Gypsum

Question 1

Key compound in gypsum

Answer 1

Calcium sulfate

Unset material: calcium sulphate hemihydrate

Set material: calcium sulphate dihydrate

Question 2

What is type 1 gypsum

Answer 2

Impression plaster

Question 3

What is type 2 gypsum

Answer 3

Model plaster

Question 4

What is type 3 gypsum

Answer 4

Denture dental stone

Question 5

What is type 4 gypsum

Answer 5

Dental stone high strength low expansion

Question 6

What is type 5 gypsum

Answer 6

Dental stone high strength high expansion

Question 7

What determines compressive strength of gypsym

Answer 7

Porosity of set material

Water powder ratio (increased w/p = more porosities = weaker)

Time for which material allowed to dry after setting
(Evaporation causes precipitation of dissolved dihydrate, removal of excess water increases strength)

Question 8

Does gypsum have high flexural strength

Answer 8

No. Brittle

Question 9

Does gypsum have good dimensional stability

Answer 9

Yes

Question 10

Is gypsum soluble in water

Answer 10

Set plaster (Type 1 and 2) slightly soluble in water

Question 11

What is the water/powder ratio of dental plaster

Answer 11

0.5

Question 12

Powder of stone vs plaster

Answer 12

Powder of stone is more regular and uniform is shape, is less porous

Question 13

What is dental stone used for

Answer 13

Casts for diagnostic purposes, for complete and partial denture construction

Question 14

What is the water/powder ratio for stone

Answer 14

0.3

Question 15

What is the equation for setting reaction of gypsum

Answer 15

Ca(so4)2.h2o +3 h2o —> 2 caso4.2h2o + heat

Dihydrate become hemihydrate!

Question 16

How does particle size affect setting time of gypsum

Answer 16

Finer particles, more nuclei, more rapid crystallisation, increase setting rate, shorter setting time

Question 17

How do impurities affect setting time of gypsum

Answer 17

Impurities poison nuclei, fewer nuclei present, lower crystallisation rate, increased setting time

Question 18

How does water powder ratio affect setting time of gypsum

Answer 18

Increased water/powder ratio, increase setting time

Question 19

How does mixing affect setting time of gypsum

Answer 19

Increased mixing, break up dihydrate crystals at early stages of setting to produce more nuclei, decrease setting time

Question 20

Why is it important to wash blood off impression before pouring cast

Answer 20

Blood is a colloid which acts as a retarder, would increase setting time

Question 21

What is the setting expandion of plaster

Answer 21

0.2-0.3%

Question 22

What is the setting expansion of stone

Answer 22

0.08-0.1%

Question 23

Why is there setting expansion of gypsum

Answer 23

Outward thrust of crystals growing against each other, outward growth from nuclei

Question 24

How does water/powder ration affect setting expansion

Answer 24

More water, less expansion because fewer nuclei of crystallisation per unit volume

Question 25

How does mixing time affect setting expansion

Answer 25

Increased mixing time, more nuclei in given volume, increase outward thrust, increase setting expansion

Question 26

What is hygroscopic expansion

Answer 26

Gypsum material soaked in water during setting process to increase setting expansion of casting investments

Question 27

Is wet or dry strength of gypsum greater

Answer 27

Dry strength 2 to 3 times greater

Question 28

Difference in manufacture of plaster vs stone

Answer 28

Plaster is heated up in open system while stone is heated up in closed system with chemicals

Question 29

How must gypsum products be stored

Answer 29

Airtight waterproof containers in dry region since they absorb moisture which will cause gradual deterioration