The finest fraction of a castable mix will usually be microsilica. Due to their small particle size, microsilica particles fill the voids between larger aggregates and give dense castables. A dense castable will have lower permeability, higher strength and higher thermal conductivity.
The reactivity of microsilica particles combined with their fine distribution allows for a more efficient cement reaction, i.e. optimising the effects of a low cement content.
The superfine particles of microsilica fill in the voids between larger aggregates. Efficient particle packing gives dense castables with low porosity. (illustrated in the figure as small particles)