Influence of slurried silica fume and water content on the microstructure and diffusion parameters of cement pastes

Abstract

Durability of silica fume cement based materials is of great importance for radioactive 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. A 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. It was observed that the addition of silica fume as a slurry –a liquid mixture of undensified powder and water- offers a better dispersion and improves diffusion properties of SF pastes compared to the use of a densified powder, often studied in literature. Test results also showed that despite an increase in total porosity, the addition of 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 Portland cement pastes where the increase is more important compared to SF pastes.