Multiscale mechanisms of KH-550-enhanced interfacial adhesion between asphalt binder and basalt/steel slag aggregates

Basalt and steel slag are widely used in asphalt pavements, yet the mechanisms governing their adhesion to asphalt remain incompletely understood. This study investigated KH-550-modified basalt/steel slag-asphalt interfaces under unaged and short-term-aged conditions through an integrated experimental-molecular simulation framework. Basalt and steel slag were treated as engineering aggregates, whereas kaolinite and hematite served only as mineralogical references for selecting simplified oxide models: kaolinite-related Si-O and Al-O environments were represented by SiO₂ and Al₂O₃, respectively, and the hematite-related Fe-O environment was represented by Fe₂O₃. Together with CaO, these surfaces were used to compare representative oxide chemistries rather than to reproduce complete mineral crystal structures. XRD, SEM, FTIR, contact-angle analysis, surface-free-energy measurements, and interfacial bonding tests were combined with SARA-based molecular models to evaluate adhesion work, interaction energy, radial distribution functions, concentration profiles, and molecular mobility. KH-550 increased the aggregate surface free energy, reduced the water contact angle, and enhanced interfacial bonding. Steel slag exhibited a stronger response than basalt because its rougher morphology and Ca-related active sites promoted mechanical interlocking and polar adsorption. Molecular simulations further indicated that hydrolyzed KH-550 formed Si-O-M linkages with mineral surfaces, while its amino-functional groups interacted with asphalt through hydrogen bonding, electrostatic attraction, and van der Waals interactions. KH-550 treatment increased the interfacial binding energy of the asphalt-basalt and asphalt-steel slag systems by 25–40% compared with the corresponding untreated systems. Aging intensified localized interactions involving carbonyl and sulfoxide groups but reduced molecular mobility and conformational adaptability. These results establish a cross-scale mechanistic framework linking aggregate characteristics, oxide surface chemistry, and macroscopic adhesion, and demonstrate the effectiveness of KH-550 in improving interfacial stability and moisture resistance.