Effect of Interlayer Cation on the Desiccation and Shrinkage Behavior of Bentonite

Abstract

Bentonite, used as a barrier material in landfill systems and the geological disposal of radioactive waste, undergoes various processes such as the cation exchange with landfill leachate and repository groundwater, desiccation shrinkage due to heat released from solid waste or nuclide decay. In the context of these engineering applications, it is crucial to consider the effect of interlayer cation composition on the desiccation-shrinkage behavior of bentonite for assessing the long-term performance of barrier systems. This study investigates this impact by conducting desiccation tests on four different cation-type bentonite samples (i.e., K-, Na-, Ca-, and Mg-bentonite) and elucidating the underlying mechanisms through mercury intrusion porosimetry test, X-ray diffraction analysis, and suction measurements. The results indicate that K-bentonite exhibits the highest water evaporation rate and the lowest shrinkage potential, with no shrinkage cracks detected, and that Na-bentonite evaporates water at a relatively fast rate but presents the most pronounced volumetric shrinkage. In contrast, Ca- and Mg-bentonite have the highest water-retention capacity, and their shrinkage potentials are less than Na-bentonite. An improved shrinkage model is proposed, which is shown to describe the measured shrinkage curves for four types of homoionic bentonite samples. Additionally, the mechanism behind the effect of different interlayer cations was interpreted in terms of the interlayer–cation–water interaction, pore structure evolution, and soil suction variation.