Dicalcium Silicate, Tetracalcium Aluminoferrite, Tricalcium Aluminate, and Tricalcium Silicate are the four main compounds present in Portland cement. These compounds react with water to form hydrated products, which contribute to the hardening and setting of the cement.

Among these compounds, Tricalcium Aluminate (C3A) is the one that reacts immediately with water and sets early. Let's understand why:

1. Tricalcium Aluminate (C3A):
- Tricalcium Aluminate is one of the main compounds in Portland cement, typically present in the range of 5-15%.
- It is represented by the chemical formula 3CaO.Al2O3.
- When water is added to cement, C3A undergoes a rapid hydration reaction, releasing a large amount of heat. This exothermic reaction leads to an increase in temperature, known as the heat of hydration.
- The reaction of C3A with water produces calcium aluminate hydrates and a calcium hydroxide compound called portlandite.
- The initial reaction of C3A with water contributes to the early strength development of the cement paste. However, it also results in a rapid setting time, which can be challenging in certain applications.

2. Dicalcium Silicate (C2S):
- Dicalcium Silicate is another major compound in Portland cement, typically present in the range of 20-30%.
- It is represented by the chemical formula 2CaO.SiO2.
- Unlike C3A, C2S reacts slowly with water. Its hydration process occurs gradually over time, contributing to the long-term strength development of the cement paste.

3. Tetracalcium Aluminoferrite (C4AF):
- Tetracalcium Aluminoferrite is also a significant compound in Portland cement, typically present in the range of 5-15%.
- It is represented by the chemical formula 4CaO.Al2O3.Fe2O3.
- Similar to C2S, C4AF reacts slowly with water and contributes to the long-term strength development of the cement paste.

4. Tricalcium Silicate (C3S):
- Tricalcium Silicate is the most abundant compound in Portland cement, typically present in the range of 40-60%.
- It is represented by the chemical formula 3CaO.SiO2.
- C3S reacts relatively quickly with water and contributes to both the early and long-term strength development of the cement paste. Its hydration process occurs faster than C2S but slower than C3A.

In summary, among the compounds in Portland cement, Tricalcium Aluminate (C3A) reacts immediately with water and sets early due to its rapid hydration reaction. Other compounds like Dicalcium Silicate (C2S), Tetracalcium Aluminoferrite (C4AF), and Tricalcium Silicate (C3S) contribute to the long-term strength development of the cement paste.