Original behavior of pore water radiolysis in cement-based materials containing sulfide: Coupling between experiments and simulations

Abstract

Blended cements with high content of blast furnace slag (CEM III/C) can be used for nuclear waste conditioning because of their low hydration heat as compared to ordinary Portland cements (CEM I). They however contain some sulfide, an impurity whose role needs to be investigated. Indeed, they can have an effect on the radiolytic H2 production under irradiation. To study the impact of sulfide species on H2 production, gamma irradiation, at a dose rate of 356 Gy h−1, was performed during 6 months in a closed system without O2 on a cement paste made with CEM III/C. At short time, the radiolytic H2 production rate is higher than that measured using CEM I. On the basis of reaction data collected in the literature on sulfur species, radiolysis simulations performed for both systems confirm this behavior. Moreover, they suggest that the sulfide concentration, initially imposed in pore solution by the slag is of the order of 180 mM, and is responsible for this H2 production. For the first two irradiation months, the following phenomena are then evidenced in CEM III/C: 1) conversion of sulfide into polysulfide anions; 2) pH increase; 3) production of H2 due to the H•+ H2S reaction having a very high rate constant. Nevertheless, in the medium term, the residual sulfide concentration is not sufficient any more for this mechanism to occur. It imposes a reducing environment, leading to a very efficient recombination of H2 in pore solution. The resulting equilibrium state is reinforced by the high liquid saturation level in the cement paste porosity. Therefore, even if the presence of sulfide species in blended cements momentarily increases the H2 production rate, it strongly reduces it at long times.