Pozzolana: Advantages and its properties

 

Pozzolana

The term pozzolana is derived from Pozzuoli, 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.

Pozzolana may be defined as a siliceous material which 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.

Pozzolanas 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.

 

THE ACTIVITY OF P0ZZOLANA -

When mixed with ordinary Portland cement the silica of the pozzolana combines with the free lime released during the hydration of cement. This action is called pozzolanic action. The pozzolanic 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 pozzolana 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

                                   ^{(Calcium silicate hydrate)                 (Lime)}

Lime produced combines with silica of pozzolana

Ca(OH)2    +    SiO2  → C-S-H

                                        ^(Silica)

Since pozzolanic action can proceed only in the presence of water, enough moisture has to be made available for a long time to complete pozzolanic action. The optimum amount of pozzolana, as replacement for cement, may normally range between 10-30% and may be as low as 4-6% for natural pozzolanas. It may be somewhat higher for some fly ashes.

 

EFFECTS OF NATURAL PUZZOLANAS -

• On heat of Hydration: the heat of hydration of a pozzolana is same as that of low heat cement.

• On strength of concrete: when pozzolanas 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 pozzolana 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 and moisture movement: It is similar to Portland cement.

APPLICATIONS -

 Pozzolana 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 pozzolana causes considerable advantage in the lean harsh mixes normally used in the construction of mass concreting. The examples are dams, retaining walls, wharf walls, breakwaters, harbor works and massive foundations.

ADVANTAGES OF ADDITION OF POZZOLANAS -

The main advantage of adding pozzolanas to cement and fat lime are the following:

1. Economy: Materials like fly ash are waste products and are cheap. Hence, as much as 10 to 25 percent of costly cement can be replaced by pozzolanas.

2. Reduces Permeability of Concrete: As these materials are very fine when mixed to make concrete, they fill up the pore of concrete fully. Moreover, all the free lime in cement reacts with these materials, thus forming a dense mass. In this case it is considered as an admixture.

3. Addition to Fat Lime: Addition of pozzolanas to lime allows the formation of cementing compounds by chemical action. This can convert fat lime into a hydraulic lime. Traditionally. surkhi was used in India to convert fat lime into hydraulic lime and this was used for construction of hydraulic works.

PROPERTIES AND SPECIFICATIONS FOR USE IN CONSTRUCTION -

When crushed or powdered bituminous coal or lignite is burnt in boilers of industrial plants like powerhouses, finely divided ash escape with the fuel gases 80% of the total ash is finely divided and by suitable technologies they are collected. This material is known as fly ash. It is also known as pulverised fuel ash. The ash collected from bottom of boilers is known as bottom ash. Large quantities of fly ash are produced in India as a waste material and the Indian Government has given special incentives for its utilization. As fly ash contains reactive silica it can be used as pozzolana. It can also be used as an admixture to concrete. There are three ways it is used namely:

(i) Mixed with clinker and ground to produce PPC

(ii) Mixed with cement as a pozzolana to produce PPC.

(iii) Added to mortar and concrete as an admixture.

 

SI 3812 deals with the use of pulverised full ash. It was published in 1981 in 3 parts but in 2003 it has been published as second revision in two parts. Part I deals with the specification for its use as an admixture to cement mortar and concrete. There are many chemical and physical requirements specified in this IS code. The main physical requirements the uses are following:

1. Fineness- The minimum specific surface by Blane's apparatus should be 320 m²/kg for use as a pozzolanas and 200 m²/kg for or admixture compared to the value of 225 m²/kg required for cement.

2. Particle Size - The maximum percentage of particles retained on 45 micron IS sieve (wet sieving) should be 34 for use as a pozzolana and 50 for use as an admixture.

3. Lime Reactivity- Minimum average compression strength in 100 g on three 7 cm mortar cubes, made with one part of hydrated lime, two parts of fly ash and three parts of standard sand by weight, should not be less than 4.5 N/mm2 for use of fly ash as a pozzolana.

4. Soundness by Autoclave Test- One testing a mixture of 4 parts of ordinary portland cement and one part of fly ash by weights, the increase in volume should not be more than 0.8 percent in both cases.

5. Compressive Strength with Cements - With a mixture of ordinary portland cement and fly ash, the compressive strength should not be less than 80% of the strength of corresponding plain cement mortar cubes.