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Progress in the Chemical Modelling of the Interactions Between Cement

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Progress in the Chemical Modelling of the Interactions Between Cement FRO302696 P-AA-06 11V PROGRESS IN THE CHEMICAL MODE LING OF THE INTERACTIONS BETWEEN CEMENT POREWATER AND CLAY BARRIER. E. Gaucher 1, P. Blanc 1, N. Michau 2, E. JacqUot 2 1. BRGM, Orlans, 45, France 2. Andra, Chfitenay-Malabry, 92, France In the program of the European Ecoclay 11 project, the predictive modelling of the interaction between cement porewater and different types of clay barriers is performed to evaluate the potential alteration of the minerals as a function of time. Indeed, according to the current concepts of future nuclear waste disposal in deep clayey geological formations, a large quantity of concrete will be potentially used as a building and confinement material. The introduction of concrete will lead to a chemical perturbation within the host clayey fori-nation and the artificial clayey barriers. Alkaline waters with a high pH and a high concentration of alkaline cations are produced during the saturation of the concrete after the closure of the repository. These alkaline waters might dissolve the minerals of the barriers and modify their ability to confine the nuclear waste. The diffusion of an alkaline plume from concrete into the bentonite of the artificial barriers or into the Callovian-Oxfordian Argillite Formation has been modelled for a 100,000-year period. The Callovian-Oxfordian Formation was selected by ANDRA (the French agency for radioactive waste management) for the installation of an underground research laboratory at Bure (Meuse Haute Marne) at a depth of 5 10 in. The calculations, carried out with the PHREEQC geochernical code, were performed in order to detennine, as a function of time: modifications of mineral surfaces, dissolution of minerals and precipitation of new mineral phases. The modelled system consists of an alkaline solution which representes the concrete porewater, and the clayey barrier (MX80 bentonite or Callovian-Oxfordian Formation) with the corresponding equilibrated pore waters. The partial pressure Of C02, which is a key point for the evolution of the pH in this system, is either fixed or is set "free" (it is determined by PHREEQC as a function of mineral equilibria). A new database derived from the Laurence Livermore National Laboratory was created including mineral-phase solubilities of a large number of zeolites and concrete minerals susceptible to precipitate in such a context. Ion-exchange constants in the Gaines-Thomas formalism for the ions Na+, K, Ca 2+ Mg 2+ and H are added to the database for an MX80 bentonite or for a smectite-I'llite mxture in the case of the Callovian-Oxfordian Formation. The thickness of the clay barrier was taken as 825 m. The mesh was refined near the concrete/clay interface to permit a better understanding of the phenomena in this zone. Simulations were carried out at a temperature and a pressure of 25 'C and I bar, respectively. Transport modelling was based on a one-dimensional diffusion model. Constant concentrations were assumed for the concrete. The clay barrier was modelled as a semi-infinite medium with a single diffusion coefficient of IO-" M2/S. For the MX80, after I 0,000 years, the simulation with a free PCO2 revealed four different zones. In the order of increasing degree of mineralogical alteration, these zones are: zone 2 to n from the concrete interface) in which the initial bentonite composition is preserved; zone 2 0.3 to 2 m) where an increase of the concentration of saponite and a partial dissolution of calcite can be observed; zone 3 (0.15 to 03 m) which is characterised by a drastic increase in pH and K concentration leading to the total illitisation of the montmorillonite and a total dissolution of calcite and cristobalite; and zone 4 O to 015 in), in which pH ranges between International Meeting, December 912, 2002, Reims, France Clays In Natural And Engineered Barriers For Radioactive Waste Confinement Page 409 P-AA-06 10 and 12.5 and induces firstly the precipitation of gismondine, gyrolite, tobermorite and calcite and secondly, just at the interface, precipitation of ettringite, hydrotalcite, afwillite, katoite and hydrogrenat. For the Callovian-Oxfordian formation, the same zones are observed. The major difference are: the extension of the zone of total illitisation of the montmorillonite-Ca (0 I to 06 m) and the total dissolution of dolomite in the zone to 025 m. It can be noticed that the mineralogy acts as a major buffer on the pH, and limits the increase of this parameter. The main conclusion is that the effects of the alkaline plume from the concrete would be limited to the first metre of the clay plug after 100,000 years. And the total modification of the mineralogy will only concern the first 15 cm from the concrete/clay interface. International Meeting, December 912, 2002, Reims, France Page 410 Clays In Natural And Engineered Barriers For Radioactive Waste Confinement.
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