Portland cement chemistry – earliest reacting compound on hydration Which compound of Portland cement reacts immediately with water and is responsible for the earliest set?

Introduction:
Understanding the hydration behavior of cement compounds is central to controlling setting time, early strength, and durability. This question targets the compound that reacts most rapidly with water and largely governs the initial set if not moderated by gypsum.

Given Data / Assumptions:

• Ordinary Portland cement (OPC) composition includes C3S, C2S, C3A, and C4AF.
• Gypsum is added to regulate flash set.
• We focus on the earliest, fastest reaction with water.

Concept / Approach:

Tricalcium aluminate (C3A) hydrates very quickly, releasing significant heat. Without gypsum, C3A would cause flash setting. Gypsum supplies sulfate to form ettringite initially, controlling the rapid reaction and enabling a workable setting time. C3S contributes mainly to early strength but does not react as instantly as C3A in terms of setting behavior.

Step-by-Step Solution:

Verification / Alternative check:

Cement chemistry literature consistently describes C3A as the phase responsible for flash set and early heat evolution, confirming it as the earliest reacting compound.

Why Other Options Are Wrong:

C3S: Important for early strength, but slower than C3A for immediate set.

C2S: Contributes to later strength; slower hydration.

C4AF: Reactivity is moderate and not dominant in setting.

Common Pitfalls:

Equating early strength with earliest set; overlooking gypsum’s regulatory role; confusing heat evolution profiles among phases.

Final Answer:

Tricalcium aluminate (C3A)