Microsilica in concrete contributes to strength and durability two ways: as a pozzolan, microsilica provides a more uniform distribution and a greater volume of hydration products; as a filler, microsilica decreases the average size of pores in the cement paste. Used as an admixture, microsilica can improve the properties of both fresh and hardened concrete. Used as a partial replacement for cement, microsilica can substitute for energy-consuming cement without sacrifice of quality.
Addition of microsilica to a concrete mix alters the cement paste structure. The resulting paste contains more of the strong calcium-silicate hydrates and less of the weak and easily soluble calcium hydroxides than do ordinary cement pastes. Because the microsilica particles are so small they disperse among and separate the cement particles. The resulting fine, uniform matrix can give markedly higher compressive, flexural, and bond strength.
Microsilica reduces the rate of carbonation, decreases permeability to chloride ions, imparts high electrical resistivity, and has little effect on oxygen transport. Therefore, microsilica concrete can be expected to be strongly protective of reinforcement and embedments.