Calcium aluminates

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.

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).