History ► Been used for several thousand years ► Greeks and Egyptians both used it.
Uses of Gypsum and Lime ► Interior finish – gypsum plaster is durable and versatile ► Plaster surfaces can be toweled smooth, stripped, or sand finished. ► Can be applied directly onto gypsum lath, metal lath, fiberboard plaster base, or over masonry surface
Where Gypsum and Lime are found ► Found in rock formations in various parts of the world US France England Italy China Russia South America
Production ► Gypsum is very seldom found in pure state Usually contains variety of clay, limestone, silica, and iron. In pure state it is white in color, with impurities it may be gray Usually found close to the surface of the earth.
Production ► The raw material is mined, and then sent to the mill, crushed to a size of 2 to 3 in. ► It is further reduced by a hammer mill to pieces of approximately ½ in. ► Next the material goes to a rotary heater where it is heated to 175 degrees to reduce the amount of water. – Which drives off 75 percent of the combined water – leaving a hemi hydrated plaster of paris. ► Or the following process takes place – ½ material goes to a Raymond mill, where it is reduced to a fine powder known as land plaster Land plaster proceeds to a claiming kettle where it is heated to about 165 c to drive off the combined water to produce plaster of paris.
Gypsum Plasters ► Plaster of Paris Plaster of paris is made from carefully selected white rock Mixed with water to form a plaster Sets in 15 to 20 minutes Used to do small patch jobs, making molds ► Keene’s Cement If gypsum is subjected to temperatures of 400 c it is completely dehydrated This material is ground and has a additive of alum added to it Mixed with water to form a plaster Highly resistant to moisture penetration and is used where sanitary conditions or excessive moisture make it necessary to specify a hard impervious surface.
Gypsum Plasters ► Casting Plaster Made from specially selected rock and ground much finer then regular plaster of paris Has a slower setting and cooler working Makes it ideal for ornamental molded plaster work ► Hard wall plaster Plaster containing hair or fiber used to form the first scratch coat and the second brown coat on plastered walls and ceilings It requires the addition of aggregate and water on the job Aggregate is usually sand or a lightweight aggregate.
Gypsum Plasters ► Cement bond plaster Intended for application to concrete surfaces Requires the addition only of water ► Finish plaster Mixed with hydrated lime putty and water Used a finish coat ► Prepared finish plaster Requires only water Contains no lime So the surface can be decorated as soon as it dries Has a off white color
Gypsum Plasters ► Texture Plaster Similar to manufacture prepared finish plaster A texture is added most likely sand. ► Texture spray Is a dry gypsum based material designed to cover minor imperfections and provide a uniform, durable white surface. ► Acoustical Plaster Calcined gypsum is mixed with lightweight mineral aggregate to make a type of finish plaster that has a high rate of sound absorption ► Joint Filler Is used to make the paste for filling nail holes and covering joints in gypsum wallboard
Gypsum Boards ► Is a highly fire resistant board ► Intended for both sheathing and finishing for interior walls and ceilings ► Made with a core of gypsum covered on each side by a heavy specially manufactured Kraft paper. ► Two classifications Standard – 30 minute fire rating X – over 30 minutes ► Sizes 4 x 4- 16 ft Thickness 3/8, ½, 5/8 ► Joints need to be taped and filled.