Steel slag is a major solid waste generated by the steelmaking industry. Its characteristics, including high hardness and large specific surface area, offer the potential to replace traditional mineral fillers in asphalt mixtures. However, the high alkalinity of unmodified steel slag often leads to unbalanced rheological properties and insufficient moisture stability in asphalt mastic. In this study, a modified steel slag filler was prepared using a process involving crushing and screening, water washing for dealkalization, and surface modification with a silane coupling agent. Using limestone powder and hydrated lime as control groups, the modification effects on base asphalt mastic were systematically investigated. Rheological properties were characterized using a dynamic shear rheometer (DSR) and bending beam rheometer (BBR). Interfacial performance was evaluated through pull-off tests and water immersion dispersion tests. Furthermore, mechanisms were elucidated using X-ray Fluorescence (XRF), BET specific surface area analysis, and surface free energy (SFE) tests. The results indicate that the modified steel slag significantly enhances the high-temperature deformation resistance of the asphalt mastic. At 58 °C, the complex modulus reached 7.3 MPa, representing increases of 43.3% compared to limestone powder mastic. At −18 °C, the creep stiffness increased by only 3.0%, suggesting that low-temperature cracking resistance remained fundamentally stable. The water immersion dispersion loss rate was 2.12%, and the attenuation rate of pull-off strength after water immersion was 12.5%, indicating that its resistance to moisture damage is superior to that of limestone powder and comparable to that of hydrated lime. Mechanism analysis reveals that the large specific surface area of the modified steel slag strengthens physical adsorption, while the basic oxides undergo a weak acid–base reaction with the acidic components of the asphalt. Additionally, surface modification improves compatibility. The preparation process for modified steel slag is simple; it can be used as a standalone substitute for traditional mineral fillers, balancing both performance and environmental benefits.

To enhance the overall performance of asphalt pavements and promote the efficient utilization of solid waste resources, this study innovatively incorporates recycled polyethylene (PE) particles and recycled ethylene-vinyl acetate copolymer (EVA) particles, each compounded with waste cooking oil (WCO), to modify base asphalt. Systematic tests were conducted to evaluate the physical and rheological properties of the composite modified asphalt. Additionally, Fourier transform infrared spectroscopy (FTIR) and thin-layer chromatography with flame ionization detection (TLC-FID) were used to analyze the microstructures and internal components of the modified asphalt. The results indicate that the optimal mixing ratio for the WPA is 5% WCO, 5% EVA, and 5% PE. With the incorporation of these modified materials, the asphalt’s high-temperature and low-temperature properties, as well as its rutting and fatigue resistance, are enhanced to some extent. Furthermore, the modification significantly improves the rheological properties of the asphalt across the full temperature range. Additionally, the modified materials lead to changes in the internal composition of the asphalt: the content of lighter components decreases, while the content of heavier components increases. These changes in the internal composition are the primary cause of the observed improvements in the rheological properties of the asphalt.

This study developed a novel waste rubber powder modified high-modulus asphalt based on composite modification technology. The preparation process was determined by orthogonal tests, and the mechanical properties of modified high-modulus asphalt were evaluated through rheological tests in comparison with three common high-modulus asphalts to verify its application potential. The modification mechanism of modified high-modulus asphalt was characterized by Fourier transform infrared spectroscopy and fluorescence microscopy tests. The results show that the recommended mixing ratio of modified high-modulus asphalt is 20% waste rubber, 6% ethylene-vinyl acetate copolymer, and 4% ammonium polyphosphate. Although the high temperature performance, rutting resistance, and fatigue performance of modified high-modulus asphalt are slightly inferior to those of high-modulus asphalts prepared with two polymer modifiers, they are significantly better than those of hard asphalt, and all mechanical properties meet the application requirements of high-modulus asphalt. Compared with other asphalts, modified high-modulus asphalt exhibits the most prominent low temperature performance. The microscopic test results indicate that the modification process of modified high-modulus asphalt is chemical modification with minimal swelling, and no obvious network structure is formed inside.