Molecular Dynamics Study on Solution Transport Properties in CSH/SiO2 Nanochannels Under High-Temperature and Water-Rich Environments

This study employs molecular dynamics simulations to construct a CSH/SiO2 nanochannel model, systematically investigating the dynamic transport behavior of pure water and Na2SO4 solution within the pores across temperatures ranging from 293 K to 368 K. The results elucidate the fluid intrusion mechanism under the combined effects of elevated temperature and sulfate ions. Results show that: (i) Water migration within the CSH/SiO2 channel exhibits wall-dependent differences. The interaction between water and the CSH surface is approximately eight times stronger than with the SiO2 side at 293 K, causing water molecules to preferentially advance along the CSH side. As the penetration depth difference between the two sides increases, local water molecules extend toward the SiO2 side, forming liquid finger-like protrusions that lead to rapid filling on the SiO2 side. (ii) Elevated temperature enhances water molecule mobility; at 368 K, the mean square displacement of water in the pure water system is about 3.7 times greater than at 293 K, and the critical time for liquid finger formation on the SiO2 side decreases from 700 ps at 293 K to 100 ps at 368 K. (iii) The introduction of Na2SO4 solution alters the transport mechanism within the nanochannel. Na+ and SO42− form ionic coordination structures with oxygen and calcium sites on the CSH surface, establishing Na-OCSH and S-CaCSH ionic bonds, which reduces the solution’s transport rate. Na+ and SO42− adsorb and accumulate near the CSH/SiO2 interface; heating weakens their hydration shells and strengthens the ion association between Na+ and SO42−, promoting the formation of local ion clusters.