Gypsum precipitation enhanced by electrokinetic method and porewater chemistry in compacted montmorillonite

An understanding of chemical reactions in bentonite is important in completing a reliable safety assessment of the geological disposal of radioactive waste. In the present study, porewater chemistry and calcium reactivity in a compacted Na/Ca-mixed montmorillonite is explored through a gypsum precipitation reaction enhanced using an electrokinetic method. Results showed that CaSO4 was sparsely precipitated as gypsum in up to 300-μm aggregates, and grew into the montmorillonite texture. A threshold ionic equivalent fraction of calcium, XCa, of the Na/Ca-mixed montmorillonite in the gypsum precipitation was experimentally found to be within a range between XCa = 0.1 and XCa = 0.25 under a dry density of 1.0 kg/dm3 saturated with 0.5 M Na2SO4 solution. The saturation indices of gypsum for specimens following the precipitation experiments were evaluated based on a model including anion exclusion and cation exchange (Gains-Thomas selectivity coefficients, KGT) between the montmorillonite and interparticle solution using the PHREEQC code. Gypsum precipitation regions obtained from the precipitation experiments are in good agreement with regions in which the saturation indices are greater than zero when assuming KGT = 200, which is much greater than the KGT = 4.9 reported from the batch experiment. The greater KGT value implied in this study indicates that the thermodynamic properties of confined solutions are different from those of diluted or bulk solutions. The reason for the KGT deviation remains unresolved; however, the precipitation tendency of gypsum may be explained by a model including anion exclusion and cation exchange between the montmorillonite and the interparticle solution when an adequate KGT value is used in the calculation.