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Manufacture of Portland cement
From the definition of Portland cement given
above, it can be seen that it is made primarly
from a calcareous material, such as limestone or
chalk, and from alumina and silica found as clay
or shale. Marl, a mixture of calcareous and argillaceous
materials, is also used. Raw materials for
the manufacture of Portland cement are found in
nearly all countries and cement plants operate all
over the world.
The process of manufacture of cement consists
essentially of grinding the raw materials,
mixing them intimately in certain proportions and
burning in a large rotary kiln at a temperature of
up to about 1450 °C when the material sinters and
partially fuses into balls known as clinker. The
clinker is cooled and ground to a fine powder,
with some gypsum added, and the resulting
product is the commercial Portland cement so
widely used throughout the world.
Some details of the manufacture of cement
will now be given, and these can be best followed
with reference to the diagrammatic representation
of the process shown in
Fig. 1.1
.
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129
Fig. 1.1. Diagrammatic representation of: (
a
)
the wet process and (
b
) the dry process of
manufacture of cement
The mixing and grinding of the raw materials
can be done either in water or in a dry condition;
hence the names ‘wet’ and ‘dry’ processes. The
actual methods of manufacture depend also on
the hardness of the raw materials used and on
their moisture content.
Let us consider first the wet process. When
chalk is used, it is finely broken up and dispersed
in water in a washmill; this is a circular pit with
revolving radial arms carrying rakes which break
up the lumps of solid matter. The clay is also
broken up and mixed with water, usually in a similar
washmill. The two mixtures are now pumped
so as to mix in predetermined proportions and
pass through a series of screens. The resulting cement
slurry flows into storage tanks.
When limestone is used, it has to be blasted,
then crushed, usually in two progressively smaller
crushers, and then fed into a ball mill with
the clay dispersed in water. There, the comminution
of the limestone (to the fineness of flour) is
completed, and the resultant slurry is pumped into
storage tanks. From here onwards, the process
is the same regardless of the original nature of the
raw materials.
The slurry is a liquid of creamy consistency,
with a water content of between 35 and 50 per
cent, and only a small fraction of material – about
2 per cent – larger than a 90
μ
m (No. 170 ASTM)
sieve size. There are usually a number of storage
tanks in which the slurry is kept, the sedimentation
of the suspended solids being prevented by
mechanical stirrers or bubbling by compressed
air. The lime content of the slurry is governed by
the proportioning of the original calcareous and
argillaceous materials, as mentioned earlier. Final
adjustment in order to achieve the required chemical
composition can be made by blending slurries
from different storage tanks, sometimes using
an elaborate system of blending tanks. Occasionally,
for example in the world’s northernmost
130
plant in Norway, the raw material is a rock
of such composition that it alone is crushed and
no blending is required.
Finally, the slurry with the desired lime content
passes into the rotary kiln. This is a large,
refractory-lined steel cylinder, up to 8 m (or 26
ft) in diameter, sometimes as long as 230 m (or
760 ft), slowly rotating about its axis, which is
slightly inclined to the horizontal. The slurry is
fed in at the upper end while pulverized coal is
blown in by an air blast at the lower end of the
kiln, where the temperature reaches about 1450
°C. The coal, which must not have too high an
ash content, deserves a special mention because
typically 220 kg (500 lb) of coal is used to make
one tonne of cement. This is worth bearing in
mind when considering the price of cement. Oil
(of the order of 125 litres (33 US gallons) per
tonne of cement) or natural gas were also used,
but since the 1980s most oil-fired plants have
been converted to coal, which is by far the most
common fuel used in most countries. It is worth
noting that, because it is burnt in the kiln, coal
with a high sulfur content can be used without
harmful emissions.
The slurry, in its movement down the kiln, encounters
a progressively higher temperature. At
first, the water is driven off and CO2 is liberated;
further on, the dry material undergoes a series of
chemical reactions until finally, in the hottest part
Ca(OH)2, some minor components, unhydrated
cement, and the residue of the water-filled spaces
in the fresh paste. These voids are called capillary
pores but, within the gel itself, there exist interstitial
voids, called gel pores. The nominal diameter
of gel pores is about 3 nm while capillary pores
are one or two orders of magnitude larger. There
are thus, in hydrated paste, two distinct classes of
pores represented diagrammatically in
Fig. 1.8
.
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