Abstract
Smectite (montmorillonite) is a major clay mineral constituent of the bentonite buffer and backfilling materials to be used for the geological disposal of high-level radioactive waste. Swelling pressure of the bentonite buffer occurring in the permeation process of moisture was estimated based on thermodynamic theory and the thermodynamic data of interlayer water in smectite in this study. The relative partial molar Gibbs free energies (ΔG_<H2O>) of water on the smectite surface were measured as a function of water content (0-83 %) in a dry density range of 0.6-0.9 Mg/m^3. Purified Na-smectite of which interlayer cations were exchanged with Na^+ ions and soluble salts were completely removed, was used in this study. Obtained ΔG_<H2O> decreased with an increase of water content in the range of water content lower than about 40 %, and similar trends were obtained to data of Kunipia-F bentonite (Na-bentonite) of which smectite content was approximately 100 wt.%. From the specific surface area of smectite (ca. 800 m^2/g) and the correlation between ΔG_<H2O> and water content, water affected from the surface of smectite was estimated to be up to approximately 2 water layers. Swelling pressure versus smectite partial density (montmorillonite partial density) was estimated based on ΔG_<H2O> from the chemical potential balance of water in equilibrium between the free water and moisturized smectite, and compared to data measured for various kinds of bentonites of which smectite contents were respectively different. The estimated swelling pressures were in good agreement with the measured data.
