Monitoring semi-free swelling and water transport in bentonites using X-ray radiography

Clay materials such as montmorillonite-rich bentonite, are planned to be used as buffer materials between waste canisters and host rock in deep geological radioactive waste repositories. To ensure the safety of the repository over very long time scales, it is essential to understand the mechanical and transport properties of the clay materials through measurements and modelling. Here, an experimental method based on X-ray radiography was used to gather temporal data on the hydro-mechanical behaviour and homogenisation of three bentonites with different initial dry densities and wetting solution chemistry. The results showed that the salinity of wetting solution and the dominant cation of montmorillonite affect the overall saturation and swelling process of the clay material thus indicating that there is a chemical coupling to the hydro-mechanical behaviour of montmorillonite-rich clay materials. In particular, higher salinity was associated with more rapid wetting and a more homogeneous distribution of bentonite material in the final state of the experiments. The results also highlighted that the new measurement method is much faster and less cumbersome than traditional gravimetric methods. The method yielded detailed data on the wetting and swelling processes, and may thus help obtain better insight on the intricate hydro-mechanical properties of clay materials.