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