Cement chemistry — compound responsible for initial setting In ordinary Portland cement, which compound primarily governs the initial setting behavior?

Introduction / Context:
The setting and hardening of Portland cement are controlled by reactions of its clinker compounds with water. Identifying which compound influences the initial set is important for mix design, hot weather concreting, and sulfate resistance.

Given Data / Assumptions:

• Main clinker phases: C3S, C2S, C3A, C4AF, with gypsum added to regulate set.
• Initial set refers to the early stiffening when paste loses plasticity.
• Gypsum moderates the rapid reaction of C3A.

Concept / Approach:
C3A hydrates very rapidly and would cause flash set if not controlled by gypsum, which forms ettringite initially and delays the reaction rate. Thus, C3A is closely linked to initial setting characteristics. C3S contributes mainly to early strength (first days), while C2S contributes to later strength.

Step-by-Step Solution:
Identify fast-reacting phase: C3A → rapid heat evolution and stiffening.Role of gypsum: added to prevent flash set by moderating C3A hydration.Therefore, compound primarily governing initial set: C3A.

Verification / Alternative check:
Cements with low C3A (sulfate-resisting) show slower initial set and reduced heat from aluminate hydration compared to ordinary cements.

Why Other Options Are Wrong:
C3S influences early strength more than initial set timing; C2S is slower-reacting; C4AF has minor impact on setting; gypsum is a regulator, not the clinker phase primarily governing the intrinsic rapid set tendency.

Common Pitfalls:
Confusing early strength (C3S) with initial set (C3A); neglecting gypsum content leading to flash set or false set issues; overlooking temperature effects on aluminate hydration.

Final Answer:
Tricalcium aluminate (C3A)