Assessing Moisture Susceptibility and Long-Term Leaching Behavior of Municipal Solid Waste Incineration Fly Ash–Modified Asphalt Mixtures

The heavy metal (HM) content and limited disposal options of municipal solid waste incineration fly ash (MSWIFA) pose significant environmental challenges. However, fly ash offers potential as a sustainable modifier in asphalt pavements. However, moisture susceptibility and long-term leaching necessitate a comprehensive evaluation of the material before its use. This research assessed the moisture-induced sensitivity and long-term leaching behavior of MSWIFA-modified asphalt mixtures, focusing on their practicality in moisture-laden environments. Dense-graded (DG) and gap-graded (GG) asphalt mixtures were prepared using conventional and MSWIFA-modified asphalt binders. The MSWIFA modification significantly enhanced moisture resistance in both aggregate gradations. The DG mixture with modified bitumen (DG-MB) achieved the highest tensile strength ratio (TSR) of 86%. The GG mixture with modified bitumen (GG-MB) did not meet the 80% TSR threshold, but outperformed the conventional GG, highlighting the role of MSWIFA in improving binder stiffness and asphalt–aggregate bonding. Dynamic modulus |E*| test results showed reduced stiffness loss and a higher |E*| stiffness ratio (ESR) in the MSWIFA-modified asphalt mixtures with DG-MB exhibiting 18% to 21% stiffness loss and ESRs up to 84%. The wheel tracking test for high-temperature performance revealed reduced rut depths in MSWIFA-modified asphalt mixtures. The overall leaching remained well below the regulatory limits for MSWIFA-modified asphalt mixtures, demonstrating effective immobilization by the asphalt binder conglomerate. Specifically, DG-MB mixtures achieved better immobilization of HM leaching compared with GG-MB. Overall, incorporating MSWIFA into asphalt mixtures was found to be a viable strategy for safely managing HM leaching and substantially reducing the environmental risk.