A Thermodynamic Approach on the Effect of Salt Concentration on Swelling Pressure of Water-saturated Bentonite
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1124-Q07-11
A Thermodynamic Approach on the Effect of Salt Concentration on Swelling Pressure of Water-saturated Bentonite Haruo Sato1 1 Japan Atomic Energy Agency, 432-2 Hokushin, Horonobe-cho, Hokkaido 098-3224, Japan ABSTRACT The effect of salt concentration on swelling pressure (dPext) of water-saturated bentonite was calculated based on the thermodynamic data of water at montmorillonite surface and of water in solutions of various salinities coming in contact with the montmorillonite. Activities (aw) of water at montmorillonite surface were obtained as a function of water content and temperature by a vapor pressure method, and the relative partial molar Gibbs free energies (dGw) of the water were determined. Water affected from the montmorillonite surface was estimated to be almost all interlayer water. dPext versus montmorillonite partial density was estimated for solutions of various salinities and compared to data measured for various montmorillonite contents and silica sand contents. The calculated dPext decreased with increasing salinity. The effect of salinity on dPext was not clear in the measured data due to the scattering in the measured data. This cause is presumed to be due to the increase of the ionic strength of porewater by dissolution of soluble minerals. INTRODUCTION In the safety assessment of the geological disposal for high-level radioactive waste in Japan, bentonite is used for its physical and chemical functions as backfilling and buffer material of the engineered barrier system. Since the major clay mineral constituent of bentonite is montmorillonite which has the nature of swelling, bentonite swells by contacting with groundwater and restricts the groundwater flow. And diffusion field is finally formed. Since swelling pressure (dPext) develops by hydration of the interlayer of montmorillonite, understanding the thermodynamic properties of water on the montmorillonite surface, especially of interlayer water, is important. It has been reported that activity (aw) of the interlayer water of montmorillonite and the chemical potential are smaller than those of free water [1-3]. It also has been reported that the moisture potential and suction increased with decreasing water content (Wc) of the bentonite [4, 5], being indicated that water at montmorillonite surface has a high-moisture potential and a high suction. Thus, in view of thermodynamics, water at montmorillonite surface and water existing in ultra-narrow spaces such as interlayer water should not be treated similarly to free water. The moisture potential of montmorillonite is closely related to dPext. Kahr et al. [6] have predicted dPext values for 2 kinds of bentonites (Montigel, MX-80) versus respective dry densities based on the water vapor adsorption-desorption isotherms, and shown that predictions are consistent with experimental results. Kanno and Wakamatsu [5] also have predicted dPext values for the Kunigel-V1 bentonite based on the moisture potential and arrived to similar conclusions. In a recent study, Tanaka et al. [7] have
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