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September-October 2012

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Connoisseur's Choice: Ettringite, N'Chwaning II Mine, Northern Cape Province, Republic of South Africa

Concrete, that manmade mix of cement with sand and/or stone, has been in use for a very long time. It was known to the Etruscans, and its basic formula was refined by the Romans. The Romans made great use of concrete. In fact concrete is part of many Roman buildings, notably the Pantheon. The critical ingredient in concrete is the cement; the chemical reactions involved in the mixing and hardening of the cement are of particular interest because of the minerals involved. Portland cement, so-named for its color resemblance to Portland Island (U.K.) granite, is a hydraulic type of cement, meaning that it hardens by reacting with water to form a water resistant end product. Portland cement is a mixture of calcium silicates and calcium carbonates with minor amounts of aluminum and iron oxides. It is produced by heating a limestone and clay mixture to 1450°–1550°C. The heating process makes a product called clinker to which gypsum is added to assist in the settling and hardening. The clinker is pulverized into a fine powder that is called Portland cement. The addition of water to Portland cement causes a series of reactions, many of which are not well understood as yet. To gain a better understanding of these reactions, as well as the ensuing hardening phase, the concrete industry is one of the biggest employers of mineralogists. These reactions produce various minerals that have natural counterparts; ettringite in fine needles is among the first of these minerals, followed by platelike crystals of portlandite. As the cement gradually sets, hardens, and cures, the ettringite is gradually transformed into other calcium minerals (Cotterill 2008; Wenk and Bulakh 2004).

Paul W. Pohwat, a comsultimg editor of Rocks & Minerals, is the collections manager (minerals) in the Division of Mineralogy at the National Museum of Natural History.

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