Montmorillonite is a layered swelling clay mineral that has the abilities to absorb water, causing the mineral to swell, and to exchange its structural cations, most commonly Na^+ and Ca^{2+}. These properties are applied in various fields including the nuclear waste management in Finland. Montmorillonite is the main component of bentonite clay which is planned to be used as a release barrier material in the final repository for spent nuclear fuel.
The aim of this work was to study how water is absorbed into the interlayer spaces of Na-montmorillonite. Molecular dynamics simulations were performed on a 3-layered montmorillonite particle surrounded by free water. The amount of water initially present between the layers was varied from none to 1 and 2 water molecules per unit cell. The simulations were performed at two temperatures, 298 K and 323 K, applying CLAYFF force field.
The evolution of water content showed practically no absorption at either temperature in the case of completely dry montmorillonite. For the other cases, montmorillonite with water initially present in the interlayers, absorption was observed and it was faster at the higher temperature. The evolution of interlayer thicknesses in each case showed a variation between the two interlayers of the system which was thought to result from the different placement of substitutions in the clay layers.
