Chemical effects on the hydro-mechanical behavior of compacted bentonite: a review

Abstract

The hydro-mechanical behavior of bentonite-based barriers plays a key role in ensuring the long-term safe operation of deep geological repositories. However, salinity of groundwater and alkaline solutions generated by concrete degradation all degrade the hydro-mechanical properties of the barrier. Based on a comprehensive review of the previous works, achievements of chemical effects on the hydro-mechanical properties of bentonite were summarized and analyzed. Hydraulic behavior shows that elevated salt concentration enhances water retention through increased osmotic suction and reduced Matric suction, though diminishes beyond 70 MPa suction in GMZ bentonite during wetting. Prolonged alkaline exposure reduces the water retention capacity, accelerated by elevated temperatures. Permeability evolution exhibits ion-specific characteristics—sodium bentonite’s hydraulic conductivity increases with salinity (diffuse double-layer thinning), yet Ca²⁺ induces lower permeability than Na⁺ due to pore-clogging, and alkaline conditions accelerate flow via dissolution-induced preferential channels. With regard to mechanical properties, the swelling behavior is jointly controlled by solution chemistry and mineral phase transitions: high salinity suppresses crystalline/double-layer swelling, cation exchange follows Na⁺< Li⁺< K⁺< Rb²⁺< Cs⁺< Mg²⁺< Ca²⁺< Ba²⁺< Al³⁺, high-density calcium bentonite generates greater swelling pressure than sodium bentonite via thickened adsorption layers, while K⁺ fixation and alkaline-induced phase transformations (e.g., illitization/kaolinization) drive swelling reduction. Mechanical responses involve coupled osmotic consolidation (reduced compression index, elevated yield stress) and chemical softening (elastic domain contraction), with unloading hysteresis governed by preserved face-to-face microstructures. Existing models achieve accurate predictions of hydraulic properties and swelling pressure through liquid limit-concentration correlations, dual-pore structure modifications, and chemically revised effective stress formulations, where hardening modulus sign inversion quantifies chemo-mechanical transitions. Future efforts should focus on three frontiers: quantifying time-dependent swelling/compression under alkaline conditions, establishing multiscale chemo-hydro-mechanical frameworks, and developing constitutive models integrating cation exchange kinetics, K-fixation thresholds, pore reconstruction, and mineral transformation thermodynamics.