Gypsum Products Flashcards
What does the heating reaction of gypsum rock result in during the manufacture of gypsum products?
Formation of hemihydrate, which is the basis of all gypsum used in dentistry.
What does calcium sulfate and dihydrate turn into during the heating reaction?
calcium sulfate and hemihydrate
What is the main difference between plaster, stone and high strength stone?
The manner of driving off the water
What are the three types of material derived from the partial dehydration of gypsum rock?
plaster, stone and high strength stone
What is the precursor to model/lab stone?
Plaster
What is the precursor to dental stone?
Hydrocal
What is the precursor to high strength stone?
Densite
What is the setting reaction of gypsum products?
Nucleated recrystallization, characterized by a continuous solution of hemihydrate, diffusion of calcium and sulfate ions to nucleation centers, and precipitation of microscopic gypsum crystals
Regarding water/powder ratio, what is the effect of less water?
Less porosity
What is the effect of less porosity?
More strength
What happens to a thick mix (low W/P ratio)?
It hardens more quickly b/c nucleation centers are concentrated in a smaller volume. The interaction of the growing solid occurs earlier and is more effective
What is the effect of porosity?
Tends to weaken the mass
What is the result of a decrease in W/P ratio?
Denser cast
What is the result of raising W/P ratio?
Lower strength (more porosity)
What is gypsum used as in materials commonly employed in casting of dental gold alloys?
A binder
Gypsum is generally not heated above what temperature?
700 degrees C
Is gypsum used for base metals or other high melting alloys?
No
What does Refractory provide in gypsum bonded investments?
Provides expansion to compensate for gold shrinkage
What are the benefits of a binder?
Used to form a coherent solid mass. Provide strength for the investment, Calcium sulfate hemihydrate (gold). Phosphate, ethyl silicate (high temp alloys)
What binder is used in gold investments?
Calcium sulfate hemihydrate
What binder is used in high temp alloys?
Phosphate, ethyl silicate
What are the forms in which silica exists?
Alpha - a low temperature phase
Beta - a high temperature phase
What are the effects of increasing amount of silica w/in practical limits on the refractory behavior of gypsum bonded investments?
Increased manipulation time
Increased setting expansion
Increased thermal expansion
Decreased compressive strength
