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[ B ] Acts as pozzolona.
Fly-ash when added to concrete does not act as aggregate because it is alumino-silicate and reacts with the hydration products of cement in secondary reaction and itself changes CSH, which means it acts as pozzolan. Since fly-ash particles are spherical, it improves the workability of concrete and because of its secondary reaction more CSH is released and due to higher percentage of CSH in the concrete it improves the appearance of concrete and improves the durability of concrete by reducing the voids & porosity in the concrete. Fly-ash when added to concrete does not reduce setting time, but rather increase setting time because some percentage of cement is replaced with fly-ash in concrete there fore delay in setting time also delay in early strength development but at28 days strength would be at par to concrete made only from cement. In pre-cast, strength may be similar after24 hour curing.
Dr. Mahfooz Soomro
Perth. Australia.
Fly ash is used as supplementary cementitious material in the concrete. In concrete, Fly ash is act as pozzolona when added to conrete
Fly ash is act as pozzolona when added to conrete
The most important benefit of fly ash 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 are reduced. This increases strength and reduces permeability.
One of the most indesirable effects of fly ash, when used with concrete, is that it reduces setting time.
When flyash is added to the concrete it acts in the following ways.
1. It fills finer voids in concrete as it is finer than cement, makes concrete dense and reduces pores. Acts as an aggregate.
2. It acts as pozzolana during hardening process of concrete and gives enhanced strength.
3. It gives smooth surface (basing on mould's contact surface with concrete undulated, special design, etc surface.) and improves appearance.
4. It gives more resistance to chloride penetration.
5. Minimizes surface cracks thereby we can reduce thickness of cover to reinforcement.
6. It slow downs hardening process and hence reduces heat of hydration. Less water need for curing process, but required for longer days. Simultaneously increases final setting time there by affects stripping time of form work. Special care is to be taken in this aspect. Flyash addition is one of the best method to control internal thermal cracks in mass concrete works.
7.Its is an essential aggregate in high performance concrete, Self Levelling Concrete. It is cheaper than MicroSilica.
8. Overall Flyash enhances durability. It is to be used after testing as per relevant standards of Indian or ACI or BS .
9. The name "Flyash" is not giving encouragement to use in concrete for a non-civil engineering persons. It could have named like "Golden Pozzolana" or some thing new.
Fly-ash when added to concrete does not act as aggregate because it is alumino-silicate and reacts with the hydration products of cement in secondary reaction and itself changes CSH, which means it acts as pozzolan. Since fly-ash particles are spherical,
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