Link between microstructure and diffusivity in silica fume mortars

The resistance of concrete to the penetration of aggressive agents generally controls concrete durability principally by diffusion. The former is closely related to the porosity of the material but also to the geometric arrangement of the pores. The present study aims to investigate the influence of mortar's microstructure, especially the effect of aggregates content and silica fume replacement on tritiated water diffusivity in these materials. To this end, several formulations of mortars were prepared and tritiated water diffusion tests were conducted. Sand volume fraction was varied from 0 to 65% while silica fume replacement and water to binder ratio were respectively set at 10% and 0.41. In parallel, microstructure of these materials was characterized by water porosimetry, mercury porosimetry (MIP), thermogravimetric analysis (TGA) and by scanning electron microscopy (SEM) associated to (EDS) analysis. It was observed that silica fume clusters exist in cement paste and in the mortar with a low sand content (10%). These agglomerations reduce the effectiveness of silica fume (SF) on diffusion properties of these materials because they have a larger diameter, a smaller specific surface area and a lower pozzolanic reactivity than the unitary grains. However, it was proved that the presence of sand during mixing of mortar (more than 30% of sand volume fraction), offers a better dispersion of silica fume and helps to break up agglomerates providing a good homogenization of the material and improving the properties of materials. (authors)