Pozzolanas: rice husk ash and pulverised fuel ash
Small incinerators have a number of advantages: they are simple and inexpensive to
construct, easy to operate and will produce ash of an acceptable quality. On the other hand,
their output is rather small. They also require constant supervision and, perhaps most
importantly, they make no use of the energy value of the husk.
Weight for weight rice husk has an energy value about half that of coal and is therefore an
important potential energy source. Although rice husk is still burnt as waste, this practice is
likely to become less common, as other more traditional fuel sources become less readily
available and/or more expensive. Recently attempts have been made to design kilns or
furnaces for husks which will utilize the potential energy value of the husk by making it avail-
able for useful work, and which control the temperature of combustion to below 700°C.
The transportation of rice husk is not an economically viable option, even over quite short
distances, due to its low bulk density and the fact that only 20% of its weight can be utilized
as a pozzolana. The location of incinerators or kilns has to be close to a rice mill with
sufficient capacity to supply the quantity of husk required for cement production.
The second step in processing is grinding the RHA to a fine powder, and ball or hammer mills
are usually used for this purpose. Crystalline ash is harder and will require more grinding in
order to achieve the desired fineness.
A fineness similar to or slightly greater than that of OPC is usually recommended for
pozzolanas although some have been ground considerably finer. The minimum fineness
recommended by the Indian Standards for pozzolana (1344) is 320 and 250m2/kg for grade
1 and 2 pozzolanas respectively, measured by the Blaine air permeability test. Although this
standard is for calcined clay, the fineness requirements are also suitable for RHA.
Pulverised fuel ash
PFA is a residue from the process of combustion in the boilers of coal fired power stations. It
is extracted as a fine powder from the flue gasses and hence its other common name 'fly
ash'. The ash extracted from the bottom of power station boilers, furnace bottom ash, is less
suitable as a pozzolana.
There are two types of PFA, depending upon the type of coal used. These are high lime and
low lime, with the former having a lime content above 10% and therefore possessing some
cementing properties on its own. Low lime PFA has a lime content below 10%. Both types of
PFA can be used as a pozzolana.
PFA is available, in large quantities, in countries or regions using coal fired electricity
generating stations. These include most of Europe, North America, the Indian sub-continent,
China and southern Africa.
The chemical composition of PFA will depend upon the type of coal used and can vary
considerably, as can pozzolanic reactivity. Table 2 gives typical compositions of British, US
and Indian PFA’s.
Acceptable limits of composition, derived from the various national standards are:
The percentage of the main oxides, SiO2 + Al2O3 + Fe2O3, should not fall below 70%;
The SO3 content should not exceed 5% (Some Standards specify 2.5%);
The MgO content should not exceed 5%;
The loss on ignition should not exceed 12% (Some Standards give a 5 or 6% limit).
In addition some standards specify that the alkali metal (Na2O) content should not exceed
1.5 %, although this is only relevant if it is used with reactive aggregate.
Physically PFA is a fine (less than 75 micron) powder, with a rounded particle shape and a
colouring ranging from cream to dark grey. Its loose bulk density is approximately 800kg/m3,