When coal is burnt (e.g. in coal fed power plants), two types of ash is produced. One that falls in the bottom of the boiler is called bottom ash. Another is made of fine particles that are driven out along with the flue gases. These fine particles are captured using the electrostatic precipitators or some other filtration equipments when the flue gases reach at Chimney of the plant. This is called Fly Ash and is made of Silica, Aluminium oxide, calcium oxide and some potentially toxic elements such as arsenic, beryllium, cadmium, barium, chromium, copper, lead, mercury, molybdenum, nickel, radium, selenium, thorium, uranium, vanadium, and zinc etc.
Uses of Fly Ash
Fly Ash was once discarded as a waste but today, it is increasingly emerging as a major input material in several industries including cement, agriculture, construction, Paints etc.
- Fly Ash is added as a mixture to cement for building construction, laying of roads and landfills.
- The bricks and blocks made of fly-ash are stronger and cheaper than conventional bricks
- Fly ash can help transform barren wasteland into lush green forests
- Pigments extracted from Fly Ash can be very useful in manufacturing of paints
Fly Ash bricks
In construction industry, Fly ash is used in making of bricks and concrete. Fly Ash Bricks are well proven building material and can be used as an alternate to the ordinary burnt clay bricks with equivalent strength. The Government promotes the use of Fly Ash Bricks. BIS (Bureau of Indian Standards) has issued the code IS:12894-2002 for ash bricks.
Further, an MOEF notification mandates that every construction agency engaged in the construction of buildings within a radius of 100 km. from a coal or lignite based thermal power plant shall use only fly ash based products for construction.
Construction of Fly Ash Bricks
The raw material to make Fly Ash bricks includes Fly Ash, Sand and Lime (or Cement). All of these are mixed with water in a pan mixer to produce a semi dry mix. It is then placed in moulds of a machine (a hydraulic or vibratory press) to produce bricks of uniform size. These bricks are air dried for one/two days in a shed and then water cured for 14-21 days.
Advantages of Fly Ash Bricks
Fly ash bricks are produced with/without frog. Fogged Bricks have depression in one face as shown below:
These bricks require less plaster thickness and save cement mortar. They are environment friendly because they are produced by by-product of thermal power stations. They save agriculture land used in brick kilns. In comparison to clay bricks, the Fly Ash bricks are less energy intensive. Further, they can be made at the construction site itself.
Fly Ash Concrete
In comparison to cement only, the Fly Ash improves the performance and quality when used in concrete. Fly ash as a part of the concrete saves water ; lowers heat of hydration; reduces the corrosion of steel and makes the concrete becomes strong quickly. This is because the fly ash reaches its maximum strength more slowly than concrete made with only Portland cement.