Gypsum

Question 1

What is Gypsum?

Answer 1

Calcium Sulphate Dihydrate

Question 2

What are the 5 types of gypsum

Answer 2

1. Type 1 - Impression plaster
2. Plaster
3. Stone
4. Die stone low expansion (fixed pros)
5. Die stone high expansion (rem pros)

Question 3

Name three factors that will increase the setting time for gypsum

Answer 3

1. Cold water
2. More water
3. Decreased spatulation

Question 4

What is the gypsum reaction

Answer 4

Hemihydrate + water = dihydrate

Question 5

How long does gypsum take to set

Answer 5

45-60 mins

Question 6

Why do the different gypsum types have different porosity/strength

Answer 6

The manufacture process.

Question 7

Notes on the setting reaction for gypsum

Answer 7

• Hemihydrate dissolves and dihydrate precipitates
• Exothermic reaction
• Hygroscopic expansion

Question 8

Adding more water to gypsum powder will result in:

Answer 8

• Lower strength
• Increased porosity
• Less expansion
• Increased setting time

Question 9

List 6 factors which influence setting of gypsum

Answer 9

1. Impurities
2. Temp
3. Manipulation technique
4. Water/powder ratio
5. Humidity
6. Accelerators/retarders