Dissolution kinetics of AFm-Cl as a function of pH at room temperature.
Résumé
Estimation of reliable weathering/dissolution rates of cement phases is of fundamental importance for modelling the temporal evolution of radioactive waste repositories and well for CO2 geological storage. In this context dissolution kinetics of AFm (hydrated calcium aluminates) have been studied with flow-through experiments at pH ranging from 9.2 to 13. Dissolution kinetics of a pure AFm-Cl, i.e. a mix between the Friedel's salt and its naturally-occurring form, hydrocalumite, have been investigated. Mineralogical (XRD) and chemical (EPMA, TEM/EDX) analyses have been performed to characterize the reacting minerals prior and after dissolution experiments. For pH values ranging from 10 to 13, flow-through experiments indicate congruent dissolutions of AFm-Cl (i.e. Ca/Al ratios close to 2 both for solids and outlet concentrations). In contrast a precipitation of Al-phases and mixed Al/Ca phases are expected at pH 9.2 leading to high Ca/Al ratios of outlet solutions. However, estimated rate in such condition appears to be weakly affected by these secondary-phase formations. Accordingly, far-from-equilibrium dissolution rates were normalized to the final specific surface areas (ranging from 6.1 to 35.4 m 2 g-1). None significant effect of pH on dissolution kinetics can be draw and therefore the far-from-equilibrium dissolution kinetics at pH ranging from 9.2 to 13 and room temperature is expressed as:
log R (mol m-2 s-1) = -9.23 ±0.18
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