CHAPTER THREE
PART - II
Gypsum Plasters
GYPSUM
Generals
Gypsum is a rock like mineral commonly found in the
earth’s crust, extracted processed and used in
construction or decoration in the form of plaster and
alabaster.
Gypsum is a combination of sulfate of lime with
water of crystallization.
Gypsum occurs naturally as:
➢ Hydrous sulfate of lime (Ca SO4 2H2O) which is generally 76%
CaSO4 and 24% H2O,
➢ Anhydrate (Ca SO4).
Pure gypsum is known as alabaster and it is a white
translucent crystalline mineral
GYPSUM
Generals
Color transparent to white and can be of grey, green ,pink,
blue and red because of impurities
GYPSUM
PRODUCTION
➢Excavating
➢Crushing (~25 mm dia.)
➢Grinding
➢Heating (calcining)
➢Cooling and Pulverizing
➢Marketing in Bags
GYPSUM
Calcination
➢ Gypsum rock when heated to 100-190°C looses ¾ of its water.
CaSO4.2H2O → CaSO4.½H2O + 3/2 H2O
Plaster of Paris
✓ This is low burning process and named as INCOMPLETE
CALCINATION.
➢ When calcination is carried out at temperatures above 190°C all
water is removed.
CaSO4.2H2O → CaSO4 + 2H2O
Gypsum anhydrite ( Hard Finish Plaster)
✓ This is high-burning process & COMPLETE CALCINATION.
➢ Both of these products form gypsum rock by recombining with water.
CaSO4.½H2O + 3/2H2O → CaSO4.2H2O
CaSO4 + 2H2O → CaSO4.2H2O
GYPSUM
Gypsum Plasters
Gypsum plasters are used in the arts and
in building construction.
Gypsum plasters are manufactured by
heating the raw material gypsum at either
moderate or high temperatures the
results being plaster of Paris or hard-
finish plaster respectively.
GYPSUM PLASTERS
Plaster Of Paris
If some of the water of crystallization is driven off
by incomplete dehydration at a temperature just
above the boiling point of water 1000c pure, finely
ground gypsum, a semi-hydrated plaster is
obtained which is known as plaster of Paris.
(CaSO4.2H2O)+Moderate Heat (CaSO4 .1/2 H2O) +1/2 H2O
GYPSUM PLASTERS
Plaster Of Paris
➢ is a white powder having a specific gravity of 2.57.
➢ is also known as low-temperature gypsum derivative
or semi-hydrated plasters (hemi hydrate).
➢ is used for small patching jobs on plaster walls.
➢ When mixed with sufficient water to form a plastic
paste it sets very rapidly (retarder must be added)
GYPSUM PLASTERS
Hard Finish Plaster
➢ Produced by burning gypsum to a considerably
higher temperature .
Hard-finish plaster is also known as anhydrous
plaster or high-temperature gypsum derivative.
(CaSO4. 2H2O)+High Heat CaSO4+ 2H2O
➢ Thisplaster is less soluble with consequent
reluctance to absorb water in the process of re
crystallization.
GYPSUM
Important Properties
Plaster of Paris
⚫ Setting time ~5-20 min.
⚫ Used for sculpturing, ornamental work, small repair works.
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.
Fire resistance
⚫ 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.
Light in weight
Good sound and heat insulation
Good adhesion
Solubility: It is slightly soluble in water
GYPSUM
Uses of Gypsum Plaster
The use is limited due to its
inherent character to absorb
moisture.
For ornamental casting
To prepare gypsum board and
blocks
To prepare base coat to
receive various finishing coats
Questions ?
