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Gypsum plaster, or plaster of Paris, is produced by heating __________ to about 300 °F.

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Gypsum plaster, or plaster of Paris, is produced by heating __________ to about 300 °F.

(a) gypsum

(b) clinker

(c) calcium

(d) lime

I got this question in an interview for job.

My doubt is from Analysis of Rates of Plastering and Pointing in division Analysis of Rates of Civil Engineering Drawing

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Right option is (a) gypsum

The explanation is: CaSO4•2H2O + heat → CaSO4•0.5H2O + 1.5H2O (released as steam).

When the dry plaster powder is mixed with water, it re-forms into gypsum. The setting of unmodified plaster starts about 10 minutes after mixing and is complete in about 45 minutes; but not fully set for 72 hour. If plaster or gypsum is heated above 266 °F (130 °C), hemihydrate is formed, which will also re-form as gypsum if mixed with water.

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