Used since ancient times, plaster of paris is so called because gypsum was early used near Paris to make plaster and cement. Plaster of paris is also used to precast and hold parts of ornamental plasterwork placed on ceilings and cornices and is used in medicine to make plaster casts to immobilize broken bones while they heal. Some modern sculptors work directly in plaster of paris. The speed at which the plaster sets gives the work a sense of immediacy and enables the sculptor to achieve the original idea quickly.
In modern days this material is also used for False Ceiling. In this the powder form is converted in a sheet form and the sheet is then attached to the basic ceiling with the help of fasteners. It is done in various designs containing various combinations of lights and colors.
Many of the greatest mural paintings in Europe, like Michelangelo's Sistine Chapel ceiling are executed in fresco, meaning they are painted on a thin layer of wet plaster, called intonaco; the pigments sink into this layer so that the plaster itself becomes the medium holding them, which accounts for the excellent durability of fresco. Additional work may be added a secco on top of the dry plaster, though this is generally less durable.
Plaster is widely used as a support for broken bones; a bandage impregnated with plaster is moistened and then wrapped around the damaged limb, setting into a close-fitting yet easily removed tube, known as an orthopedic cast.
Plasters have been in use in passive fire protection, as fireproofing products, for many decades.
It is used in glass making, tanning, bleaching powder making and purification of sugar.
CaSO4•2H2O + heat → CaSO4•0.5H2O + 1.5H2O (released as steam).
When the dry plaster powder is mixed with water, it re-forms into gypsum. The setting of unmodified plaster starts about 10 minutes after mixing and is complete in about 45 minutes; but not fully set for 72 hours If plaster or gypsum is heated above 392degreeF (200degree C), anhydrite is formed, which will also re-form as gypsum if mixed with water.
Lime plaster is a mixture of calcium hydroxide and sand (or other inert fillers). Carbon dioxide in the atmosphere causes the plaster to set by transforming the calcium hydroxide into calcium carbonate (limestone). Whitewash is based on the same chemistry.
To make lime plaster, limestone (calcium carbonate) is heated to produce quicklime (calcium oxide). Water is then added to produce slaked lime (calcium hydroxide), which is sold as a wet putty or a white powder.
Cement plaster is a mixture of suitable plaster, sand, portland cement and water which is normally applied to masonry interiors and exteriors to achieve a smooth surface. Interior surfaces sometimes receive a final layer of gypsum plaster. Walls constructed with stock bricks are normally plastered while face brick walls are not plastered. Various cement-based plasters are also used as proprietary spray fireproofing products. These usually use vermiculite as lightweight aggregate. Heavy versions of such plasters are also in use for exterior fireproofing, to protect LPG vessels, pipe bridges and vessel skirts.
You must be logged in to post a comment.
© Copyright 2013-15, All Rights Reserved by Rich Field Minerals