Extra Rapid Hardening Cement-Definition, Uses and Advantages
Extra rapid hardening cement is obtained by inter grinding calcium chloride with rapid hardening Portland cement. The normal addition of calcium chloride should not exceed 2 percent by weight of the rapid hardening cement. It is necessary that the concrete made by using extra rapid hardening cement should be transported, placed and compacted and finished within about 20 minutes.
It is also necessary that this cement should not be stored for more than a month. Extra rapid hardening cement accelerates the setting and hardening process. A large quantity of heat is evolved in a very short time after placing. The acceleration of setting, hardening, and evolution of this large quantity of heat in the early period of hydration makes the cement very suitable for concreting in cold weather.
The strength of extra rapid hardening cement is about 25 percent higher than that of rapid hardening cement at one or two days and 10–20 percent higher at 7 days. The gain of strength will disappear with age and at 90 days the strength of extra rapid hardening cement or the ordinary portland cement may be nearly the same.
There is some evidence that there is a small amount of initial corrosion of reinforcement when extra rapid hardening cement is used, but in general, this effect does not appear to be progressive and as such, there is no harm in using extra rapid hardening cement in reinforced concrete work.
However, its use in prestressed concrete construction is prohibited. In Russia, the attempt has been made to obtain the extra rapid hardening property by grinding the cement to a very fine degree to the extent of having a specific surface between 5000 to 6000 sq. cm/gm.