Английская Википедия:Ettringite
Шаблон:Short description Шаблон:Infobox mineral Ettringite is a hydrous calcium aluminium sulfate mineral with formula: Шаблон:Chem2. It is a colorless to yellow mineral crystallizing in the trigonal system. The prismatic crystals are typically colorless, turning white on partial dehydration.[1][2] It is part of the ettringite-group which includes other sulfates such as thaumasite and bentorite.[3]
Discovery and occurrence
Ettringite was first described in 1874 by Шаблон:Nobreak,[4] for an occurrence near the Ettringer Bellerberg Volcano, Ettringen, Rheinland-Pfalz, Germany.[1][2] 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.[1] It occurs associated with portlandite, afwillite and hydrocalumite at Scawt Hill, Ireland and with afwillite, hydrocalumite, mayenite and gypsum in the Hatrurim Formation.[1] 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[5] and in the Lucky Cuss mine, Tombstone, Arizona.[1][2]
Ettringite is also sometimes referred in the ancient French literature as Candelot salt, or Candlot salt.[6]
Occurrence in cement
In concrete chemistry, ettringite is a hexacalcium aluminate trisulfate hydrate, of general formula when noted as oxides:
or
Ettringite is formed in the hydrated Portland cement system as a result of the reaction of tricalcium aluminate (Шаблон:Chem) with calcium sulfate, both present in Portland cement.[7]
The addition of gypsum (Шаблон:Chem2) to clinker during the grinding operation to obtain the crushed powder of Portland cement is essential to avoid the flash setting of concrete during its early hydration. Indeed, the tricalcium aluminate (Шаблон:Chem) is the most reactive phase of the four main mineral phases present in Portland cement ([[alite|Шаблон:Chem2]], [[belite|Шаблон:Chem2]], Шаблон:Chem2, and Шаблон:Chem2). Шаблон:Chem2 hydration is very exothermic and also occurs very fast in the fresh concrete mix as the temperature quickly increases with the progress of the hydration reaction. The effect of gypsum addition is to promote the formation of a thin impervious film of ettringite at the surface of the Шаблон:Chem grains, passivating their surface, and so slowing down their hydration.[8] The addition of gypsum to Portland cement is needed to control the concrete setting.[8]
Ettringite, the most prominent representative of AFt phases or (Шаблон:Chem2), can also be synthesized in aqueous solution by reacting stoichiometric amounts of calcium oxide, aluminium oxide, and sulfate.
In the cement system, the presence of ettringite depends on the ratio of calcium sulfate to tri-calcium aluminate (Шаблон:Chem2); when this ratio is low, ettringite forms during early hydration and then converts to the calcium aluminate monosulfate (AFm phases or (Шаблон:Chem2)). 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 standard abbreviations are used to designate the different oxide phases in the cement chemist notation (CCN):[9]
Шаблон:Col-begin
Шаблон:Col-break
- C = CaO
- S = Шаблон:Chem2
- A = Шаблон:Chem2
- F = Шаблон:Chem2
- Шаблон:Overline = Шаблон:Chem2
- H = Шаблон:Chem2
- K = Шаблон:Chem2
- N = Шаблон:Chem2
- m = mono
- t = tri
AFt and AFm phases
- AFt: abbreviation for "alumina, ferric oxide, tri-substituted" or (Шаблон:Chem2). It represents a group of calcium aluminate hydrates. AFt has the general formula Шаблон:Chem where X represents a doubly charged anion or, sometimes, two singly charged anions. Ettringite is the most common and prominent member of the AFt group (X in this case denoting sulfate), and often simply called Alumina Ferrite tri-sulfate (AFt).
- AFm: abbreviation for "alumina, ferric oxide, mono-substituted" or (Шаблон:Chem2). It represents another group of calcium aluminate hydrates with general formula Шаблон:Chem 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 (Шаблон:Chem2), sulfate (Шаблон:Chem2), and carbonate (Шаблон:Chem2).
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 Шаблон: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.[10] The first X-ray diffraction crystallographic study was done by Bannister, Hey and 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 Шаблон:Chem/mmc and Z = 2, where Z is a number of formula units per unit cell. From observations on dehydration and chemical formulas there were suggestions of the structure being composed of Шаблон:Chem and Шаблон:Chem, were between them lie Шаблон:Chem ions and Шаблон:H2O molecules. Further X-ray studies ensued; namely Wellin (1956) which determined the crystal structure of thaumasite, and Besjak and Jelenic (1966) which gave confirmation of the structure nature of ettringite.[10]
An ettringite sample extracted from Scawt Hill was analysed by C. E. Tilley, the crystal was Шаблон:Nowrap, with specific gravity of Шаблон:Val, possessed five prism faces of the form m{10Шаблон:Overline0} and a small face a{11Шаблон:Overline0}, with no pyramidal or basal faces. Upon X-ray diffraction a Laue diagram 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.[11] Further studies conducted on synthetic ettringite by use of X-ray and powder diffraction confirmed earlier assumptions and analyses.[12]
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 Шаблон:Chem2, 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 containing a hexacoordinated silicon complex of Шаблон:Chem (a rare octahedral configuration for Si) consists of a cylindrical column of Шаблон:Chem2 in the c axis, with sulfate and carbonate anions in channels between these columns which contain water molecules as well.[13]
Further research
Ongoing research on ettringite and cement phase minerals is performed to find new ways to immobilize toxic anions (e.g., borate, selenate and arsenate) and heavy metals to avoid their dispersion in soils and the environment; this can be achieved by using the proper cement phases whose crystal lattice can accommodate these elements. For example, copper immobilization at high pH can be achieved through the formation of C-S-H/C-A-H and ettringite.[14] The crystal structure of ettringite Ca6Al2(SO4)3(OH)12·26H2O can incorporate a variety of divalent ions: Cu2+, Pb2+, Cd2+ and Zn2+, which can substitute for Ca2+.[14]
See also
References
- ↑ 1,0 1,1 1,2 1,3 1,4 Ошибка цитирования Неверный тег
<ref>; для сносокHBMне указан текст - ↑ 2,0 2,1 2,2 Ошибка цитирования Неверный тег
<ref>; для сносокMindatне указан текст - ↑ Ettringite-group. Mindat.org
- ↑ Lehmann, J. (1874). Über den Ettringit, ein neues Mineral in Kalkeinschlüssen der Lava von Ettringen (Laacher Gebiet). N. Jb. Mineral. Geol. Paläont., 273–275.
- ↑ Carpenter, A.B. (1963). Oriented overgrowths of thaumasite on ettringite. Am. Mineral. 48
- ↑ Шаблон:Cite journal
- ↑ Шаблон:Cite journal
- ↑ 8,0 8,1 Шаблон:Cite journal
- ↑ Шаблон:Cite book
- ↑ 10,0 10,1 Шаблон:Cite journal
- ↑ Шаблон:Cite journal
- ↑ Goetz-Neunhoeffer, F. and Neubauer, J. (2006). Refined ettringite (Ca6Al2(SO4)3(OH)12·26H2O) structure for quantitative X-ray diffraction analysis. Powder Diffraction 21, 4–11.
- ↑ Rachel L. Norman, Sandra E. Dann, Simon C. Hogg, Caroline A. Kirk. (2013). Synthesis and structural characterisation of new ettringite and thaumasite type phases: Ca6[Ga(OH)6·12H2O]2(SO4)3·2H2O and Ca6[M(OH)6·12H2O]2(SO4)2(CO3)2, M = Mn, Sn. Solid State Sciences 25.
- ↑ 14,0 14,1 Moon D.H., Park J.W., Cheong K.H., Hyun S., Koutsospyros A., Park J.H., Ok Y.S. (2013). Stabilization of lead and copper contaminated firing range soil using calcined oyster shells and fly ash, Environ Geochem Health 35.
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