Compacted bentonite–sand mixtures are commonly considered as the key materials for the construction of engineered barrier systems (EBSs) in deep geological repositories (DGRs) in Canada and other countries. These bentonite-based barriers can be simultaneously exposed to a wide range of chemical and thermal (nuclear waste heat decay) conditions of underground water. Thus, an understanding of the effects of groundwater chemistry and temperature and their interactions on the changes in the swelling properties as well as the microstructure of bentonite-based barriers is essential for assessing the durability and safety of an EBS in a DGR. This study experimentally investigates the impact of a Canadian deep underground water chemistry (C; total dissolved solids up to 300 g/L), temperature (T; 23 °C, 40 °C, and 80 °C), and the coupled effects of C and T on the swelling properties and microstructure of bentonite–sand barrier mixtures (30 to 70% by weight). The results show that the groundwater chemistry of southwestern Ontario (Canada) has a significant impact on the swelling characteristics of the examined bentonite–sand material. The swelling ability significantly decreases with respect to the groundwater chemistry. The mineralogical composition of the bentonite changes. Most of the Na-montmorillonite is mainly transformed to Ca-montmorillonite and traces of illite with a lower swelling capacity when the bentonite–sand barrier material is exposed mixed to saline water solutions. The temperature (alone) has a limited effect. However, the temperature–groundwater chemistry interactions significantly affect the swelling capacity of the studied bentonite–sand material. Future safety assessments and the final engineering design of engineered barriers for DGRs in Ontario should take into consideration the possibility of the long-term deterioration of bentonite–sand barriers from chemical attacks and/or thermochemical attacks induced by the high salinity of the groundwater and by simultaneous impact of the chemistry of the groundwater and the heat generated by the nuclear wastes, respectively.
