gypsum Flashcards
what is gypsum made up of
calcium sulphate dihydrate (CaSO4.2H2O)
how to make model plaster from gypsum
- beta
- DRY calcination
- NO pressure
model plaster qualities
- irregular shape and size
- porous particles
- needle-like crystals
- loosely packed
- needs lots of water
- weak (35% porosity)
how to make dental stone from gypsum
- alpha
- WET calcination
- UNDER pressure
dental stone qualities
- uniform, dense, small particles
- rods and prism crystals
- pack tightly
- not lots of water
- organized crystals growth
- strong (20% porosity)
plaster vs stone
- chemically same
- different in form and physical structures
how to make die stone (jade stone) from gypsum
-no pressure
main cause in differences in physical shape/size of particles between gypsum types is due to
different methods of deriving off the water from gypsum rocks
water molecules in dihydrate are in
alternating double layers
driving force for setting reaction
stable at room temp because dehydration req lots of energy (due to alternating double layers)
what happens when you add water to stone/plaster powder
CaSO4 hemihydrate becomes CaSO4 dihydrate and releases heat
gypsum is ___ while plaster and stone is ___
gypsum is a dihydrate while plaster and stone are hemihydrates
what type of reaction is it when you add water to stone/powder and make it gypsum again
exothermic reaction (through crystallization)
dissolution-precipitation theory
dissolution of hemihydrate and recrystallization of dihydrate results in interlocking of crystals to form the set solid gypsum
4 mechanisms for dissolution-precipitation theory
- workable (particles only moderately dissolved in water)
- dissolution forms super-saturated hemihydrate soln
- super-saturated forms dihydrate crystals (gradual precipitation)
- growth of dihydrate crystal and soln becomes non-saturated. cycle repeats until set
is calcium sulphate dihydrate soluble or insoluble
insoluble
*hemihydrate is way more soluble so it allows dissolution-precipitation theory
physical starting spot for dihydrate crystals
nucleation centers in suspension
what can act as a nucleation center
impurities (residual gypsum)
or pre-existing fine dihydrate particles
gypsum particles can also be added to act as seeds to
accelerate setting
4 stages of crystallization
- fluid
- plastic
- friable
- carvable
what stage of crystallization is when it is exothermic
friable stage
interlocking of each crystal results in
expansion
what stages does expansion occur
plastic and friable stages
result of low water/powder ratio
- quick set
- high expansion
- stronger
- higher surface hardness
- less porosity
result of high water/powder ratio
- slow setting
- low expansion
- less strength
- poor surface hardness
- more porosity
changing setting time by which 2 properties
- solubility of hemihydrate (high solubility = shorter time)
- number of nuclei of crystallization (more nuclei = shorter time)
in practice, how can you decrease the setting time
- add slurry water to mixture
- using mixing bowl with gypsum particles
- mixing faster and vigorously
what can be added to alter expansion, setting time, dimesional stability etc.
- accelerators
- retarders
- colour pigments
- filler particles
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)
Properties of jade stone
- uniform, larger, dense particles
- little water
- organized crystal growth
- strong and smooth (10% porosity)
- high setting expansion (for type V)
