L2: Gypsum

Question 1

What is the chemical makeup of gypsum?

Answer 1

Hydrous calcium sulfate; (CaSO4*2H2O)

Question 2

What is the basic effect of heat on gypsum? What are the two forms, before & after heat addition?

Answer 2

Drives water from gypsum.

Calcium Sulfate Dihydrate –> Calcium Sulfate Hemihydrate

Question 3

What is the difference b/t Type I & II Beta hemihydrate forms of gypsum & types III, IV & V Alpha hemihydrate forms?

Answer 3

Difference is in how they are made.

• Beta hemihydrate gypsum has high amount of water remaining in mixture of irregularly shaped crystals. When this water evaporates as the model sets it leaves voids resulting in spongy porous models
• Alpha has prismatic & regularly shaped crystals that pack more readily, leaving less water. Thus, when model evaporates & hardens there are fewer voids remaining in the model making it harder & more resistant to abrasion

Question 4

What is the reaction, by which, calcium sulfate hemihydrate is converted to calcium sulfate dihydrate?

Answer 4

1.5H2O + CaSO4 + 1/2H20 CaSO4 + 2H20

Question 5

What are the 3 very general steps that occur during the setting of gypsum?

Answer 5

1. Nuclei Formation
2. Crystal Formation
3. Setting Expansion

Question 6

What is a usual working time with Gypsum?

Answer 6

3 minutes

Question 7

What are some factors that can affect setting time of Gypsum? Does it increase or decrease setting time?

Answer 7

1. Impurities - Increase
2. Fineness - Decrease
3. W/P ratio -
4. Mixing time - If takes too long will decrease working time
5. Temperature - Increase temp causes increase working time but must be really hot (Above 122 F)

Question 8

Accelerators & retarder are used in gypsum for what purpose?

Answer 8

Accelerators: To decrease setting time
Retarders: Increase working time

Question 9

How do accelerators shorten setting time? What is the most common accelerator used to set gypsum?

Answer 9

They act as nucleating sites for dihydrate crystal growth. Terra Alba is the most common used for setting gypsum.

Question 10

Does Terra Alba affect the setting expansion of gypsum?

Answer 10

NO

Question 11

How do Retarders work? What is an important retarder that dentists must be aware of with every impression taken?

Answer 11

Lengthen setting time by interfering w/ dihydrate crystal formation. Colloids - ‘poison’ nuclei - include dry blood, alginate, agar, & saliva. Note that colloids are found normally on impressions, thus the impression material MUST be cleaned before it is used to make a model or it will be inaccurate b/c of the colloid retarders.

Question 12

During the setting process of gypsum, why do certain materials expand?

Answer 12

Setting expansion occurs b/c adjacent crystals push each other apart. Ranges from 0.06-0.5%. Half a percent is a significant amount when it comes to models.

Question 13

In what way does each variable affect expansion?

1. Mixing time
2. Water to Powder ratio
3. Addition of chemicals

Answer 13

1. Increased
2. More water –> less expansion
3. Reduced

Question 14

What are 3 ways to increase strength & surface hardness of a gypsum model?

Answer 14

1. Low water/powder ratio - Decrease water
2. Increased spatulation time - Increase mixing
3. Gypsum hardening solutions

Question 15

What are 2 ways to decrease hardness of gypsum?

Answer 15

1. High W/P ratio

2. Addition of accelerators & retarders - results in reduced inter-crystalline cohesion