Introduction:
Ordinary Portland Cement (OPC) is the most commonly used type of cement in construction. It is known for its initial setting and early strength gain properties, which play a crucial role in the strength development of concrete. The compound largely responsible for these properties in OPC is C3S (tricalcium silicate).

Explanation:
The setting and hardening of cement is a complex process influenced by various compounds present in the cement. Among these compounds, C3S has the most significant impact on the initial setting and early strength gain of OPC.

Initial Setting:
- The initial setting time of cement is the time taken for the cement paste to change from a plastic state to a solid state.
- C3S is responsible for the early hydration reactions, which lead to the initial setting of cement.
- It starts reacting with water as soon as water is added to cement, resulting in the formation of calcium silicate hydrate (C-S-H) gel.
- The C-S-H gel is responsible for the early strength development in cement.

Early Strength Gain:
- Early strength gain is desirable in construction as it allows for faster formwork removal and early loading of structures.
- C3S contributes to the early strength gain through its hydration reactions.
- The C-S-H gel formed during the hydration of C3S provides a strong and dense matrix, resulting in the development of early strength.
- The C-S-H gel also acts as a binder, holding the aggregates together and providing strength to the concrete.

Other Compounds:
While C3S is largely responsible for the initial setting and early strength gain of OPC, other compounds present in cement also contribute to these properties:
- C2S (dicalcium silicate) also contributes to the early strength development, but its contribution is relatively lower than C3S.
- C3A (tricalcium aluminate) and C4AF (tetracalcium aluminoferrite) have different roles in cement hydration and do not play a significant role in initial setting and early strength gain.

Conclusion:
In conclusion, the compound largely responsible for the initial setting and early strength gain of Ordinary Portland Cement (OPC) is C3S (tricalcium silicate). Its rapid hydration reactions lead to the formation of a strong and dense calcium silicate hydrate (C-S-H) gel, which provides the initial setting and early strength to the cement. While other compounds in cement also contribute to these properties, C3S has the most significant impact.