Swelling pressure of montmorillonite with multiple water layers at elevated temperatures and water pressures: A molecular dynamics study

The swelling of clay at high temperature and pressure is important for applications including nuclear waste storage but is not well understood. A molecular dynamics study of the swelling of Na montmorillonite in water at several temperatures (T = 298, 400, and 500 K) and water environment pressures (P-e = 5 and 100 MPa) is reported here. Adopting a rarely used setup that enables swelling pressure to be resolved with an accuracy of similar to 1 MPa, the swelling pressure was computed at basal spacings 1.6-2.6 nm (or 2-5 water layers between neighboring clay sheets), which has not been widely studied before. At T = 298 K and P-e = 5 MPa, swelling pressure P-s oscillates at d-spacing d smaller than 2.2 nm and decays monotonically as d increases. Increasing T to 500 K but keeping P-e at 5 MPa, P-s remains oscillatory at small d, but its repulsive peak at d = 2.2 nm shifts to similar to 2.0 nm and P-s at different d-spacings can grow more attractive or repulsive. At d > 2.0 nm, P-s is weakened greatly. Keeping T at 500 K and increasing P-e to 100 MPa, P-s recovers toward that at T = 298 K and P-e = 5 MPa, however, the repulsive peak at d = 2.0 nm remains the same. The opposite effects of increasing temperature and pressure on the density and dielectric screening of water, which control ion correlations and thus double layer repulsion, are essential for understanding the observed swelling pressure at elevated temperatures and its response to environment pressures.

Automated summary

The study conducted molecular dynamics simulations to investigate the swelling of Na montmorillonite under high temperature and pressure conditions. It found that the effects of temperature and pressure on water density and dielectric screening are essential for understanding the observed swelling pressure at elevated temperatures and its response to environment pressures.