What is pozzolana or pozzolanic material?
Pozzolana is a volcanic powder found in Italy near Vesuvius. A pozzolanic material can be a natural or artificial which contains silica and aluminous in a reactive form. This materials usually doesnt posses any cementitious properties, but when it is mixed with water or moisture or lime to undergo reaction with calcium hydroxide to form compounds possessing cement properties.
Fig 1: Use of PPC in marine structure.
Types of pozzolana materials :
1. Artificial pozzolana
Fly ash, silica fume, rice husk, blast furnace slag.2. Natural pozzolana
Burnt clay, pumicite, diatomaceous Earth.Manufacture of Portland pozzolana cement
- The primary raw materials used for this cement manufacture are limestone (CaCO2) and clay (SiO2,AI2,O3,Fe2O3). Rocks are loaded into trucks and transported to the crushers, where the crushed into fine particles.
- Fine particles of clay and limestone are fed into the air-swept ball mills in desired proportions as per requirement and mixed very well before it is sent to silos for storing.
- This mixture is then pre-heated upto 800-1000c where calcinations of CACO3 to CaO takes place.
- The pre heated mixture is then sent into kiln where the mixture is heated to 1450C in rotary Kiln. The modules formed from the burning process called clinker. The clinker is cooled by a rotary cooler.
- This clinker is now mixed with gypsum and pozzolana materials in the required proportion and thus the Portland Pozzolana Cement is obtained.
Properties of Portland Pozzolana Cement
- Initial setting time = 30 min (minimum)
- Final setting time = 600 min (maximum).
- At 3 days 13MPa (minimum)
- At 7 days 22 MPa (minimum)
- At 28 days 33 MPa (minimum)
- Drying shrinkage should not be more than 0.15%
- Fineness should not be less than 300 m2/kg
- Initial strength of PPC is less but final strength is equal to the 28 days strength of OPC
- PPC has lower rate of development of strength than OPC
Uses of Portland Pozzolana Cement
- Used in hydraulic structures, marine structures, construction near the sea shore, dam construction etc.
- Used in pre-stressed and post-tensioned concrete members.
- Used in masonry mortars and plastering.
- As it gives better surface finish, it is used in decorative and art structures.
- Used in manufacture of precast sewage pipes.
- Used under harsh concreting conditions.
Advantages of Portland Pozzolana Cement
- it is an eco-friendly cement as the material used in the manufacture are made of natural recycled waste.
- It is very fine cement hence very good when used for plastering works.
- Pozzolano consists of silica material which makes it cheap and hence reduces the cost of the cement making it economical to use.
- Pozzolana cement has very good resistance against sulphate attack hence is used in hydraulic structures, marine structures, construction near the sea shore, dam construction etc.
- PPC used in pre-stressed and post-tensioned concrete members.
- It reduces the carbon monoxide emission from the concrete making it environmental friendly.
- As the pozzolano materials are very fine, it can fill gaps between the reinforcement and aggregate , thus reducing the shrinkage, honeycomb formation and bleeding can be reduced, which in turn increases the strength and durability of concrete.
Disadvantages of Portland Pozzolana Cement
- The initial strength obtained is less, which effect the de-shuttering of supports early.
- As it contains more fine material, handing of concrete is difficult.
- When compared to the OPC setting time is less for PPC
- Reduction in alkanity reduces the resistance to corrosion of steel reinforcement
- As the strength of this concrete gains slowly, curing process is very important. Any error in this could cause durability problems.