Building Materials, Third Edition

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—— All puzzolanas are rich in silica and alumina and contain only a small
quantity of alkalis. Following are some of the naturally occurring puzzolanas:

1. Clays and shales which must be calcined to become active.


2. Diatomaceous earth and opaline cherts and shales which may or may not need calcination
   (most active).


3. Volcanic tuffs and pumicites. Fine grained ashes form better puzzolana. However, tuffs—
   solidified volcanic ash—may be ground to desired fineness for use.


4. Rhenish and Bavarian trass.

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—— Some of the examples of artificial puzzolanas are:

1. Fly ash


2. Ground blast-furnace slag


3. Silica fume


4. Surkhi


5. Rice husk ash

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When mixed with ordinary Portland cement the silica of the puzzolana combines with the free
lime released during the hydration of cement. This action is called puzzolanic action. The
puzzolanic activity is due to the presence of finely divided glassy silica and lime which
produces calcium silicate hydrate similar to as produced during hydration of Portland cement.
The silica in the puzzolana reacts with the lime produced during hydration of Portland cement
and contributes to development of strength. Slowly and gradually additional calcium silicate
hydrate is formed which is a binder and fills up the space, gives impermeability, durability and
ever increasing strength.
Hydration of Portland cement may be expressed as
C 3 S + H 2 O
C-S-H + Ca(OH) 2
(Calcium silicate hydrate) (Lime)
Lime produced combines with silica of puzzolana
Ca(OH) 2 + SiO 2
C-S-H
(Silica)
Silicas of amorphous form react with lime readily than those of crystalline form and this
constitutes the difference between active puzzolanas and materials of similar chemical
composition which exhibit little puzzolanic activity. Since puzzolanic action can proceed only
in the presence of water, enough moisture has to be made available for a long time to complete
puzzolanic action.
It is commonly thought that lime-silica reaction is the main or the only one that takes place,
but recent information indicates that alumina and iron if present also take part in the chemical
reaction.
The optimum amount of puzzolana, as replacement for cement, may normally range between
10-30% and may be as low as 4-6% for natural pouzzolanas. It may be somewhat higher for
some fly ashes.