Experimental characterization of the contact seam between salt cement and rock salt

Cement-based salt concrete (concrete with crushed salt filler) is a potential building material for seals in rock salt formations. The contact seam between sealing element and the excavation damaged zone (EDZ) of the host rock is a critical path way for transport of aqueous solutions and the release of radionuclides to the biosphere. For an assessment for the long-term integrity of seals it is therefore important to know at which rate the seam closes up upon contact with saline solution and confining stress.
The investigation of the sealing capacity and the closure of the contact seam influenced by saline solutions and confining stress was investigated in laboratory tests at GRS. The available core material originates from a sealing element of a former German salt mine and was exposed to confining pressure of the host rock for about ten years. The rock salt originates from the EZD of the sealing element. The salt concrete core was coated with salt slurry and inserted in a hollow rock salt cylinder for the preparation of a combined sealing element at laboratory scale.
Afterwards the combined sample was dried and installed in an isostatic cell. The development of permeability by percolation of saturated NaCl solution and confining pressure was investigated on various combined samples at GRS. All samples showed a significant reduction of permeability with time. One sample was dismantled and prepared for investigation with PET.
Following, transport of radiotracers through the sample and along the seam was investigated with positron emission tomography (PET). PET could clearly and quantitatively display the propagation of the tracer along the seam and inside the salt concrete matrix. It is thus a laboratory-scale method that is capable of non-destructive quantitative visualization and parametrization of this particular heterogeneous tracer transport . The investigation also substantiated the issue of unfavorable conditions of the seam for the safety of the barrier.
The method of GeoPET has been developed and advanced at HZDR. A number of show case experiments could prove its capability for spatiotemporal quantification of tracer transport experiments (Kulenkampff et al., 2016) with very high sensitivity (“picomolar”) and reasonable spatial resolution (about 1 mm).
Here, a small volume of brine, labelled with the PET-tracer 22Na, was injected into the head space between the end cap and the sample. It was observed that a portion of the tracer was sucked into the contact zone between rock salt and cement immediately after injection. The spatial tracer distribution rapidly stabilized as a patchy structure (Fig. 2). Then, during the observation period of 71 days it diffused into the cement and reached a mean depth of 5 mm.