Hydration kinetics of compacted low-density bentonite

Abstract

The hydration kinetics of a compacted low-density bentonite is investigated from both experimental and numerical points of view. A water retention curve and water permeability under saturated conditions were determined to characterize the hydraulic behaviour of the compacted material. The evolution of swelling strains developed during a soaking test performed under oedometer conditions is interpreted as a measure of the hydration kinetics. While soaking was immediate, the vertical strains appeared to develop very slowly during the first two hours of testing. Numerical simulations of the soaking test were performed to achieve a better understanding of the hydration mechanisms. It is shown that the numerical results overestimate the hydration rate of the sample. Two explanations for the very slow hydration are proposed and discussed in the paper, namely the formation of clay gel and an initial drying of the sample.In all concepts of disposal, the engineered barrier will be initially unsaturated and will be subjected to hydration from the surrounding saturated host rock. During this process, it will tend to expand and will develop swelling stresses, hence modifying the structure and the water transfer properties of the material. Although transient, this stage is very important as it is believed to directly impact the final state and effectiveness of the system. A good understanding of the hydration mechanisms is thus essential to assess the performance of the engineered barriers and the time needed for their full saturation.Given the very low permeability of the material and the complex phenomena involved in the problem, it has proved useful to perform elementary laboratory experiments on small samples. These tests generally provide useful qualitative and quantitative information along a given stress path. This information should be complemented with numer-