Cement chemistry — compound responsible for long-term strength Which compound in Ordinary Portland Cement contributes most to strength gain after about two to three years of curing?

Introduction / Context:
Portland cement has several clinker phases that hydrate at different rates. Understanding which compound governs early versus long-term strength helps in selecting cement for diverse structural and durability needs.

Given Data / Assumptions:

• Cement phases: C3S, C2S, C3A, C4AF with gypsum added for set control.
• Strength age range considered: beyond 2 years (long-term).

Concept / Approach:
C3S hydrates rapidly, dominating early strength (1–28 days). C2S hydrates more slowly, with strength contribution rising at later ages (months to years). Aluminate and ferrite phases primarily affect setting, heat evolution, and color rather than long-term compressive strength.

Step-by-Step Solution:

Verification / Alternative check:
Textbook hydration kinetics attribute late strength to C2S producing C-S-H with lower heat evolution, beneficial for mass concrete.

Why Other Options Are Wrong:

• C3S (A): early strength former; effect tapers later.
• C3A (C): influences setting and sulfate resistance, not long-term strength.
• C4AF (D): minor effect on strength; affects color and some reactions.
• Gypsum (E): set regulator; not a strength-forming clinker phase.

Common Pitfalls:
Assuming the phase that gives high early strength also controls very late strength. In reality, hydration kinetics and microstructure evolution shift the contribution to C2S with time.

Final Answer: