Add time:09/05/2019         Source:sciencedirect.com

The movement of water and ions in the CALCIUM SILICATE HYDRATE (cas 1344-96-3) (C-S-H) gel pores determines the durability for the material. In this study, molecular dynamics was utilized to study aqueous NaCl solution capillary transport through the C-S-H gel pore with pore size of 3.5 nm, 2.5 nm, 1.5 nm and 1 nm. The penetration depth for the solution advancing frontier with menisci shape follows a parabolic relation as the function of time, matching well with classic LW capillary adsorption theory. The progressively reduction for the solution/C-S-H contact angle reflects the hydrophilic nature of the C-S-H surface. With decreasing of C-S-H gel pore size, the ions are filtered by the small C-S-H gel pore, with water invading deeply in the gel pore, and chloride and sodium ions remaining in the region of gel pore channel entry. The ultra-slow transport mechanism for ions in the nanometer channel has been further explained by the local structure and dynamics of ions hydrated ultra-confined in the gel pore. While the silicate chains in C-S-H surface can provide non-bridging oxygen sites to associate with the sodium ions by Na-Os connection, the courter calcium ions in C-S-H surface can capture the chloride ions, forming the ionic pair. Both Na-Os and Ca-Cl pairs have longer resident time that the unstable H-bond connection between water and solid oxygen. Furthermore, the transition region with slow mobility is formed at the channel entry region due to the coupling immobilization effect by both surfaces normal and parallel to the transport pathway. The presence of transition zones results in the necking of channel entry, which inhibits the ions with larger hydration shell from penetration. Additionally, at channel with size less than 2 nm, the Ca-Cl and Na-Cl ionic pairs accumulate to cluster and are highly concentrated in the center of the gel pore, blocking transport pathway for water molecules and ions.

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