What Is Fly Ash ?

Fly ash, also referred to as coal combustion residuals (i.e., CCRs), is the finely divided residuals that mostly come from the burning of pulverised coal. With the help of exhaust gases, Fly ash is transported from the combustion chamber. It can also be considered as the by-product of coal-fired power stations, which is a fine grey powder consisting mostly of spherical glassy particles. In a research, scientists found that fly ash was severely produced by over a hundred million metric tons in quantity as of 2011. That is why Fly Ash has become a matter of concern in our environment, and we should be aware of its consequences.

Causes of Fly Ash

Fly Ash is primarily the byproduct in coal-fired power stations, a fine grey powder consisting mostly of spherical glassy particles. It is produced in the plants generated by coal-fired electric and steam elements. Typically, in the boiler’s combustion chamber, coal is pulverised and blown with air where it ignites immediately, generates a huge amount of heat, and produces a molten mineral residue. Boiler tubes succumb to heat from the boiler, cool the flue gas, and cause the molten mineral residue to harden and form the ash. High-density ash particles, generally referred to as slag or bottom ash, get stuck to the bottom of the combustion chamber. In contrast, the lighter fine ash particles, called fly ash, remain suspended in the flue gas. Fly ash is removed by some particulate emission control devices, such as filter fabric baghouses or electrostatic precipitators.

Classification of Fly Ash

There is a popular society in America for Testing and Materials, ASTM, which defines two classes of fly ash, namely Class F and Class C fly ash. The amount of silica, calcium, alumina, and iron content differentiate these classes. The fly ash’s chemical properties are highly influenced by the chemical content of the burned coal (i.e., bituminous, anthracite, lignite).

Class F 

The burning of harder and older bituminous and anthracite coal produces Class F fly ash. This fly ash is pozzolanic and contains less than 7% lime (CaO or Calcium Oxide or Oxocalcium). This ash possesses pozzolanic properties; the glassy silica and alumina of Class F require a beautiful cementing agent, such as Portland cement, quicklime, or hydrated lime mixed with water to react and produce complex compounds. Similarly, adding a chemical activator such as sodium silicate (water glass) to a Class F ash can form a geopolymer. 

Class C

Fly ash, a fine grey powder consisting mostly of spherical glassy particles and sub-bituminous or younger lignite coal, has pozzolanic properties and has some self-cementing properties. Class C fly ash gets stronger and hardens over time. Generally, Class C fly ash contains more than 20% lime (CaO or Calcium Oxide or Oxocalcium) in the presence of water. Unlike class F, self-cementing class C fly doesn’t require an activator. Here Alkali and Sulphate contents are generally higher than class F fly ashes. A fly ash brick has been found in the United States containing up to 50 per cent fly ash of Class C. Testing shows bricks exceed or meet the standard performance listed in ASTM C 216 for conventionally made clay brick. In ASTM C 55, Standard Specifications for Concrete Building Brick, it is also within the allowable shrinkage limits for concrete brick. It is estimated that the production method of using Class C fly ash bricks will reduce the embodied energy of masonry construction by up to 90%. Pavers and bricks were expected to be available in commercial quantities before 2009.

Benefits

Fly ash, a fine grey powder consisting mostly of spherical glassy particles, can be a cost-effective alternative for Portland cement in many markets. It’s also an environmentally friendly material because it’s a byproduct with low embodied energy. Fly ash always requires less water than usual cement and is pretty much easier to use in cold weather. Other benefits include:

• Produces various amounts of set times.
• Resistance to cold weather.
• High strength gains which are depending on the use.
• Fly ash, a fine grey powder consisting mostly of spherical glassy particles, can be used as an admixture.
• Fly ash is considered a non-shrink material. 
• Thick concrete pavers with a smooth surface and sharp detail can be made efficiently from the fly ash.
• It is less-water property-related material, allowing for a lower water-cement ratio.
• It can Reduce Carbon-di-oxide emissions.

Conclusion

Fly Ash is primarily the byproduct in coal-fired power stations and is a fine grey powder consisting mostly of spherical glassy particles. Fly ash can be a cost-effective alternative for Portland cement in many markets. It’s also an environmentally friendly material because it’s a byproduct with low embodied energy. Fly ash, a byproduct of coal-fired power stations, particles can be involved in the deepest part of our human body affecting the lungs and other organs, resulting in immunological reactions, asthma, and inflammation. That’s why it’s our responsibility to understand technological development and related programs, which can be a huge game-changer in making the most use of the Fly Ash.