Tricalcium aluminate (C3A) is one of the four major compounds present in cement, along with tricalcium silicate (C3S), dicalcium silicate (C2S), and tetracalcium aluminoferrite (C4AF). C3A is responsible for several properties of cement, including its early strength development and setting time. In this answer, we will focus on the three properties of C3A mentioned in the question: its weak resistance against sulfate attack, its high heat of evolution, and its fast reaction with water.

Weak Resistance Against Sulfate Attack

Sulfate attack is a chemical reaction in which sulfates in the environment react with the components of cement, leading to the formation of expansive products that can damage concrete. C3A is particularly susceptible to sulfate attack because it reacts with sulfate ions to form ettringite, a mineral that can cause expansion and cracking in concrete. This reaction is more likely to occur in environments with high levels of sulfates, such as soil and groundwater near mines, chemical plants, or agricultural areas.

High Heat of Evolution

When cement is mixed with water, a chemical reaction called hydration occurs, in which the cement compounds react with water to form a hardened mass. This reaction is exothermic, meaning it releases heat. C3A is responsible for the highest heat of evolution of all the cement compounds, because it reacts quickly with water to form calcium aluminate hydrates. This property can be both advantageous and problematic, depending on the application of the cement. For example, in cold weather concreting, the high heat of evolution can help accelerate the setting and hardening of the concrete, but in mass concrete structures, where large volumes of concrete are placed at once, the high heat can cause thermal cracking.

Fast Reaction with Water

C3A is the most reactive of all the cement compounds, meaning it reacts the fastest with water. This property is responsible for the early strength development of cement, because C3A reacts quickly to form calcium aluminate hydrates, which contribute to the strength of the cement paste. However, the fast reaction can also cause problems such as flash set, which occurs when the cement sets too quickly and does not have enough time to be properly mixed and placed. Flash set can lead to poor quality concrete with reduced strength and durability.

Conclusion

In summary, C3A is an important compound in cement, contributing to its early strength development and setting time. However, it also has some drawbacks, such as weak resistance against sulfate attack, high heat of evolution, and fast reaction with water. Understanding the properties of C3A is crucial for selecting the right type of cement for a given application and for ensuring the quality and durability of concrete structures.