Ettringite

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.^[2][3] It is part of the ettringite-group which includes other sulfates such as thaumasite and bentorite.^[4]

Discovery and occurrence

Ettringite, 6.5×3.2 cm. N'Chwaning Mines, Kalahari manganese fields, Northern Cape Province, South Africa

Ettringite was first described in 1874 by J.Lehmann,^[5] 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 US, occurrences were found in spurrite-merwinite-gehlenite skarn at the 910 level of the Commercial quarry, Crestmore, Riverside County, California^[6] and in the Lucky Cuss mine, Tombstone, Cochise County, Arizona.^[2][3]

Occurrence in cement

SEM image of fractured hardened cement paste, showing plates of calcium hydroxide and needles of ettringite (micron scale)

In concrete chemistry ettringite is a hexacalcium aluminate trisulfate hydrate, of general formula:

(CaO)₆(Al₂O₃)(SO₃)₃·32H₂O

or

(CaO)₃(Al₂O₃)(CaSO₄)₃·32H₂O.

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.^[7] 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.

The following characters designate standard notations:^[8]

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·nH₂O 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·nH₂O 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.

Structure

The mineral ettringite has a structure that runs parallel to the c axis -the needle axis-; in the middle of these two lie the sulfate ions and H₂O molecules, the space group is P31c. Ettringite crystal system is trigonal, crystals are elongated and in a needle like shape, occurrence of disorder or twining is common, which affects the intercolumn material.^[9] The first X-ray study was done by Bannister, Hey & Bernal (1936), which found that the crystal unit cell is of a hexagonal form with a=11.26 and c=21.48 with space group P63/mmcand Z=2. From observations on dehydration and chemical formulas there were suggestions of the structure being composed of Ca²⁺ and Al(OH)₆³⁻, were between them lie SO₄²⁻ ions and H₂O molecules. Further X-ray studies ensued; namely Wellin (1956) which determined the crystal structure of thaumasite, and Besjak & Jelenic (1966) which gave affirmation of the structure nature of ettringite.^[9]

An ettringite sample extracted from Scawt Hill was analysed by C. E. Tilley, the crystal was 1.1 x 0.8 0.5 mm, with specific gravity of 1.772±0.002, the density was determined by immersing the sample in a solution of carbon tetrachloride mixed with bromoform. The crystal showed five prism faces of the form m {1010} and a small face a{1120}, with no pyramidal or basal faces, upon x-ray diffraction a Lauer gram along the c-axis revealed a hexagonal axis with vertical planes of symmetry, this study showed that the structure has a hexagonal and not a rhombohedral lattice.^[10] further studies conducted on synthetic ettringite by use of x-ray and powder diffraction confirmed earlier assumptions and analyses.^[11]

Upon analyzing the structure of both ettringite and thaumasite, it was deduced that both minerals have hexagonal structures, but different space groups. Ettringite crystals have a P31c with a=11.224 Å, c= 21,108 Å, while thaumasite crystals fall into space group P6₃ with a= 11.04 Å, c= 10.39 Å. While these two minerals form a solid solution, the difference in space groups lead to discontinuities in unit cell parameters. Differences between structures of ettringite and thaumasite arise from the columns of cations and anions: Ettringite cation columns are composed of Ca₃[Al(OH)₆·12H₂O]³⁺, which run parallel to the c axis, and the other columns of sulfate anions and water molecules in channels parallel to these columns. In contrast, thaumasite consists of a cylindrical column of Ca₃[Si(OH)₆·12H₂O]³⁺ in the c axis, with sulfate and carbonate anions in channels between these columns which contain water molecules as well.^[12]

Further research

Ongoing research on ettringite and cement phase minerals is in general to find ways to immobilize wastes and heavy metals from soils and the environment; this can be done by use of the proper cement phase forming mineral by use of lattice to extract according elements. For example, it is reported that copper immobilization at high pH can be achieved through formation of CSH/CAH and ettringite.^[13] It is suggested that the crystal morphology of ettringite Ca₆Al₂(SO₄)₃(OH)₁₂·26H₂O can incorporate a variety of divalent ions: Cu²⁺, Pb²⁺, Cd²⁺ and Zn²⁺, which can substitute for the Ca²⁺ by incorporating these ions into the lattice.^[13]

See also

• Cement
• Cement chemists notation
• Concrete

References

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