The government in Maharashtra has been trying to make the use of coal dust or fly ash - a by-product of burning pulverized coal, mandatory for all sorts of construction. In a recent update, the Brihanmumbai Municipal Corporation (BMC) has finally proposed to use fly ash to make concrete and bricks for constructing buildings, in its Development Control and Promotional Rules (DCPR) released a month ago. It is yet to see if the proposal turns into a rule of law.

Earlier, Maharashtra government did make fly ash mandatory for all government infrastructure and construction within 300-km radius of any power plant.

What is coal dust or fly ash?

As per a research by two Indian students titled, "Fly ash utilization in different sectors in Indian scenario," coal mining operations result in two general types of by-products. One is coarse coal refuse and another is fine coal refuse which is also termed as coal mine dust or fly ash. Fly ash is a fine powder which is a by-product of burning pulverized coal. It is a pozzolan, meaning a substance containing aluminous and siliceous material that forms cement in the presence of water.

Classes of fly ash

According to ASTM C-618, fly ash is broadly classified into two major categories: Class F and Class C fly ash. The chief difference between these two classes is the amount of calcium, silica, alumina, and iron content. The chemical properties of the fly ash are largely influenced by the chemical content of the coal burned (i.e., anthracite, bituminous, and lignite).

Class F fly ash

The burning of old anthracite and bituminous coal typically produces Class F fly ash which contains less than 10% lime (CaO). This class of fly ash requires a cementing agent, such as Portland cement, quicklime, or hydrated lime, with the presence of water in order to react and produce cement compounds.

Class C fly ash

Class C Fly ash produced from the burning of younger lignite or sub-bituminous coal generally contains more than 20% lime (CaO). This type of ash does not require an activator to produce cement compounds as compared to the Class F Fly ash.

The industrial world is dependent on coal. Over 40% of all electricity used worldwide is generated from coal. Both the mining of coal and its use as fuel are highly problematic.

During the formation of coal, clay is entrapped through natural process. During the combustion, carbon is burnt leaving the incombustible clay particles as ash. In age old grate boilers, through prolonged residential time this incombustible ash used to agglomerate as cinder. But, ever since pulverised coal technology is put to practice in order to improve energy-efficiency, the ground clay remains in hearth for a fraction of second and flies (escapes) along with flue gases, settling as ash in bag filters or electro static precipitators (ESPs). Since it is 'flied ash' it is named as 'fly ash'.

Coal dust suspended in air is explosive coal dust has far more surface area per unit weight than lumps of coal, and is more susceptible to spontaneous combustion. But the idea of using coal dust or fly ash in construction serves as a good alternative than to letting it exist as a waste product.

Due to its pozzolanic nature, fly ash can produce a stronger, more durable and more environment friendly concrete. It minimizes the amount of carbon dioxide released into the atmosphere and can also be used for construction underwater due to the hydrophobic nature of fly ash also allowing better resistance to pavements from deterioration by water.

Bricks produced from fly ash are superior in quality as they offer higher cold crushing strength and smooth, uniform size. They are denser, lighter and have greater compressive strength than regular clay bricks. It reduces crack problems, permeability, bleeding and the water absorption is also lesser than that of a regular clay brick.

Major developers today are using fly ash bricks instead of the traditional clay bricks which can easily break and are also needed in large quantities for completion of projects. Though fly ash has certain disadvantages like longer setting time and dull cosmetic quality of the concrete in its kitty, advantages like reduction in soil erosion, pollution, greenhouse gas emissions, resistance to water damage outweigh the drawbacks.