Tensile and Compressive Strength of Silica Fume-Cement Pastes and Mortars

Abstract The effect of silica fume on the compressive and uniaxial direct tensile strength of portland cement paste and mortar is reported. Sixteen and 25% of the cement used in the paste and in the mortar, measured by mass, was replaced by silica fume. Four different W/C ratio mixtures were tested: 0.22, 0.25, 0.28, and 0.31. The super-plasticizer content was adjusted for each mixture to provide a sufficient amount for efficient dispersion of the cement and silica fume particles, but also to make sure that there would be no excess of this additive that might lead to effects such as bleeding. Results show that the partial replacement of cement by silica fume increases the compressive strength of mortar but has no effect on the compressive strength of paste. Results also show that the partial replacement of portland cement by silica fume decreases the tensile strength of both paste and mortar. The reduction in the strength of paste was greater than the reduction in the strength of mortar. Results were verified by statistical analysis using hypothesis testing at a 95% confidence level. The uniaxial tensile strength results were obtained using the cementitious composites axial tensile technique (CCATT). The relation between the tensile strength and compressive strength of materials with and without silica fume was obtained. The ratios of tensile to compressive strength of paste and of mortar were decreased with increasing silica fume content.