Civil Engineering - Building Materials - Discussion

Tricalcium aluminate results into flash setting of cement and in order to prevent flash setting Gypsum is added.

C4af is involve first in rate of hydration.

C3A is characteristically fast reacting with water and may lead to an immediate stiffening of paste and this process is termed as a flash set. The role of gypsum added in the manufacture of cement is to prevent such a fast reaction.

Yes, the correct answer A. I agree.

C3a qukist react btt c3s fst react with water.

The Rate of hydration is more for C4AF then how C3A react first?

It is not di calcium silicate its tri calcium aluminate because it has highest affinity toward water and generate more heat about 809 kJ/kg more than C2S.

It react with h2o to form false set rich hinder other strength giving reaction. That hy gypsum is added to react it c3a to form tranzite.

Definitely its di-calcium silicate not tri-calcium aluminate.

What about tetracalcium alumino ferrite C4AF?

C 3A :

Tricalcium Aluminate reacts most strongly with water of all the calcium aluminates. Its hydration leads to the phenomenon of "flash set" (instantaneous set) , and a large amount of heat is generated. To avoid this, Portland-type cements include a small addition of calcium sulfate (typically 4-8%). Sulfate ions in solution lead to the formation of an insoluble layer of ettringite over the surface.

Tricalcium aluminate is associated with three important effects that can reduce the durability of concrete:

1. "Overheating" in large masses of concrete.

2. Formation ettringite in sulfate attack, This reaction is expansive, and can "disrupt mature concrete".

3. Delayed ettringite formation, where "concrete is cured at temperatures above the decomposition temperature" of ettringite (about 65'C). On cooling, expansive ettringite formation takes place.

Because they are even more basic, the alkali-loaded polymorphs are correspondingly more reactive. Appreciable amounts (>1%) in cement make set control difficult, and the cement becomes excessively hygroscopic. The cement powder flowability is reduced, and air-set lumps tend to form.

They withdraw water from gypsum on storage of the cement, leading to false set. For this reason, their formation is avoided.