Belite is an industrial mineral important in Portland cement manufacture. Its main constituent is dicalcium silicate, Ca₂SiO₄, sometimes formulated as 2 CaO · SiO₂ (C₂S in cement chemist notation).

Etymology

The name was given by Törnebohm 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, Northern Ireland, 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 is the chemical compound Ca2SiO4, also known as calcium orthosilicate and is sometimes formulated as 2CaO·SiO2. It is also referred to by the shortened trade name Cal-Sil or Calsil. It occurs naturally as the mineral larnite. ...

. It is a solid solution and contains minor amounts of other

oxide An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of the E ...

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 Ca_1.94Mg_0.02Na_0.01K_0.03Fe_0.02Al_0.07Si_0.90P_0.01O_3.93. In practice, the composition varies with the bulk composition of the

clinker Clinker may refer to: *Clinker (boat building), construction method for wooden boats *Clinker (waste), waste from industrial processes *Clinker (cement), a kilned then quenched cement product * ''Clinkers'' (album), a 1978 album by saxophonist St ...

, 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 Ultrama ...

() ions can be replaced with sulfate () ions, provided that for each sulfate ion, two aluminate () 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. 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: Physical strength *Physical strength, as in people or animals *Hysterical strength, extreme strength occurring when people are in life-and-death situations *Superhuman strength, great physical strength far above human ca ...

. The other silicate, alite contributes "early" strength, due to its higher reactivity. Belite reacts with water (roughly) to form calcium silicate hydrates (C-S-H) and

portlandite Portlandite is a hydroxide-bearing mineral typically included in the oxide mineral class. It is the naturally occurring form of calcium hydroxide (Ca(OH)2) and the calcium analogue of brucite (Mg(OH)2). Occurrence Portlandite occurs in a variety ...

(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 end-member of the olivine solid solution series. It is isomorphous with the iron-rich end-member, fayalite. Forsterite crystallizes in the orthor ...

(the magnesium end-member of olivine) leading to the formation of

serpentine Serpentine may refer to: Shapes * Serpentine shape, a shape resembling a serpent * Serpentine curve, a mathematical curve * Serpentine, a type of riding figure Science and nature * Serpentine subgroup, a group of minerals * Serpentinite, a ...

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. \underbrace_ +\underbrace_ -> \underbrace_ + \underbrace_{{brucite 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 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

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.

Detection

The minerals in Portland cement 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 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 Hydrogen fluoride (fluorane) is an inorganic compound with the chemical formula . This colorless gas or liquid is the principal industrial source of fluorine, often as an aqueous solution called hydrofluoric acid. It is an important feedstock i ...

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 accelerated electrons as a source of illumination. As the wavelength of an electron can be up to 100,000 times shorter than that of visible light photons, electron microscopes have a hi ...

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 X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...

on the powdered clinker, using the Rietveld analysis technique. Belite is much harder to grind in a

cement mill A cement mill (or finish mill in North American usage) is the equipment used to grind the hard, nodular clinker from the cement kiln into the fine grey powder that is cement. Most cement is currently ground in ball mills and also vertical rolle ...

than alite.

References