Silica Fume typically replaces around 10% of the Portland
cement in a concrete mix and is a
by-product of producing silicon alloys.
Concretes containing silica fume have the following
properties compared to concretes containing 100% Portland cement:
- Increased water demand due to the greater
surface area from the spherical particles.
Concrete mixes containing silica fume generally require a high range
water reducing admixture.
- Increased short term and long term strength
can be achieved.
- Decreased workability for same water / cement
ratio – it is rarely used without the addition of a superplasticiser to
increase the workability to a normal level.
- Reduced segregation and bleeding.
- Decreased porosity – this leads to improved
durability and increased resistance to corrosion of the reinforcement due to
the low chloride diffusivity.
- Can lower the chance of a damaging alkali-silica
reaction and helps reduce or even prevent sulphate attack.
Even though silica
fume is a waste product, it is still more expensive than Portland cement, as its
strengthening qualities are in such high demand. For this reason, silica fume
is usually only used in structures where a very high performance concrete is