Compacted bentonite and sand-bentonite mixtures are attracting greater attention as buffer material for repositories of high-level nuclear waste. This buffer material is expected to fill up the space between the canisters containing the waste and the surrounding ground by swelling. To produce the specifications, such as dry density, sand-bentonite mass ratio, and dimensions, of the buffer material, the swelling characteristics of compacted bentonite and sand-bentonite mixtures must be evaluated quantitatively. New equations for evaluating the swelling behavior of compacted bentonite and sand-bentonite mixtures are presented that can accommodate the influences of the sand-bentonite mass ratio and the exchangeable-cation composition of bentonite. The new method for predicting swelling characteristics is presented by combining the new equations with the theoretical equations of the Gouy-Chapman diffuse double layer theory and of the van der Waals force, which can evaluate the repulsive and attractive forces of montmorillonite mineral (i.e., the swelling clay mineral in bentonite). Furthermore, the applicability of the new prediction method has been confirmed by comparing the predicted results with laboratory test results on the swelling deformation and swelling pressure of compacted bentonites and sand-bentonite mixtures.
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