Microstructural Evolution, Water Infiltration, and Swelling Pressure in Expansive Clays: Insights from a Multiphase Molecular Model

JP Du and AN Zhou and Y Zhong and ZZ Wang, JOURNAL OF PHYSICAL CHEMISTRY C, 129, 21496-21507 (2025).

DOI: 10.1021/acs.jpcc.5c06827

The performance of engineered barrier systems critically relies on the swelling behavior of compacted bentonite, yet the microscale mechanisms coupling the pore structure of bentonite, hydration dynamics, swelling pressure and internal force development remain poorly understood. Here, we develop a multiphase molecular model to simulate confined hydration of montmorillonite (MMT, dominated clay mineral in bentonite) platelet assemblies with atomistic resolution. Results reveal that dry density and initial water content jointly control both structural and hydraulic responses. Increasing dry density from 1.2 to 1.8 g/cm3 reduces lateral pore connectivity by over 60% (for w 0 = 0%), while higher initial water content suppresses chemical potential gradients and slows water ingress. Consequently, denser and wetter assemblies exhibit restricted, anisotropic infiltration and reach hydration equilibrium earlier, whereas looser and drier systems show rapid, isotropic infiltration with up to 50% higher water uptake rates. Swelling pressure evolves in three stages, including contraction, expansion, and stabilization, where denser and drier assemblies show higher equilibrium values. Force analysis captures the transition from contraction to expansion during wetting: initial interlayer attractions dominate during the contraction stage, while progressive hydration enhances repulsive interactions that trigger structural expansion and sharp stress buildup. Platelet orientation shows that assemblies with high S-order transmit stresses more uniformly and achieve higher swelling pressures, while those with lower S-order show heterogeneous force distributions and weaker responses. This study establishes a mechanistic framework that links microscale swelling mechanisms of compacted bentonite, providing predictive insight into bentonite buffer performance in deep geological repositories.

Return to Publications page