Fly Ash Analysis

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Figure 2. 3. Role of fly ash in concrete

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Source: NTPC (2014).

2.3.4. BENEFITS OF USING FLY ASH IN CONCRETE

2.3.4.1. WORKABILITY OF THE CONCRETE

Fly ash particles are spherical in shape with high fineness and low carbon content this reduces the quantity of water required for a given slump (Zulu, 2014). This means that a mix with Portland cement would have a higher water content of the same workability compared to the fly ash mix. It is estimated that each 10% of fly ash added to a mix reduces the water content by 3%, this is dependent on the nature of the ash and other parameter of the mix (Thomas, 2007).
Its spherical shape helps to reduce the friction between aggregates and between concrete and pump line and thus increases
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As this is taking place, heat is liberated which increases the temperature of the mass. The whole process is an exothermic one. The fly ash plays a dual role for the strength development when present in a concrete mix. Firstly, it reacts with the lime and produces a binder gel as explained above and gives the concrete additional strength. Secondly, the unreactive portion of fly ash acts as micro aggregates and fills up the matrix to render packing effect and results in increased strength (ibid, …show more content…
To a small extent calcium carbonate is also formed when calcium silicate and aluminates of the hydrated cement react with carbon dioxide from the atmosphere. Carbonation process in concrete results in two deleterious effects (I) shrinkage may occur (ii) concrete immediately adjacent to steel reinforcement may reduce its resistance to corrosion. Thus the rate of carbonation depends on permeability of concrete, quantity of surplus lime and environmental conditions such as moisture and temperature. When fly ash is available in the concrete; it reduces availability of surplus lime by way of pozzolanic reaction, reduces permeability and as a result improves resistance of concrete against carbonation occurrence (Ibid,

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