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.

What is false setting reaction?

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.