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What is Fly Ash?

Updated: Oct 27, 2022

Fly Ash is a pozzolanic material, captured in electro-static precipitators as a by-product from coal-fired thermal power stations. It consists of small, hard and spherical particles rich in silica (glass) and alumina, with pozzolanic properties. It complements Portland cement upon hydration by reacting with free lime (calcium hydroxide) to form an additional durable binder, namely calcium silicate hydrate. The spherical particles also have a ‘ball-bearing’ effect, which allows concrete to be produced using less water.

Fly Ash is subject to processing/ quality control to regulate its fineness (particle size and distribution, shape and glass content) and unburnt carbon (loss-on-ignition), thus giving quality-assured material which is consistent, predictable, and suitable for varied applications. Fly Ash is mainly used as a partial replacement of cement in concrete, and has unique properties that benefit both fresh and hardened concrete. Its key advantages include reduced heat of hydration, improved workability, enhanced durability and longer setting time. It can be effectively used where aggregates have poor grading and shape, resulting in bleeding and segregation. It is also suited for mass concrete applications, and in hot and aggressive environments where early-age thermal cracking and durability are major concerns. General replacement of cement with Fly Ash in concrete can be 15-35% by weight for normal structures. However, for special structures such as dams, the replacement levels can increase up to 70%. The service life of structures using Fly Ash is enhanced by more than 100% on account of superior durability of concrete. Fly Ash use in concrete qualifies for credit under the U.S. Green Building Council's popular LEED® rating system for sustainable construction.

Applications of Fly Ash encompass structural and ready mix concrete, cement, plastering and brick mortar, pre-cast and pre-stressed concrete, pumped concrete, bricks and blocks. Fly Ash can either be pre-blended with the cement or sold separately for subsequent blending at the concrete batching plant. It is recommended that the volume of water may be reduced by 2.5% and quantity of sand may be reduced by 0.5% for every 10% of Fly Ash used instead of cement in concrete. Apart from the use of Fly Ash in blended cements or as a direct addition in concrete manufacture, Fly Ash may also be used as a raw material ingredient in clinker production where the silica and alumina contents are of particular interest. Non cementitious uses for Fly Ash are wide ranging and include use in roads, asphalt, soil and waste stabilisation, lightweight aggregates, etc. It can also be used to replace the filler ingredient of asphalt mix, which is usually 7-10%, leading to reduction in bitumen consumption by 10-15% resulting in a superior asphalt mix and cost-savings.



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