Researchers are trying to reduce carbon dioxide emissions in the manufacture of cement by finding alternatives to limestone
PORTLAND cement may acquire a
new, environmentally friendly,
chemical composition. Researchers
are focussing on how to reduce emissions from cement manufacture and
also use less energy. Another area of
research that has emerged recently
examines the use of bauxite (aluminiuni oxide) and wastes such as fly ash,
scrap iron and blast furnace slag in
cement manufacture.
Large amounts of carbon dioxide
and nitrous oxides are produced in
manufacture of cement. Della M Roy
of the Materials Research Laboratory
of Pennsylvania State University estimates 30 per cent of limestone used
in cement production becomes carbon dioxide. Emissions can be
reduced by lowering temperatures at
which different cement varieties are
manufactured, but this requires altering the composition of cement.
Belite cement, for example, uses
gypsum and bauxite to replace 10 per
cent of the. limestone. This has
reduced energy use by 16 per cent
and carbon dioxide emissions by 10
per cent. But a disadvantage is that it
takes longer to set than Portland
cement. This can be solved by adding
superplasticizers that reduce the
quantity of water needed to mix
belite.cement into concrete.
Alinite cement, developed in the
former Soviet Union in 1977, also
contains less limestone and uses calcium chloride, which makes constituents react at a lower temperature. Its drawback is that the chlorine
from calcium chloride corrodes steel
and other metals used in reinforced
concrete. 16 use is, therefore, limited
to unreinforced concrete or foundations.
Portland cement is
manufactured in a
rotary kiln. If the kiln
is replaced by a fluidised - bed reactor,
energy use would be
cut by more than one-third and construction
costs by one-fourth in a
plant producing 330
tonnes of cement per
day. Although fluidised bed reactors are
not new, they have
been adapted by the
cement industry only
recently.
Whether produced
in a rotary kiln or a flu- Ulm
idised bed reactor,
cement has Ito be
ground to a fine powder. Grinding can be
improved if the conventional ball mill is
complemented with a
roller mill as the latter has a greater
contact area on the grinding surface
than a ball mill. Therefore, smaller
ball-and-roller mills that use less
electricity can be used.
The disposal of fly ash is a major
problem in India. Industries and
power plants in the country produce
40 million tonnes of this fine powder
every year. While some companies
use fly ash as landfill material, disposal remains a vexatious process.
Fly ash does not require to be ground
and can be used instead of clay in the
manufacture of cement. Fly ash ran
also be used in concrete - a mixture
of cement, stones, sand and water.
Says Roy, "Fly ash can replace up to
50 per cent of the cement in concrete
without any noticeable fall in
strength. Such a mixture does, however, take longer to set than concrete
without fly ash."
Blast furnace slag is used to manufacture another type of cement
called belite-sulphoaluminate
cement. "This type of cement does
not save energy but has desirable
characteristics such as no shrinkage
on drying, no efflorescence
and a short setting time. It is used for
glass fibre, reinforced concrete and
precast concrete in Japaii," says
Uchikawa.
Indian cement manufacturers,
who have yet to introduce new varieties of cement developed abroad,
use the dry process in which the constituents of cement react in a dry
kiln. While this process uses much
less energy than those in Which the
constituents are moistened and made
to react, it is more polluting as it is
far dustier.
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