Question

What is the effect of fly ash in concrete?
1. Reduces the heat of hydration
2. Increases the amount of air entrainment
3. Increases the heat of hydration
4. Reduces the secondary hydration process

Answer 1

Correct Answer - Option 1 : Reduces the heat of hydration

Concept:

Effects of Fly Ash on cement concrete:

The following are the effects of Fly Ash on the properties of cement concrete:

• Permeability of concrete reduces.
• Improves resistance of concrete to sulphate attack
• Reduces heat of hydration in concrete such that substitution of 30% fly ash results in the reduction of 50-60% of the heat of hydration.
• Reduces the amount of air-entraining agent
• Increases the initial setting time such that 30% substitution can increase setting time upto 2 hours.
• Coarser fly ashes having high carbon content are more liable to increase dry shrinkage than finer fly ashes having low carbon content.
• Modulus of elasticity of altered concrete is lower at early ages but higher at later ages.
• Lesser compressive strength at early ages but equal strength at later ages.

Fly Ash:

• It is a fine powder that is a byproduct of burning pulverized coal in electric generation power plants.
• It is a pozzolanic material which when mixed with lime and water forms a compound similar to portland cement.
• It is supplied in two grades; Grade I and Grade II. However, only Grade I is recommended for the manufacturing of cement.