gypsum Flashcards

1
Q

what is gypsum made up of

A

calcium sulphate dihydrate (CaSO4.2H2O)

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2
Q

how to make model plaster from gypsum

A
  • beta
  • DRY calcination
  • NO pressure
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3
Q

model plaster qualities

A
  • irregular shape and size
  • porous particles
  • needle-like crystals
  • loosely packed
  • needs lots of water
  • weak (35% porosity)
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4
Q

how to make dental stone from gypsum

A
  • alpha
  • WET calcination
  • UNDER pressure
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5
Q

dental stone qualities

A
  • uniform, dense, small particles
  • rods and prism crystals
  • pack tightly
  • not lots of water
  • organized crystals growth
  • strong (20% porosity)
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6
Q

plaster vs stone

A
  • chemically same

- different in form and physical structures

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7
Q

how to make die stone (jade stone) from gypsum

A

-no pressure

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8
Q

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

A

different methods of deriving off the water from gypsum rocks

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9
Q

water molecules in dihydrate are in

A

alternating double layers

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10
Q

driving force for setting reaction

A

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

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11
Q

what happens when you add water to stone/plaster powder

A

CaSO4 hemihydrate becomes CaSO4 dihydrate and releases heat

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12
Q

gypsum is ___ while plaster and stone is ___

A

gypsum is a dihydrate while plaster and stone are hemihydrates

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13
Q

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

A

exothermic reaction (through crystallization)

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14
Q

dissolution-precipitation theory

A

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

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15
Q

4 mechanisms for dissolution-precipitation theory

A
  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
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16
Q

is calcium sulphate dihydrate soluble or insoluble

A

insoluble

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

17
Q

physical starting spot for dihydrate crystals

A

nucleation centers in suspension

18
Q

what can act as a nucleation center

A

impurities (residual gypsum)

or pre-existing fine dihydrate particles

19
Q

gypsum particles can also be added to act as seeds to

A

accelerate setting

20
Q

4 stages of crystallization

A
  1. fluid
  2. plastic
  3. friable
  4. carvable
21
Q

what stage of crystallization is when it is exothermic

A

friable stage

22
Q

interlocking of each crystal results in

23
Q

what stages does expansion occur

A

plastic and friable stages

24
Q

result of low water/powder ratio

A
  • quick set
  • high expansion
  • stronger
  • higher surface hardness
  • less porosity
25
result of high water/powder ratio
- slow setting - low expansion - less strength - poor surface hardness - more porosity
26
changing setting time by which 2 properties
1. solubility of hemihydrate (high solubility = shorter time) 2. number of nuclei of crystallization (more nuclei = shorter time)
27
in practice, how can you decrease the setting time
1. add slurry water to mixture 2. using mixing bowl with gypsum particles 3. mixing faster and vigorously
28
what can be added to alter expansion, setting time, dimesional stability etc.
- accelerators - retarders - colour pigments - filler particles
29
colloids (like alginate and agar) can act as ____ by ___
retarders; they are adsorbed on nucleating sites and interfere with crystallization on surface (chalky look; powder texture)
30
Properties of jade stone
- uniform, larger, dense particles - little water - organized crystal growth - strong and smooth (10% porosity) - high setting expansion (for type V)