Silica fume

Silica fume, also known as microsilica, is a fine-grain, thin, and very high surface area silica.

It is sometimes confused with fumed silica (also known as colloidal silica and pyrogenic silica). These materials have different derivations, technical characteristics, and applications.

Properties

Silica fume consists of fine vitreous particles with a surface area on the order of 215,280 ft²/lb (20,000 m²/kg) when measured by nitrogen adsorption techniques, with particles approximately one hundredth the size of the average cement particle.^[1]

Production

Silica fume is a byproduct in the reduction of high-purity quartz with coke in electric arc furnaces in the production of silicon and ferrosilicon alloys.

Applications

Concrete

Because of its extreme fineness and high silica content, silica fume is a very effective pozzolanic material.^[2][3] Standard specifications for silica fume used in cementitious mixtures are ASTM C1240^[4], EN 13263^[5].

Silica fume is added to Portland cement concrete to improve its properties, in particular its compressive strength, bond strength, and abrasion resistance. These improvements stem from both the mechanical improvements resulting from addition of a very fine powder to the cement paste mix as well as from the pozzolanic reactions between the silica fume and free calcium hydroxide in the paste.^[6]

Addition of silica fume also reduces the permeability of concrete to chloride ions, which protects the reinforcing steel of concrete from corrosion, especially in chloride-rich environments such as coastal regions and those of northern roadways and runways (because of the use of deicing salts) and saltwater bridges.^[7]

Prior to the mid-1970s, nearly all silica fume was discharged into the atmosphere. After environmental concerns necessitated the collection and landfilling of silica fume, it became economically viable to use silica fume in various applications, in particular high-performance concrete^[8].

See also

• Fumed silica

Notes

References

• U.S. Federal Highway Administration. "Silica Fume". http://www.fhwa.dot.gov/infrastructure/materialsgrp/silica.htm. Retrieved 2007-01-24. 

External links

• Silica Fume Association