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 intermediate-level radioactive waste, pore-fluids chemically equilibrated with cementious components of the engineered barriers will migrate into the surroundings. Secondary calcium silicate hydrate (CSH) phases might be formed in the micro pore system of the rocks and change the available porosity for radionuclides and hence permeability. Since the retardation of radionuclides in host rock is dependent on the sorption and matrix diffusion, these chemical reactions are of concern. Experiments have been undertaken to react discs of Aspo diorite with alkaline cement porewaters. These experiments provided aged material for through diffusion experiments and samples for studies of chemical reactions. 1 cm thick discs of Aspo diorite were reacted with waters representative for fresh and leached concrete at 70C in nitrogen filled vessels for six months. Analysis of the waters indicates dissolution 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 alkaline 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.