Fly ash is generated as waste in thermal power plants that run on coal. An ideal use for fly ash is in making Portland Pozzolana Cement (ppc). Though cement companies are producing ppc , the market appears to be biased against it. Why? Because the predominantly used Ordinary Portland Cement (opc) is projected to be stronger and takes less time to set.
Cement is graded according to its compressive strength, which is defined in terms of kilogrammes per square centimetre a cube made of 1 part cement, 3 parts sand and water can withstand 28 days after being kept in water. For instance, 53-grade opc will have the minimum compressive strength of 530 kg/sq cm. While only one grade of ppc is available, opc has three grades -- 33, 43 and 53. Most buyers prefer 53-grade opc. Says Ashok Gupta, manager-sales, Vismai properties and builders, Delhi, "We stock only 53-grade cement. Our customers prefer this over fly ash based cement."
But a look at the compressive strengths of opc and ppc shows they are comparable (see table: Worthy rival). Then why the preference for 53-grade opc ? Says Satish Sharma, production manager, Associated Cement Company (acc), Wadi, Karnataka, "The consumers hesitate to use cement made from a waste product. Besides, consumers also feel a cement costing more might be of a better quality." The price difference between the two is about Rs 10 per a 50-kg bag. At Rs 140 a bag (in Delhi), ppc is the cheaper option.
Experts do not consider such a viewpoint objective. "The perception of the people is completely wrong. If we use fly ash up to a maximum of 35 per cent (the rest being clinker, the raw material that contains limestone), the cement behaves much better. ppc is much more durable and has high resistance to cracks," stresses Shiban Raina, director-general, National Council for Cement and Building Materials (ncb). Also, though 53-grade opc sets faster than any other type or grade of cement, it requires intensive curing. Otherwise it develops cracks.
According to cement manufacturers, however, 53-grade opc controls the market for cement in India. "The cement industries are pushing for 53-grade cement purely to capture this market. Companies have made 'strength of cement' their usp," says K S Chandrashekaraiah, general manager, production at Ambuja Cements, Rabriyawas, Rajasthan (see interview: There is no need to...).
A typical television advertisement shows an entire village razed after a natural calamity except, of course, the house constructed with the "sturdy" 53-grade opc. "People want to buy popular brands of cement based purely on tv commercials. Sometimes, it is impossible to convince the consumers to use ppc. The second most influential source is the contractors," says P C Jinda, proprietor, Jindal Italia Mamo Inc, a shop that stocks cement and other construction material in Delhi. But even contractors "prefer to use this grade of cement because it sets quickly as it is finely ground and makes their work far easier," says Raina. The result is that most Indian cement plants mainly produce 53-grade opc .
The high demand for 53-grade opc is draining limestone deposits. Says Sharma of acc, "To produce 100 tonnes of opc, we need 112 tonnes of limestone. While, for every 100 tonnes of ppc, we need only 77 tonnes of limestone. Besides this, 53-grade cement is extremely energy intensive: an additional 4 kilowatt hour per tonne of cement is needed to grind the clinker." Raina recommends replacing opc completely with ppc without grading it. He explains: "In a house, to do the chores, do we need a person with a normal physique or do we need a wrestler? It is the same case with cement also. Why are we using a resource intensive cement when it does not give any additional advantage to the customer?"
Despite the preference for 53-grade opc, the production of ppc shows a gradual increase. In 2003, 43.19 per cent of the total cement produced in India was ppc -- a jump of 25 per cent in 15 years (see graph: Slow but steady). Experts believe ppc has a much higher potential.
The scepticism regarding ppc owes in part to the Bureau of Indian Standards (bis). The norms for ppc that bis set in 1991 require it to have a minimum compressive strength of 330 kg/sq cm after 28 days -- the equivalent of 33-grade opc. This despite ppc being capable of compressive strength matching that of 53-grade opc.
Although bis has not revised its standards for cement for over a decade, the government, in the meantime, has started using 43-grade cement instead of 33-grade cement for all government constructions. With the earlier government norms, ppc could have been used for all construction works where the 33-grade cement was being used. But no more.
Says Raina, "It is very clear that clinker of 43-grade can also absorb a lot of fly ash without compromising on the compressive strength. We have been pushing bis to revise the standards and upgrade the minimum requirements for ppc." Let's hope bis pays heed.
Comparison of compressive strength
OPC 43 grade*
OPC 53 grade*
|* in kg/sq
cm, after 28 days
Source: Data available with the Green Rating Project, Centre for Science and Environment,
New Delhi, 2004
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