Sr, C and O isotopes as markers of alkaline disturbances in the Toarcian argillites of the Tournemire experimental platform (France). Case of a 15-years old engineered analogue

In France, the concept of a geological disposal of high-activity and long-period nuclear wastes requires the use of concrete and cement-bearing materials as building structures or as waste containment packages, in conjunction with clayey barriers (e.g., compacted bentonite as an engineered barrier and/or argillite-type rocks as a geological barrier). Hydrolysis of cementitious phases is however known to produce hyper-alkaline pore fluids with pH ranging from 10 to 13.5 that will be in disequilibrium with the geological setting environment (argillite pore-water pH around 8). The disturbance of clayey rocks in contact with such materials is thus an important task in safety assessment studies of deep geological storage. This concerns the knowledge of mineral / solution paths but also the spatial extent of the alkaline plume into the clayey material. Experimental and modelling approaches were performed this last decade to answer these questions. In addition to these approaches, natural or engineered contexts in which a clayey formation has been in contact with cementitious materials can be considered as analogues of a deep geological storage for the study of argillite /cement interaction. Such contexts can be found in the IRSN Tournemire experimental platform in Aveyron (France). This platform is based on a tunnel, excavated between 1882 and 1886 through Domerian marls and Toarcian argillites, which is dedicated since 1990 to multidisciplinary research programs. In the frameworks of these programs, exploration boreholes were realized from the basement of the tunnel in the 1990 years. The boreholes were then filled with concrete and cement that are presently in contact with the Toarcian argillites for 15-20 years. One of this borehole - DM borehole - was overcored in 2005 in order to collect the Toarcian argillites in contact with the cement and the concrete. Mineralogical, petrographic and microstructural analyses have argued for a clear disturbance of the Toarcian argillites close to the cement-concrete contact. These data are detailed in another session of this meeting (Techer et al., ERE6 session). In order to precise the spatial extent of the disturbances and to discuss the nature of the responsible fluids, a systematic chemical and isotopic study was performed focusing on the Sr, C and O isotopes known to be very good markers of alkaline fluids percolation (Fourcade et al. 2006). Four studied levels were selected according to their location on the DM over