The reprocessing of spent fuel designed for natural uranium – graphite – gas reactors has produced some waste with aluminum alloys, which need to be stabilized and solidified before their final disposal. Portland cement is extensively used for the conditioning of low-level and intermediate-level radioactive waste; however, its high alkalinity is a serious obstacle to aluminum stabilization, as it is oxidized by the mixing solution, with production of dihydrogen. This work investigates a new solution consisting in using magnesium potassium phosphate cement (MKPC) instead of Portland cement (PC). Gas chromatography and electrochemical impedance spectroscopy (EIS) are used to monitor the corrosion of pure aluminum and aluminum-magnesium alloys containing 2 to 4.5 wt.% of Mg in MKPC mortar. EIS provides qualitative information about the corrosion, but also makes it possible to assess the corrosion current using an equivalent electrical circuit linked to the kinetic parameters of the postulated corrosion mechanism. It is shown that the corrosion current of the alloys, regardless of their composition, is reduced by about two orders of magnitude in MKPC mortar as compared to Portland cement mortar. This result opens up new prospects for increasing the incorporation rate of reactive Al metal in a cementitious matrix.