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 stability in nuclear waste storage.