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Modeling in situ interactions between concrete and Tournemire argillite

Congress title :Clays in natural and engineered barriers for radioactive waste confinement
Congress location :Lille
Congress date :17/09/2007


 Experimental investigations of a vertical borehole sealed by concrete ("engineered analogue") overcored at the Tournemire site allow to model in situ mineralogical evolutions of argillite on an unusual timescale of 15 y, i.e. longer than standard durations of laboratory experiments. The reactive transport code HYTEC was used to simulate the perturbation observed in the argillite matrix and along small decompression fractures, assuming either local thermodynamic equilibrium or kinetic control for all dissolution and precipitation reactions. The kinetic simulations show a better agreement with the experimental characterization than thermodynamic equilibrium ones, in particular with respect to the expansion of the perturbation. The global trend of kinetics is to smooth the sharpness and intensity of the mineralogical transformation fronts, to delocalize them deeper inside the argillite matrix (< 1 cm) and the fracture ( 2 cm) as well as to lessen clogging within argillite. In fair agreement with the experimental observations, the calculated mineralogical changes are globally similar in the argillite matrix and the fracture filling. At 15°C, the secondary mineral sequence simulated from concrete to clay is: CSH/ettringite/carbonates at the concrete/argillite interface, followed by a zone of neoformation of clay-like phases and calcite, and a last zone with precipitation of macrocrystalline calcite and (scarce) zeolite. The main discrepancy between modeling and experimental results is the precipitation of muscovite instead of K-feldspar. Calculations also show that the carbonated precipitates observed at the concrete/argillite interface, which are more specifically attributed to concrete alteration, follows the Ostwald's rule with predominance of vaterite over calcite.
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