Alite

Alite is the mineralogical name for Tricalcium Silicate, Ca₃.SiO₅. It is the major, and characteristic, mineral in Portland cement.

Composition and structure

The alite found in Portland cement differs in composition from simple tricalcium silicate. It is a solid solution and contains minor amounts of other oxides besides CaO and SiO₂. A typical composition [H F W Taylor, "Cement Chemistry", Academic Press, 1990, ISBN 0-12-683900-X, pp 10-11] is:

Based on this, the formula can be expressed as Ca_2.90Mg_0.06Na_0.01Fe_0.03Al_0.04Si_0.95P_0.01O₅. 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

Tricalcium silicate is thermodynamically unstable below 1250°C, but can be preserved in a metastable state at room temperature by fast cooling: on slow cooling it tends to revert to belite (Ca₂SiO₄) and CaO.

As the temperature changes, it passes through several polymorphic states:

The polymorphs differ structurally by minor deformations from the basic hexagonal structure.

Hydration

Alite is the mineral in Portland cement responsible for setting and development of "early" strength. The other silicate, belite contributes "late" strength, due to its lower reactivity. Alite is more reactive because of its higher Ca content, and the presence of an oxide ion in the lattice. It reacts with water (roughly) according to the reaction:

2Ca₃SiO₅ + 6H₂O → 3CaO.2SiO₂.3H₂O + 3Ca(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. High alite reactivity is desirable in Portland cement manufacture, and this is achieved by retaining, as far as possible, high temperature polymorphs, in crystals that are small, distorted and highly defective. Defects provide sites for initial water attack.

Detection

See the article on belite.

Notes

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