Question 75 from UPSC CSE 2025 - Environme nt & Ecology

Question

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

Accepted Answer

Correct Option: A. Both Statement II and Statement III are correct and both of them explain Statement I

Strategy: Identify how Statements II and III relate to Statement I about CO2 emissions from cement manufacturing. Both mention critical steps in cement production: mixing silica-bearing clay with limestone, and conversion of limestone (CaCO3) to lime (CaO) during clinker formation. Both processes directly contribute to the high carbon emissions, as the calcination of limestone releases CO2 and significant energy is used, much from fossil fuels. Since both technical processes explain the large carbon footprint (Statement I), the only fitting logical conclusion is that both II and III are correct and explanatory.

Key Info: The cement industry is a major source of anthropogenic CO2 emissions, contributing over 5%—estimates commonly range around 7–8%—of global CO2 emissions according to agencies like the International Energy Agency (IEA). Cement manufacturing primarily uses limestone (calcium carbonate, CaCO3) and clay rich in silica (silicon dioxide, SiO2) as raw materials. During clinker production, these raw materials are heated together in rotary kilns at approximately 1,450 °C. The process involves calcination, where limestone decomposes into lime (calcium oxide, CaO) and carbon dioxide gas (CO2): CaCO3 → CaO + CO2. This chemical reaction is responsible for about half the cement industry's CO2 emissions, while the rest comes mainly from fuel combustion to heat the kiln. High-purity limestone and silica-bearing clay are essential raw materials; the resulting clinker is then ground to make cement. These steps—mixing silica-based clay with limestone and thermally converting limestone into lime—underpin the significant carbon footprint of cement manufacturing and directly explain the global emissions noted in Statement I.

Answer

Option B: Both Statement II and Statement III are correct but only one of them explains Statement I

Answer

Option C: Only one of the Statements II and III is correct and that explains Statement I

Answer

Option D: Neither Statement II nor Statement III is correct