Bentonite and alkaline solutions were reacted in batch experiments at 90 °C during 365 days to determine the effect of the chemical composition and pH of the solution in stability of the smectite. Bentonite and concrete are considered for use as backfilling and sealing materials in a deep geological repository of high level radioactive waste. The pH of the pore-water leached from concrete during degradation is above 11 for a long period of time. In these alkaline conditions, reactions take place in smectite, affecting the properties of bentonite. The aim of this work is to identify crystalchemical changes in smectite resulting from its interaction with synthetic cement pore water, taking into account the temperature effect in order to consider the thermal impact originated by radioactive decay. The bentonite used comes from the \\"La Serrata de Nijar\\" deposit (Almeria, Spain). The bentonite comprised mainly high-charge montmorillonite. Alkaline solutions are representative of cement pore water leached from concrete during the first stages of degradation. Crystalchemistry characterization of smectite (< 0.5 µm fraction of bentonite) was performed before and after the experiments by differential thermal analysis, X ray diffraction (Hofmann & Klemen test, simplified alkylammonium method), infrared spectroscopy and chemical analysis. The results suggest a preferential dissolution of some smectite layers, more specifically octahedrally-charged layers.