UPSC Prelims 2015 · Question 18 of 98

UPSC Prelims 2015 question on Fly Ash Bricks Cement

With reference to fly ash produced by the power plants using coal as fuel, which of the following statements is/are correct?

1. Fly ash can be used in the production of bricks for building construction.

2. Fly ash can be used as a replacement for some of the Portland cement contents of concrete.

3. Fly ash is made up of silicon dioxide and calcium oxide only, and does not contain any toxic elements.

Select the correct answer using the codes given below.

  1. 1 and 2
  2. 2 only
  3. 1 and 3
  4. 3 only
Show answer

Answer: A. 1 and 2

Verdict

The answer is 1 and 2. Fly ash is used in bricks and as a partial replacement for Portland cement, but it does contain toxic trace elements.

Analysis

Statement 1 is CORRECT. Fly ash bricks are a standard construction material and their use is mandated in the vicinity of thermal power stations. Statement 2 is CORRECT. Fly ash is a pozzolanic material and replaces a portion of the cement in concrete, improving durability and lowering cost. Statement 3 is INCORRECT on two counts. Fly ash is not composed of silicon dioxide and calcium oxide only, and it carries trace levels of toxic elements including arsenic, barium, beryllium, boron, cadmium, chromium, thallium, selenium, molybdenum and mercury.

Source

Rural roads research material and National Thermal Power Corporation material on fly ash concrete.

How to crack it

Statement 3 contains the word only and then adds an absolute denial of any toxic content, and a double absolute in one sentence is close to a guarantee of falsity. Test it with a single question: fly ash is the residue of burning coal, and coal contains heavy metals, so where would those metals have gone. Reasoning from where the material came from is more reliable than recalling its composition.

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