Exercise unit 7

Plaster of Paris is

• Calcium sulphate dihydrate

• Calcium sulphate Anhydrate

• Calcium sulphate hemihydrate

• Calcium sulphate trihydrate

When consistency higher

• Mechanical strength higher

• Viscosity lower

• Setting time higher

• Porosity lower

Plaster of paris is from mineral

• Dolomite

• Talc

• Gypsum

• Fluorite

Setting time need to be control because too long setting time

• will restrict production

• will damage the mould

• will make the product more harden

• will lower the cost

The setting expansion will are effected by factor below:

• ratio of mixing

• Temperature of the mixture

• setting time

• Type of mould used

Calcination under atmospheric pressure will produce

• Beta plaster

• Alpha plaster

For casting mould, normally consistency is between

• 80-90

• 60-70

• 40-50

• 70-80

Consistency affected below properties EXCEPTED

• Viscosity

• Setting time

• Strength

• cost

Two important characteristics of plaster is

• Ability to be cast and short setting time

• Long setting time and high hardness

• Short setting time and easy to handled

• Easy to cast and low cost

Below is the application of plaster EXCEPTED

• Fertilizer and soil

• Surgical

• Plaster ceiling

• Glass