Introduction:
The initial setting time of cement refers to the period of time between the addition of water to the cement and the point at which the cement paste starts to lose its plasticity and hardens. This is an important property of cement as it determines the workability and handling characteristics of the cement during construction.

Explanation:
The initial setting time of cement is mainly influenced by the chemical composition of the cement. Among the various compounds present in cement, tri-calcium aluminate (C3A) has the most significant effect on the initial setting time.

Tri-calcium aluminate (C3A):
Tri-calcium aluminate is one of the primary compounds found in cement. It reacts with water to form calcium aluminate hydrates, which contribute to the strength and hardening of the cement paste. However, the hydration of C3A is an exothermic reaction, which means it releases heat during the process. This heat generation accelerates the setting time of cement.

Effect on initial setting time:
When water is added to cement, the C3A compound hydrates rapidly, releasing a large amount of heat. This rapid hydration and heat release lead to the early setting of cement. As a result, the initial setting time is significantly reduced.

Influence of other cement compounds:
While tri-calcium aluminate has the most significant effect on the initial setting time, other cement compounds also play a role. Tri-calcium silicate (C3S) and di-calcium silicate (C2S) are the other major compounds in cement. However, they have a relatively minor influence on the initial setting time compared to C3A.

Conclusion:
In conclusion, the initial setting time of cement depends most on the presence of tri-calcium aluminate (C3A) in the cement composition. The rapid hydration and heat release of C3A accelerate the setting time, leading to the cement paste losing its plasticity and hardening quickly. While other cement compounds such as tri-calcium silicate (C3S) and di-calcium silicate (C2S) also contribute to the setting time, their influence is relatively minor compared to C3A.