Bentonite homogenization with technological voids upon hydration

Abstract

During the construction of the nuclear waste geological repository, the inevitable creation of technological gaps will lead to the redistribution of bentonite material along the wetting path upon water intake, thus impacting its localized hydraulic and mechanical properties. It is therefore of the utmost importance to be able to understand and predict the dry density inhomogeneous distribution throughout the buffer zone. This work proposes a microscopic dynamic force balance framework, incorporating the influence of van der Waals force, diffuse double layer force, and boundary friction force, to explore the underlying mechanism of the bentonite homogenization process upon hydration. The microscopic assumption postulates that bentonite materials are an assembly of numerous parallel thin sheets with other dimensions substantially exceeding the thickness, establishing an association between the aforementioned force and the material’s volume fraction. Particular attention is directed toward exploring the mechanism of boundary friction's effects on the inhomogeneous distribution of dry density with hydration time. The application of the framework to examine the impact of engineered voids with 10% and 20% of the total volume yields a satisfactory reproduction in terms of the spatial and temporal evolution of dry density, validating the robust prediction capacity of the established model. The investigation indicates that compacted blocks in proximity to technological voids experience a rapid reduction in dry density upon hydration, leading to the homogenization process characterized by elevated values in the initial void-affected region and diminished values in the innermost area. Whereas full homogenization is yet to be achieved within the observation period of up to 2160 h, and the greater the variability of the dry density within the specimens containing larger initial voids. Further analysis reveals that as the hydration process advances, the friction force within the portion featuring the initial void exhibits a gradual increase, while emergence gradually decreases in the inner swelling zone. Notably, the high friction factor at the bentonite and steel interface allows for the maintenance of large stress gradients and the associated density gradients.