AbstractThis study concerned the field of radioactive waste conditioning by cementation, involving the optimization of cemented radioactive spent resin form quality by the development of new formulations based on mixtures of Class 45 portland cement, sand, and water, with metakaolin (MK) as a mineral additive. Here, the MK contribution to the cemented-waste form quality was evaluated. In this context, four new formulations were prepared containing 2%, 4%, 6%, and 8% MK relative to a mixture of cementing materials without MK. The mechanical quality of each of the cemented-resin forms was determined, after 28 days of curing, by measuring the compressive strength and analyzing the porosity. Characterizations of the prepared cemented-resin forms were completed using X-ray fluorescence, X-ray diffraction spectrometry, and scanning electron microscopy. It was found that, for each formulation, the phases of early and secondary pozzolanic hydration were marked by accelerations in the hydrate formation reactions. These phases, characterized by an increase in mechanical resistance following the silicate and aluminate dissolution phase, justify the inclusion of the MK and prove its participation in the early formation of hydrates. Among the four developed formulations, the one with 6% MK produced a cementitious mixture that allowed the containment of 14.38% of the spent ion-exchange resins, with a mechanical resistance to compression equivalent of to 15.92 MPa and a low-porosity equivalent to 34.98%. This formulation allowed the identification of the course of pozzolanic reactions favored by the addition of MK.