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{{Short description|Silicon dioxide nano particles}}{{distinguish|fumed silica}}
[[File:Silica fume agglomerate, Transmission Electron Microscopy.jpg|thumb|Silica fume particles viewed in a [[transmission electron microscope]]]]

'''Silica fume''', also known as '''microsilica''', (CAS number 69012-64-2, EINECS number 273-761-1) is an [[amorphous]] (non-crystalline) polymorph of [[silicon 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&nbsp;nm. The main field of application is as [[pozzolan]]ic material for high performance concrete.

It is sometimes confused with [[fumed 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.

== History ==

The first testing of silica fume in Portland-[[cement]]-based [[concrete]]s 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, an [[amorphous]] form of silica made by combustion of [[silicon 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 elemental [[silicon]] or [[ferrosilicon]] alloys in electric arc furnaces. Before the late 1960s in [[Europe]] and the mid-1970s in the [[United 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 low [[Porosity|porosities]]. 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.

== Properties ==
{{Components of Cement, Comparison of Chemical and Physical Characteristics}}
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.<ref>{{cite web|title=Chapter 3 Fly Ash, Slag, Silica Fume, and Natural Pozzolans|publisher=The University of Memphis |url=http://www.ce.memphis.edu/1101/notes/concrete/PCA_manual/Chap03.pdf}}</ref> The bulk density of silica fume depends on the degree of densification in the silo and varies from 130 (undensified) to 600&nbsp;kg/m<sup>3</sup>. 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 the [[BET theory|BET method]] or nitrogen adsorption method. It typically ranges from 15,000 to 30,000 m<sup>2</sup>/kg.<ref>{{cite web|title=Silica Fume User's Manual|publisher=Silica Fume Association |url=http://www.silicafume.org/pdf/silicafume-users-manual.pdf}}</ref>

== Production ==
Silica fume is a byproduct in the [[carbothermic]] reduction of high-purity [[quartz]] with carbonaceous materials like coal, coke, wood-chips, in [[electric arc furnace]]s 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.<ref>ACI Committee 226. 1987b. "Silica fume in concrete: Preliminary report", ''ACI Materials Journal'' March–April: 158–66.</ref><ref>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)</ref> Standard specifications for silica fume used in cementitious mixtures are ASTM C1240,<ref>ASTM C1240. Standard Specification for Silica Fume Used in Cementitious Mixtures, http://astm.org</ref> EN 13263.<ref>EN 13263 Silica fume for concrete. http://www.cen.eu</ref>

Silica fume is added to Portland cement concrete to improve its properties, in particular its [[compressive strength]], [[bond strength]], and [[abrasion (mechanical)|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 reaction]]s between the silica fume and free [[calcium hydroxide]] in the paste.<ref>Detwiler, R.J. and Mehta, P.K., ''[http://concreteinternational.com/pages/featured_article.asp?ID=2281 Chemical and Physical Effects of Silica Fume on the Mechanical Behavior of Concrete]'', ''Materials Journal'' Nov. 1989</ref>

Addition of silica fume also reduces the [[permeability (fluid)|permeability]] of concrete to [[chloride]] [[ion]]s, which protects the [[rebar|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 [[seawater|saltwater]] bridges.<ref>{{cite web|title=Use of supplementary cementing materials to increase the resistance to chloride ion penetration of concretes cured at elevated temperatures|author1=Rachel J. Detwiler |author2=Chris A. Fapohunda |author3=Jennifer Natale  |name-list-style=amp |publisher=Materials Journal|date=January 1994|url=http://www.concreteinternational.com/pages/featured_article.asp?ID=4451 }}</ref> 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. <ref>{{Cite web |title=Silica Fume for Oil and Gas {{!}} R-E-D Industrial Products |url=https://www.redindustrialproducts.com/silica-fume-for-oil-and-gas |access-date=2023-02-06 |website=R-E-D Industrial |language=en}}</ref>

Prior to the mid-1970s, nearly all silica fume was discharged into the atmosphere. After environmental concerns necessitated the collection and [[landfill]]ing of silica fume, it became economically viable to use silica fume in various applications, in particular high-performance concrete.<ref>''ACI 234R-06. Guide to Silica Fume in Concrete'', [http://www.concrete.org American Concrete Institute]</ref> Effects of silica fume on different properties of fresh and hardened concrete include:

* Workability: With the addition of silica fume, the [[Concrete slump test|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.
* [[Segregation in concrete|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.

=== Silicon carbide ===
The silica fumes, as byproduct, may be used to produce [[silicon carbide]].

== See also ==
{{div col|content=
* [[Alkali–silica reaction]]
* [[Engineered cementitious composite]]
* [[Energetically modified cement]] (EMC)
* [[Fly ash]]
* [[Kaolinite]]
* [[Pozzolan]]
* [[Rice husk ash]]
* [[Metakaolin]]
}}

== References ==

{{reflist|35em}}

== Further reading ==
* {{cite web| author = U.S. Federal Highway Administration| authorlink = Federal Highway Administration| title = Silica Fume| url = http://www.fhwa.dot.gov/infrastructure/materialsgrp/silica.htm| access-date = 2007-01-24| url-status = dead| archive-url = https://web.archive.org/web/20070122022403/http://www.fhwa.dot.gov/infrastructure/materialsgrp/silica.htm| archive-date = 2007-01-22}}
* {{ cite web | author = Portland Cement Association|authorlink=The University of Memphis | title = Chapter 3 Fly Ash, Slag, Silica Fume, and Natural Pozzolans| url = http://www.ce.memphis.edu/1101/notes/concrete/PCA_manual/Chap03.pdf|access-date=2012-04-07 }}
* {{ cite web | author = Silica Fume Association | title = Silica Fume User's Manual| url = http://www.silicafume.org/pdf/silicafume-users-manual.pdf|access-date=2012-05-05 }}

== External links ==
{{commons category|Silicon dioxide}}
* [http://silicafume.org/ Silica Fume Association]

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