Alabaster is a form of gypsum used both in building and as a plaster material. Gypsum was discovered near Paris, and “Plaster of Paris” became popular building material. Gypsum plaster is investigated experimentally at fire temperatures with respect to their thermophysical properties i.e., thermal conductivity, effective heat capacity, and density. It is shown that depending on different ingredients (carbonates) different endothermic reactions occur between room temperature and 900°C. These reactions strongly influence the temperature dependence of the mentioned properties which in turn affect the response of the material to fire. Based on these experimental results the thermal reaction of a gypsum-protected steel column in fire is modeled for the four types of gypsum. The results of numerical simulations demonstrate clearly the advantages caused by certain ingredients of the chemical composition of gypsum plaster.