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. It is part of the ettringite-group which includes other sulfates such as thaumasite and bentorite.Ettringite was first described in 1874 by J.Lehmann, for an occurrence near the Ettringer Bellerberg Volcano, Ettringen, Rheinland-Pfalz, Germany. 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. It occurs associated with portlandite, afwillite and hydrocalumite at Scawt Hill, Ireland and with afwillite, hydrocalumite, mayenite and gypsum in the Hatrurim Formation. 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 and in the Lucky Cuss mine, Tombstone, Arizona.In concrete chemistry ettringite is a hexacalcium aluminate trisulfate hydrate, of general formula: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. 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.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. 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.