Cement manufacture — which raw material is most commonly used as the principal source of calcium in Portland cement production? Select the correct rock.

Introduction / Context:
Portland cement clinker is produced by sintering a precisely proportioned raw mix dominated by calcium oxide, along with silica, alumina, and iron oxides. The main calcium source is a decisive choice for plant location and quarrying strategy.

Given Data / Assumptions:

• We consider standard Portland cement manufacture.
• Primary requirement: CaCO3-rich feedstock.
• Auxiliary sources supply silica, alumina, and iron.

Concept / Approach:
Limestone (calcium carbonate) is the principal raw material supplying CaO upon calcination. It is abundant, quarried easily, and blends well with shale/clay/iron correctives to achieve kiln feed chemistry. Slate, sandstone, and basalt lack the necessary high CaCO3 content to serve as the main calcium source in typical cement plants.

Step-by-Step Solution:

Verification / Alternative check:
All major cement plants are co-located with limestone quarries to secure long-term, high-CaCO3 feed.

Why Other Options Are Wrong:

• Slate/Sandstone/Basalt: Do not provide the high calcium carbonate content required.

Common Pitfalls:
Confusing corrective additives with the principal calcium-bearing raw rock.

Final Answer:
Limestone