Pozzolanic Materials (Pozzolanas) and their Uses

Pozzolanas are materials containing reactive silica which in themselves posses little or no cementitious value but which combines with lime in finely divided form in the presence of water to produce cementing compounds.

The name pozzolana was given to such material in honour of the place Puzzolini in Italy where it was found that Roman had built hydraulic structures by mixing lime and the volcanic ash, thus producing hydraulic lime which can set under water.

Pozzolanic Materials:

Pozzolanic Materials are classified into two groups:

  1.  Natural Pozzolanas such as volcanic ash
  2. Artificial Pozzolanas such as fly ash.
In India, we do not have many deposits of natural Pozzolanas. The commonly used pozzolanic materials are:

1. Surki:

Surki was once very popular in use and readily available in India. Nowadays it is not readily available. Ordinary Surki is made by grinding well burn broken brickbats, tiles etc. Reactive surki, which has superior lime combining properties is obtained by calcining selected clays within optimum temperature range and grinding the product into a fineness of 90 % passig through IS Sieve no.15

2. Flyash: 

Flyash is finely divided ash resulting from the burning of pulverized coal or lignite in boilers. The flyash obtained from lignite is considered superior because of its higher lime content. Flyash is a waste product and today, it is available in all parts of India.

3. Ground Blast Furnace slag:

From slow cooling of the slag obtained from blast furnace, we get crystalline material which can be crushed and used as aggregates. Rapid cooling of slag produces amorphous glassy pellets with about 35 % silica which when grind to fine powder can form a pozzolanic material. It can chemically react with free lime to produce cementing materials.

4. Rice Husk Ash: 

If rush husk is burnt, the resulting ash contain about 80 % silica varying with the type of rice husk. If it is burnt with free access to atmosphere, then the resulting silica is crystalline. However, if it is burnt under controlled condition with limited access to to air on temperature of about 700 C for 2 to 3 hours, then the silica produced is non-crystalline. Such burnt rice husk ash has very good pozzolanic properties.

Uses of Pozzolans:

  1. In making of  Portland Pozzolana Cement:  Pozzolana materials are mixed upto 35% inn finely ground form as a mixture in ordinary portland cement to manufacture PPC.
  2. As replacement upto 20% of fine aggregates in lime or cement mortar.
  3. Mixed as a fine powder to fat lime to produce hydraulic lime.