Reactivity of α-Alumina Powder and Fibres in Highly Alkaline Hydrothermal Solutions at 70 °C and 150 °C

This research examines the hydrothermal corrosion behaviour of ceramic matrix composites (CMCs) under highly alkaline conditions (pH > 11.5) in the framework of a deep geological repository for high-level radioactive waste (HLW). The study focuses on the degradation of alumina powder and fibres, key constituents of an oxide/oxide CMC material. Accelerated ageing experiments were conducted in a highly alkaline aqueous environment (pH > 11.5, T = 70 °C for 220 days and T = 150 °C for 30 days). The research used a cross-disciplinary approach integrating thermodynamic calculations and physicochemical analyses to determine the degradation mechanisms of alumina powder and fibres induced by contact with the aqueous ageing solution. Characterisation of the aged alumina powders and fibres revealed the presence of unaltered alumina, hydrated alumina, amorphous phases, and calcium carbonate precipitates from the aqueous solution. Thermodynamic calculations indicate (1) the hydrolysis of alumina to diaspore and (2) the formation of an aluminosilicate phase and calcium carbonate. However, experimental results reveal kinetic limitations, such as the preferential formation of boehmite over diaspore, and morphology-dependent degradation pathways (protective-layer formation on fibres and partial dissolution of powders).