gypsum Flashcards
what does the hemihydrate dissolves to form in the setting of dental stone
highly saturated CaSO4
when water is removed from calcium sulphate dihydrate, what does it form
B- hemihydrate
which is more soluble near room temp, hemihydrate or dihydrate
hemihydrate
what are 3 factors that can allow you to control setting time
solubility of hemihydrate, no. of nuclei( impurities), rate of crystal growth
what are some factors influencing setting time
- particle size
- impurities
- water/powder ratio
- mixing
- accelerators and retarders
what are some examples of accelerators for setting time to decrease
Nacl (<20%), Na2SO4(<12%), K2SO4
how does accelerators work
increases rate of dissolution of hemihydrate–> saturated solution form faster and crystallises rapidly
why is there setting expansion
outward thrust of growing crystals against each other
what results from expansion
increasing porosity
how to control setting expansion
change crystalline form of dihydrate, mkae initial rate of crystallisaiton rapid, so that subsequent growht is resisted
what are some factors influencing setting expansion
- water powder ratio
- accelerators and retarders both decrease expansion
- mixing time decrease, expansion decrease
what are some factors influencing strength
- water powder ratio
- porosity
- spatulation time
- accelerators and retarders
how does a high water powderr ratio cause a decrease in strength
low density of crystal strength, particles cannot interlock, mechanical properties decrease
why does a decrease in spatulation time causes an increase in strength
more crystals, more interlocking
what are the effects of increased water on setting time and hygroscopic expansion
increases setting time, decreases hygroscopic expansion
