Pozzolanas - Calcined clays and shales, and volcanic ash
Practical Action
However this large-scale utilization has declined in the last three decades, due to the
availability of pozzolanas which require less processing and are therefore cheaper, such as
volcanic ash and pulverized fuel ash. Where these are not available, the use of calcined clay
still has considerable potential. Although sandy clays are often used as a pozzolana,
frequently in the form of crushed fired clay bricks, the coarser sand is not reactive. The
pozzolanic activity resides in the finer clay mineral fraction, and sandy clays may not
produce the best pozzolanas. Despite their variable pozzolanic performance, the use of
ground underfired or reject bricks and tiles as a pozzolana is likely to continue on a small
scale due to the low cost of these waste materials.
Plastic clays, as used in tile manufacture or for pottery, can produce better pozzolanas,
although the composition of good pozzolanic clays is variable. Table 1 gives the chemical
composition (on an oven-dry basis} of some clays in India which produce pozzolanas
conforming to the Indian Standard for calcined clay pozzolana (IS 1344.1981).
Constituents
Contents by weight
Silica + Alumina + Iron Oxide
Not less than 70%
Silica
Not less than 49%
Calcium Oxide
Not more than 10%
Magnesium Oxide
Not more than 3%
Sulphur trioxide
Not more than 3%
Water soluble alkali
Not more than 0.1%
Water soluble material
Not more than 1%
Loss on ignition
Not more than 10%
Table 1. Chemical composition of clay suitable for use in calcined clay pozzolanas
Raw materials for
calcined clay
pozzolanas should be
free from coarse sand
or gravel greater than
about O.6mm in
diameter.
In tropical climates,
clay deposits are
often subjected to a
form of chemical
weathering which
leaches out the silica
and alkalis resulting
in a accumulation of
ferric and aluminium
hydroxides. The soils
Figure 2: Volcanic ash pozzolana being excavated, Rwanda.
produced are bauxitic
(aluminium bearing) and lateritic (iron bearing). Although these soils are low in silica,
normally considered essential for a pozzolanic reaction, both will exhibit some pozzolanic
reactivity when calcined. In general, the reactivity of laterite is low but bauxite can show
reasonable results and its use as a pozzolana should be considered if silica-bearing clays are
not available.
Processing of calcined clays and shales
Calcining
The early stages of processing calcined clay pozzolanas are similar to the moulding and firing
process for clay bricks, tiles or pottery, and traditionally the rejects from these industries
have been used as pozzolanas. The optimum calcining temperatures for clay pozzolanas are
2