Gypsum Cement Plaster of Paris

When gypsum, a sedimentary mineral (composed of hydrated calcium sulfate) is calcinated, most of its volatile constituent, water, evaporates and is therefore removed. The friable material remaining after the product of the calcination process cools down to ambient temperature is commonly known as plaster of Paris (composed of calcium sulfate hemihydrate) (see Textbox 33). 

Plaster of Paris has long been used as a casting material, a cement, and a mortar. If mixed with water, plaster of Paris forms a very soft and pliable mixture. After a very short time, lasting only 5-8 minutes, the wet, pliable mixture sets, that is, it hardens into a stable, firm solid. The setting process entails the incorporation of water molecules (a process known as hydration) into the calcium sulfate hemihydrate and the consequent formation and crystallization of hydrated sulfate of calcium, in other words, when water is added to plaster of Paris, the two combine, again forming gypsum, which soon crystallizes into a hard solid mass  

Hydraulic Cements. To build constructions submerged in a sea, lake, or river, it is necessary to use waterproof cements, generally known as hydraulic cements, which harden even in the presence of excessive amormfs of wafer. In the past, such cements were prepared by heating a nuxture of limestone and a considerable amormf of clay or other powdered siliceous material. At high temperature (above 650°C), the quicklime, formed when fhe limestone 

There are various reasons to study the composition of ancients cements. The actual composition of a cement, for example, provides information on its nature, the technology used for making it, and the provenance of its components (Middendorf et al. 2005). It may also elicit differences between the nature of an original cement used for building and that used for later repairs (Streicher 1991 Jedrzejewska 1990). Most analytical work concerning ancient cement in the recent past has been based mainly on the use of optical microscopy and classical analysis techniques. Sometimes, such studies are complemented with information derived by instrumental techniques (Blauer-Bohm and Jagers 1997). 

Archaeological Chemistry, Second Edition By Zvi Goffer Copyright 2007 John Wiley Sons, Inc. 

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