Gypsum Products

Question 1

What is the chemical composition of gypsum?

Answer 1

• Calcium sulphate dihydrate

- (CaSO4)2H2O

Question 2

What can gypsum based materials be used for? (5)

Answer 2

• Impression materials
• Model (poured from an impressionl)
• Dies
• Moulds (construction of dentures)
• Refractory investments

Question 3

Where is gypsum obtained from?

Answer 3

White Cliffs of Dover

Question 4

What is gypsum converted to by the manufacturers?

Answer 4

• Calcium sulphate hemihydrate

- (CaSO4) H2O

Question 5

Depending on the treatment process what can conversion too hemihydrate result in? (3)

Answer 5

• Plaster
• Stone
• Improved stone

Question 6

Heating gypsum in an open kettle at 120° results in production of?

Answer 6

Plaster of Paris

b-hemihydrate
(CaSO4)2H2O

Question 7

Heating gypsum in an autoclave under steam and pressure at 120/130° results in production of? (2)

Answer 7

• Autoclaved Artificial Stone
• Hydrocal Calcined

α-hemihydtare
(CaSO4)2H2O

Question 8

Heating gypsum in 30% boiling solution of magnesium or calcium chloride results in production of?

Answer 8

Improved stone (densite)

α-hemihydrate
(CaSO4)2H2O

Question 9

Comment on the strength of product produced when a b-hemihydrate is mixed with water

Why is this?

Answer 9

• Forms a weak product
• Due to the large irregular & porous particles, which do not pack closely, thus resulting in large pores within the set material

Question 10

Comment on the strength of product produced when a α-hemihydrate is mixed with water

Why is this?

Answer 10

• Forms a strong product

- Due to it’s small, regular & non-porous particles, which pack closely together within the set material

Question 11

What are the advantages of plaster? (3)

Answer 11

• Cheap
• Soft
• Easy to use and shape

Question 12

When is plaster generally used? (3)

Answer 12

• Mounting models into articulators
• Flasking procedures
• Basing models

Question 13

What is stone generally used for?

Answer 13

Models of the mouth

• Replica of individual teeth
• Construction of crowns bridges and dentures

Question 14

What is used in dentate casts when maximum strength is essential?

Answer 14

Dental stone

Question 15

What is the general composition of gypsum products? (5)

Give rough percentages of these

Answer 15

• Hemihydrate (75-80%)
• Dihydrate (5-8%)
• Insoluble anhydrites (5-8%)
• Impurities (4%)
• Accelerators/retarders (4%)

Question 16

How do accelerators and retarders affect setting time?

Answer 16

These chemicals increase or decrease the solubility of gypsum in water, thus affecting it’s setting time

Question 17

Name an accelerator for gypsum

What product is produced because of this?

Answer 17

• 2% K2SO4 solution

- Syngenite [K2(CaSO4)2H2O]

Question 18

How does syngenite speed up a reaction? (3)

Answer 18

• Rapid crystallisation
• Reduces overall expansion
• Accelerates setting reaction

Question 19

Name 3 retarders for gypsum

Answer 19

• > 20% NaCl
• Borax
• Potassium citrate

Question 20

Comment on the solubility of hemihydrate over dihydrate

Answer 20

Hemihydrate is more soluble

Question 21

What happens during the setting reaction of hemihydrate

Answer 21

• Water is added
• Some hemihydrate dissolves in water and forms dihydrate
• The dihydrate’s lower water solubility results in a supersaturated solution being formed
• This is unstable so the dihydrate precipitates to form stable crystals

Question 22

What is the role of unchanged gypsum dihydrate in the setting mechanism?

Answer 22

Acts as crystallisation nuclei for growth of dihydrate crystals

Question 23

How much water is required to set 100g of

Plaster?
Stone?
Improved stone?

Answer 23

• 50ml
• 20ml
• 20ml

Question 24

What will occur if you add too much water in the setting process of gypsum products?

Setting time?
Consistency of mix?
Model strength?

Answer 24

• Slower setting time
• The mix will be runny
• Model will be weaker

Question 25

What will occur if you add too much powder in the setting process of gypsum products?

Answer 25

• Difficult to mix

- Result in porosity within the set material

Question 26

Increasing spatulation time and or rate in gypsum products will result in? (2)

Answer 26

• Reduction in setting time

- Increases expansion

Question 27

What is the result of increasing temperature of the water in gypsum product setting?

What is the ideal temperature water?

Answer 27

• Slow the reaction down
• Will stop completely at 100°
• 0°-50°

Question 28

How is the setting time of different gypsum products measured? (2)

Answer 28

• Vicant needles

- Gillmore needles
¼lbs needle for initial
(1lbs needle for final)

Question 29

Why is setting expansion in gypsum products important?

Answer 29

Allowing models and dies to be slightly larger than oral anatomy so crowns and dentures aren’t too tight

Question 30

Comment on the dry strength of gypsum products compared to wet strength

Answer 30

Dry strength is twice that of wet strength

Question 31

What is the tensile strength of plaster

Answer 31

Very low 2Mpa

Question 32

What is the tensile strength of stone

Answer 32

4 Mpa

Question 33

Comment on the surface hardness of

• Plaster
• Stone
• Improved stone

Answer 33

• Low
• Improved
• Best out of 3

Question 34

What is setting time for plaster as an impression material

Answer 34

3-5 minutes

Question 35

What is the setting time for plaster in the laboratory

Answer 35

5-10 minutes

Question 36

What is the setting time for stone

Answer 36

7-15 minutes

Question 37

What is the setting time for improved stone

Answer 37

5-12 minutes