Claystones are considered as potential host rocks for deep geological waste repositories. During excavation, a Damaged Zone ( \(\text{EDZ}\) ) is created. This \(\text{EDZ}\) is expected to be sealed with underground water seepage. During the operational phase, the degradation of concrete structures will act as a source of alkaline plumes increasing the \(\text{pH}\) of the ground water, further affecting the sealing of the fractures within the \(\text{EDZ}\) . This study focuses on the effect of alkalinity on the self-sealing of the Opalinus Clay from the lower sandy facies ( \(\text{LSF}\) ) of Mont Terri site, considered as potential host rock in Switzerland. Both hydraulic conductivity and X-ray computed microtomography (μ-CT) tests were performed. Series of initially unsaturated and artificially fractured \(\text{LSF}\) Opalinus Clay samples were exposed to the synthetic water or the concrete pore water. It was found that the hydraulic conductivity of the initially unsaturated samples exposed to the synthetic water varies non-monotonically with time. While at first the hydraulic conductivity increased suggesting the generation of secondary fractures with hydration, a decrease was then observed underlining the self-sealing. After \(183\) days of exposure, the hydraulic conductivity was one order of magnitude lower than the one at \(47\) days and two orders of magnitude greater than the one for the undisturbed material. Based on µ-CT observations, the self-sealing of the Opalinus Clay after \(32\) days of exposure to the synthetic water and to the alkaline solution was of \(37\%\) and \(33\%\) , respectively, suggesting that the self-sealing of the investigated claystone occurs even under high \(\text{pHs}\) .
