NUMERICAL EVALUATION OF CALCIUM LEACHING FROM CEMENTITIOUS MATERIALS IN CONTACT WITH BENTONITE

Abstract

The use of concrete with bentonite is being investigated as a barrier material for the radioactive nuclear waste in Japan. Because such waste contains materials with long half-lives, the barrier must remain stable for several tens of thousands of years, which is far longer than the lifetimes of conventional infrastructures under normal use. Here, it is important to investigate the effect of interaction between concrete and bentonite. In this study, a unified approach that can be used for both concrete and bentonite is developed in order to simulate the long-term durability of cementitious composites considering the influence of surrounding bentonite on the degradation process of concrete. To predict the calcium leaching, a multi-phase equilibrium of calcium ions and their transport are formulated on the basis of thermodynamics. For the bentonite, the proposed equilibrium formulation considers the absorbed ions as well as the exchanged ions as the bound ions. The relation was determined based on the experimental results for block sample of compacted dense bentonite. The proposed model is verified by comparing to experimental results that investigate calcium leaching of concrete with bentonite. The analysis revealed that the deterioration of concrete developed rapidly when the bound effect of the surrounding bentonite was taken into account. This is because the pores in the bentonite maintained a low concentration of free calcium ions due to the ion binding effect and the constantly high concentration gradient between the concrete and the surrounding bentonite. In addition, the sensitivity analysis shows that consideration of additional absorbed ions as the bound ions has significant effect on the calcium leaching from concrete with bentonite. They mean that it is important to take into account the effect of bentonite on the degradation process of concrete by the proper model.