Gypsum

Question 1

What are the uses of gypsum

Answer 1

cast (plaster/stone)​

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die (stone / improved stone)​

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mould material (stone)​

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investment binder (stone)

Question 2

What determines the type of gypsum

Answer 2

The manufacturing conditions
dihydrate –(heat)–> hemihydrate

open vessel
autoclave
CaCl2

Question 3

What are the types of gypsum

Answer 3

Plaster ( beta-hemihydrate)​

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Dental stone ( alpha-hemihydrate)​

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Densite (improved stone)

Question 4

What does manufacturing CaSO43H2O in an open vessel produce

Answer 4

plaster

Question 5

What does manufacturing CaSO43H2O in an autoclave produce

Answer 5

stone

Question 6

What does manufacturing CaSO43H2O with CaCl2 produce

Answer 6

Improved stone

Question 7

What is the structure of plaster

Answer 7

large porous, irregular crystals

Question 8

What is the structure of stone

Answer 8

non-porous, regular crystals, requires less water​

Question 9

What is the structure of densite (improved stone)

Answer 9

compact smoother particles

Question 10

What is the gypsum setting reaction

Answer 10

reverse of manufacture

(CaSO4)2 .H2O + 3H2O —–> 2CaSO4 . 2H2O

Question 11

What are the mixing ratios for plaster and stone

Answer 11

Stone - 20/35ml per 100g
Plaster 50/60ml per 100g

Question 12

What is the setting process

Answer 12

Hemihydrate dissolves as dihydrate forms and the crystals precipitate on impurities
Hemihydrate fully disolves

Initial set - dihydrate crystals come into contact and expansion starts (properties of a weak solid)

Final set - strong and hard enough to be worked strength continues to develop

Question 13

How are initial and final setting times measured

Answer 13

Using Gilmore needles

Question 14

What role does water play in setting

Answer 14

During setting excess water is trapped in the powder mass

On completion of setting excess water evaporates and voids are produced

Question 15

Is gypsum reliable for reproducing surface details

Answer 15

Ideally GYPSUM reproduces the fine detail on the impression material​

BUT gypsum is inherently porous, resulting in a relatively rough surface – about 28 to 40um

Question 16

What does increased spatulation do

Answer 16

increased spatulation breaks down growing crystals​

fragments act as nuclei of crystallisation​

more growing crystals -come into contact sooner​

Hence
decreased setting time
increased expansion

Question 17

Which is the strongest form

Answer 17

Densite (imrpoved stone)

Question 18

What is the compressive strength of gypsum

Answer 18

28MPa - 38 MPa

Question 19

What is the compressive strength of gypsum after 24 hrs

Answer 19

75MPa

Question 20

What are the properties of Gypsum

Answer 20

Surface hardness low (not ideal - easy abrasion)

Setting time - varied

Expansion on setting should be low

Brittle

Compatable with impression materials

Question 21

What does increasing and decreasing the powder in ratio do

Answer 21

increasing powder​
-more nuclei of crystallisation per unit volume​
-crystals come into contact sooner​
-faster set & greater expansion​

decreasing powder​
-converse

Question 22

What effect does temperature have on setting

Answer 22

Conflicting actions
-Rate of diffusion of ions increases with temp
-While solubility of the hemihydrate decreases as temp increases

Question 23

What is gypsums flexural strangth

Answer 23

15-20 MPa

Question 24

What are the advantages and disadvantages of gypsum

Answer 24

Advantages​
-dimensionally accurate and stable
-low expansion (<0.1%) of stone/densite​
-good colour contrast

Disadvantages​
-low tensile strength​
-poor abrasion resistance​
-very brittle​
-surface detail less than elastomer impression (~20um groove)​
-poor “wetting” of some impression materials

Question 25

What effect does potassium sulphate and borox additives have on setting

Answer 25

Potassium Sulphate - decreases setting time

Borox - increases setting time