hydration is very exothermic and also occurs very fast in the fresh concrete mix as the temperature quickly increases with the progress of the hydration reaction. The effect of gypsum addition is to promote the formation of a thin impervious film of ettringite at the surface of the  grains, passivating their surface, and so slowing down their hydration.
Hydration is an exothermic reaction that occurs quickly in fresh concrete mix due to the formation of ettringite, which acts as a protective barrier on the concrete grains. Gypsum addition slows down this process by creating a thin film of ettringite.