Effect of different high surface area silicas on the rheology of cement paste

This work studies the effect of nanosilica (NS) on the rheology of cement paste by comparing it with two high specific surface area silicas: silica fume (SF) and pyrogenic silica (PS). Portland cement pastes were produced with different water-to-cementing material ratios and different solid substitutions of cement by silica. Water demand, setting time, and rheology tests were performed. Results showed that NS and SF decreased plastic viscosity, while PS increased it. Only PS was found to have an effect on yield stress. NS showed the most decreasing effect on viscosity, regardless of its higher water demand. It was concluded that the behavior of pastes containing NS and SF is governed by the “ball-bearing” effect from silica particles, by their agglomeration degree, and their impact on the solid volume fraction. The behavior of pastes containing PS is governed by its ability to absorb a portion of the mixing water.