Gypsum Products (about setting time) Quiz 2 Flashcards
What is guaging water?
the amount of water mixed with the powder
Guaging water includes the water needed for what two things?
water needed for the setting reaction and that needed to wet the powder particles
What 3 things determine guaging water amount?
1) particle size aka therefore total surface area of powder particles
2) particle size distribution
3) adhesion between particles (surfactants)
particle size distribution determines?
packing efficiency of the powder
surfactants are?
surface active agents used to reduce the water (gauging water) requirements
Purpose of surfactants?
reduce the water requirements
what happens to hemihydrate in water?
it dissolves
What happens to dihydrate in water? What does this allow?
it precipitates out. This allows for more hemihydrate to dissolve
What allows for the material to thicken when added to water?
dihydrate precipitates out and hemihydrate dissolves. As the mass hardens, needle like clusters are formed
What can you use excess water for?
1) form a useable slurry
2) act as a lubricant to reduce strength in the SET product
3) make a porous final product (pores left behind when water evaporate)
Why is a wet gypsum product less strong?
some of its material is still in water (less crystals). SO when water finally evaporates, more crystals form reinforcing those already present
What is the water/powder ratio? Important why?
# of milliliters of water per 100 grams of powder *important in determining physical properties of the set gypsum product
increase W/P results in increased or decreased strength and setting time?
LONGER setting time (direct relationship) and LOWER strength (indirect relationship)
typical W/P ranges for plaster, stone and die stone?
plaster: 0.48-0.50
stone: 0.28-0.30
die stone: 0.25-0.30
What is the theoretical W/P?
0.19
Closer to 0.0 is best
What is steps in the setting process
1) mixing time
2) working time
3) loss of gloss
4) setting time
5) ready for use
mixing time
time from addition of powder to water until mixing is completed. 1 minute for hand mixing or 20-30 for machine
working time
time available to use WORKABLE mix. About 3 min
Loss of Gloss? How long does it take?
caused by excess water removed from the surface during formation of the dihydrate. Usually 9min from start of MIXING time
setting time
time elapsed from the beginning of mixing until the material completly hardens, 13-20 minutes
When is a gypsum material “ready for use”?
varies from batch to batch but between 30-60 minutes
What are 7 things that control setting time?
1) impurities
2) fineness
3) W/P ratio
4) Mixing
5) Temperature
6) retarders
7) accelerators
Impurities
the presence of gypsum particles in stone or plaster act as nuclei of crystallization, thereby reducing setting time
What is fineness and what does it affect?
How small a powder particle is. Afiner particle size allows the powder to go into solution faster, crystallization to occur sooner (Shorter setting time)
Setting time is altered by W/P ratio how?
more water means fewer nuclei of crystallization per unit of volume. Setting time is increased
Setting time is altered by mixing how?
the longer and more rapidly the plaster is mixed, the shorter the setting time
Setting time is altered by temperature how?
setting time is slowed (retarded) at temps above 120F; stops around 212
What do retarders do? Explain how
increase setting time. act by absorbing to the surface of the hemihydrate powder and reducing its solubility
examples of retarders?
citrates, acetates, borates
Purpose of accelerators?
To shorten setting time by inreasing solubility of hemihydrate
The accelerator must NOT affect?
the solubility of dihydrate
typical accelerators?
K2SO4, NaCl, slurry water, increasing speed or mixing time
according to theoretical calculations, a net linear ____ of 2% should occur during setting of gypsum
shrinkage
In reality there is expansion
In reality, there is no shrinkage of gypsum during setting. There is?
expansion. caused by unimpeded growth of crystals
increased expansion occurs with what W/P?
lower W/P ratios
Hygroscopic expansion? What does this do to expansion?
if gypsum sets under water, the expansion is MORE THAN DOUBLE
is there a chemical different between air expansion or hygroscopic expansion?
No. purely physical
Why is there more hygroscopic expansion?
crystallites are not drawn together by surface tension like they are in air
hygroscopic expansion is most important in?
investments, which are used for fabrication of cast restorations
unimpinged crystal growth causes _______ setting expansion
increased
