Effects of Hyperalkaline Solutions on the Water Retention Properties and Microstructure of the Opalinus Clay from the Lower Sandy Facies of Mont Terri Site, Switzerland

In Switzerland, the Opalinus Clay has been selected as a potential host rock for the deep geological disposal of radioactive waste due to its low hydraulic conductivity and favorable swelling properties. During the operational phase of the repository, the host rock will be exposed to pH values as high as 13.5 due to concrete degradation, which will certainly affect its hydraulic properties. This study investigates the effect of pH increase on the water retention properties of Opalinus Clay. A series of samples from the lower sandy facies of the Mont Terri site in Switzerland, at initial dry state, were exposed to a hyperalkaline solution of pH=13.5 and to the synthetic water of pH=7.5 at different water contents. After equilibrium, the total suction was measured with a dew point potentiometer and microstructural analyses were conducted via mercury intrusion porosimetry (MIP) and nitrogen adsorption-desorption technique. It was found that the total suction decreased with hydration and pH increase. Since the two investigated solutions have the same osmotic suction, the decrease in total suction was attributed to the decrease in matric suction. Indeed, the total porosity increased with saturation and pH increase. This was confirmed by MIP data that evidenced an increase in the proportion of macropores, and by Barret–Joyner–Halenda (BJH) data that showed mesopore generation. The specific surface area (SSA) also increased. The previous results were due to mineral hydration and, exclusively in the case of alkaline solution, to (1) the dissolution of quartz and calcite and (2) the acid-base reactions, which were concentrated at the edges of the clay particles, leading to an increase in negatively charged groups and thus to a face-to-face association of the clay particles (dispersion), causing an increase in the repulsive forces between the clay particles. In addition, the weakening of covalent bonds led to the primary dissolution of clay minerals, i.e. silicon and aluminum detachment.