GYPSUM GYPSUM In nature Gypsum Rock Pure gypsum

GYPSUM

GYPSUM Ø In nature : Gypsum Rock Ø Pure gypsum rock : Ca. SO 4. 2 H 2 O Ø Impurities : Mg. O, Al 2 O 3, Fe 2 O 3, Si. O 2, Ca. CO 3, Mg. CO 3. . .

PRODUCTION STEPS Excavating Ø Crushing (~25 mm diameter) Ø Grinding Ø Heating (calcining) Ø Cooling and Pulverizing Ø Marketing in Bags Ø

CALCINATION Gypsum rock wahen heated to 100 -190°C looses ¾ of its water. Ca. SO 4. 2 H 2 O → Ca. SO 4. ½H 2 O + 3/2 H 2 O Plaster of Paris This is low burning process and named as INCOMPLETE CALCINATION. Ø When calcination is carried out at temperatures above 190°C all water is removed. Ca. SO 4. 2 H 2 O → Ca. SO 4 + 2 H 2 O gypsum anhydrite This is high-burning process & COMPLETE CALCINATION. Ø

Ø Both of these products form gypsum rock by recombining with water. Ca. SO 4. ½H 2 O + 3/2 H 2 O → Ca. SO 4. 2 H 2 O Ca. SO 4 + 2 H 2 O → Ca. SO 4. 2 H 2 O Calcination process is carried out in two types of kilns. n Kettle Kilns n Rotary Kilns Ø

KETTLE KILN MIXER FIRE BRICK ~ 2 -3 m GYPSUM ROCK DISCHARGE HEAT ~ 2. 5 -3 cm

ROTARY KILN

GYPSUM PLASTERS Ø Obtained by Incomplete Calcination (Ca. SO 4. ½H 2 O) 1. Plaster of Paris : is formed by incomplete calcination at 100 -190°C. No admixtures are found. Hard Wall Plaster of Paris + Admixtures (Glue, Sand. . . ) 2.

Ø Obtained by Complete Calcination (Ca. SO 4) 1. Flooring Plaster (Ca. SO 4 with no impurities) 2. Hard Finish Plaster (Ca. SO 4 + Al 2(SO 4)3) (Ca. SO 4 + Na 2 B 4 O 7)

PROPERTIES & USES OF GYPSUM PLASTERS Ø Plaster of Paris – Setting time ~5 -20 min. – Used for sculpturing, ornamental work, small repair work

PROPERTIES & USES OF GYPSUM PLASTERS Ø Hard Wall Plaster – Setting time ~1 hr – Compressive strength ~7 MPa – Admixtures result in increased plasticity & setting time & reduced shrinkage – Can be used for plastering walls – Production of prefabricated structural units – Masonry bricks & blocks

PROPERTIES & USES OF GYPSUM PLASTERS Ø Flooring, Hard Finish Plaster – Setting time ~1 -16 hrs – Compressive strength > 7 MPa – Can be used for producing prefabricated units, masonry bricks & blocks & flooring & pavement bricks & tiles.

PROPERTIES & USES OF GYPSUM PLASTERS Gypsum often serves as a fire proofing material even though its strength is destroyed by long continuous heat. It forms a powder covering the surface which acts as an effective insulator. Ø Gypsum products tend to disintegrate when exposed to moisture. Therefore, they should not be used for exterior work & for moist interiors. (NON-HYDRAULIC) Ø

BY-PRODUCT GYPSUM Phospogypsum – Major by-product of phosporic acid production n Desulfogypsum – Obtained from the desulfurization of combustion gases in coal burning power plants (Harmful SO 2 gas is turned into Ca. SO 4. 2 H 2 O n