Admixtures are organic or inorganic materials added in limited quantities to improve or achieve the properties of the mortar.
Accelerators and Retarders are two types of admixtures.
- Mainly accelerators are used to reduce the setting time, thus gear up initial setting time.
- Early strength development in concrete.
- Usually used in cold weather conditions.
- Ex: Refined silicates, Calcium Chloride etc.
- Helps in speed up of construction work.
- Mainly retarders are used to increase the setting time, thus delay in initial setting time.
- Hydration reducing agent which keep concrete workable during placement.
- Usually used in hot weather conditions.
- Ex: Gypsum, Starch, Sugar etc.
- Increase strength by decreasing W/C ratio.
Gypsum is the retarding agent in cement to prevent it from premature hardening. The addition of gypsum in cement not only influences its setting time but other properties as well. Gypsum is fire resistant. It influences the strength, strength development, and volume stability of cement.