Types of Cement
There are various types of cement which are available in market are described herewith.
- Portland Pozzolona Cement
Pozzolonas are siliceous or alumino siliceous materials,which in themselves posses little or no cementations properties,but in finely divided form and in presence of moisture (water),chemically react with Ca(OH)2 liberated on hydration at ordinary temperature and forms compounds having cementations properties.
Pozzolona as defined as a material other than cement , water and aggregates and is used as one ingredient of concrete. Pozzolona is added to the batch immediately before or during mixing.
Additive is a material which is added at the time of grinding cement clinker at the cement factory.
Portland,Pozzolona cement is manufactured by intergrinding of OPC clinker with 15% to 35% of pozzolonic materials. Fly ash is a waste material generated in thermal power station,when powdered coal is used as a fuel. These are collected in the Electrostatic Precipitator(ESP).
The pozzolonic action is shown below,
Ca(OH)2+Pozzolona+Water——> C-S-H Gel.
Portland Pozzolona cement produces less heat of hydration and offer great resistance to the attack of impurities of water than OPC.Moreover it reduces the leaching of Ca(OH)2 when used in hydraulic structure.
It is important to note that the addition of pozzolona does not contribute to the strength at the early ages. If PPC is manufactured with right type of reactive ,pozzolonic material,PPC will not be in any way inferior to OPC, except for the rate of development of strength up to 7 days. It is only when inferior pozzolonic materials, which are not reactive type and which does not satisfy the specification limits for pozzolonic materials are used ,the cement would be of doubtful quantity.
Technically PPC has considerable advantages over OPC when made by using optimum percentage of right quality of Fly Ash.
Advantages of PPC
- Economically as costly clinker is by cheaper pozzolonic material.
- Soluble Ca(OH)2 is converted into insoluble cementations products resulting in improvment of permeability.It therefore, improves durability characteristics, particularly in hydraulic and marine construction.
- PPC consumes Ca(OH)2 and does not produce as much as that of OPC.
- It generates reduced heat of hydration and that too at a low rate.
- PPC being finer than OPC, it improves the pore size distribution.
- With reduction of permeability ,PPC offers many other advantages.
- Fly ash being finer and of lower density, the bulk volume of 50 Kg bag is slightly more than OPC. Therefore PPC gives more volume of mortar than OPC.
- The long term strength of PPC beyond a couple of month ,is higher than OPC, if enough moisture is available for continued pozzolonic action.