Calcite Precipitation at Cement–Bentonite Interface. Part 2: Acceleration of Transport by an Electrical Gradient

Abstract

Calcium leaching from cementitious materials in contact with bentonite in nuclear waste repositories can alter the functionality of an engineered barrier system. In this study, we contribute to the fundamental understanding of calcite precipitation at cement–bentonite interfaces by adding carbonate to bentonite. In addition, we accelerate the transport of charged reactants towards the interface using an electrochemical migration method. The carbonate admixture successfully promotes calcite precipitation at the surface of cement paste. The analysis also revealed that the amount of precipitated calcite is not simply correlated to the amount of added carbonate or the applied electrical potential. Experiments in which bentonite pore water contains high initial contents of carbonate exhibit rapid calcite precipitation in a very narrow region at the cement–bentonite interface, resulting in pore clogging. This is the second of two papers; the system evolution without an electrical gradient was discussed in the first paper. This paper is the extended version in English from the authors’ previous works [Watanabe and Nakarai, (2008). “Effect of NaHCO3 in bentonite on calcium leaching from cementitious material.” Proc. of the JCI, 30(1), 717-722. Nakarai, et al., (2010). “Effect of carbonate mixing into bentonite on calcium leaching of cementitious material.” Proc. of the JCI, 32(1), 713-718. (in Japanese)]