Colloidal silica is a pozzolan, made up of the same silicon dioxide that constitutes the major active ingredient in fly ash, slag, metakaolin, and silica fume.
Silicon dioxide has been used in concrete for centuries, only now do we have the understanding of delivery systems and mechanisms to effectively use this particle size.
This sizing makes it the most reactive pozzolan known.
Colloidal silica reacts to form Calcium Silicate Hydrate
(C-S-H) as any other pozzolanic reaction.
The nano sized particles fill in void space previously left alone in typical concrete with traditional pozzolans
More complete particle packing through hydration, not through just "pore filling", thus creating a less permeable conventional concrete = concrete that is more durable.