Calcium aluminates

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12CaO • 7Al 2 O 3 (C12A7).

Calcium aluminates are compounds of calcium oxide (or calcium hydroxide ) and aluminum oxide (or aluminum hydroxide ) of different compositions. They play a major role in construction chemistry as reactive compounds and, as anhydrous oxides, are part of cements . Basically there are burnt (sintered) calcium aluminates and chemically precipitated calcium aluminate hydrates (hydroxides).

Manufacturing

Calcined calcium aluminates are produced by burning limestone and aluminum oxide or bauxite in rotary furnaces or in electric arc furnaces at temperatures of over 1500 ° C. "Alumina cements" with different proportions of CaO: Al 2 O 3 are formed :

  • CaO * Al 2 O 3        [CA]
  • 3CaO * Al 2 O 3      [C 3 A]
  • CaO * 2Al 2 O 3      [CA 2 ]
  • 12CaO * 7Al 2 O 3   [C 12 A 7 ]

(Alternatively in the spelling of construction chemistry )

Chemically precipitated calcium aluminates are obtained from sodium aluminate solution by adding calcium oxide or calcium hydroxide as a precipitate. Here, too, different compounds can arise depending on the CaO: Al 2 O 3 ratio.

  • Ca 3 Al 2 (OH) 12 = tricalcium aluminate hydrate or [3CaO * Al 2 O 3 * 6 H 2 O]
  • Ca 4 Al 2 (OH) 14 = tetracalcium aluminate hydrate or [4CaO * Al 2 O 3 * 7 H 2 O]

use

The fired calcium aluminates are used as high-quality, quick-setting calcium aluminate cements with high strength, combined with Portland cement as a quick-setting cement . The anhydrous calcium aluminates hydrate ( hydration ) when cement sets (exothermic process, similar to the quenching of CaO) and form various hydrates.

Tricalcium aluminate hydrate also occurs as a mineral “katoite” and, as a precipitated precipitate, serves as the starting substance for the production of calcium aluminate sulfate (synthetic ettringite ). As a reaction product and phase component, it plays an important role in cement chemistry as an accelerator for setting and solidifying concrete . The calcium aluminate hydrates react with sulphate carriers such as gypsum or anhydrite and gradually form the ettringite, which is important for the solidification of cement.

literature

Individual evidence

  1. ↑ H. Hosono, K. Tanabe, E. Takayama-Muromachi, H. Kageyama, S. Yamanaka, H. Kumakura, M. Nohara, H. Hiramatsu, S. Fujitsu: Exploration of new superconductors and functional materials, and fabrication of superconducting tapes and wires of iron pnictides . In: Science and Technology of Advanced Materials . 16, No. 3, 2015, p. 033503. doi : 10.1088 / 1468-6996 / 16/3/033503 .
  2. ↑ Jochen Stark: Durability of concrete. Springer-Verlag, 2013, ISBN 978-3-642-35278-2 , p. 263 ( limited preview in Google book search).