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Name: Mike
Status: Teacher
Grade: 9-12
Location: South Africa
Country: South Africa
Date: June 2006

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?

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.


Bob Wilson.

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