Silica fume

Once a nuisance waste product, silica fume is now a valuable addition to cement, creating concrete that is high strength, such as that used to build 311 South Wacker Drive, Chicago, once the world’s tallest reinforced-concrete building.

Silica fume is a by-product of the silicon metal/ferrosilicon alloy industry. Collected in baghouse filters, it is a very fine pozzolanic material. The addition of silica fume to concrete was first tested in Norway in the 1950s. Although positive results were recorded in terms of improved strength, at that time no method of retaining large quantities of silica fume was available, preventing its industrial-scale use.

This changed in the 1970s, when environmental regulations that required the removal of silica fume from exhaust gases were introduced. This led to significant improvements in filtration technology and, for the first time, the production of large amounts of silica fume.

Building on the earlier work of the 1950s, intensive research was undertaken by Norwegian smelting companies to promote the use of silica fume. This work forms the basis for its use in concrete today.

The advantages of silica fume include significantly increasing concrete strength, a reduction in concrete bleeding and segregation (although this increases the need for proper curing), improved resistance to chemically-aggressive environments, and inhibition of alkali-silica reactions, which can severely damage concrete structures.

It is however a costly and difficult-to-handle material that requires special transportation and dosing equipment to avoid dust formation. It may also reduce resistance to carbonation, raising the risk of corrosion to carbon (black) steel reinforcement.

The very fine nature of silica fume particles (and thus its high surface area) brings both advantages and challenges. It develops strength far more quickly than other pozzolanic materials, allowing its use in the precast industry; however, it also results in a need for significantly more water in concrete production and the use of water-reducing admixtures.

For these reasons, cements containing silica fume are more rarely produced than those containing other pozzolanic industrial by-products, such as fly ash or natural pozzolans, its use being restricted to high-strength applications. For example, high-strength silica fume concrete was used in one of the world’s tallest reinforced-concrete buildings, 311 South Wacker Drive in Chicago, helping to significantly reduce the quantity of concrete and reinforcing steel that was required for construction.