Transport Properties of Sulfate and Chloride Ions Confined between Calcium Silicate Hydrate Surfaces: A Molecular Dynamics Study

The migration of sulfate and chloride ions in porous cementitious materials influences the durability of the concrete material. To understand transport behavior of ions in concrete, molecular dynamics is utilized to investigate the capillary adsorption of in the nanometer gel pore of calcium silicate hydrate (C–S–H), the main cement hydrate. In NaCl solution, the invading depth of water in the C–S–H gel pore follows the parabolic relation as the function of time. The Na and Cl ions closely pursue the advancing front of water and frequently penetrate into the gel pore. However, the SO42– ions in Na2SO4 solution migrate dramatically slower than that of Cl ions. The different transport rates of ions in NaCl and Na2SO4 solution in gel pore can be attributed to the immobilization effect from C–S–H surface and ionic cluster aggregation formation. On the one hand, as compared with chloride ions, the immobilization effect from the C–S–H surface is quite stronger from the surface calcium ions and nonbridging oxygen in silicate chains to elongate the resident time of sulfate ions with higher negativity in the surface. On the other hand, the sodium ions can form ionic pairs with chloride and sulfate ions in the confined gel pore. Because of the strong connection between Na–SO4, the Na–SO4 pair can further attract neighboring ions, accumulate, and grow to ionic cluster with maximum size around 2 nm. The mobility of ions caged in cluster state is significantly reduced as compared with free sulfate ions. Furthermore, in the mixed Na2SO4 and NaCl solution, the presence of sulfate ions can inhibit the transport of chloride ions. This can be explained by fact that the formed large cluster of Na–SO4 blocks the nanometer channel and prevents the water from invading in the gel pore, which further retards the ingress of solution species. Hopefully, the transport mechanism of chloride and sulfate ions in C–S–H gel can provide molecular insight on durability study of concrete material.