Accelerators are admixtures that can be added in concrete during mixing to accelerate the hardening of concrete or gaining the initial strength. It is very useful when there is a time constraint, and you need to erect the structure as fast as possible. It is also useful for colder reasons where the setting time of concrete is low, which can be increased using the accelerator.
Retarders also knew as water reducers, are admixture to reduce the water-cement ratio without affecting slump value. It is used when high strength concrete is required, which can be achieved by reducing the water-cement ratio, which in turn increases the slump value and hence not workable, so by using readers, this problem can be solved easily. Retarders also reduce the setting time of concrete and hence are ideal to be used for mass concreting or in a very hot climate to overcome the heat of hydration, which can result in cracks.
Gypsum is added into cement to slow down the hydration process while the addition of water to it. It allows the mixture to be workable for more time periods.