Belite
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Belite is an
industrial mineral Industrial resources (minerals) are geological materials that are mined for their commercial value, which are not fuel ( fuel minerals or mineral fuels) and are not sources of metals (metallic minerals) but are used in the industries based on t ...
important in
Portland cement Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar (masonry), mortar, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in th ...
manufacture. Its main constituent is dicalcium silicate, Ca2SiO4, sometimes formulated as 2
CaO Cao or CAO may refer to: Mythology *Cao (bull), a legendary bull in Meitei mythology Companies or organizations * Air China Cargo, ICAO airline designator CAO * CA Oradea, Romanian football club * CA Osasuna, Spanish football club * Canadian ...
· SiO2 (C2S in
cement chemist notation Cement chemist notation (CCN) was developed to simplify the formulas cement chemists use on a daily basis. It is a shorthand way of writing the chemical formula of oxides of calcium, silicon, and various metals. Abbreviations of oxides The main ...
).


Etymology

The name was given by
Alfred Elis Törnebohm Alfred Elis Törnebohm (16 October 1838 – 21 April 1911) was a Swedish geologist, best known today for his study of the overthrust of the Caledonian range. After studies at the Royal Institute of Technology (KTH) 1855-1858, he worked at ...
in 1897 to a crystal identified in microscopic investigation of Portland cement. 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 Larne (, , the name of a Gaelic Ireland, Gaelic territory)Larne/Latharna
Placenames Database of Ireland.
is a to ...
,
Northern Ireland Northern Ireland ( ; ) is a Countries of the United Kingdom, part of the United Kingdom in the north-east of the island of Ireland. It has been #Descriptions, variously described as a country, province or region. Northern Ireland shares Repub ...
, the closest town to Scawt Hill where it was discovered.


Composition and structure

The belite found in Portland cement differs in composition from pure di
calcium silicate 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 calci ...
. It is a
solid solution A solid solution, a term popularly used for metals, is a homogeneous mixture of two compounds in solid state and having a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The word "solutio ...
and contains minor amounts of other
oxide An oxide () is a chemical compound containing at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion (anion bearing a net charge of −2) of oxygen, an O2− ion with oxygen in the oxidation st ...
s besides CaO and SiO2. A typical composition:Taylor H.F.W. (1990), ''Cement Chemistry'', Academic Press, 1990, , pp. 10-11. Based on this, the formula can be expressed as Ca1.94Mg0.02Na0.01K0.03Fe0.02Al0.07Si0.90P0.01O3.93. In practice, the composition varies with the bulk composition of the 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 In chemistry, orthosilicate is the anion , or any of its salts and esters. It is one of the silicate anions. It is occasionally called the silicon tetroxide anion or group.C. A. Kumins, and A. E. Gessler (1953), "Short-Cycle Syntheses of Ultr ...
() ions can be replaced with
sulfate The sulfate or sulphate ion is a polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many ...
() ions, provided that for each sulfate ion, two
aluminate In chemistry, an aluminate is a compound containing an oxyanion of aluminium, such as sodium aluminate. In the naming of inorganic compounds, it is a suffix that indicates a polyatomic anion with a central aluminium atom. Aluminate oxyanions ...
() ions are also substituted.


Polymorphs

Dicalcium silicate is stable, and is readily prepared from reactive CaO and SiO2 at 300 °C. The low temperature form is γ-belite, or lime
olivine The mineral olivine () is a magnesium iron Silicate minerals, silicate with the chemical formula . It is a type of Nesosilicates, nesosilicate or orthosilicate. The primary component of the Earth's upper mantle (Earth), upper mantle, it is a com ...
. This form does not hydrate, and is avoided in cement manufacture. As the temperature rises, it passes through several polymorphic states:


Hydration

Belite is the mineral in Portland cement responsible for development of "late"
strength Strength may refer to: Personal trait *Physical strength, as in people or animals *Character strengths like those listed in the Values in Action Inventory *The exercise of willpower Physics * Mechanical strength, the ability to withstand ...
. The other silicate,
alite Alite is an impure form of tricalcium silicate, , sometimes formulated as ( in cement chemist notation), typically with 3-4% of substituent oxides. It is the major, and characteristic, phase in Portland cement. The name was given by Alfred Elis ...
contributes "early" strength, due to its higher reactivity. Belite reacts with water (roughly) to form
calcium silicate hydrate Calcium silicate hydrates (CSH or C-S-H) are the main products of the hydration of Portland cement and are primarily responsible for the strength of cement-based materials. They are the main binding phase (the "glue") in most concrete. Only well de ...
s (C-S-H) and portlandite (Ca(OH)2) according to the reaction: \underbrace_ + \underbrace_ -> 3 CaO . 2 SiO2. 3 H2O + \underbrace_ This rapid reaction is "chemically analogue" to the slow natural hydration of
forsterite Forsterite (Mg2SiO4; commonly abbreviated as Fo; also known as white olivine) is the magnesium-rich Endmember, end-member of the olivine solid solution series. It is Isomorphism (crystallography), isomorphous with the iron-rich end-member, fayalit ...
(the magnesium end-member of
olivine The mineral olivine () is a magnesium iron Silicate minerals, silicate with the chemical formula . It is a type of Nesosilicates, nesosilicate or orthosilicate. The primary component of the Earth's upper mantle (Earth), upper mantle, it is a com ...
) leading to the formation of serpentine and
brucite Brucite is the mineral form of magnesium hydroxide, with the chemical formula Magnesium, Mg(hydroxyl, OH)2. It is a common alteration product of periclase in marble; a low-temperature hydrothermal Vein (geology), vein mineral in metamorphosed li ...
in nature, although the kinetic of hydration of poorly crystallized artificial belite is much faster than the slow weathering of well crystallized Mg-
olivine The mineral olivine () is a magnesium iron Silicate minerals, silicate with the chemical formula . It is a type of Nesosilicates, nesosilicate or orthosilicate. The primary component of the Earth's upper mantle (Earth), upper mantle, it is a com ...
under natural conditions. \underbrace_ +\underbrace_ -> \underbrace_ + \underbrace_ The
hydrate In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was understo ...
phase, CaO · 2 SiO2 · 3 H2O is referred to as the " C-S-H" phase. It grows as a mass of
interlocking In railway signalling, an interlocking is an arrangement of signal apparatus that prevents conflicting movements through an arrangement of tracks such as junctions or crossings. In North America, a set of signalling appliances and tracks inte ...
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 Alite is an impure form of tricalcium silicate, , sometimes formulated as ( in cement chemist notation), typically with 3-4% of substituent oxides. It is the major, and characteristic, phase in Portland cement. The name was given by Alfred Elis ...
. This is observed macroscopically as "dusting": the clinker
nodules Nodule may refer to: *Nodule (geology), a small rock or mineral cluster *Manganese nodule, a metallic concretion found on the seafloor *Nodule (medicine), a small aggregation of cells *Root nodule Root nodules are found on the roots of plants, p ...
fall to a fine
dust Dust is made of particle size, fine particles of solid matter. On Earth, it generally consists of particles in the atmosphere that come from various sources such as soil lifted by wind (an aeolian processes, aeolian process), Types of volcan ...
.


Detection

The minerals in
Portland cement Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar (masonry), mortar, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in th ...
clinker may be observed and quantified by petrographic microscopy. Clinker nodules are cut and ground to a flat, polished surface. The exposed minerals are made visible and identifiable by
etching Etching is traditionally the process of using strong acid or mordant to cut into the unprotected parts of a metal surface to create a design in intaglio (incised) in the metal. In modern manufacturing, other chemicals may be used on other type ...
the surface. The surface can then be observed in reflected light by
optical microscopy Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultravio ...
. In the example, a clinker nodule has been polished and etched with
hydrogen fluoride Hydrogen fluoride (fluorane) is an Inorganic chemistry, inorganic compound with chemical formula . It is a very poisonous, colorless gas or liquid that dissolves in water to yield hydrofluoric acid. It is the principal industrial source of fluori ...
vapour. The alite shows as brown, the belite as blue, and the melt phases as white.
Electron microscopy An electron microscope is a microscope that uses a beam of electrons as a source of illumination. It uses electron optics that are analogous to the glass lenses of an optical light microscope to control the electron beam, for instance focusing i ...
can also be used, in which case the minerals may be identified by
microprobe A microprobe is an instrument that applies a stable and well-focused beam of charged particles (electrons or ions) to a sample. Types When the primary beam consists of accelerated electrons, the probe is termed an electron microprobe, when the pr ...
analysis. The preferred method to quantify the minerals accurately is
X-ray diffraction X-ray diffraction is a generic term for phenomena associated with changes in the direction of X-ray beams due to interactions with the electrons around atoms. It occurs due to elastic scattering, when there is no change in the energy of the waves. ...
on the powdered clinker, using the Rietveld analysis technique. Belite is much harder to grind in a cement mill than alite.


See also

* Hydration reaction of forsterite (olivine) in serpentinisation * CCN, cement chemist notation


References

{{reflist Chemistry of construction methods Silicates Calcium compounds Cement