Page 218 - Encyclopedia of Chemical Compounds 3 Vols

CALCIUM SULFATE

Paris) has also been known and used for a very long period of
time. Archaeologists have learned that the Egyptians devel-
oped a method for converting gypsum to plaster of Paris,
which was then used as mortars to join blocks in buildings,
more than 5,000 years ago.

HOW IT IS MADE
Gypsum is an abundant mineral providing a ready nat-
ural supply of calcium sulfate. The mineral usually consists
of a mixture of the anhydrous and dihydrate forms of cal-
cium sulfate, which can be separated into its component
parts. Clay, sand, limestone, and other impurities are also
present in most gypsum deposits. The process of separation
and purification begins by crushing natural gypsum and

heating it in an open kettle to temperatures of 100 Cto

125 C (212 F to 257 F) for a few hours. Heating converts
the gypsum to the hemihydrate or anhydrous calcium sulfate
depending on the temperature of the reaction. The solubility
of the final product can also be controlled by the temperature
used in the heating container and the time during which the
material is heated.
Large amounts of calcium sulfate are also produced as
the by-product of other reactions or by synthetic processes.
The production of phosphoric acid (H 3 PO 4 ) from sulfuric acid
(H 2 SO 4 ) and phosphate rock, for example, results in the for-
mation of calcium sulfate, which can be recovered and pur-
ified. Calcium sulfate can also be produced in the reaction
between other calcium compounds, such as calcium hydro-
xide (Ca(OH) 2 ) and sulfuric acid.

COMMON USES AND POTENTIAL HAZARDS
The most widely used form of calcium sulfate is the
dihydrate, gypsum, which is an important raw material in
the construction industry. It is used in the manufacture of
Portland cement, in specialized plasters (known as gypsum
plasters) for walls, in the production of wallboard, and in
cement blocks and mortars. Gypsum is also used extensively
in agriculture as a conditioning agent that adds both cal-
cium ions (Ca2+) and sulfate ions (SO 4 2-) to the soil. The
compound is also used as a raw material in the synthesis of
other calcium compounds and in the production of plaster
of Paris.

CHEMICAL COMPOUNDS 167