Fire proof wall boards are made with gypsum because it suppresses the fire

In the event of fire the dehydrate will dehydrate to form the hemi hydrate and released water vapour. This reaction is endothermic $\left({\Delta }_{r}H=+117kJmo{l}^{-1}\right)$ So it absorbs heat from the fire. In addition, the water produced absorbs heat and evaporates and than the gaseous water provides an inert barier reducing the supply of oxygen to the fire.Oxygen is non in Hammable, cheaper and also have low thermal conductivity,