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

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 SO₄^2- form ionic coordination structures with oxygen and calcium sites on the CSH surface, establishing Na-O_CSH and S-Ca_CSH ionic bonds, which reduces the solution's transport rate. Na⁺ and SO₄^2- adsorb and accumulate near the CSH/SiO₂ interface; heating weakens their hydration shells and strengthens the ion association between Na⁺ and SO₄^2-, promoting the formation of local ion clusters.