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An overview of fly ash; classification, advantage, and utlization

Fly ash is a byproduct of burning pulverized coal in electric power generating plants. During combustion, mineral impurities in the coal (clay, feldspar, quartz, and shale) fuse in suspension and float out of the combustion chamber with the exhaust gases. As the fused material rises, it cools and solidifies into spherical glassy particles called fly ash. Fly ash is collected from the exhaust gases by electrostatic precipitators or bag filters. The fine powder does resemble Portland cement but it is chemically different. Fly ash chemically reacts with the byproduct calcium hydroxide released by the chemical reaction between cement and water to form additional cementitious products that improve many desirable properties of concrete.

The most important benefit is reduced permeability to water and aggressive chemicals. Properly cured concrete made with fly ash creates a denser product because the size of the pores is reduced. This increases strength and reduces permeability. The utilization of Fly ash in concrete as a partial replacement of cement is gaining immense importance today, mainly on the account of improvement in durability of concrete with its ecological benefits. Another considerable interest in many parts of the world is the utilization of Flyash in brick manufacturing. The components of fly ash vary significantly, but all fly ash includes considerable amounts of silicon dioxide (SiO2), and calcium oxide (CaO), aluminum oxide (Al2O3).

Classification of Fly ash

Classification of Fly ash ASTM C618 classifies Fly ash into two classes:

a) Class F

b) Class C

Two types of fly ash are commonly used in concrete: Class C and Class F. Class C are often high-calcium fly ashes with carbon content less than 2%; whereas, Class F are generally low-calcium fly ashes with carbon contents less than 5% but sometimes as high as 10%. In general, Class C ashes are produced from burning sub-bituminous or lignite coals and Class F ashes bituminous or anthracite coals. Performance properties between Class C and F ashes vary depending on the chemical and physical properties of the ash and how the ash interacts with cement in the concrete.

Class F Fly ash: Fly ash normally produced from burning anthracite or bituminous coal falls in this category. This class of Fly ash exhibits pozzolanic property but rarely if any, self-hardening property. In Class F Fly ash, total calcium typically ranges from 1 to 12 percent, mostly in the form of calcium hydroxide, calcium sulphate, and glassy components in combination with silica and alumina.

Class C Fly ash: Fly ash normally produced from lignite or sub-bituminous coal is the only material included in this category. This class of Fly ash has both pozzolanic and varying degree of selfcementitious properties.

Advantages of Fly Ash

  • Makes the concrete highly dense and reduces the permeability of concrete. It can add greater strength to the building.
  • The concrete mixture with fly ash generates a very low heat of hydration which prevents thermal cracking.
  • Resistant to acid and sulphate attacks.
  • The shrinkage of fly ash concrete is very less.
  • The use of fly ash gives concrete good workability, durability and finish.
  • Reduces the requirement of clay, sand, limestone in cement manufacturing and hence conserves natural resources.
  • Reduces the cement requirement and hence carbon-dioxide liberation during cement manufacturing.
  •  Reduces the topsoil requirement for land filling/brick manufacturing and saves agricultural land.
  • The use of fly ash in concrete improves the workability of plastic concrete and the strength and durability of hardened concrete.
  • Lightweight as compared to commonly used fill material (local soils), therefore, causes lesser settlements.
Fly ash bricks

Utilization of Fly Ash

The utilization of fly ash in the construction industry is not a new technology but it is a growing technology in improving the construction quality as well as the environment quality. Adding fly ash in concrete give benefits in term of economical, ecological and technical Currently, fly ash is used by cement industries as a pozzolanic material for manufacturing of Portland Pozzolana Cement since the SiO2 and Al2SiO3 content is very similar to Portland Cement. In the presence of moisture and at room temperature, it reacts chemically with calcium hydroxide to derive compounds possessing cementitious properties.

Fly ash can be used as a partial cement replacement by cement weight or as an admixture to the concrete mix. The utilization fly ash as a component of blended cement can save a significant amount of energy and cost in cement manufacturing. Normally fly ash was used in blended cement to produce in-situ concrete mix however fly ash also can be used in high strength precast and prestressed concrete.

Utilization of fly ash in road and embankment construction has many advantages compared with the conventional method. It saves topsoil which otherwise is conventionally used, avoids the creation of low-lying areas. Fly Ash may be also used in road construction for filling purposes, stabilizing and constructing sub-base or base.

Bricks may be made from several different kinds of material, but they must usually possess (can be capable of developing) a certain amount of plasticity. Fly ash is one of them. The use of Fly ash with soil and firing the bricks with agricultural waste will reduce the cost of the building material i.e. Clay bricks. Fly ash can be utilized in the manufacture of fired, unfired, and steam cured bricks. Fly ash bricks have several advantages over conventional burnt clay bricks. Fly ash can be used in brick production and the fly ash brick is technically acceptable, economically viable and environment-friendly. Blocks and paving stones can also be manufactured by adding an appropriate amount of coarse aggregate to the mixture.

In summary, the fly ash can be utilized for;

  • Portland Pozzolana Cement
  • Fly Ash Concrete
  • Manufacturing of Clay-Flyash burnt bricks/ Fly ash lime bricks
  • Roller Compacted Concrete (RCC)
  • Soil Stabilization
  • Embankments and backfills – Reinforced or unreinforced
  • Stabilization of subgrade, sub-base and base course in roads
  •  For replacing a part of OPC in Concrete pavements, paving blocks, kerbstones
  • Production of flowable fill.
  • As the component in geopolymers.
  • Stabilization of aggregate bases;
  • Soil stabilization
  • Soil drying

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