Gypsum Products

Question 1

What is the setting reaction of gypsum?

Answer 1

Gypsum sets by an exothermic reaction

1. When gypsum reacts with water, a suspension of hemihydrate is formed
2. Suspension dissolves to form a supersaturated solution of calcium sulfate
3. Supersaturated solution of calcium sulfate diffuses to precipitate/crystallise on nuclei of crystallisation

Question 2

What are some factors influencing setting time?

Answer 2

1. W:P = Lower W:P, faster the setting (takes shorter for solution to saturate and precipitate)
2. Particle size = Smaller the particle size, faster the setting
3. Impurities = Presence of impurities, faster the setting (provides nuclei of crystallisation for crystal formation)
4. Spatulation = Increased spatulation, faster set (break up crystals to form more nuclei of crystallisation)
5. Addition of accelerators = Increase rate of crystal growth + increased dissolution of hemihydrate
6. Addition of retarders = poisoning of nuclei decreases rate of crystallisation

Question 3

What are the effects of increasing W:P ratio on these properties of gypsum:

1. Setting time
2. Setting expansion
3. Compressive strength
4. Surface hardness
5. Amount of heat released (dental stone VS dental plaster)

Answer 3

1. Setting time: Longer
2. Setting expansion: Decrease
3. Compressive strength: Decrease
4. Surface hardness: Decrease
5. Amount of heat released (dental stone VS dental plaster): Greater heat released per unit volume of dental plaster than dental stone due to the W:P ratio required for each type of gypsum