Gypsum Products

Question 1

What is a gypsum product?

Answer 1

refers to various forms of calcium sulfate

Question 2

What is the manufacture form of gypsum product?

Answer 2

Calcium sulfate dihydrate

Question 3

Calcium sulfate dihydrate + heat —–> ???

Answer 3

Calcium sulfate hemihydrate

Question 4

What’s the basis of hemihydrate?

Answer 4

Basis for all gypsum used in dentistry

Question 5

What are the three types of materials derived from partial dehydration of gypsum rock?
What is the main difference btwn these derived materials?

Answer 5

Plaster —> model/lab (formulation)
hydrocal –> dental stone
densite –> high strength stone
* Main difference is the manner of driving off the water.

Question 6

What are some accelerators used in gypsum products?

(to set the dihydrate)?

Answer 6

terra alba (K2so4), NACL (speed up the rxn)

Question 7

What’s the setting reaction for gypsum product?

Answer 7

going from hemihydrate back to dihydrate +heat.
-nucleated recrystalization, characterized by continuous solution of hemihydrate, diffusion of calcium and sulfate ions to nucleation centers (impurities), and precipitation of microscopic gypsum crystals.

Question 8

What’s the water/powder ratio?

Answer 8

• amt h20 added to powder to allow for best workability
• less water = less porosity
• less porosity = more strength

Question 9

What does less porosity mean?

Answer 9

more strength.

Question 10

Whats the effects on rate of setting?

Answer 10

Thick mixes (low W/P ratio): hardens more quickly because nucleation centers are concentrated in a small volume.
- the interaction of the growing solid occurs earlier and is more effective.

Question 11

What kind of cast do you get when you decrease the water/powder ratio?

Answer 11

denser cast (more solid)

Question 12

What happens when you increase the water/powder ratio?

Answer 12

More porosity = decreased strength.

Question 13

What’s the purpose of gympsum in dentistry?

• What kind of casting does it normally employ?
• degree of heat that it does not heat above?

Answer 13

used as a binder in materials commonly employed in the casting of dental gold alloys and is generally not heated above 700-c.

Question 14

What is refractory in gympsum bond investments?

function?

Answer 14

• a form of silicone dioxide (refractory present in gold or high melting alloys)
• function: PROVIDES EXPANSION to compensate for gold shrinkage.

Question 15

Refractory silica exists in two phases. What are they?

-What’s the purpose of the changes in these 2 phases?

Answer 15

alpha) Low temperature phase
beta) High temperature phase
- The change btwn the two is rapid and reversible and is responsible for most of the thermal expansion.

Question 16

Within practical limits, increasing amount of silica will do four things.What are they?

Answer 16

1. Increase manipulation time
2. Increase setting expansion
   3, Increase thermal expansion
3. Decrease compressive strength

Question 17

What’s the purpose of a binder?

(What does it do for the investment)?

Answer 17

Form a coherent solid mass.

Provide strength for the investment

Question 18

Binder:

Calsium sulfate hemihydrate is used for casting what type of material?

Answer 18

• gold alloys

Question 19

Binder:

Phosphate, ethyl silicate is used for casting what type of material?

Answer 19

• high temp alloys