Plaster setting processes

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 ... 

Pastes made from plaster of Paris exhibit fast setting and hardening, and the hydration process is typically completed within hours. Small amounts of chemical additives may be added to the calcined material to modify its properties. These may include set accelerators (such as soluble sulfates or prehydrated plaster), set retarders (such as citric acid or ker-atin), or modifiers of rheology (such as methyl cellulose or caibo methyl cellulose). 

Hydraulic cements. These cements are formed from two constituents one of which is water. Setting comprises a hydration and precipitation process. Into this category fall Portland cement and plaster of Paris. 

Monosodium and disodium phosphates are used for pH control in food manufacture and in textile processing. The trisodium and disodium salts are frequently incorporated in detergent compositions (Chapter 12.5). The trisodium salt can be used to retard the setting rate of calcium sulphate plasters (Table 5.15). 

Antioch Process. Plaster moulds are produced by pouring an aqueous plaster of Paris slip over a mould, steam treating, allowing to set in the air and oven drying. 

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