View source for Ettringite

{{Short description|Hydrous calcium sulfo-aluminate}}
{{Infobox mineral
| name        = Ettringite
| boxwidth    =
| boxbgcolor  = #bfbf50
| image       = Ettringite-Oyelite-Ettringite-Kalahari-Namibia-136mm 0762closeup.jpg
| imagesize   = 260
| alt         =
| caption     = Ettringite, Kalahari manganese fields, [[Northern Cape Province]], South Africa
| category    = [[Sulfate minerals]]
| formula     = {{chem2|Ca6Al2(SO4)3(OH)12*26H2O}}
| IMAsymbol   = Ett<ref>{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3|pages=291–320|doi=10.1180/mgm.2021.43|bibcode=2021MinM...85..291W|s2cid=235729616|doi-access=free}}</ref>
| molweight   =
| strunz      = 7.DG.15
| dana        =
| system      = [[Trigonal]]
| class       = Ditrigonal pyramidal (3m) <br/>[[H-M symbol]]: (3m)
| symmetry    = ''P''31c
| unit cell   = a = 11.23, c = 21.44&nbsp;[Å]; Z&nbsp;=&nbsp;2
| color       = Colorless, pale yellow, milky white
| colour      =
| habit       = Acicular growth, striated prismatic crystals; fibrous to cottonlike
| twinning    =
| cleavage    = Perfect on {10{{overline|1}}0}
| fracture    =
| tenacity    =
| mohs        = 2–2.5
| luster      = Vitreous
| streak      = White
| diaphaneity = Transparent to opaque
| gravity     = 1.77
| density     = 
| polish      =
| opticalprop = Uniaxial (−)
| refractive  = n<sub>ω</sub> = 1.491, n<sub>ε</sub> = 1.470
| birefringence = δ = 0.021
| pleochroism =
| 2V          =
| dispersion  =
| extinction  =
| length fast/slow =
| fluorescence = Non-fluorescent
| absorption  =
| melt        =
| fusibility  =
| diagnostic  =
| solubility  = Partially soluble in water
| impurities  =
| alteration  = Partially dehydration on atmospheric exposure, becomes opaque
| other       =
| prop1       =
| prop1text   =
| references  = <ref name=Webmin>[http://webmineral.com/data/Ettringite.shtml Ettringite data, Webmineral]</ref><ref name=HBM>[http://rruff.geo.arizona.edu/doclib/hom/ettringite.pdf Ettringite in Handbook of Mineralogy]</ref><ref name=Mindat>[https://www.mindat.org/min-1414.html Ettringite]. Mindat.org</ref>
}}
'''Ettringite''' is a hydrous [[calcium]] [[aluminium]] [[sulfate mineral]] with formula: {{chem2|Ca6Al2(SO4)3(OH)12*26H2O}}. It is a colorless to yellow mineral crystallizing in the [[trigonal]] [[crystal system|system]]. The prismatic crystals are typically colorless, turning white on partial dehydration.<ref name=HBM/><ref name=Mindat/> It is part of the ettringite-group which includes other sulfates such as [[thaumasite]] and [[bentorite]].<ref name=Ettringite-group>[https://www.mindat.org/min-29289.html Ettringite-group]. Mindat.org</ref>

==Discovery and occurrence==
[[File:Ettringite.jpg|thumb|left|170px|Ettringite, 6.5 × 3.2 cm. N'Chwaning Mines, Kalahari manganese fields, [[Northern Cape Province]], South Africa]]
Ettringite was first described in 1874 by {{nobreak|J. Lehmann}},<ref name="Lehmann_1874">Lehmann, J. (1874). Über den Ettringit, ein neues Mineral in Kalkeinschlüssen der Lava von Ettringen (Laacher Gebiet). N. Jb. Mineral. Geol. Paläont., 273–275.</ref> for an occurrence near the Ettringer [[Bellerberg Volcano]], [[Ettringen, Mayen-Koblenz|Ettringen]], [[Rheinland-Pfalz]], [[Germany]].<ref name=HBM/><ref name=Mindat/> It occurs within [[contact metamorphism|metamorphically]] altered [[limestone]] adjacent to [[igneous]] intrusive rocks or within [[xenolith]]s. It also occurs as [[weathering]] crusts on [[larnite]] in the [[Hatrurim Formation]] of [[Israel]].<ref name=HBM/> It occurs associated with [[portlandite]], [[afwillite]] and [[hydrocalumite]] at [[Scawt Hill]], [[Ireland]] and with afwillite, hydrocalumite, [[mayenite]] and [[gypsum]] in the Hatrurim Formation.<ref name=HBM/> It has also been reported from the Zeilberg quarry, [[Maroldsweisach]], [[Bavaria]]; at [[Ceyrat|Boisséjour]], near Clermont-Ferrand, [[Puy-de-Dôme]], [[Auvergne (region)|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]]<ref name="Carpenter_1963">Carpenter, A.B. (1963). Oriented overgrowths of thaumasite on ettringite. Am. Mineral. 48</ref> and in the Lucky Cuss mine, [[Tombstone, Arizona]].<ref name=HBM/><ref name=Mindat/>

Ettringite is also sometimes referred in the ancient French literature as Candelot salt, or Candlot salt.<ref name="Thiery_2021">{{Cite journal| last1 = Thiery| first1 = Vincent| last2 = Rica| first2 = Brunilda| date = 2021| title = Minerals explained 59: Ettringite| journal = Geology Today| volume = 37| issue = 2| pages = 70–76| doi = 10.1111/gto.12346| s2cid = 233817487| issn = 1365-2451| url = https://onlinelibrary.wiley.com/doi/abs/10.1111/gto.12346| accessdate = 2023-04-06| url-access = subscription}}</ref>

==Occurrence in cement==
[[File:CaOH2SEM.jpg|thumb|left|170px|[[Scanning electron microscopy|SEM]] image of fractured hardened cement paste, showing thin hexagonal plates of [[portlandite]] ([[calcium hydroxide]]) and needles of ettringite (micron scale)]]
In concrete chemistry, ettringite is a hexacalcium aluminate trisulfate [[hydrate]], of general formula when noted as oxides:
:{{chem2|6CaO*Al2O3*3SO3*32H2O}}
or
:{{chem2|3CaO*Al2O3*3CaSO4*32H2O}}.

Ettringite is formed in the hydrated [[Portland cement]] system as a result of the reaction of [[tricalcium aluminate]] ({{chem|C|3|A}}) with [[calcium sulfate]], both present in Portland cement.<ref>
{{cite journal|last=Merlini|first=Marco|author2=Artioli, Gilberto|author3=Cerulli, Tiziano|author4=Cella, Fiorenza|author5=Bravo, Anna|year=2008|title=Tricalcium aluminate hydration in additivated systems. A crystallographic study by SR-XRPD|journal=Cement and Concrete Research|publisher=Elsevier|volume=38|issue=4|pages=477–486|doi=10.1016/j.cemconres.2007.11.011}}<!--| accessdate = 01-03-2008-->
</ref>

:{{chem2|C3A + 3 CaSO4 → ettringite}}

The addition of [[gypsum]] ({{chem2|CaSO4*2H2O}}) to [[Cement clinker|clinker]] during the [[Mill (grinding)|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 ({{chem|C|3|A}}) is the most reactive phase of the four main mineral phases present in Portland cement ([[alite|{{chem2|C3S}}]], [[belite|{{chem2|C2S}}]], {{chem2|C3A}}, and {{chem2|C4AF}}). {{chem2|C3A}} 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 {{chem|C|3|A}} grains, [[Passivation (chemistry)|passivating]] their surface, and so slowing down their hydration.<ref name="Divet2000">{{cite journal |last1=Divet |first1=Loïc |date=2000 |title=Etat des connaissances sur les causes possibles des réactions sulfatiques internes au béton |trans-title=State of knowledge on the possible causes of sulfate reactions internal to concrete |journal=Bulletin de Liaison des Laboratoires des Ponts et Chaussées |volume=227 |pages=71–84 |url=https://www.ifsttar.fr/collections/BLPCpdfs/blpc_227_71-84.pdf}}</ref> The addition of gypsum to Portland cement is needed to control the concrete setting.<ref name="Divet2000" />

Ettringite, the most prominent representative of [[AFt phases]] or ({{chem2|Al2O3 \s Fe2O3 \s tri}}), 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 ({{chem2|C3A}}); when this ratio is low, ettringite forms during early hydration and then converts to the calcium aluminate monosulfate ([[AFm phases]] or ({{chem2|Al2O3 \s Fe2O3 \s 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.
[[File:Ettringite-99753.jpg|thumb|260px|Fibroradiated ettringite needles on [[slag]] (Concordia [[smelter]], [[Eschweiler]], [[Aachen]], Germany)]]
The following standard abbreviations are used to designate the different oxide phases in the [[cement chemist notation]] (CCN):<ref>{{cite book
  | last1 = Bazant
  | first1 = Z.P.
  | last2 = Wittmann
  | first2 =  F.H.
  | title = Creep and shrinkage in concrete structures
  | publisher = John Wiley and Sons
  | year = 1982
  | isbn = 0-471-10409-4}}
</ref><br>
{{col-begin|width=30%}}
{{col-break|width=50%}}
:C = CaO
:S = {{chem2|SiO2}}
:A = {{chem2|Al2O3}}
:F = {{chem2|Fe2O3}}
:{{overline|S}} = {{chem2|SO3}}
{{col-break|width=50%}}
:H = {{chem2|H2O}}
:K = {{chem2|K2O}}
:N = {{chem2|Na2O}}
:m = mono
:t = tri
{{col-end}}

=== AFt and AFm phases ===

* [[AFt phases|AFt]]: abbreviation for "alumina, [[ferric oxide]], tri-substituted" or ({{chem2|Al2O3 \s Fe2O3 \s tri}}). It represents a group of calcium aluminate hydrates. AFt has the general formula {{chem|[Ca|3|(Al|,Fe)|(|O|H|)|6|•12H|2|O|]|2|•X|3|•nH|2|O|}} 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 phases|AFm]]: abbreviation for "alumina, ferric oxide, mono-substituted" or ({{chem2|Al2O3 \s Fe2O3 \s mono}}). It represents another group of calcium aluminate hydrates with general formula {{chem|[Ca|2|(Al|,Fe|)|(|O|H)|6|]|2|•X•nH|2|O}} 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]] ({{chem2|OH-}}), [[sulfate]] ({{chem2|SO4(2-)}}), and [[carbonate]] ({{chem2|CO3(2-)}}).

==Structure==
[[File:EttringitREM.JPG|thumb|260px|[[Scanning electron microscope]] (SEM) photograph of intertwined ettringite needles]]
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.<ref name="Moore 1970">{{cite journal |last1=Moore |first1=A.E. |last2=Taylor |first2=H.F.W. |year=1970 |title=Crystal structure of ettringite |journal=Acta Crystallographica Section B |volume=26 |issue=4 |pages=386–393 |doi=10.1107/S0567740870002443|s2cid=4188234 }}</ref> The first X-ray diffraction [[Crystallography|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 {{chem|P6|3}}/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 {{chem|Ca|2+}} and {{chem|Al(OH)|6|3−}}, were between them lie {{chem|SO|4|2−}} ions and {{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.<ref name="Moore 1970"/>

An ettringite sample extracted from [[Scawt Hill]] was analysed by [[Cecil Edgar Tilley|C. E. Tilley]], the crystal was {{nowrap|1.1 × 0.8 × 0.5&nbsp;mm}}, with [[Relative density|specific gravity]] of {{val|1.772|0.002}}, possessed five prism faces of the form ''m''{10{{overline|1}}0} and a small face ''a''{11{{overline|2}}0}, 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 crystal family|hexagonal]] and not a [[Rhombohedral crystal system|rhombohedral]] lattice.<ref name= Bannister >{{cite journal |last=Bannister |first=F.A. |year=1935 |title=Ettringite from Scawt Hill, Co. Antrim. |journal=Mineralogical Magazine |volume=24 |issue=153 |pages=324–329 |url=https://rruff.info/doclib/MinMag/Volume_24/24-153-324.pdf |doi=10.1180/minmag.1936.024.153.05}}</ref> Further studies conducted on synthetic ettringite by use of [[X-ray crystallography|X-ray]] and [[powder diffraction]] confirmed earlier assumptions and analyses.<ref>Goetz-Neunhoeffer, F. and Neubauer, J. (2006). Refined ettringite (Ca<sub>6</sub>Al<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>(OH)<sub>12</sub>·26H<sub>2</sub>O) structure for quantitative X-ray diffraction analysis. ''Powder Diffraction'' '''21''', 4–11.</ref>

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 P6<sub>3</sub> 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 {{chem2|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 containing a hexacoordinated silicon complex of {{chem|Si(OH)|6|2–}} (a rare octahedral configuration for Si) consists of a cylindrical column of {{chem2|Ca3[Si(OH)6*12H2O](4+)}} in the ''c'' axis, with sulfate and carbonate anions in channels between these columns which contain water molecules as well.<ref>Rachel L. Norman, Sandra E. Dann, Simon C. Hogg, Caroline A. Kirk. (2013). Synthesis and structural characterisation of new ettringite and thaumasite type phases: Ca<sub>6</sub>[Ga(OH)<sub>6</sub>·12H<sub>2</sub>O]<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>·2H<sub>2</sub>O and Ca<sub>6</sub>[M(OH)<sub>6</sub>·12H<sub>2</sub>O]<sub>2</sub>(SO<sub>4</sub>)<sub>2</sub>(CO<sub>3</sub>)<sub>2</sub>, M = Mn, Sn. Solid State Sciences 25.</ref>

==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 [[Calcium silicate hydrate|C-S-H]]/C-A-H and ettringite.<ref name="Moon_2013">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.</ref> The crystal structure of ettringite Ca<sub>6</sub>Al<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>(OH)<sub>12</sub>·26H<sub>2</sub>O can incorporate a variety of divalent ions: Cu<sup>2+</sup>, Pb<sup>2+</sup>, Cd<sup>2+</sup> and Zn<sup>2+</sup>, which can substitute for Ca<sup>2+</sup>.<ref name="Moon_2013"/>

==See also==
{{Commons category|Ettringite}}
* [[Cement]]
* [[Cement chemists notation]]
* [[Concrete]]

== References ==
{{Reflist}}

[[Category:Aluminium minerals]]
[[Category:Calcium minerals]]
[[Category:Cement]]
[[Category:Concrete]]
[[Category:Hydrate minerals|26]]
[[Category:Sulfate minerals]]
[[Category:Geology of Riverside County, California]]
[[Category:Crestmore Heights, California]]
[[Category:Trigonal minerals]]
[[Category:Minerals in space group 159]]
[[Category:Minerals described in 1874]]