Combined Effect of Enriched Air-Cooled Blast-Furnace Slag Aggregate and Microsilica-Modified Bitumen on Asphalt Concrete Properties

Air-cooled blast-furnace slag (ACBFS) is a promising secondary aggregate for asphalt concrete, but its heterogeneous porosity and mechanical properties can limit performance. This study evaluates a combined approach based on selective crushing of ACBFS and modification of bitumen with microsilica. Selective crushing reduced the proportion of weak, highly porous slag particles, while microsilica was added to bitumen at 0, 3, 6, 9, 12, 15, and 18% by mass of bitumen. Asphalt concrete mixtures were assessed by density, water saturation, compressive strength, indirect tensile strength, Marshall stability and flow, water resistance, freeze–thaw resistance, and SEM–EDS analysis. The best overall performance was obtained at 12% microsilica. At this content, compressive strength reached 5.92 MPa, indirect tensile strength 3.15 MPa, Marshall stability 20.10 kN, and the water resistance coefficient 0.81, while compressive strength loss after 50 freeze–thaw cycles decreased to 8.50%. SEM observations showed fewer large interfacial voids and a more continuous aggregate–mastic contact region in the modified mixture, while EDS indicated a more continuous Si distribution. The results show that aggregate enrichment and binder modification can complement each other in improving ACBFS-based asphalt concrete.