Coupled processes across a 10-year-old clayrock/concrete interface: results of a combined X-ray CT and PET transport experiment

Interfaces between clay and cementitious materials are studied in the context of deep disposal of radioactive waste. Contrasting porewater chemistries lead to transport and chemical reactions that modify the pore network and affect transport. Research efforts were directed towards mineralogical and physical characterisation of interface regions (e.g. Mäder et al. 2017, Swiss J. Geosci. 110, 307) but little evidence exists on direct observations of transport behaviour across complex skins. We aim at providing evidence on how mineralogical-physical changes at such an interface affect transport of water and solutes, and linking mineralogical-physical characterisation.
A core was recovered at the Mont Terri rock laboratory (CI Experiment), containing a physically preserved interface between Opalinus Clay and Portland cement (PC) concrete reacted for 10 years. A long-term transport experiment was set up by injecting a synthetic claystone pore water into the core on the clay side, and forcing advection/diffusion across the interface and out of the cement side.
Before injection, the core was tomographed by X-ray CT; the clay part showed pre-existing bedding-parallel weak jointing and the PC concrete contains aggregates and gas pores. Figure 1 (left) shows the core skeleton during the infiltration, i.e. only the aggregates in the concrete and the dense Opalinus Clay.
A series of X-ray CT scans over time showed the change in porosity, while PET (positron emission tomography) directly images the mobile phase in 3D, and its penetration as a function of time (Kulenkampff et al., 2016, Solid Earth, 7, 1217). The sample was monitored frequently by high resolution X-ray CT during the first 4 months. 124I was used as PET tracer in the infiltrating synthetic claystone pore water, and the chosen dose allowed for continuous PET scanning during two weeks. Figure 1 (right) shows the flow observed by PET superimposed to the skeleton of the core. PET captured some preferential flow across claystone along some remaining joints, a large spreading of the tracer plume at the clay/cement interface, and some preferential flow across the PC.
The mineralogical and chemical changes coupled to the time-resolved 3D X-ray CT and PET scans (imaging both the stationary and the mobile phase) provide detailed information of coupled processes in complex porous media, e. g. how the dissolution of hydroxide cement phases and the precipitation of carbonates are influenced by advection/diffusion and vice versa.
Partial funding from the European Union's (Euratom) Horizon 2020 Programme under grant agreement 662147 – Cebama is acknowleged, and contributions by Nagra and the Mont Terri Consortium (CI Experiment) to Uni Bern.