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Q: What is the effect of fly ash in concrete?
  • A. Reduces the heat of hydration
  • B. Increase the heat of hydration
  • C. Increase the amount of air entrainment
  • D. Reduces the secondary hydration process
Correct Answer: Option A - Effect of fly ash on cement concrete. (i) On strength of compression– Since the action of pozzolana is slow, addition of fly ash upto 30% may result in lower strength at 7 and 28 days, but may be about equal at 3 months and may further increase at ages greater than 3 months providing curing period is continued. (ii) On modulus of elasticity– It is lower at early ages and higher at later ages. (iii) On permeability– It reduces permeability of concrete because of its fineness. (iv) On resistance of chemical attack– It improve resistance against chemical attack. (v) On heat of hydration– It reduce heat of hydration. (vi) On setting time– It increases initial setting time of cement upto 2 hours.
A. Effect of fly ash on cement concrete. (i) On strength of compression– Since the action of pozzolana is slow, addition of fly ash upto 30% may result in lower strength at 7 and 28 days, but may be about equal at 3 months and may further increase at ages greater than 3 months providing curing period is continued. (ii) On modulus of elasticity– It is lower at early ages and higher at later ages. (iii) On permeability– It reduces permeability of concrete because of its fineness. (iv) On resistance of chemical attack– It improve resistance against chemical attack. (v) On heat of hydration– It reduce heat of hydration. (vi) On setting time– It increases initial setting time of cement upto 2 hours.

Explanations:

Effect of fly ash on cement concrete. (i) On strength of compression– Since the action of pozzolana is slow, addition of fly ash upto 30% may result in lower strength at 7 and 28 days, but may be about equal at 3 months and may further increase at ages greater than 3 months providing curing period is continued. (ii) On modulus of elasticity– It is lower at early ages and higher at later ages. (iii) On permeability– It reduces permeability of concrete because of its fineness. (iv) On resistance of chemical attack– It improve resistance against chemical attack. (v) On heat of hydration– It reduce heat of hydration. (vi) On setting time– It increases initial setting time of cement upto 2 hours.