Guides for gypsum powder and building plaster plant investors and factory managers: calcination, calciners, alpha and beta plaster, grinding, standards and plant planning.
A gypsum powder test report records how a sample of plaster performed in the laboratory: combined and free water, phase composition, fineness, water demand, setting time and strength, each with its method. A useful report also identifies the sample, the lot it represents and the rule used to pass it. Without the method and the sample, the numbers cannot be compared.
Gypsum powder bulk density, measured loose, is usually around 600–900 kg/m³ for building plaster. Published tests on commercial products give 594, 636, 693 and 899 kg/m³, and recycled plasters come out lower. The same powder packs denser when tapped and much lighter when aerated, so a plant designer needs more than one figure.
A gypsum powder packing machine fills fine plaster into valve bags through a spout, usually 25 kg each, and weighs every fill as it goes. Impeller, air and screw packers differ in how they push the powder into the bag. How well a line works depends on three things: weighing accuracy, the bag and its valve, and dust control at the spout.
Gypsum plaster is made by grinding calcined gypsum to the required fineness, then batch-mixing it with retarder, water-retention agent, lime, perlite or limestone in measured doses. The blend goes through intermediate bins to a bag packer or bulk silo. Calcination makes the binder, but this dry-mix section decides how the plaster pumps, spreads, sets and stores.
A gypsum plaster retarder delays setting by interfering with the growth of gypsum crystals. Producers mostly use citric or tartaric acid, their salts, or protein hydrolysates. Published doses are small: about 0.04–0.06% in one machine-plaster formulation. Too much cuts strength, so the dose is set by trial on your own stucco, against a measured setting time.
Phosphogypsum can be used for building plaster and gypsum board, but only after treatment and a radiological check. Its soluble phosphate, fluoride and acidity upset setting and strength, so it is usually sieved, washed and neutralized with lime before it is dried and calcined. Washing does not remove radium-226, so the material must also meet building-material radioactivity limits such as China’s GB 6566.
A gypsum plant grinds in two places. Before calcination, roller mills (pendulum or vertical) or hammer mills grind dry rock to about 90% finer than 149 µm for kettles and fluidized beds. After calcination, ball, tube, impact or pin mills grind plaster to the fineness a product needs. Heated mills can also dry the gypsum, or dry, grind and calcine it in one unit.
An alpha gypsum powder manufacturing plant converts gypsum to hemihydrate in saturated steam or hot water under pressure, not in dry air. Lump or pressed gypsum is autoclaved at roughly 110–180 °C, or fine gypsum is reacted as a slurry with crystal modifiers. The wet alpha is then dewatered, dried while still hot so it cannot rehydrate, and ground to a controlled fineness.
EN 13279 is the European standard for gypsum binders and gypsum plasters. Part 1 (EN 13279-1:2008) defines the product types (binders A1–A3, plasters B1–B7 and special plasters C1–C7) and sets requirements for binder content, setting time, strength, adhesion and reaction to fire. Part 2 (EN 13279-2:2014) gives the test methods. A revision of Part 1 has been drafted, but BSI still lists the 2008 edition as current.
Plaster of Paris is made by crushing and grinding gypsum, then heating it to roughly 120–165 °C so that it loses about three-quarters of its chemically bound water and becomes calcium sulfate hemihydrate. The hot plaster is cooled, aged, ground or classified, blended with additives such as retarders, and packed or sent to a board or block line.
