·
Linear Formula CaSO4 ·
2H2O
·
Molecular Weight 172.17
Calcium sulfate (or calcium sulphate) is the
inorganic compound with the formula CaSO4 and related hydrates.
In the form of γ-anhydrite (the anhydrous form), it is used as a desiccant.
Detailed Description:
One particular hydrate is
better known as plaster of Paris, and another occurs naturally as the
mineral gypsum. It has many uses in industry. All forms are
white solids that are poorly soluble in water.[6] Calcium sulfate causes permanent hardness in water.
Uses
The main use of calcium sulfate is to produce
Plaster of Paris and stucco. These applications exploit the fact that calcium
sulfate forms a moldable paste upon hydration and hardens as a hemihydrate. It
is also convenient that calcium sulfate is very poorly soluble in water, so
structures do not dissolve.
Hydration and dehydration reactions
With judicious heating, gypsum converts to the
partially dehydrated mineral called calcium sulfate hemihydrate, calcined
gypsum, and plaster of Paris.This
material has the formula CaSO4·(nH2O), where 0.5
≤ n ≤ 0.8.[9] Temperatures
between 100 °C and 150 °C (302 °F) are required to drive off the
water within its structure. The details of the temperature and time depend on
ambient humidity. Temperatures as high as 170 °C are used in industrial
calcination, but at these temperatures γ-anhydrite begins to form. The heat
energy delivered to the gypsum at this time (the heat of hydration) tends to go
into driving off water (as water vapor) rather than increasing the temperature
of the mineral, which rises slowly until the water is gone, then increases more
rapidly. The equation for the partial dehydration is:
The endothermic property of
this reaction is relevant to the performance of drywall, conferring fire
resistance to residential and other structures. In a fire, the structure behind
a sheet of drywall will remain relatively cool as water is lost from the
gypsum, thus preventing (or substantially retarding) damage to the framing (through combustion of wood members or loss of strength of steel at high temperatures) and consequent structural collapse.
But at higher temperatures, calcium sulfate will release oxygen and act as
an oxidizing agent.
This property is used in aluminothermy. In contrast to
most minerals, which when rehydrated simply form liquid or semi-liquid pastes,
or remain powdery, calcined gypsum has an unusual property: when mixed with
water at normal (ambient) temperatures, it quickly reverts chemically to the
preferred dihydrate form, while physically "setting" to form a rigid
and relatively strong gypsum crystal lattice:
This reaction is exothermic and is
responsible for the ease with which gypsum can be cast into various shapes
including sheets (for drywall), sticks (for
blackboard chalk), and molds (to immobilize broken bones, or for metal
casting). Mixed with polymers, it has been used as a bone repair cement. Small
amounts of calcined gypsum are added to earth to create strong structures
directly from cast earth, an alternative
to adobe (which loses its strength when wet). The
conditions of dehydration can be changed to adjust the porosity of the
hemihydrate, resulting in the so-called alpha and beta hemihydrates (which are
more or less chemically identical).
On heating to 180 °C, the nearly water-free
form, called γ-anhydrite (CaSO4·nH2O where n =
0 to 0.05) is produced. γ-Anhydrite reacts slowly with water to return to the
dihydrate state, a property exploited in some commercial desiccants. On heating above
250 °C, the completely anhydrous form called β-anhydrite or
"natural" anhydrite is formed.
Natural anhydrite does not react with water, even over geological timescales,
unless very finely ground.
The variable composition of the hemihydrate and γ-anhydrite,
and their easy inter-conversion, is due to their possessing nearly identical
crystal structures, containing "channels" that can accommodate
variable amounts of water, or other small molecules such as methanol.
Other uses
Up to the 1970s, commercial quantities of sulfuric acid were produced
from the anhydride of calcium sulfate. Upon being mixed with shale or marl, and roasted, the
sulfate liberates sulfur dioxide gas, a
precursor in sulfuric acid production,
the reaction also produces calcium silicate, a precursor
in cement production.[11]
When sold as a color-indicating variant under the name
Drierite, it appears blue or pink due to impregnation with cobalt(II) chloride, which functions as a moisture indicator.
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