Pozzolanas: An introduction
Practical Action
The only disadvantage of these blended cements is that their early strength gain is slightly
slower. This might mean that the dismantling of formwork on structural concrete may need to
be delayed by a day or so, but this disadvantage is far outweighed by the advantages. These
technical and economic advantages are well recognized by many 'enlightened' engineers and
Portland-pozzolana blends are now commonly specified, particularly on major civil
engineering works, in both the developed and developing world.
Composition and chemistry of pozzolanas
The chemical composition of pozzolanas varies considerably but the following can be taken as
a rough guide.
Silica + Alumina + Iron Oxide - Not less than 70%
Other Oxides and alkalis - Not more than 15%
Loss on ignition - Not more than 15%
Of the active oxides, silica is normally considered to be the most important and should not
normally fall below 40 per cent of the total, indeed some of the best pozzolanas have silica
contents above 90%.
Carbon is a common constituent in pozzolana and its content should be as low as possible,
below 12 per cent is normally recommended. Plant ashes will often have higher carbon
contents, unless the airflow on combustion is carefully controlled. Higher carbon contents
can be tolerated but will result in lower strength cements.
Types of pozzolana
Calcined clays
Calcined or burnt clays were the earliest pozzolanas used, in the form of crushed reject clay
bricks, tiles, or pottery which were mixed with lime to produce a cement for mortars. This
process is still used in many countries today and is known as surkhi in India, homra in Egypt
and semen merah in Indonesia. In general, the best clays for pozzolanas will be plastic clays
of the type normally used for pottery and clay tile manufacture. The optimum calcining
temperature is normally taken to be between 700 and 800°C, although for some clay types it
may be higher or lower than this.
Volcanic ash
Deposits of volcanic ash or tuffs 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 natural state of these deposits varies considerably as does their
pozzolanic reactivity. Volcanic ashes normally require no heating to enhance pozzolanic
reactivity and, if they are already in a powdered state, may need little or no grinding.
Volcanic ash pozzolanas are commercially exploited in many countries, for example in
Germany, Italy, Kenya, Rwanda, Vanuatu and Indonesia. Other volcanic materials such as
pumice may also be pozzolanic.
Pulverized fuel ash (PFA)
Pulverized fuel ash, often referred to as fly ash or PFA, is probably the pozzolana in greatest
use globally today. In 1976, it was estimated that some 30 million tonnes were used
annually and that the annual increase in usage was about ten per cent. PFA is the residue
from the combustion of pulverized coal in power stations and is essentially a waste material.
PFA is already in a fine powdered form and requires no further processing for use as a
pozzolana and this, combined with its availability in bulk and low cost, makes it ideal for
blending at cement factories or at large civil engineering projects. Its reactivity is not as high
as other commonly used pozzolanas and it is, therefore, less frequently used in combination
with lime.
3