The reaction of cement starts immediately when the water is mixed. In it’s pure and finely ground form, it is extremely sensitive to water. Out of the three main compounds of cement, i.e. C3A, C3S, C2S, reacts quickly with water. They produce a jelly-like mixture that starts solidifying. This changing action from fluid to solid is called ‘Setting’ (not ‘hardening’).
In the next stage of hydration, cement paste starts hardening itself to the reaction of C3S and C2S with water. After that it become more harden and dominant. Once initial stiffing has taken place, it should be hardened or gain strength as soon as possible so the risk of frost damage in frozen climate is minimized. The exact timing should always be followed.
“Initial setting time” is defined the period elapsing between the times when water is added to the cement and the required section fails to pierce the test block to a depth from the bottom of the mould.
“The final setting time” is defined as the period elapsing between the time when water is added to cement and the time at which the needle of certain section with required attachment makes an impressive on the test block.
The compressive strength test is based on two functions to fulfill. 1. Final check on the quality of the cement. 2. In case of any doubt, it also helps to classify the cements according to their strength.
The difference between setting and hardening cement is: cement is said to harden when the paste further reacts with water into action. Hardening is associated with the development of strength.