Calcium silicate
Template:Short description Template:For Template:Chembox
Calcium silicate can refer to several silicates of calcium including:
- CaO·SiO2, wollastonite (CaSiO3)
- 2CaO·SiO2, larnite (Ca2SiO4)
- 3CaO·SiO2, alite or (Ca3SiO5)
- 3CaO·2SiO2, (Ca3Si2O7).
This article focuses on Ca2SiO4, also known as calcium orthosilicate, or by the shortened trade name Cal-Sil/Calsil. All calcium silicates are white free-flowing powders. Being strong, cheap and nontoxic, they are components of important structural materials.
Production and occurrenceEdit
Calcium silicates are produced by treating calcium oxide and silica in various ratios. Their formation is relevant to Portland cement.<ref>H. F. W. Taylor, Cement Chemistry, Academic Press, 1990, Template:ISBN, p. 33–34.</ref>
Calcium silicate is a byproduct of the Pidgeon process, a major route to magnesium metal. The process converts a mixture of magnesium and calcium oxides as represented by the following simplified equation:<ref>Template:Cite book</ref>
The calcium oxide combines with silicon as the oxygen scavenger, yielding the very stable calcium silicate and releasing volatile (at high temperatures) magnesium metal.
Via the carbonate–silicate cycle, carbonate rocks convert into silicate rocks by metamorphism and volcanism and silicate rocks convert to carbonates by weathering and sedimentation.<ref>Template:Cite journal</ref><ref name=Walker1981>Template:Cite journal</ref>
The production of sulfuric acid from anhydrous calcium sulfate produces calcium silicates.<ref>Whitehaven Cement Plant</ref> Upon being mixed with shale or marl, and roasted at 1400 °C, the sulfate liberates sulfur dioxide gas, a precursor to sulfuric acid. The resulting calcium silicate is used in cement clinker production.<ref name="anhydrite process">Anhydrite Process</ref>
StructureEdit
As verified by X-ray crystallography, calcium silicate is a dense solid consisting of tetrahedral orthosilicate (SiO44-) units linked to Ca2+ via Si-O-Ca bridges. There are two calcium sites. One is seven coordinate and the other is eight coordinate.<ref>Template:Cite journal</ref>
UseEdit
As a component of cementEdit
Calcium silicates are the major ingredients in Portland cements.<ref>Template:Cite book</ref>
| Clinker | CCN | Mass | Mineral |
|---|---|---|---|
| Tricalcium silicate (CaO)3 · SiO2 | C3S | 25–50% | alite |
| Dicalcium silicate (this article) (CaO)2 · SiO2 | C2S | 20–45% | belite |
| Tricalcium aluminate (CaO)3 · Al2O3 | C3A | 5–12% | |
| Tetracalcium aluminoferrite (CaO)4 · Al2O3 · Fe2O3 | C4AF | 6–12% | |
| CaSO4 · 2 H2O | CS̅H2 | 2–10% | gypsum |
High-temperature insulationEdit
Calcium silicate is commonly used as a safe alternative to asbestos for high-temperature insulation materials. Industrial-grade piping and equipment insulation is often fabricated from calcium silicate. Its fabrication is a routine part of the curriculum for insulation apprentices. Calcium silicate competes in these realms against rockwool and proprietary insulation solids, such as perlite mixture and vermiculite bonded with sodium silicate. Although it is popularly considered an asbestos substitute, early uses of calcium silicate for insulation still made use of asbestos fibers.
Passive fire protectionEdit
It is used in passive fire protection and fireproofing as calcium silicate brick or in roof tiles. It is one of the most successful materials in fireproofing in Europe because of regulations and fire safety guidelines for commercial and residential building codes. Where North Americans use spray fireproofing plasters, Europeans are more likely to use cladding made of calcium silicate. Template:Why High-performance calcium-silicate boards retain their excellent dimensional stability even in damp and humid conditions and can be installed at an early stage in the construction program, before wet trades are completed and the building is weather-tight. For sub-standard products, silicone-treated sheets are available to fabricators to mitigate potential harm from high humidity or general presence of water. Fabricators and installers of calcium silicate in passive fire protection often also install firestops.Template:Citation needed
While the best possible reaction to fire classifications are A1 (construction applications) and A1Fl (flooring applications) respectively, both of which mean "non-combustible" according to EN 13501-1: 2007, as classified by a notified laboratory in Europe, some calcium-silicate boards only come with fire classification of A2 (limited combustibility) or even lower classifications (or no classification), if they are tested at all.Template:Citation needed
Acid mine drainage remediationEdit
Calcium silicate, also known as slag, is produced when molten iron is made from iron ore, silicon dioxide and calcium carbonate in a blast furnace. When this material is processed into a highly refined, re-purposed calcium silicate aggregate, it is used in the remediation of acid mine drainage (AMD) on active and passive mine sites.<ref name="steel slag">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Calcium silicate neutralizes active acidity in AMD systems by removing free hydrogen ions from the bulk solution, thereby increasing pH. As its silicate anion captures H+ ions (raising the pH), it forms monosilicic acid (H4SiO4), a neutral solute. Monosilicic acid remains in the bulk solution to play other important roles in correcting the adverse effects of acidic conditions. As opposed to limestone (a popular remediation material),<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> calcium silicate effectively precipitates heavy metals and does not armor over, prolonging its effectiveness in AMD systems.<ref name="steel slag"/><ref>Template:Cite journal</ref>
As a product of sealantsEdit
It is used as a sealant in roads or on the shells of fresh eggs: when sodium silicate is applied as a sealant to cured concrete or egg shells, it chemically reacts with calcium hydroxide or calcium carbonate to form calcium silicate hydrate, sealing micropores with a relatively impermeable material.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
AgricultureEdit
Calcium silicate is often used in agriculture as a plant available source of silicon. It is "applied extensively to Everglades mucks and associated sands planted to sugarcane and rice" <ref>Template:Cite journal</ref>
OtherEdit
Calcium silicate is used as an anticaking agent in food preparation, including table salt<ref name="mortonsalt1">[1] Template:Webarchive</ref> and as an antacid. It is approved by the United Nations' FAO and WHO bodies as a safe food additive in a large variety of products.<ref> {{#invoke:citation/CS1|citation |CitationClass=web }} Codex General Standard for Food Additives (GSFA) Online Database, FAO/WHO Food Standards Codex alimentarius, published by the Food and Agricultural Organization of the United Nations / World Health Organization, 2013.</ref> It has the E number reference E552.
See alsoEdit
- Template:Annotated link
- Template:Annotated link
- Template:Annotated link
- Template:Annotated link
- Template:Annotated link
- Template:Annotated link