Concrete setting time divided into initial setting time and final setting time, initial setting time is the time at which concrete losing plasticity, if the concrete stiffs quickly before the elapsed of initial setting time. This indicates that the concrete setting is abnormal, the abnormal setting of concrete occurs due to several reasons, and it is divided into:
- False setting: in false setting, concrete will lose plasticity and stiffen rapidly, then after remixing concrete will restore fluidity and set as usual. This occurs due to the nature of producing cement, in the last step of cement production, a small amount of gypsum mild with clinker by intergrounding. The temperature of the mix may increase due to the heat generated by friction and reaches up to 120 c. this will cause the following reaction
The product of this reaction is half-water gypsum or plaster, at time of adding water for concrete mix another reaction will occur, and half-water gypsum will rehydrate back to two-water gypsum. This will cause concrete to stiffen due to the tinny amount of gypsum. The gained strength is little, and concrete will restore fluidity by remixing.
- Flash setting: flash setting occurs due to the formation of large amounts of monosulfoaluminate or other calcium aluminate hydrate. This will occur due to the reactivities of C3A in the absense of gypsum. Adding of a small amount of gypsum 3% to 5% will prevent the flash setting of cocnrete, gypsum will react with C3A and water to form an AFt as a barrier layer that will stop further recation of C3A. Flash setting is a rapid setting, and it is worse than the false setting.