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Gypsum Product Types — MFDS Part 1 MCQ

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ModerateDental MaterialsGypsum Product TypesMFDS Part 1

Commercial dental gypsum Types I–V are principally based on calcium sulphate hemihydrate, although additives may modify their handling. Compared with Type II model plaster, Types III–V dental stones generally require less mixing water and produce stronger casts. Which underlying material difference best explains this pattern?

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Correct answer: D — Denser, more regular hemihydrate particles with lower porosity and water demand

The correct answer is D. Type II plaster is predominantly beta-calcium sulphate hemihydrate, with relatively irregular, porous particles that require more gauging water. Dental stones are predominantly alpha-hemihydrate forms with denser, more regular particles and therefore need less water to obtain a workable mix. Less excess water leaves fewer pores after drying, producing a denser and stronger cast; water/powder ratio also influences crystal interaction and setting expansion. The essential calcium sulphate hemihydrate chemistry is not distinguished by a different calcium-to-sulphate ratio. Colour is mainly identification and does not confer strength. Setting time may be modified by additives but does not alone explain the systematic strength difference. Cost is a commercial consequence rather than a defining material property.

Reference: Otani H, Adachi M, Morino Y. Effect of porosity on strength and hardness of dental stone. 1978. https://pubmed.ncbi.nlm.nih.gov/282381/