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UPSC Prelims 2025, Question 31

The concept being tested: Limestone and silica-bearing clay chemistry in cement manufacturing and carbon emissions.

Consider the following statements: Statement I: Studies indicate that carbon dioxide emissions from cement industry account for more than 5% of global carbon emissions. Statement II: Silica-bearing clay is mixed with limestone while manufacturing cement. Statement III: Limestone is converted into lime during clinker production for cement manufacturing. Which one of the following is correct in respect of the above statements?

Options

  1. ABoth Statement II and Statement III are correct and both of them explain Statement I
  2. BBoth Statement II and Statement III are correct but only one of them explains Statement ICorrect answer
  3. COnly one of the Statements II and III is correct and that explains Statement I
  4. DNeither Statement II nor Statement III is correct

The answer: Both Statement II and Statement III are correct but only one of them explains Statement I

Cement manufacturing starts by grinding and blending limestone (CaCO3) with silica-bearing clay to form the raw meal fed into the kiln. Inside the kiln, at roughly 900 degrees Celsius, the limestone undergoes calcination and breaks down into lime (CaO), releasing carbon dioxide in the process; this lime then combines with silica and alumina from the clay at still higher temperatures to form clinker. Statement II and Statement III both describe genuine, well-documented steps of this process and are correct. But only the calcination reaction in Statement III actually produces carbon dioxide — it is this chemical breakdown of limestone into lime that accounts for the bulk of the cement industry's process emissions, which studies put above 5% of global carbon emissions. Mixing clay with limestone, as Statement II describes, is a physical blending step and releases no gas on its own, so it cannot explain Statement I.

What UPSC was testing

The trap here is treating factual correctness and causal explanation as the same thing — UPSC wants aspirants to see that a statement can be true about the process while still failing to explain a separate claim about emissions, since only the calcination step actually generates carbon dioxide.

How often this comes up

This concept has appeared 7 times in UPSC Prelims General Studies Paper I between 2015 and 2025 — in 2015, 2017, 2021, 2022, 2023 and 2025.

The option-by-option autopsy for this question — why each of the other three is wrong, and which neighbouring concept each one was built from — is in the app.