why gypsum added to cement?

How gypsum delays cement setting

Tricalcium aluminate reacts with water to form calcium aluminate hydrate ions.

C3A     +    water    =    calcium aluminate hydrate 

This reaction occurs very fastly and releases more heat.This reaction causes irreversible stiffening of cement paste. So cement sets within minutes. This set is called quick set or false set of cement. To delay or increase this setting gypsum was added to cement while manufacturing.

 

(C4AF reactions and products are similar to C3A,but this reaction is very slow.)

 

Generally cement will be mixed at one place and it will be consumed at some other place. So the mixed cement mortar to be carried from mixed site to usage site which consumes some time until which cement should not set. If set the cement cannot be used. So only gypsum was added to delay this setting time.

Water dissolves calcium sulphate ions in gypsum and mixes with lime in cement to form calcium sulphate solution.

Calcium sulphate + Calcium aluminate hydrate = Ettringite

This calcium sulphate solution reacts with calcium aluminate hydrate from C3A hydration to form a insouble compound called ettringite.

This ettringite as insoluble, deposits on C3A ion surface and thus reducing the rate of water diffusing into C3A ion.

The following diagram makes you to understand clearly about how setting time of cement delayed by gypsum:

 

Thus C3A hydration time increases as C3A reaction with water slows due to low diffuse of water and correspondingly setting time increases.

 The reactions continues until presence of water and C3A compounds.