Development of lightweight roof tiles using spent mushroom material: enhancing thermal insulation and sustainability

2.1 Materials

Kaolin clay was collected from Aswan, Egypt. This material, classified as a plastic mineral, contributes to the development of strength in green tiles due to its kaolinite content. SMW was sourced from local mushroom cultivation facilities in Cairo, Egypt. The chemical composition of the raw materials was analyzed using an X-ray fluorescence (XRF) system (MXF 2400, Shimadzu) and an X-ray diffraction (XRD) system (Rigaku Co., Japan, with a wavelength of 1.54056 Å for Cu Kα radiation). The particle size distribution of the materials was determined using standard ASTM C136 sieve analysis [49].

2.2 Sample preparation

After drying at 110 °C for 24 h, the SMW and kaolin clay were ground into fine powders using a laboratory ball mill. The ball mill, with D = 0.48 m and L = 0.8 m, was operated at 400 rpm for 4 h. Steel balls with diameters ranging from 25 to 125 mm were used as the grinding medium with a filling ratio of 0.2. The ground materials were sieved through a Tyler sieve modified to comply with ASTM D422 standards [50]. Both materials were mixed with water in a laboratory planetary mixer to achieve the desired consistency. Different percentages of SMW (5–25 %) were thoroughly mixed with clay using 18 % water for better compaction. The resulting pastes were shaped into specimens (150 × 30 × 30 mm³) using steel molds and subjected to uniaxial pressure of 10 MPa. The formed samples were left at room temperature for 24 h before being dried at 110 °C overnight to ensure complete removal of water. The dried samples were fired at a rate of 10 °C min⁻¹ at four different temperatures: 1,000, 1,050, 1,100, and 1,150 °C. The firing process involved three stages (Figure 1) [1]: heating to 400 °C and holding for 1 h to oxidize organic matter [2]; heating to 700 °C and holding for 1 h for clay dehydroxylation; and [3] heating to the final target temperature in a muffle furnace and holding for 1 h. Each experiment was conducted using three specimens per sample, with average results reported. The experiment involved preparing six tile formulations by partially replacing clay with 0–25 wt% SMW. Each mixture was designated according to its replacement level and fired at four different temperatures (1,000–1,150 °C) to evaluate the influence of SMW content and sintering temperature on the final properties of the tiles. The sample designations and corresponding firing regimes are summarized in Table 1.

Figure 1:

Schematic representation of the three-stage firing regime used for SMW-incorporated clay tiles.

2.3 Characterization of tile samples

The dimensional stability of fired specimens was evaluated by measuring linear shrinkage in accordance with ASTM C326 [51]. Additionally, the fired specimens were analyzed following the BS EN ISO 10545 to assess the extent of densification and structural integrity [52]. The mechanical properties, including breaking strength and modulus of rupture (MOR), were determined for fired samples in compliance with ASTM C1167-22 [53]. Furthermore, a comprehensive set of durability tests, including permeability and efflorescence resistance, was conducted in accordance with ASTM C1167 [53]. This standard categorizes clay roof tiles into three grades based on their weathering index (WI), which is a crucial parameter in evaluating their long-term performance under varying climatic conditions. According to ASTM C1167, roof tiles are classified as follows: Grade 1 (WI > 500), suitable for severe weathering conditions; Grade 2 (WI between 50 and 500), appropriate for moderate weathering; and Grade 3 (WI < 50), recommended for areas with minimal weathering exposure.

3.1 Characterization of raw materials

Figure 2 illustrates XRD patterns of the clay used in this study. The analysis reveals that silica (SiO₂) and kaolinite are the dominant crystalline phases present in the clay samples. Additionally, smaller quantities of hematite, anatase, and illite were detected, further contributing to the clay’s mineralogical composition. These findings align with the expected mineralogical characteristics of clays typically used in ceramic applications. The chemical composition of the raw materials, determined through XRF analysis, is summarized in Table 2. The results indicate that SiO₂ and Al₂O₃ are the primary oxides present in the clay, with SiO₂ content of 48.931 %, and Al₂O₃ content of 32.906 %. These values fall within the recommended ranges for clays suitable for tile and brick production [54]. The high SiO₂ content contributes to the structural stability and thermal resistance of the final products, while Al₂O₃ plays a critical role in enhancing mechanical strength and densification during the firing process. The SMW used as a replacement in this study is grown on the SiO₂-rich rice straw as a substrate. As a result, the SMW exhibits a high SiO₂ content, which is a significant factor influencing the properties of the manufactured tiles. Furthermore, the SMW is characterized by a high loss on ignition (LOI), indicative of its substantial organic matter content. This organic content contributes to pore formation during the firing process, which can influence the thermal and mechanical properties of the tiles. The mineralogical and chemical compositions of the clay and SMW suggest their potential to produce tiles with desirable thermal and mechanical properties. Figure 3 illustrates the particle size distribution of the raw materials, highlighting the variation in their median particle sizes (D50). The results reveal that the median particle size for clay is 0.24 mm, whereas for SMW, it is 0.37 mm. This indicates that clay has a finer particle size compared to SMW. Hence, due to the presence of SMW particles in the mixture, the mixture would become more porous. The powder densities of the clay and SMW were found to be 2.56 g/cm³ and 0.565 g/cm^3, respectively. As the specific gravity of SMW is less than that of clay, replacing clay with SMW would help to produce lighter roof tiles compared to conventional roof tiles. Figure 4 shows a photo image of the manufactured tiles.

Figure 2:

XRD of clay.

Figure 3:

Particle size distribution of clay and SMW.

Figure 4:

Photo-image of the manufactured tiles.

The thermogravimetric (TG) and derivative thermogravimetric (DTG) profiles of the SMW are presented in Figure 5. The thermal decomposition progresses through several distinct stages typical of lignocellulosic biomass, extending from room temperature up to approximately 950 °C. The total recorded mass loss was approximately 69.6 %, leaving a 30.4 % char residue. The first stage (50–200 °C) is characterized by an initial 14.8 % weight reduction with a DTG peak near 91 °C (−1.96 % min⁻¹). This corresponds to the evaporation of both free and bound moisture, along with the release of light organic volatiles. The broad DTG peak suggests a gradual loss of physically adsorbed water retained in the porous matrix [31], 55], 56]. The second stage (200–600 °C) represents the principal decomposition zone, accounting for approximately 61 % of the total mass loss and exhibiting two main sub-stages. The second stage is divided into two regions: First region (200–400 °C), the most intense DTG peak appears at 314 °C (−3.28 % min⁻¹), corresponding to the thermal depolymerization and breakdown of hemicellulose and cellulose into gaseous products such as CO₂, CO, CH₄, and low-molecular-weight volatiles. Second region (400–600 °C), a smaller DTG peak at 459 °C (−0.36 % min⁻¹) is attributed to the slower degradation of lignin and the formation of more condensed aromatic carbon structures. The overlapping nature of these two peaks reflects the simultaneous decomposition of different biopolymers, a behavior also observed in previous studies on lignocellulosic residues [56]. The third stage (600–950 °C) involves an additional 3.5 % weight reduction with a weak DTG shoulder around 696 °C (−0.77 % min⁻¹), primarily corresponding to the oxidation of fixed carbon and decarbonation of mineral carbonates. Beyond this temperature, the TG curve becomes nearly horizontal, indicating the formation of a thermally stable carbonaceous residue.

Figure 5:

TG–DTG results of SMW.

3.2 Linear drying shrinkage (LDS) and linear firing shrinkage (LFS)

Figure 6 illustrates the variation in LDS and LFS of tiles incorporating different proportions of SMW. As shown in Figure 6a, the LDS values increased progressively from 4.21 % for the control sample (SMW-0) to 7.28 % for the SMW-25 sample. This rise can be attributed to the higher organic and volatile content of SMW, which enhances shrinkage during the drying stage due to the evaporation of absorbed moisture and partial decomposition of organic matter [57]. SMW undergoes thermal degradation and loses structural integrity upon heating, unlike natural clays, which retain their crystalline framework and exhibit minimal volumetric change [58]. Similar shrinkage behavior has been reported when other agro-wastes were introduced into clay systems; for instance, the replacement of 7 % potato peel powder resulted in a 45.95 % increase in LDS [59]. In this study, LDS improved to 15.6 % at SMW-5, 29.5 % at SMW-10, 36.2 % at SMW-15, and 46.5 % at SMW-20, demonstrating a comparable trend in pore formation and shrinkage induction. Figure 6b presents the influence of firing temperature (1,000–1,150 °C) on LFS for all compositions. At 1,000 °C, the LFS values ranged from 1.95 % (SMW-0) to 7.02 % (SMW-25). Increasing the firing temperature to 1,150 °C resulted in higher shrinkage values of 4.96 % and 8.26 %, respectively. The results confirm that both firing temperature and SMW content strongly affect volumetric contraction. The increase in LFS with temperature is primarily due to enhanced sintering and partial vitrification, which promote particle rearrangement and densification. Meanwhile, the combustion of organic constituents in SMW produces gaseous by-products (mainly CO₂ and H₂O), leading to pore formation and additional shrinkage. Excessive shrinkage, however, may cause microcracking or dimensional distortion in the fired body if not adequately controlled. According to ASTM C326, maintaining the LFS below 8 % ensures the structural integrity and mechanical reliability of ceramic tiles. In the present study, all samples except the SMW-25 composition fired at 1,150 °C satisfied this criterion, with a marginally exceeded limit of 0.2 %. The obtained shrinkage values confirm that SMW substitution up to 20 % can produce dimensionally stable tiles suitable for industrial manufacture. The shrinkage behavior provides critical insight into optimizing extrusion die design and firing schedules, ensuring dimensional accuracy and minimizing production defects.

Figure 6:

Drying and firing shrinkage behavior of clay roof tiles incorporating SMW. (a) Linear drying shrinkage (LDS, %) and (b) linear firing shrinkage (LFS, %) of clay tiles containing varying proportions of SMW and fired at 1,000 °C, 1,050 °C, 1,100 °C, and 1,150 °C.

3.3 Bulk density (BD) of tiles

Figure 7 illustrates that the incorporation of SMW significantly affects the BD and apparent porosity (AP) of the manufactured tiles, resulting in the formation of lightweight structures. The reduction in BD is primarily attributed to the generation of pores within the tile matrix, caused by the decomposition of organic matter and mineral hydrates in SMW during the firing process [60]. Additionally, the inherently lower specific gravity of SMW compared to clay contributes to the reduced overall density of the fired specimens. As illustrated in Figure 7a, BD exhibits an inverse relationship with SMW content. The control tile (SMW-0) displayed the highest BD of approximately 2,204 kg/m³, while the BD decreased progressively to 1830 kg/m³ at SMW-5 and further to 1,390 kg/m³ at SMW-25 after firing at 1,150 °C. This reduction is closely linked to the volatilization of organic components and the release of gases during firing, which create internal voids and expand the pore network [59], 60]. The coarse SMW particles also contribute to the formation of larger pores due to their poor compaction within the clay matrix. The influence of firing temperature on BD is also evident. At SMW-25, increasing the firing temperature from 1,000 °C to 1,150 °C resulted in a modest increase in BD (∼8 %), which can be attributed to partial vitrification and the formation of a glassy phase. These mechanisms enhance viscous flow and pore closure, thereby partially compensating for the porosity induced by organic burnout. Similar densification effects have been reported for clay ceramics containing organic or agro-waste additives fired above 1,100 °C [61]. According to ASTM C90, materials with BD values below 1,680 kg/m³ are classified as lightweight. The developed tiles achieved this benchmark at higher SMW contents, demonstrating their potential as lightweight roofing materials. Such tiles offer practical advantages – reducing structural dead load, transportation costs, and environmental footprint – while improving ease of handling and installation. As shown in Figure 7b, AP increased consistently with higher SMW contents and firing temperatures. The elevated LOI of SMW (64.99 %) confirms its high organic fraction, which, upon oxidation, leads to increased pore formation [62]. The direct correlation between AP and BD, as depicted in Figure 7c, confirms that higher porosity is associated with lower density. These findings align with those of Hall and Hamilton [51], who reported that increased porosity in stone and brick materials corresponds to a reduced bulk density.

Figure 7:

Effect of SMW content and firing temperature on (a) bulk density (kg m⁻³), (b) apparent porosity (%), and (c) their correlation for clay tiles incorporating 0–25 wt% SMW and fired at temperatures ranging from 1,000 °C to 1,150 °C.

3.4 Water absorption (WA) and apparent porosity (AP)

Figure 8a reveals that increasing the proportion of SMW in clay mixtures significantly enhances the AP of the fired tiles, which in turn leads to higher WA values. WA is a critical durability indicator for tiles, reflecting the degree of pore connectivity and the matrix’s capacity to retain water after firing. The observed linear increase in WA with SMW substitution confirms that SMW acts as an effective pore-forming [60]. At firing temperatures of 1,000 °C, 1,050 °C, 1,100 °C, and 1,150 °C, the WA increased progressively with higher SMW contents. The maximum WA values reached 37.8 %, 35.6 %, 29.8 %, and 25.6 % for SMW-25, while the corresponding control samples (SMW-0) exhibited much lower WA values of 11.55 %, 10.03 %, 9.2 %, and 8.3 %, respectively. This nearly twofold increase in WA is primarily associated with the enhanced formation of open pores. The correlation between AP and WA, as shown in Figure 8b, further validates this mechanism, demonstrating that higher porosity directly contributes to elevated water uptake. Similar relationships have been reported by Farhana et al. [63], who observed that reducing porosity in geopolymer-based blocks from 11.95 % to 3.77 % decreased WA from 4.65 % to 3.81 %. Likewise, Kizinievič et al. [64] found that the incorporation of 20 % paper sludge increased WA to 26.3 % relative to 15 % in control bricks. Other studies also confirm this trend – WA increased from 7.2 % to 20.9 % with rising tannery sludge content [65], from 0 % to 22.5 % with 17 % spent mushroom compost [66], and up to 32.5 % with 10 % olive mill waste (OMW) [67]. Firing temperature also plays a decisive role in controlling porosity and WA. Higher sintering temperatures promote partial vitrification and the formation of a glassy phase, which helps seal smaller pores and reduce capillary water absorption. Consequently, tiles fired at 1,150 °C consistently showed lower WA compared to those fired at lower temperatures. However, excessive WA beyond standard limits may compromise the long-term durability of ceramic tiles. According to ASTM C1167, the maximum allowable WA is 13 % for Grade 2 (moderate weathering) and 15 % for Grade 3 (negligible weathering). In the present study, tiles with 5–10 % SMW fired at 1,100–1,150 °C met the Grade 2 standard, demonstrating practical viability for moderate climates [68]. Although higher porosity and WA may reduce mechanical strength, they simultaneously offer functional advantages. The presence of interconnected pores enhances thermal insulation by entrapping air within the ceramic body, thereby improving the energy efficiency of buildings. Additionally, the micro-porous network allows controlled vapor diffusion through the tiles, which can mitigate crack propagation in humid environments [69], 70]. Thus, optimizing the SMW content balances both durability and thermal performance in lightweight, sustainable ceramic roofing materials.

Figure 8:

Effect of spent mushroom waste (SMW) content and firing temperature on water absorption behavior of clay roof tiles. (a) Water absorption (%) of clay tiles incorporating 0–25 wt% SMW and fired at temperatures ranging from 1,000 °C to 1,150 °C, and (b) correlation between water absorption and apparent porosity (%).

3.5 Transverse breaking strength (TBS) and modulus of rupture (MOR)

Figure 9a demonstrates that incorporating SMW into clay matrices considerably affects the TBS of the fired tiles. While the SMW addition enhances several physical and thermal properties, it tends to reduce mechanical strength due to its influence on porosity and microstructural integrity. At a firing temperature of 1,000 °C, the TBS decreased from 1,297 N for the control sample (SMW-0) to 1,102 N, 1,026 N, 943 N, 847 N, and 623 N for 5 %, 10 %, 15 %, 20 %, and 25 % SMW, respectively. This reduction can be attributed to the increase in AP. The produced voids interrupt particle contact and reduce the effective load-bearing cross-section. Additionally, the decrease in clay proportion weakens the crystalline mineral framework, thereby compromising the material’s densification and overall strength [71]. Raising the firing temperature mitigated these losses by promoting sintering and the formation of a glass phase. At 1,050 °C, the TBS values increased to 1,945 N, 1,486 N, 1,210 N, 1,165 N, 1,023 N, and 823 N, respectively, while further firing at 1,100 °C and 1,150 °C improved strength to 2,662 N–3,067 N for the SMW-0 and 923 N–987 N for the SMW-25 samples. The enhancement results from the viscous flow of molten phases, which densify the matrix and seal interparticle voids, thus compensating for porosity-induced weakening. The observed mechanical performance is consistent with previous findings for other agro-waste and industrial additives. For instance, adding recycled cigarette butts reduced the strength by 88 % (from 25.65 MPa to 3.00 MPa) [72]; the incorporation of RHA and wood ash decreased the strength to 13.5 MPa and 17.5 MPa at a 30 % substitution due to porosity development [72]. Similarly, integrating 17 % spent mushroom compost reduced brick strength by 65 %, while 10 % olive-mill waste decreased compressive strength from 36.9 MPa to 10.26 MPa [66], 67]. According to ASTM C1167, the minimum TBS requirement for Grade 2–3 roofing tiles (mild to moderate weathering; weather index <500) is 890 N [73]. All SMW-substituted tiles in this study satisfied this criterion at all firing temperatures, except for the SMW-25 composition fired at 1,000 °C. Thus, while SMW addition reduces strength through porosity enhancement, proper control of firing temperature can effectively counterbalance this effect, enabling acceptable performance for construction applications. A similar trend was observed for MOR values (Figure 9b), which also declined with increased SMW incorporation due to reduced particle bonding and increased pore concentration. Although no explicit standard for MOR exists, the results remain within the acceptable range for lightweight ceramic tiles, affirming the feasibility of SMW as a sustainable additive in fired clay products.

Figure 9:

Effect of SMW content and firing temperature on the mechanical properties of clay roof tiles. (a) Transverse breaking strength (N) and (b) modulus of rupture (MPa) of tiles containing 0–25 wt% % SMW and fired at temperatures ranging from 1,000 °C to 1,150 °C.

Table 3 summarizes the mechanical performance of roof tiles made from various industrial and agricultural waste materials compared with the present SMW-based tiles. Among the reported systems, tannery waste and RHA ceramics achieved moderate strengths of 956 N and 1,136.55 N, respectively, when fired between 850 °C and 1,150 °C [10], 74]. A combined mixture of RHA and CS produced an even higher strength of 1,519 N at 1,000 °C due to synergistic SiO₂–Al₂O₃ interactions that promoted liquid-phase sintering [8]. In contrast, highly organic wastes such as corncob powder and bagasse ash yielded much lower strengths (0.23–3.5 MPa) [27], 75], primarily because their excessive volatile content generated a porous, poorly bonded structure. Similarly, 7.5 wt% % RHA yielded only 0.67 MPa, attributed to incomplete vitrification and high residual porosity [64]. In the present work, the SMW-incorporated tiles reached a maximum TBS of 987 N at SMW-25 and 1,150 °C, placing them on par with or above the performance of several other waste-based ceramics. This mechanical strength, combined with reduced bulk density and improved thermal insulation, demonstrates that SMW provides a balanced performance profile suitable for lightweight construction. The relatively high SiO₂ content and controlled organic burnout of SMW contribute to a well-graded porous network that retains sufficient structural cohesion while enhancing heat-insulating efficiency. Furthermore, the results confirm that all SMW-incorporated tiles, except for the SMW-20 and SMW-25 samples, fired at 1,000 °C, comply with ASTM C1167 breaking strength requirements for Grade 2–3 roofing tiles. This indicates that the developed SMW-based ceramics not only achieve standard mechanical strength but also deliver superior thermal performance.

3.6 Microstructure analysis

Figure 10 presents the SEM micrographs of tile specimens containing 0 %, 10 %, and 20 % SMW fired at 1,150 °C. The control specimen (Figure 10a, 0 % SMW) exhibits a dense and homogeneous microstructure with minimal micro-voids, indicating an average pore size of approximately 4.8 µm, which suggests efficient sintering and particle bonding. The specimen with 10 % SMW (Figure 10b) demonstrates a noticeable increase in pore development, forming a network of interconnected pores with an average diameter of about 15.4 µm. This increase results from the thermal decomposition of organic matter within SMW during firing, which releases gases and disrupts particle packing, thereby increasing the void fraction. At 20 % SMW (Figure 10c), the surface morphology reveals a highly porous and irregular structure, characterized by large and unevenly distributed pores with an average size of approximately 28.7 µm. The greater AP and pore size reflect the intensified burnout of the organic fraction and reduced clay content, leading to diminished particle densification. These microstructural features align with the AP measurements, confirming that higher SMW contents generate more voids within the matrix. The increased AP improves thermal insulation due to the presence of trapped air pockets, but reduces mechanical strength due to weaker interparticle bonding. Hence, controlled incorporation of SMW can be effectively utilized to produce lightweight and thermally efficient ceramic roof tiles [76], 77].

Figure 10:

SEM images of (a) 0 % SMW, (b) 10 % SMW, and (c) 20 % SMW tile specimens fired at 1,150 °C.

3.7 Thermal conductivity (TC)

Figure 11 illustrates the TC behavior of the fired tile specimens containing various proportions of SMW. The control sample (SMW-0) fired at 1,150 °C exhibited the highest TC value (0.95 W m⁻¹ K⁻¹), reflecting its dense and compact microstructure with minimal air-filled pores. Increasing the firing temperature enhances sintering and densification, thereby decreasing AP and improving heat transfer. However, progressive addition of SMW markedly reduced the TC of the tiles due to pore generation during firing. As the organic matter in SMW decomposes, volatile gases evolve, creating interconnected voids that interrupt heat-transfer pathways. Consequently, TC declined to 0.47, 0.293, 0.267, and 0.237 W m⁻¹ K⁻¹ for SMW-10, SMW-15, SMW-20, and SMW-25 substitutions, respectively, corresponding to reductions of 50–75 % compared with the control (Table 4). This trend agrees with earlier findings for other bio-waste additives: the TC of bricks decreased from 0.638 W m⁻¹ K⁻¹ to 0.436 W m⁻¹ K⁻¹ upon the inclusion of 10 % olive-mill waste [67]; from 0.738 to 0.208 W m⁻¹ K⁻¹ with 17 % vine-shoot waste [66]; and by 61 % when sunflower-seed cake and wheat-straw were incorporated [78]. Binici et al. [79] further demonstrated that fiber-reinforced bricks reduced indoor summer temperatures by 56.3 % and improved winter heat retention by 41.5 %, emphasizing the energy-saving potential of porous composites. The observed linear correlation between porosity and TC is attributed to phonon-scattering mechanisms. In crystalline solids, heat transfer primarily occurs through lattice vibrations (phonons). The presence of air-filled pores introduces numerous interfaces that shorten the phonon mean free path, thereby impeding the propagation of energy. The firing temperature also influences pore evolution. Higher sintering temperatures (>1,100 °C) promote viscous flow and partial glass-phase formation, which can seal micro-pores and locally enhance TC. Nevertheless, the overall effect of SMW addition dominates, resulting in reduced TC values at all substitution levels. From an energy-efficiency standpoint, lowering the TC of roofing materials directly reduces conductive heat gain through building envelopes.

Figure 11:

Thermal conductivity of SMW-incorporated tile specimens fired at 1,150 °C.

3.8 Specific heat (C_p)

Figure 12 shows the relationship between C_p and AP of the fired tiles. The results reveal an inverse relationship between C_p and SMW content: as SMW incorporation increases, the material’s ability to store heat decreases. The control specimen (SMW-0) exhibited the highest C_p (2.2 MJ m⁻³ K⁻¹), whereas tiles containing 5–25 % SMW showed progressive declines of 18.4 %, 26.7 %, 35.3 %, 40.6 %, and 44.8 %, respectively. This decline stems from the formation of a porous network that lowers the solid-phase volume fraction and thermal mass of the tiles. Because air has a negligible heat-storage capacity compared with the ceramic matrix, the overall volumetric heat capacity decreases as porosity rises. Consequently, while SMW addition improves insulation by reducing TC, it simultaneously diminishes thermal inertia – an effect that should be considered for applications requiring delayed heat release. The negative correlation between C_p and porosity (Figure 11) confirms that increased void volume reduces the effective number of phonon vibration modes capable of storing thermal energy.

Figure 12:

Relationship between specific heat and apparent porosity of tiles containing 0–25 % SMW.

3.9 Permeability

The water permeability of the fired roof tiles was evaluated according to the ASTM C1167 standard to determine their resistance to water penetration [53]. The results revealed that, after 24 h of testing, no water droplets were observed on the underside of tiles containing up to SMW-20 fired at 1,150 °C. This confirms that all specimens – irrespective of SMW substitution level – successfully satisfied the standard’s permeability criteria. Although the incorporation of SMW into the clay matrix increases the overall pore volume, as discussed in the microstructural and porosity analyses, the resulting pores are primarily closed or discontinuous in nature as shown in SEM images. Such a microstructure effectively inhibits the capillary transport of water through the tile body. Moreover, the formation of a partial vitrified surface layer during firing at 1,150 °C further enhances water resistance by sealing surface pores and limiting liquid infiltration. Therefore, the substitution of clay with up to 20 % SMW does not compromise the waterproofing performance of the tiles. On the contrary, it maintains structural integrity and functional durability under wet conditions. These findings indicate that SMW-modified tiles exhibit excellent water-resistance characteristics, while offering environmental and thermal advantages, making them highly suitable for sustainable roofing applications in humid or rainy climates.

3.10 Efflorescence test

The efflorescence resistance of the fired roof tiles was evaluated in accordance with ASTM C67 across all substitution levels of SMW (0–20 wt%) and firing temperatures ranging from 1,000 °C to 1,150 °C [54]. The test results confirmed that no visible efflorescence appeared on the surface of any specimens, regardless of SMW content or firing temperature. This absence of salt deposition indicates that the concentration of soluble alkali and alkaline-earth salts within both the clay and SMW phases is too low to permit migration to the surface during drying. Furthermore, the stable mineral composition of SMW and the partial vitrification achieved at higher firing temperatures likely immobilize these ionic species within the ceramic matrix. Such immobilization restricts the formation of capillary networks that would otherwise facilitate moisture-driven salt transport. The results, therefore, confirm the chemical stability and microstructural integrity of the developed tiles. The lack of efflorescence also suggests that the SMW additive does not introduce undesirable soluble compounds, ensuring that the tiles retain their aesthetic quality and long-term durability under humid or weather-exposed conditions. Hence, the SMW-incorporated roof tiles satisfy the efflorescence resistance requirements for commercial and architectural applications, reinforcing their suitability for sustainable building construction.

3.11 Environmental impact

The embodied impact encompasses the total energy and greenhouse gas emissions associated with the extraction, transportation, processing, and thermal manufacturing of raw materials. Conventional fired clay tiles typically exhibit embodied CO₂ emissions in the range of 0.40–0.50 kg CO₂ eq/kg, consistent with the reference value of 0.46 kg CO₂ eq/kg adopted in this study [80]. Experimental results revealed that substituting 25 wt% of SMW reduced the tile bulk density from 2,204.67 kg/m³ to 1,390.88 kg/m³, corresponding to a 36.9 % decrease in bulk density. Given that embodied CO₂ and embodied energy scale approximately linearly with material mass, this reduction directly translates to a similar saving in embodied impact per unit volume. If the waste-based SMW is treated as a low- or near-zero-carbon input – a standard assumption in waste valorization frameworks – the embodied CO₂ per cubic meter of the developed tile can be estimated as follows:

CO₂(conventional) = 0.46 × 2,204.67 = 1,014.1 kg CO₂/m³

CO₂(SMW tile) = 0.46 × 1,390.88 = 639.8 kg CO₂/m³

Thus, the embodied CO₂ reduction equals approximately 37 % on a volumetric basis.

Similarly, using the reference embodied energy of 6.6 MJ/kg [81], the total embodied energy decreases from 14,550 MJ/m³ (for conventional tiles) to 9,180 MJ/m³ (for SMW-based tiles), also indicating a 37 % reduction.