Thermochemical Effect on Swelling Pressure and Microstructure of Monovalent and Divalent Indian Bentonites

Abstract

During the long-term operation of deep geological nuclear waste repositories, the swelling properties of compacted bentonites can be affected by canister temperature and electrolyte concentration in groundwater. The present article highlights the combined effect of temperature and electrolytes on the swelling pressure of monovalent Na-bentonite (predominant Na-montmorillonite) and divalent Ca-Mg bentonite (dominated by Ca-Mg montmorillonite and palygorskite minerals). A series of isochoric swelling pressure tests were carried out on compacted bentonites at a dry density of 1.7 Mg/m3 by applying temperatures of 25°C, 55°C, and 95°C and infiltrating with 0.1 and 1.0 M NaCl and CaCl2 electrolytes. The changes in the microstructure of bentonites were examined by using a Field Emission Scanning Electron Microscope (FESEM), X-ray diffraction (XRD), and mercury intrusion porosimetry (MIP). The experimental results indicated that the elevated temperature caused a marginal decrease and increase in the swelling pressure of monovalent and divalent bentonites, respectively. The infiltrated electrolytes caused a significant decrease in the swelling pressure of monovalent bentonite, whereas the effect was found to be negligible for divalent bentonite. At electrolyte concentrations of 0.1 and 1.0 M, the impact of CaCl2 was noted to be higher than that of NaCl. The coupled thermochemical effect caused a significant decrease in the swelling pressure of monovalent bentonite, whereas the effect was nominal for divalent bentonite. The FESEM, XRD, and MIP results confirmed that the thermochemical effect leads to a significant change in morphology, basal position of Na-montmorillonite, and pore structure of monovalent bentonite. The performance of Ca-Mg montmorillonite and palygorskite minerals as buffer material was noted to be better than that of Na-montmorillonite subjected to coupled conditions of elevated temperature and electrolyte concentration.