Gypsum and Lime History Been used for several

Gypsum and Lime

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.