Explanation

The cement industry is one of the largest industrial sources of carbon dioxide (CO₂) globally, accounting for approximately 6.5% to 8% of total global CO₂ emissions. In 2022 alone, cement manufacturing produced around 1.6 billion metric tonnes of CO₂. Statement I is therefore correct — emissions from the cement industry do exceed 5% of global carbon emissions.

Statement II is correct. Cement production involves mixing limestone with silica-bearing clay or shale, along with other minerals containing alumina and iron oxides. This mixture is heated in a kiln to form clinker — the key intermediate product in cement manufacturing. The silica reacts with calcium compounds to form calcium silicates, which give cement its binding properties. However, this mixing process itself is not the primary driver of CO₂ emissions, so Statement II does not directly explain Statement I.

Statement III is correct and directly explains Statement I. During clinker production, limestone (CaCO₃) is heated to approximately 1450°C in a process called calcination. This causes limestone to decompose into lime (CaO) and carbon dioxide (CO₂) — represented by the reaction: CaCO₃ → CaO + CO₂. This chemical decomposition is the primary source of CO₂ emissions in cement manufacturing, directly explaining why the industry contributes so significantly to global carbon emissions.

Thus, both Statement II and Statement III are correct, but only Statement III directly explains Statement I.

UPSC Insight
This question tests the ability to distinguish between a statement being factually correct versus explanatorily relevant. Statement II describes a manufacturing step but does not cause significant CO₂ emissions. Statement III identifies the actual emission mechanism — calcination. Always ask: does this statement merely describe the process, or does it explain the outcome mentioned in Statement I?