Silica fume, also known asmicrosilica, (CAS number 69012-64-2, EINECS number 273-761-1) is anamorphous (non-crystalline) polymorph ofsilicon dioxide,silica. It is an ultrafine powder collected as a by-product of the silicon and ferrosilicon alloy production and consists of spherical particles with an average particle diameter of 150 nm. The main field of application is aspozzolanic material for high performance concrete.

It is sometimes confused withfumed silica (also known as pyrogenic silica, CAS number 112945-52-5). However, the production process, particle characteristics and fields of application of fumed silica are all different from those of silica fume.

The first testing of silica fume in Portland-cement-basedconcretes was carried out in 1952. The biggest drawback to exploring the properties of silica fume was a lack of material with which to experiment. Early research used an expensive additive called fumed silica, anamorphous form of silica made by combustion ofsilicon tetrachloride in a hydrogen-oxygen flame. Silica fume on the other hand, is a very fine pozzolanic, amorphous material, a by-product of the production of elementalsilicon orferrosilicon alloys in electric arc furnaces. Before the late 1960s inEurope and the mid-1970s in theUnited States, silica fumes were simply vented into the atmosphere.

With the implementation of tougher environmental laws during the mid-1970s, silicon smelters began to collect the silica fume and search for its applications. 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 lowporosities. Since then the research and development of silica fume made it one of the world's most valuable and versatile admixtures for concrete and cementitious products.

Silica fume is an ultrafine material with spherical particles less than 1 μm in diameter, the average being about 0.15 μm. This makes it approximately 100 times smaller than the average cement particle.^[4] The bulk density of silica fume depends on the degree of densification in the silo and varies from 130 (undensified) to 600 kg/m³. The specific gravity of silica fume is generally in the range of 2.2 to 2.3. The specific surface area of silica fume can be measured with theBET method or nitrogen adsorption method. It typically ranges from 15,000 to 30,000 m²/kg.^[5]

Silica fume is a byproduct in thecarbothermic reduction of high-purityquartz with carbonaceous materials like coal, coke, wood-chips, inelectric arc furnaces in the production of silicon and ferrosilicon alloys.

Because of its extreme fineness and high silica content, silica fume is a very effective pozzolanic material.^[6][7] Standard specifications for silica fume used in cementitious mixtures are ASTM C1240,^[8] EN 13263.^[9]

Silica fume is added to Portland cement concrete to improve its properties, in particular itscompressive strength,bond strength, andabrasion 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 thepozzolanic reactions between the silica fume and freecalcium hydroxide in the paste.^[10]

Addition of silica fume also reduces thepermeability of concrete tochlorideions, which protects thereinforcing steel of concrete fromcorrosion, especially in chloride-rich environments such as coastal regions and those ofhumid continental roadways and runways (because of the use ofdeicing salts) andsaltwater bridges.^[11] Furthermore, Silica Fumes has important uses in oil and gas operations. Silica fume can be used for a primary placement of grout as a hydraulic seal in the well bore, or secondary applications such as remedial operations including leak repairs, splits, and closing of depleted zones.^[12]

Prior to the mid-1970s, nearly all silica fume was discharged into the atmosphere. After environmental concerns necessitated the collection andlandfilling of silica fume, it became economically viable to use silica fume in various applications, in particular high-performance concrete.^[13] Effects of silica fume on different properties of fresh and hardened concrete include:

• Workability: With the addition of silica fume, theslump 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.
• 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.

The silica fumes, as byproduct, may be used to producesilicon carbide.

• U.S. Federal Highway Administration."Silica Fume". Archived fromthe original on 2007-01-22. Retrieved2007-01-24.
• Portland Cement Association."Chapter 3 Fly Ash, Slag, Silica Fume, and Natural Pozzolans"(PDF). Retrieved2012-04-07.
• Silica Fume Association."Silica Fume User's Manual"(PDF). Retrieved2012-05-05.

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