The fire-proofing materials used to make fire breaks are usually calcium sulphate dihydrate or calcium silicate panels. During fire exposure, the gypsum panels are characterized by an isothermal stage on the unexposed side due to the latent heat consumption. This latent heat is induced by two endothermic phase changes during transformation of gypsum into hemihydrate and anhydrite. These phase changes cause thermal shrinkage that provokes panel cracking and the increased temperature on the unexposed side. To improve the thermomechanical properties of gypsum panels, a specific filler IS dded to the plaster before hydration. Thermomechanical analyses show that this filler reduces the shrinkage in gypsum during heating. Fire tests show that this decrease in thermal shrinkage reduces crack appearance. Unlike gypsum plaster without filler, the heat flow is reduced even after gypsum dehydration and the temperature of the unexposed side after the isothermal stage is stabilized. The inert filler decreases the latent heat and the thermal conductivity of the materials which also modifies the thermal behaviour of panels.