In situ X-ray diffraction observation of smectite hydration under constant volume

Smectite (especially, montmorillonite) is the major clay mineral constituent of bentonite, which is designed to play a key role as a buffer material in geological repositories for the final disposal of radioactive waste in Japan. It is therefore crucial to understand the hydration behavior of smectite in terms of swelling during hydration and saturation processes.Against such a background, the authors simultaneously observed behaviors of smectite swelling at the micro-level (i.e., both the generation of swelling pressure and the change of hydration state). In the experiments, deionized water was allowed to permeate into a dried specimen of smectite (named Kunipia-F®) with different dry densities (ρ_d: 0.97, 1.23, 1.43, 1.64 and 1.88 Mg/m³) under conditions of constant temperature and volume. The swelling pressure was measured using an in situ uniaxial consolidation apparatus during the water feeding process. Changes in local hydration states (i.e., one-molecular-layer hydration states to three-molecular-layer hydration states) were also simultaneously observed. Hydration among these different states propagated from the inlet side to the outlet side of the specimen. The authors discussed the relationships governing the hydration state, swelling pressure, the number of hydration moles, dry density, equilibrium final pressure, and then the dynamic mechanism behind pressure propagation.