Fineness-Dependent Rheology and Chemothermal Modification Mechanism of RHB-SBS Composite-Modified Asphalt

This study investigates the synergistic and fineness-dependent modification of base asphalt using rice husk biochar (RHB) and styrene–butadiene–styrene (SBS), aiming to achieve the efficient utilization of agro-waste resources while markedly improving the high-temperature performance and durability of green pavement materials and sustainable transportation infrastructure. Through conventional performance tests, rheological measurements, and microstructural analyses, the performance behavior of RHB-SBS composite-modified asphalt and the interaction mechanisms between the modifiers were systematically examined. The results indicate that the fineness of RHB has a significant effect on the performance of the composite-modified asphalt, with 300 mesh identified as the optimal particle size that provides the best balance between high-temperature stiffness, low-temperature ductility, and storage stability. When the RHB fineness is fixed at 300 mesh, increasing the RHB content from 0 to 16 wt% markedly enhances the high-temperature performance of the composite asphalt, while its low-temperature performance slightly decreases. Scanning electron microscopy (SEM) analysis reveals that the porous structure and large specific surface area of RHB enable it to form a stable spatial network within the asphalt matrix, thereby improving high-temperature stability. Fourier-transform infrared spectroscopy (FTIR) results show that the incorporation of RHB alters the chemical structure of the asphalt and increases the degree of crosslinking, while thermogravimetry–differential scanning calorimetry (TG-DSC) analysis further confirms that the thermal stability of the composite-modified asphalt is significantly enhanced.