The present study evaluates the formation of magnesium silicates phases as a result of the alkaline alteration of FEBEX bentonite in long-term experiments. The results are relevant in the context of radioactive waste disposal since the bentonite barrier will partly change its original mineralogy and condition the geochemical behavior. Bentonite samples from an in situ experiment of interaction with a CEM-II-type concrete performed for 13 years in a rock gallery and a laboratory experiment of interaction with a CEM-I-type concrete performed for 10 years provide some experimental evidences on the mineralogical alteration. Results required multiple analytical techniques to resolve the nature of the Mg silicates. X-ray diffraction, thermogravimetric analyses, scanning electron microscopy, infrared spectroscopy and ²⁷Al and ²⁹Si nuclear magnetic resonance have been used. The mineralogical alteration is complex since several Mg silicates may coexist in the same region not only with themselves but also with carbonates and calcium (aluminium) silicates hydrates. Brucite intercalation in the interlayer of smectite, previously reported as chlorite-like phase in a former study, is observed. In addition, a serpentine-type mineral phase was better observed in the in situ samples, at least by XRD, and a 2:1 trioctahedral phyllosilicates was better observed in the laboratory samples. Formation of Mg silicates in the bentonite barrier may buffer the Ca alkaline front originated in concrete and decreases the porosity at the concrete-bentonite interface.