Permeability Properties of Cement Mortars Blended with Silica Fume, Fly Ash, and Blast Furnace Slag

Deterioration of concrete structures due to reinforcement corrosion because of chloride ingress is a growing problem in many countries throughout the world. Partial replacement of portland cement by mineral additions, such as ground granulated blast furnace (GGBS), silica fume and fly ash influences the resistance of the pastes and mortars to the chloride environments. The rate of chloride ingress into mortar depends on the pore structure and the capacity of the hydration products to bind chlorides

This paper reports the comparative results of mechanical and permeability properties of blended mortars. Mortar specimens were made with slag replacement levels of 60%, 70%, and 80%, fly ash replacement levels of 20%, 30%, 40%, and 50% and silica fume replacement levels of 5%, 10%, 15%, and 20%. The following tests were performed: compressive strength, water absorption, rapid chloride permeability, mercury intrusion porosimetry, and X-ray diffraction.

Mortars with fly ash and slag reported lower strengths than silica fume mortars. The silica fume mortars show a 50% pore size of that for the mortar without addition. The densifying effect of these materials on the microstructure is attributed to the sealing of pore openings and the narrowing of pore channels by the hydration products of pozzolanic reactions. This results in a reduction in permeability. In general, the use of these admixtures improves the resistance of portland cement mortars against chloride attack. The slag added mortar obtained the best performance.