Influence of slurried silica fume on microstructure and tritiated water diffusivity of cement pastes

Abstract

Silica fume cement based materials have long been used in nuclear industry particularly for low-level waste (LLW) and intermediate-level waste (ILW) management. The durability of these materials is therefore of a great importance for the safety of nuclear waste storage.This study investigates the durability in the aspect of tritiated water (HTO) diffusion in relation with the porosity and the microstructure within silica fume cement pastes. Silica fume (SF) was used as a slurry; a liquid mixture of undensified powder and water. Due to its better dispersion, it was found that slurried silica fume improves the pore structure and diffusion properties of SF pastes compared to the use of a densified form of SF.The effect of silica fume and water content were also explored. Silica fume (SF) replacement varying from 0 to 20% by weight of cement was used to prepare cement pastes at water-to-binder ratios (w/b) ranging from 0.3 to 0.5. The manufactured pastes were tested by through-out diffusion and their microstructure characterized by water and mercury porosimetry (MIP), thermogravimetric analysis (TGA) and by scanning electron microscopy (SEM) associated to (EDS) analysis.Test results showed that despite an increase in total porosity, the addition of slurried silica fume has a positive impact on pore refinement and on reducing the effective diffusion coefficients.Finally, HTO diffusion coefficients seem to increase linearly with w/b ratio especially for cement pastes without SF where the increase is more important compared to SF pastes.