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.