The correct answer is: Di-calcium silicate.
Di-calcium silicate (C2S) is the main compound responsible for the early strength of cement. It is a relatively fast-reacting compound that starts to hydrate within minutes of mixing with water. This initial hydration reaction produces a gel that fills the voids between the cement particles, providing early strength and setting.
Tetra-calcium alumino-ferrite (C4AF) is a slower-reacting compound that contributes to the long-term strength of cement. Tri-calcium silicate (C3S) is also a fast-reacting compound, but it is not as important for early strength as C2S. Tri-calcium aluminate (C3A) is a very fast-reacting compound that can cause problems with setting and strength if it is not properly controlled.
In summary, di-calcium silicate is the main compound responsible for the early strength of cement. It is a relatively fast-reacting compound that starts to hydrate within minutes of mixing with water. This initial hydration reaction produces a gel that fills the voids between the cement particles, providing early strength and setting.