Cement setting control – compound responsible for retarding the initial set To retard the initial setting time of ordinary Portland cement during hydration, which added compound is primarily responsible?

Introduction / Context:
Controlling the setting time of cement is crucial for handling, placing, and finishing concrete. Without regulation, certain clinker phases cause cement to set too rapidly (flash set). This question asks which compound is deliberately present to moderate the earliest reactions so that workable time (initial set) is adequate for construction operations.

Given Data / Assumptions:

• Ordinary Portland cement contains C3S, C2S, C3A, and C4AF.
• Gypsum is interground with clinker during manufacture.
• We focus on initial set retardation during early hydration.

Concept / Approach:

Tricalcium aluminate (C3A) reacts very quickly with water. If not controlled, this rapid reaction leads to flash setting. Gypsum (calcium sulfate) provides sulfate ions that react with C3A to form ettringite initially, which slows the reaction rate and thereby retards the initial set. C3S and C2S contribute to strength development, not primary set retardation.

Step-by-Step Solution:

Verification / Alternative check:

Standard cement chemistry notes show that removing gypsum causes immediate stiffening (flash set), confirming gypsum’s role as a set regulator.

Why Other Options Are Wrong:

C3S / C2S: Govern strength (early and later) rather than retarding set.

C3A: Causes rapid reaction; it is the reason a retarder is needed, not the retarder itself.

Common Pitfalls:

Confusing early strength (C3S) with set regulation; assuming any sulfate source is acceptable without dosage control; overlooking temperature’s influence on set time.

Final Answer:

Gypsum (calcium sulfate)