Damage on asphalt surfaces caused by ionic solution erosion and salt crystallization at molecular scale

The damage mechanism of severe salt corrosion on asphalt surfaces should be revealed at molecular scale. This study utilized molecular dynamics (MD) and density functional theory (DFT) simulations to investigate damage degree and mechanism of ionic solution erosion and salt crystallization on asphalt surfaces. Various asphalt-solution and asphalt-salt interfaces models were built, while interfacial interaction, diffusion, adhesion, and electronical properties were calculated. The electrostatic interaction played a critical role both in solution-asphalt and crystal-asphalt systems, particularly between anions and asphalt. All ions adsorbed onto unaged asphalt surfaces and eroded into unaged asphalt interiors while only cations penetrated into aged asphalt due to electronical repulsions between anions and SO/CO, indicating cation erosion contributed more to salt damage during aging. Salt crystals participated and expanded on asphalt surfaces, resulting in micro-cracks and micro-voids. Sulfates adhered asphalt more than chlorides because sulfates had closer interval distance and larger free volumes with asphalt. Asphalt molecules with high polarity and more charge were more susceptible to interact by ions. The salt damage on asphalt mainly included solution perpetration and crystal adhesion. The order of salt damage of cations to asphalt is: Mg2+ > Na+ > Ca2+, and of anions is: SO42- > Cl-.