Gypsum and Lime

1. Gypsum and Lime

2. History • Been used for several thousand years • Greeks and Egyptians both used it.

3. 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

4. Where Gypsum and Lime are found • Found in rock formations in various parts of the world • US • France • England • Italy • China • Russia • South America

5. 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.

6. 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.

7. 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.

8. 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.

9. 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

10. 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

11. 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.