Reactivity of constitution vs. crystallization water under irradiation: Insights from tobermorites

Understanding radiolytic H2 production in irradiated cement is crucial for nuclear waste safety, yet the role of solid cement phases remains unclear. This study examines the behavior of model minerals –tobermorite 11 Å (Ca5Si6O17.5H2O)and tobermorite 9 Å (Ca5Si6O16(OH)2)- under electron irradiation. When fully dried, these minerals retain only crystallization water or structural hydroxyl groups, respectively. The results reveal that while crystallization water decomposes under irradiation, it does not lead to H2 formation, as hydrogen atoms react with radiation-induced defects to form SiO–H bonds. In contrast, tobermorite 9 Å produces H2 only when surface SiO–H bonds are present, indicating that radiolytic dihydrogen arises from surface bond breakage, while the cleavage of the bonds in the material does not ultimately lead to H2 production. These findings enhance our understanding of irradiation effects on cementitious materials, aiding in the assessment of their long-term sta bility in nuclear waste storage.