Cement Compounds and Their Roles

Cement is a complex material primarily composed of four main compounds, also known as Bogue compounds. These compounds hydrate upon mixing with water, and each contributes differently to the setting and hardening process of cement. The four main compounds are:

• Tri-calcium Silicate ($\text{C}_3\text{S}$ or Alite): Hydrates quickly, contributes significantly to early strength (up to 7 days).
• Di-calcium Silicate ($\text{C}_2\text{S}$ or Belite): Hydrates slowly, contributes to strength gain at later ages (beyond 7 days) and provides ultimate strength and durability.
• Tri-calcium Aluminate ($\text{C}_3\text{A}$ or Celite): Hydrates very rapidly, contributes to flash setting. Its hydration releases a lot of heat. Gypsum is added to control its setting time.
• Tetra-calcium Aluminoferrite ($\text{C}_4\text{AF}$ or Ferrite): Hydrates moderately fast, contributes slightly to strength, and is mainly responsible for the colour of cement.

Role of Di-calcium Silicate ($\text{C}_2\text{S}$) in Cement

The question asks about the main role of di-calcium silicate ($\text{C}_2\text{S}$) in cement. As discussed above, di-calcium silicate hydrates slowly over time. This slow hydration process is crucial for the long-term strength development of cement.

While tri-calcium silicate ($\text{C}_3\text{S}$) provides the initial strength that allows concrete structures to be handled and worked on relatively early, di-calcium silicate ($\text{C}_2\text{S}$) continues to hydrate over weeks, months, and even years. This prolonged hydration reaction leads to the formation of more calcium silicate hydrate (C-S-H) gel, which is the primary binding material in hardened cement paste.

Therefore, the main role of di-calcium silicate is to provide the ultimate strength to the cement paste and concrete at later ages, contributing significantly to the long-term durability and structural integrity.

Analyzing the Options

Let's examine the given options based on the roles of the cement compounds:

• to provide ultimate strength at the latter age: This aligns perfectly with the known role of di-calcium silicate ($\text{C}_2\text{S}$) which hydrates slowly and contributes to strength over extended periods.
• to retard the flash setting of cement: Flash setting is primarily caused by the rapid hydration of tri-calcium aluminate ($\text{C}_3\text{A}$). Gypsum is added to control this. Di-calcium silicate does not play this role.
• to provide colour effect in cement: The colour of cement is mainly influenced by tetra-calcium aluminoferrite ($\text{C}_4\text{AF}$), which contains iron. Di-calcium silicate is a white compound.
• to flash set: Flash setting is caused by $\text{C}_3\text{A}$, not $\text{C}_2\text{S}$. Di-calcium silicate's hydration is very slow.

Based on the analysis, the option describing the role of providing ultimate strength at a latter age is the correct one for di-calcium silicate.