The porous and irregular crystal form of CaSO4·12H2O, commonly known as dental plaster or type II gypsum product.
[hemi-, half, + hydrate]

alpha-hemihydrate (al´fə-hem´ēhī´drāt),

n a physical form of the hemihydrate of calcium sulfate (CaSO4)-H2O; dental artificial stone.
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The slurry process calcines a mixture of gypsum and ligno-cellulosic or other materials under elevated temperature and pressure to generate an unusually uniform composite mixture of alpha-hemihydrate gypsum and fibers.