Molecular dynamics simulation of swelling properties of Ca-montmorillonite at high temperatures
Authors: Yang Ya-Fan , Wang Jian-Zhou , Shang Xiang-Yu , Wang Tao , Sun Shu-Yu ,   State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining and Technology, Xuzhou 221116, China,   Physical Science and Engineering Division, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia,   Institute of Geophysics and Geomatics, China University of Geosciences, Wuhan 430074,  China
Published: February 2022

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 experimentally observed swelling behaviors of Ca-montmorillonite contacting liquid water and the temperature effects on the swelling pressure are not well understood. In this work, molecular dynamics simulations are carried out to study the swelling of Wyoming Ca-montmorillonite with a d-spacing (<i>d</i>) range of 1.40–4.00 nm at 5 MPa and various temperatures (298–500 K). The ClayFF and SPC are adopted for modeling Ca-montmorillonite and water, respectively. The simulation box is measured to be 11.15, 3.66, and 28.00 nm in the <i>x-</i>, <i>y-</i>, and <i>z</i>-direction. Atomistic pistons are used to control the bulk pressure of the water environment, and the implicit walls are implemented for preventing the ions from leaking from the pore into the water environment. The clay atoms are fixed during the simulation and the swelling pressure is calculated through dividing the force by the area. The equilibrium time is at least 20 ns and the production time falls in a range of 50–88 ns. The swelling pressure results show that for small <i>d</i>, high temperature reduces the magnitude of the oscillating curve of swelling pressure and also reduces the range of <i>d</i> where hydration force dominates the swelling pressure. This temperature effect is due to the weakened hydration force as evidenced from the weakened water density distributions inside the pore. For large <i>d</i>, high temperature reduces the swelling pressure, which is consistent with the experimental result, and increases the range of <i>d</i> where double layer force dominates the swelling pressure. The reduction of the swelling pressure can be explained by the enhanced ion correlation that reduces the double layer force according to the strong coupling theory, given that the calculated coupling parameters at higher temperatures are smaller. The swelling pressures are negative at elevated temperatures and large <i>d</i>, which prevents the clay from further swelling. However, the classical Poisson-Boltzmann (PB) equation predicts the 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 <i>d</i>-spacing of around 1.9 nm, which is in good agreement with the experimental result. The energy barrier for Ca-montmorillonite to swell to large <i>d</i> is larger than that for Na-montmorillonite, which means that it is more difficult for Ca-montmorillonite to swell to large <i>d</i>. 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.