Experimental and molecular dynamics studies on the transport and adsorption of chloride ions in the nano-pores of calcium silicate phase: The influence of calcium to silicate ratios

Chloride transport in the gel pore determines the durability of concrete material. In this paper, experiments and molecular dynamics (MD) simulation were utilized to systematically investigate chemical composition (calcium to silicate ratio C/S) influence on the transport and adsorption behavior of water, calcium and chloride ions confined in the nano-pores of calcium silicate hydrate (C-S-H). The immersion experiments indicate C-S-H samples with higher C/S ratio have better chloride adsorption capability and the zeta potential measurements proved the calcium ions can determine the surface potential in the system and strongly affect the chloride adsorption. In the MD simulation, with increasing C/S ratio, the long surface silicate chains are broken to defective short chains that can provide more non-bridging oxygen sites to accumulate surface water molecules and calcium atoms. There existed strong spatial correlation between surface calcium and chloride ions in the radial distribution function patterns, which confirms that the surface calcium plays critical role in chloride adsorption. Furthermore, the interaction mechanism between calcium and chloride ions can be categorized into two parts, the stronger one caused by the formation of ionic pairs Ca-Cl (within 3 Å), and the weaker interaction at larger distance (around 4-6Å). Therefore, high calcium concentration in the vicinity of surface contributes to stronger chloride adsorption, more Ca-Cl accumulation and longer resident time.