Previous Year Question
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 Statements II and III is correct and that explains Statement I.
Neither Statement II nor Statement III is correct.
Correct Answer: B
Detailed Solution
Statement I is correct: The cement industry is one of the major industrial sources of global carbon dioxide emissions, and its contribution exceeds 5%.
The two principal sources of CO₂ emissions from cement production are the chemical decomposition of limestone and the use of fuel and energy for cement kilns.
Statement II is correct: Limestone is mixed with clay or other materials that supply silica, alumina and iron oxide for cement manufacture.
This mixture is ground into a homogeneous raw meal, which is heated in the preheater and cement kiln.
Silica-bearing clay reacts with calcium oxide and assists in the formation of the principal calcium-silicate compounds of clinker.
Therefore, mixing silica-bearing clay with limestone is a correct process in cement manufacturing.
However, the act of mixing clay with limestone is not itself the direct reason for the cement industry’s high CO₂ emissions.
Thus, although Statement II is correct, it does not directly explain Statement I.
Statement III is correct: During clinker production, limestone or calcium carbonate is heated and converted into lime or calcium oxide.
This process is called calcination, and its chemical reaction is:
CaCO₃ → CaO + CO₂During this reaction, carbon dioxide chemically bound within the limestone is directly released.
Therefore, Statement III directly explains the high carbon dioxide emissions of the cement industry.
After calcination, the material is heated to approximately 1,450°C to form clinker minerals.
The combustion of coal, petcoke or other fuels to provide such high-temperature heat also releases additional CO₂.
Clinker is ground with gypsum and other materials as required to produce cement.
Measures to reduce cement-industry emissions include lowering the clinker content, using supplementary materials such as fly ash and slag, improving energy efficiency, adopting alternative fuels and renewable energy and applying carbon capture.