Chemistry Reference
In-Depth Information
deposits on the surface of the earth or in bituminous or asphalt rock, from
which it is extracted. When used as a building material, to join and consol-
idate or to cover stone or brick, bitumen is applied only after it is heated and
turns into a hot, viscous liquid. In the molten state it penetrates into the
pores, cracks, and holes of stones and bricks, cementing and consolidating
them together and providing an effective and lasting cementing and/or cov-
ering material. Bitumen is highly waterproof; it is unharmed by most acids
and salts, and thus prevents weathering of the structures it covers (Serpico
and White 2000a; Marschner and Wright 1978).
Lime and Gypsum Cements
The two other main types of human-made building cements,
lime cement
and
gypsum cement
, have been and still are used in many areas of the world. Both
these cements require quite elaborate thermal procedures for producing their
main components, which are
slaked lime
in lime cement and
plaster of Paris
in
gypsum cement. Making them involves the calcination of an appropriate
type of stone, a process that has been practiced since prehistoric times.
Slaked lime is made by the calcination of limestone; plaster of Paris, by the
calcination of gypsum (see Textbox 33) (Coburn et al. 1990; Lea 1962).
TEXTBOX
33
CALCINATION AND ROASTING
Calcination
and
roasting
are two thermal operations that entail heating
minerals to high temperatures (above 650°C) so as to alter their composi-
tion and render them friable.
Calcination
Calcinating
a mineral removes its volatile components, such as water or
carbon dioxide and leaves an usually crumbly solid residue. Calcinated sec-
ondary minerals such as limestone are the basic components of building
cements, and in extractive metallurgy operations they facilitate the smelt-
ing of metals. Calcinating
limestone
(composed of calcium carbonate), for
example, drives away carbon dioxide, leaving a solid, friable residue of
quicklime
(composed of calcium oxide):
heat
CaCO
→
CaO
+
CO
↑
3
2
(
)
over 650 C
°
limestone
calcium carbonate
quicklime
calcium oxide
