9 - Gypsum Products

Question 1

9.5 In what chemical form is gypsum found in its natural state?

Answer 1

Calcium sulfate dihydrate (CaSO4`2H2O)

Question 2

9.5 In what chemical form is gypsum used for dentistry?

Answer 2

Calcium sulfate hemihydrate (CaSO4`1/2 H2O + 1/2 H2O)

The hemihydrate form is the basis for all gypsum used in dentistry

Question 3

9.8 What are three examples of materials that dihydrate calcium sulfate (gypsum rock) can be dehydrated into for use in dentistry?

Answer 3

1. Plaster (model/lab)
2. Hydrocal (dental stone)
3. Densite (high strength stone)

Question 4

Recognize that this sentence is tied to calcium sulfate dihydrate being dehydrated into calcium sulfate hemihydrate:

Nucleated re-crystallization, characterized by a continuous solution of hemihydrate, diffusion of calcium and sulfate ions to nucleation centers, and precipitation of microscopic gypsum crystals

Answer 4

Nucleated re-crystallization, characterized by a continuous solution of hemihydrate, diffusion of calcium and sulfate ions to nucleation centers, and precipitation of microscopic gypsum crystals

Question 5

9.21 Water/powder ratios determine the shape and form of hemihydrate crystals

• Less water = more or less porosity?
• Less porosity - more or less strength?

Answer 5

Less water = less porosity

Less porosity = more strength

Question 6

9.22 Why do thick mixes (low W/P ratio) harden more quickly?

Answer 6

With low W/P ratio nucleation centers are concentrated in a small volume. The interaction of the growing solid occurs earlier and is more effective.

Question 7

9.26 Decreasing the W/P ratio results in a ________ cast.

Answer 7

Results in a denser cast

Question 8

9.31 What kind of relationship does the W/P ratio have with strength?

Answer 8

An inverse relationship

As you increase W/P ratio, you decrease the strength

Question 9

9.31 What kind of relationship does the W/P ratio have with porosity?

Answer 9

A direct relationship

As you increase W/P ratio, you increase the porosity

Question 10

9.35 Describe gypsum use in bonded investments

Answer 10

Gypsum is used as a binder in materials commonly employed in the casting of dental gold alloys and is generally not heated above 700 degrees Celsius

Question 11

9.39 What is the purpose of the refractory material in investments?

Answer 11

To provide expansion to compensate for gold shrinkage

Typically silicone dioxide quartrz, crystobalite, or tridymite

Question 12

9.40 What is the role that binder plays in investments?

What is an example of a chemical used for gold investments that functions as a binder?

Answer 12

To help the investment form a coherent solid mass and provides strength

Gold - typically uses Calcium sulfate hemihydrate

Question 13

4.44 Why is silica used as a refractory material in vestments?

Answer 13

Silica exists in a low temperature phase (α) and a high temperature phase (β).

The change between the two is rapid and reversible and is responsible for most of the thermal expansion

Question 14

4.45 When increasing the refractory composition (silica) of an investment, what properties are adjusted?

Answer 14

1. Increases manipulation time
2. Increases setting expansion
3. Increases thermal expansion
4. Decreases compression strength