portland pozzolana cement pdf (PPC GRADE)

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portland pozzolana cement pdf (PPC GRADE)
Portland pozzolana cement pdf (PPC GRADE)

Portland Pozzolana Cement (IS 1489–1991)

The history of pozzolanic material goes back to Roman’s time. The descriptions and details of pozzolanic material will be dealt separately under the chapter ‘Admixtures’. However a brief description is given below.

Portland Pozzolana cement (PPC) is manufactured by the intergrinding of OPC clinker with 10 to 25 per cent of pozzolanic material (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). More information on fly ash as a mineral admixture is given in chapter 5.

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)

Portland pozzolana cement produces less heat of hydration and offers greater resistance to the attack of aggressive waters than ordinary Portland cement. Moreover, it reduces the leaching of calcium hydroxide when used in hydraulic structures. It is particularly useful in marine and hydraulic construction and other mass concrete constructions. Portland pozzolana cement can generally be used where ordinary Portland cement is usable. However, it is important to appreciate that the addition of pozzolana does not contribute to the strength at early ages. 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 India there is apprehension in the minds of the user to use the Portland pozzolana cement for structural works. It can be said that this fear is not justified. If the Portland pozzolana cement is manufactured by using the right type of reactive pozzolanic material, the Portland pozzolanic cement will not be in any way inferior to ordinary Portland cement except for the rate of development of strength upto 7 days. It is only when inferior pozzolanic materials, which are not of reactive type and which do not satisfy the specifications limit for pozzolanic materials, are used the cement would be of doubtful quality. The advantages of PPC can be summerised as follows.

Technically PPC has considerable advantages over OPC when made by using optimum percentage of right quality of fly ash.

Schematic representation of the formation of fly ash
Schematic representation of the formation of fly ash

Portland pozzolana cement uses / Advantages of PPC

  1. In PPC, costly clinker is replaced by cheaper pozzolanic material - Hence economical.
  2. Soluble calcium hydroxide is converted into insoluble cementitious products resulting in improvement of permeability. Hence it offers, alround durability characteristics, particularly in hydraulic structures and marine construction.
  3. PPC consumes calcium hydroxide and does not produce calcium hydroxide as much as that of OPC.
  4. It generates reduced heat of hydration and that too at a low rate.
  5. PPC being finer than OPC and also due to pozzolanic action, it improves the pore size distribution and also reduces the microcracks at the transition zone.
  6. Reduction in permeability of PPC offers many other alround advantages.
  7. As the fly ash is 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.
  8. The long term strength of PPC beyond a couple of months is higher than OPC if enough moisture is available for continued pozzolanic action.

All the above advantages of PPC are mainly due to the slow conversion of calcium hydroxide
in the hydrated cement paste into cementitious product. In one investigation, 20 per cent calcium hydroxide in one year old OPC paste was found to be only 8.4 per cent calcium hydroxide
in a similarly hydrated paste containing 30 per cent pozzolana. It may be noted that due to the
dilution and leaching also certain reduction in calcium hydroxide may have taken place. Giving
consideration to that effect, the calcium hydroxide should have been 14%. But the fact is that only
8.4% has remained goes to prove that 5.6% of calcium hydroxide was converted by the
pozzolanic activity. below image shows the typical reduction of Ca(OH)2.

Typical reduction of Ca(OH)2
Typical reduction of Ca(OH)2

A few of the disadvantages are that the rate of development of strength is initially slightly
slower than OPC. Secondly reduction in alkalinity reduces the resistance to corrosion of steel
reinforcement. But considering the fact that PPC significantly improve the permeability of concrete, increases the resistance to corrosion of reinforcement. The setting time is nominal longer 

Status of PPC in India 

Over 60 million tones of fly ash is generated from over 75 thermal power stations. But the qualities of such fly ash are generally not satisfactory to be used in PPC. In western countries fly ash generated in thermal power plants are further processed to render it fit for using in PPC. Because of the poor quality of fly ash, lack of awareness and fear psychics on the part of users, PPC is not popular. In India only 19% of total cement production is PPC. (1998-1999) and about 10% is slag cement. Government of India has set up an organisation called Fly Ash mission to promote the use of fly ash as mineral admixture or in manufacturing PPC. It has been realised by all experts in the world that more and more blended cement has to be used for sustainable development of any country.

Due to the shortage of electrical power, many cement factories have their own dedicated thermal power plant. They use their own fly ash for manufacturing PPC. As they know the importance of the qualities of fly ash, they take particular care to produce fly ash of good qualities to be used in PPC. The PPC produced by such cement plant is of superior quality. The chemical and physical qualities of properties of such PPC show much superior values than what is prescribed in BIS standard.

Portland pozzolana cement grade / Grading of PPC

In many countries, PPC is graded like OPC depending upon their compressive strength at 28 days. In India, so far PPC is considered equivalent to 33 grade OPC, strengthwise, although some brand of PPC is as good as even 53 grade OPC. Many cement manufacturers have requested BIS for grading of PPC just like grading of OPC. They have also requested for upper limits of fly ash content from 25% to 35%. 

Recently BIS has increased the fly ash content in PPC from 10–25% to 15–35%.

Application

Portland pozzolana cement can be used in all situations where OPC is used except where high early strength is of special requirement. As PPC needs enough moisture for sustained pozzolanic activity, a little longer curing is desirable. Use of PPC would be particularly suitable for the following situations:

  1. For hydraulic structures;
  2. For mass concrete structures like dam, bridge piers and thick foundation;
  3. For marine structures;
  4. For sewers and sewage disposal works etc.
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