Calcium silicate can refer to several silicates of calcium including:
This article focuses on Ca
2SiO
4, 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
silicon dioxide in various ratios. Their formation is relevant to
Portland cement.
[H. F. W. Taylor, Cement Chemistry, Academic Press, 1990, , p. 33–34.]
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:
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 Carbonate rock by weathering and sedimentation.
The production of sulfuric acid from anhydrous calcium sulfate produces calcium silicates.[ Whitehaven Cement Plant] 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.[ Anhydrite Process]
Structure
As verified by X-ray crystallography, calcium silicate is a dense solid consisting of
tetrahedral orthosilicate (SiO
44-) units linked to Ca
2+ via Si-O-Ca bridges. There are two calcium sites. One is seven coordinate and the other is eight coordinate.
Use
As a component of cement
Calcium silicates are the major ingredients in
.
+ Typical constituents of portland clinker plus gypsum
showing cement chemist notation (CCN)
! Clinker
! CCN
! Mass
! Mineral |
| alite |
| belite |
| 5–12% |
| 6–12% |
| gypsum |
High-temperature insulation
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
. Calcium silicate competes in these realms against
Mineral wool 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
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
use spray fireproofing
, 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
.
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.
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.
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 (H
4SiO
4), 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),
calcium silicate effectively precipitates heavy metals and does not armor over, prolonging its effectiveness in AMD systems.
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.
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"
Other
Calcium silicate is used as an
anticaking agent in food preparation, including table salt
[[3] ] 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.
[
]''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.
It has the [[E number]] reference '''E552'''.
See also
