Gypsum Flashcards
What are the uses of gypsum?
Cast - plaster, stone
Die - stone, improved stone
Mould material - stone
Investment binder - stone
What are the three types of gypsum?
Plaster - B-hemihydrate
Dental Stone - A-hemihydrate
Improved stone - densite
How is gypsum manufactured?
Calcium sulphate dihydrate is exposed to heat producing calcium sulphate hemihydrate.
What is the difference in manufacturing the different types of gypsum?
Plaster - heated in an open vessel
Stone - heated in an autoclave
Improved stone - heated in the presence of Ca & Mg chloride
What is the difference in properties of the types of gypsum?
Plaster - large porous, irregular crystals
Stone - non porous, regular crystals, requires less water
Improved stone - compact, smoother particles
What is the setting reaction of gypsum?
The reverse of the manufacturing process.
Hemihydrate -> dihydrate
What are the mixing ratios of gypsum?
Plaster - 50/60ml water and 100g powder
Stone - 20/35ml water and 100g powder
What are the mechanical properties of gypsum?
Compressive strength - 20-35MPa
What is the expansion of gypsum?
Plaster: 0.2-0.3%
Stone: 0.08-0.1%
Densite: 0.05-0.07%
What effect does spatulation of gypsum have on setting time and expansion?
Decreased setting time and increased expansion.
This is because the growing crystals are forced to come into contact sooner which in turn creates more growing crystals.
What effect does increasing powder have on the setting time and expansion of gypsum?
Faster set and greater expansion
What chemical additives can be added to gypsum to alter the setting time?
Potassium sulphate - decreases setting time
Borax - increases setting time
What are the advantages of gypsum?
Dimensionally accurate and stable
Good colour contrast
What are the disadvantages of gypsum?
Low tensile strength Poor abrasion resistance Very brittle Surface detail less than impression Poor wetting of some rubber impression materials
