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Fly ash can be used in concrete either as an additive or in-part replacement of cement. The presence of finely divided glassy silica and lime which produce calcium-silicate results in pozzolanic activity, very similar to the hydration of Portland cement. This process provides mechanical strength. Also the calcium hydroxide Ca(OH)2 produced in Portland cement hydration enable pozzolanic action to proceed. The combination of these two reactions results in improved mechanical properties.
Fly Ash can be used in the following ways:
1. As a part replacement of cement
This simple and direct replacement for up to 15-30% of cement reduces the strength at ages up to 3 months. But once calcium hydroxide has been liberated to start the pozzolanic action, the rate of development of strength increases rapidly and equality can be attained after 1-3 months. Beyond this stage even higher strength can be achieved. The optimum replacement is between 1-30%, normally nearer the lower limit. Higher workability can be observed which can be used to reduce water content directly increasing strength. Hence, the water content can be sufficiently reduced to limit the loss of early strength up to 25%.
2. As a part replacement of fine aggregates
This method has a beneficial effect on the strength even at early ages but is rather uneconomical hence is rarely used.
3. As a simultaneous replacement of cement and fine aggregates
This method enables the strength at a specified age to be equaled depending on the water content. By adjusting the water content and the total amount of cement and fly ash the same workability and 28-day compressive strength can be achieved. Suggested multiplication factor for this adjustment is shown below.
Source: Gambhir, M.L., Concrete Technology, New Delhi, Tata McGraw Hill Education Private Limited, 2009, Print.