Hydrated cement and concrete are major components of the engineered barrier system in proposed underground repositories for radioactive waste. Concrete was in contact with a clay-rich rock during 15 years in a borehole at the Tournemire Underground Rock Laboratory in France. Overcoring of the borehole and mineralogical analyses have shown a reduction of porosity at the interface due to the precipitation of ettringite, C-S-H/C-A-S-H and calcium carbonate, together with dissolution of portlandite in the cement.
In the framework of the GTS-LCS project (JAEA, Japan; NAGRA, Switzerland; NDA, UK; POSIVA, Finland; SKB, Sweden), new reactive transport modeling (solute diffusion + mineral reaction) has been performed, including a sensitivity analysis with respect to several compositional and kinetic parameters. Results using the CrunchFlow code show sealing of porosity at the rock side of the interface (mm scale) due to the precipitation of C-A-S-H (calcium aluminum silicate hydrate), calcite and ettringite, together with some clay dissolution. The location of sealing is influenced by cation exchange. Inclusion of cation exchange results in sealing at the rock side of the interface. Without cation exchange, sealing is at the concrete side of the interface.
Calculated alteration profiles along cm-scale fractures show increased alteration distances. Sealing of porosity is at the concrete side of the interface, due to the smaller effect of cation exchange.