Silica fume

Not to be confused with fumed silica.

Silica fume, 380m²/g

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 pyrogenic silica) and colloidal silica. These materials have different derivations, technical characteristics, and applications.

History

The history of silica fume is relatively short, the first recorded testing of silica fume in Portland cement based concretes was in 1952 and it wasn’t until the early 1970’s that concretes containing silica fume came into even limited use. The biggest drawback to discovering the unique properties of silica fume and its potential was a lack of silica fume to experiment with. Early research used an expensive additive called Fumed silica, a colloidal form of silica made by combustion of silicon tetrachloride in hydrogen-oxygen furnaces. Silica fume on the other hand, is a by-product or a very fine pozzolanic material, composed of mostly amorphous silica produced by electric arc furnaces during the production of elemental silicon or ferro silicon alloys. Before the late 1960’s in Europe and the mid1970’s in the United States, silica fume simply went up the stack as smoke vented into the atmosphere .

Only with the implementation of tougher environmental laws during the mid-1970’s did silicon smelters begin to capture and collect the silica fume, instead of sending it to the landfill. Thus the push was on to find uses for it. Obviously, the early work done in Norway received most of the attention, since it had shown that Portland cement based concretes containing silica fumes had very high strengths and low porosities. Since then silica fume usage and development has continued making it one of the world’s most valuable and versatile admixtures for concrete and cementitous products. Typically purchased by the refractory market in a 50 lb. bag this very fine gray powder has come along way from being known only as “smoke”.

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 humid continental 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] effect of silica fume on different properties of fresh and harden concrete:-

a) Workability: With the addition of silica fume, the slump loss with time is directly proportional to increase in the silica fume content due to the introduction of large surface area in the concrete mix by its addition. Although the slump decreases, the mix remains highly cohesive.

b) Segregation and Bleeding: Silica fume reduces bleeding significantly because the free water is consumed in wetting of the large surface area of the silica fume and hence the free water left in the mix for bleeding also decreases. Silica fume also blocks the pores in the fresh concrete so water within the concrete is not allowed to come to the surface.

See also

• Engineered Cementitious Composite
• Fly ash
• Kaolinite
• Pozzolan
• Rice husk ash
• Metakaolin

Notes

1. ^ "Silica Fume". U.S. Department of Transportation, Federal Highway Administration. http://www.fhwa.dot.gov/infrastructure/materialsgrp/silica.htm. 
2. ^ ACI Committee 226. 1987b. "Silica fume in concrete: Preliminary report", ACI Materials Journal March–April: 158–66.
3. ^ 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)
4. ^ ASTM C1240. Standard Specification for Silica Fume Used in Cementitious Mixtures, http://astm.org
5. ^ EN 13263 Silica fume for concrete. http://www.cen.eu
6. ^ Detwiler, R.J. and Mehta, P.K., Chemical and Physical Effects of Silica Fume on the Mechanical Behavior of Concrete, Materials Journal Nov. 1989
7. ^ 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. http://www.concreteinternational.com/pages/featured_article.asp?ID=4451. 
8. ^ ACI 234R-06. Guide to Silica Fume in Concrete, American Concrete Institute

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
• Microsilica Union