Calcium sulfate hemihydrate (CaSO4 • 0.5H2O) is the dehydrated product of gypsum, calcium sulfate dihydrate (CaSO4•2H2O). It is obtained by heating gypsum to approximately 150°C. It is known as gypsum plaster or plaster of Paris (POP). On adding water, it solidifies by an exothermic reaction into hydrated calcium sulphate. It is commonly used to precast and hold parts of ornamental plasterwork placed on ceilings and cornices. Plaster of Paris casts can be used as a treatment of fractures to immobilize the limb. It is also used as a desiccant, and in medicine as a tablet excipient.
CI 77231 is classed chemically as an inorganic colour.
white or slightly off-white powder or granules
Gypsum originates from a supersaturated aqueous solution in the shallow sea, which first evaporates and deposits carbonates, then sulphates and finally chlorides. Gypsum and anhydrite are almost always
found in the Permian, Triassic, and Tertiary formations. The Permian includes the Upper Permian (Zechstein deposits, Germany). The Triassic consists of Lower (Buntsandstein), Middle (Muschelkalk), and Upper Triassic (Keuper).
The Tertiary deposits include the Eocene (Spain,
Persia, Near East), the Oligocene (Paris), and the
Miocene deposits (Mediterranean area, North
africa). The Jurassic and Cretaceous formations,
which lie between the Triassic and Tertiary, are
almost devoid of useful gypsum deposits, as are
the older pre-Permian formations – at least in the
Old World – as, for example, the Carboniferous
formation. The accepted view is that gypsum, CaSO4
· 2 H2O, was formed upon initial crystallization.
The best known deposits of primary gypsum are
those of the Paris Basin and the areas around the
Mediterranean. Under increased pressure and
thus temperature, anhydrite could have developed locally from the original gypsum.
However, uptake of surface water can convert
the anhydrite back into gypsum. This gypsum is
therefore a secondary rock, especially in formations older than the Middle Tertiary.
The various gypsum and anhydrite deposits
differ in purity, structure, and color. The major impurities are calcium carbonate (limestone),
dolomite, marl, clay; less frequently, silica, bitumen, glauberite, syngenite, and polyhalite. Gypsum can be pure white. If it contains iron
oxide, it is reddish to yellowish. If clay and/or bitumen is present, it is gray to black. Very pure anhydrite is bluish white, but usually it is gray with
a bluish tinge. The white veins sometimes found
on the boundary between gypsum and anhydrite
consist largely of glauberite (CaSO4 · Na2SO4)
or Glauber’s salt (Na2SO4 · 10 H2O).
ChEBI: Calcium sulfate hemihydrate is a calcium salt and a hydrate.
Gypsum is a naturally hydrated calcium sulfate. It can also be easily synthesized in a partially hydrated or anhydrous form. The partially hydrated form is called hemihydrate calcium sulfate, while the anhydrous form is called anhydrite (Jorgensen 1994). In the past, a gypsum fiber product made of long, strong, and thin fibers that had good reinforcing properties was used. These fibers were nonabrasive and stable up to a temperature of 1,000°C. They were also stable in mild acids and bases (Virta 1994).
Gypsum is the most common sulfate mineral,characterized by the chemical formulaCaSO4 · 2H2O; it shows little variation from thiscomposition.
Gypsum is one of the several evaporite minerals.This mineral group includes chlorides,carbonates, borates, nitrates, and sulfates. Theseminerals precipitate in seas, lakes, caves, andsalt flats due to concentration of ions by evaporation.When heated or subjected to solutionswith very large salinities, gypsum converts tobassanite (CaSO4.H2O) or anhydrite (CaSO4).Under equilibrium conditions, this conversionto anhydrite is direct. The conversion occursabove 42°C in pure water.
Gypsum is used for making building plaster,wallboard tiles, as an absorbent for chemicals,as a paint pigment and extender, and forcoating papers. Natural gypsum of California,containing 15 to 20% sulfur, is used for producingammonium sulfate for fertilizer. Gypsumis also used to make sulfuric acid by heatingto 1093°C in an air-limited furnace. Theresultant calcium sulfide is reacted to yield limeand sulfuric acid. Raw gypsum is also used tomix with portland cement to retard the set.Compact massive types of the mineral are usedas building stones.
Gypsum
is slightly soluble in water. Gypsum is readily
soluble in glycerol and sugar solutions and in
aqueous solutions of chelating agents, such as
EDTA.
Its solubility in H2O is 0.2parts/100 at 18.75o. It dehydrates completely >650o. Dry it below 300o to give a solid with estimated pore size ca 38% of volume. Anhydrous CaSO4 (Drierite) has a high affinity for H2O and will absorb 6.6% of its weight of H2O to form the hemihydrate (gypsum). It sets to a hard mass with H2O; hence it should be kept in a tightly sealed container. The solubility of gypsum in H2O is unusual: 0.176% at 0o, 0.209% at 30o, 0.210 at 40o, 0.204 at 50o and 0.200 at 60o. [Hulett J Am Chem Soc 27 49 1905, James & Partington J Chem Soc 107 1019 1915, Namba J Soc Chem Ind 40 2797 1920.]
