Swelling deformation of Na-bentonite in solutions containing different cations

Abstract

Investigation of the effect of salt solution on the swelling properties of compacted bentonite is of great importance for evaluation of the properties of behavior of engineering barrier in deep geological repository for disposal of high-level radioactive waste. In the present work, a multitude of swelling deformation tests were performed on sodic bentonite using NaCl, KCl, and CaCl2 solutions. It is revealed from the results that the swelling strains of the tested samples in variety of saline solutions follow the order of NaCl > KCl > CaCl2 at the concentration of 0.1 mol/L (M), whereas the order was NaCl > CaCl2 > KCl at 1.0 M concentration. The fractal em–pe relations for swelling deformation of Na-bentonite in different solutions were estimated, where em is the smectite void ratio at saturation, pe is effective stress incorporating osmotic suction depicting the effects of ionic concentration on swelling. In addition, the effect of the cation exchange is described using the smectite swelling coefficient K. The estimation values fit well with the experimental data. In NaCl solutions, the coefficient K is basically unchanged due to the weak cation exchange reaction. On the contrary, Ca2+ can much easily replace Na+ even at low concentrations and the CaCl2 solution exhibits the highest values of osmotic suction, the swelling coefficient and swelling deformation of the samples in the 0.1 M CaCl2 solution are smaller than that of samples in other 0.1 M solutions. However, at high concentrations, Na+ can be fully replaced by both Ca2+ and K+. The hydration energy of K+ is comparatively lower due to which the swelling coefficient of the samples in the 1.0 M KCl solution significantly decreased thereby resulting in least swelling deformation.