The correct answer is D. All of the above.
Dicalcium silicate (C2S), tricalcium silicate (C3S), and tricalcium aluminate (C3A) are the three main constituents of cement. They are responsible for the initial setting and hardening of cement.
Dicalcium silicate is the most abundant constituent of cement, making up about 60% of its mass. It is responsible for the initial setting of cement. When dicalcium silicate comes into contact with water, it reacts to form calcium silicate hydrate (C-S-H), a gel that binds the cement particles together.
Tricalcium silicate is the second most abundant constituent of cement, making up about 25% of its mass. It is responsible for the hardening of cement. When tricalcium silicate comes into contact with water, it reacts to form calcium silicate hydrate (C-S-H) and calcium aluminate hydrate (C-A-H). C-A-H is a stronger gel than C-S-H, and it is responsible for the long-term strength of cement.
Tricalcium aluminate is the least abundant constituent of cement, making up about 15% of its mass. It is responsible for the early strength of cement. When tricalcium aluminate comes into contact with water, it reacts to form calcium aluminate hydrate (C-A-H) and ettringite (AFt). Ettringite is a large, needle-shaped crystal that expands as it forms. This expansion helps to interlock the cement particles together, which increases the early strength of cement.
In conclusion, all three of the main constituents of cement (dicalcium silicate, tricalcium silicate, and tricalcium aluminate) are responsible for the initial setting and hardening of cement.