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Gypsum Setting Expansion — MFDS Part 1 MCQ

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HardDental MaterialsGypsum Setting ExpansionMFDS Part 1

Two batches of the same Type IV dental stone are vacuum-mixed using water-to-powder ratios within the manufacturer's recommended range. Batch 1 has the lower water-to-powder ratio and demonstrates greater net linear setting expansion. Measurements are made without contact with additional water and are corrected for temperature change. Which mechanism best explains this difference?

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Correct answer: E — Intercrystalline thrust as growing dihydrate crystals impinge on one another

The correct answer is E. Calcium sulfate hemihydrate dissolves and calcium sulfate dihydrate crystals nucleate and grow. As these crystals interlock and impinge on neighbouring crystals, crystallisation pressure produces an outward thrust and measurable setting expansion. A lower water-to-powder ratio places more crystals within a given volume, increasing crystal impingement and therefore expansion. Thermal effects cannot explain the result because the measurements were corrected for temperature. Hygroscopic expansion requires contact with additional water, which the stem excludes. Gas evolution is not a feature of the gypsum hydration reaction, and vacuum mixing further reduces incorporated air. Contraction of residual water would not produce the characteristic crystallisation-related expansion.

Reference: Alberto N, Carvalho L, Lima H, et al. Characterization of different water/powder ratios of dental gypsum using fiber Bragg grating sensors. Dental Materials Journal. 2011;30(5):700-706. https://pubmed.ncbi.nlm.nih.gov/8621042/