UPSC Prelims 2025Question 31Environment

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

(a)Both Statement II and Statement III are correct and both of them explain Statement I
(b)Both Statement II and Statement III are correct but only one of them explains Statement I
(c)Only one of the Statements II and III is correct and that explains Statement I
(d)Neither Statement II nor Statement III is correct

Correct answer: b

Decision rule

A raw-material composition fact does not explain an emissions fact; only the step that chemically releases carbon dioxide does the explaining.

Explanation

Correct Answer:

(b) Both Statement II and Statement III are correct but only one of them explains Statement I

Explanation of the Correct Answer:

Statement I: Studies indicate that carbon dioxide emissions from cement industry account for more than 5% of global carbon emissions.

Correct.
Cement production contributes ~7–8% of global CO₂ emissions, making it a major industrial source of greenhouse gases.
Emissions are due to both:
- Combustion of fossil fuels for high-temperature kilns.
- Calcination process (chemical decomposition of limestone).

Statement II: Silica-bearing clay is mixed with limestone while manufacturing cement.

Correct.
Cement is made by heating limestone (CaCO₃) with silica-rich materials (like clay/shale).
These materials provide silica (SiO₂), alumina (Al₂O₃), and iron oxide (Fe₂O₃) — all essential for cement composition.

Statement III: Limestone is converted into lime during clinker production for cement manufacturing.

Correct.
This process is called calcination, where calcium carbonate (CaCO₃) is heated and decomposes into calcium oxide (CaO) and CO₂:

\[CaCo_3\rightarrow CaO+CO_2\]

This chemical reaction is responsible for a large share of CO₂ emissions in cement production.

Why (b) is the correct option:

Both Statement II and Statement III are factually correct.
However, only Statement III explains Statement I, because it directly accounts for CO₂ emissions via calcination, which is a major source of carbon emissions from the cement industry.

Why Other Options Are Incorrect:

(a) Both explain: Incorrect. Statement II is about raw material composition, not emissions.
(c) Only one is correct: Incorrect. Both Statements II and III are correct.
(d) Neither correct: Incorrect, both Statements II and III are factually valid.

Relevant Supplementary Information

Table: Key Components in Cement Manufacturing

Input	Source	Purpose in Cement
Limestone (CaCO₃)	Natural rock	Provides calcium for strength
Clay/Shale	Sedimentary rock	Provides silica, alumina, iron
Gypsum	Mineral additive	Regulates setting time
Fuel	Coal, pet coke, alternative	Provides heat for clinker production

Table: Emissions in Cement Industry

Source of CO₂	Contribution to Total Emissions
Calcination of limestone	~60%
Fuel combustion in kilns	~30–35%
Electricity consumption, others	~5–10%

Current Relevance:

The Global Cement and Concrete Association (GCCA) and UN agencies are focusing on low-carbon alternatives like:
- Blended cement (with fly ash, slag)
- Carbon capture, utilization, and storage (CCUS)
- Green hydrogen and alternative fuels in kiln operations

Table 1: Key Raw Materials in Cement Manufacturing

Material	Chemical Component	Function
Limestone	Calcium Carbonate (CaCO₃)	Primary source of lime (CaO)
Clay/Shale	Silica (SiO₂), Alumina (Al₂O₃), Iron Oxide (Fe₂O₃)	Provides silicate, aluminate, and ferrite compounds
Gypsum	Calcium Sulfate (CaSO₄·2H₂O)	Controls the setting time of cement
Iron Ore (if added)	Fe₂O₃	Added to adjust the composition

Table 2: Major Steps in Cement Manufacturing

Step	Description
1. Raw Material Preparation	Crushing and mixing of limestone, clay, and other additives
2. Calcination	Heating CaCO₃ at high temperatures to produce CaO and release CO₂
3. Clinker Production	Formation of nodules (clinker) in rotary kilns at ~1400–1500°C
4. Grinding	Clinker is ground with gypsum to produce cement

Table 3: CO₂ Emission Sources in Cement Industry

Source of Emission	Cause	Estimated Share of Total Emissions
Calcination	Decomposition of limestone (CaCO₃ → CaO + CO₂)	~60%
Combustion of Fossil Fuels	Burning coal, pet coke, etc., in kilns for heat	~30–35%
Electricity Use & Others	Machinery operation, transportation, etc.	~5–10%

Table 4: Environmental Impact and Mitigation Measures

Impact	Details	Mitigation
High CO₂ emissions	Contributes ~7–8% of global carbon emissions	Use of blended cement, CCUS, alternative fuels
High energy consumption	Rotary kilns require large amounts of energy	Improve kiln efficiency, use renewables
Dust and particulate pollution	From crushers and grinders	Use of filters, enclosures, wet scrubbers

Table 5: Role of Clinker in Cement and Its Chemical Reactions

Clinker Component	Chemical Formula	Function in Cement
Alite	3CaO·SiO₂ (C₃S)	Early strength development
Belite	2CaO·SiO₂ (C₂S)	Later strength development
Aluminate	3CaO·Al₂O₃ (C₃A)	Reacts with water and gypsum; affects setting time
Ferrite	4CaO·Al₂O₃·Fe₂O₃ (C₄AF)	Contributes to color and minor strength

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