gypsum

Question 1

what is gypsum made up of

Answer 1

calcium sulphate dihydrate (CaSO4.2H2O)

Question 2

how to make model plaster from gypsum

Answer 2

• beta
• DRY calcination
• NO pressure

Question 3

model plaster qualities

Answer 3

• irregular shape and size
• porous particles
• needle-like crystals
• loosely packed
• needs lots of water
• weak (35% porosity)

Question 4

how to make dental stone from gypsum

Answer 4

• alpha
• WET calcination
• UNDER pressure

Question 5

dental stone qualities

Answer 5

• uniform, dense, small particles
• rods and prism crystals
• pack tightly
• not lots of water
• organized crystals growth
• strong (20% porosity)

Question 6

plaster vs stone

Answer 6

• chemically same

- different in form and physical structures

Question 7

how to make die stone (jade stone) from gypsum

Answer 7

-no pressure

Question 8

main cause in differences in physical shape/size of particles between gypsum types is due to

Answer 8

different methods of deriving off the water from gypsum rocks

Question 9

water molecules in dihydrate are in

Answer 9

alternating double layers

Question 10

driving force for setting reaction

Answer 10

stable at room temp because dehydration req lots of energy (due to alternating double layers)

Question 11

what happens when you add water to stone/plaster powder

Answer 11

CaSO4 hemihydrate becomes CaSO4 dihydrate and releases heat

Question 12

gypsum is ___ while plaster and stone is ___

Answer 12

gypsum is a dihydrate while plaster and stone are hemihydrates

Question 13

what type of reaction is it when you add water to stone/powder and make it gypsum again

Answer 13

exothermic reaction (through crystallization)

Question 14

dissolution-precipitation theory

Answer 14

dissolution of hemihydrate and recrystallization of dihydrate results in interlocking of crystals to form the set solid gypsum

Question 15

4 mechanisms for dissolution-precipitation theory

Answer 15

1. workable (particles only moderately dissolved in water)
2. dissolution forms super-saturated hemihydrate soln
3. super-saturated forms dihydrate crystals (gradual precipitation)
4. growth of dihydrate crystal and soln becomes non-saturated. cycle repeats until set

Question 16

is calcium sulphate dihydrate soluble or insoluble

Answer 16

insoluble

*hemihydrate is way more soluble so it allows dissolution-precipitation theory

Question 17

physical starting spot for dihydrate crystals

Answer 17

nucleation centers in suspension

Question 18

what can act as a nucleation center

Answer 18

impurities (residual gypsum)

or pre-existing fine dihydrate particles

Question 19

gypsum particles can also be added to act as seeds to

Answer 19

accelerate setting

Question 20

4 stages of crystallization

Answer 20

1. fluid
2. plastic
3. friable
4. carvable

Question 21

what stage of crystallization is when it is exothermic

Answer 21

friable stage

Question 22

interlocking of each crystal results in

Answer 22

expansion

Question 23

what stages does expansion occur

Answer 23

plastic and friable stages

Question 24

result of low water/powder ratio

Answer 24

• quick set
• high expansion
• stronger
• higher surface hardness
• less porosity

Question 25

result of high water/powder ratio

Answer 25

• slow setting
• low expansion
• less strength
• poor surface hardness
• more porosity

Question 26

changing setting time by which 2 properties

Answer 26

1. solubility of hemihydrate (high solubility = shorter time)
2. number of nuclei of crystallization (more nuclei = shorter time)

Question 27

in practice, how can you decrease the setting time

Answer 27

1. add slurry water to mixture
2. using mixing bowl with gypsum particles
3. mixing faster and vigorously

Question 28

what can be added to alter expansion, setting time, dimesional stability etc.

Answer 28

• accelerators
• retarders
• colour pigments
• filler particles

Question 29

colloids (like alginate and agar) can act as ____ by ___

Answer 29

retarders; they are adsorbed on nucleating sites and interfere with crystallization on surface (chalky look; powder texture)

Question 30

Properties of jade stone

Answer 30

• uniform, larger, dense particles
• little water
• organized crystal growth
• strong and smooth (10% porosity)
• high setting expansion (for type V)