Cement chemistry—early hydration reactivity: Among the principal clinker phases in Portland cement, which compound reacts the fastest with water immediately after gauging, strongly affecting initial set and heat evolution?

Introduction / Context:

Portland cement hydration involves several clinker phases with distinct reaction rates and products. The earliest reactions determine initial set, flash-set control, and early heat release—parameters crucial for workability and thermal stress considerations in mass concrete.

Given Data / Assumptions:

• Typical OPC containing C₃S, C₂S, C₃A, and C₄AF with gypsum as set regulator.
• Normal curing temperatures and water-to-cement ratios.
• Question focuses on reaction speed at early times.

Concept / Approach:

Tricalcium aluminate (C₃A) hydrates very rapidly and can cause flash set if gypsum is absent. Gypsum moderates this by forming ettringite initially. Although C₃S (alite) contributes strongly to early strength, its hydration rate is slower than the instantaneous reactivity of C₃A at the moment of mixing. C₂S (belite) is slow-reacting and contributes to later strength, while C₄AF is moderately reactive with less pronounced heat.

Step-by-Step Solution:

Verification / Alternative check:

Calorimetry curves show a pronounced early peak associated with aluminate reactions (tempered by gypsum), confirming C₃A’s rapid hydration.

Why Other Options Are Wrong:

• C₃S: fast and key for early strength but not as instantaneous as C₃A.
• C₂S: slow; linked to later strength.
• C₄AF: moderate; lesser heat and reactivity early on.
• Gypsum: additive/regulator, not a clinker phase controlling intrinsic reactivity.

Common Pitfalls:

• Equating ‘‘fastest reaction’’ with ‘‘earliest strength’’—C₃S dominates early strength, but C₃A reacts fastest chemically.

Final Answer:

Tricalcium aluminate