Calcium silicate

For the metasilicate, see Calcium metasilicate.

Calcium silicate can refer to several silicates of calcium including:

• 2CaO·SiO₂, larnite (Ca₂SiO₄)
• 3CaO·SiO₂, alite or (Ca₃SiO₅)
• 3CaO·2SiO₂, (Ca₃Si₂O₇)
• CaO·SiO₂, wollastonite (CaSiO₃).

Calcium silicate

Names
Preferred IUPAC name Calcium silicate
Systematic IUPAC name Dicalcium silicate
Other names Belite Calcium monosilicate Calcium hydrosilicate Calcium metasilicate Calcium orthosilicate Micro-cell Silene Silicic acid calcium salt
Identifiers
CAS Number	1344-95-2 Y 111811-33-7 hydrate Y 12168-85-3 calcium oxide Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:190294
ChemSpider	14235 N 23811 calcium oxide Y
ECHA InfoCard	100.014.282
EC Number	235-336-9
E number	E552 (acidity regulators, ...)
KEGG	D03309 N
MeSH	Calcium+silicate
PubChem CID	14941 44154858 hydrate 25523 calcium oxide
UNII	S4255P4G5M Y 404G39282C calcium oxide Y
CompTox Dashboard (EPA)	DTXSID5049570
InChI InChI=1S/2Ca.O4Si/c;;1-5(2,3)4/q2*+2;-4 N Key: JHLNERQLKQQLRZ-UHFFFAOYSA-N N
SMILES [Ca++].[Ca++].[O-][Si]([O-])([O-])[O-]
Properties
Chemical formula	Ca2O4Si
Molar mass	172.237 g·mol−1
Appearance	White crystals
Density	2.9 g/cm3 (solid)[1]
Melting point	2,130[2] °C (3,870 °F; 2,400 K)
Solubility in water	0.01% (20 °C)[1]
Thermochemistry
Std molar entropy (S⦵298)	84 J/(mol·K)[3]
Std enthalpy of formation (ΔfH⦵298)	−1630 kJ/mol[3]
Pharmacology
ATC code	A02AC02 (WHO)
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	Irritant
NFPA 704 (fire diamond)	2 0 0
Flash point	Not applicable
NIOSH (US health exposure limits):
PEL (Permissible)	TWA 15 mg/m3 (total) TWA 5 mg/m3 (resp)[1]
REL (Recommended)	TWA 10 mg/m3 (total) TWA 5 mg/m3 (resp)[1]
IDLH (Immediate danger)	N.D.[1]
Safety data sheet (SDS)	[4]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

This article focuses on Ca₂SiO₄, also known as calcium orthosilicate. It is also referred to by the shortened trade name Cal-Sil or Calsil. All calcium silicates are white free-flowing powders. They are components of important structural materials because they are strong, cheap, and nontoxic.

Production and occurrence

Calcium silicates are produced by treating calcium oxide and silica in various ratios. Their formation is relevant to Portland cement.^[5]

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:^[6]

MgO·CaO +Si → 2 Mg + Ca₂SiO₄

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.^[7][8]

The production of sulfuric acid from anhydrous calcium sulfate produces calcium silicates.^[9] 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.^[10]

2 CaSO₄ + 2 SiO₂ + C → 2 CaSiO₃ + 2 SO₂ + CO₂

Structure

Unit cell of Ca₂SiO₄. Color code: red (O), blue (Ca), gold (Si).

As verified by X-ray crystallography, calcium silicate is a dense solid consisting of tetrahedral orthosilicate (SiO₄^4-) units linked to Ca²⁺ via Si-O-Ca bridges. There are two calcium sites. One is seven coordinate and the other is eight coordinate.^[11]

Use

As a component of cement

Calcium silicates are the major ingredients in Portland cements.^[12]

Typical constituents of portland clinker plus gypsum
showing cement chemist notation (CCN)

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 insulation

Calcium-silicate passive fire protection board being clad around steel structure in order to achieve a fire-resistance rating

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 protection

Circuit integrity fireproofing of cable trays in Lingen/Ems, Germany using calcium-silicate board system qualified to DIN 4102. Other methods for exterior protection of electrical circuits include boards made of sodium silicate bonded and pressed vermiculite and flexible wraps made of ceramic fibre and rockwool.

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. ^[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.^{[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.^{[citation needed]}

Acid mine drainage remediation

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.^[13] 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),^[14] calcium silicate effectively precipitates heavy metals and does not armor over, prolonging its effectiveness in AMD systems.^[13][15]

As a product of sealants

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.^[16][17]

Agriculture

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" ^[18]

Other

Calcium silicate is used as an anticaking agent in food preparation, including table salt^[19] 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.^[20] It has the E number reference E552.

See also

• Alite – chemical compoundPages displaying wikidata descriptions as a fallback
• Jaffeite – Sorosilicate mineral
• Plaster – Broad range of building and sculpture materials
• Perlite – Amorphous volcanic glass
• Vermiculite – Hydrous phyllosilicate mineral
• Brick#Calcium-silicate bricks – Block or a single unit of a ceramic material used in masonry construction

References

1. NIOSH Pocket Guide to Chemical Hazards. "#0094". National Institute for Occupational Safety and Health (NIOSH).
2. R. B. Heimann, Classic and Advanced Ceramics: From Fundamentals to Applications, Wiley, 2010 ISBN 352763018X
3. Zumdahl, Steven S. (2009). Chemical Principles 6th Ed. Houghton Mifflin Company. p. A21. ISBN 978-0-618-94690-7.
4. "SDS Sheet Library". BNZ Materials. Archived from the original on 2012-03-04. Retrieved 2017-07-19.
5. H. F. W. Taylor, Cement Chemistry, Academic Press, 1990, ISBN 0-12-683900-X, p. 33–34.
6. Amundsen, Ketil; Aune, Terje Kr.; Bakke, Per; Eklund, Hans R.; Haagensen, Johanna Ö.; Nicolas, Carlos; Rosenkilde, Christian; Van Den Bremt, Sia; Wallevik, Oddmund (2003). "Magnesium". Ullmann's Encyclopedia of Industrial Chemistry. doi:10.1002/14356007.a15_559. ISBN 978-3-527-30385-4.
7. Berner, Robert; Lasaga, Antonio; Garrels, Robert (1983). "The carbonate-silicate geochemical cycle and its effect on atmospheric carbon dioxide over the past 100 million years". American Journal of Science. 283 (7): 641–683. Bibcode:1983AmJS..283..641B. doi:10.2475/ajs.283.7.641.
8. Walker, James C. G.; Hays, P. B.; Kasting, J. F. (1981). "A negative feedback mechanism for the long-term stabilization of Earth's surface temperature". Journal of Geophysical Research: Oceans. 86 (C10): 9776–9782. Bibcode:1981JGR....86.9776W. doi:10.1029/JC086iC10p09776. ISSN 2156-2202.
9. Whitehaven Cement Plant
10. Anhydrite Process
11. Jost, K. H.; Ziemer, B.; Seydel, R. (1977). "Redetermination of the structure of β-dicalcium silicate". Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry. 33 (6): 1696–1700. Bibcode:1977AcCrB..33.1696J. doi:10.1107/S0567740877006918.
12. Sprung, Siegbert (2008). "Cement". Ullmann's Encyclopedia of Industrial Chemistry. doi:10.1002/14356007.a05_489.pub2. ISBN 978-3-527-30385-4.
13. Ziemkiewicz, Paul. "The Use of Steel Slag in Acid Mine Drainage Treatment and Control". Wvmdtaskforce.com. Archived from the original on 20 July 2011. Retrieved 25 April 2011.
14. Skousen, Jeff. "Chemicals". Overview of Acid Mine Drainage Treatment with Chemicals. West Virginia University Extension Service. Archived from the original on 24 May 2011. Retrieved 29 March 2011.
15. Hammarstrom, Jane M.; Philip L. Sibrell; Harvey E. Belkin. "Characterization of limestone reacted with acid-mine drainage" (PDF). Applied Geochemistry (18): 1710–1714. Archived from the original (PDF) on 5 June 2013. Retrieved 30 March 2011.
16. Giannaros, P.; Kanellopoulos, A.; Al-Tabbaa, A. (2016). "Sealing of cracks in cement using microencapsulated sodium silicate". Smart Materials and Structures. 25 (8): 8. Bibcode:2016SMaS...25h4005G. doi:10.1088/0964-1726/25/8/084005.
17. Passmore, S. M. (1975). "Preserving eggs". Nutrition & Food Science. 75 (4): 2–4. doi:10.1108/eb058634.
18. Gascho, Gary J. (2001). "Chapter 12 Silicon sources for agriculture". Studies in Plant Science. 8 (8): 197–207. doi:10.1016/S0928-3420(01)80016-1. ISBN 9780444502629.
19. Archived 2008-12-25 at the Wayback Machine
20. "Food Additive Details: Calcium silicate". Archived from the original on June 5, 2012. Retrieved July 28, 2013. 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.