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Guy Barnes
Asked a question 2 years ago

What are various functions of retarders in Concrete? What is meant by accelerator?

Accelerator means what ? Retarder means what ? Why gypsum is used in cement ?

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Manuel Mason
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  • It acts as a retarding agent in cement, which prevents the flash setting of cement.
  • It has another advantage that it is fire resistant also.
  • It also influences the strength, strength development, and volume stability of cement. It simply means that the addition of gypsum in cement not only influences its setting time property but other properties as well.

Gypsum – CaSO4 . 2H2O


Added to cement to increase the setting time of cement so that it can be easily worked with, transported & placed.

If we don’t add gypsum to cement, cement dries or hardens very quickly as it is mixed with water.

By adding gypsum, it gives high strength & hardness to the cement.

Cement + water is a exothermic process, by adding gypsum, it acts as a coolant.

C3A is the compound responsible for initial setting of cement & is the first compound to react with water.

Gypsum reacts with C3A & turns it into Calcium sulphoaluminate

Since Gypsum reacts with C3A & C3A reduces.

Thus C3A is not available for reaction with water & hence setting action reduces.

Reduction in setting action means elongation of setting of cement i.e., setting time increases.

Accelerator :-

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.

Retarder :-

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.

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.


  • These admixtures increases rate of hydartion & concrete gains strength faster.
  • Dose: 0.1-0.2%
  • Applications: Prefab structures, pavement construction, cold weather concreting
  • Eg: Cal. chloride, silicates, Triethanol amine.


  • These admixtures slow down hydration so that concrete can remain workable for a longer duration.
  • These are used in areas with high temp to slow down the hydration process.
  • Dose: 0.05-0.2% by wt of cement
  • Hydration process can be delayed by 72hrs.
  • Applications: When transportation period is more
  • Eg: Cal. sulphate, sugar, starch, tartaric acid.

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.