Belite

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).

The name was given by Alfred Elis Törnebohm in 1897 to a crystal identified in microscopic investigation of Portland cement.

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.

[2] The belite found in Portland cement differs in composition from pure dicalcium silicate.

As the temperature rises, it passes through several polymorphic states: Belite is the mineral in Portland cement responsible for development of "late" strength.

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 (Ca(OH)2) according to the reaction:

This rapid reaction is "chemically analogue" to the slow natural hydration of forsterite (the magnesium end-member of olivine) leading to the formation of 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.

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.

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.

The minerals in Portland cement clinker may be observed and quantified by petrographic microscopy.

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 analysis technique.