Effect of Pyrogenic Silica and Nanosilica on Portland Cement Matrices

In this work, the effect of pyrogenic silica and nanosilica on the properties of portland cement matrices is compared. Two chemically and mineralogically similar mineral additions (amorphous silica) with different particle size and specific surface area were used to prepare pastes and mortars with different solids substitutions of cement by silica. These samples were used to measure water and superplasticizer demand, setting time, hydration kinetics, water absorption by capillary suction, and compressive strength. It was found that specific surface area, rather than particle size, played a crucial role in the amount of water and superplasticizer necessary to obtain a desired workability in pastes and mortars. Such water and superplasticizer demands had a delaying effect on the setting time and hydration kinetics of pastes. Nevertheless, compressive strength results at different curing ages of mortars were found to have a direct correlation with the porous structure of the matrix, rather than with the specific surface area of the silica particles. It was concluded that regardless of its higher specific surface area and greater effect on the fresh state properties of pastes, pyrogenic silica was less efficient than nanosilica to increase the compressive strength of mortars, being considered a less efficient pozzolanic material.