Geochemical constraints on the stability of zeolites and C–S–H in the high pH reaction of bentonite

Abstract Cement–bentonite interactions will occur in deep geological repositories for high level radioactive waste. We performed laboratory experiments in order to constrain some previous geochemical and mineralogical uncertainties in the formation of secondary minerals in the high pH reaction of FEBEX bentonite at moderate temperatures. The reactivity of a Mg-homoionic FEBEX bentonite was studied at 25, 60 and 90 °C in batch reactors using two solutions representative of short and long time evolution stages of cement degradation. These solutions were periodically renewed in order to maintain a constant pH regime for a period of almost 2 years. Chabazite and merlinoite were observed to be the main zeolites that formed at 60 °C, while merlinoite was the dominant zeolite formed at 90 °C with the hyperalkaline (K–Na–OH) solution. The alkaline experiments (Ca–OH solution) showed minor reactivity and negligible mineralogical alteration, but the overall results suggest that the reaction tends to equilibrium conditions between C–S–H and montmorillonite at high temperature and metastable conditions at low temperature. The montmorillonite dissolution rates were calculated for all experiments, showing good agreement with previous calculations.