Over the past two decades, silica fume has increasingly been used as a concrete property enhancer, either as a partial replacement for Portland cement or as an additive.

It was the Scandinavian countries - particularly, Norway - which pioneered the use of silica fume in concrete in the early seventies. Since then, the concrete industry in many countries has gained wide experience in using silica fume and numerous structures are constructed with enhanced silica concrete.

Scancem Chemicals is a large exporter of silica fume, other silica-based products and micro-cements for injection/rock stabilisation and concrete admixtures. These products are marketed under the brand names Scancem Silica Fume and Micropoz.

Due to the harsh climatic conditions in the Arabian Gulf, silica fume has become a major component for high-performance concrete used in the area. Chlorides - either in the ground or in the humid air of coastal areas - represent a serious deterioration factor by causing extensive corrosion of reinforcement. In addition, the concrete also suffers from sulphate intrusion.

Over the past few years, a large number of high-rise buildings and other high-performance constructions have been erected in the Gulf. These require concrete with high compressive strengths and other high performance characteristics, such as high workability without separation or bleeding. The use of silica fume helps meet all these requirements.

The most important property afforded by silica fume concrete in the Gulf area is protection against chloride-based corrosion.

Durability

Mercury intrusion porosimetry has shown that silica fume makes the pore structure of paste more homogeneous by decreasing the number of large pores without affecting the total porosity. This in turn reduces the number of continuous pores. Permeability is directly related to the presence of continuous capillary pores and silica fume therefore reduces the risk of intrusion of liquids and vapours into the concrete.

Silica fume in concrete will alter the microstructure of the transition zone between aggregate and matrix in it. In normal concrete, the transition zone contains voids from bleeding and a massive layer of weak calcium hydroxide. Silica fume transforms the zone to a much more homogenous and dense microstructure and therefore reduces permeability.

Because of its low permeability, silica fume concrete has a substantially increased resistance to attack by various chemicals compared to ordinary concrete. In addition to reduced intrusion of chloride ions, silica concrete increases the electrical resistivity, thereby reducing the rate of galvanic corrosion.

In Norway, the Public Road Department has specified the use of silica fume in concrete in the construction of all structures. The quantity of silica fume required depends upon how aggressive the environment is.

In addition, the reduced permeability of silica fume concrete will reduce the ingress of sulphate ions and reaction between tricalcium aluminate (C3A) in cement, resulting in enhanced durability.

Gothenburg wharves

A new and interesting investigation of the 23-year-old wharves by the Gothia River in Gothenburg on the west coast of Sweden proves the connection between theory and in-situ tests conducted over a long time. The wharves were built using a total of five different concrete qualities.

"Low" quality concrete was used in the lower part of the structure, while higher quality concrete was used in the upper sections. Different mixtures were used for each of the situations, with or without silica fume. Thus, there is equal exposure of otherwise comparable concretes, giving a unique opportunity to study the effect of silica fume addition. The table summarises water/binder relations (relation between water and cement + silica fume) (1); in-situ compressive strength 22-24 years 95 mm cores (2); in-situ estimated chloride diffusion coefficient (3); laboratory chloride profiles (NT Build 443) (4); electrical resistivity (internal SINTEF procedure) (5); and resistance number from capillarity suction and porosity measurements (6).

Conclusion

The results show a dramatic increase in the concrete's resistance to chloride penetration produced by silica fume. Low chloride transport and high electrical resistivity of the silica fume concretes indicate that the corrosion protection of the steel reinforcement should be very good.

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