By mixing 10 to 25 percent pozzolanic material with with OPC clinjer Portland Pozzolana cement (PPC) is manufactured (as per the latest amendment, it is 15 to 35%). A pozzolanic material is essentially a silicious or aluminous material which while in itself possessing no cementitious properties, which will, in finely divided form and in the presence of water, react with calcium hydroxide, liberated in the hydration process, at ordinary temperature, to form compounds possessing cementitious properties. The pozzolanic materials generally used for manufacture of PPC are calcined clay (IS 1489 part 2 of 1991) or fly ash (IS 1489 part I of 1991). Fly ash is a waste material, generated in the thermal power station, when powdered coal is used as a fuel. These are collected in the electrostatic precipitator. (It is called pulverised fuel ash in UK).
It may be recalled that calcium silicates produce considerable quantities of calcium hydroxide, which is by and large a useless material from the point of view of strength or durability. If such useless mass could be converted into a useful cementitious product, it considerably improves quality of concrete.The use of fly ash performs such a role. The pozzolanic action is shown below
Calcium hydroxide + Pozzolana + water → C – S – H (gel)
Strengths similar to those of ordinary Portland cement can be expected in general only at later ages provided the concrete is cured under moist conditions for a sufficient period.
In some countries there is apprehension in the minds of the user to use the Portland pozzolana cement for structural works. But the fear is not proper. If correct pozzoloanic materials are used to make the Pozzolana Cement it will not be inferior to the ordinary portland cement except for the rate of development of strength upto 7 days. The doubtfully quality is only can be the result of use of inferior pozzolanic materials, which are not of reactive type and which do not satisfy the specifications limit for pozzolanic materials.
Advantages:-
Technically PPC has considerable advantages over OPC when made by using optimum percentage of right quality of fly ash.
1.In PPC cost of cement is reduced by replacing costly clinker with cheaper pozzolanic material.
2.In hydraulic structures and marine constructions PPC gives alround durability characters.
3.PPC consumes Calcium Hydroxide and produces it less than that of OPC.
4.PPC reduces heat of hydration and and the heat generation rate is also very low.
5.Reduction of permeability of PPC offers many other advantages.
6.The ultimate strength of PPC is more than OPC cement if enough curing is done for its pozzolanic actions.
7.Being finer than OPC it improves the pore size distribution and reduces the micro-attacks at transition zone.
Uses of PPC:-
PCC cement can be used in all the places except where early high strength is required.It is generally used in following cases
1.For hydraulic structures like canals,concrete pipelines,water tankers etc.
2.For dam,bridges or heavy foundations where mass concreting works is done.
3.For Marie structures.
4.For sewage and sewage disposal works.
