Enhancing the potential of industrial use of the Indian fly ashes through echano-chemical activation – prospects and problems

The generation of fly ash from the coal-fired thermal power stations in India is very large. At the same time, its quality is highly variable and predominantly crystalline. Since the generation is likely to touch 170 Mt in 2011-12 and about 600 Mt in 2031-32, one will have to look for newer avenues of bulk usage that have not been tried so far. This strategy requires technologies for upgradation of fly ash quality. Currently fine grinding of fly ash and cyclone classification are the technologies adopted and the product particle size reduces from a mean of 30 µm to 3-9 µm. With these particulate characteristics the properties of mortars and concrete significantly improve due to enhancement of packing density and increased number of nucleation points for cement hydration in the paste. In other words the improvement in properties is consistent with the increase in surface generation. But in Materials Science it is known that a decrease in the size of the crystals below some threshold value may result in a disproportionately large change of properties. This observation has been made profusely in the context of nanomaterials with mean size of particles below 100 nm. But there is always a region in the submicrocrystalline field of materials going up to 300 nm.
This paper addresses the issues pertaining to the prospects of finding a threshold value for the Indian fly ash particles which may lead to large change of properties, which, in turn, may unfold possibilities of newer bulk applications of fly ash by virtue of such mechano-chemical activation.