Which of the following benefits is most commonly associated with the use of fly ash in concrete production? i. Reduces water demand ii. Increases the concrete setting time iii. Increases the permeability of concrete iv. Reduces efflorescence
- AOnly (i)
- BOnly (i) and (ii)Correct
- COnly (ii), (iii) and (iv)
- D(i), (ii), (iii) and (iv)
Solution
Concept
Benefits of fly ash in concrete
Formula
NONE
Step-by-step solution
1. Fly ash consists of spherical particles that reduce the water demand in fresh concrete mix. 2. The pozzolanic reaction of fly ash slows hydration, thereby increasing concrete setting time. 3. Therefore, statements i and ii represent the correct benefits. Answer: B
Explanation
The spherical shape of fly ash particles improves concrete workability, which reduces the required water demand for a given slump. Furthermore, fly ash delays early cement hydration reactions, thereby increasing the setting time of the concrete mix. Because statements i and ii are valid benefits while fly ash actually reduces permeability, option B is correct.
Why the other options are wrong
A) Statement (ii) is also a recognized benefit, making statement (i) alone incomplete. C) Statement (iii) is incorrect because fly ash decreases concrete permeability. D) Statement (iii) is false, so the option containing all four statements is incorrect.
Shortcut
Fly ash reduces water demand, delays setting time, lowers permeability, and reduces efflorescence.
Common mistake
Selecting option D by incorrectly assuming that fly ash increases concrete permeability rather than reducing it.
Correct answer: B) Only (i) and (ii)