In cement manufacturing for civil engineering works, which raw material is most commonly used as the principal source of calcium carbonate (the main ingredient in clinker)?

Introduction / Context:
Cement clinker is produced by heating a precisely proportioned mix of calcareous and argillaceous materials to form the compounds that give cement its hydraulic properties. Understanding which natural rock supplies the bulk of calcium carbonate is essential for materials engineers and site supervisors who evaluate raw material quality and quarry suitability.

Given Data / Assumptions:

• The question refers to ordinary Portland cement manufacturing.
• We assume standard kiln processes where raw meal is burned to clinker.
• Typical raw mix requires a calcareous component (rich in CaCO3) and a clayey/siliceous component.

Concept / Approach:
The dominant oxide in clinker is CaO, obtained primarily by calcining CaCO3. The most abundant and economical natural source of CaCO3 is limestone. During clinkering, CaCO3 decomposes to CaO and CO2 and then combines with SiO2, Al2O3, and Fe2O3 to form alite, belite, aluminate, and ferrite phases. Other rocks listed either lack sufficient CaCO3 or are not practical sources for the calcareous component.

Step-by-Step Solution:
Identify the needed raw constituent: high CaCO3 for CaO in clinker.Match to rock types: limestone is rich in CaCO3 (often 70–95%).Eliminate unsuitable rocks: sandstone (mostly SiO2), slate (metamorphosed shale, siliceous/aluminous), graphite (carbon, not a carbonate), laterite (may supply Fe/Al, low in CaCO3).Therefore, the principal material is limestone.

Verification / Alternative check:
Standard cement plant practice uses limestone quarries located near the kiln; quality control targets for the raw mix (LSF, SM, AM) presume limestone as the CaCO3 source.

Why Other Options Are Wrong:

• Slate/Sandstone: inadequate CaCO3; used, if at all, as minor corrective materials.
• Graphite: elemental carbon; irrelevant for CaO supply.
• Laterite: iron/alumina contributor, not principal calcareous component.

Common Pitfalls:
Confusing any sedimentary rock with limestone; overlooking the need for high CaCO3 content and low impurities like MgO and organic matter.

Final Answer:
Limestone