Internal Forces within the Layered Structure of Na-Montmorillonite Hydrates: Molecular Dynamics Simulation

The hydration swelling property of bentonites, which mainly depends on the content and swelling capacity of montmorillonite (MMT), greatly contributes to many engineering applications such as grouting and barrier materials. Much efforts have been made to investigate the process of crystalline swelling by experimental tests, analytical models, and numerical simulations. However, most of the studies are limited to the microscopic structure of the hydrates and the variation of free energy of the system. Via molecular dynamics simulation, this paper discusses the internal forces within the layered structure of Na-MMT hydrates for the mechanical analysis of crystalline swelling. The repulsive force between crystal layers fLL, the repulsive force of interlamellar water molecules to crystal layer fWL, and the attractive force of interlamellar cations to crystal layer fCL in different hydration phases are calculated to illustrate the relationships among the microscopic structure of hydrates, internal forces, and the macroscopic characteristics of swelling. For fully dry Wyoming MMT, fLL is (3.85 ± 0.39) × 10–9 N/uc (positive for repulsion) and fCL is (−3.93 ± 0.34) × 10–9 N/uc (negative for attraction). With the increase of water content, the three forces reach relatively stable values, which are 3.43 × 10–9, −4.99 × 10–9, and 1.25 × 10–9 N/uc for fLL, fCL, and fWL, respectively. The quantitative analysis shows that the total repulsive force from fLL and fWL is approximately equal to the attractive force fCL during the process of crystalline swelling. The Coulomb electrostatic force term controls the fLL and fCL, while it is comparable to but less than the van der Waals force term for fWL. The Coulomb electrostatic force term of fLL may be regarded as an interaction between "infinite parallel charged plates" when the water content is greater than 6 H2O/uc. The Coulomb electrostatic force term of fWL is mainly caused by the polarization effect of interlamellar water molecules. The crystalline swelling can be presented as a stepwise "expansion–filling–saturation" process via the analysis of the change of average density of interlamellar water, whose values are close to the mass density of bulk water at the "saturation" stages.