Fly ash is a product of burning finely ground coal in a boiler to produce electricity. It is removed from the plant exhaust gases primarily by electrostatic precipitators, or baghouses and secondarily by scrubber systems. Physically, fly ash is a very fine, powdery material, composed mostly of silica nearly all particles are spherical in shape. Fly ash is generally light tan in color and consists mostly of silt-sized and clay-sized glassy spheres. This gives fly ash a consistency somewhat like talcum powder. 
Fly ash applications:
- Raw material in concrete products and grout
- Feed stock in the production of cement
- Fill material for structural applications and embankments
- Ingredient in waste stabilization and/or solidification
- Ingredient in soil modification and/or stabilization
- Component of flowable fill
- Component in road bases, sub-bases, and pavement
- Mineral filler in asphalt
How much flyash does India produce?
The 80-odd utility thermal power stations in India use bituminous coal and produce large quantities of fly ash. According to one estimate, up to 150 million tonnes of fly ash will be produced in India in the year 2000, primarily by thermal power plants and, to a lesser extent, by cement and steel plants and railways. This poses problems in the form of land use, health hazards, and environmental dangers. Both in disposal and in utilization utmost care has to be taken to safeguard the interest of human life, wild life, and such other considerations. – TERI
What does the Indian Government have in mind for flyash?
Protocol has been signed between India and Russia for export of Indian expertise and know-how, including technologies to Russia and to set up “Fly Ash Mission” on the lines of “Fly Ash Mission – India”. – DST, India
With all the good things it can do for us, why is it not an option sometimes?
Drawbacks of flyash:
Fly ash has been gaining acceptance by Federal Government, specifically FHWA, however, smaller builders and housing contractors are not that familiar with fly ash products. Fly ash could have different properties depending on where it was obtained or how it was obtained. For this reason, fly ash applications are encountering resistance on traditional builders due to tendency to effloresce and major concerns about freeze/thaw performance.
Other major concerns about using fly ash concrete are:
- Slower strength gain
- Seasonal Limitation, and
- Increase in air entraining admixtures.
- Higher fly ash contents can produce an increase of salt scaling.
Fly ash: Goor or bad for the environment?
- Ramesh Singh said the heap of fly ash has started taking its toll on the environment. “This place used to be like a dense forest even two weeks ago. But, the green cover is fast disappearing,” he added. – The Telegraph
- “Addition of fly ash in plantation pit improves survival rate from about 70 per cent (without fly ash) to more than 90 per cent (with fly ash). The growth and timber formation has been observed to increase by about 35 per cent in four years at plantation done with fly ash at Kiajhara under Tamka Range and at Barabati under Sukinda Range in Cuttack Forest Division,” say the findings. – The Hindu
- “Fly ash improves the soil texture and increases its water holding capacity, which can ensure survival of the plants,” said C-FARM advisor Vimal Kumar.”Under normal circumstances, the survival of plants in case of bulk plantation is low. But the use of fly ash has made wonders. Use of fly ash in plantation can not only check pollution but it can ensure effortless greenery,” said the scientist. – Times of India
Of all the applications of fly ash, fly ash bricks gets most of my attention.
What is fly ash brick?
Fly ash brick (FAB) is a building material, specifically masonry units, containing class C fly ash and water. Compressed at 28 MPa (272 atm) and cured for 24 hours in a 66 °C steam bath, then toughened with an air entrainment agent, the bricks last for more than 100 freeze-thaw cycles. Owing to the high concentration of calcium oxide in class C fly ash, the brick is described as “self-cementing”. The manufacturing method saves energy, reduces mercury pollution, and costs 20% less than traditional clay brick manufacturing. 
Economics of fly ash bricks:
Plagued by the imposition of various taxes, and rising raw material and labour costs, the Andhra Pradeshfly ash brick industry is witnessing a slow down with only 250 new units being set up in the state during the last two years….. Due to this additional tax burden, the fly ash brick industry, which is the most environment-friendly, is unable to compete with the clay brick industry, he added. – Business Standard
- USA EPA Wastes – Resource Conservation – Industrial Materials Recycling
- Wikipedia article on fly ash brick
- Flyash applications -About.com
Informative video on fly ash bricks: