Chloride ions transport and adsorption in the nano-pores of silicate calcium hydrate: Experimental and molecular dynamics studies

Abstract

Water and ions transport in the nano-meter channel of the porous calcium silicate hydrate (C-S-H) gel was investigated by the experiments and molecular dynamics. The C-S-H gel was first synthesized and the purity, composition and chemical structure were characterized systematically by X-ray diffraction (XRD), Thermogravimetric analysis (TG) and Fourier transform infrared spectroscopy (FTIR). The synthesized C-S-H sample was immersed in the two types of chloride solution for the ions adsorption study. It indicates that the calcium ions can promote binding ability of the C-S-H sample. Furthermore, the interface model of the chloride solution in C-S-H gel pore of 4.5nm was constructed by the molecular dynamics to unravel chloride ions adsorption mechanism. As the carrier of the chloride ions, the water molecules in the vicinity of C-S-H substrate show ordered layered packing and extremely low diffusion coefficient due to the strong chemical restriction from silicate chains. The structural and dynamic behavior of chloride ions are greatly influenced by the states of the calcium ions near the C-S-H surface. Radial distribution function of Ca-Cl shows strong spatial correlation at 2.7Å, indicating the Ca-Cl cluster formation. The interfacial calcium ions are defined as solid and liquid state to represent the increase of calcium to silicate ratio (C/S) of C-S-H samples and the increase of concentration of calcium ions in the solution. In the former case, the calcium ions penetrate into the silicate channel and transform to the solid state, which contributes to the chlorides adsorption near the surface significantly. In the latter case, the calcium ions, adsorbed by oxygen atoms in the silicate chains, resemble the feature of solvated ions in the bulk solution. Because of the unstable connection of Ca-O-Si, the Ca-Cl cluster cannot remains in the C-S-H surface for long time.