Solute transport across an aged concrete-claystone interface (CI-D): tracking diffusion after 4.4 years of tracer circulation

Designs for deep storage of radioactive waste foresee cementitious materials as structural elements, backfill or for waste conditioning. Interactions will take place at interfaces of contrasting materials driven by chemical gradients in pore water causing diffusive fluxes of dissolved species. This may lead to mineralogical alterations in the barrier system, which in turn locally influence properties like permeability, swelling pressure or specific retention properties in the case of concrete in contact with claystone or compacted bentonite. The Cement-Clay Interaction (CI) experiment carried out at the Mont Terri rock laboratory aims to reduce uncertainties in the understanding of processes at cement-clay interfaces by analysing mineralogical changes over relatively long time periods under realistic boundary conditions. This has provided a good understanding of the mineralogical alterations at such interfaces (Mäder et al. 2017, 2026) and its modelling interpretation (Jenni et al. 2017, 2026). The CI experiment included different concrete types emplaced in 2007 and subsequently served as a site for the CI-D experiment which investigated diffusion across a ten-year-old concrete-claystone interface (2018–2023).