Pozzolanas: An introduction
Practical Action
Ash from agricultural wastes
Many plant ashes have a high silica content and are therefore suitable as a pozzolana. In
recent years considerable research has gone into identifying plant wastes whose ashes
produce good pozzolanas and which are available in exploitable quantities. Rice husk, a
waste product of rice milling has been shown to have the greatest potential:
it is available in large quantities in many parts of the world.
when burnt, it produces a large quantity of ash - about one tonne for every five
tonnes of husk.
the ash typically contains approximately 90 per cent silica and is therefore an
excellent pozzolana.
The disadvantage of rice husk is that for its ash to be highly pozzolanic it has to be burnt
under controlled conditions at temperatures below 700°C, otherwise the silica becomes
crystalline and loses a degree of reactivity. Rice husk ash has been utilized as a pozzolana in
cement production in Asia, particularly India, and is under research in rice growing areas of
Africa. Other agricultural wastes which have been identified as having potential as a
pozzolana include rice straw and bagasse.
Other pozzolanas
Other sources of pozzolanas worth mentioning include shale, diatomite, bauxite and blast
furnace slag. Shale requires similar treatment to clay, as does bauxite, although the latter's
temperature of calcination is lower at between 250 and 300°C. Diatomites are usually highly
reactive but are rarely used as pozzolanas because they require a large quantity of water to
produce a plastic mix, due to their porous nature. Blast furnace slag is a by-product of the
iron and steel industry which has a limited pozzolanic reactivity but has been very successful
when blended with OPC.
Processing and production
Some pozzolanas require calcining to activate their reactivity and the best results are
obtained from calcining in conditions where the temperature is controlled to the optimum for
that particular pozzolana. In most cases, calcining can be undertaken in a simple and
inexpensive purpose-built kiln.
Materials which have been calcined or burnt for other purposes at non-optimum
temperatures can also be utilized, although a drop in reactivity may have to be accepted. The
use of reject bricks and pottery, and rice husk ash from a boiler are examples of this.
Unless the pozzolana is already in a fine powdered form (as is the case with PFA and some
volcanic ashes), it should be ground in a ball or rod mill. The greater the amount of milling,
the finer the pozzolana will become and the rate of reaction will increase. The benefits of
increased fineness must however be balanced against the additional cost of grinding. The
pozzolana and cement and/ or lime must be mixed as thoroughly as possible. This is best
achieved by intergrinding in a ball mill for a short period. If this is not possible, dry mixing in
a concrete mixer should produce reasonable results. Less efficient methods of mixing may be
acceptable but the strength and consistency of the resultant cement is likely to be adversely
affected. ,
Pozzolanas can be used beneficially with either lime and/ or OPC. With the latter,
replacement of up to 50 per cent is possible. With lime pozzolana cements, mixtures of 1:1
to 1:4 (lime: pozzolana) are used depending upon the quality of the lime and pozzolana, and
the end-use of the cement. The addition of between five and ten per cent of OPC in a lime-
pozzolana mixture will increase its strength and decrease its setting time. A higher
percentage addition of OPC may be necessary if the pozzolana is of poor quality .The
addition of up to four per cent of powdered gypsum accelerates the strength development of
some lime-pozzolana mixes.
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