Mechanisms of crystalline swelling in bentonite buffer materials: Insights from molecular dynamics

Abstract

Compacted bentonite has been extensively studied due to its critical role in high-level radioactive waste (HLRW) disposal. Understanding the mechanism of crystalline swelling at the molecular level is essential for optimizing bentonite as a buffer material. To investigate the interplay between interlayer molecular conformation, migration, and interactions, molecular models of Na-montmorillonite (Na-Mt) were developed, and hydration behavior was examined through molecular dynamics (MD) simulations. The findings reveal that interlayer water molecules form distinct layers influenced by cation solvation and hydrogen bonding, with hydration layer conformations closely correlated to basal spacings. Na⁺ ions transition from inner-sphere to outer-sphere complexes due to hydration shell formation. The diffusion of Na⁺ ions is restricted by interlayer blocking and adsorption on clay surfaces. Under extreme drying conditions, strong attractions between Na⁺ ions and clay sheets result in interlayer collapse, with interlayer repulsion primarily driven by Coulomb electrostatic forces between adjacent clay sheets. Subsequently, the influx of water molecules acts as the driving force for crystalline swelling. Finally, the relationship between the hydration behavior of bentonite and montmorillonite was analyzed, providing valuable insights into the mineralogical mechanisms underlying Na-Mt swelling. These results contribute to a deeper understanding of bentonite's performance as a buffer material in HLRW repositories.