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?
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- ABoth Statement II and Statement III are correct and both of them explain Statement I
- BBoth Statement II and Statement III are correct but only one of them explains Statement I
- COnly one of the Statements II and III is correct and that explains Statement I
- DNeither Statement II nor Statement III is correct
Show answer
Answer: (A) Both Statement II and Statement III are correct and both of them explain Statement I
Another assertion-reason question about cement industry emissions.
Statement I (Assertion):
Cement industry accounts for more than 5% of global CO₂ emissions.
— This is widely documented; some estimates put it at 7-8%.
Statement II:
Silica-bearing clay is mixed with limestone while manufacturing cement.
— CORRECT. In cement manufacturing, limestone (CaCO₃) is mixed with silica-bearing clay and heated in a kiln to about 1450°C. This extremely high temperature requires burning large quantities of fossil fuel, which releases CO₂. Additionally, the clay-limestone mixture undergoes chemical reactions that form clinker. ✓ Correct and contributes to explaining emissions.
Statement III:
Limestone is converted into lime during clinker production.
— CORRECT. This is the critical chemistry:
CaCO₃ (limestone) → CaO (lime) + CO₂.
This 'calcination' reaction directly releases CO₂ as a chemical byproduct — not from burning fuel, but from the raw material itself. This is why cement emissions are so hard to reduce even with renewable energy, because the CO₂ comes from the limestone, not just the fuel. ✓ Correct and directly explains emissions.
Both statements are correct and both explain why cement production is so carbon-intensive (Statement II explains the high energy needed, Statement III explains the chemical CO₂ release).
Answer is (a).
Cement production generates CO2 in two ways: from burning fossil fuels for extreme heat (1450°C) and from the chemical breakdown of limestone itself (CaCO3 → CaO + CO2).
Climate commitments and net-zero targets have made industrial emissions a major policy focus, with cement being one of the hardest sectors to decarbonize because CO2 comes from the raw material limestone, not just energy use.
The question tests whether students understand that industrial emissions have both energy-related and process-related sources, which require different solutions.
Cement Industry Carbon Emissions
Environment cement industry carbon dioxide emissions global carbon emissions
Cement Industry Carbon Emissions: Why It Accounts for 8% of Global CO₂
Cement industry contributes 7-8% of global CO₂ emissions — one of the largest industrial sources
Emissions come from two sources: fossil fuel burning (40%) and limestone calcination (60%)
Process emissions from limestone make cement a 'hard-to-abate' sector
India is the second-largest cement producer globally after China
Why Cement Is Carbon-Intensive
Cement manufacturing is uniquely carbon-intensive because emissions come from both energy use and chemical reactions. Unlike other industries that could theoretically switch to renewable energy, cement faces unavoidable process emissions from its raw materials.
Sources of Cement Industry Emissions
Emission Source | Contribution | Chemical Process | Can Renewable Energy Fix It? |
|---|---|---|---|
Fossil Fuel Burning | ~40% | Heating kilns to 1450°C | Yes - can use renewable electricity/hydrogen |
Limestone Calcination | ~60% | CaCO₃ → CaO + CO₂ | No - CO₂ comes from limestone itself |
Transportation | ~5% | Moving raw materials | Partially - electric vehicles |
Global Context
China produces ~55% of world cement, followed by India (~7%) and Vietnam (~3%)
Cement demand is driven by urbanization and infrastructure development in developing countries
Concrete (made from cement) is the most-used material on Earth after water
Industry faces pressure under Paris Agreement to achieve net-zero by 2050
Trap: Confusing cement with concrete — cement is the binding agent, concrete is cement + sand + gravel
Trap: Thinking all emissions come from fuel burning — 60% actually comes from limestone chemistry
Trap: Underestimating the percentage — cement is 7-8% of global emissions, not 3-4%
Cement Manufacturing Process
Science And Technology limestone silica-bearing clay clinker production
Cement Manufacturing: Raw Materials to Clinker
Limestone (CaCO₃) and silica-bearing clay are the main raw materials
Materials are heated to 1450°C in rotary kilns to form clinker
Calcination: CaCO₃ → CaO + CO₂ directly releases greenhouse gases
Final cement is clinker ground with gypsum to control setting time
Cement Production Steps
%%{init: {"flowchart": {"wrappingWidth": 460}}}%%
flowchart TD
s1["`**Raw Material Preparation**
**Limestone** (80%) + **silica-bearing clay** (15%) + iron ore + alumina`"]
s2["`**Grinding & Mixing**
Materials ground into **raw meal** and mixed in precise proportions`"]
s3["`**Kiln Heating**
Raw meal heated to **1450°C** in rotary kiln using coal/gas/oil`"]
s4["`**Calcination Reaction**
CaCO₃ → CaO + CO₂ (limestone becomes lime, releasing CO₂)`"]
s5["`**Clinker Formation**
Hot lime reacts with silica/alumina to form **clinker nodules**`"]
s6["`**Final Grinding**
Clinker ground with **gypsum** (3-5%) to make cement powder`"]
s1 --> s2
s2 --> s3
s3 --> s4
s4 --> s5
s5 --> s6Key Raw Materials
Material | Chemical Formula | Purpose | Typical % |
|---|---|---|---|
Limestone | CaCO₃ | Provides calcium oxide (lime) | 80% |
Silica-bearing Clay | Al₂O₃·2SiO₂·2H₂O | Provides silica & alumina | 15% |
Iron Ore | Fe₂O₃ | Provides iron oxide for clinker chemistry | 3% |
Gypsum | CaSO₄·2H₂O | Controls setting time of final cement | 3-5% |
Question Connection
Both Statement II and Statement III from the question correctly describe cement manufacturing. The high-temperature process (Statement II) and limestone calcination (Statement III) together explain why cement production is so carbon-intensive.
Trap: Thinking gypsum is a main raw material — it's only added at the end (3-5%)
Trap: Confusing lime (CaO) with limestone (CaCO₃) — limestone becomes lime
Trap: Missing that both clay and limestone are needed — not just limestone alone
Calcination & Chemical Reactions
Science And Technology limestone lime calcination
Calcination: Why Limestone Chemistry Drives Cement Emissions
Calcination: CaCO₃ → CaO + CO₂ is the key reaction in cement kilns
This thermal decomposition occurs at 900-1000°C and is endothermic
Process emissions from calcination are unavoidable — CO₂ comes from limestone itself
For every 1 ton of cement, about 0.6 tons of CO₂ are released from calcination alone
The Chemistry
Calcination is the thermal decomposition of limestone into lime and carbon dioxide. This reaction requires sustained high temperatures and directly converts the carbon in limestone into atmospheric CO₂ — making it impossible to eliminate even with clean energy.
Calcination vs Combustion Emissions
Emission Type | Source | Chemical Reaction | Reduction Strategy |
|---|---|---|---|
Process Emissions | Limestone calcination | CaCO₃ → CaO + CO₂ | Carbon capture, alternative materials |
Combustion Emissions | Fuel burning | Coal/Gas + O₂ → CO₂ + H₂O | Renewable energy, hydrogen, biomass |
Why This Matters for Climate
Process emissions make cement a 'hard-to-abate' sector unlike steel or aluminum
Even with 100% renewable energy, cement would still release CO₂ from limestone
Solutions require carbon capture or alternative cement chemistries
Supplementary materials like fly ash can partially replace clinker and reduce emissions
Calcination Reaction
Source: ScienceDirect.com — An environment-friendly process for limestone calcination ... · www.sciencedirect.com
Trap: Thinking calcination removes CO₂ from the atmosphere — it releases CO₂
Trap: Confusing endothermic (requires heat input) with exothermic (releases heat)
Trap: Missing that this reaction happens at 900-1000°C, not room temperature
Hard-to-Abate Industrial Sectors
Environment
Hard-to-Abate Sectors: Industries Beyond Renewable Energy
Hard-to-abate sectors have emissions from chemical processes, not just energy
Include cement, steel, chemicals, shipping, and aviation
Account for ~30% of global CO₂ emissions and are critical for net-zero goals
Solutions require carbon capture, hydrogen, or alternative materials
Definition & Challenge
Hard-to-abate sectors are industries where emissions cannot be eliminated through renewable electricity alone. They face technical, economic, or physical barriers that make decarbonization extremely challenging compared to sectors like power generation.
Major Hard-to-Abate Sectors
Sector | Main Challenge | Global Emissions % | Decarbonization Path |
|---|---|---|---|
Cement | Limestone calcination (process emissions) | 8% | Carbon capture, alternative materials |
Steel | Coal needed for iron reduction | 7% | Hydrogen-based steel, electric arc furnaces |
Chemicals | Feedstock from fossil fuels | 5% | Bio-based feedstock, recycling |
Shipping | Energy density for long voyages | 3% | Ammonia, hydrogen, methanol fuels |
Aviation | Weight constraints for batteries | 2% | Sustainable aviation fuels, hydrogen |
Policy & Investment Focus
Mission Innovation focuses R&D funding on breakthrough technologies for these sectors
EU Carbon Border Adjustment will impact cement, steel, and chemical imports
Green premium — extra cost for low-carbon alternatives — remains high
India's climate targets require addressing these sectors beyond renewable energy expansion
Trap: Thinking renewable energy alone can decarbonize all industries
Trap: Underestimating their importance — they're 30% of global emissions
Trap: Confusing with easy-to-abate sectors like power generation and buildings