Ettringite

Ettringite is found in hydrated Portland cement system as a result of the reaction of calcium aluminate with calcium sulfate, both present in Portland cement. 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. In the cement system, the presence of ettringite depends on the ratio of calcium sulfate to aluminate; 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.

The chemical formula used by cement scientists is C₆AS₃H₃₂;

The characters designate standard notations:
C = CaO,
S = SiO₂,
A = Al₂O₃,
F = Fe₂O₃,
S = 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 as general formula: [Ca₂(Al,Fe)(OH)₆)] · X · n H₂O 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.

References