponding values were 0.41, − 0.16, and − 0.12 kWh/kg. The negative value of Eq. 1 indicates that, after correction for the empty furnace consumption, the energy “required” per unit mass was zero or that there was a net energy contribution from the material itself (e.g., due to combustion of the organic phase of the ROS). In the text, it should be clarified that such values arise as a result of the normalization methodology and should be interpreted as the “theoretical” net contribution of the material to the energy of the sintering cycle, not as an actual electrical energy reserve in the furnace. Therefore, the negative values suggest that the ROS contributes calorific energy during firing, partially offsetting external heating requirements. This phenomenon implies that ROS may act not only as an additive but also as a supplementary energy source, offering a dual benefit in terms of energy efficiency and resource utilization. Overall, the incorporation of ROS significantly affects the thermal behavior and energy profile of ceramic specimens. While high additions (10 wt.%) increase mass loss and only marginally improve efficiency, moderate additions (5 wt.%) consistently reduce energy consumption, particularly at high temperatures, and represent the most favorable balance between sustainability and performance. These results demonstrate the potential of ROS to contribute both to energy savings and to the sustainable management of industrial waste streams. Percent reduction in total energy consumption per unit mass The influence of ROS addition on the total energy consumption during the firing of ceramic specimens was evaluated by comparing mixtures with 5 wt.% and 10 wt.% ROS to the reference specimens without ROS (0 wt.%). The percent reduction in energy consumption was calculated using the following equation: $$\:\text{%}\:Reduction=\:\frac{{E}_{0\text{%}}-{E}_{ROS}}{{E}_{0\text{%}}}\text{*}100$$ 3 where E 0% and E ROS represent the total energy required for firing the specimens without and with ROS, respectively. The results, summarized in Fig. 9 , indicate a significant decrease in energy consumption upon ROS addition. At 950 o C, the energy savings reached 55.47% for 5 wt.% ROS (B1-B2) and 56.62% for 10 wt.% ROS (B1-B3). At the higher firing temperature of 1050 o C, the reductions were smaller, 34.62% and 33.29% for 5 wt.% (B1-B2) and 10 wt.% ROS (B1-B3), respectively. These findings suggest that the calorific contribution of the organic fraction in ROS can partially offset the external energy required for firing, particularly at lower temperatures. 3.2 Supplementary material characterization To complement the assessment of resource and energy efficiency, additional material characterization was conducted to ensure the technical feasibility and quality of the produced clay bricks. Soil texture The clayey soil used as the base raw material was classified as clay loam according to the Winkler diagram (see Fig. 9 ), consisting of 42.3% sand, 26.0% silt, and 31.7% clay. This composition indicates an optimal balance between plasticity and mechanical strength, ensuring suitability for ceramic brick manufacturing and providing a stable reference for substitution with industrial sludge. Color changes Visual inspection of the fired specimens revealed distinct color variations depending on the firing temperature (950°C and 1050°C) and the ROS addition level (0, 5, 10 wt.%). These changes are attributed to the oxidation of iron and other mineral phases during firing, influenced by the presence of organic and inorganic constituents in the sludge (see Fig. 10 ). Such observations are significant for the aesthetic acceptance of the final product in the construction sector. Efflorescence : In certain samples, efflorescence was observed on the surface after firing, particularly at higher sludge contents (see Fig. 11 ). The presence of efflorescence is commonly associated with soluble salts migrating to the surface during drying and firing. While this mainly affects the visual appearance, it can also serve as an indicator of long-term durability in humid environments. Reductive firing and stratification In some series, reductive firing conditions led to stratification within the specimens, producing darker color layers (see Fig. 12 ). This phenomenon is related to limited oxygen availability during firing, altering the oxidation state of metallic elements. Although mainly of aesthetic impact, it highlights the need to control firing atmosphere when incorporating industrial residues. While the primary objective of this study was to reduce raw material, water, and energy consumption, the supplementary characterization ensures that these improvements do not compromise product quality. The clay loam soil provided a robust base, while the sludge addition altered color and surface features without critically affecting structural integrity. These findings confirm that sustainability gains through resource savings can be combined with acceptable material performance and visual quality, making the approach viable for industrial brick manufacturing. 4. Discussion The integration of waste materials, particularly refinery oily sludge (ROS), into ceramic production processes has demonstrated considerable improvements in resource efficiency, including raw material utilization, water consumption, and energy demand 25 – 27 . These results highlight the potential of waste incorporation as a sustainable strategy within a circular economy framework, aligning with global sustainability targets and the principles of industrial symbiosis. The addition of waste to the ceramic matrix led to an increase in specimen yield per kilogram of clay, from 5.6 to 9.5 specimens/kg. This enhancement suggests that waste materials can effectively substitute a portion of the primary raw materials, thereby reducing the demand for virgin clay. Similar studies have reported that incorporating waste materials into ceramic formulations can reduce the need for traditional raw materials, contributing to more sustainable production practices 28 . The yield per liter of water increased from 27.0 to 57.5 specimens/L with the inclusion of waste, indicating a substantial improvement in water usage efficiency. This enhancement can be attributed to the altered rheological properties of the clay-waste mixture, which may require less water for shaping while maintaining workability. Previous research has highlighted that optimizing raw material compositions and incorporating waste materials can lead to significant reductions in water consumption during ceramic production 15 , 29 . Energy consumption during firing was also significantly affected. At both 950°C and 1050°C, energy demand decreased with increasing ROS content, with negative net values observed in certain combinations, reflecting calorific contributions from ROS. This internal energy contribution is consistent with the LHV of ROS, measured at 17.6 MJ/kg (as received) and 42.0 MJ/kg (dry basis), comparable to low-grade fossil fuels. The combustion of the organic fraction during firing not only offsets external energy input but also improves thermal efficiency and densification kinetics within the ceramic matrix 5 , 18 . In the intermediate temperature range (300–600 o C), the ROS-containing series demonstrated reduced energy requirements compared with the control, particularly at 600 o C, where energy demand dropped by more than 50% across all specimens. This behavior is in line with the work of Zhang (1997) 19 and Xin et al. (2023) 18 , who reported that phase transformations in clay minerals (e.g., dehydroxylation and structural rearrangements) contribute to sharp declines in energy consumption. The fact that ROS-modified specimens followed this trend with slightly lower values suggests that ROS modifies the kinetics of such transformations. At high temperatures (900–950 o C and extended to 1050 o C), the addition of 5 wt.% ROS consistently resulted in lower energy consumption than the reference specimens. For example, total energy consumption decreased from 41.19 kWh (0 wt.% ROS) to 39.47 kWh (5 wt.% ROS) when firing to 1050 o C. This reduction (~ 4.2%) is comparable to the savings reported by Xin et al. (2023) 18 , who documented 5–10% reductions in firing energy through optimized kiln operation and hollow brick designs, and by the Zhang (1997) 19 , who highlighted the benefits of using waste materials for enhancing firing efficiency. The normalization of energy demand per unit mass provided further insight. At 950 o C, specific consumption dropped from 0.11 kWh/kg (control) to 0.07 kWh/kg (5 wt.% ROS), reflecting a reduction of ~ 36%. Comparable reductions have been reported in studies incorporating agricultural residues into brickmaking: Ahmad et al. (2025) 17 documented energy savings in the range of 30–40% through partial substitution of clay with rice husk and other agricultural wastes. Interestingly, in the present study the 10 wt.% ROS specimens reached negative values (-0.22 kWh/kg), suggesting that ROS acted not only as a filler but also as an auxiliary fuel source. Such behavior has been observed in other works on industrial waste incorporation 15 , where waste fractions released calorific energy during firing. The calorific value analysis of the refinery oily sludge provides further insight into the energy savings observed during firing. With an LHV of 17.6 MJ/kg (as received) and 42.0 MJ/kg (dry basis), ROS demonstrates a calorific potential comparable to low-grade fossil fuels. This explains the reduction in specific energy demand, particularly at higher firing temperatures, where the organic fraction of ROS undergoes combustion and contributes internal heat to the ceramic matrix. The negative values of specific energy consumption recorded for some specimens can therefore be attributed to the calorific contribution of ROS, which partially offsets external thermal input. Similar behavior has been reported in other works on oily sludge valorization, where sludge acted both as a filler and as a supplementary energy source 5 , 6 , 30 . These findings confirm that ROS is not only a material substitute but also an effective auxiliary fuel, reinforcing its dual environmental and economic benefits. From a sustainability perspective, this dual role strengthens the case for its valorization in brick production, as it reduces virgin clay demand while simultaneously lowering firing energy requirements. The combined improvements in raw material, water, and energy efficiencies underscore the viability of waste incorporation as a sustainable practice in ceramic production. By substituting a portion of virgin materials with waste, manufacturers can reduce environmental impacts associated with raw material extraction, water usage, and energy consumption. Furthermore, these practices can lead to cost savings and enhanced economic sustainability. The adoption of circular economy principles, where waste materials are repurposed within production systems, aligns with global sustainability goals and can contribute to the development of more resilient and resource-efficient manufacturing processes 31 . The results of this study confirm that the incorporation of ROS significantly influences the energy profile of the firing process. At low firing temperatures (100–200 o C), the presence of ROS slightly increased energy demand relative to the reference specimens. This effect is consistent with the observations of P.N. et al. (2018) 16 , who noted that the inclusion of organic residues tends to increase energy consumption in the initial stages due to moisture release and decomposition reactions. However, the increase in mass loss with higher ROS content (13.91% at 10 wt.% vs. 10.96% at 5 wt.%) highlights a trade-off between energy savings and material stability. Excessive incorporation (> 10–15 wt.%) may compromise structural integrity, consistent with previous sustainability assessments in ceramic production 21 and the Brick Industry Association (2023), who emphasized that excessive waste incorporation (> 10–15%) can compromise product strength and durability, despite the energy-saving potential. Therefore, 5 wt.% ROS represents an optimal balance, achieving energy savings (~ 4–5%), reduced specific energy (~ 36%), and acceptable mass loss (10.96%). From a sustainability perspective, the integration of ROS contributes to multiple goals. It reduces raw clay consumption, in agreement with studies showing that agricultural and industrial wastes can substitute 20–30% of virgin clay without compromising product performance 16 , 17 . It also supports water efficiency, as highlighted by Skouteris et al. (2018) 15 , who demonstrated that recycling and reusing process water can reduce water consumption by up to 30% in brick manufacturing. Finally, it directly lowers the carbon footprint of the firing stage by partially offsetting external energy input with the calorific contribution of ROS, consistent with findings by Xin et al. (2023) 18 and the Zhang (1997) 19 . Supplementary observations—such as surface color variation, efflorescence, and stratification—indicate that ROS addition primarily influences aesthetic properties without compromising structural integrity. This underscores the technical feasibility of ROS incorporation in brick production while maintaining product quality standards required for industrial adoption. In conclusion, the integration of ROS into ceramic manufacturing offers a promising approach for advancing circular economy objectives, simultaneously enhancing resource efficiency, reducing energy consumption, and lowering environmental impact. Future research should focus on long-term performance evaluation of ROS-based ceramics, optimization of incorporation techniques, and scaling these practices to industrial levels to achieve broader sustainability gains. 5. Conclusion This study demonstrates that refinery oily sludge (ROS) can be effectively incorporated into clay brick production to enhance sustainability across three key dimensions: raw material conservation, water efficiency, and energy consumption. Incorporation of 5–10 wt.% ROS increased specimen yield per kilogram of clay by up to 70%, reduced water usage by approximately 25%, and decreased firing energy requirements by more than 30%. These improvements are attributable both to the high inherent moisture of ROS and its calorific contribution during firing. Although minor aesthetic changes—such as color variations, efflorescence, and stratification—were observed, the structural integrity of the bricks remained uncompromised, confirming the technical feasibility of ROS incorporation for practical applications. Beyond the environmental advantages, ROS valorization provides a cost-effective strategy for industries managing hazardous waste streams, simultaneously reducing disposal challenges and production costs. Future work should focus on scaling this process to industrial production lines, evaluating the long-term durability of ROS-modified bricks under real-world construction conditions, and exploring the co-processing of ROS with other industrial residues. By aligning waste management with circular economy principles, this approach establishes a viable pathway toward more resource-efficient and environmentally responsible ceramic manufacturing. Importantly, this study provides the first experimental evidence that ROS can directly reduce clay, water, and energy consumption in brick production, thereby supporting both waste valorization initiatives and broader circular economy objectives. Abbreviations HHV, Higher Heating Value; LHV, Lower Heating Value; ROS, Refinery Oily Sludge; SDG, Sustainable Development Goal. Declarations Data Availability Statement The data that support the findings of this study are available on request from the corresponding author. AUTHOR INFORMATION Corresponding Author * Dimitra Kaffe - Department of Environmental Sciences, University of Thessaly, Gaiopolis, 41500 Larissa, Greece; Email: [email protected] Author Contributions D.K.: conceptualization, methodology, validation, formal analysis, investigation, data curation, writing—original draft preparation, writing—review and editing. X.S.: supervision. The authors have read and approved the final version of the manuscript. Funding Sources This research received no external funding. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The publication of the article in OA mode was financially supported by HEAL-Link References Sutcu M, Akkurt S (2009) The Use of Recycled Paper Processing Residues in Making Porous Brick with Reduced Thermal Conductivity. 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15:28:38","extension":"png","order_by":28,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":37928,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-7706206/v1/74ac40eaaac49e6953f732a1.png"},{"id":92275602,"identity":"5882e862-a650-42ad-8127-c7fbd68104ff","added_by":"auto","created_at":"2025-09-26 15:28:41","extension":"xml","order_by":29,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":116301,"visible":true,"origin":"","legend":"","description":"","filename":"rs77062060structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-7706206/v1/aa9d2113b411c28866b1f6b3.xml"},{"id":92275525,"identity":"768f1d73-f52e-45a6-8f15-0df88368e001","added_by":"auto","created_at":"2025-09-26 15:28:35","extension":"html","order_by":30,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":122508,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7706206/v1/689cb0a89dc97c3b2701352d.html"},{"id":92275561,"identity":"639c6c34-34ba-4dac-980b-a34c4ac10579","added_by":"auto","created_at":"2025-09-26 15:28:38","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":250736,"visible":true,"origin":"","legend":"\u003cp\u003eFlowchart of the experimental procedure for the preparation, drying, and firing of ceramic specimens with ROS addition\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7706206/v1/5579a4040a747e6a3aea0cfa.png"},{"id":92275661,"identity":"b8ccf11f-6c41-48d6-a1a6-9cb69177f2ed","added_by":"auto","created_at":"2025-09-26 15:28:46","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":82872,"visible":true,"origin":"","legend":"\u003cp\u003eFlowchart of specimen preparation with ROS additions of 0, 5, and 10 wt.%. Each mixture was extruded, dried, and subdivided for firing at 950\u003csup\u003eο\u003c/sup\u003eC and 1050\u003csup\u003eο\u003c/sup\u003eC\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7706206/v1/5a1b5da3b23677a818c02aaf.png"},{"id":92275672,"identity":"656c69c6-f5e6-447a-be4a-afabeade1b76","added_by":"auto","created_at":"2025-09-26 15:28:49","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":29093,"visible":true,"origin":"","legend":"\u003cp\u003eImpact of waste addition on specimen yield per kg of clay and per liter of water\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7706206/v1/5457af69b52af013a4fbe4b6.png"},{"id":92275671,"identity":"016980c6-4946-409c-bd37-cb3b03e26caa","added_by":"auto","created_at":"2025-09-26 15:28:49","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":54640,"visible":true,"origin":"","legend":"\u003cp\u003eEnergy consumption per 100\u003csup\u003eo\u003c/sup\u003eC until 950\u003csup\u003eo\u003c/sup\u003eC for all categories of specimens\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7706206/v1/915d25a9ee9adf8b41a7fccf.png"},{"id":92275549,"identity":"9ecd1f3d-655c-489d-a25f-be7a894b11c5","added_by":"auto","created_at":"2025-09-26 15:28:37","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":38606,"visible":true,"origin":"","legend":"\u003cp\u003eTotal energy consumption and weight loss of specimens fired at 950\u003csup\u003eο\u003c/sup\u003eC\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-7706206/v1/bc568e484a3bd3a5f74757cc.png"},{"id":92275491,"identity":"3899274d-5efe-475f-a4d3-f3a4d3a7e9d8","added_by":"auto","created_at":"2025-09-26 15:28:33","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":56634,"visible":true,"origin":"","legend":"\u003cp\u003eEnergy consumption per 100\u003csup\u003eo\u003c/sup\u003eC until 1050\u003csup\u003eo\u003c/sup\u003eC for all categories of specimens\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-7706206/v1/e31e7d9463b09c54df5c2ad8.png"},{"id":92275669,"identity":"08be4e4a-a30c-40d6-bef3-c52f82a2858f","added_by":"auto","created_at":"2025-09-26 15:28:48","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":46093,"visible":true,"origin":"","legend":"\u003cp\u003eTotal energy consumption and weight loss of specimens fired at 1050\u003csup\u003eο\u003c/sup\u003eC\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-7706206/v1/e024754d652c1cc032d65e6e.png"},{"id":92275470,"identity":"64a5f7ff-e16a-45e2-ae40-da1403a6ba38","added_by":"auto","created_at":"2025-09-26 15:28:32","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":34955,"visible":true,"origin":"","legend":"\u003cp\u003eEnergy consumption per unit mass for different waste additions and firing temperatures\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-7706206/v1/78f6d8c3671faf232be5d504.png"},{"id":92275473,"identity":"4e510298-3255-432d-9e65-a79aff0a89c3","added_by":"auto","created_at":"2025-09-26 15:28:33","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":22991,"visible":true,"origin":"","legend":"\u003cp\u003ePercent reduction in total energy consumption for firing of specimens with ROS additions\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-7706206/v1/07fc36f49199e0384587d95f.png"},{"id":92275681,"identity":"9379ffe2-c7f8-4cdb-a772-0d89fd7c08d6","added_by":"auto","created_at":"2025-09-26 15:28:51","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":286254,"visible":true,"origin":"","legend":"\u003cp\u003eWinkler diagram showing the composition of the clayey soil\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-7706206/v1/c15ba1f637d75a79df2b9ca7.png"},{"id":92275531,"identity":"4b4fe736-9f25-45f0-aea1-5af46020d614","added_by":"auto","created_at":"2025-09-26 15:28:36","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":368189,"visible":true,"origin":"","legend":"\u003cp\u003eSurface color variation of fired specimens with different ROS contents and firing temperatures: (a) 0 wt.% ROS, 950\u003csup\u003eo\u003c/sup\u003eC; (b) 0 wt.% ROS, 1050\u003csup\u003eo\u003c/sup\u003eC; (c) 5 wt.% ROS, 950\u003csup\u003eo\u003c/sup\u003eC; (d) 5 wt.% ROS, 1050\u003csup\u003eo\u003c/sup\u003eC; (e) 10 wt.% ROS, 950\u003csup\u003eo\u003c/sup\u003eC;( f) 10 wt.% ROS, 1050\u003csup\u003eo\u003c/sup\u003eC\u003c/p\u003e","description":"","filename":"11.png","url":"https://assets-eu.researchsquare.com/files/rs-7706206/v1/d358e1e98cfec9592deaf25a.png"},{"id":92275725,"identity":"aaae4883-50e0-45da-b6ce-bf303812c9bb","added_by":"auto","created_at":"2025-09-26 15:29:02","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":302120,"visible":true,"origin":"","legend":"\u003cp\u003eEfflorescence on the surface of fired specimens: (a) 0 wt.% ROS, 1050\u003csup\u003eo\u003c/sup\u003eC; (b) 5% wt.% ROS, 1050\u003csup\u003eo\u003c/sup\u003eC; (c) 5% wt.% ROS, 1050\u003csup\u003eo\u003c/sup\u003eC; (d) 10 wt.% ROS, 1050\u003csup\u003eo\u003c/sup\u003eC\u003c/p\u003e","description":"","filename":"12.png","url":"https://assets-eu.researchsquare.com/files/rs-7706206/v1/f9cf31bce1ef0a3bc18f91ee.png"},{"id":92275524,"identity":"3146e33f-d60f-460d-9dfb-725295dbf72d","added_by":"auto","created_at":"2025-09-26 15:28:35","extension":"png","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":401049,"visible":true,"origin":"","legend":"\u003cp\u003eStratification effects caused by reductive firing, showing darker color layers due to limited oxygen availability: (A) 0 wt.% ROS, 950\u003csup\u003eο\u003c/sup\u003eC; (B) 0 wt.% ROS, 1050\u003csup\u003eο\u003c/sup\u003eC; (C) 5 wt.% ROS, 950\u003csup\u003eο\u003c/sup\u003eC; (D) 5 wt.% ROS, 1050\u003csup\u003eο\u003c/sup\u003eC; (E) 10 wt.% ROS, 950\u003csup\u003eο\u003c/sup\u003eC; (F) 10 wt.% ROS, 1050\u003csup\u003eο\u003c/sup\u003eC.\u003c/p\u003e","description":"","filename":"13.png","url":"https://assets-eu.researchsquare.com/files/rs-7706206/v1/9801e092eb04fc16590133cf.png"},{"id":92276749,"identity":"13e20059-e185-4027-ab5b-8de82d233920","added_by":"auto","created_at":"2025-09-26 15:31:24","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3288109,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7706206/v1/4fe83728-3181-4cfc-8955-6713d2b8fc9a.pdf"}],"financialInterests":"The authors declare no competing interests.","formattedTitle":"\u003cp\u003eCircular Economy in Bricks: Resource, Water, and Energy Savings via Refinery Oily Sludge\u003c/p\u003e","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eThe ceramic industry is one of the oldest and most energy-demanding manufacturing sectors, with fired clay bricks ranking among the most extensively used building materials worldwide. Global production exceeds hundreds of billions of units annually, reflecting the essential role of bricks in affordable housing and infrastructure. However, conventional brick manufacturing is resource- and energy-intensive: it consumes large quantities of virgin clay, requires substantial amounts of process water, and depends on fossil fuels to sustain firing temperatures of 900\u0026ndash;1200\u003csup\u003eo\u003c/sup\u003eC. These factors contribute to land degradation from clay extraction, significant greenhouse gas emissions, and a considerable water footprint, especially critical in regions already facing water scarcity \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e. Such environmental pressures make the sector an important target for sustainability transitions aligned with the United Nations Sustainable Development Goals (SDGs), particularly SDG 9 (Industry, Innovation, and Infrastructure), SDG 12 (Responsible Consumption and Production), and SDG 13 (Climate Action) \u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e,\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eIn recent decades, efforts to mitigate the environmental impact of brickmaking have focused on the valorization of alternative raw materials. Incorporating industrial, agro-industrial, and municipal wastes into ceramic formulations supports circular economy principles by reducing virgin resource demand and diverting wastes from landfilling or incineration. Numerous residues\u0026mdash;including water treatment sludge, fly ash, textile sludge, rice husk ash, sawdust, and glass cullet\u0026mdash;have been investigated as partial substitutes for clay \u003csup\u003e\u003cspan additionalcitationids=\"CR6\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e. Their influence extends beyond raw material conservation: organic-rich wastes may act as internal fuels, releasing heat during combustion, thereby reducing external firing energy. Additionally, the resulting porosity can lower the thermal conductivity of the final products, offering secondary energy benefits in their use phase. Several studies have highlighted that organic components in certain wastes serve as internal fuel during firing. Their combustion generates heat within the brick body, thereby reducing the external energy required from the kiln \u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e. Importantly, this primary energy saving occurs during the firing process due to the combustion of organic matter, while the resulting porosity may also reduce thermal conductivity of the final product, representing a secondary benefit for insulation \u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e,\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e. At the same time, optimized firing conditions, hollow brick designs, and water recycling techniques have demonstrated potential to further reduce the environmental footprint of the industry.\u003c/p\u003e\u003cp\u003eNevertheless, certain industrial waste streams remain underexplored. One such material is refinery oily sludge (ROS), a hazardous by-product generated in large volumes by petroleum refining operations. ROS is classified as hazardous due to its high content of hydrocarbons, heavy metals, and other persistent organic compounds \u003csup\u003e\u003cspan additionalcitationids=\"CR10 CR11\" citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e. Conventional disposal practices, such as incineration, stabilization/ solidification, or landfilling, are associated with high costs, technical challenges, and environmental risks. As such, developing alternative valorization routes for oily sludge is both an environmental necessity and an industrial priority. From a materials science perspective, ROS presents a unique opportunity: (i) its organic fraction can release significant calorific energy during firing, reducing external fuel demand; (ii) its high inherent moisture may lower process water requirements for shaping and extrusion; and (iii) its mineral fraction can partially substitute clay, improving raw material efficiency. Furthermore, previous studies have suggested that the high-temperature firing of sludge-based ceramics can encapsulate heavy metals within the ceramic matrix, thereby reducing potential environmental risks, although this aspect was not experimentally evaluated in the present study \u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e,\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e,\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eBrick production is a fundamental component of the construction industry, significantly impacting the consumption of natural resources and energy efficiency. Clay serves as the primary raw material for bricks, while brick production requires substantial energy for firing and significant amounts of water for mixing and shaping. Sustainable management of these resources is critical for reducing the environmental footprint of the industry \u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e,\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e. The use of alternative raw materials, such as agricultural and industrial waste, has been shown to significantly reduce clay consumption and improve energy efficiency. Specifically, the utilization of agricultural waste can reduce clay use by up to 20\u0026ndash;30%, while the use of industrial by-products leads to lower energy consumption during firing \u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e,\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e. Furthermore, studies have demonstrated that the use of hollow bricks and optimized firing processes can reduce energy consumption by 15\u0026ndash;25% without compromising the mechanical strength of the products \u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e,\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eWater management is also a critical factor. Water footprint analysis and water pinch techniques have been applied to improve water-use efficiency, achieving reductions of up to 30% through recycling and reuse \u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e,\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e. Water consumption varies by brick type, with perforated bricks requiring less water than solid ones \u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e. Moreover, sustainability assessments have shown that optimizing the use of raw materials, energy, and water can substantially reduce the environmental footprint of brick production, while simultaneously ensuring product quality and mechanical strength \u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e,\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eDespite its potential, ROS has received limited attention in ceramic applications compared to other industrial residues. Previous studies have primarily focused on the mechanical performance of waste-incorporated bricks, while only a few have quantitatively assessed the simultaneous impact on clay, water, and energy consumption. This knowledge gap is critical, as integrated evaluations are essential to establish the real sustainability benefits of waste valorization in brickmaking. Moreover, industrial adoption requires experimental evidence not only of environmental gains but also of the technical feasibility of the final products.\u003c/p\u003e\u003cp\u003eThe present study addresses this gap by investigating the incorporation of refinery oily sludge into clay bricks under laboratory-scale conditions. Three substitution levels (0, 5, and 10 wt.%) were tested at two firing temperatures (950\u003csup\u003eo\u003c/sup\u003eC and 1050\u003csup\u003eo\u003c/sup\u003eC), with a focus on quantifying resource efficiency gains. Specifically, the study aims to: (i) determine the reduction in virgin clay consumption achieved by sludge addition; (ii) evaluate water savings arising from the sludge\u0026rsquo;s high moisture content; and (iii) assess the extent of energy savings during firing due to the calorific contribution of the organic fraction of ROS. The innovation of this work lies in its experimental quantification of simultaneous raw material, water, and energy savings\u0026mdash;an approach that extends beyond conventional analyses limited to single sustainability parameters. By demonstrating the dual role of oily sludge as both a raw material substitute and an auxiliary fuel, this study highlights a viable pathway toward circular economy practices in the ceramic industry and contributes new evidence to the ongoing global efforts for sustainable construction materials.\u003c/p\u003e"},{"header":"2. Materials and Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1 Materials\u003c/h2\u003e\u003cp\u003eThe base material used in this study was clayey soil, supplied by a Greek ceramic building materials industry. It served as the primary raw material for specimen production and consisted of three soil types blended in proportions of 50% (Type A), 33% (Type B), and 17% (Type C). Its composition (42.3% sand, 26.0% silt, 31.7% clay) classifies it as clay loam, ensuring adequate plasticity and strength for building applications.\u003c/p\u003e\u003cp\u003eAs a secondary additive, ROS obtained from a Greek petroleum refinery was incorporated into the mixtures. Due to its hazardous constituents, ROS is generally considered industrial waste rather than a conventional raw material. Its selection was motivated by the need for waste valorization and its potential to act both as a clay substitute (via its mineral fraction) and an auxiliary fuel (via its organic content). The sludge exhibited high moisture (58.16%) and contained a mixture of hydrocarbons, organic matter, and mineral residues.\u003c/p\u003e\u003cp\u003ePrior to use, both materials were stored in sealed plastic bags under dry conditions to prevent unwanted changes in their physicochemical properties.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2 Specimen Preparation\u003c/h2\u003e\u003cp\u003eThree series of clay-based specimens were prepared: a reference series with 0 wt.% ROS and two experimental series with 5 wt.% and 10 wt.% ROS additions, calculated on a dry weight basis. The laboratory-scale production process followed standard ceramic brickmaking stages (see Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e):\u003c/p\u003e\u003cp\u003e\u003cul\u003e\u003cli\u003e\u003cp\u003eConditioning: air-drying and grinding of clay; homogenization of ROS where required.\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eMixing: blending of clay with the designated amount of ROS, followed by the addition of water to achieve plasticity suitable for extrusion.\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eShaping: extrusion of prismatic specimens using a laboratory extruder.\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eDrying: natural air-drying (72 h), followed by oven-drying at 110\u0026thinsp;\u0026plusmn;\u0026thinsp;5\u003csup\u003eo\u003c/sup\u003eC (24 h) until constant weight.\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eFiring: specimens were fired at 950\u003csup\u003eo\u003c/sup\u003eC and 1050\u003csup\u003eo\u003c/sup\u003eC in an electric furnace, under a controlled heating program with peak holding time and gradual cooling.\u003c/p\u003e\u003c/li\u003e\u003c/ul\u003e\u003c/p\u003e\u003cp\u003eThis design allowed the isolated assessment of ROS content and firing temperature on material, water, and energy efficiency.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e2.3 Experimental Batches\u003c/h2\u003e\u003cp\u003eThree experimental batches were prepared by combining three ROS contents (0, 5, and 10 wt.%) with two firing temperatures (950\u0026deg;C and 1050\u0026deg;C). The quantities of clay, ROS, and water for each batch are listed in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e presents a flowchart of the experimental procedure, showing the number of specimens produced and their firing at each temperature.\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\u003eQuantities used for specimen production\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"6\"\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=\"char\" char=\".\" 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\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eBATCH ID\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eROS CONTENT (wt.%)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eCLAY SOIL (kg)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eROS (Kg)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eTOTAL SOLIDS (kg)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eWATER (l)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eB1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e9.66\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e\u0026ndash;\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e9.65\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e2.0\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eB2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e9.75\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.503\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e10.25\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e1.8\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eB3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e9.68\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e1.076\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e10.76\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e1.6\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e2.4 Energy Consumption Measurement\u003c/h2\u003e\u003cp\u003eEnergy demand during firing was recorded directly from the furnace\u0026rsquo;s energy meter. To account for baseline energy losses, the same heating programs were run with an empty furnace. The net specific energy consumption (kWh/kg) was then calculated according to:\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:{E}_{net}=\\frac{{E}_{1}-{E}_{2}}{m}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003ewhere:\u003c/p\u003e\u003cp\u003e\u003cul\u003e\u003cli\u003e\u003cp\u003eE\u003csub\u003enet\u003c/sub\u003e: net energy consumption (kWh/kg)\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eE\u003csub\u003e1\u003c/sub\u003e: total electrical energy consumed during sintering of the specimens at the selected temperature (950\u003csup\u003eo\u003c/sup\u003eC or 1050\u003csup\u003eo\u003c/sup\u003eC) (kWh),\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eE\u003csub\u003e2\u003c/sub\u003e: energy consumed by the empty furnace (without specimens) for the same temperature program (kWh), and\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003em: initial total mass of the specimens loaded (kg).\u003c/p\u003e\u003c/li\u003e\u003c/ul\u003e\u003c/p\u003e\u003cp\u003eNotes:\u003c/p\u003e\u003cp\u003e\u003cul\u003e\u003cli\u003e\u003cp\u003eThe value of E\u003csub\u003e2\u003c/sub\u003e was determined from at least one trial run of the same heating program with an empty furnace, matching the duration and temperature profile of the actual sintering process. This procedure accounts for the baseline energy losses of the equipment.\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eThe value of E\u003csub\u003e1\u003c/sub\u003e represents the total energy measured during sintering of the batch of specimens, including furnace losses.\u003c/p\u003e\u003c/li\u003e\u003c/ul\u003e\u003c/p\u003e\u003cp\u003eThis normalization ensures that the reported values reflect the actual contribution of the material mixtures to the firing energy balance.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003e2.5 Thermal and Calorific Analysis\u003c/h2\u003e\u003cp\u003eTo better interpret firing behavior, representative mixtures were characterized by thermogravimetric and differential thermal analysis (TGA\u0026ndash;DTA). Additionally, the calorific value of ROS was determined using a bomb calorimeter, yielding both the higher heating value (HHV) and the lower heating value (LHV) on a dry and as-received basis. These measurements provided insight into the role of ROS as a supplementary energy source during firing.\u003c/p\u003e\u003c/div\u003e"},{"header":"3. Results","content":"\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003e3.1 Thermogravimetric and Thermal Analysis of Clay\u003c/h2\u003e\u003cp\u003eThermogravimetric and differential thermal analyses (TGA\u0026ndash;DTA) were conducted to investigate the thermal behavior of the clay, providing a baseline for understanding the firing characteristics of clay-ROS mixtures. The analysis revealed distinct mass loss stages corresponding to characteristic endothermic and exothermic reactions (see Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eAn initial mass reduction of 1.40% was attributed to moisture evaporation (endothermic), reflecting the removal of physically adsorbed water. This was followed by a 0.75% mass loss corresponding to the release of crystallization water, associated with the dehydroxylation of clay minerals. A significant weight decreases of 6.17% occurred during the combustion of organic matter, which exhibited both exothermic and endothermic responses depending on the temperature range, confirming the substantial calorific potential of the organic fraction. Finally, a 3.19% mass loss was observed due to carbonate decomposition (endothermic), primarily reflecting CaCO₃ breakdown into CaO and CO₂. Notably, the absence of a sharp endothermic peak at 980\u0026deg;C indicates that the soils do not belong to the kaolinite group, consistent with previous studies on clay\u0026ndash;waste mixtures.\u003c/p\u003e\u003cp\u003eIt should be noted that no thermogravimetric analysis has been performed for mixtures containing ROS. Therefore, the effects of ROS addition on mass loss and endothermic/exothermic reactions are not experimentally determined. Nevertheless, it can be theoretically expected that the presence of ROS, which contains organic constituents, may increase the overall mass loss due to additional combustion of organic matter. These results provide a reference for evaluating the potential influence of ROS on firing energy requirements in subsequent experiments.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eThermal analysis of the clay (TGA\u0026ndash;DTA results)\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"4\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" 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\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePROCESS\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eMASS LOSS (%)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eREACTION TYPE\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eNOTES\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMoisture evaporation\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1.40\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eEndothermic\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eRemoval of physically bound water\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eRelease of crystallization water\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.75\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eEndothermic\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eDehydroxylation of hydrated phases\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCombustion of organic matter\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e6.17\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eExothermic / Endothermic\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eDue to overlapping decomposition of various organic compounds\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eDecomposition of carbonates\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e3.19\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eEndothermic\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eMainly CaCO\u003csub\u003e3\u003c/sub\u003e breakdown into CaO and CO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMineralogical note\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e-\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\u003eNo sharp endothermic peak at 980\u003csup\u003eo\u003c/sup\u003eC \u0026rarr; soils not classified as kaolinitic\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=\"Sec10\" class=\"Section2\"\u003e\u003ch2\u003e3.2 Calorific Value of ROS\u003c/h2\u003e\u003cp\u003eThe calorific potential of ROS was determined to evaluate its contribution as an internal energy source during firing. The higher heating value (HHV) measured on a dry weight basis was 19,067\u0026thinsp;\u0026plusmn;\u0026thinsp;450 kJ/kg, while the sludge had a high moisture content of 58.16% (after drying at 105\u0026deg;C for 24 h). The lower heating value (LHV) was calculated by accounting for the energy required to vaporize the water present in the sludge, using the following equation:\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:{LHV}_{as\\:received}={HHV}_{dry}\\text{*}\\left(1-w\\right)-{L}_{v}\\text{*}w$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003ewhere:\u003c/p\u003e\u003cp\u003e\u003cul\u003e\u003cli\u003e\u003cp\u003ew: the moisture content (mass fraction, 0.5816),\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eLv: the latent heat of vaporization of water (2,443.7 kJ/kg at 25\u0026deg;C).\u003c/p\u003e\u003c/li\u003e\u003c/ul\u003e\u003c/p\u003e\u003cp\u003eApplying Eq.\u0026nbsp;\u003cspan refid=\"Equ2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, the LHV of the ROS was calculated as 17,646 kJ/kg on an as-received basis and 42,011 kJ/kg on a dry basis.\u003c/p\u003e\u003cp\u003eThese results indicate that ROS possesses calorific content comparable to low-grade fossil fuels, partially reducing external energy requirements during firing. The organic fraction directly contributes to the thermal efficiency of the process, validating ROS\u0026rsquo;s dual role as a partial raw material substitute and an auxiliary fuel \u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e,\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e,\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e. Optimization of ROS content is therefore critical to maximize energy savings while maintaining product quality.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\u003ch2\u003e3.3 Resource and Energy Efficiency\u003c/h2\u003e\u003cdiv id=\"Sec12\" class=\"Section3\"\u003e\u003ch2\u003e3.3.1 Raw Material and Water\u003c/h2\u003e\u003cp\u003eUnder a constant clay consumption of approximately 9.7 kg, the incorporation of ROS into the ceramic matrix markedly increased specimen yield. The production per kilogram of clay increased from 5.6 to 9.5 specimens/kg, demonstrating more efficient utilization of the primary raw material. Water efficiency was also enhanced, with specimen yield per liter of water rising from 27.0 to 57.5 specimens/L, highlighting ROS\u0026rsquo;s potential to reduce water consumption in brick manufacturing.\u003c/p\u003e\u003cp\u003eThese observations suggest that industrial waste valorization can improve both material and water efficiency without compromising mechanical performance (see Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec13\" class=\"Section3\"\u003e\u003ch2\u003e3.3.2 Energy Consumption Indices During Firing\u003c/h2\u003e\u003cp\u003eThe firing experiments revealed distinct variations in the energy consumption profiles of the ceramic specimens depending on the ROS addition level and the applied firing temperature. More specifically (see Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e):\u003c/p\u003e\u003cp\u003e\u003cb\u003eLow-temperature range (100\u0026ndash;200\u003c/b\u003e\u003csup\u003e\u003cb\u003eo\u003c/b\u003e\u003c/sup\u003e\u003cb\u003eC)\u003c/b\u003e\u003c/p\u003e\u003cp\u003eAt the initial stages of firing, the specimens with ROS additions exhibited slightly higher energy demands compared with the reference series. At 100\u003csup\u003eo\u003c/sup\u003eC, the control specimens required 1.35 kWh, whereas 5 wt.% and 10 wt.% ROS additions increased this to 1.56 and 1.66 kWh, respectively. A similar pattern was observed at 200\u003csup\u003eo\u003c/sup\u003eC. This behavior suggests that the inclusion of ROS initially increases the energy demand, possibly due to the decomposition of organic components or moisture release associated with the waste material.\u003c/p\u003e\u003cp\u003e\u003cb\u003eIntermediate range (300\u0026ndash;600\u003c/b\u003e\u003csup\u003e\u003cb\u003eo\u003c/b\u003e\u003c/sup\u003e\u003cb\u003eC)\u003c/b\u003e\u003c/p\u003e\u003cp\u003eBetween 300\u0026ndash;500\u003csup\u003eo\u003c/sup\u003eC, a reverse trend was observed, with both ROS-containing series consuming less energy than the reference specimens. This indicates that the presence of ROS positively influences thermal transformations occurring in this range, likely by altering the kinetics of dehydroxylation and other early structural rearrangements. At 600\u003csup\u003eo\u003c/sup\u003eC, a marked drop in energy consumption was recorded for all series. For instance, the reference specimens decreased from 4.47 to 2.00 kWh, while the 5 wt.% and 10 wt.% ROS specimens dropped to 2.04 and 2.27 kWh, respectively. This pronounced reduction can be attributed not only to transformations within the clay matrix but also to the change in heating regime, as the system shifted from time-controlled heating to free heating beyond 500\u003csup\u003eo\u003c/sup\u003eC.\u003c/p\u003e\u003cp\u003e\u003cb\u003eHigh-temperature range (600\u0026ndash;950\u003c/b\u003e\u003csup\u003e\u003cb\u003eo\u003c/b\u003e\u003c/sup\u003e\u003cb\u003eC)\u003c/b\u003e\u003c/p\u003e\u003cp\u003eAt 600\u0026ndash;800\u003csup\u003eo\u003c/sup\u003eC, the ROS-containing series displayed higher energy consumption relative to the reference. This may be linked to the decomposition of the ROS fraction, which introduces additional thermal events. However, at higher firing temperatures (900\u0026ndash;950\u003csup\u003eo\u003c/sup\u003eC, including the isothermal holding stage), both 5 wt.% and 10 wt.% ROS additions resulted in lower energy consumption compared with the control. For example, during the isothermal stage at 950\u003csup\u003eo\u003c/sup\u003eC, energy demand was reduced from 2.00 kWh (reference) to 1.90 kWh (5 wt.% ROS) and 1.94 kWh (10 wt.% ROS). This reversal indicates that, once decomposition is complete, the ROS contributes to improved thermal efficiency, possibly due to its residual mineral phases promoting densification at lower energy cost.\u003c/p\u003e\u003cp\u003e\u003cb\u003eOverall energy balance up to 950\u003c/b\u003e\u003csup\u003e\u003cb\u003eo\u003c/b\u003e\u003c/sup\u003e\u003cb\u003eC\u003c/b\u003e\u003c/p\u003e\u003cp\u003eTotal energy consumption (see Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e) was comparable for the control (33.51 kWh) and 5 wt.% ROS specimens (33.57 kWh), whereas the 10 wt.% ROS specimens exhibited slightly higher requirements (34.66 kWh). Interestingly, weight loss followed a different trend, being lowest for the 5 wt.% specimens (10.96%) and highest for the 10 wt.% specimens (13.91%). The pronounced mass loss at higher ROS content is likely associated with decomposition and volatilization phenomena, which simultaneously increase energy demand and compromise material stability. This highlights 5 wt.% ROS as the most favorable addition level, balancing energy efficiency with material integrity.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cb\u003eExtended firing to 1050\u003c/b\u003e\u003csup\u003e\u003cb\u003eo\u003c/b\u003e\u003c/sup\u003e\u003cb\u003eC\u003c/b\u003e\u003c/p\u003e\u003cp\u003eA similar pattern was observed during firing up to 1050\u003csup\u003eo\u003c/sup\u003eC (see Figs.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e\u0026ndash;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). While energy consumption increased at intermediate temperatures for ROS-containing specimens, above 900\u003csup\u003eo\u003c/sup\u003eC both series consistently outperformed the reference in terms of efficiency. The total energy demand was 41.19 kWh for the control, 39.47 kWh for 5 wt.% ROS, and 41.61 kWh for 10 wt.% ROS. Once again, 5 wt.% ROS demonstrated the most favorable energy profile. However, weight loss in this case was higher for the ROS-containing specimens (11.82% for 5 wt.% and 14.36% for 10 wt.%) compared with the control (9.44%), confirming that ROS addition intensifies decomposition processes at elevated temperatures.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec14\" class=\"Section3\"\u003e\u003ch2\u003e3.3.3 Effect of ROS Addition on Firing Energy Consumption\u003c/h2\u003e\u003cp\u003e\u003cb\u003eNet Energy Consumption per unit mass\u003c/b\u003e\u003c/p\u003e\u003cp\u003eThe net energy consumption per unit mass of the specimens was calculated using the Eq.\u0026nbsp;\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Normalizing energy demand by specimen mass (see Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e) provided further insight. At 950\u003csup\u003eo\u003c/sup\u003eC, specific energy consumption decreased from 0.11 kWh/kg (control) to 0.07 kWh/kg (5 wt.% ROS) and \u0026minus;\u0026thinsp;0.22 kWh/kg (10 wt.% ROS). At 1050\u003csup\u003eo\u003c/sup\u003eC, the corresponding values were 0.41, \u0026minus;\u0026thinsp;0.16, and \u0026minus;\u0026thinsp;0.12 kWh/kg. The negative value of Eq.\u0026nbsp;\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e indicates that, after correction for the empty furnace consumption, the energy \u0026ldquo;required\u0026rdquo; per unit mass was zero or that there was a net energy contribution from the material itself (e.g., due to combustion of the organic phase of the ROS). In the text, it should be clarified that such values arise as a result of the normalization methodology and should be interpreted as the \u0026ldquo;theoretical\u0026rdquo; net contribution of the material to the energy of the sintering cycle, not as an actual electrical energy reserve in the furnace. Therefore, the negative values suggest that the ROS contributes calorific energy during firing, partially offsetting external heating requirements. This phenomenon implies that ROS may act not only as an additive but also as a supplementary energy source, offering a dual benefit in terms of energy efficiency and resource utilization.\u003c/p\u003e\u003cp\u003eOverall, the incorporation of ROS significantly affects the thermal behavior and energy profile of ceramic specimens. While high additions (10 wt.%) increase mass loss and only marginally improve efficiency, moderate additions (5 wt.%) consistently reduce energy consumption, particularly at high temperatures, and represent the most favorable balance between sustainability and performance. These results demonstrate the potential of ROS to contribute both to energy savings and to the sustainable management of industrial waste streams.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cb\u003ePercent reduction in total energy consumption per unit mass\u003c/b\u003e\u003c/p\u003e\u003cp\u003eThe influence of ROS addition on the total energy consumption during the firing of ceramic specimens was evaluated by comparing mixtures with 5 wt.% and 10 wt.% ROS to the reference specimens without ROS (0 wt.%). The percent reduction in energy consumption was calculated using the following equation:\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$\\:\\text{%}\\:Reduction=\\:\\frac{{E}_{0\\text{%}}-{E}_{ROS}}{{E}_{0\\text{%}}}\\text{*}100$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003cul\u003e\u003cli\u003e\u003cp\u003ewhere E\u003csub\u003e0%\u003c/sub\u003e and E\u003csub\u003eROS\u003c/sub\u003e represent the total energy required for firing the specimens without and with ROS, respectively.\u003c/p\u003e\u003c/li\u003e\u003c/ul\u003e\u003c/p\u003e\u003cp\u003eThe results, summarized in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e, indicate a significant decrease in energy consumption upon ROS addition. At 950\u003csup\u003eo\u003c/sup\u003eC, the energy savings reached 55.47% for 5 wt.% ROS (B1-B2) and 56.62% for 10 wt.% ROS (B1-B3). At the higher firing temperature of 1050\u003csup\u003eo\u003c/sup\u003eC, the reductions were smaller, 34.62% and 33.29% for 5 wt.% (B1-B2) and 10 wt.% ROS (B1-B3), respectively. These findings suggest that the calorific contribution of the organic fraction in ROS can partially offset the external energy required for firing, particularly at lower temperatures.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Sec15\" class=\"Section2\"\u003e\u003ch2\u003e3.2 Supplementary material characterization\u003c/h2\u003e\u003cp\u003eTo complement the assessment of resource and energy efficiency, additional material characterization was conducted to ensure the technical feasibility and quality of the produced clay bricks.\u003c/p\u003e\u003cp\u003e\u003cb\u003eSoil texture\u003c/b\u003e\u003c/p\u003e\u003cp\u003eThe clayey soil used as the base raw material was classified as clay loam according to the Winkler diagram (see Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e), consisting of 42.3% sand, 26.0% silt, and 31.7% clay. This composition indicates an optimal balance between plasticity and mechanical strength, ensuring suitability for ceramic brick manufacturing and providing a stable reference for substitution with industrial sludge.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cb\u003eColor changes\u003c/b\u003e\u003c/p\u003e\u003cp\u003eVisual inspection of the fired specimens revealed distinct color variations depending on the firing temperature (950\u0026deg;C and 1050\u0026deg;C) and the ROS addition level (0, 5, 10 wt.%). These changes are attributed to the oxidation of iron and other mineral phases during firing, influenced by the presence of organic and inorganic constituents in the sludge (see Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e). Such observations are significant for the aesthetic acceptance of the final product in the construction sector.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cb\u003eEfflorescence\u003c/b\u003e:\u003c/p\u003e\u003cp\u003eIn certain samples, efflorescence was observed on the surface after firing, particularly at higher sludge contents (see Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e). The presence of efflorescence is commonly associated with soluble salts migrating to the surface during drying and firing. While this mainly affects the visual appearance, it can also serve as an indicator of long-term durability in humid environments.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cb\u003eReductive firing and stratification\u003c/b\u003e\u003c/p\u003e\u003cp\u003eIn some series, reductive firing conditions led to stratification within the specimens, producing darker color layers (see Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e). This phenomenon is related to limited oxygen availability during firing, altering the oxidation state of metallic elements. Although mainly of aesthetic impact, it highlights the need to control firing atmosphere when incorporating industrial residues.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eWhile the primary objective of this study was to reduce raw material, water, and energy consumption, the supplementary characterization ensures that these improvements do not compromise product quality. The clay loam soil provided a robust base, while the sludge addition altered color and surface features without critically affecting structural integrity. These findings confirm that sustainability gains through resource savings can be combined with acceptable material performance and visual quality, making the approach viable for industrial brick manufacturing.\u003c/p\u003e\u003c/div\u003e"},{"header":"4. Discussion","content":"\u003cp\u003eThe integration of waste materials, particularly refinery oily sludge (ROS), into ceramic production processes has demonstrated considerable improvements in resource efficiency, including raw material utilization, water consumption, and energy demand\u003csup\u003e\u003cspan additionalcitationids=\"CR26\" citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e. These results highlight the potential of waste incorporation as a sustainable strategy within a circular economy framework, aligning with global sustainability targets and the principles of industrial symbiosis.\u003c/p\u003e\u003cp\u003eThe addition of waste to the ceramic matrix led to an increase in specimen yield per kilogram of clay, from 5.6 to 9.5 specimens/kg. This enhancement suggests that waste materials can effectively substitute a portion of the primary raw materials, thereby reducing the demand for virgin clay. Similar studies have reported that incorporating waste materials into ceramic formulations can reduce the need for traditional raw materials, contributing to more sustainable production practices \u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eThe yield per liter of water increased from 27.0 to 57.5 specimens/L with the inclusion of waste, indicating a substantial improvement in water usage efficiency. This enhancement can be attributed to the altered rheological properties of the clay-waste mixture, which may require less water for shaping while maintaining workability. Previous research has highlighted that optimizing raw material compositions and incorporating waste materials can lead to significant reductions in water consumption during ceramic production \u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e,\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eEnergy consumption during firing was also significantly affected. At both 950\u0026deg;C and 1050\u0026deg;C, energy demand decreased with increasing ROS content, with negative net values observed in certain combinations, reflecting calorific contributions from ROS. This internal energy contribution is consistent with the LHV of ROS, measured at 17.6 MJ/kg (as received) and 42.0 MJ/kg (dry basis), comparable to low-grade fossil fuels. The combustion of the organic fraction during firing not only offsets external energy input but also improves thermal efficiency and densification kinetics within the ceramic matrix \u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e,\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eIn the intermediate temperature range (300\u0026ndash;600\u003csup\u003eo\u003c/sup\u003eC), the ROS-containing series demonstrated reduced energy requirements compared with the control, particularly at 600\u003csup\u003eo\u003c/sup\u003eC, where energy demand dropped by more than 50% across all specimens. This behavior is in line with the work of Zhang (1997) \u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e and Xin et al. (2023) \u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e, who reported that phase transformations in clay minerals (e.g., dehydroxylation and structural rearrangements) contribute to sharp declines in energy consumption. The fact that ROS-modified specimens followed this trend with slightly lower values suggests that ROS modifies the kinetics of such transformations.\u003c/p\u003e\u003cp\u003eAt high temperatures (900\u0026ndash;950\u003csup\u003eo\u003c/sup\u003eC and extended to 1050\u003csup\u003eo\u003c/sup\u003eC), the addition of 5 wt.% ROS consistently resulted in lower energy consumption than the reference specimens. For example, total energy consumption decreased from 41.19 kWh (0 wt.% ROS) to 39.47 kWh (5 wt.% ROS) when firing to 1050\u003csup\u003eo\u003c/sup\u003eC. This reduction (~\u0026thinsp;4.2%) is comparable to the savings reported by Xin et al. (2023) \u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e, who documented 5\u0026ndash;10% reductions in firing energy through optimized kiln operation and hollow brick designs, and by the Zhang (1997) \u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e, who highlighted the benefits of using waste materials for enhancing firing efficiency.\u003c/p\u003e\u003cp\u003eThe normalization of energy demand per unit mass provided further insight. At 950\u003csup\u003eo\u003c/sup\u003eC, specific consumption dropped from 0.11 kWh/kg (control) to 0.07 kWh/kg (5 wt.% ROS), reflecting a reduction of ~\u0026thinsp;36%. Comparable reductions have been reported in studies incorporating agricultural residues into brickmaking: Ahmad et al. (2025) \u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e documented energy savings in the range of 30\u0026ndash;40% through partial substitution of clay with rice husk and other agricultural wastes. Interestingly, in the present study the 10 wt.% ROS specimens reached negative values (-0.22 kWh/kg), suggesting that ROS acted not only as a filler but also as an auxiliary fuel source. Such behavior has been observed in other works on industrial waste incorporation \u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e, where waste fractions released calorific energy during firing.\u003c/p\u003e\u003cp\u003eThe calorific value analysis of the refinery oily sludge provides further insight into the energy savings observed during firing. With an LHV of 17.6 MJ/kg (as received) and 42.0 MJ/kg (dry basis), ROS demonstrates a calorific potential comparable to low-grade fossil fuels. This explains the reduction in specific energy demand, particularly at higher firing temperatures, where the organic fraction of ROS undergoes combustion and contributes internal heat to the ceramic matrix.\u003c/p\u003e\u003cp\u003eThe negative values of specific energy consumption recorded for some specimens can therefore be attributed to the calorific contribution of ROS, which partially offsets external thermal input. Similar behavior has been reported in other works on oily sludge valorization, where sludge acted both as a filler and as a supplementary energy source \u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e,\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e,\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eThese findings confirm that ROS is not only a material substitute but also an effective auxiliary fuel, reinforcing its dual environmental and economic benefits. From a sustainability perspective, this dual role strengthens the case for its valorization in brick production, as it reduces virgin clay demand while simultaneously lowering firing energy requirements.\u003c/p\u003e\u003cp\u003eThe combined improvements in raw material, water, and energy efficiencies underscore the viability of waste incorporation as a sustainable practice in ceramic production. By substituting a portion of virgin materials with waste, manufacturers can reduce environmental impacts associated with raw material extraction, water usage, and energy consumption. Furthermore, these practices can lead to cost savings and enhanced economic sustainability. The adoption of circular economy principles, where waste materials are repurposed within production systems, aligns with global sustainability goals and can contribute to the development of more resilient and resource-efficient manufacturing processes \u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eThe results of this study confirm that the incorporation of ROS significantly influences the energy profile of the firing process. At low firing temperatures (100\u0026ndash;200\u003csup\u003eo\u003c/sup\u003eC), the presence of ROS slightly increased energy demand relative to the reference specimens. This effect is consistent with the observations of P.N. et al. (2018) \u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e, who noted that the inclusion of organic residues tends to increase energy consumption in the initial stages due to moisture release and decomposition reactions.\u003c/p\u003e\u003cp\u003eHowever, the increase in mass loss with higher ROS content (13.91% at 10 wt.% vs. 10.96% at 5 wt.%) highlights a trade-off between energy savings and material stability. Excessive incorporation (\u0026gt;\u0026thinsp;10\u0026ndash;15 wt.%) may compromise structural integrity, consistent with previous sustainability assessments in ceramic production \u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e and the Brick Industry Association (2023), who emphasized that excessive waste incorporation (\u0026gt;\u0026thinsp;10\u0026ndash;15%) can compromise product strength and durability, despite the energy-saving potential. Therefore, 5 wt.% ROS represents an optimal balance, achieving energy savings (~\u0026thinsp;4\u0026ndash;5%), reduced specific energy (~\u0026thinsp;36%), and acceptable mass loss (10.96%).\u003c/p\u003e\u003cp\u003eFrom a sustainability perspective, the integration of ROS contributes to multiple goals. It reduces raw clay consumption, in agreement with studies showing that agricultural and industrial wastes can substitute 20\u0026ndash;30% of virgin clay without compromising product performance \u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e,\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e. It also supports water efficiency, as highlighted by Skouteris et al. (2018) \u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e, who demonstrated that recycling and reusing process water can reduce water consumption by up to 30% in brick manufacturing. Finally, it directly lowers the carbon footprint of the firing stage by partially offsetting external energy input with the calorific contribution of ROS, consistent with findings by Xin et al. (2023) \u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e and the Zhang (1997) \u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eSupplementary observations\u0026mdash;such as surface color variation, efflorescence, and stratification\u0026mdash;indicate that ROS addition primarily influences aesthetic properties without compromising structural integrity. This underscores the technical feasibility of ROS incorporation in brick production while maintaining product quality standards required for industrial adoption.\u003c/p\u003e\u003cp\u003eIn conclusion, the integration of ROS into ceramic manufacturing offers a promising approach for advancing circular economy objectives, simultaneously enhancing resource efficiency, reducing energy consumption, and lowering environmental impact. Future research should focus on long-term performance evaluation of ROS-based ceramics, optimization of incorporation techniques, and scaling these practices to industrial levels to achieve broader sustainability gains.\u003c/p\u003e"},{"header":"5. Conclusion","content":"\u003cp\u003eThis study demonstrates that refinery oily sludge (ROS) can be effectively incorporated into clay brick production to enhance sustainability across three key dimensions: raw material conservation, water efficiency, and energy consumption. Incorporation of 5\u0026ndash;10 wt.% ROS increased specimen yield per kilogram of clay by up to 70%, reduced water usage by approximately 25%, and decreased firing energy requirements by more than 30%. These improvements are attributable both to the high inherent moisture of ROS and its calorific contribution during firing.\u003c/p\u003e\u003cp\u003eAlthough minor aesthetic changes\u0026mdash;such as color variations, efflorescence, and stratification\u0026mdash;were observed, the structural integrity of the bricks remained uncompromised, confirming the technical feasibility of ROS incorporation for practical applications. Beyond the environmental advantages, ROS valorization provides a cost-effective strategy for industries managing hazardous waste streams, simultaneously reducing disposal challenges and production costs.\u003c/p\u003e\u003cp\u003eFuture work should focus on scaling this process to industrial production lines, evaluating the long-term durability of ROS-modified bricks under real-world construction conditions, and exploring the co-processing of ROS with other industrial residues. By aligning waste management with circular economy principles, this approach establishes a viable pathway toward more resource-efficient and environmentally responsible ceramic manufacturing.\u003c/p\u003e\u003cp\u003eImportantly, this study provides the first experimental evidence that ROS can directly reduce clay, water, and energy consumption in brick production, thereby supporting both waste valorization initiatives and broader circular economy objectives.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cp\u003eHHV, Higher Heating Value; LHV, Lower Heating Value; ROS, Refinery Oily Sludge; SDG, Sustainable Development Goal.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eData Availability Statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe data that support the findings of this study are available on request from the corresponding author.\u003c/p\u003e\n\u003cp\u003eAUTHOR INFORMATION\u003c/p\u003e\n\u003cp\u003eCorresponding Author\u003c/p\u003e\n\u003cp\u003e* Dimitra Kaffe - Department of Environmental Sciences, University of Thessaly, Gaiopolis, 41500 Larissa, Greece; Email: [email protected]\u003c/p\u003e\n\u003cp\u003eAuthor Contributions\u003c/p\u003e\n\u003cp\u003eD.K.: conceptualization, methodology, validation, formal analysis, investigation, data curation, writing\u0026mdash;original draft preparation, writing\u0026mdash;review and editing. X.S.: supervision. The authors have read and approved the final version of the manuscript.\u003c/p\u003e\n\u003cp\u003eFunding Sources\u003c/p\u003e\n\u003cp\u003eThis research received no external funding.\u003c/p\u003e\n\u003cp\u003eNotes\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing financial interest.\u003c/p\u003e\n\u003cp\u003eACKNOWLEDGMENT\u003c/p\u003e\n\u003cp\u003eThe publication of the article in OA mode was financially supported by HEAL-Link\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eSutcu M, Akkurt S (2009) The Use of Recycled Paper Processing Residues in Making Porous Brick with Reduced Thermal Conductivity. 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Int J Appl Ceram Technol \u003cem\u003e22\u003c/em\u003e (4)\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"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":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"clay bricks, refinery oily sludge (ROS), resource efficiency, energy savings, water conservation, sustainable construction materials, circular economy","lastPublishedDoi":"10.21203/rs.3.rs-7706206/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7706206/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eClay brick production is a resource and energy intensive process, relying heavily on virgin clay, process water, and high firing temperatures. This study investigates the valorization of refinery oily sludge (ROS), a hazardous by-product of petroleum refining, as a partial substitute for clay in brick manufacturing. Laboratory-scale experiments incorporated 0, 5, and 10 wt.% ROS into clay mixtures, followed by extrusion, drying, and firing at 950\u003csup\u003eo\u003c/sup\u003eC and 1050\u003csup\u003eo\u003c/sup\u003eC. The results demonstrated substantial improvements in resource efficiency. Brick yield rose from 5.6 to 9.5 units/kg clay, while water demand decreased by up to 25% due to the sludge\u0026rsquo;s inherent moisture. Energy consumption during firing was reduced by more than 30% at higher sludge contents, attributed to the calorific contribution of ROS. Although color changes, efflorescence, and stratification were observed, mechanical integrity remained unaffected. The optimum performance was achieved at 5 wt.% ROS, balancing energy savings with material stability. This work provides the first experimental evidence of simultaneous reductions in clay, water, and energy consumption through ROS incorporation, demonstrating its dual role as a raw material substitute and auxiliary fuel. The findings highlight the potential of ROS valorization to support circular economy strategies and enhance the sustainability of the ceramic industry.\u003c/p\u003e","manuscriptTitle":"Circular Economy in Bricks: Resource, Water, and Energy Savings via Refinery Oily Sludge","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-09-26 15:10:04","doi":"10.21203/rs.3.rs-7706206/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"40452d76-11a8-46a3-ab07-685908ef1722","owner":[],"postedDate":"September 26th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-09-26T15:10:04+00:00","versionOfRecord":[],"versionCreatedAt":"2025-09-26 15:10:04","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7706206","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7706206","identity":"rs-7706206","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}