Molecular dynamics simulation of sodium montmorillonite hydration under complex buried conditions

The phenomenon of rock structure disintegration caused by the hydration of clay minerals is closely related to engineering safety problems such as slope landslide, tunnel collapse and borehole collapse. In order to reveal the hydration mechanism of sodium montmorillonite under deep and complex burial conditions, molecular dynamics simulations on the hydration process of sodium montmorillonite at different burial depths were carried out by using molecular simulation techniques, verify the hydration expansion characteristics of sodium montmorillonite, and realize the quantitative analysis of key physical and chemical parameters in the hydration process. The simulation results show that with the increase of interlayer water content, the crystal layer spacing rises in a step pattern and presents a stratification phenomenon. With the increase of burial depth and interlayer water content, the volume of sodium montmorillonite increases, while the density decreases correspondingly. The increase of interlayer water content promotes the increase of hydrogen bonds, and significantly increases the self-diffusion coefficients of water molecules and Na⁺ ions. The self-diffusion coefficients of water molecules and Na⁺ ions in deep buried complex conditions are significantly higher than those at normal temperature and pressure. With the increase of water molecular layer, the main peaks of Na-Ow, Na-Hw, Ow-Hw, Ow-Ow and Os-Hw showed a tendency to gradually weaken, and the peak values of these main peaks were different under different burial depths. Simultaneous, the degree of water polymerization first increased and then decreased, the coordination number, the polymerization degree, the ionic hydration number and hydration radius of Na⁺ ions decreased. With the increase of burial depth, the degree of water polymerization is little different, and the hydration characteristics of Na⁺ ions only decrease slightly. The research results can be used to guide the theoretical analysis and engineering practice such as oil drilling, coal seam mining, slope stability evaluation and tunnel excavation.