Interactions of cement pore fluids with host rock and the effects on HTO, Na and Cs diffusion

In the post-closure period of a deep disposal facility for low-and intermed iate-level radioactive waste, pore-fluids chemically equilibrated with ceme ntitious components of the engineered barriers will migrate into the surrou ndings. Secondary calcium silicate hydrate (CSH) phases might be formed in the micro pore system of the rocks and change the available porosity for ra dio-nuclides and hence permeability. Since the retardation of radionuclides in host rock is dependent on the sorption and matrix diffusion, these chem ical reactions are of concern. Experiments have been undertaken to react di scs of Aspo diorite with alkaline cement porewaters. These experiments prov ided aged material for through diffusion experiments and samples for studie s of chemical reactions. 1 cm thick discs of Aspo diorite were reacted with waters representative for fresh and leached concrete at 70 degrees C in ni trogen filled vessels for six months. Analysis of the waters indicates diss olution of primary minerals and precipitation of secondary CSH phases. HTO, Na and Cs diffusion experiments were then undertaken with both reacted and unreacted discs in a nitrogen-flushed glovebox facility. The effects of al kaline degradation of Aspo diorite on tracer through-diffusion was found to be small and of the same order of magnitude as natural variations in rock heterogenity between undegraded samples.