Milled Plaster of Paris

name         Mike
status       Teacher
grade        9-12
location     South Africa

Question -   Once Plaster of Paris (hemihydrate) has set (as the
dihydrate), can the hemihydrate be obtained again, i.e. by milling the
set materials and dehydrating it by heating?  If so, are there any guide
lines as to the suitable fineness (particle size) of the milled
materials and the dehydration conditions to be used?  Will the resultant
hemihydrate (if obtained) be as good as the original in using again as
Plaster of Paris, and if so how many times can this recycling and reuse
of Plaster of Paris be continued?>
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Hi Mike,

Plaster of Paris (gypsum hemihydrate) is in fact created
industrially by heating fully hydrated gypsum (the
dihydrate). Typically the dihydrate must be heated moderately
above the boiling point of water; usually about 120°C to 170°
for a specific time to drive off excess water of
crystallization and result in the hemihydrate. Since Plaster
of Paris that has "set" is the same thing a gypsum, the
results will be the same, whether heating gypsum, or "set"
Plaster of Paris.... the hemihydrate will result.

So in answer to your question, Plaster of Paris can be
"recycled" endlessly by the above method. As to how finely it
needs to be reground, I suspect that the hemihydrate has
essentially no structural strength, and thus will crumble
easily to powder, so significant effort to regrind it is
probably not needed.

Regards,

Bob Wilson.
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