Chemical Properties
white or slightly off-white powder or granules
Definition
ChEBI: Calcium sulfate hemihydrate is a calcium salt and a hydrate.
Purification Methods
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.]
General Description
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).
Industrial uses
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.
