The concrete–bentonite interface has been studied under the geochemical conditions expected in a repository for high-level radioactive waste. The alkaline conditions emanating from concrete will modify the mineralogy, pH and cation exchange properties of bentonite. The reactive transport geochemical model CrunchFlow was used to simulate column experiments with cement mortar in contact with compacted bentonite carried out at laboratory scale for a period of one year at 25 °C and 120 °C. The thermodynamic data and compositional properties of the minerals involved have been selected and adapted where necessary. An important improvement, compared with previous work, is the inclusion of an extended formula for montmorillonite in the database that fits the experimentally determined composition of FEBEX bentonite, and this allowed to tie the exchangeable cations specifically to this mineral phase. Kinetic rate laws have been selected and evaluated at both temperatures to predict the system behaviour at long timescales (105 years). Results predict the precipitation of hydroxides, zeolites, secondary clay minerals and cement hydration phases in bentonite with different distributions as a function of time and temperature. The predicted alteration produced at long timescales is not very significant under the boundary conditions of the modelling. The expected extension of significant mineralogical changes in the bentonite barrier is in the order of a few centimetres from the concrete interface, leaving the bulk of the bentonite unchanged in its barrier function.