Accelerator are the admixtures for the use in concrete, mortar, rendering or screeds. The addition of accelerator in concrete speeds the setting time and thus cure time starts earlier. Accelerator allows concrete to be placed in winter with reduced risk of frost damage. The chemicals used for acceleration today are calcium nitrate, calcium nitrite, calcium formate and aluminium compounds. Calcium chloride is the most efficient and least expensive accelerator and is also very popular.
Retarders helps and work as water reducing agent & cement retarder keep concrete workable during placement and delay the initial set of concrete. Retarder work for slowing the setting rate of concrete & also used to counteract the accelerating effect of hot weather on concrete setting. They are used for increasing the setting time of cement.
Gypsum is called retarding agent of cement which is mainly used for regulating the setting time of cement and is an indispensable component. Gypsum is used in cement because it slow down the hydration rate of cement once it is mixed with water.