Reactive transport modeling of geochemical interactions at a concrete/argillite interface, Tournemire site (France)

Mineralogical and petrographical investigations of a borehole sealed by concrete ("engineered analogue" of concrete/clay interactions) overcored at the Tournemire site offer the possibility to perform reactive transport modeling over a timescale of 15 years, which is significantly longer than standard durations of laboratory experiments. The HYTEC code was used to simulate mineralogical evolutions in the Tournemire argillite matrix and along small fractures, as well as in concrete, assuming either thermodynamic equilibrium approach or kinetic control coupled to diffusive transfer. The mineralogical profiles calculated in thermodynamic equilibrium and with kinetics are very similar in the concrete. In contrast, kinetics of dissolution and precipitation reactions is found to be necessary for a better reproduction of the experimental observations in the argillite samples compared to thermodynamic equilibrium calculations. in particular with respect to the extent of the alkaline perturbation. The major effects observed when considering kinetics in the calculations are to smooth the sharpness and intensity of the mineralogical transformation fronts, these fronts being predicted to occur deeper inside the argillite matrix (< 1 cm) and the fractures (<= 2 cm), as well as to lessen clogging of pores within the argillite. At 15 degrees C, the secondary mineral sequence simulated from the concrete to the argillite 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 calcite. Main discrepancies between the modeling results and the experimental observations are the precipitation of muscovite instead of K-feldspar and the neoformation of zeolites. Calculations also correctly predict that carbonates form at the concrete/argillite interface, which most probably result from concrete alteration and follow Ostwald's step rule of mineral precipitation with a predominance of metastable vaterite over calcite. (C) 2008 Elsevier Ltd. All rights reserved.