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{{About|the mineral|the aircraft manufacturer|Belite Aircraft}}
'''Belite''' is an [[industrial mineral]] important in [[Portland cement]] manufacture. Its main constituent is dicalcium silicate, Ca<sub>2</sub>SiO<sub>4</sub>, sometimes formulated as 2 CaO · SiO<sub>2</sub> (C<sub>2</sub>S in [[cement chemist notation]]).

== Etymology ==
The name was given by [[Alfred Elis Törnebohm]] in 1897 to a crystal identified in microscopic investigation of Portland cement.<ref>Jean-Pierre Bournazel, Yves Malier, Micheline Moranville Regourd, 1998, ''Concrete, from Material to Structure '' RILEM Publications, {{ISBN|2-912143-04-7}}.</ref> Belite is a name in common use in the cement industry, but is not a recognised mineral name. It occurs naturally as the mineral [[larnite]], the name being derived from [[Larne]], [[Northern Ireland]], the closest town to [[Scawt Hill]] where it was discovered.<ref>{{cite book
|chapter-url = https://books.google.com/books?id=XT3cj5U2WEIC&pg=PA248
|pages = 248–249
|chapter = Larnite
|title = Disilicates and ring silicates
|isbn = 978-1-897799-89-5
|last1 = Deer
|first1 = William Alexander
|last2 = Howie
|first2 = R. A
|last3 = Zussman
|first3 = J
|date = 1997-05-01}}</ref>

== Composition and structure ==
[[File:BeliteXtalStruc.jpg|thumb|250px|Simplified crystal structure of belite]]
The belite found in Portland cement differs in composition from pure di[[calcium silicate]].  It is a [[solid solution]] and contains minor amounts of other [[oxide]]s besides CaO and SiO<sub>2</sub>.  A typical composition:<ref>Taylor H.F.W. (1990), ''Cement Chemistry'', Academic Press, 1990, {{ISBN|0-12-683900-X}}, pp. 10-11.</ref>

{| class="wikitable"
!width="60"|Oxide
!width="60"|Mass %
|-
|CaO||63.5
|-
|SiO<sub>2</sub>||31.5
|-
|Al<sub>2</sub>O<sub>3</sub>||2.1
|-
|Fe<sub>2</sub>O<sub>3</sub>||0.9
|-
|MgO||0.5
|-
|SO<sub>3</sub>||0.1
|-
|Na<sub>2</sub>O||0.1
|-
|K<sub>2</sub>O||0.9
|-
|TiO<sub>2</sub>||0.2
|-
|P<sub>2</sub>O<sub>5</sub>||0.2
|}
{{Clear}}

Based on this, the formula can be expressed as Ca<sub>1.94</sub>Mg<sub>0.02</sub>Na<sub>0.01</sub>K<sub>0.03</sub>Fe<sub>0.02</sub>Al<sub>0.07</sub>Si<sub>0.90</sub>P<sub>0.01</sub>O<sub>3.93</sub>.  In practice, the composition varies with the bulk composition of the [[Clinker (cement)|clinker]], subject to certain limits.  Substitution of calcium ions or orthosilicate ions requires that electric charges be kept in balance.  For instance, a limited number of [[orthosilicate]] ({{chem|SiO|4|4−}}) ions can be replaced with [[sulfate]] ({{chem|SO|4|2−}}) ions, provided that for each sulfate ion, two [[aluminate]] ({{chem|AlO|4|5−}}) ions are also substituted.

== Polymorphs ==
Dicalcium silicate is stable, and is readily prepared from reactive CaO and SiO<sub>2</sub> at 300&nbsp;°C.  The low temperature form is γ-belite, or lime [[olivine]].  This form does not hydrate, and is avoided in cement manufacture.

As the temperature rises, it passes through several [[Polymorphism (materials science)|polymorphic]] states:
{| class="wikitable" style="margin:1em auto; float:left"
! style="width:100px;"|Temp°C
!width="70"|Name
! style="width:120px;"|Crystal
|-
|>1425||α||Hexagonal
|-
|1160–1425||α'<sub>H</sub>||Orthorhombic
|-
|680-1160||α'<sub>L</sub>||Orthorhombic
|-
|500-680||β||Monoclinic
|-
|<500||γ||Orthorhombic
|}
{{Clear}}

== Hydration ==
Belite is the mineral in Portland cement responsible for development of "late" [[strength of materials|strength]].  The other silicate, [[alite]] contributes "early" strength, due to its higher reactivity.  Belite reacts with water (roughly) to form [[calcium silicate hydrate]]s (C-S-H) and [[portlandite]] (Ca(OH)<sub>2</sub>) according to the reaction:

<chem>\underbrace{{2 Ca2SiO4}}_{{Belite}} + \underbrace{{4 H2O}}_{{Water}} -> 3 CaO . 2 SiO2. 3 H2O + \underbrace{{Ca(OH)2}}_{{Portlandite}}</chem>

This rapid reaction is "chemically analogue" to the slow natural hydration of [[forsterite]] (the magnesium end-member of [[olivine]]) leading to the formation of [[serpentinite|serpentine]] and [[brucite]] in nature, although the kinetic of hydration of poorly crystallized artificial belite is much faster than the slow weathering of well crystallized Mg-[[olivine]] under natural conditions.

<chem>\underbrace{{2 Mg2SiO4}}_{{Forsterite}}
 +\underbrace{{3H2O}}_{{Water}} -> \underbrace{{Mg3Si2O5(OH)4}}_{{serpentine }} + \underbrace{{Mg(OH)2}}_{{brucite }}
</chem>

The [[hydrate]] phase,  [3 CaO · 2 SiO<sub>2</sub> · 3 H<sub>2</sub>O], is referred to as the "[[calcium silicate hydrate|C-S-H]]" phase.  It grows as a mass of [[interlocking]] needles that provide the strength of the hydrated cement system.  Relatively high belite reactivity is desirable in Portland cement manufacture, and the formation of the unreactive γ-form must be rigorously avoided.  This is achieved by rapid cooling, forming crystals that are small, distorted and highly defective.  Defects provide sites for initial water attack.  Failure to cool the clinker rapidly leads to inversion of belite to the γ-form.  The γ-form has a substantially different structure and density, so that inversion leads to degradation of the crystal and its surrounding matrix, and can also trigger decomposition of the neighboring [[alite]].  This is observed macroscopically as "dusting": the clinker [[nodule (geology)|nodules]] fall to a fine [[dust]].

== Detection ==
[[File:ClinkerPetrog01.jpg|thumb|250px|Clinker section 0.15 x 0.15 mm]]
The minerals in [[Portland cement]] clinker may be observed and quantified by [[petrography|petrographic]] microscopy.  Clinker nodules are cut and ground to a flat, polished surface.  The exposed minerals are made visible and identifiable by [[etching]] the surface.  The surface can then be observed in reflected light by [[optical microscopy]].  In the example, a clinker nodule has been polished and etched with [[hydrogen fluoride]] vapour.  The alite shows as brown, the belite as blue, and the melt phases as white.  [[Electron microscopy]] can also be used, in which case the minerals may be identified by [[microprobe]] analysis.  The preferred method to quantify the minerals accurately is [[X-ray diffraction]] on the powdered clinker, using the [[Rietveld refinement|Rietveld]] analysis technique.  Belite is much harder to grind in a [[cement mill]] than alite.

== See also ==
* [[Serpentinite#Formation of serpentine minerals|Hydration reaction of forsterite (olivine) in serpentinisation]]
* [[cement chemist notation|CCN]], cement chemist notation

== References ==
{{reflist}}

[[Category:Chemistry of construction methods]]
[[Category:Silicates]]
[[Category:Calcium compounds]]
[[Category:Cement]]