Chemistry Reference
In-Depth Information
slaked lime sets into a hard, stony material in two consecutive stages. The
first stage lasts only a few hours after the addition of water to the quick-
lime; during this stage the excess water evaporates, which results in the
slaked lime setting into a solid, relatively hard and stable mass. The second
stage is a much longer one, often lasting many years after evaporation of
the water; during this time the already dry and hard lime absorbs carbon
dioxide from the surrounding atmosphere and is thus converted back to
calcium carbonate, that is, to limestone:
(
)
Ca OH
+
CO
→
CaCO
+
H O
2
3
2
2
calcium hydroxide
dry slaked lime
calcium carbonate
fully set lime cement
(
)
(
)
TEXTBOX
35
GYPSUM CEMENT (PLASTER OF PARIS)
When
gypsum
, a sedimentary mineral (composed of hydrated calcium
sulfate) is calcinated, most of its volatile constituent, water, evaporates and
is therefore removed. The friable material remaining after the product of
the calcination process cools down to ambient temperature is commonly
known as
plaster of Paris
(composed of calcium sulfate hemihydrate) (see
Textbox 33).
Plaster of Paris has long been used as a casting material, a cement,
and a mortar. If mixed with water, plaster of Paris forms a very soft and
pliable mixture. After a very short time, lasting only 5-8 minutes, the wet,
pliable mixture
sets
, that is, it hardens into a stable, firm solid. The setting
process entails the incorporation of water molecules (a process known as
hydration
) into the calcium sulfate hemihydrate and the consequent for-
mation and crystallization of hydrated sulfate of calcium. In other words,
when water is added to plaster of Paris, the two combine, again forming
gypsum, which soon crystallizes into a hard solid mass:
⋅
⋅
CaSO
1
2
H O
1
1
2
H O
→
CaSO
2H O
+
4
2
2
4
2
plaster of Paris
calcium sulfate
hemihydrate
gypsum
hydrated calcium
sulfate
⎛
⎝
⎞
⎠
⎛
⎝
⎞
⎠
