Atomistic mechanisms of salt-induced interfacial failure in asphalt–aggregate systems under multiphase salt solution/crystal environments: molecular simulation and laboratory observation

Asphalt pavements in coastal, frigid, and saline–alkali soil regions face durability threats due to complex saline environments. Salts exist in asphalt mixtures in the form of solutions or crystals, weakening asphalt–aggregate adhesion and accelerating mixture deterioration. In this study, the molecular dynamics simulations were performed to explore how NaCl, MgCl₂, and Na₂SO₄, present as either solutions or crystals, and alter the asphalt–aggregate interface. The interfacial mechanical response and failure modes were examined for virgin, mildly aged, and severely aged asphalt binders. The combined influence of pull-off rate, temperature, salt environment, and aging level was quantified by simulated pull-off tests, and validated by macroscopic experiments. The results showed that salt solutions significantly decreased interfacial bond strength and fracture energy, typically leading to adhesive failure; the deterioration caused by NaCl solution is essentially irreversible. In contrast, salt crystals exhibited divergent effects: Na₂SO₄ and NaCl crystals promoted cohesive failure and preserved or improved adhesion, whereas MgCl₂ crystals strongly adsorbed onto the asphalt interface, inducing adhesive failure. Moreover, asphalt aging exhibited dual effects on interfacial strength, enhancing it in cohesive failure scenarios but reducing it in adhesive ones. These insights clarify the molecular mechanisms of salt-induced damage in asphalt interfaces.