Diffusion Calculations on Reconstructed Bentonite Microstructures

In the context of radioactive or domestic waste storage management, the use of compacted bentonite as a buffer material seems particularly appropriate due to its exceptional swelling and retention properties. In complementarity with existing molecular or macroscopic approaches, real microstructures are employed to investigate ionic transport by diffusion through water-saturated Wyoming bentonite at an intermediate scale. A particular attention is given to the contribution of electrostatic interactions in the vicinity of the montmorillonite layers’ surface and the presence of clay gels of variable density to the effective ion diffusion. Homogenization computations have been performed using a dual-porosity description of bentonite, with two distinct microstructures involved, at the scale of montmorillonite layers and at the scale of clay gels, mineral grains, and micropores. To construct the Finite Element microstructures, the contours of the phases have been extracted from TEM images using dedicated image processing algorithms. At both scales, asymptotic developments are written for each solute concentration and the electric potential. Several boundary conditions are considered to express electrostatic interactions on the montmorillonite layers’ surface. Anion exclusion and cation inclusion are displayed by ion distribution maps. Through upscaling, an advective and a source term appear in the transport equation, while the effective diffusion tensor derived is similar to the uncharged case, with a strong anisotropy induced by the elongated shape of the platelets. The effective diffusion tensor obtained at the mesoscopic scale of the montmorillonite layers is then used to simulate diffusion at the scale of clay gels and micropores. Qualitative comparisons are made with existing diffusion data at both scales, showing that the orders of magnitude of diffusion are correctly reproduced.