Salt Solution Erosion of Asphalt Colloids on Metal Oxide Surface at Molecular Scale

Salt erosion damage on asphalt pavement is closely related to the compositions of both aggregate and solution. However, recent modeling studies have mostly focused on building systems by using quartz or calcite as aggregate and NaCl as salt, but overlooking other important minerals and ions. In this study, the diversity of interfacial models was increased in computational investigation for more comprehensive understanding in erosion mechanism. Molecular dynamics (MD) simulation was applied to analyze debonding processes of salt solutions at asphalt-aggregate interfaces to deeply analyze erosion mechanism. The distributions, debonding energies, and adhesion energies of systems showed aged asphalt is more susceptible to erosion by salt solutions. The adhesion between asphalt and aggregate was obstructed by salt solutions which contact to minerals more closely and strongly. The water molecules dominate the adsorption of solutions on aggregates due to H-bonds between H2O, Cl-, or SO42-. Unaged asphalt has a more stable structure than aged asphalt and can be detached with greater difficulty from mineral surfaces. The structural stability of asphalt on minerals is ranked as MgO ˃ CaO ˃ Al2O3 ˃ Fe2O3, while on solutions is ranked as CaCl2 ˃ CaSO4 ˃ MgCl2 ˃ MgSO4 ˃ NaCl ˃ Na2SO4, which is associated to radius of ions. The interaction energy between asphalt and solutions indicates that Mg-salts and sulfates have stronger erosion ability to separate asphalt colloid and aggregate. The S=O / C=O in aged asphalt strongly adsorb solution through H-bonds, leading to more severe debonding on aggregate surfaces.