Behavior of Portlandite upon Exposure to Ionizing Radiation: Evidence of Delayed H2 Production

When used under radiation (reactor vessel well concretes, cemented radioactive waste), cementitious materials undergo radiolysis with O–H bonds leading to dihydrogen production. In order to best assess the dihydrogen (H2) risk in nuclear facilities, it is then mandatory to check whether the solid phases of the cement matrix constitute a significant source of radiolytic H2 in addition to that that arises from pore water. Herein, we focus on portlandite (Ca(OH)2) as the main hydration product of Portland cement together with hydrated calcium silicate. In the absence of water molecules, the Ca(OH)2 powder leads to an apparent H2 radiolytic yield under accelerated electrons and γ irradiation of (3.0 ± 0.7) × 10–9 mol·J–1 and (8.1 ± 0.5) × 10–9 mol·J–1, respectively. Interestingly, the H2 production measured at the end of irradiation does not represent the whole H2 produced; a portion of it remains trapped in the crystal lattice as evidenced by 1H NMR measurements. These trapped H2 molecules can be released after heating the sample at 453 K, leading to a total H2 radiolytic yield for accelerated electrons and γ irradiation of (1.1 ± 0.2) × 10–8 mol·J–1 and (1.4 ± 0.2) × 10–8 mol·J–1, respectively. The immediate H2 production corresponds to the fast diffusion, if possible, of H2 precursors to the surface with H2 formation near or at the surface; the delayed H2 production is due to the slow release of trapped H2 within the material. Our work shows that the fundamental knowledge of these samples under irradiation is crucial to a better description of materials of interest in the nuclear industry. Furthermore, the results highlight that measuring H2 production in a solid such as portlandite is a delicate task