CEMENT MANUFACTURING

Cement-making process (Credits: Eureka)
Step 1: extraction of raw materials
The raw materials needed to produce cement
(calcium carbonate, silica, alumina and iron
ore) are generally extracted from limestone
rock, chalk, shale or clay. These raw materials
are won from the quarry either by extraction
or through blasting. These naturally occuring
minerals are then crushed through a milling
process. At this stage, additional minerals are
added to ensure the correct chemical
composition to make cement is in place. These
minerals can be obtained from waste or by-
products of other industries, such as paper
ash. After milling, the raw meal (as it is
known) is transported to the plant where it is
stored.
CO2 and cement
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Step 2: raw grinding and burning
Grinding produces a fine powder, known as
raw meal, which is preheated and then sent to
the kiln. The kiln is at the heart of the
manufacturing process. Once inside the kiln,
the raw meal is heated to around 1,500°C - it
is of a similar temperature to molten lava. At
this temperature, chemical reactions take place
to form cement clinker, containing hydraulic
calcium silicates.
In order to heat the materials to this very high
temperature, a 2,000°C flame is required,
which can be produced through the use of
fossil and waste-derived fuels. The kiln itself is
angled by 3° to the horizontal to allow the
material to pass through it, over a period of
between 20-30 minutes.
Upon exiting, the clinker is cooled and stored
ready for grinding to produce cement.
Industrial ecology in the UK
Inside a kiln
Step 3: cement grinding and shipping
A small amount of gypsum (3-5%) is added to
the clinker to regulate how the cement will set.
The mixture is then very finely ground to
obtain ‘pure cement'. During this phase,
different mineral materials, called ‘additions',
may be added alongside the gypsum. Used in
varying proportions, these additions, which are
of natural or industrial origin, give the cement
specific properties such as reduced
permeability, greater resistance to sulfates and
aggressive environments, improved
workability, or higher-quality finishes.
Finally, the cement is stored in silos before
being shipped in bulk or in bags to the sites
where it will be used.