Aluminium corrosion reactivation in MKPC and Portland-based wasteforms under simulated alkaline repository conditions

The long-term stability of aluminium (Al) in cementitious wasteforms is challenged by matrix alkalinisation resulting from interaction with alkaline waters from engineered barriers in low- to intermediate-level radioactive waste (LILRW) repositories. This study evaluates the corrosion of Al (A1050) and Al-Mg (AA5754) alloys and the physicochemical evolution of magnesium potassium phosphate cement (MKPC) and Portland-based matrices (CEM I and CEM I + 50% silica fume (SF)) under simulated alkaline exposure. Over 250 days, specimens were subjected to synthetic alkaline water (SPAW), while MKPC was also embedded in a CEM I mortar (AM) to simulate repository contact conditions. In MKPC, alkaline plume diffusion from the external source triggered ion exchange with the matrix, leading to alkalinisation (pH >11), reactivation of Al corrosion, and H₂ release to levels comparable to those observed in CEM I. K-struvite dissolution, Ca–P-rich phase formation, and fly ash hydration were also observed, evidencing long-term destabilisation of the MKPC matrix. In contrast, CEM I + 50%SF, despite being exposed to a highly alkaline plume, maintained the pore solution pH at 10.7 over time due to the advanced hydration of SF, effectively limiting Al reactivity and H₂ generation without matrix alteration.