Cement manufacturing — why is gypsum added to clinker during grinding? In Portland cement production, gypsum is interground with clinker primarily for:

Introduction / Context:
Freshly ground clinker hydrates very rapidly, especially the tricalcium aluminate (C3A) phase, which can cause flash set. Gypsum addition is a standard control to achieve workable setting times suitable for mixing, transporting, and placing concrete.

Given Data / Assumptions:

• Ordinary Portland cement with typical C3A content.
• Gypsum added in the range of a few percent by mass.
• Normal temperature mixing and curing.

Concept / Approach:

Gypsum provides sulphate ions that react with C3A to form ettringite initially, slowing down the otherwise very fast C3A hydration. This control prevents flash setting and gives adequate working time before stiffening.

Step-by-Step Solution:

Verification / Alternative check:

Standards limit sulphate content to balance set control against risks of late ettringite formation.

Why Other Options Are Wrong:

• Colour and ultimate strength are not the primary reasons for gypsum addition.
• “No purpose” is incorrect; gypsum is essential for set control.

Common Pitfalls:

• Assuming more gypsum always helps; excessive sulphate can cause expansion-related distress.

Final Answer:

Controlling the setting time by moderating C3A hydration.