Calcium aluminates explained

Calcium aluminates are a range of materials^[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 (once known as mayenite^[2])
• Monocalcium aluminate, CaO·Al₂O₃ (CA) (occurring in nature as krotite and dmitryivanovite – two polymorphs^[3])
• Monocalcium dialuminate, CaO·2Al₂O₃ (occurring in nature as grossite ^[4])
• Monocalcium hexa-aluminate, CaO·6Al₂O₃ (occurring in nature as hibonite, a representative of magnetoplumbite group^[5])

In addition, other phases include:

• Dicalcium aluminate, 2CaO·Al₂O₃, which exists only at pressures above 2500 MPa.^[6] 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.^[7]
• Pentacalcium trialuminate, 5CaO·3Al₂O₃, 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, is a metastable phase formed by dehydrating 4CaO·3Al₂O₃·3H₂O .

Hydration reaction

In contrast to Portland cements, calcium aluminates do not release calcium hydroxide (portlandite or lime) during their hydration.

See also

• Calcium aluminate cements
• Cement
• Cement chemist notation (CCN)
  in which the following abbreviations for calcium and aluminium oxides are defined as:
  • C = CaO
  • A =
• Hydrocalumite
• Mayenite
• Ye'elimite,, a rare natural anhydrous calcium sulfoaluminate

Further reading

• Buttler . F. G. . Dent Glasser . L. S. . Taylor . H. F. W. . Studies on 4CaO·Al₂O₃·13H₂O and the related natural mineral hydrocalumite . Journal of the American Ceramic Society . March 1959 . 42 . 3 . 121–126 . 0002-7820 . 1551-2916 . 10.1111/j.1151-2916.1959.tb14078.x .

Notes and References

1. Taylor H.F.W (1990) Cement Chemistry, Academic Press,, pp. 34–38.
2. Web site: Mayenite Supergroup.
3. Web site: Krotite.
4. Web site: Grossite.
5. Web site: Hibonite.
6. Taylor H.F.W (1990) Cement Chemistry, Academic Press,, pp. 28, 29.
7. Web site: Brownmillerite.