Cement Industry — Primary Raw Materials In ordinary Portland cement manufacture, which combination correctly lists the principal raw material inputs used for making the clinker before gypsum is added for final setting control?

Introduction / Context:
Cement questions test recognition of base materials that form clinker minerals (alite, belite, aluminate, ferrite). Understanding the raw mix prevents confusion between ingredients added during clinkering and minor additives used after grinding for control of setting time.

Given Data / Assumptions:

• Cement type: ordinary Portland cement (OPC).
• Clinker formation requires calcium oxide and silica/alumina/iron oxides.
• Gypsum is typically a post-clinker additive for setting regulation.

Concept / Approach:

The primary calcium source is limestone (CaCO3). Silica and alumina come largely from clay/shale. The raw mix is proportioned and burned to form clinker; only after cooling and grinding is gypsum (CaSO4·2H2O) added (about 3–5%) to moderate flash set by controlling C3A hydration. Therefore, the correct pair for raw materials is limestone and clay.

Step-by-Step Solution:

Identify CaO source → limestone.Identify SiO2/Al2O3 sources → clay/shale.Note: gypsum is not a principal clinkering raw; it is added after.Select “Limestone and Clay.”

Verification / Alternative check:

Industry process flowsheets consistently list limestone + clay/shale as core feed to kiln for clinker formation.

Why Other Options Are Wrong:

Gypsum and Clay: Gypsum is post-kiln additive, not principal clinker raw.Clay only / Limestone only: Incomplete; both CaO and SiO2/Al2O3 sources are needed.Silica sand and gypsum: Omits main CaCO3 source and misuses gypsum.

Common Pitfalls:

Assuming the prominent mention of gypsum in cement bags means it is a main raw material; it is not—it controls setting after grinding.

Final Answer:

Limestone and Clay