A macroscopic model for predicating stepwise crystalline swelling of montmorillonite

Abstract

Bentonite, as an optimal buffer material in high-level radioactive waste (HLRW) repository, is usually unsaturated and sensitive to relative humidity (RH) conditions. Accurately modelling unsaturated crystalline swelling of bentonite is crucial for estimating the performance of a geological repository. In this study, we attempt to extend Laird model, a classical model for crystalline swelling, from saturated to unsaturated. For this purpose, a binary system that comprising air phase and water phase is introduced into Laird model, then the concept of effective diabattivity for atmosphere with a certain RH is proposed. On the basis, the net hydration repulsions under various RHs conditions can be determined. To calibrate the effective diavattivity, the oriented montmorillonite (Mt) tactoids is prepared and the RH controlled X-ray diffraction test is carried out. Moreover, a coupling approach of molecular dynamics (MD) simulation and fluctuation–dissipation dielectric theorem is employed to gain interlayer dielectric from dipolar fluctuations, which further improves the model accuracy. The novel model is validated by the experimental results for 3 typical buffer materials, including MX-80, Kunipia and FEBEX bentonites. Comparisons between the predicted and measured basal spacings indicate the proposed model can effectively capture the stepwise nature of crystalline swelling. Besides, the model also provides a quantitative approach to track the evolution of interaction energy during hydration process.