Thermal Dehydration of Hydrated Salts During Fire Exposure and Its Role in Predicting the Performance of Gypsum Based Systems

Gypsum based fire protection relies on thermally activated dehydration, where chem-ically bound water is released and evaporated, producing an endothermic heat sink and delaying heat penetration through assemblies. In parallel, non-organic hydrated salts are increasingly used as flame retardant additives in gypsum-based systems to enhance heat absorption over targeted temperature ranges. Fire simulation tools and performance-based fire engineering methods require dehydration kinetics and reaction enthalpies that can be implemented as coupled thermal chemical source terms. How-ever, additive specific kinetic datasets suitable for such implementation remain limited, especially under restricted vapor exchange conditions representative of porous con-struction materials. The present study investigates the thermal decomposition behavior and dehydration kinetics of selected non-organic hydrated salts—aluminium trihydrate (ATH), magne-sium hydroxide (MDH), calcium aluminate sulphate (CAS), and magnesium sulphate heptahydrate (ESM)—commonly used as flame-retardant additives in gypsum-based construction materials. Differential scanning calorimetry (DSC) experiments were conducted at three heating rates (10, 20, and 30 K/min for MDH, CAS and ESM and 20, 40 and 60 K/min for GB-ATH) up to 600 °C using pinhole crucibles to simulate autogenous vapor pressure. Thermal analysis revealed that ATH, MDH, and CAS undergo single-step dehydration, while ESM exhibits a complex multi-step mechanism involving the formation of in-termediate meta-stable hydrates. Kinetic parameters were determined using both model-free (Starink) and model-fitting approaches. The derived activation energy profiles confirmed the single-step nature of ATH and MDH and identified CAS and ESM as multi-stage systems. All reactions were well described using the Avrami–Erofeev model, indicating nucleation-and-growth mechanisms. The extracted kinetic triplets were validated through numerical simulation, showing close agreement with experimental α(t) and dα/dt(T) data. The resulting kinetic triplets and dehydration enthalpies form a directly usable dataset for coupled heat transfer and dehydration models of gypsum-based assemblies, enabling improved parameter-ization of endothermic heat sinks and bound water release in fire safety engineering simulations.