Unveiling the Interfacial Strengthening Mechanism of the Epoxy Asphalt–Steel Slag System via Molecular Dynamics Simulation

Steel slag (SS), a widely produced industrial solid waste, presents a sustainable alternative to natural pavement aggregates. However, its broader utilization is often restricted by moisture-induced interfacial debonding. To elucidate the microscopic interfacial mechanisms of epoxy asphalt (EA) in SS systems, this study integrates molecular dynamics simulations with laboratory experimental validation. Representative SS mineral crystal models were constructed to build atomistic EA–SS interfaces, facilitating the systematic investigation of adhesion, moisture stability, and molecular diffusion. Natural basalt and limestone aggregates were also examined for comparison. Results indicate that epoxy modification significantly improves interfacial thermodynamic stability and shifts the primary adhesion mechanism from van der Waals forces to electrostatic interactions, particularly at the Ca2+-rich surfaces characteristic of SS. Under moisture intrusion, the energy ratio (ER) of EA systems exhibits a notable increase. Specifically, the ER of the EA40 formulation on SS is higher than that of conventional asphalt, indicating enhanced resistance to moisture displacement. Furthermore, the cross-linked EA network is found to restrict molecular diffusion at the interface, establishing a negative correlation between adhesion strength and diffusion coefficients. Relative concentration profiles reveal that lighter fractions tend to accumulate at the interface, while the cross-linked network introduces steric hindrance that limits asphaltene migration. These molecular-level insights are consistent with macroscopic surface energy measurements and pull-off tests, confirming the improved moisture stability and interfacial durability of EA–SS composites. This study offers a molecular-level perspective on polymer–mineral adhesion and outlines a viable pathway for utilizing steel slag in sustainable pavements.