The swelling of Ca-montmorillonite at elevated temperatures is important for many applications including geological disposal of radioactive waste, subsurface carbon sequestration, and shale gas exploration. However, the experimental observations regarding the swelling behaviors of Ca-montmorillonite in contact with liquid water and the temperature effects on the swelling pressure are not well understood. In this work, molecular dynamics simulations were carried out to study the swelling of Wyoming Ca-montmorillonite with a d-spacing (d) range of 1.40-4.00 nm at 5 MPa and various temperatures (298-500 K). ClayFF and SPC were adopted for modeling Ca-montmorillonite and water, respectively. The simulation box measured 11.15, 3.66, and 28.00 nm in the x, y, and z-directions. Atomistic pistons were used to control the bulk pressure of the water environment and implicit walls were implemented for preventing the ion leakage from the pore into the water environment. The clay atoms were fixed during the simulation and the swelling pressures were calculated by dividing the force by the area. The equilibrium time is at least 20 ns and the production time falls in the range of 50-88 ns. The swelling pressure results show that for small d, high temperature decreases the magnitude of the oscillating curve of swelling pressure and decreases the range of d where hydration force dominates the swelling pressure. This temperature effect is due to the weakened hydration force as evident from the weakened water density distributions inside the pore. For large d, high temperature decreases swelling pressure consistent with experiment and increases the range of d where double layer force dominates the swelling pressure. The reduction of the swelling pressure can be explained by the enhanced ion correlation that decreases double layer force according to the strong coupling theory given the calculated coupling parameters at higher temperatures are smaller. The swelling pressures are negative at elevated temperatures and large d, which prevents further swelling of clay. However, the classical Poisson-Boltzmann (PB) equation predicts positive double layer force since the ion correlation effect is not considered in the PB equation. Furthermore, the calculated swelling free energy curve shows that at 298 K and 5 MPa, it is difficult for Ca-montmorillonite to swell beyond a d-spacing of around 1.9 nm, which is in good agreement with the experiment. The energy barrier for Ca-montmorillonite to swell to large d is larger than that of Na-montmorillonite, which means it is more difficult for Ca-montmorillonite to swell to large d. This behavior is consistent with experimental observation and can be explained by the larger ion correlation effect in the Ca-montmorillonite system. These findings enhance the understanding of swelling of Ca-montmorillonite at elevated temperatures and could help to engineer better barrier materials for nuclear waste storage.
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