SETTING REACTIONS

According to the crystaline theory , the difference in the solubilty of calcium sulphate dihydrate and hemihydrate causes the setting of these materials

Untitled Slide

Untitled Slide

As the dihydrate is less soluble unstable condition happens leading to the formation of nuclei of crystalization (at the molecular level)
And precipitation of dihydrate molecules around it (begining of solidification)

Water/powder ratio

• 1 gram molecule of hemihydrate react with 1.5 gm molecule of water to produce 1 gram of hemihydrate
• Exess water should be added for the solution to be workable

Water/powder ratio

• Model plaster 50-60/100
• Dental stone 30/100
• High strength stone 19-22/100

Setting time

• Mixing time
• Working time
• Loss of gloss
• Initial setting time
• Final setting time

Mixing time

• The time taken from the addition of powder to the water until we obtain homogenous workable mixing
• Hand mixing = 1 minute
• Mechanical mixing=20-30 seconds

Working time

• It is the time available to use a workable mix while maintaining its even consistency
• Time available for mixing

Loss of gloss

• Some of the mixed water is taken up in forming dihydrate and the mix loses its gloss
• Can not be used(partial setting has taken place)

Initial setting time

• The time elapsed from the begining of mixing until partial setting occurs
• It takes an average of 12 minutes to reac this stage

Final setting time

• It is the time elapsed from the beginning of mixing until complete setting takes place
• After 30 minutes