Mechanical Activation of Fly Ash
Fly ash is a finely divided residue, resulting from the combustion of ground or powdered bituminous coal or sub-bituminous coal (lignite) and is transported by the flue gases of boilers fired by pulverised coal or lignite. It comprises spherical particles, mostly amorphous, in addition to unburnt carbon, crystalline mullite, quartz and hematite. It has cementitious properties which exhibit when mixed with hydrated lime and alkalis. The mineralogical and chemical composition of n¼éy ash depends on the source of the coal and design engineering of fly ash collection at the power stations.
Fly ash is a very fine powder and tends to travel far in the air. When not properly disposed, it is known to pollute air and water, and causes respiratory problems when inhaled. When it settles on leaves and crops in fields around the power plant, it lowers the yield. The conventional method used to dispose of both fly ash and bottom ash is to convert them into slurry for impounding in ash ponds around the thermal plants. The construction of ash ponds requires vast tracts of land. This depletes land available for agriculture over a period of time. When one ash pond fills up, another has to be built, at high cost and with further loss of agricultural land. Huge quantity of water is required to convert ash into slurry. During rains, numerous salts and metallic content in the slurry can leach down to the groundwater and contaminate it. Of late (early 1970s), due to development of technology, collection of dry fly ash has started.
Fly ash generation in India
- 75 per cent of installed power generation in India - coal-based
- 200 million MT of fly ash is generated every year
- 300-500 million MT coal used every year
- Ash generation likely to reach 300+ million MT by 2017-18
- Average ash contents up to 35 per cent in Indian coal
- Presently as per the figures from the Ministry of Environment & Forest, only 25-35 per cent of ash is being used in fillings, embank-¡ments, construction/cement industry, block and tiles, etc.
- Poor understanding of the chemistry of fly ash and its derivatives for proper end applications
- Absence of standards and specifications for fly ash products
- Lack of reliable quality assurance for fly ash products
- Poor public awareness about the products and their performance
- Availability of dry fly ash collection facilities is far off from consumption places.
- Easy availability of land with top soil at cheap rates for manufacturing conventional bricks
- Lack of proper coordination between thermal plants and ash users.
Low fly ash utilisation - technical facts
In addition to factors discussed above, there are some technical factors also, which hinder the optimum utilisation of fly ash. The prime factors are:
Coarseness: Despite having cementitiouss and pozzolanic properties, consumption of available fly ash has limitation due to its coarseness. Coarse fly ash gives a low compressive strength when used up to 30 per cent of cement replacement.
Spherical shape: Further, the large particle sizes carry the spherical form, which is inactive to pozzolanic reaction.
Inconsistency in particle size distribution (large variation in particle size in coarse fly ash), lower fineness and spherical shape result in limited availability of surface for reactivity.
Spherical particles also contribute to lower density of the mass.
Thus, lower density hinders the development of strength at early stage.
So, how mechanical activation would overcome these limitations: As discussed above, the mechanical activation is a method of obtaining very fine fly ash (up to or +8,000 Blaine). In addition, it also destroys the spherical morphology of raw fly ash and converts fly ash particles from spherical shape to crystalline shape.
Fineness and conversion result in larger surface area availability for lime reactivity, which is vital to any material having cementitious and pozzolanic properties.
Smaller particles fill voids between larger particles, thereby giving the final product increased density and strength.
Crystalline form of the particles itself leaves no room for voids as in the case of spherical particles. This results in reduction of water consumption, which otherwise gets accumulated in the voids.
Lab tests have shown increase in lime reactivity
Improved durability of concrete, even with 50 per cent replacement of OPC
Increase in tensile strength of cement product
Reduction in water consumption
Difference between superfine fly ash (SFAF) and normal fly ash: Finer particle size of SFAF and change in its morphology allow higher dissolution rate of SFAF. Thus, Geo polymer paste made with SFAF when cured at ambient temperature leads to an increase of about 80 per cent in compressive strength when compared with Geo polymer made from normal fly ash.
Beside environmental issues discussed above, one of the major elements, which cause global warming, is emission of carbon dioxide (CO2). It is observed that production of one tonne of Portland cement releases an equal amount of CO2 into atmosphere, thus utilisation of fly ash in cement will help to contain CO2 emission. Therefore, use of fly ash will increase the cement production which in turn will slow down the rate of depletion of available natural resources.
An estimate says replacing 15 per cent cement worldwide by fly ash will reduce CO2 emission by 227 million tonne.
The target is replacing 50 per cent of cement worldwide by fly ash will reduce CO2 by 750 million tonne. Refer to Table 3. This may be equal to removing 25 per cent of all automobiles in the world.
Full utilisation of generated fly ash in India will provide employment potential for more than 3,00,000 people. This will generate a business volume of over Rs 4,000 crore.
I shall now give my experiences on improving the performance of fly ash through mechanical ways. Fly ash used for study purpose is procured from the Gandhinagar Thermal Power Station in Gujarat and Parli Thermal Power Station at Parli Vaijnath in Beed district of Maharashtra. Around 80 tonnes of samples is collected for study assessment.
After mechanical activation, both the samples were analysed for the particle sizes, fineness (blains air permeability) and lime reactivity. The analysis was conducted at National Council for Cement and Building Materials (NCCBM) at Ahmedabad, Gujarat. Standard method of testing were done as per IS 3812- Part 1 and 2 (2003) as Specification for Pulverised Fuel Ash, Part 1: for use as pozzolanic additive in cement, cement mortar and concrete [CED 2: Cement and Concrete] and specification for pulverised fuel ash, Part 2: for use as admixture in cement mortar and concrete [CED 2: Cement and Concrete] and IS 1727 (1967) for methods of test for pozzolanic materials [CED 2: Civil Engineering].
Test results of parameter for fineness, lime reactivity and compressive strength are mentioned in the tables Refer to Table 1 and 2.
Fly Ash is a wonderful material, however need of the hour is to explore its properties scientifically and make use of the same in various spheres.
Mechanically activated fly ash has vast potential to improve upon the utilization of fly ash in no. of fields viz cement, concrete, cement products like paver blocks bricks.
Mechanically activated fine fly ash can also be used in paint industry, cement roof sheeting industry
Consistent quality of activated fly ash can be a good substitute for concrete/cement strength enhancing material viz micro silica alcofine etc.
Activation facility if set up near thermal power plant it would generate employment opportunity to the local labour force.
Activation of fly ash may a good tool to combat the problem of Fly Ash disposal which is likely to increase in the days to come.
Up to 110 per cent of strength activity index can be achieved when normal fly ash is ground to smaller size.
Strength activity index of normal fly ash can be improved by grinding and coarse fly ash is not in crystalline phase.
For a good quality of micro fine fly ash, by classifying or grinding, the important factor is its fineness. Fly ash with finer particle size increases ultimate strength as well as rate of strength gain of fly ash cement mortar.
When keeping the same work-¡ability of mortal, the use of finer fly ash demands less water than the use of normal coarse one.
Excerpt from the former President, APJ Abdul Kalam´s address to the nation on the eve of the country´s 56th Republic Day
Conversion of fly ash into wealth generator: Fly ash can become a wealth generator by making use of it for producing ´green building´ materials, roads, agriculture etc.
Full utilisation of the generating stock will provide employment potential for three hundred thousand people and result in a business volume of over Rs 4,000 crore.´(ENVIS newsletter vol. 2, no.6 Jan 2007)
The author is an expert with over 35 years of experience working in cement, concrete and allied fields.