Pozzolanas - Calcined clays and shales, and volcanic ash
Practical Action
Regular testing is therefore required if volcanic ash is to be used as a pozzolana and this has
been a restraint on its commercial exploitation. However, volcanic ash is, or has recently
been, successfully used as a pozzolana in many countries including the US, Germany, Japan,
Italy, Kenya and Indonesia, with pilot plants tested in Tanzania and Rwanda. For example,
200,000 tonnes of volcanic pozzolana were used in the construction of the Glen Canyon dam
in the US, completed in 1964.
Volcanic pozzolanas usually have chemical composition within the limits shown on table 2.
Processing of volcanic ash
Once the deposits have been excavated most volcanic ashes will require only minor
processing before use as a pozzolana. Many ashes are only loosely cemented and can easily
be excavated by hand, although others may need mechanical or pneumatic equipment.
Some lithic tuffs may require blasting with explosives. The ash may require drying, and in dry
sunny climates this can simply be achieved by spreading the ash in a thin layer on a
specially prepared drying floor, similar to those commonly used to dry crops. Alternatively, in
wet climates, and for large quantities, inclined rotary driers are normally used.
If the ash is cemented it will need to be crushed before entering the dryer. Some volcanic
ashes will already be in a very fine, loose powdered form and may not require crushing or
grinding. Other ashes may be of sufficient fineness but be cemented together. These will
require milling or crushing. Coarse ashes and lithic tuffs will need to be ground in a ball mill
or similar.
Utilization of pozzolanas
A fineness similar 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 (IS 1344. 1981. Calcined clays) is 320 and 250m2/kg
for grade 1 and 2 pozzolanas respectively, measured by the Blaine air permeability test.
Once the pozzolana has been ground, it must be blended with lime and/or OPC to produce a
pozzolanic cement. This can be accomplished by human or animal-powered methods but full
homogeneity is unlikely to be achieved and the strength and consistency of cements blended
in this manner will be variable. Mechanical techniques, preferably intergrinding in a ball mill
or, as a second option, dry blending in a pan or concrete mixer, will give better results in
terms of both strength and consistency.
Pozzolanas can be used with either lime and/or OPC. With the latter, replacement of up to
25-30% is common, although research has suggested that for non-structural purposes
replacement of up to 50% can be used. With lime pozzolana cements mixtures of 1:1 to 1:4
(lime:pozzolana) by weight are used. The addition of 5-10% of OPC will improve strength
and decrease setting times. A larger percentage of OPC may be required if only poor quality
pozzolanas are available. The exact ratio of the ingredients will depend upon the quality of
the respective raw materials and on the required characteristics of the concrete or mortar
made from the cement.
A good calcined clay or volcanic ash pozzolana should produce a cement, when mixed with a
good quality lime, capable of producing concretes and mortars with 7 and 28 days strengths
in excess of 2 and 4 mega pascals (MPa) respectively. Pozzolana strength development is
slow and long-term strengths should be considerably higher, as much as 15MPa by 2 years.
Strength development can be accelerated if up to 4 per cent of fine gypsum is added to the
lime-pozzolana mix.
Other leaflets in this series deal in more detail with the production, properties and utilization
of pozzolana cements, and with testing and performance standards.
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