Calcium aluminates are a range of materials [2] obtained by heating calcium oxide and aluminium oxide together at high temperatures. They are encountered in the manufacture of refractories and cements.
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The stable phases shown in the phase diagram (formed at atmospheric pressure under an atmosphere of normal humidity) are:
- Tricalcium aluminate, 3CaO·Al2O3 (C3A)
- Dodecacalcium hepta-aluminate, 12CaO·7Al2O3 (C12A7) (once known as mayenite [3] )
- Monocalcium aluminate, CaO·Al2O3 (CA) (occurring in nature as krotite and dmitryivanovite – two polymorphs [4] )
- Monocalcium dialuminate, CaO·2Al2O3 (CA2) (occurring in nature as grossite [5] )
- Monocalcium hexa-aluminate, CaO·6Al2O3 (CA6) (occurring in nature as hibonite, a representative of magnetoplumbite group [6] )
In addition, other phases include:
- Dicalcium aluminate, 2CaO·Al2O3 (C2A), which exists only at pressures above 2500 MPa. [7] The crystal is orthorhombic, with density 3480 kg·m−3. The natural dicalcium aluminate, brownmillerite, may form at normal pressure but elevated temperature in pyrometamorphic zones, e.g., in burning coal-mining heaps. [8]
- Pentacalcium trialuminate, 5CaO·3Al2O3 (C5A3), forms only under an anhydrous 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 (C4A3), is a metastable phase formed by dehydrating 4CaO·3Al2O3·3H2O (C4A3H3).