Ettringite

Ettringite (also Woodfordite ) is a rarely occurring mineral from the mineral class of hydrous sulfates with foreign anions. It crystallizes in the monoclinic crystal system with the chemical composition

And usually develops well-defined, prismatic or needle-like, pseudo-hexagonal crystals, which can range from yellowish to greenish, playing color, but also very white. A synthetic precipitated product, it is also known as satin white and " Casul ". After more common in construction chemistry notation is the oxide Molecular formula:

Special Features

Ettringite, with a share of about 46 percent by weight of water is one of the minerals with the highest crystal water content and therefore relatively voluminous and light. A part of crystal water from escaping from a temperature of 80 ° C. At a temperature of 250 ° C a substantial dehydration ( Kalcination ) is reached, starting from 500 ° C complete. Ettringite decomposes at the Kalcination in calcium aluminate, calcium oxide and calcium sulfate anhydrite. In conjunction with water formed from the Dehydraten again ettringite.

Ettringite is one with a Mohs hardness of 2 to 2.5 to the soft minerals, it is still can be scratched with a fingernail. The crystals show vitreous luster, fibrous aggregates, however, rather silky shine. The molar mass of ettringite is 1255.11.

Ettringite is sensitive to acids and decomposes already at a pH value of less than 9th Basically, ettringite can form only in strongly alkaline medium ( pH approximately 12).

Etymology and history

Ettringite was named after its first location Ettringen in the Eifel. Described it in 1874 by J. Lehmann in his book " About the ettringite, a new mineral in Kalkeinschlüssen the lava of Ettringen ( Laacher area ) ".

In 1890 identified and Michaelis Candlot the mineral for the first time as tricalcium aluminate - trisulfathydrat.

Classification

In the old ( 8th edition ) and new classification of minerals according to Strunz ( 9th edition ) of ettringite is part of the Department of the " water -containing sulfates with foreign anions ." The new Strunz'sche Mineral classification divided here, however, precisely according to the size of the cations involved, and therefore the mineral is now in accordance with the subdivision "With large to medium-sized cations; with NO3, CO3, B (OH ) 4, SiO4 or IO3 "where he forms the group 7.DG.15 with Bentorit, Birunit, Buryatit, Carrarait, Charlesit, Jouravskit, Korkinoit, Sturmanit and thaumasite.

The commonly used in English-speaking classification of minerals according to Dana assigns the ettringite also in the class of sulfates, there, however, in the department of " Hydrated sulfates with hydroxyl or halogen" and the sub-division " Various hydrated sulfates with hydroxyl or halogen", where he Bentorit and Buryatit forms the unnamed group 31.10.2.

Education and Locations

Ettringite formed either by precipitation of a hydrothermal solution, or by precipitation from aluminum sulfate solution with hydrated lime or by precipitation / crystallization of calcium aluminate and gypsum, as well as through metamorphosis along with the very similar Sturmanit. Known - and important for the curing of cements - is the formation of ettringite during hydration of cement from the constituents of tricalcium aluminate, gypsum and water. Accompanying minerals are next Afwillit, gypsum, hydrocalumite, mayenite and portlandite, you still calcite, brucite, hematite, hausmannite, manganite and Oyelith

In addition to the volcanic deposits in the Eifel ettringite was detected at about 60 localities worldwide so far, so among other things, in Limburg and Liège in Belgium; Gera, Maroldsweisach, Oberwolfach and Richelsdorfer Mountains in Germany; Clermont- Ferrand in France, Kloch and Brixlegg in Austria, Kuruman in South Africa, as well as Franklin / New Jersey and Crestmore / Riverside County in the United States.

In the concrete structure it usually comes first to the formation of mono-sulphate, which is then recrystallized to ettringite. This process is associated with a three-fold increase in volume and is referred to as driving Ettringittreiben or sulfate. Triggered this is often due to penetration of sulfate-containing water in concrete structures. Ettringite can also be synthetically produced as calcium aluminate. Impure ettringite is formed from sulfate-containing wastewaters by addition of aluminate and lime .. why sulfates can be removed from wastewater by addition of cement slurries ( " ettringite precipitation ").

Crystal structure

Ettringite crystallized trigonal space group P31c with the lattice parameters a = 11.26 Å and c = 21.48 Å and two formula units per unit cell.

Use

Ettringite solid ( granules / powder) is used in the construction industry as an additive to cement source or fireproofing plasters.

Ettringite slurry (Satin White / Casul ) is preferably used as a white pigment for coating paper ( coated paper ) were used. The Painted with satin white / Casul papers are characterized mainly by high whiteness, opacity and gloss. They are used as art paper with excellent print quality and food packaging. Another application is the use in emulsion paints and liquid plasters.