This study investigates the evolutions of the mineralogy, pore solution and porosity of magnesium potassium phosphate cement pastes (MKPC) over time in neutral and basic environments, media that MKPC pastes might encounter when used as patch repair materials or as stabilization materials for hazardous wastes. Mineralogy evolutions were characterized by X-ray diffraction/Rietveld (XRD) and thermogravimetric analyses (TGA) and completed with scanning electron microscopy (SEM/EDX) microanalyses. Pore solution compositions (ICP-OES, IC) and the porosity of the materials were also analyzed. The results showed an amorphization of MKPC pastes over time that might be related to structural changes for the K-struvite phase. Achieving chemical equilibrium between the cementitious waters and the MKPC pore solution can result in porosity evolutions within the materials. Transport and mechanical changes might be expected, especially for MKPC pastes kept in basic media, since K-struvite [MgKPO4·6H2O], could be partially dissolved, leading to a higher total pore volume.