Composition and Hydration reactions of Fly ash used in Concrete

A. Introduction

Fly ash is one of the most extensively used by-product materials in the construction field resembling Portland cement. It is an inorganic incombustible, finely divided residue collected or precipitated from the exhaust gases of any industrial furnace.

Fly ash is extracted from flue gases through Electrostatic Precipitator in dry form. It is a fine material & possesses good pozzolanic property.

Fly ash produced in modern power stations is of good quality as it contains low sulphur & very low unburnt carbon i.e. less loss on ignition. In order to make fly ash available for various applications, most of the new thermal power stations have set up dry-fly ash evacuation & storage system.

B. Chemical Composition

The composition varies with type of fuel burnt, load on the boiler and type of separation. The fly ash consists of spherical glassy particles ranging from 10 to 15 micron in diameter. The constituents of fly ash are mentioned below.

Silicon dioxide               —– SiO2               — 30 – 60 %

Aluminum oxide           —– Al2O3           — 15 -30 %

Unburnt fuel                    —– (Carbon)     — up to 30 %

Calcium oxide                 —– CaO                — 1-7%

Magnesium oxide          —– (MgO)           — small amounts

Sulphur trioxide             —– (SO3)            — small amounts

C. Hydration Reactions when Fly ash is used in OPC

As mentioned in section 2; OPC is made up of four principal mineralogical phases symbolically represented by C3S, C2S, C3A and C4AF. The hydration reactions of these chemical compounds as mentioned in the respective section are as follows:

For C3S:

2C3S  +  6H        —> C3S2H3 + 3Ca(OH)2

For C2S:                                                                                            

2C2S  +  4H        —>   C3S2H3 + Ca(OH)2

For C3A:

C3A   +  6H        —>   C3AH6

One of the primary benefits of fly ash is its reaction with available lime and alkali in concrete, producing additional cementitious compounds. The following equations illustrate the pozzolanic reaction of fly ash with lime to produce additional calcium silicate hydrate (C-S-H) binder:

Cement Reaction       :           C3S/ C2S   +    H   →   C-S-H + Ca(OH)2

Pozzolanic Reaction  :            Ca(OH)2   +    S    →   C-S-H

S —- Silica from Fly ash constituents

So, clearly from the above equation we can interpret that the excess lime content produced by the hydration reactions of cement, which weakens cement by leaching and other processes, is reduced by the use of silica. Also due to the formation of additional calcium silicate hydrate (C-S-H) binder, the strength of the mix as a whole increases.

D. Advantages of using Fly ash

The benefits of using Fly ash along with OPC in mix design are as follows:

  • Improved workability
  • Decreased water demand
  • Reduced heat of hydration
  • Increased ultimate strength
  • Reduced permeability
  • Improved durability



One thought on “Composition and Hydration reactions of Fly ash used in Concrete

  1. Fake Oakleys says:

    Fake Oakleys

    Composition and Hydration reactions of Fly ash used in Concrete | sarat

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