Molecular-scale insights into the freeze-thaw process of aqueous and saline solutions in hydrated‑calcium-silicate gel pores

Abstract

The bulk expansion and solute migration caused by ice-water phase change are the main causes for the damage and deterioration of cement-based materials in the cold region. To better understand the degradation mechanism of calcium silicate hydrate (C-S-H) at low temperatures, the freeze-thaw process of aqueous and saline solutions along the pore axis of C-S-H gel was investigated using the molecular dynamics method. The study found that during the freezing process, the spatial confinement effect of the C-S-H substrate significantly lowers the freezing point of the pore solution and reduces the growth rate of ice crystals. The differential repulsion of salt ions by ice crystals, in conjunction with the selective adsorption of ions by the C-S-H substrate, results in increased adsorption of sodium ions onto the C-S-H surface, while chloride ions are repelled into deeper pores. During thawing, the C-S-H matrix facilitates the thawing of ice crystals at higher temperatures, while inhibiting their thawing at lower temperatures. As the ice crystals thaw, chloride ions near the C-S-H surface rapidly dissolve into the solution, whereas sodium ions adsorbed on the surface are less likely to diffuse back into the solution. Temperature variations affect ion diffusion and ice crystal growth, influencing salt ion migration. These findings provide valuable scientific insights for improving the durability of cement-based materials in harsh environments.