Valorizing waste materials as biomass fillers in the production of asphalt mixtures

Environmental wastes are contaminants that unfavorably pollute our surroundings, partly or wholly, owing to economic activity, consumption and population growth (USEPA, 2024). Almost every person, industry and human activity on the planet produces waste. Waste is produced in various forms: sewage sludge, construction and demolition debris, hazardous and non-hazardous industrial waste, medical waste, radioactive waste, agricultural and animal waste and waste from the exploitation and mining of fossil fuels (USEPA, 2024). If not properly managed, environmental wastes harm the air, water and land qualities. The ever-expanding growth of diverse sectors has resulted in the constant exploitation of natural resources and the massive production of solid wastes. The global generation of solid waste is projected to rise to 2.2 billion tons in 2025 from the annual generation of 1.30 billion tons in 2012 (Daniel and Perinaz, 2012). In addition to environmental considerations, researchers and decision-makers are using waste and secondary materials instead of conventional construction materials because of the rising cost of virgin materials and the steadily depleting natural resource base. Therefore, wastes must be properly managed once generated through reuse, recycling, storage, treatment and energy recovery (USEPA, 2024).

Conventionally, limestone powder (LP), pulverized quarry dust (PQD) and Portland Cement (PC) are used as mineral fillers in asphalt mixtures (Choudhary et al., 2018). However, limestone, shale and clay are prepared, pulverized and calcined at 1200°C–1450°C to produce powders (Ige et al., 2024). Additionally, PC manufacturing contributes 2%–8% of global energy consumption and generates about one ton of greenhouse gas emissions per ton (Ige et al., 2024). Large-scale quarry dust (QD) production from quarries, aggregates and decorative stone industries also poses a serious environmental threat (Choudhary et al., 2018). Therefore, valorizing waste materials as substitute fillers is crucial while making asphalt mixtures. Valorization helps conserve natural resources by reducing the need for new materials, which translates to substantial savings in aggregate resources, especially during supply interruptions. The use of recycled waste materials in construction works is beneficial, reducing the amount of waste materials requiring disposal, providing construction materials with sufficient savings and promoting environmental friendliness in pavement works (Arbelaez Perez et al., 2022; Oyebisi and Alomayri, 2022).

Waste materials are valorized as alternative mineral fillers to conventional fillers. Reusing waste in pavement construction is one of the best strategies to increase sustainability because it has proven to be environmentally compliant under economic limits without compromising technical or societal goals. Biomass wastes such as shells, rice husks, oil palm leaves, bamboo leaves and coconut husks have recently been recycled and used as alternative fillers in asphalt mixture production, exhibiting promising results. For instance, studies on the possible use of two biomass ashes, date seed ash (DSA) and rice husk ash (RHA), as filler materials in hot mix asphalt (HMA) at 0–100 Wt. % of conventional filler (stone dust) revealed greater stiffness modulus and stability with 100 Wt. % DSA and 75Wt. % RHA than the control mixture (Tahami et al., 2018). Additionally, adding biomass ashes increased the mixtures’ thermal sensitivity and the adhesive force between aggregates and asphalt binder, improving the HMA’s resilience to rutting and fatigue life (Tahami et al., 2018). The use of wood ash as an alternative biomass filler at 50 Wt. % replacement with conventional mineral filler (limestone powder) improved asphalt’s tensile strength, plastic deformation resistance and water action resistance (Dimter et al., 2021). Using bamboo leaf ash (BLA) as a biomass filler in tonnage of asphalt concrete production is 4.35% and 4.13% cheaper for wearing and binder courses than control filler (quarry dust) (Osuolale et al., 2023). However, the moisture resistance, fatigue and resilience moduli of the control filler (quarry dust) were 15%, 20% and 25% higher than BLA (Osuolale et al., 2023). Choudhary et al. (2018) investigated the suitability of seven distinct waste materials as fillers in place of conventional filler (OPC) in Dense graded Bituminous Macadam (DBM) mix. All wastes examined exhibited the chemical and physical properties of a good filler, and the DBM mixtures made using those wastes yielded mechanical and durability characteristics that were on par with conventional mixtures. The recycled asphalt mixtures manufactured with a commercial polymer-modified binder yielded the best mechanical performance and an aging resistance equivalent to the conventional binder modified with 5% styrene-butadiene-styrene (Torres et al., 2020). Despite several studies on using waste materials in asphalt mixture manufacture, there has been little to no research on the binary blending of sawdust ash (SDA) and corn cob ash (CCA) as biomass fillers in place of PQD for asphalt mixture production. This is the justification for the research.

Sawdust and corncob ashes are widely used as PC replacements in concrete production. Studies show that SDA can be used in place of PC, improving concrete’s sustainability by reducing waste and production costs (Dominguez-Santos et al., 2019). Similarly, the potential of corncob ash in concrete applications led to improved sustainability and enhanced concrete performance (Oyebisi et al., 2022; Oyebisi and Alomayri, 2023).

The novelty of this research is the binary blends of SDA and CCA as partial biomass filler replacements of PQD for asphalt mixture production. This is the first time a blended strategy of these waste materials would be used, and the results would aid in identifying the possible manufacturing of asphalt mixture that would attain optimal biomass utilization.

This research valorizes SDA and CCA as biomass fillers for use in partial replacement with PQD for asphalt mixture production. Sawdust and corncob were calcined at 650°C for 2 h, obtaining SDA and CCA. The PQD (conventional filler) was replaced with SDA and CCA at 0–100 Wt. %. Volumetric properties (percentage of air voids, voids in mineral aggregates and voids filled with binder) and the performance of modified asphalt mixtures against different distresses (rutting, cracking and moisture) were examined. The microstructures and elemental compositions of the asphalt mixture samples were analyzed using Field Emission Scanning Electron Microscopy (FESEM) and Energy Dispersive X-ray spectroscopy (EDX). Findings from this research would reduce reliance on conventional PQD fillers, promote environmental sustainability and improve the quality of flexible road pavement.

Sawdust was obtained from a local sawmill at Ededimeji Area in Ede, Nigeria, while corn cobs were sourced from a nearby farmyard at Egbeda Loogun in Ede, Nigeria. The materials were sorted and dried to aid valorization. Sawdust and corncobs were calcined at 650°C for 2 h, obtaining SDA and CCA. The SDA and CCA were milled and sieved below 75 µm, as shown in Figures 1(a) and (b), . Quarry dust was pulverized and sieved below 75 µm, obtaining PQD as a conventional mineral filler (Figure 1(c)). Coarse aggregate (granite, 4.75–12.5 mm sizes) and fine aggregate (quarry dust, ≤ 4.75 mm size), as shown in Figures 1(d) and (e), , were obtained from a local quarry in Ede, Nigeria. The asphalt binder used (VG-30 or 60/70 grade), as indicated in Figure 1(f), was obtained from Construction Products Nigeria Limited, Ilorin, Nigeria. The chemical compositions of SDA, CCA and PQD, analyzed by the X-ray fluorescence analyzer, are shown in Table 1.

The sum of SiO₂, Al₂O₃ and Fe₂O₃ is a typical index for evaluating the pozzolanic and reactive properties of fillers. From Table 1, SDA, CCA and PQD had SiO₂, Al₂O₃ and Fe₂O₃ totals of 67.17, 74.22 and 89%, which are higher than 50% per ASTM C618-19 (2022). The higher value suggests better reactivity and pozzolanicity, improving the performance of asphalt mixtures. The LOI values obtained for SDA, CCA and PQD were below the 6% stipulated by ASTM C618-19 (2022). The SDA, CCA and PQD had specific gravities of 1.50, 2.00 and 2.55.

Table 2 shows the physical and mechanical properties of the aggregates used. All samples were prepared and tested following the American Society for Testing and Materials (ASTM). Table 3 presents the aggregate gradation obtained from the Nigerian Standards’ pavement mineral aggregate mix design and evaluation (Federal Ministry of Works and Housing, 2016).

A total of 11 samples of asphalt mixtures were made, with SDA and CCA replacing PQD as the fillers in the contents of 0, 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 Wt. %. Marshall tests were performed per ASTM D1559-89 (2021) and Asphalt Institute (2014). A specimen weighing 1200 g was prepared after sieving, cleaning and drying. The fillers and aggregates were heated to 160°C. After fully mixing the asphalt binder, fillers and aggregates at 160°C, the material was compacted using an automatic compactor (with 75 blows on each side) that produced Marshall samples measuring 50 mm in height and 100 mm in diameter. The Marshall samples were cooled at room temperature before extruding from the molds. The Optimum Binder Content (OBC) is considered as binder content that corresponds to 4% of the percentage of air voids (VA) in prepared samples (Asphalt Institute, 2014). The OBC of 6.3% was obtained using the Marshall testing method by varying the binder content from 5 to 8% at intervals of 0.5% as specified by the Federal Ministry of Works and Housing (FMWH) (2016). This OBC was constantly maintained for all mixtures. Three samples of each type of asphalt mixture were made and the mean values were used.

Binder compliance with permissible values of certain engineering properties needed for the asphalt mixtures was examined. Various tests were performed on the asphalt binder following ASTM standards, as seen in Figure 2. The tests included penetration, softening point, flash point, ductility, viscosity and specific gravity. Table 4 lists the binder tests for a comparative examination of the findings using ASTM specifications. For each test, three samples were made, and the mean values were determined and used.

The Marshall properties of the asphalt mixtures with varying percentages of SDA and CCA were examined. Marshall stability and flow of 11 asphalt mixture samples (3 for each binder content), as indicated in Figure 3, were determined per ASTM D6927 (2022). Stability measures the resistance of the asphalt mixtures to deformation under load; a greater stability value signifies a better resistance to deformation. Flow measures the deformation of the asphalt mixtures under a standard load. Volumetric properties, namely, the percentage of air voids (VA), voids in mineral aggregates (VMA) and voids filled with binder (VFB), were measured following the Asphalt Institute (2014) procedure. Three specimens were produced for each test, and the mean values were determined and used.

A Marshall Quotient (MQ) metric is used to gauge a mixture’s resistance to rutting, shear stress and permanent deformation (Zoorob and Suparma, 2000). The Marshall stability (kN) ratio to flow (mm) at OBC is used to calculate MQ. Higher MQ asphalt mixtures are stiffer, better at distributing applied load and more resistant to creep deformation (Choudhary et al., 2018). Three specimens were produced for each test, and the mean values were determined and used.

The indirect tensile strength is a commonly used measure to evaluate the mechanical properties of asphalt mixtures, particularly their resistance to cracking and deformation. In this study, the indirect tensile strength (σ, kPa) of compacted asphalt mixtures was determined per ASTM D6931 (2017) and AASHTO T 283 (Kandhal et al., 1998). The Marshall samples were divided into two groups: wet (conditioned) and dry (unconditioned) subsets. The conditioned samples were partially vacuum saturated with water and sealed. After that, the samples were placed in a plastic bag containing about 10 ml of potable water, and subjected to a freezing cycle for 16 h at −18°C. This was followed by placing the samples in 60°C water bath (samples had a minimum of 25 mm of water above their surface) for 24 h. At the end of the cycle, samples were placed in a water bath for 2 h at 25°C, and their indirect tensile strengths were determined. The samples of the dry (unconditioned) subsets were immersed in a water bath at 25°C for 2 h, and their strengths were tested. All tests were conducted at 25°C by loading the Marshall samples diametrically in compression [Figure 3(b)] with steel strips having a steady pace of 50 mm/min. Three specimens were made for each test, and the mean values were determined and used. The indirect tensile strength was computed using equation (1):

where P_max is the peak load, N

D is the diameter of the Marshall sample, mm

T is the thickness of the Marshall sample, mm

Using unsuitable filler in the modified asphalt mixtures leads to asphalt binder-aggregate bond failure due to moisture susceptibility. Tensile strength ratio (TSR) test per AASHTO T 283 (Kandhal et al., 1998) determines the moisture susceptibility of both conditioned and unconditioned samples. A higher TSR value signifies a mixture that is more resistant to moisture. Three specimens were produced for each test, and the mean values were determined and used. Thus, TSR is determined by the relationship illustrated in equation (2):

Field emission scanning electron microscopy (FESEM) is an innovative technique for capturing the microstructure image of materials. Zeiss Crossbeam 340 was used to obtain the microstructure picture to characterize the modified asphalt mixture samples in this investigation per ASTM E986 (2024). Before morphological examination, the samples were sliced into approximately 5 × 5 × 5 mm pieces and coated with aurum (Jaya, 2020). Besides FESEM, the same samples were subjected to the EDX using Zeiss Smart EDX. EDX is an X-ray technique that identifies the elemental compositions of materials. These devices are add-ons for electron microscopes, using the microscope’s image power to identify samples of interest.

Table 5 provides the properties of the binder used for the asphalt mixtures. The outcomes were suitable for the intended use. The penetration value (66 mm) was within the Nigerian Federal Ministry of Works and Housing (FMWH) (2016) and ASTM D5 (2017) of 60–70 mm, demonstrating that the asphalt binder had the appropriate degree of hardness. The obtained value of 53°C softening met the FMWH (2016) of 48°C–56°C, ASTM D36 (2020) of 49°C–56°C, and Asphalt Institute (AI) (2014) of above 50°C, indicating the proper softening point of the binder. The ductility measures the ability of asphalt binder to expand without breaking. Furthermore, viscosity refers to the thickness and flow resistance of the binder. The obtained value of 102 cm indicated good ductility, surpassing the minimum requirement of 100 cm following FMWH (2016) and ASTM D113 (2023). The obtained values of viscosity at 60°C (1950P) and 135°C (305 cst) fell within the ASTM D2171 (2022) and ASTM D2170 (2018) recommendations, indicating favorable flow characteristics. The lowest temperature at which the binder releases enough vapor to ignite is known as the flash point. The binder’s strong resistance to ignite was demonstrated by the obtained value of 255°C, which is higher than the minimum requirement of 230°C and 250°C per ASTM D92 (2018) and FMWH (2016). The specific gravity of the binder influences its density in relation to water. From Table 5, the specific gravity obtained (1.04) was within the standard range of 1.01–1.06 recommended by ASTM D70 (2021) and FMWH (2016).

The properties of asphalt binders (grade 60/70 Pen) used in producing asphalt mixtures modified with steel slag powder and BLA were conducted. The results showed 60/70, 47.67°C, 100 cm, 255°C and 1.02 as penetration at 25°C, softening point, ductility, flash point and specific gravity at 25°C (Osuolale et al., 2023). The penetration, ductility and softening point of pure binder used in expanded polystyrene bead-based asphalt mixtures demonstrated values of 68.10 mm, 95 cm and 50°C, supporting the findings shown in Table 5 (Anwar et al., 2020). The results shown in Table 5 are further supported by penetration, ductility, specific gravity and softening point of VG-30 grade binder employed in asphalt mixtures, which produced values of 62 mm, 100 cm, 1.02 and 52.50°C (Choudhary et al., 2018). Ultimately, the properties highlighted in Table 5 are crucial for ensuring the durability, workability and efficacy of the asphalt mixture pavement.

Figure 4 shows the outcomes of volumetric and Marshall properties of the asphalt mixtures. From Figure 4(a), the Bulk Specific Gravity (BSG) of the mixture decreased as SDA and CCA contents increased, with values ranging from 2.14–2.27 g cm⁻³ compared to 2.29 g cm⁻³ for the control sample. The reason can be attributable to a low specific gravity of SDA (1.50) and CCA (2.00) compared to PQD with a specific gravity of 2.55. The higher surface areas of SDA and CCA compared to PQD resulted in a lower BSG for modified asphalt mixtures. The increased specific gravity of PQD can be ascribed to the higher Al₂O₃ concentration in PQD (14.59) compared to SDA (6.53) and CCA (5.96), as Table 1 indicates (Choudhary et al., 2018).

Figure 4(b) indicates the effects of SDA and CCA in asphalt mixtures on air void content. Small air pockets or spaces exist between the coated aggregate particles in the final compacted mixtures. Thus, Figure 4(b) exhibits a decrease in the percentage of air void with increasing SDA and CCA contents. The percentage of air void (VA) value varied between 4% and 5% at 100 and 10 Wt. % SDA and CCA substitution compared to the control mixture (100 Wt. % PQD) with 5.15%. The decrease in the VA can be attributable to the filler effects of SDA and CCA, filling interstitial space and reducing the air voids in the mixture (Gao et al., 2018; Mistry et al., 2019). A related study found that an asphalt wearing course modified with BLA had comparable air spaces (about 3%) (Osuolale et al., 2023). The air-void content affects the durability of asphalt mixtures. This is because the mixture gets less permeable as the air spaces decrease. Excessive air voids allow harmful air and water to permeate the mixture (Kandhal et al., 1998). Ultimately, the results obtained satisfied 3%–8% and 3%–5% VA recommendations specified by FMWH (2016) and AI (2014). Additionally, the results align with the 2%–5% air void criteria for dense-graded asphalt mixtures for different traffic scenarios (Mclbod, 1955).

Figure 4(c) displays the voids in the mineral aggregate (VMA) of the asphalt mixtures incorporating varied proportions of SDA and CCA. The highest and lowest spaces that accommodated the aggregate particles, the asphalt binder and the volume of air voids necessary in the asphalt mixture were 20.56 and 17.76% attained at 30 and 50 Wt. % of SDA and CCA. This signifies that the addition of SDA and CCA at 50 Wt. % substitution causes a decrease in VMA. This might be because aggregate absorbs more binder than filler, which lowers the filler’s composition and increases the interlocking flexibility of asphalt mixtures (Mistry et al., 2019). Relevant research noted a reduction in VMA for wearing course from 12% to 11.50%, with 100% quarry dust yielding the highest value and alternative mineral fillers (BLA and steel slag powder) with the same value exhibiting the lowest value (Osuolale et al., 2023). One of the physical requirements for dense graded asphalt mixtures is the minimum volume of VMA of the compacted mixture to provide room for the specified air voids and the required volume of asphalt binder for a durable pavement (Mclbod, 1955). The amount of space accessible for the asphalt film increases with the amount of VMA in the dry aggregate. Most specifications include explicit minimum requirements for VMA since the more durable the mixture is, the thicker the asphalt coating on the aggregate particles. Adherence to minimum VMA values is necessary for a durable asphalt film thickness. Thus, a 50 Wt. % SDA and CCA exhibited the best VMA because of their lowest VMW. The outcomes satisfied the minimum VMA requirements of 15% for dense-graded asphalt mixtures under various traffic conditions (Mclbod, 1955).

The effects of SDA and CCA on voids filled with binder (VFB) of asphalt mixtures are presented in Figure 4(d). The results indicated that the conventional asphalt mixture yielded the highest VFB, with 79.98%. However, the VFB decreased as SDA and CCA contents in the mixtures increased from 10 to 50 Wt. %. After 50 Wt. % substitution, the VFB increased marginally but below the VFB of the control mixture. These results indicate that adding more than 50 Wt. % of SDA and CCA to the asphalt mixtures results in a binder deficiency, brittleness, raveling and wear. This signified that a 50 Wt. % SDA and CCA demonstrated the best performance with the asphalt-void ratio of about 4% higher than the control mixture. The VFA property is strongly associated with percent density, making it a valuable tool for assessing relative durability. Insufficient asphalt mixture prevents over-densification under traffic and bleeds if the VFA is too low. Consequently, the VFA is a crucial design characteristic. A higher VFB indicates a greater percentage of void filled, promoting better strength and durability. Thus, these results met FMWH (2016) and AI (Asphalt Institute, 2014)’s recommendations, specifying 65%–82% and 65%–80% VFB.

The Marshall stability of the asphalt mixtures is depicted in Figure 4(e). In contrast to the control mixture, which had a stability of 7.72 kN, the stability rose with increasing SDA and CCA contents at 10–70 Wt. %, with values ranging from 7.88 to 10.58 kN. This indicated that SDA and CCA exhibit better stability than PQD. Low specific gravity particles concentrate in the space between the aggregate and the binder matrix, creating a “filler effect” that improves adhesion and coating (Khan et al., 2017). These findings are consistent with earlier research showing that samples containing 25 and 100% DSA filler had 12% and 36% higher Marshall stability than samples containing 100% conventional filler. Furthermore, the stability value of the asphalt mixture with 25% and 100% RHA filler was 10% and 28% greater than the control sample (Tahami et al., 2018). According to previous research, the wearing stability of steel slag (filler) was shown to be almost 19% higher than that of quarry dust (control filler) (Osuolale et al., 2023). However, from Figure 4(e), the stability declined marginally after 70 Wt. % SDA and CCA substitution. About 1%–4% lower stability occurred at 80–100 Wt. % SDA and CCA compared to the control mixture. Because of their diminished affinity for asphalt, SDA and CCA were shown to reduce matrix cohesion and adhesion above 70 Wt. % replacement limit, resulting in decreased stability. Ultimately, the stability of all asphalt mixtures satisfied the minimum stability values of 3 and 9 kN specified by FMWH (2016) and AI (Asphalt Institute, 2014). Also, the results align with the minimum light-heavy duty (3.45–10.3 kN) criteria for dense-graded asphalt mixtures for different traffic scenarios (Mclbod, 1955).

Figure 4(f) depicts the flow of asphalt mixtures. Marshall stability and flow index are one of the three essential characteristics of the physical specifications recommended for densely graded asphalt mixtures. From Figure 4(f), the flow decreased with increasing SDA and CCA content in the asphalt mixtures. This result showed the effective performance of SDA and CCA as fillers and their ability to reduce the brittleness and plastic deformation susceptibility of the mixtures. This can be attributable to the SDA and CCA’s pozzolanic reactions, lessening lubrication between the aggregate particles (Gao et al., 2018; Mistry et al., 2019). These findings are consistent with related research revealing lower flow with BLA as a filler than steel slag powder and quarry dust fillers in the wearing and binder courses (Osuolale et al., 2023). Rice husk ash, carbide lime and brick dust yielded about 17, 2 and 3% lower flow than OPC as fillers in asphalt mixtures (Choudhary et al., 2018). All flow values satisfied the 2–6 mm requirement stated by FMWH (2016). Besides, the flow results were within the 2.54–5.08 mm range of requirements for densely graded asphalt mixtures under various traffic conditions (Mclbod, 1955).

Figure 5 shows the MQ, or rutting resistance, of the asphalt mixtures. The MQ increased as SDA and CCA contents in the mixtures increased. The porous nature of SDA and CCA fillers can be attributable to their favorable effect on enhancing aggregate adhesion in modified asphalt mixtures, providing a strong propensity to absorb asphalt binder. This can result in a thicker layer of asphalt binder on aggregates, lowering the proportion of free asphalt binder. The overall load-bearing capability of the asphalt mixtures increased because of the greater adhesive force between the aggregates and asphalt binder. In contrast to the control mixture, which had an MQ of 2.29 kN mm⁻¹, the MQ values of SDA-CCA mixtures ranged from 2.42 to 3.65 kN mm⁻¹. These findings are consistent with previous research that asphalt mixtures with biomass fillers (DSA and RHA) showed greater resilience to rutting at 100 Wt. % substitution than mixtures containing conventional mineral filler (stone dust) (Tahami et al., 2018). Previous studies have shown that adding fine fillers makes the asphalt mixtures more rigid (Choudhary et al., 2020a; Islam et al., 2021; Modarres et al., 2015). In comparison to BLA (5.12 kN mm⁻¹) and quarry dust (5.05 kN mm⁻¹), steel slag powder had the highest MQ (5.74 kN mm⁻¹) (Osuolale et al., 2023). A higher MQ value suggests better overall performance. As the proportion of SDA and CCA varied, the results of Marshall characteristics in Figure 5 also differed. The mixture with 50 Wt. % SDA-CCA demonstrated higher stability and lower flow than other replacement levels, resulting in the highest MQ (3.65 kN mm⁻¹).

The indirect tensile strengths of asphalt mixtures under wet and dry conditions are presented in Figure 6. The results revealed a higher cracking resistance to applied stress under the dry condition than the wet condition. This is because water weakens the bond between the aggregates and binder, making asphalt mixtures less resilient to traffic stresses (Omar et al., 2020). The cracking resistance of asphalt mixtures to applied stress increased with increased SDA and CCA content. The SDA-CCA-based asphalt mixtures outperformed the control mixture under wet and dry conditions. At 10–100 Wt. % SDA-CCA substitution, wet and dry indirect tensile strengths varied between 700 and 835 kPa and 994–1105.74 kPa compared to the control sample with 593.31 and 829.92 kPa. More homogenous distribution of finer fillers leads to void filling and the development of an integrated structure in the asphalt mixtures, enhancing the stiffness and indirect tensile strength (Choudhary et al., 2018; Modarres et al., 2015). A higher indirect tensile strength value of asphalt mixtures results in higher resistance to low-temperature and fatigue cracking (Choudhary et al., 2018). As the amount of filler increases and the percentage of binder decreases simultaneously, the indirect tensile strength of a bituminous mix increases (Huang et al., 2007). Thus, a 50 Wt. % SDA-CCA substitution exhibited superior performance with 835.26 and 1105.74 kPa wet and dry indirect tensile strengths compared to other filler levels and control mixtures. These findings align with earlier research revealing that the stiffness modulus of asphalt mixtures increased with increased RHA and DSA fillers. The test temperatures demonstrated that using 100% DSA and RHA in place of the conventional filler (stone dust) was the most effective way to increase stiffness modulus (Tahami et al., 2018). The finer nature of red mud filler resulted in 14% higher indirect tensile strengths than the PC filler. Asphalt mixtures modified with copper tailings as a filler exhibited higher indirect tensile strength (1222 kPa) than stone dust filler (1085 kPa) (Choudhary et al., 2020b). Nonetheless, it was discovered that mixtures comprising brick dust and rice straw ash had lower indirect tensile strengths than PC-based asphalt mixtures (Choudhary et al., 2018). Ultimately, this research recommends the use of 50 Wt. % SDA and CCA as optimum fillers in the production of asphalt mixtures.

A greater TSR indicates an improved resistance to moisture in the mixture. Figure 7 displays the TSR values for each blend. Compared to the control mixture, which had a TSR of 0.71, the results showed improved TSR with increasing biomass filler content in the mixtures with the optimum ratio of 0.72 at 100 Wt. % replacement level. Nonetheless, the optimal replacement for higher moisture resistance was found to be at 50 Wt. % SDA-CCA substitution with a TSR of 0.76. The increased TSR can be attributable to the calcium oxide (CaO) content in SDA and CCA, which was about 75% higher than PQD’s in Table 1. Fillers with a primarily calcium-based insoluble composition showed excellent moisture resistance and increased adhesion between asphalt binder and filler (Pasandín et al., 2016). Superior TSR values were produced by the presence of Portlandite and calcite minerals in carbide lime, PC and limestone dust mixtures (Choudhary et al., 2018). Similarly, the predominance of CaO in steel slag powder (13.04%) composition was attributable to its highest TSR values compared to quarry dust (2.70%) (Osuolale et al., 2023). However, the rice straw ash mix had the lowest TSR value due to its high porosity and silica dominance in its chemical compositions. In addition, the high concentration of silica and active clay in the brick dust mix resulted in a low TSR value (Choudhary et al., 2018). The comparatively higher active clay concentration in the oxide composition of limestone sludge-based asphalt mixtures was attributable to its relatively lower TSR value than mixtures modified with stone dust (Choudhary et al., 2020a). Similar to this, the stone dust-based mixtures, which contain a significant amount of calcium-based water-insoluble dolomite, produced a slightly higher TSR value (89.26%) than the copper tailing mix (84.24%) (Choudhary et al., 2020b).

Figures 8(a) and (b), display the FESEM micrographs for the modified asphalt mixture (50 Wt. % SDA-CCA) and the control mixture. The FESEM micrographs provided performance at working distances of 11.28 and 8.53 mm, with a resolution of 100 µm at 15 kV. An amorphous structure with various particle sizes and shapes was visible in the micrographs. In contrast to the control mixture [Figure 8(a)], the internal structure of the asphalt mixture incorporating SDA-CCA at 50 Wt. % replacement level, as indicated in Figure 8(b), was more compact. This proved the filler effects of SDA and CCA, filling voids in the matrix, improving adhesion and cohesion between the asphalt binder and filler and strengthening the mixture (Wei et al., 2021). These resulted in improved cracking resistance, indirect tensile strength and TSR, as evident in Figures 5–7.

Figures 9(a) and (b), indicate the EDX of the control mixture and the modified asphalt mixture. Multiple significant peaks in Figure 9(a) indicated the presence of different components in the sample. The identified peaks with weight percentages of 25.34, 27.69, 2.41, 15.34, 7.07, 5.70, 10.10 and 6.34 correspond to the characteristic X-ray energies of elements of silicon (Si), oxygen (O), sodium (Na), bromine (Br), calcium (Ca), potassium (K), molybdenum (Mo) and rubidium (Rb). These components (silicon, calcium and molybdenum), in particular, improved the pore structures and mechanical characteristics of the control mixture (Quan et al., 2022). Similarly, several significant peaks in Figure 9(b) indicated the presence of different elements in the sample. These elements were aluminium (Al), silicon (Si), potassium (K) and magnesium (Mg) with 15.94, 44.42, 25.49 and 14.16 Wt. %. These elemental compositions can be responsible for higher rutting, cracking and moisture resistance in the modified asphalt mixtures than that of the control mixture.

This study examined the effects of biomass fillers (SDA and CCA) as alternatives for conventional mineral filler (PQD) in asphalt mixture production. The following inferences can be made in light of the physical, volumetric, Marshall, durability and microstructural findings:

• SDA-CCA-based asphalt mixtures showed better Marshall properties than the control mixture. The higher the SDA and CCA levels in the mixture, the greater the Marshall stability and MQ (rutting resistance). The mix incorporating SDA and CCA (at 50 wt. %) outperformed the control mix in terms of stability and rutting resistance by 27.03 and 25.65%.

• The cracking resistance of asphalt mixtures increased with increased SDA and CCA contents. An optimum resistance to crack was attained at 50 Wt. % SDA-CCA substitution with 28.97 and 24.95% higher wet and dry cracking resistance than the control mixture.

• SDA and CCA enhanced the resistance of asphalt mixtures to moisture action. The best performance was achieved at 50 Wt. % SDA-CCA substitution with about 6% higher moisture resistance than the control mixture.

• The filler effects of SDA and CCA filled the voids of asphalt mixtures, increasing asphalt binder-filler adhesion and strengthening the mixture.

This finding is significant as it valorizes SDA and CCA at a 50 Wt. % optimal substitution improves asphalt mixtures and promotes sustainability by reducing reliance on conventional filler (PQD), which has a higher environmental impact, potentially revolutionizing the construction industry.

The indirect tensile strength provides insight into cracking resistance; however, it does not directly measure crack propagation or growth, which are critical to understanding the durability of asphalt mixtures. Thus, future studies can evaluate the effects of SDA and CCA on long-term performance analysis, such as fatigue cracking, thermal cracking, fatigue resistance and aging effects over time. While the MQ can be a useful preliminary indicator of rutting resistance in asphalt mixtures, it is essential to consider dynamic modulus tests to predict asphalt mixture performance incorporating SDA and CCA under real-world conditions. Future studies can investigate the effects of binary blends of SDA and CCA on the economic and environmental feasibility of asphalt mixtures modified with PQD.