The correct answer is A. Tri-calcium silicate.
Tri-calcium silicate (C3S) is the most abundant compound in Portland cement, accounting for about 60% of its weight. It is the main compound that contributes to the strength of Portland cement. C3S reacts with water to form calcium silicate hydrate (C-S-H), which is a gel that fills the voids in the cement paste and provides strength.
Di-calcium silicate (C2S) is the second most abundant compound in Portland cement, accounting for about 25% of its weight. It reacts with water to form calcium silicate hydrate (C-S-H), but it does so more slowly than C3S. C2S also forms ettringite, which is a needle-like crystal that can cause expansion and cracking in concrete.
Tri-calcium aluminate (C3A) is the third most abundant compound in Portland cement, accounting for about 8% of its weight. It reacts with water to form calcium aluminate hydrate (C-A-H), which is a gel that provides strength. However, C3A is also the most reactive compound in Portland cement and can cause expansion and cracking in concrete if it is not properly hydrated.
Tetra-calcium alumino ferrite (C4AF) is the least abundant compound in Portland cement, accounting for about 3% of its weight. It reacts with water to form calcium alumino ferrite hydrate (C-A-F-H), which is a gel that provides strength. However, C4AF is also the least reactive compound in Portland cement and does not contribute significantly to the strength of Portland cement.
In conclusion, the compound of Portland cement which contributes to the strength after two to three years is A. Tri-calcium silicate.