Pozzolanas - Calcined clays and shales, and volcanic ash
Practical Action
slightly below those for clay bricks or tiles and therefore better results are likely to be
obtained if the moulding and firing process are designed specifically for pozzolana
production. The Indian Standard (IS1344 1981) gives the following range of temperatures
for different types of clays:
Montmorillonite type 600 to 800°C
Kaolinite type
700 to 800°C
Illite type
900 to 1000°C
In practice, most clay soils consist of a mixture of minerals, and a calcination temperature of
700-800°C is normally considered suitable. The optimum period of calcination will vary with
clay type but is normally around one hour or less.
Rotary kilns have been the most common means of calcining clays and have been extensively
used in the US and Brazil. Natural gas or oil is normally used as a fuel and outputs vary from
12.5 to 100 tonnes per day.
Several Indian institutions, including the Central Road Research Institute and the National
Building Organization, have undertaken studies to design and test kilns specifically for clay
pozzolana production with production rates of between 5 and 20 tonnes per day. Two of the
kilns are coal fired and are designed on the natural down-draught and forced air vertical shaft
concepts similar to those commonly used, respectively, in the ceramics and lime industries.
A third kiln utilizes the fluidized bed system. Dried and pulverized clay is allowed to fall
through a series of fins against an updraft of hot gases generated by oil burners. Although
the contact time is extremely short, of the order of a few minutes, it is apparently sufficient
to calcine the ground clay feed. The cost of production, despite the cost of using oil as a
fuel, is reported to be competitive with other methods.
Grinding
The second step in processing is the grinding of the calcined clay to a fine powder. On a
small scale, this has traditionally been performed with human or, more commonly, animal-
powered methods. Ball mills are more suited to large-scale applications. Some calcined
clays, such as kaolin, will be softer than others and will therefore require less grinding in
order to achieve the desired fineness.
Volcanic ash
Deposits of volcanic ash are likely to be found wherever there are active or recently active
volcanoes, for example in the Mediterranean, the Pacific region, and central and eastern
Africa. The physical condition of volcanic ashes may range from loose fine material to coarse
deposits containing quite large particles. Deposits may be loose, with an appearance and
texture similar to a compacted coal or wood ash. Other deposits are cemented, sometimes
with appearance and properties similar to stone, and in this form they are normally referred
to as tuffs or trassy. The colour of deposits can vary from off-white to dark grey.
Constituents
Contents by weight
Silica
Alumina + Iron Oxide
Calcium + Magnesium Oxide + alkalis
Loss on ignition
45-65%
15-30%
Up to 15%
Up to 12%
Table 2: Composition of volcanic ash suitable for use as a pozzolana
The pozzolanic reactivity of ash deposits can vary considerably. The quality of material may
also vary within a single deposit or a single geologically consistent stratum, with variations in
depth being common.
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