POZZOLANAS
RICE HUSK ASH (RHA) AND
PULVERISED FUEL ASH (PFA)
Pozzolanas are materials containing reactive silica and/or alumina which on their own have
little or no binding property but, when mixed with lime in the presence of water, will set and
harden like a cement.
Pozzolanas are an important ingredient in the production of alternative cementing materials
to Portland cement (OPC). (See the leaflets in this series Alternatives to Portland cement -
an introduction and Pozzolanas - an introduction.)
Alternative cements provide an excellent technical option to OPC at a much lower cost and
have the potential to make a significant contribution towards the provision of low-cost
building materials and consequently affordable shelter.
Pozzolanas can be used in combination with lime and/or OPC. When mixed with lime,
pozzolanas will greatly improve the properties of lime-based mortars, concretes and renders
for use in a wide range of building applications. Alternatively, they can be blended with OPC
to improve the durability of concrete and its workability, and considerably reduce its cost.
A wide variety of siliceous or aluminous materials may be pozzolanic, including the ash from
a number of agricultural and industrial wastes. Of the agricultural wastes, rice husk has been
identified as having the greatest potential as it is widely available and, on burning, produces
a relatively large proportion of ash, which contains around 90% silica.
Pulverized fuel ash (PFA), which is often referred to as fly ash, has probably the greatest
potential of the industrial wastes due to its widespread availability in spite of its only
moderate pozzolanic reactivity. It is probably the pozzolana in greatest use today with a
worldwide estimate of over 30 million tonnes in use and an annual increase in usage of
about 10%.
Rice husk ash
About one tonne of husk is produced from five tonnes of rice paddy and it has been
estimated that some 120 million tonnes of husk could be available annually on a global
basis for pozzolana production. As the ash content by weight is about 20%, there are
potentially 24 million tonnes of RHA available as a pozzolana.
Rice is grown in large quantities in many Third World countries including China, the Indian
sub-continent, South-east Asia and, in smaller quantities, in some regions of Africa and
South America. Table 1 gives the rice production of some of the principal rice growing
countries of the world in the early 1980s. Production is likely to have more than doubled in
most of these countries by 2004.
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