Bentonite swelling behavior under physicochemical controls: A critical review on thermal–electrolytic conditions and mineralogical determinants for nuclear waste management

Abstract

High-level radioactive waste in deep geological repositories relies on bentonite buffers to ensure long-term containment by sealing canisters and limiting groundwater transport. Swelling behavior of bentonite, arising from its unique mineralogical and physico-chemical characteristics, is therefore central to repository integrity. Current knowledge gaps governing its swelling capacity include, (i) exploring individual and coupled effects of temperature and electrolyte concentration and (ii) understanding key physico-chemical properties (montmorillonite content, cation exchange capacity (CEC), specific surface area (SSA), and exchangeable sodium percentage (ESP)) influence on thermo-chemical stress mechanisms. This review critically examines the understanding of how temperature, electrolyte concentration, and mineralogical parameters interact to influence bentonite swelling, hydraulic sealing, and long-term stability. Experimental results, international repository programs and case studies consistently show that montmorillonite content, multi-barrier microstructural states, and electrolyte chemistry are decisive in predicting buffer performance. Evidence indicates that swelling pressure follows a temperature-dependent duality, governed by interlayer and interparticle mechanisms, while salinity and pH strongly suppress swelling through diffused double layer (DDL) compression and ion exchange. Comparisons of globally prominent monovalent cations (Na+) dominated bentonites with divalent cations (Ca+-Mg2+) bentonites revealed that Na-rich bentonites exhibited superior swelling, however, its performance declines sharply in electrolyte conditions. At intermediate dry densities, collapse yields isotropic swelling

low and high densities preserve anisotropy, modulating crystalline vs. DDL expansion. Despite clear trends, anomalies remain, this review aims to improve understanding of thermo-chemical swelling responses to identify the most suitable buffer material for nuclear waste repositories, thereby supporting environmental safety and protection. © 2025 Elsevier B.V., All rights reserved.