A thermodynamic description of the evolution of pore water chemistry and uranium speciation during the degradation of cement

Portland cement, used as solidification-, backfill- and construction-material, is considered to determine the chemical behaviour of a low- and intermediate level radioactive waste repository for very long periods of time. In order to describe the long term chemical behaviour of such cement based structures, a series of submodels including the hydration of cement, the thermodynamic properties of calcium-silicate-hydrates and the degradation of hydrated cements in natural groundwaters have been developed. As an application, the leaching of uranium from a cement based structure is modelled. Cement hydration is modelled using Bogue's method. The incongruent solubility behaviour of calcium-silicate-hydrates (CSH-gels) is described using an approach directly applicable to common geochemical speciation codes. Within a mixing tank model, the results of the hydration and of the solubility model are used to describe the degradation of cement in natural groundwaters. As a representative for actinides, the solubility behaviour of uranium in degrading cement is modelled assuming an Eh of -300 mV. The uranium concentration in the pore solution is determined by the 'choice' of the solubility limiting phase. If the speciation is calculated by using the thermodynamic solubilities of the pure solids, concentration levels of about 10-4 M are found for U3O8, USiO4, UO2, U4O9 and 10-14 M for CaUO4. Cumulative uranium leaching from hydrated cement is found to be nearly independent of cement and groundwater composition, but strongly dependent on the properties of the solubility limiting phase. An amount of 0.1 nmoles U/kg hydrated cement is completely leached out after 5 to 10 cycles using U3O8 as the solubility limiting solid, after ∼100 cycles using UO2 and after ∼5000 (to 1000) cycles using CaUO4. (author) 23 figs., 15 tabs., 43 refs