Résumé
This work focuses on understanding the leaching behavior of magnesium phosphate cement (MPC) pastes by using demineralized water as the leaching solution. The main constituents of MPC are magnesium oxide (MgO) and potassium dihydrogen phosphate (KH2PO4). Their hydration mainly yields K-struvite (MgKPO4·6H2O), a low-solubility mineral that sets the pore solution pH to a value close to 8 (i.e., within the passivation domain of aluminum). Recent work has shown that magnesium phosphate cements (MPC) may present higher chemical compatibility with aluminum metal as compared to usual calcium silicate cements. Therefore, MPC hold promise for the the conditioning of low- or intermediate-level radioactive waste with aluminum metal. Nonetheless, understanding the evolution of these binders under leaching conditions is limited and thus, there is the need to gain insights into their long-term durability. To this end, a semi-dynamic leaching test was performed for 90 days. The chemical composition of the leachates was determined over time, and mineralogical and microscopic characterizations were performed on the solid before and after leaching to assess the degradation mechanisms. Results indicated that leaching was kinetically controlled by the diffusion of dissolved species through the pore network of the cement paste. Moreover, the flux of dissolved elements was different, which may indicate the release of elements from fly ash, incongruent dissolution of K-struvite, and/or the reprecipitation of other phosphate phases.