A Review of Interfacial Interaction Mechanisms of Steel Slag in Asphalt Concrete: Multi-Scale Characterization and Performance Implications

Steel slag, as a high-strength, alkaline, and rough-textured industrial solid waste, has been increasingly considered a sustainable alternative to natural aggregates in asphalt concrete. However, the hydration expansion risks induced by free calcium oxide and free magnesium oxide impose critical constraints on interfacial compatibility and long-term durability. This review systematically examines the interfacial interaction mechanisms between steel slag and asphalt from a multi-scale perspective. The physicochemical basis of compatibility is first analyzed based on the physical, chemical, and mineralogical characteristics of steel slag and the compositional and rheological properties of asphalt binders. Key interfacial mechanisms are then discussed, including physical adsorption, chemical bonding, formation and evolution of the interfacial transition zone, selective migration and redistribution of asphalt components, and the initiation and propagation of pores and microcracks. The capabilities and limitations of major characterization and simulation methods, such as SEM, AFM, X-CT, FT-IR, XPS, molecular dynamics, and discrete element modeling, are critically reviewed. Furthermore, the influence of interfacial bond strength, micromechanical heterogeneity, and structural stability on mechanical performance, moisture resistance, aging resistance, fatigue behavior, and durability is elucidated. Finally, existing research gaps are identified, particularly regarding long-term performance evaluation, in-situ characterization of interfacial evolution, multi-scale model coupling, and the lack of systematic life-cycle assessment and environmental benefit analysis. This review aims to provide a structured theoretical basis and practical reference for the interfacial design, performance optimization, and engineering application of steel slag asphalt concrete.