Gypsum Flashcards
Key compound in gypsum
Calcium sulfate
Unset material: calcium sulphate hemihydrate
Set material: calcium sulphate dihydrate
What is type 1 gypsum
Impression plaster
What is type 2 gypsum
Model plaster
What is type 3 gypsum
Denture dental stone
What is type 4 gypsum
Dental stone high strength low expansion
What is type 5 gypsum
Dental stone high strength high expansion
What determines compressive strength of gypsym
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)
Does gypsum have high flexural strength
No. Brittle
Does gypsum have good dimensional stability
Yes
Is gypsum soluble in water
Set plaster (Type 1 and 2) slightly soluble in water
What is the water/powder ratio of dental plaster
0.5
Powder of stone vs plaster
Powder of stone is more regular and uniform is shape, is less porous
What is dental stone used for
Casts for diagnostic purposes, for complete and partial denture construction
What is the water/powder ratio for stone
0.3
What is the equation for setting reaction of gypsum
Ca(so4)2.h2o +3 h2o —> 2 caso4.2h2o + heat
Dihydrate become hemihydrate!
How does particle size affect setting time of gypsum
Finer particles, more nuclei, more rapid crystallisation, increase setting rate, shorter setting time
How do impurities affect setting time of gypsum
Impurities poison nuclei, fewer nuclei present, lower crystallisation rate, increased setting time
How does water powder ratio affect setting time of gypsum
Increased water/powder ratio, increase setting time
How does mixing affect setting time of gypsum
Increased mixing, break up dihydrate crystals at early stages of setting to produce more nuclei, decrease setting time
Why is it important to wash blood off impression before pouring cast
Blood is a colloid which acts as a retarder, would increase setting time
What is the setting expandion of plaster
0.2-0.3%
What is the setting expansion of stone
0.08-0.1%
Why is there setting expansion of gypsum
Outward thrust of crystals growing against each other, outward growth from nuclei
How does water/powder ration affect setting expansion
More water, less expansion because fewer nuclei of crystallisation per unit volume
How does mixing time affect setting expansion
Increased mixing time, more nuclei in given volume, increase outward thrust, increase setting expansion
What is hygroscopic expansion
Gypsum material soaked in water during setting process to increase setting expansion of casting investments
Is wet or dry strength of gypsum greater
Dry strength 2 to 3 times greater
Difference in manufacture of plaster vs stone
Plaster is heated up in open system while stone is heated up in closed system with chemicals
How must gypsum products be stored
Airtight waterproof containers in dry region since they absorb moisture which will cause gradual deterioration
