Molecular Dynamics Simulation of the Diffusion Mechanism of Chloride Ion between Different Phases of Cement-Based Concrete

Abstract

The diffusion of chloride ions can cause corrosion of concrete buildings, thus reducing their service life. The diffusion of chloride ion in concrete is mainly carried out between the interfaces of different phases. In this article, Hamid model and Jennite model are used to represent C-S-H gel phase with different ratios of calcium to silicon (C/S), and Fe crystal is used to represent stainless steel phase. The diffusion behavior of chloride ion in calcium chloride (CaCl2) solution layer with a certain thickness, which is between gel phases, gel phase and stainless steel phase, was studied. A supercell molecular model containing upper and lower surface phases and CaCl2 solution in the middle was established. The effect of upper and lower surface type, the thickness of solution layer, and concentration of CaCl2 solution were investigated by molecular dynamics simulation. The results show that when the upper and lower surfaces are Fe and C-S-H crystals, respectively, the diffusion coefficient of chloride ion is one order of magnitude larger than that of both C-S-H crystals, where the diffusion coefficient of chloride ions decreases with the increase of C/S on the whole. The relative concentration distribution of calcium ion and chloride ion in the solution layer were analyzed, demonstrating that the surface of C-S-H crystals are negatively charged near the side of the solution and the diffusion of chloride ions is related to the number of charges; the diffusion coefficients of chloride ions increase with both the concentration and thickness of CaCl2 solution layer.