UPSC Prelims 2025 Q31: Cement CO2 Emissions & Clinker — Explained
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?
- Both Statement II and Statement III are correct and both of them explain Statement I
- Both Statement II and Statement III are correct but only one of them explains Statement I
- Only one of the Statements II and III is correct and that explains Statement I
- Neither Statement II nor Statement III is correct
Answer: (b) Both Statement II and Statement III are correct but only one of them explains Statement I
Statement-wise explanation
- STATEMENT-I: correct — Cement is one of the largest industrial sources of carbon dioxide. Most estimates put its share of global man-made CO2 emissions at about 7–8%, which is above the 5% threshold in the statement. The IEA and the Global Cement and Concrete Association both place it in this range. Cement is called a 'hard-to-abate' sector because much of its CO2 comes from chemistry, not from fuel.
- STATEMENT-II: correct — Portland cement is made by grinding limestone with clay or shale and heating the mix. The clay provides silica (SiO2), alumina (Al2O3) and iron oxide, which react with lime to form the clinker minerals. Clay is a raw material, but it does not give off the CO2 that makes the industry a big emitter. So Statement II is correct but does not explain Statement I.
- STATEMENT-III: correct — Inside the kiln, at about 900°C, limestone (calcium carbonate, CaCO3) breaks down into lime (calcium oxide, CaO) and carbon dioxide. This step is called calcination. It produces roughly 60% of a cement plant's CO2. The rest comes mainly from burning fuel to heat the kiln to about 1,450°C. This process CO2 is the main reason for the high emissions in Statement I, so Statement III explains it.
How to eliminate
Statements II and III can both be checked with school chemistry. Portland cement is limestone plus clay, and heating limestone gives lime plus CO2. Both are true, so the two options that call one or both of them false drop out. That leaves the two options where both are correct. To pick between them, ask what actually releases CO2. Clay supplies silica and alumina and gives off no carbon. Limestone (CaCO3) gives off CO2 when it turns into lime. Only the calcination statement explains the emissions. Statement I sets a low bar ('more than 5%'). If you remember that cement is often quoted at about 7–8% of global CO2, it is safe to accept. Fact to remember: about 60% of cement CO2 comes from calcining limestone and about 40% from fuel.
Know more
Cement making in brief
- Raw mix: limestone (about 75–80%) is ground with clay or shale. Small amounts of bauxite, iron ore or sand may be added to get the right chemistry.
- Calcination (about 900°C, in the preheater or precalciner): CaCO3 → CaO + CO2.
- Clinkering (about 1,450°C, in the rotary kiln): lime reacts with silica, alumina and iron oxide to form clinker minerals such as alite and belite.
- Grinding: clinker is ground with about 4–5% gypsum, which slows down setting, to make cement.
Where the CO2 comes from
| Source | Approx. share of plant CO2 | Can it be avoided? |
|---|---|---|
| Calcination of limestone (process) | ~60% | Not by changing fuel. Needs less clinker or CCUS |
| Fuel burnt to heat the kiln | ~35–40% | Alternative fuels, waste heat recovery |
| Electricity for grinding | small | Renewable power |
Ways to cut emissions
- Lower the clinker factor: blend cement with fly ash (PPC), blast-furnace slag (PSC) or calcined clay and limestone (LC3).
- Alternative fuels: refuse-derived fuel, biomass, co-processing of waste.
- Carbon capture, utilisation and storage (CCUS): the only way to deal with process CO2 at scale.
- Policy: cement is among the sectors covered by India's Carbon Credit Trading Scheme and by the EU's CBAM.
India is the world's second-largest cement producer after China. Limestone, clay, coal and markets largely decide where the industry is located.
NCERT Concepts to Revise
- Carbon capture, utilisation and storage: Calcination CO2 cannot be removed by switching fuels, which is why CCUS keeps coming up for cement plants.
- Carbon Border Adjustment Mechanism: Cement is one of the carbon-intensive goods covered by the EU's CBAM, which links cement emissions to trade.