Civil Engineering - Building Materials - Discussion

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.

Yes, the sequence of the rate of hydration and heat of hydration for the four main constituents of cement is C3A > C3S > C4AF > C2S.

Rate of hydration:
The rate of hydration for the four constituents of cement is C3A > C4AF > C3S > C2S. C3S reacts more than five times faster than C2S.

Heat of hydration:
The heat of hydration for the four constituents of cement is C3A > C3S > C4AF > C2S. The heat of hydration for each compound is:
C3A: 867 J/kg
C3S: 502 J/kg
C4AF: 419 J/kg
C2S: 260 J/kg
The main cement constituent, C3S, contributes the most to the early strength of Portland cement, while C2S contributes the most to its long-term strength.

Water reacts with cement is true, but it is dicalcium silicate is the wrong one you predicted, its made up of 4 contents, dicalcium silicate, tricalcium silicate, tricalcium aluminate and tetracalcium alumino-ferrate. Dicalcium silicate gives the strength up to last stage, while tricalcium silicate gives the strength in initial phase like upto firth some days, while tricalcium silicate react instantly as false setting but its role in giving strength is very poor. Mainly dicalcium silicate and tricalcium silicate gives the strength to the concrete in later stage and initial stage respectively. But false setting reaction first is done by tricalcium aluminate.

Cement has 4 compounds which are known as Bogue compound.

1. Tricalcium silicate: It gives early strength.
2. Dicalcium silicate: It gives ultimate strength.
It also decreases the heat of hydration.

3. Tricalcium Aluminate:it rapidly reacts with.
Water. It is also responsible for the initial setting. It has a high heat of hydration.

4: Tetracalcium Amino Ferrite: It produces less heat. If increase the content reduce the strength slightly.

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.

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.

I think C4AF is the right one because the quantity of alumina in it is more as compare to C3A, alumina is responsible for quick setting for this it reacts instantly with water, so that's why it reacts quickly with water.

Cement has 4 component.

1. C3S it is quickly hydrated and a short time in give strength.
2. C2s it is a set time long periods and alternative setting time.
3. C3af training Calcium Alumino-ferrite.

I think C4AF is the right one because it reacts instantly with water where C3A produces the highest heat of hydration so it sets quickly and produces volume Change.

The Initial setting of Portland cement is due to tricalcium aluminate. Tricalcium silicate hydrates quickly and contributes more to the early strength.