Building Materials, Third Edition

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X The heat of hydration of a puzzolana is same as that of low heat cement.

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™X When puzzolanas are used the addition of an air entraining agent
may enable a reduction in the amount of water than if the air entraining agent was added to
concrete containing cement only. This may lead to an increase in strength and consequently
less cement may be permitted for the same strength. At early ages the replacement of cement
by a puzzolana usually results in a decrease in the compressive strength, but the difference
becomes less and may disappear at ages of 3 months or more.

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X It is similar to Portland cement.

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Puzzolana finds its chief application where the reduction in the heat of hydration is of great
importance and the slower rate of gain in strength is not of much conscience, i.e., where mass
concreting is to be done. Also, the improvement in workability obtained by using puzzolana
causes considerable advantage in the lean harsh mixes normally used in the construction of
mass causes concreting. The examples are dams, retaining walls, wharf walls, breakwaters,
harbour works and massive foundations. Lime-puzzolana mixtures are used for masonry
mortars, plasters and for foundation concrete. Their types and physical requirements are given
in Appendix 1.

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Fly ash or pulverized fuel ash (PFA) is the residue from the combustion of pulverized coal
collected by mechanical or electrostatic separators from the flue gases or power plants. It
constitutes about 75 per cent of the total ash produced. The properties and composition of fly
ash vary widely, not only between different plants but from hour to hour in the same plant. Its
composition depends on type of fuel burnt and on the variation of load on the boiler. Fly ash
obtained from cyclone separators is comparatively coarse and contains a large proportion of
unburnt fuel, whereas that obtained from electrostatic precipitators is relatively fine having a
specific surface of about 3500 cm^2 /g and may be as high as 5000 cm^2 /g. Normally it is rather
finer than Portland cement. Fly ash consists generally of spherical particles, some of which may
be like glass and hollow and of irregularly shaped particles of unburnt fuel or carbon. It may
vary in colour from light grey to dark grey or even brown.
Carbon content in fly ash is important consideration for use with cement; it should be as low
as possible. The fineness of fly ash should be as high as possible. The silica contained in fly ash
should be present in finely divided state since it combines slowly over a very long period with
the lime liberated during the hydration of the cement. Curing at a temperature of 38°C has
been found to greatly accelerate its contribution to the strength of concrete. Curing at high
pressure and temperature in autoclave promotes the reaction between the lime liberated
during hydration of cement and the silica in the fly ash. However, this reaction should tend to
prevent the release of free lime to reduce efflorescence.
Fly ash is supplied in two grades; grade I and grade II. There general use is incorporating it
in cement mortar and concrete and in lime pozzolana mixture. However, only grade I is
recommended for manufacture of Portland pozzolana cement.