Rheological and Microstructural Characterization of Steel Slag Powder-Modified Asphalt Mastics: Insights into High-Temperature Performance Enhancement

This study systematically investigates the rheological modification mechanism of steel slag powder (SSP) as an alternative filler in asphalt mastics, with comparative analysis against conventional limestone powder (LP). Four filler-to-asphalt (F/A) ratios (0.6–1.2) were employed to prepare modified mastics. Comprehensive characterization through laser diffraction analysis, BET nitrogen adsorption, and scanning electron microscopy (SEM) revealed SSP’s significant microstructural advantages: a 29.2% smaller median particle size (D50) and 7.06% larger specific surface area compared to LP, accompanied by enhanced interparticle connectivity and morphological complexity. Rheological evaluation via dynamic shear rheology (DSR) demonstrated SSP’s superior performance enhancement—particularly at elevated F/A ratios (1.0–1.2), where multiple stress creep recovery (MSCR) tests showed a 6.9–46.06% improvement in non-recoverable creep compliance (Jnr) over LP-modified counterparts. The temperature sweep analysis indicated SSP’s effectiveness in reducing the temperature susceptibility index by 9.37–18.06% relative to LP. Fourier-transform infrared spectroscopy (FTIR) combined with two-dimensional correlation analysis (2D-COS) confirmed the dominance of physical interactions over chemical bonding in the SSP–asphalt interface. The results establish SSP’s dual functionality as both a rheological modifier and sustainable construction material, providing mechanistic insights for optimizing steel slag utilization in pavement engineering.