Chemistry:Calcium silicate

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Short description: Chemical compound naturally occurring as the mineral larnite
For the metasilicate, see Calcium metasilicate.
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
3D model (JSmol)
ChEBI
ChemSpider
EC Number
  • 235-336-9
KEGG
MeSH Calcium+silicate
UNII
Properties
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)
0.01% (20 °C)[1]
Thermochemistry
84 J/(mol·K)[3]
−1630 kJ/mol[3]
Pharmacology
1=ATC code }} A02AC02 (WHO)
Hazards
Main hazards Irritant
Safety data sheet [4]
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondFlammability code 0: Will not burn. E.g. waterHealth code 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g. chloroformReactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
0
2
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]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Calcium silicate can refer to several silicates of calcium including:

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 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 + Ca
2SiO
4

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 CaSO
4 + 2 SiO
2 + C → 2 CaSiO
3 + 2 SO
2 + CO
2

Structure

Unit cell of Ca
2SiO
4. Color code: red (O), blue (Ca), gold (Si).

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.[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. 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.

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

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 NIOSH Pocket Guide to Chemical Hazards. "#0094". National Institute for Occupational Safety and Health (NIOSH). https://www.cdc.gov/niosh/npg/npgd0094.html. 
  2. ↑ R. B. Heimann, Classic and Advanced Ceramics: From Fundamentals to Applications, Wiley, 2010 ISBN 352763018X
  3. ↑ 3.0 3.1 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. http://www.bnzmaterials.com/msds/msds218.html. 
  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 et al. (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. doi:10.2475/ajs.283.7.641. Bibcode: 1983AmJS..283..641B. 
  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" (in en). Journal of Geophysical Research: Oceans 86 (C10): 9776–9782. doi:10.1029/JC086iC10p09776. ISSN 2156-2202. Bibcode: 1981JGR....86.9776W. 
  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. doi:10.1107/S0567740877006918. Bibcode: 1977AcCrB..33.1696J. 
  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. ↑ 13.0 13.1 Ziemkiewicz, Paul. "The Use of Steel Slag in Acid Mine Drainage Treatment and Control". http://wvmdtaskforce.com/proceedings/98/98zie/98zie.htm. 
  14. ↑ Skousen, Jeff. "Chemicals". Overview of Acid Mine Drainage Treatment with Chemicals. West Virginia University Extension Service. http://www.wvu.edu/~agexten/landrec/chemtrt.htm#Chemicals. 
  15. ↑ Hammarstrom, Jane M.; Philip L. Sibrell; Harvey E. Belkin. "Characterization of limestone reacted with acid-mine drainage". Applied Geochemistry (18): 1710–1714. http://mine-drainage.usgs.gov/pubs/AG_18-1705-1721.pdf. 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. doi:10.1088/0964-1726/25/8/084005. Bibcode: 2016SMaS...25h4005G. 
  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. ↑ [1]
  20. ↑ "Food Additive Details: Calcium silicate". http://www.codexalimentarius.net/gsfaonline/additives/details.html?id=315.  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.



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