Calcium aluminates

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Calcium aluminates are a range of minerals^[1] obtained by heating calcium oxide and aluminium oxide together at high temperatures. They are encountered in the manufacture of refractories and cements.

The stable phases shown in the phase diagram (formed at atmospheric pressure under an atmosphere of normal humidity) are:

• Tricalcium aluminate, 3CaO.Al₂O₃
• Dodecacalcium hepta-aluminate, 12CaO.7Al₂O₃
• Monocalcium aluminate, CaO.Al₂O₃
• Monocalcium dialuminate, CaO.2Al₂O₃
• Monocalcium hexa-aluminate, CaO.6Al₂O₃

In addition, other phases include:

• Dicalcium aluminate (2CaO.Al₂O₃), which exists only at pressures above 2500 MPa^[2]. The crystal is orthorhombic, with density 3480kg.m^-3.
• Pentacalcium trialuminate 5CaO.3Al₂O₃, forms only under an ahydrous and oxygen-free atmosphere. The crystal is orthorhombic, with a density of 3067 kg.m^-3. It reacts rapidly with water.
• Tetracalcium trialuminate 4CaO.3Al₂O₃, is a metastable phase formed by dehydrating 4CaO.3Al₂O₃.3H₂O.

References

1. ^ H F W Taylor, Cement Chemistry, Academic Press, 1990, ISBN 0-12-683900-X, pp 34-38
2. ^ ibid, p 28, 29