Civil Engineering Reference
In-Depth Information
while fossil meal mixed with clay needs to be fired to a temperature of 600°C to
be mixed with slaked lime. Higher temperatures reduce the reactivity of the lime.
Very few experiments have been undertaken with this cement.
Calcined clay/calcined lime
Most clays react with lime after they are calcined. Clays to be used as pozzolana
must be calcined to sintering level, which is usually around 550-650°C. Firing
time is about half an hour, but the reactivity and viability of different types of
clay varies. All ceramic clays are suitable for pozzolana. Clay and lime cements
are used today in several parts of Asia. In India this cement is called Surkhi, and
consists of lime ground with pulverized brick. It is weaker than Portland
cement, but has better waterproof properties and has been used widely in dam
building.
Blast furnace slag/calcined lime
The starting point for a reactive blast furnace slag is granulation. The glowing
slag is tipped into a vessel filled with cold water. It is then ground into powder
and mixed with calcined lime. An alternative is a mixture with dolomite calcined
at 800°-900°C which also works well. The strength of slag and lime cements is
good, but the mixture cannot be stored for long periods and must therefore be
used shortly after production.
Hydraulic lime
Hydraulic lime is produced from natural limestone containing 6-20 per cent clay
impurities. The firing is done in the same way as with lime. After hydraulic lime
is mixed with water, it begins to set in the air. It will also eventually set underwa-
ter, and can be used for casting underwater in the same way as hydraulic cement.
The strength in this concrete is from about half to two-thirds that of normal
Portland cement.
Portland cement
The main constituent of Portland cement is lime, which is 1.7-2.2 parts for each
part of the other substances. The limestone is broken up and ground with quartz
sand and clay or just clay:
CaO + SiO
2
+ Al
2
O
2
+ Fe
2
O
2
(4)
The content of sulphur compounds must not be more than 3 per cent. Water is
added during grinding so that it becomes a slimy gruel. Next it is is fired in kilns
at 1400°-1500°C and sintered to small pellets called cement clinker.
