H<sub>2</sub> Production\nMechanisms in Irradiated\nPortlandite: Surface and Volume Contributions

Calcium hydroxide or portlandite (Ca(OH)<sub>2</sub>)\nis the second\nmost abundant hydrate in cementitious materials. The latter form the\nbasis of the coating matrices of some forms of radioactive waste.\nUnder ionizing radiation, this results in the production of radiolytic\nmolecular hydrogen, which, when it accumulates in the environment,\nmay raise safety concerns. It is important, therefore, to understand\nhow radiation interacts with these materials, and especially with\nportlandite. This hydroxide is of particular interest: on exposure\nto irradiation, it not only leads to the immediate production of molecular\nhydrogen but also to a delayed production over long periods of time\n(days or even weeks) after the irradiation stops. It follows that\nit is important to determine the reaction mechanisms operating in\nthis system. This was carried out here based on experiments with electron\nparamagnetic resonance spectroscopy, which identified the different\nradicals generated under radiation. As a result, we have been able\nto show that the immediate production of H<sub>2</sub> is due to the\nstabilization of electrons on the surface of the portlandite, followed\nby surface reactions that encourage the immediate release of H<sub>2</sub> into the gaseous atmosphere. When the dose increases, the\nnumber of these sites drops, leading to a fall in the production of\nthe molecular hydrogen immediately released. The delayed production\nof H<sub>2</sub> is due to the formation of hydrogen atoms followed\nby their dimerization within the portlandite crystal lattice, with\nthe result that the molecular hydrogen molecules are trapped. In this\ncase, the movement of the hydrogen atoms in the crystal lattice sets\nthe CaO<sup>•</sup> radicals in motion. At low doses, these\nradicals dimerize to form CaO-OCa peroxides. At higher doses, the\nCaO<sup>•</sup> radicals can react with hydrogen atoms, which\nrestricts the formation of the trapped H<sub>2</sub>. The range of\ndifferent reaction intermediates identified reflects the richness\nof the chemical processes involved in the mechanisms, which accounts\nfor the behavior of portlandite on exposure to irradiation.