| Ettringite | |
|---|---|
Ettringite, 3.3×2.6×2.5 cm. Kalahari manganese fields, Northern Cape Province, South Africa |
|
| General | |
| Category | Sulfate minerals |
| Chemical formula | Ca6Al2(SO4)3(OH)12·26H2O |
| Strunz classification | 07.DG.15 |
| Crystal symmetry | Trigonal 3m ditrigonal pyramidal |
| Unit cell | a = 11.23 Å, c = 21.44 Å; Z = 2 |
| Identification | |
| Color | Colorless, pale yellow, milky white |
| Crystal habit | Striated prismatic crystals; fibrous to cottonlike |
| Crystal system | Trigonal |
| Cleavage | Perfect on {1010} |
| Mohs scale hardness | 2-2.5 |
| Luster | Vitreous |
| Diaphaneity | Transparent to opaque |
| Specific gravity | 1.77 |
| Optical properties | Uniaxial (-) |
| Refractive index | nω = 1.491, nε = 1.470 |
| Birefringence | δ = 0.021 |
| Solubility | Partially soluble in water |
| Alters to | Partially dehydration on atmospheric exposure, becomes opaque |
| References | [1][2][3] |
Ettringite is a hydrous calcium aluminium sulfate mineral with formula: Ca6Al2(SO4)3(OH)12·26H2O. It is a colorless to yellow mineral crystallizing in the trigonal system. The prismatic crystals are typically colorless, turning white on partial dehydration.[3][2]
Contents |
Ettringite was first described in 1874 for an occurrence near the Ettringer Bellerberg volcano, Ettringen, Rheinland-Pfalz, Germany.[2][3] It occurs within metamorphically altered limestone adjacent to igneous intrusive rocks or within xenoliths. It also occurs as weathering crusts on larnite in the Hatrurim Formation of Israel.[2] It occurs associated with portlandite, afwillite and hydrocalumite at Scawt Hill, Ireland and with afwillite, hydrocalumite, mayenite and gypsum in the Hatrurim Formation.[2] It has also been reported from the Zeilberg quarry, Maroldsweisach, Bavaria; at Boisséjour, near Clermont-Ferrand, Puy-de-Dôme, Auvergne, France; the N’Chwaning mine, Kuruman district, Cape Province, South Africa; in the USA, at the Crestmore quarry, Riverside County, California and in the Lucky Cuss mine, Tombstone, Cochise County, Arizona.[2][3]
In concrete chemistry ettringite is a hexacalcium aluminate trisulfate hydrate, of general formula:
or
Ettringite is formed in hydrated Portland cement system as a result of the reaction of calcium aluminate with calcium sulfate, both present in Portland cement. [4] 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 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.
The characters designate standard notations:[5]