Civil Engineering Reference
In-Depth Information
Vertical shaft kilns or rotating kilns can be used, but the rotating kiln is domi-
nant in the industry. Rotating kilns, at their most efficient, yield 300-3000 tons a
day; shaft kilns produce 1-200 tons a day. Modern shaft kilns have a higher effi-
ciency and certain functional advantages, such as low energy consumption
(Spence, 1980).
After firing, the mass is ground again and usually a little finely ground glass
or gypsum is added to regulate setting. Pure Portland cement is seldom used
today - it is usually mixed with lime or pozzolana.
Portland pozzolana cements
Pozzolanas also react with lime in Portland cement, resulting in cements that not
only use less energy in production but also have higher strength and elasticity. In
fossil meal/Portland cement, fossil meal is mixed in a proportion of 20-30 per
cent. In calcined clay/Portland cement, clay is mixed-in, in a proportion of 25-40
per cent.
Industrial pozzolanas can also be used. For the production of blast furnace
slag/Portland cement, the slag is granulated and ground with Portland cement
in a proportion of 1-85 per cent. So-called Trief-cement consists of 60 per cent
slag, 30 per cent Portland cement and 2 per cent cooking salt. It is usually rec-
ommended to use far less slag - preferably under 15 per cent. Fly ash/Portland
cement has about 30 per cent ground in fly ash. The same proportions are used
if mixing with industrial silicate dust, microsilica.
Blast furnace slag often slightly increases radioactive radiation from the mate-
rial. Particles of poisonous beryllium can be emitted from fly ash, and easily-sol-
uble sulphates can leach out from pollute waste and the ground water.
Lime/cement mortar
Lime/Portland cement is made by grinding larger or smaller amounts of slaked
lime or hydrated lime into Portland cement. This mixing can also take place on
the building site. The mix has a better elasticity than normal Portland cement,
both during use and in the completed brickwork.
Non-hydraulic binders
Lime
Lime reacts as a binder with carbon dioxide in the air to form a stable compound.
Ca(OH) 2 + CO 2 = CaCO 3 + H 2 O
(5)
This reaction is exothermic in the same way as slaking, in that the energy used in
firing is now released. It takes a long time for the lime to set, and the process is
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