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Description
Concretes are materials of primary importance for the storage of nuclear waste in geological formation. They have a high specific surface area and a low hydraulic conductivity that confer them unique confining properties. Nevertheless, concretes may also be very reactive due to the high pH of their pore water. Therefore, understanding their chemical reactivity in situ is a key point. In this work, we propose the use of a geophysical method, the induced polarization (IP) to monitor in the laboratory the evolution of the mineralogy, water chemistry and pore size distribution of low-pH concrete and cement paste. The measured spectra show a high resistivity (> 10 kOhm m) and phase shift between injected current and measured voltage (superior to 40 mrad and above 100 mrad for frequencies > 100 Hz). The cement paste was observed to be more resistive than the concrete containing additional carbonates and silicates grains, this last one exhibiting a peak in the phase spectra at a frequency around 1 Hz. These observations were described using a mechanistic membrane polarization model based on the work of Bucker and Hordt (2013) that simulates the electrical potential distribution and resulting ion transport in the nano, meso and macropores of cement, cations being repulsed from the nanopores due to the positive surface charge of calcium-silicate-hydrate (C-S-H) phase (Labbez et al., 2006). The high resistivity and phase shift were respectively explained by the presence of dead pores corresponding to the interlayer space of C-S-H and the strong cationic exclusion within it (Gaboreau et al., 2011). The sensitivity of IP to the electrochemical and microstructural properties of cement proves that it is very promising to monitor its reactivity and confining properties. Nevertheless, some work should still be done to correctly relate the IP response to the reactive transport properties of cement.
References
Bucker, M., Hordt, A., 2013. Analytical modelling of membrane polarization with explicit parametrization of pore radii and the electrical double layer. Geophys J Int 194, 804-813.
Gaboreau, S., Prêt, D., Tinseau, E., Claret, F., Pellegrini, D., Stammose, D., 2011. 15 years of in situ cement–argillite interaction from Tournemire URL: Characterisation of the multi-scale spatial heterogeneities of pore space evolution. Appl Geochem 26, 2159-2171.
Labbez, C., Jonsson, B., Pochard, I., Nonat, A., Cabane, B., 2006. Surface charge density and electrokinetic potential of highly charged minerals: Experiments and Monte Carlo simulations on calcium silicate hydrate. Journal of Physical Chemistry B 110, 9219-9230.
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