Ettringite

Ettringite is a hexacalcium aluminate trisulfate hydrate, of general formula:

(CaO)₆(Al₂O₃)(SO₃)₃ · 32 H₂O,

or;

(CaO)₃(Al₂O₃)(CaSO₄)₃ · 32 H₂O.

Ettringite, a naturally-occurring mineral, is formed in hydrated Portland cement system as a result of the reaction of calcium aluminate with calcium sulfate, both present in Portland cement. [1] Ettringite, the more prominent representative of AFt phases or (Al2O3-Fe2O3-tri), can also be synthesized in the laboratory by reacting stoichiometric amounts of calcium, aluminium and sulfate in water.

C₃A + 3 CaSO₄ —> ettringite

In the cement system, the presence of ettringite depends on the ratio of calcium sulfate to tri-calcium aluminate (C₃A); when this ratio is low, ettringite forms during early hydration and then converts to the calcium aluminate monosulfate (AFm phase or (Al₂O₃-Fe₂O₃-mono)). When the ratio is intermediate, only a portion of the ettringite converts to AFm and both can coexist, while ettringite is unlikely to convert to AFm at high ratios.

Ettringite is named after Ettringen, Germany, the place where it was discovered in 1874.

The chemical formula used by cement scientists is C₆AS̅₃H₃₂;

The characters designate standard notations: [2]
C = CaO,
S = SiO₂,
A = Al₂O₃,
F = Fe₂O₃,
S̅ = SO₃,
H = H₂O,
K = K₂O,
N = Na₂O,
m = mono,
t = tri.

AFt and AFm Phases

* AFt: abbreviation for "Alumina, Ferric oxide, tri-sulfate" or (Al₂O₃ – Fe₂O₃ – tri). It represents a group of calcium sulfoaluminate hydrates. AFt has the general formula [Ca₃(Al,Fe)(OH)₆ · 12 H₂O]₂ · X₃ · n H₂O where X represents a doubly-charged anion or, sometimes, two singly-charged anions. Ettringite is the most common and important member of the AFt group (X in this case denoting sulfate).

* AFm: abbreviation for "Alumina, Ferric oxide, mono-sulfate" or (Al₂O₃ – Fe₂O₃ – mono). It represents another group of calcium aluminate hydrates with general formula [Ca₂(Al,Fe)(OH)₆)] · X · n H₂OO where X represents a singly charged anion or 'half' a doubly-charged anion. X may be one of many anions. The most important anions involved in Portland cement hydration are hydroxyl, sulfate and carbonate.

Sea also

* Cement
* Cement chemists notation
* Concrete

References

1. ^ Merlini, Marco; Artioli, Gilberto; Cerulli, Tiziano; Cella, Fiorenza; and Bravo, Anna (2008). "Tricalcium aluminate hydration in additivated systems. A crystallographic study by SR-XRPD". Cement and Concrete Research (Elsevier) 38 (4): 477–486. doi:10.1016/j.cemconres.2007.11.011.
2. ^ Bazant, Z.P.; Wittmann F.H. (1982). Creep and shrinkage in concrete structures. John Wiley and Sons. ISBN 0 471 10409 4.

External links

* Understanding cement
* Ettringite data, Webmineral
* Ettringite in Handbook of Mineralogy

List of minerals