Classification, Activity, Effects and Applications of Puzzolanas

1. INTRODUCTION TO PUZZOLANAS

Capture of Silica Fumes

The term puzzolana is derived from Puzzouli, a town in Italy on the Bay of Naples near Mount Vesuvious. The sand (volcanic dust) around this town when mixed with hydrated lime was found to possess hydraulic properties. Puzzolana may be defined as a siliceous material which whilst itself possessing no cementitious properties, either processed or unprocessed and in finely divided form, reacts in the presence of water with lime at normal temperatures to form compounds of low solubility having cementitious properties. Puzzolanas may be natural or artificial, fly ash being the best known in the latter category. Before the advent of cement these were used with lime to make concrete. Currently its principal use is to replace a proportion in cement when making concrete. The advantages gained are economy, improvement in workability of concrete mix with reduction of bleeding and segregation. Other advantages are greater imperviousness, to freezing and thawing and to attack by sulphates and natural waters.In addition the disruptive effects of alkali-aggregate reaction and heat of hydration are reduced. It is generally held that the addition of natural puzzolanas reduce the leaching of soluble compounds from concrete and contributes to the impermeability of the concrete at the later ages. 

The main justification for using puzzolanas is the possibility of reducing costs. If they are to reduce costs, they must be obtained locally and it is for this reason that they have not so far been much in use.

2. CLASSIFICATION OF PUZZOLANAS

Volcanic Tuff

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.

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

3. ACTIVITY OF PUZZOLANAS

Fly ash

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

C₃S + H₂O ➟ C-S-H + Ca(OH)₂

C-S-H = Calcium silicate hydrate

Ca(OH)_{2 =} Lime

Lime produced combines with silica of puzzolana

Ca(OH)₂  + SiO₂ ➟ C-S-H

SiO₂ = Lime

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.

Rice husk ash

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.

4. EFFECTS OF NATURAL PUZZOLANAS

• On heat of hydration: The heat of hydration of a puzzolana is same as that of low heat cement.
• On strength of concrete: 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.
• On shrinkage & moisture movement: It is similar to Portland cement.

 5. APPLICATIONS

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.