In this article we will discuss about:- 1. Advantages of Portland Pozzolana Cement (PPC) 2. Disadvantages of Portland Pozzolana Cement (PPC) 3. Use.
Advantages of Portland Pozzolana Cement (PPC):
If the right quality fly ash is used in optimum percentage, the following advantages of Portland pozzolana cement (PPC) have been observed over ordinary Portland cement.
1. It is economical as in PPC a part (25%) of costly clinker of OPC is replaced by cheaper pozzolanic material.
2. Improvement in permeability as soluble calcium hydroxide is converted into insoluble cementitious product. Hence it offers all-round durability characteristics, specially in the construction of hydraulic and marine structures.
3. Reduction in the production of calcium hydroxide as PPC consumes calcium hydroxide. Thus the calcium hydroxide is not produced as much as in the case of ordinary Portland cement.
4. It generates less heat of hydration at a low rate than ordinary Portland cement.
5. Reduction in permeability of PPC offers many other advantages, such as durability.
6. Portland pozzolana cement improves pore size distribution and also reduces the micro cracks in the cement paste at the transition zone due to pozzolanic action and being finer than OPC.
7. A 50 kg bag of the Portland pozzolana cement gives more volume of mortar than ordinary Portland cement as the fly ash is finer and of lower density.
8. If enough moisture is available for continued pozzolanic action the long term strength of Portland pozzolana cement beyond 90 days is higher than ordinary Portland cement.
The above advantages of Portland pozzolana cement have been attributed due to the slow conversion of calcium hydroxide Ca(OH)2 in the hydrated cement paste into cementitious product.
In one of the recent studies in an ordinary Portland cement paste only 20% calcium hydroxide was found after 1 year hydration where as in a similarly hydrated paste containing 30% pozzolana, only 8.8% calcium hydroxide was found. It may be noted that due to leaching and dilution certain reduction in calcium hydroxide may have taken place. Considering this loss also, the amount of calcium hydroxide should have been 14%. The difference in calcium hydroxide (14.0 – 8.4) = 5.6 proves that 5.6% calcium hydroxide was converted due to pozzolanic activity.
Disadvantages of Portland Pozzolana Cement (PPC):
Following disadvantages have been observed of Portland pozzolana cement:
1. Less rate of development of strength initially than ordinary Portland cement.
2. It reduces the resistance to corrosion of steel reinforcement due to alkalinity reduction. But it improves permeability of concrete considerably.
3. Setting time is longer.
Use of Portland Pozzolana Cement (PPC):
The Portland pozzolana cement has been found useful under the following conditions:
1. For the construction of hydraulic structures.
2. For the construction of massive structures as dams, thick foundations, bridge piers etc.
3. For marine structures.
4. For the construction of sewers and sewage disposal works etc.