Evolution of pore water chemistry during degradation of cement in a radioactive waste repository environment

Abstract

Portland cement, used as a solidification, backfill, and construction material, is considered to determine the chemistry of a low and intermediate level radioactive waste repository for very long periods of time. A thermodynamic description of the main constituents of hydrated Portland cement, calcium silicate hydrates (CSH-gels), is presented. The incongruent solubility behaviour of CSH-gel is described by introducing several independent model solids. The thermodynamic properties of these model solids are extracted from literature data and their solubility products are given as a function of the gel-composition. This solubility model is then used to predict the behaviour of cement structures which degrade in groundwater, including the chemical evolution of their pore waters. A volume of fresh groundwater corresponding to 5,000 to 10,000 times the total pore volume is required to completely degrade the cement. The total carbonate concentration of the groundwater is found to be a key parameter as far as the lifetime of the chemical inventory is concerned. As a representative for the antinides, the solubility behaviour of uranium in degrading cement is modelled. Cumulative uranium leaching from the cement is found to be nearly independent of cement and groundwater composition, but strongly dependent on the properties of the solubility limiting phase.