Q. Consider the following statements:

1. Studies indicate that carbon dioxide emissions from 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? (UPSC Prelims 2025)
Answer: Both Statement 2 and Statement 3 are correct and both of them explain Statement 1
Notes: The correct answer is [B] Both Statement 2 and Statement 3 are correct but only one of them explains Statement 1. This question analyzes the chemical process of cement manufacturing and its environmental impact.

• Statement 1 (Correct): The cement industry is a major contributor to global warming. It is estimated to account for approximately 7% to 8% of global CO₂ emissions. Since the statement says "more than 5%," it is factually accurate.
• Statement 2 (Correct): In the manufacturing process, raw materials like limestone (calcium carbonate) are mixed with silica-bearing clay (or shale/sand) to provide the necessary chemical components—calcium, silica, alumina, and iron—required for cement.
• Statement 3 (Correct): During the production of clinker (the intermediary product), the raw mix is heated in a kiln to about 1450°C. In a process called calcination, limestone (CaCO₃) is thermally decomposed into lime (CaO) and carbon dioxide (CO₂).

Explanation of the Link:While both Statements 2 and 3 describe the manufacturing process, only Statement 3 explains Statement 1. The CO₂ emissions in cement production come from two main sources:

1. Chemical Decomposition (Calcination): As described in Statement 3, the conversion of limestone to lime inherently releases CO₂ as a byproduct of the chemical reaction. This accounts for about 50% to 60% of the industry's total emissions.
2. Fuel Combustion: The energy required to heat the kiln.

Statement 2 merely describes the ingredients being mixed; it does not explain why the process generates carbon emissions. Therefore, only Statement 3 serves as a causal explanation for Statement 1.