In cement chemistry, which compound present in ordinary Portland cement predominantly contributes to the ultimate long term strength of hardened cement paste?

Introduction / Context:
Ordinary Portland cement contains several important clinker minerals, and each one contributes differently to strength development over time. Civil engineering and construction exam questions often ask which specific compound is responsible for early strength and which one controls ultimate long term strength. This question focuses on identifying the compound that mainly contributes to the ultimate strength of hardened cement paste after long curing periods.

Given Data / Assumptions:

• We are dealing with ordinary Portland cement composition.
• Major clinker phases include tricalcium silicate (C3S), dicalcium silicate (C2S), tricalcium aluminate (C3A) and tetracalcium aluminoferrite (C4AF).
• Gypsum is present as a set regulator.
• Ultimate strength refers to long term strength, typically after many weeks or months of curing.

Concept / Approach:
Tricalcium silicate, C3S, hydrates relatively quickly and is mainly responsible for early strength development in the first few days. Dicalcium silicate, C2S, hydrates more slowly but continues to contribute strength over a longer period, playing a major role in the ultimate strength. The aluminates and ferrites contribute to early reactions, setting behaviour and colour but are not the primary sources of long term compressive strength. The approach is to recall the distinct roles of C3S and C2S in the hydration of cement.

Step-by-Step Solution:
Step 1: Recall that tricalcium silicate (C3S) hydrates rapidly and is the main contributor to strength in the first seven days. Step 2: Remember that dicalcium silicate (C2S) hydrates slowly and continues to form calcium silicate hydrate gel over extended curing periods. Step 3: Understand that long term or ultimate strength, often measured after 28 days and beyond, depends significantly on the continued hydration of C2S. Step 4: Recognise that C3A and C4AF have more influence on setting time, heat evolution and other properties rather than on ultimate compressive strength. Step 5: Therefore, the compound that mainly provides ultimate strength to cement is dicalcium silicate (C2S).

Verification / Alternative check:
Standard texts on concrete technology provide strength development curves showing rapid early strength gain attributable to C3S, followed by gradual long term strength growth due in large part to C2S. Microstructural studies of hardened cement paste confirm ongoing hydration of C2S at later ages. In practical terms, concretes with higher C2S content tend to gain strength more slowly but may reach higher ultimate strengths and lower heat of hydration, which is desirable for massive structures.

Why Other Options Are Wrong:
Option B, C3S, is mainly responsible for early strength, not ultimate strength, although it does contribute. Option C, C4AF, influences colour and some early properties but is not the main strength giving phase. Option D, C3A, reacts rapidly with water and gypsum to control setting and can generate considerable heat, but it is not the principal source of long term strength. Option E, gypsum, is added to regulate setting and prevent flash set; it does not control ultimate compressive strength. Only option A correctly identifies dicalcium silicate (C2S) as the main contributor to ultimate strength.

Common Pitfalls:
A common error is to select C3S for any strength related question because students remember that it gives high early strength. It is important to separate early strength from long term strength. Another pitfall is ignoring the slower but continuous hydration of C2S over weeks and months. Remembering that C3S is for early strength and C2S is for ultimate strength is a useful mnemonic in civil engineering examinations.

Final Answer:
The component that predominantly provides long term compressive strength is Dicalcium silicate (C2S), making it the correct answer for ultimate strength of cement.