Low-grade magnesium hydroxide is being used with very promising results as flame retardant filler in polymeric materials and as aggregate in the formulation of mortars for passive fire protection, combining an economic and sustainable solution. Simultaneous TGA–DSC was used to evaluate low-grade magnesium hydroxide thermal decomposition in order to examine the suitability of this product for a broad range of temperatures. Thermal analysis in air shows an unexpected exothermic peak, as well as the endothermic peaks corresponding to the decomposition of magnesium hydroxide, magnesite and dolomite. Thermal decomposition using nitrogen as gasflowdoes not show the exothermic peak, whereas anewendothermic peak corresponding to decomposition of calcite appears. In situXRDpatterns collected in air atmosphere at high temperature allows determining the presence of anhydrite, which was related to direct sulphation of calcite from the sulphur trioxide generated during the combustion of petcoke adsorbed on the particle surface of the low-grade magnesium hydroxide