Technical Report NTB 90-12

A thermodynamic description of the evolution of porewater 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 variety of possible hydrated cement minerals is classified into several groups of model solids. Cement additives (e.g. silica fume, trass etc.) are assumed to react completely with the cement.

The incongruent solubility behaviour of calcium-silicate-hydrates (CSH-gels) is described using an approach directly applicable to common geochemical speciation codes. Based on the assumption that CSH-gels may be described by independent model components, their solubility properties are extracted from a large amount of literature data. Solubility constants are given as a function of the CSH-gel composition.

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. The mixing tank model assumes thermodynamic equilibrium between hydrated cement and groundwater and recal­culates the speciation of solutes in the pore solution and the remaining chemical inventory in consecutive steps (cycles). It takes some 5000 to 10'000 cycles to completely degrade the hydrated cement. The particular "lifetime" depends mainly on the groundwater compo­sition. The composition of the cement has a secondary effect on lifetime, but it determines the absolute pH level and the relative time period of high pH conditions (pH ≥ 12) in the pore solution. Carbonate concentration in the groundwater is found to be a very important parameter, due to transformation of hydrated cement minerals into calcite.

As a representative for the 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 of 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 mmoles 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 10'000) cycles using CaUO4.

Recommendations for further development of the submodels and appropriate priorities are given in the conclusions.