Compliance of concrete – bentonite barriers is a key issue in the performance assessment of long-term underground storage of radioactive waste. The reaction of synthetic alkaline solutions (K–Na–OH and Ca–(OH)2) interacting by diffusion with a Mg-saturated compacted FEBEX-bentonite column of 2.1 cm thickness was investigated through closed-system experiments at 60 °C over 6 and 12 months. The first few millimeteres near the surface were grated to resolve mineralogical changes at sub-mm resolution determined by XRD, SEM, BET and the analysis of exchangeable cations. Alkaline cations diffused beyond the mineralogical alteration zone (2–2.5 mm), and were exchanged by Mg2+ in the interlayer region of montmorillonite, but no K-silicates were formed. The presence of minor zeolites in the alteration assemblage was only detected by XRD on samples treated with ethylene-glycol. A cemented rim mixed with poorly ordered clay materials, mainly brucite, a chlorite-like phase and Mg-smectite was found at the interface. Montmorillonite was partially dissolved and a part of it remained shielded by the newly-formed cementation crusts. Cation diffusion through the interlayer of montmorillonite is inferred to be the dominant transport pathway in compacted bentonite equilibrated with external high pH solutions. The high pH front diffused at a much slower rate due to buffering of mineral reactions.