Involved mechanisms when exposing Opalinus Clay of the lower sandy facies (LSF) of the Mont Terri URL to highly saline and alkaline solutions

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Yara Barakat*, Yujun Cui, Nadia Mokni, Frédéric Bernier

In Switzerland, the Opalinus Clay is considered as potential host rock for deep geological disposal of radioactive wastes due to its favorable hydraulic and mechanical properties as well as its good retention properties for radionuclides. During the operational phase of the repository, alkaline plumes may seep from concrete degradation and saline plumes may seep from saline wastes increasing the pH and salinity of the underground water. Within this context, it is of great significance to study the chemical-hydro-mechanical behavior of a natural claystone by identifying the different involved mechanisms. In this study, two Opalinus Clay samples from the lower sandy facies of Mont Terri site were exposed in an oedometer cell for 31 days (1) to a synthetic water having the same chemical composition as the underground porewater and (2) to a sodium nitrate solution having an osmotic suction of 34 MPa. A third sample was exposed for 17 days to an alkaline solution at pH=12.5. The pore size distributions of the three samples were determined through mercury intrusion porosimetry and nitrogen adsorption-desorption tests. It was found that, while a higher density of macropores was detected at 34 MPa, a higher mesopore density and a more pronounced peak were found at 1 MPa. These results were due to the transfer of water from the meso to the macro pores under the imposed osmotic gradient and to the contraction of the clay particles as a result of the shrinkage of the diffuse double layer. As for the impact of alkalinity, a higher macropore density and a bigger peak were recorded at pH=12.5. These results are due to the acid-base attack, concentrated at the edges of the clay particles that have led to a dispersed structure with higher repulsive forces and to the dissolution of the clay minerals. In addition, the dissolution of non-clay minerals (quartz and calcite) is to be considered.  

Keywords: Opalinus clay; salinity; alkalinity; dissolutions; quartz;, calcite; clay minerals; diffuse double layer. 

Proscience vol. 9

Pp 1-7

DOI: 10.14644/EC2023.001