Bulk density (BD) changes due to temperature variations during testing is one of the prime concerns in eco-friendly brick manufacturing. Generally, MSW addition results in a decrease in the density of bricks. Multiple studies report density reductions following the addition of solid waste. Addition of paper mill sludge (PMS), as done by Goel and Kalamdhad (2018), noted a significant decrease in the density from 1560 to 640 kg/m³ on inclusion of 30% sludge by weight. Similar results were drawn for other research incorporating paper mill or sludge waste. This was also inferred by Singh et al. (2018). The reduction in the density is probably due to the burning of de-inking PMS, leaving the pores behind as a residue. The addition of food waste, like tea waste (TW), also resulted in a decrease in the density of fired clay brick (Hussien et al., 2024). The density decreased from 1963.91 kg/m³ to 1602.18 kg/m³ on adding 0%–10% TW, respectively. Similarly, bricks made with spent oyster mushrooms as additives exhibited a reduction in density, with values decreasing from 1870 at 0% to 1370 kg/m³ at 15% oyster mushrooms by volume, indicating that higher concentrations of organic waste contribute to a less dense structure (Chung et al., 2021).

Adding eggshell powder to waste glass-based bricks also decreased density, with bricks incorporating 5% eggshell powder showing a notable decline in density compared to the control (Tangboriboon, 2019). Furthermore, TW in fired clay bricks led to a marked reduction in BD, dropping from 1860 at 0% TW to 1580 kg/m³ at 12% TW when the firing temperature was 1250°C and 1590 at 0% TW to 1370 kg/m³ at 12% TW when firing temperature was 950°C, demonstrating that higher TW content reduces the compactness of the material (Ozturk et al., 2019). Similarly, the use of wine lees (WL) and grape seeds (GS) in clay bricks decreased the BD, with the values for WL ranging between 1320–1460 kg/m³ at 10% content, while GS resulted in even lower density values of 1170–1200 kg/m³ at the same concentration (Taurino et al., 2019). When considering plastic-based waste materials like plastic dust and high density polyethylene (HDPE), the value for BD ranged from 1654 to 1298 kg/m3 for addition of 0%–15% plastic dust by volume and 2000 to 1360 kg/m³ on 0%–50% by volume incorporation of HDPE (Idrees et al., 2023; Sarwar et al., 2023). Introducing cassava peel bio-solid waste (CP) into clay bricks decreased density, with the lowest values observed at 16% CP, highlighting the effect of organic additives in reducing material density (Adazabra et al., 2024) Furthermore, in the case of incorporation of sago fine waste (SFW) mixed with cement decreased the density of the resulting bricks, with values dropping from 2103–2127 kg/m³ at 0% SFW to the range of 1687–1796 kg/m³ at 10% SFW, further emphasizing the influence of waste materials on the BD of construction materials (Norhayati et al., 2023). These findings collectively suggest that the incorporation of various waste materials into construction bricks and pellets reduces BD, which could impact the final product’s structural and thermal properties.

Figure 7 illustrates the variation in BD with increasing concentrations of MSW for a diverse set of waste-derived ashes, each treated at varying calcination temperatures. A consistent trend is evident across all materials: as the MSW content increases, the BD of the composite material significantly decreases. This behavior is primarily attributed to the lower specific gravity and increased internal porosity of MSW ashes compared to conventional cementitious binders or aggregates. The porous structure of MSW-based ashes, often resulting from the combustion of organic matter and volatile components during thermal treatment, leads to a looser particle packing arrangement, thus reducing the overall density. At 0% MSW, the highest bulk densities are observed in materials like high-density polyethylene and leather buffing dust, both exceeding 2200 kg/m³ and reaching up to 2400 kg/m³. These materials initially offer compact and denser structures, likely due to their thermoplastic or fibrous origins, which result in tighter interparticle packing and less void content. However, as MSW increases, their bulk densities drop markedly, indicating that the incorporation of MSW disrupts the matrix and introduces greater void space. On the other end of the spectrum, waste-paper ash and oyster mushroom ash exhibit considerably lower bulk densities even at minimal MSW levels, with values falling below 1200 kg/m³ at higher MSW concentrations. This suggests these ashes are inherently lighter and more porous, and their structural contribution in terms of density is minimal. PMS and paper pulp residue (PPR) follow a similar trend, showing significant reductions from approximately 1800–1900 kg/m³ to below 1400 kg/m³ as MSW percentages reach 20%–30%. TW ash, calcined at various temperatures ranging from 950°C to 1250°C, displays a more gradual and controlled reduction in BD. This indicates that higher calcination temperatures may promote partial sintering or particle densification, which slightly stabilizes BD despite increasing MSW content. Additionally, CP ash treated at both 800°C and 1000°C demonstrates intermediate behavior, starting from moderate densities and showing a steady decrease, reflecting the influence of organic content and thermal reactivity.

Overall, the incorporation of MSW-derived ash leads to a reduction in BD across all types of waste materials, highlighting their potential application in the development of lightweight construction materials. While this property is advantageous for reducing dead loads and enhancing thermal insulation, it is critical to balance these benefits with the need to maintain acceptable levels of strength, durability, and overall performance for structural applications. The observed data supports the idea that material selection and processing temperature play a crucial role in controlling BD and tailoring mix designs to meet specific engineering requirements.

Incorporating waste materials into construction significantly affects their porosity, often increasing the void space within bricks and pellets. For instance, bricks containing TW showed a substantial increase in porosity, with values rising from 16.14% at 0% TW to 28.87% at 10% TW (Hussien et al., 2024). This suggests that the organic nature of TW enhances the porous structure of the bricks, which may improve thermal insulation but reduce overall strength. Similarly, bricks incorporating SCG and TW also exhibited increased porosity. With SCG content rising from 0% to 10%, the porosity increased from 12.27% to 32.62%, highlighting the influence of organic waste in creating more air pockets within the material (Chung et al., 2021). In another study, eggshell powder used in waste glass-based bricks led to a higher porosity in the resulting materials, with the control brick showing a porosity of 9.47%, while the brick containing 5% eggshell powder had a porosity of 17.73%, illustrating how the addition of waste materials can create a more porous structure (Tangboriboon, 2019).

In bricks made with TW, the porosity increased as the TW percentage rose, from 25.1% at 0% TW to 33.3% at 12.5% TW, further confirming the role of organic materials in expanding the internal void spaces of the brick (Ozturk et al., 2019). Similarly, including wine lees (WL) and grape seeds (GS) in clay bricks resulted in a rise in porosity. For WL, the porosity ranged from 37.5% at 10% content to 43.9% at 20% content, while GS-based bricks exhibited even higher values, reaching up to 50.1% at 10% GS (Taurino et al., 2019). This increase in porosity will likely reduce the material’s weight and potentially improve its insulation properties, though it may also affect its mechanical strength. The effect of plastic waste on porosity depends on the type of plastic and the mix design. While hydrophobic and dense plastics like HDPE and LDPE often reduce porosity when combined with other materials such as bottom ash, copper slag, ceramic or foundry sand (Monish et al., 2021; Aneke and Shabangu, 2021), others, such as plastic dust or mixed waste, as done by Subhani et al. (2024) increase the value for porosity.

Thermal conductivity is an essential property for assessing the insulating qualities of building materials. Waste materials frequently help lower thermal conductivity, which is advantageous for energy efficiency when mixed into clay bricks or pellets. Because of their larger porosity and air pockets that act as thermal insulators, organic waste materials have been shown in numerous studies to considerably impact thermal conductivity in construction materials, usually lowering it. The incorporation of paper-based materials also influences the thermal conductivity of bricks. Waste paper and paper sludge (PS) reduce the thermal conductivity of bricks, making them more efficient as thermal insulators. For instance, PS bricks had a thermal conductivity range of 0.396–0.555 W/mK, with higher paper content leading to a lower thermal conductivity (Ospina Salazar et al., 2023) This reduction in thermal conductivity is beneficial for energy-efficient construction, as it helps maintain a stable indoor temperature. Similarly, other studies reported decreased thermal conductivity, ranging from 0.15 W/mK to 0.39 W/mK depending on the paper content and the firing temperature (Goel and Kalamdhad, 2018; Sutcu et al., 2014). Additionally, wine lees (WL) and grape seeds (GS) reduced thermal conductivity in bricks constructed with these components. While GS-based bricks showed even more notable improvements, with values as low as 0.73 W/mK at 10% GS, the thermal conductivity for WL decreased from 1.05 W/mK at 10% content to 0.84 W/mK at 20% content (Taurino et al., 2019). By increasing the bricks’ porosity, these organic waste ingredients improve their insulation properties and slow the pace heat moves through them. The impact of organic waste on thermal conductivity was also noted in clay pellets. The thermal conductivity of clay pellets decreased by adding groundnut shells, coffee grinds, and cork powder. Thermal conductivity decreased from 0.68 W/mK to 0.46 W/mK in pellets containing larger percentages of groundnut shells, demonstrating the organic material’s insulating properties (Cobo-Ceacero et al., 2023). A similar pattern was seen when CP biosolid was added to clay bricks; the thermal conductivity dropped from 1.02 W/mK at 0% CP to 0.92 W/mK at 16% CP, confirming the notion that waste materials might enhance the thermal performance of construction materials (Adazabra et al., 2024). The incorporation of plastic waste into construction bricks significantly impacts thermal conductivity, typically leading to a reduction due to the low thermal conductivity of plastic materials. This property enhances the insulation performance of plastic-based bricks, making them suitable for energy-efficient construction applications (Aneke and Shabangu, 2021; Alaloul et al., 2020). Also, as micro-voids serve as insulating barriers, bricks manufactured with larger percentages of plastic dust exhibit better thermal performance (Idrees et al., 2023).

Water absorption (WA) is another critical factor impacted by paper-based materials. Bricks with higher paper content tend to exhibit increased water absorption, which is linked to the higher porosity of the bricks. For example, bricks with PPRs demonstrated water absorption values ranging from 20% to 35%, depending on the content and curing conditions (Akinwande et al., 2021). Similarly, other studies observed 8% and 37% water absorption rates, with PS content increasing the brick’s porosity and water uptake (Goel and Kalamdhad, 2018; Sarkar et al., 2017). Regarding paper waste, it has been reported that the water absorption in bricks with wastepaper inclusion is much more than that of PMS (Sutcu et al., 2014; Shibib, 2015; Yaras, 2020; Kizinievič et al., 2018a). In the case of food waste-based MSW incorporation, the value generally varied for a range of approximately 8%–15% on inclusion of 10% waste as reported by the study through TW and CP (Adazabra et al., 2024; Ibrahim et al., 2023). Due to plastic’s hydrophobic nature, adding plastic trash to bricks and other building materials considerably lowers water absorption. For instance, bricks manufactured from recycled HDPE or PP plastic waste have remarkably low water absorption rates—0.752% for HDPE and 0.370% for PP (Kulkarni et al., 2022). Similarly, water absorption rates as low as 1.5%–4.9% are achieved by LDPE-based composites with bottom ash, ceramic, or copper slag, significantly lower than traditional clay bricks (Monish et al., 2021). Preserving compatibility with second-class brick standards, including plastic dust as a partial substitute for clay also keeps water absorption within acceptable bounds, with values staying below 20% even at greater plastic dust levels (Idrees et al., 2023). Conversely, leather and tannery result in comparatively less water absorption.

Figure 8 presents the variation in WA with increasing MSW concentration for a range of waste-derived ashes treated at different calcination temperatures. A clear upward trend is observed across nearly all materials, indicating that as the proportion of MSW increases, the WA capacity of composite materials also rises. This increase in WA is largely attributed to the porous and hydrophilic nature of MSW-derived ashes, which contain a high volume of micro-voids, unburnt organic residues, and loosely packed particles. These characteristics promote moisture penetration and retention, thereby increasing the overall water uptake. The most prominent increase is observed in high-density polyethylene ash, where WA exceeds 80% at 50% MSW concentration, highlighting its extremely porous and water-retentive structure. Similarly, tannery sludge ashes, particularly those treated at 900°C and 950°C, show high absorption values, surpassing 40% at higher MSW content. These results suggest that thermally treated organic-rich wastes tend to produce ashes with large pore networks, which significantly elevate capillary absorption. Waste-paper ash and plastic dust also display steep increases, indicating that these materials are less dense and contain higher internal voids, making them more susceptible to water ingress. In contrast, materials such as CPs, PS, and TW ash exhibit more moderate increases in WA with MSW addition. Their lower absorption rates, even at higher MSW percentages, suggest relatively better thermal transformation and possibly denser ash morphology. The behavior of TW ash, particularly at higher treatment temperatures (up to 1250°C), indicates that controlled calcination can improve ash quality by reducing unburnt carbon and decreasing pore connectivity, thus restraining excessive water absorption.

Overall, the rising trend of WA with increasing MSW concentration demonstrates the influence of ash structure and composition on moisture interaction. While elevated WA may enhance internal curing in certain lightweight or non-structural applications, excessive absorption poses challenges in terms of durability, dimensional stability, and long-term strength retention. Therefore, careful optimization of MSW content and calcination conditions is essential when developing sustainable construction composites using waste-derived ashes.

Compressive strength is deemed the most essential quality index for a brick (Shi and Zheng, 2007; Saikia and De Brito, 2012). For normal weather conditions, the minimum compressive strength of brick according to ASTM C62-13a standard is 10.3 MPa. The addition of paper-based admixtures to bricks generally results in lower compressive strength (Tables 1–5). For example, bricks made from a mixture of paper pulp (47.5%–50%) and banana fiber exhibited compressive strength values up to 6.7 MPa, significantly lower than conventional clay bricks (Akinwande et al., 2021). Similarly, other studies found that increasing paper content in brick mixtures led to decreased mechanical strength, with compressive strengths ranging from 2.4 MPa to 36.7 MPa depending on the type and concentration of paper admixture (Yaras, 2020; Sutcu et al., 2023). Notably, certain combinations of paper and other materials, such as fly ash or banana fibers, can partially offset the reduction in strength by enhancing the bonding properties of the matrix. For food waste like TW, the compressive strength tends to decrease as the percentage of waste increases. For example, bricks with 10% TW content exhibited a compressive strength reduction from 29.55 MPa (at 0% TW) to 17.71 MPa (at 10% TW), indicating that the addition of organic waste reduces the brick’s overall load-bearing capacity (Hussien et al., 2024). Conversely, adding waste materials such as eggshell powder (EP) in glass-based bricks increased compressive strength. For example, adding 5% eggshell powder to waste glass-based bricks raised the compressive strength from 25.73 MPa (for the control brick) to 27.83 MPa, representing a modest improvement in strength (Tangboriboon, 2019). The control mix using glass and plastic waste achieves 40 MPa, while the 55% by mass additional of glass reduces it to 22 MPa. A combined mix of 25% glass and 2% plastic achieves a balanced 25 MPa (Zhang et al., 2022) Rauniyar et al. (2024) used polypropylene waste fibers and noted that the strength is maximum at 10% addition of waste, and reported it to be 16.85 MPa, compared to the 13.44 MPa at 5% and 12.62 MPa at 15%. Similarly, an addition of 30% SPW noted a compressive strength of 30% SPW (Aneke and Shabangu, 2021). Overall, plastic waste usage is deemed beneficial for clay brick manufacture.

Figure 9 illustrates the variation in compressive strength of bricks with increasing concentrations of MSW for a variety of waste-derived ashes treated at different temperatures. A general downward trend is observed across all materials, indicating that higher MSW content typically results in reduced compressive strength. This decline can be attributed to several factors, including the reduced binding capacity, poor particle interlocking, and increased porosity introduced by the incorporation of lightweight and organic-rich ash materials. As MSW content rises, the structural matrix becomes less compact and more heterogeneous, leading to a decrease in load-bearing capacity. Among the materials tested, leather buffing dust and high-density polyethylene initially show the highest compressive strengths, with values exceeding 45 MPa and 40 MPa respectively at 0% MSW, primarily reflecting their higher baseline control strengths rather than inherent material advantages. These materials also maintain relatively higher strengths even at elevated MSW contents, which may be attributed to their stronger initial matrix in addition to favorable particle morphology, better bonding characteristics, or partial sintering effects during calcination. In contrast, materials such as oyster mushroom ash and paper sludge exhibit significantly lower compressive strength values from the outset, which decline rapidly with increasing MSW. This suggests that these ashes may lack sufficient pozzolanic reactivity or cohesive properties to form a dense and durable brick matrix. Tannery sludge, across various temperatures (900°C–1000°C), demonstrates a moderately strong initial performance but shows noticeable reductions in compressive strength beyond 20%–30% MSW, indicating limited tolerance to high ash content. Tea waste ash, processed at different calcination temperatures, reveals varied results: while higher temperatures (e.g., 1250°C) slightly improve initial compressive strength, the overall trend still indicates a loss in strength as MSW concentration increases. Similarly, paper mill sludge and paper waste ashes follow a consistent downward pattern, with compressive strength dropping to as low as 10 MPa or below at higher MSW levels.

The normalized strength analysis reveals distinct performance categories when baseline differences are eliminated. High-density polyethylene demonstrates exceptional retention (98% at 10% MSW, 85% at 50% MSW), while most paper-based materials maintain 68%–88% of control strength at 10% MSW content. Organic waste shows moderate performance with tea waste retaining 82%–88% and cassava peels 78%–80% at 10% MSW. Conversely, paper mill sludges exhibit the steepest decline (65%–68% retention at 10% MSW), whereas leather buffing dust uniquely shows initial improvement (110% at 10% MSW) before declining. This normalized comparison eliminates baseline bias and provides unbiased material performance ranking essential for practical engineering applications.

The observed reductions in compressive strength highlight the critical need for optimizing MSW replacement levels to balance sustainability with structural performance. While incorporating waste materials supports circular economy goals and reduces the environmental footprint of construction materials, excessive MSW content can compromise the mechanical integrity of the bricks. Therefore, establishing optimal replacement thresholds and refining calcination conditions are essential for ensuring that waste-derived bricks meet the required standards for load-bearing applications.

Fibrous additives like banana fiber or cellulose improve tensile properties by reinforcing the material matrix. However, porosity limits tensile performance at higher waste levels. Therefore, more porous materials often lead to less tensile strength. On the addition of 0%–12% of deinking PS (DPS), the value for tensile strength is not reported. Still, the characteristic indicated by increasing porosity reflects that the value for tensile strength will be depleted (Makni et al., 2024). At 10% TW content, the tensile strength of bricks decreased significantly from 1.45 MPa (for control bricks) to 0.85 MPa, reflecting the negative impact of TW on the material’s ability to resist tensile forces (Hussien et al., 2024). This reduction is attributed to introducing porous spaces within the material, which weakens the overall matrix and makes it more susceptible to stretching and pulling. Similarly, SCG also causes a decrease in tensile strength. Bricks with 10% SCG content exhibited a reduction from 1.66 MPa to 1.10 MPa (Chung et al., 2021). The addition of such waste materials introduces micro-voids, which reduce the material’s resistance to stretching forces, thus lowering its overall tensile strength. On the other hand, certain inorganic waste materials, such as EP, can enhance tensile strength when mixed with other construction materials. Adding 5% eggshell powder to waste glass-based bricks increased tensile strength from 1.60 MPa to 1.80 MPa (Tangboriboon, 2019). Meanwhile, the variability of tensile strength for clay bricks depends on the plastic type used and the particle size. PET and PU mixed waste decreased the tensile strength of clay bricks as noted by Alaloul et al. (2020). Conversely, the tensile strength increased from 7.36 MPa to 9.51 MPa, increasing the scrap plastic waste (SPW) from 20% to 30%. Subhani et al. (2024) conducted a study investigating the mixture of plastic waste, including HDPE, PET, and LDPE. They inferred that the value of tensile strength is about 1.35 times that of traditional clay bricks. This result is further supported by the results drawn from research on LDPE by Arun Solomon et al. (2023).

Overall, as shown in Tables 1–5, the impact of MSW-derived additives on tensile strength varies based on material type, porosity, and bonding characteristics. Organic wastes such as TW and spent coffee grounds (SCG) generally reduce tensile strength due to the formation of internal voids and weak matrix cohesion, with declines reaching up to 40%–50% at higher replacement levels. Paper-based residues, while not always directly measured for tensile strength, exhibit similar tendencies due to increased porosity. In contrast, certain inorganic additives like eggshell powder and scrap plastic waste have demonstrated improvements in tensile properties. For example, bricks with eggshell powder showed slight increases in tensile strength, while those with optimized levels of plastic waste exhibited significantly higher values compared to traditional clay bricks. These outcomes highlight the critical role of material compatibility, particle structure, and bonding efficiency in determining tensile performance in MSW-integrated masonry units.

In most cases, waste materials improve flexural strength at lower contents by enhancing bonding and densification. Excessive content often leads to porosity increases, reducing bending resistance. On addition of recycled PS (RPS) and expanded perlite (EP), the value of flexural strength ranged from 10.2 MPa to 34.6 MPa, depending on firing conditions. The combination of RPS and EP provides strength and reduces weight while maintaining flexibility (Sutcu et al., 2023). The flexural strength value ranges from 2 MPa to 4 MPa, with higher PPR (PPR) contents moderately increasing bending strength due to the material’s elasticity (Muñoz et al., 2020a). Bricks made with TW exhibited a reduction in flexural strength as the percentage of TW increased. At 10% TW, the flexural strength decreased from 3.62 MPa (for the control bricks) to 2.12 MPa (Hussien et al., 2024). This reduction is consistent with the observed decrease in compressive and tensile strength, as the TW particles likely introduce voids and disrupt the overall bond strength of the material, making it more prone to bending and failure under load. In a similar study, bricks made with SCG showed a decrease in flexural strength from 4.20 MPa (at 0% SCG) to 2.95 MPa (at 10% SCG), which is again consistent with the negative impact of organic waste on the flexural properties of the material (Chung et al., 2021). The presence of SCG in the brick mix likely reduces the overall cohesion between the particles, which weakens the material’s resistance to bending. On the other hand, some inorganic wastes can have a strengthening effect. For instance, bricks containing EP demonstrated improved flexural strength. Adding 5% EP to waste glass-based bricks increased the flexural strength from 3.40 MPa to 3.90 MPa (Tangboriboon, 2019). This improvement can be attributed to the reinforcing nature of eggshell powder, which enhances the material’s structural integrity and helps resist bending forces. On the other hand, the mixture of different types of plastic waste (HDPE, LDPE, and PET) showed approximately double the value of conventional clay bricks, i.e., 8 MPa.

Generally, flexural strength in MSW-incorporated masonry units (Tables 1–5) generally benefits from low to moderate waste content, which can enhance bonding, matrix cohesion, and elasticity. Materials like recycled PS, expanded perlite, and PPRs have demonstrated moderate to significant increases in flexural strength, especially under controlled firing conditions. However, organic additives such as TW and spent coffee grounds tend to reduce flexural performance at higher concentrations due to the introduction of voids and disruption in particle bonding. This reduction is often in line with decreases observed in compressive and tensile strengths, reflecting the overall weakening of the structural matrix. In contrast, inorganic additives like eggshell powder and mixed plastic wastes (e.g., HDPE, LDPE, PET) have shown improved flexural strength, with some formulations achieving values nearly double that of conventional clay bricks. These results suggest that the type, proportion, and physical interaction of the waste material with the binder matrix are critical in determining flexural performance outcomes.