Calcium aluminates

Calcium aluminates phase diagram.

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 (C3A)
• Dodecacalcium hepta-aluminate, 12CaO·7Al₂O₃ (C12A7) (mayenite)
• Monocalcium aluminate, CaO·Al₂O₃ (CA)
• Monocalcium dialuminate, CaO·2Al₂O₃ (CA2)
• Monocalcium hexa-aluminate, CaO·6Al₂O₃ (CA6)

In addition, other phases include:

• Dicalcium aluminate, 2CaO·Al₂O₃ (C2A), which exists only at pressures above 2500 MPa.^[2] The crystal is orthorhombic, with density 3480 kg·m⁻³.
• 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).

See also

• Cement
• Cement chemist notation (CCN)
  in which the following abbreviations for calcium and aluminium oxides are defined as:
  • C = CaO
  • A = Al₂O₃

References