CEMENT TECHNOLOGY — Role of gypsum. Question: In Portland cement manufacture, gypsum is interground with clinker primarily to:

Introduction / Context:
Portland cement sets through hydration reactions of clinker phases (notably tricalcium silicate and tricalcium aluminate). Controlling the early setting time is essential for handling, placing, and finishing concrete. Gypsum plays a specific chemical role in moderating the very fast reaction of aluminate phases.

Given Data / Assumptions:

• Gypsum (calcium sulfate dihydrate) is added in a few percent by mass.
• Objective: regulate initial set, not long-term strength directly.
• We consider early hydration chemistry, especially of C₃A.

Concept / Approach:
Without gypsum, the highly reactive C₃A phase hydrates rapidly, causing “flash set.” Sulfate ions from gypsum combine with C₃A to form ettringite initially, which slows down aluminate hydration, providing workable time. Later, silicate hydration dominates strength development; gypsum’s main effect is on setting regulation rather than tensile strength per se.

Step-by-Step Solution:
Recognize the problem: C₃A reacts too quickly → flash set.Add gypsum → provides sulfate → forms ettringite with C₃A.Ettringite formation moderates the reaction rate → workable setting time.Therefore, gypsum acts primarily as a set retarder.

Verification / Alternative check:
Standards limit SO₃ content to balance workability and avoid excessive sulfate, confirming gypsum’s regulatory role.

Why Other Options Are Wrong:
A) Tensile strength results from hydration products over time, not from gypsum addition directly.C) “Colloidal gel” is not the targeted mechanism here.D) Gypsum does not act as a binder of calcium silicate particles.E) Color effects are incidental; the function is chemical control of set.

Common Pitfalls:
Confusing setting control with strength gain. Gypsum prevents flash set; it is not a plasticizer or strengthener in itself.

Final Answer:
decrease the rate of setting of cement (act as a retarder)