Tri-calcium aluminate (C3A)
The constituent of cement responsible for all the undesirable properties is tri-calcium aluminate (C3A). It is one of the four main components of cement, along with di-calcium silicate (C2S), tri-calcium silicate (C3S), and tetra-calcium alumino ferrite (C4AF). However, C3A is known to cause various issues in cement due to its reactivity and hydration characteristics.

Undesirable Properties of C3A:
- Rapid setting: C3A hydrates quickly, leading to rapid setting of cement, which can be problematic during construction.
- Sulphate attack: C3A reacts with sulphates present in the environment, leading to the formation of expansive products that can cause cracking and deterioration of concrete.
- Alkali-silica reaction: C3A can also contribute to alkali-silica reaction, which results in the formation of a gel that can lead to cracking and reduced durability of concrete structures.
- Heat generation: Hydration of C3A generates more heat compared to other components, which can result in thermal cracking in large concrete structures.

Impact on Cement Performance:
- Decreased workability: Rapid setting caused by C3A can reduce the workability of concrete, making it challenging to place and finish.
- Reduced strength: Sulphate attack and alkali-silica reaction can compromise the strength and durability of concrete over time.
- Cracking: Heat generation and rapid setting associated with C3A can lead to early-age cracking in concrete structures.
In conclusion, while tri-calcium aluminate is an essential component of cement, its reactivity and undesirable properties make it crucial to carefully control its content in cement formulations to ensure the desired performance and durability of concrete structures.