Research progress in mechanisms of swelling pressures of compacted bentonite under chemical conditions

Chemical compositions affect the long-term stabilization of the swelling properties of bentonites, which is significant to the safety of deep geological repositories. To review the chemical effects on the swelling pressures of bentonites, the latest advances in the hydration tests, microscopic mechanisms and numerical models are summarized. The results show that the swelling process of the compacted bentonite is influenced by the coupling of chemical solutions (ion type, concentration) and bentonite properties (exchangeable cation, compacted dry density). The salinity and cation exchange reaction work on the three main hydration mechanisms, including the crystalline swelling, breakup of quasicrystals, and diffuse double-layer swelling. Compared to the hydration swelling models, the elastoplastic constitutive models are more applicated and accurate, which quantify the salinity by osmotic suction, and the cation exchange reaction by stiffness of crystal layers. The deficiencies of the current mechanisms and model studies include the failure to determine the boundary between crystalline and diffuse double-layer swelling, due to the special structure of bentonites (unit layers, quasicrystals, and aggregates), the failure to establish the relationship between microscale and macroscale based on the theories of hydration mechanisms, and the neglecting of the synergies between multi-field conditions and C-H-M behaviors of bentonites in the engineering scale. Thus, further multi-scale systematic tests, multi-field coupling theories and dynamic co-evolution models are expected.