The correct answer is: A. Tri-calcium silicate.
Tri-calcium silicate (C3S) is the most important constituent of Portland cement, accounting for about 50-60% of its weight. It is the main component responsible for the initial setting of cement. When C3S is mixed with water, it reacts to form a gel that hydrates and hardens over time. This process is known as hydration.
Di-calcium silicate (C2S) and tri-calcium aluminate (C3A) are also important constituents of Portland cement, but they play a lesser role in the initial setting of cement. C2S reacts more slowly than C3S, and C3A reacts very quickly but forms a gel that is not as strong as the gel formed by C3S.
Tetra calcium alumino ferrite (C4AF) is the least important constituent of Portland cement, accounting for about 5-10% of its weight. It does not play a significant role in the initial setting of cement.
In summary, the initial setting of cement is caused by the hydration of tri-calcium silicate (C3S). This process is responsible for the formation of a gel that hydrates and hardens over time.