Silica fume — also known as microsilica — is an extremely fine, amorphous silicon dioxide powder added to concrete to increase strength and density. It is a by-product of silicon and ferrosilicon manufacture, captured as fume in the filters of the furnace off-gas.
The powder is roughly 85 to 97 per cent amorphous silicon dioxide (SiO₂). Its particles are about 0.1 micrometer across, roughly a hundred times finer than cement grains. That extreme fineness is exactly where the effect comes from.
How it works in concrete
Silica fume does two things at once:
- Pozzolanic reaction. The amorphous SiO₂ reacts with the calcium hydroxide released during cement hydration, forming additional calcium silicate hydrate — the same phase that gives cement paste its strength. Calcium hydroxide is the weak, leachable component of concrete, so converting it is a double gain
- Filler effect. The tiny spheres pack into the voids between cement grains. The particle skeleton becomes far denser and the pores in the hardened paste get smaller
The result is higher compressive strength, sharply reduced permeability, better bond to the aggregate, and greater resistance to chemical attack and chloride ingress. Kilogram for kilogram, silica fume contributes roughly two to three times as much to strength as cement.
Points to watch
- Dosage. In practice 5 to 10 per cent of the cement mass is used. More gives little extra benefit and makes the mix unworkable
- Water demand. The enormous specific surface area means silica fume is thirsty. Without a superplasticiser the mix simply will not be workable — that pairing is standard
- Curing. Silica fume concrete is dense and dries rapidly at the surface. Consistent curing is essential, otherwise plastic shrinkage cracking follows
- Placing. The mix is sticky and cohesive; it barely bleeds or segregates, but it does need an adjusted placing and finishing method
Where it is used
Silica fume is mainly used in high-strength concrete, sprayed concrete, abrasion-resistant industrial floors, and elements exposed to severe chemical or marine attack — quay walls, tunnels and containment structures. It is not needed in ordinary housebuilding concrete.
Related terms
- Concrete
- Cement
- Pozzolan
- High-strength concrete
- Fly ash
- Superplasticiser
- Exposure class
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