Investigation on the Chloride Ion Erosion Mechanism of Cement Mortar in Coastal Areas: from Experiments to Molecular Dynamics Simulation

Chloride salts in groundwater are a serious threat to the durability of underground reinforced concrete structures in coastal areas. In order to effectively improve the durability performance of underground structural concrete in coastal areas, physical and mechanical properties such as compressive strength, water absorption rate and chloride ion permeability of cement mortar were studied. The influence laws of mineral additives such as fly ash, ground granulated blast-furnace slag and chloride ions on the relevant properties of cement mortar were analyzed. The effect of chloride ions on these physical and mechanical properties was investigated. The main hydration product categories of cement mortar were derived using microscopic analysis (XRD). The internal microstructure and the surface morphology of cement mortar were obtained by scanning electron microscopy (SEM). The effect of chloride ions in pores on C-S-H adsorption was studied by using molecular dynamics simulations. The results showed that chloride ion erosion increased the compressive strength of cement mortars doped with FA and GGBS at 28, 100, and 180 days by 2.76 %, 14.32 %, and 7.83 %, respectively. Chloride ion erosion moderately increased the compressive strength of cement mortar doped with FA and GGBS at all curing period. After chloride ion erosion, the electric fluxes of the four groups of specimens at 180 days decreased by 30.37 %, 31.64 %, 23.22 % and 17.41 %, respectively. After the addition of mineral additive, the bound chloride ions changed the influence law of water absorption rate of cement mortar, and reduced the electric flux of cement mortar. The molecular dynamics simulations revealed that the chloride ions in the pores weakened the physical adsorption of external chloride ions by C-S-H, resulting in the easier passage of external chloride ions through the pores. Microscopic analysis showed that the Friedel's salt generated by chloride ions attacking the cement -based materials filled communicating pores, which may be one of the reasons for the reduced electric flux of the cement mortar.