Silica fume

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Silica fume, also known as microsilica, is a byproduct of the reduction of high-purity quartz with coke in electric arc furnaces in the production of silicon and ferrosilicon alloys (ASTM C1240[1], EN 13263[2]. Before 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 justified to use silica fume in various applications, in particular high performance concrete[3].

Silica fume consists of very fine vitreous particles with a surface area on the order of 215,280 ft²/lb (20,000 m²/kg) when measured by nitrogen absorption techniques, with particles approximately 100 times smaller than the average cement particle. [4]

Because of its extreme fineness and high silica content, silica fume is a highly effective pozzolanic material. [5][6]

Silica fume is used as an addition in Portland cement concretes to improve properties. It has been found that silica fume improves compressive strength, bond strength, and abrasion resistance. The improvements in concrete properties from addition of silica fume stem from both the mechanical improvements resulting from addition of a very fine powder to the cement paste mix as well as from pozzolanic reactions between the silica fume and free calcium hydroxide in the paste.[7]

Addition of silica fume also reduces the permeability of concrete to chloride ions, which protects concrete's reinforcing steel from corrosion, especially in chloride-rich environments such as those of northern roadways and runways (because of the use of deicing salts) and saltwater bridges.[8]

[edit] Notes

  1. ^ ASTM C1240. Standard Specification for Silica Fume Used in Cementitious Mixtures, http://www.astm.org
  2. ^ EN 13263. Silica fume for concrete. http://www.cen.eu
  3. ^ ACI 234R-06. Guide to Silica Fume in Concrete, American Concrete Institute, http://www.concrete.org
  4. ^ Silica Fume. U.S. Department of Transportation, Federal Highway Administration.
  5. ^ ACI Committee 226. 1987b. Silica fume in concrete: Preliminary report. ACI Materials Journal March-April: 158-66.
  6. ^ Luther, M. D. 1990. High-performance silica fume (microsilica)—Modified cementitious repair materials. 69th annual meeting of the Transportation Research Board, paper no. 890448 (January)
  7. ^ Detwiler, R.J. and Mehta, P.K., Chemical and Physical Effects of Silica Fume on the Mechanical Behavior of Concrete, November 1989, Materials Journal, http://www.concreteinternational.com/pages/featured_article.asp?ID=2281
  8. ^ Rachel J. Detwiler, Chris A. Fapohunda, and Jennifer Natale (January 1994). Use of Supplementary Cementing Materials to Increase the Resistance to Chloride Ion Penetration of Concretes Cured at Elevated Temperatures. Materials Journal.

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