1.

Gypsum plaster, or plaster of Paris, is produced by heating __________ to about 300 °F.(a) gypsum(b) clinker(c) calcium(d) limeI 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

Answer»

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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