Belite

Belite is the mineralogical name for Dicalcium Silicate, Ca₂SiO₄. It is a mineral present in Portland cement. It also—rarely—occurs naturally, as larnite.

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Composition and Structure

  The belite found in Portland cement differs in composition from simple dicalcium silicate. It is a solid solution and contains minor amounts of other oxides besides CaO and SiO₂. A typical composition^[1]:

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, 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 (SiO₄^4-) ions can be replaced with sulfate (SO₄^2-) ions, provided that for each sulfate ion, two aluminate (AlO₄^5-) 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. The other silicate, alite contributes "early" strength, due to its higher reactivity. Belite reacts with water (roughly) according to the reaction:

2Ca₂SiO₄ + 4H₂O → 3CaO.2SiO₂.3H₂O + Ca(OH)₂

The hydrate 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 rigourously 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 neighbouring alite. This is observed macroscopically as "dusting": the clinker nodules 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 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 analysis technique.

Notes

1. ^ H F W Taylor, Cement Chemistry, Academic Press, 1990, ISBN 0-12-683900-X, pp 10-11