Gypsum Products

Question 1

What is Gypsum?

Answer 1

Calcium Sulphate Dihydrate

(CaSO4)2H2O

Question 2

What can Gypsum products be used for? 5

Answer 2

• Impression Materials
• Models (Positive impression)
• Dies (Positive replicas of individual teeth)
• Moulds (Dentures)
• Refractory Investments

Question 3

What do manufacturers convert Gypsum into?

Answer 3

Hemihydrate version (CaSO4)H2O

• When mixed with water on setting converts back to Dihydrate

Question 4

Depending on the type of conversion treatment what can the hemihydrate lead to?

Answer 4

• Plaster of Paris
• Stone
• Improved Stone

Question 5

What does treating Dihydrate with 120˚C heat from open kettles form?

Answer 5

Plaster of Paris

- Beta Hemihydrate

Question 6

What does treating Dihydrate with 120-130˚C heat with steam and pressure form?

Answer 6

Autoclaved Artificial Stone / Hydrocal Calcined

- Alpha Hemihydrate

Question 7

What does treating Dihydrate with a 30% solution of CaCl2 or MgCl2 form?

Answer 7

Improved Stone (Densite)
- Alpha Hemihydrate

Question 8

Comment on the strength of Beta Hemihydrate when mixed with water?

Answer 8

• Forms a weak product

- Due to large irregular and porous particles which do not pack closely, leaving large gaps.

Question 9

Comment on the strength of Alpha Hemihydrate when mixed with water?

Answer 9

• Forms a stronger product than beta

- Due to its small regular and non porous particles, which pack closely together

Question 10

Where is Plaster used?

Answer 10

• Mounting models into articulators
• Flasking procedures for dentures
• Base for models

Used where strength is not a critical requirement

Question 11

Advantages of Plaster 3

Answer 11

• Soft
• Cheap
• Easy to use and shape

Question 12

Where is Stone used?

Answer 12

• Replica of individual teeth
• Construction of crowns, bridges and dentures
• Study casts

Question 13

When maximum strength is essential for dentate casts what is used?

Answer 13

Dental Stone by itself

Question 14

Composition of Gypsum

Answer 14

• Hemihydrate a or b ( 75-85%)
• Unchanged Gypsum (5-8%)
• Insoluble Anhydrites (5-8%)
• Impurities (≈4%)
• Accelerators/ retarders (≈4%)

Question 15

What are accelerators/retarders?

Answer 15

Chemicals that increase/decrease the solubility of gypsum in water, thus affecting it’s setting time.

Question 16

Name an Accelerator for Gypsum

What does this produce?

Answer 16

K2SO4

Syngenite [K2(CaSO4)2.H2O]

Question 17

How does Syngenite work?

Answer 17

• Crystallises very rapidly
• Reduces overall expansion
• Accelerates setting reaction

Question 18

3 Examples of Accelerators for Gypsum

Answer 18

• K2SO4
• CaSO4.2H2O
• NaCl (<20%)

Question 19

3 Examples for retarders for Gypsum

Answer 19

• NaCL(>20%)
• Borax
• Potassium Citrate

Question 20

Which is more soluble: hemihydrate or Dihydrate?

Answer 20

Hemihydrate

Question 21

What happens during the setting reaction for Hemihydrate?

Answer 21

• Hemihydrate slowly forms Dihydrate
• Some Hemihydrate dissolves in water and forms dihydrate
• The dihydrates lower solubility results in a supersaturated, UNSTABLE solution
• Thus forming STABLE crystals
• As crystals form MORE hemihydrate dissolves until set

Question 22

What does unchanged Gypsum do in the mechanism?

Answer 22

• Acts as a crystallisation nuclei for growth of Dihydrate Crystals

Question 23

How much water is needed to mix 100g Gypsum products?

Answer 23

50ml for PLASTER
20ml for STONE
20ml for IMPROVED STONE

Question 24

What will happen if you add MORE WATER to the mixing process of gypsum products?

Answer 24

• Slower Setting time
• Runny mix
• Weaker model

Question 25

What will happen if you add MORE POWDER to the mixing process of gypsum products?

Answer 25

- Difficult to mix | - Porosity within set material

Question 26

What will have if you increase SPATULATION TIME to the mixing process of gypsum products?

Answer 26

- Reduced setting time | - Increased setting expansion

Question 27

What is the optimum temperature for the mixing process of gypsum products?

Answer 27

0-50˚C with little change

Question 28

What will have if you increase TEMPERATURE in the mixing process of gypsum products?

Answer 28

>50˚C Gradual Retardation takes place ≥100˚C No reaction

Question 29

What can you measure setting time of Gypsum Products with?

Answer 29

Vicat Needles Gillmore Needles (1/4lb and 1lb in weight)

Question 30

Why is setting expansion important in Gypsum Products?

Answer 30

- Models are slightly bigger than the oral anatomy so crowns and dentures aren't to tight

Question 31

Comment on Dry strength compared to Wet strength of Gypsum Products

Answer 31

Dry strength is roughly twice as strong as wet strength

Question 32

Tensile Strength of Plaster

Answer 32

Very Low 2MPa

Question 33

Tensile Strength of Stone

Answer 33

Double Strength of Plaster

Question 34

Place in order of surface hardness Plaster Improved Stone Stone

Answer 34

Plaster - Weakest Stone Improved Stone

Question 35

Setting time for Plaster as an impression material

Answer 35

3-5 Mins

Question 36

Setting time for Stone

Answer 36

7-15 Mins

Question 37

Setting time for Improved Stone

Answer 37

5-12 Mins

Question 38

Setting time for Plaster as a laboratory material

Answer 38

5-10 Mins