Maximilian Pache

Gypsum boards are widely used as passive fire protection materials due to their ability to delay heat transfer through endothermic dehydration of calcium sulfate dihydrate (CaSO ₄ ·2 H ₂ O). Understanding the temperature-dependent evolution of porosity and pore size distributions in gypsum board is essential for defining microstructural parameters used in fire-scale thermal transport models. In particular, the evolution of pore structure during dehydration and high-temperature degradation influences heat transfer within the material and therefore affects its fire protection capability. Quantitative data describing how the pore structure evolves at high temperatures remain scarce, limiting the accuracy of multi-physics fire simulations. This work experimentally characterizes the temperature-dependent evolution of porosity and pore size distributions in gypsum board exposed to temperatures up to 1000°C. Samples subjected to controlled thermal treatments were analyzed using mercury intrusion porosimetry, optical microscopy, and large-area optical scanning in order to capture pore structures across multiple length scales. The resulting dataset provides a comprehensive description of pore structure evolution associated with dehydration, fragmentation, and high-temperature sintering processes. The experiments reveal a bimodal pore system consisting of micro-pores (< 10 μm) formed during dehydration and stable meso-/macro-pores (10–100 μm) generated by foaming agents. Total porosity increases with temperature, while the average pore diameter decreases, indicating pore multiplication rather than coalescence. The experimentally measured pore characteristics were incorporated into a microstructure-informed thermal transport model (GPRO: Gypsum PROperties) in order to evaluate the influence of representative pore length scales on the radiative component of heat transfer and to assess their impact on the effective thermal conductivity (k _eff ). Model predictions were compared with literature data for the effective thermal conductivity of gypsum board at elevated temperatures to assess the consistency of the derived microstructural parameters with independently measured macroscopic properties. The results show that the evolution of total porosity strongly affects the conductive component of heat transfer, while the radiative conductivity predicted by diffusion-based radiation models is primarily controlled by pore lengths in the meso-scale range identified in the experimental measurements. Sensitivity analysis indicates that variations in representative pore diameter between 1 μm and 100 μm lead to changes in predicted effective thermal conductivity below 3%. Within diffusion-based radiative heat-transfer formulations, this behaviour reflects the dominance of meso-scale pore lengths in the radiative contribution. The temperature-resolved microstructural dataset presented in this work provides experimentally grounded parameters for thermal modelling of gypsum-based fire protection materials and contributes to improved representation of porous insulation behaviour in fire safety engineering simulations. The resulting temperature-resolved microstructural dataset, linking dehydration, microstructural evolution and temperature-dependent porosity, delivers experimentally grounded parameters for thermal modelling of gypsum-based fire protection materials and contributes to improved representation of porous insulation behavior in fire safety engineering simulations.

This work presents a combined experimental and numerical investigation into the fire performance of gypsum boards enhanced with hydrated inorganic salt additives. Furnace tests were conducted on formulations containing aluminium trihydrate, magnesium hydroxide, calcium aluminium sulphate, and magnesium sulphate heptahydrate, covering temperatures up to 500 °C to assess their thermal buffering capacity. Each additive exhibited characteristic dehydration inflection that delayed temperature rise and provided additional thermal inertia. The experimental data were used to calibrate a coupled heat and mass transfer model, highlighting two critical formulation-dependent parameters: the mass transfer correction coefficient (f _mass ) and the crystal aspect ratio (L/d) _cyl . Empirical correlations linking these parameters with fundamental material properties such as bulk density, chemically bound water content, and carbonate loss were established. These correlations enable predictive modelling of new gypsum formulations based solely on recipe data, eliminating the need for extensive recalibration and experimentation. The proposed framework demonstrates strong predictive capability and provides a practical tool for virtual design and optimisation of gypsum-based systems for fire safety applications.

Gypsum-based fire protection relies on thermally activated dehydration, where chemically bound water is released and evaporated, thereby providing an endothermic heat sink that delays heat penetration through assemblies. In parallel, inorganic hydrated salts are increasingly used as flame-retardant additives in gypsum-based systems to enhance heat absorption over specific temperature ranges. Fire simulation tools and performance-based fire engineering approaches require reliable kinetic data and reaction enthalpies that can be implemented as coupled thermal–chemical source terms. However, additive-specific kinetic datasets remain limited, particularly under restricted vapor exchange conditions representative of porous construction materials. This work investigates the thermal decomposition behavior and dehydration kinetics of Aluminum Trihydrate (Al(OH)₃, ATH), Magnesium Hydroxide (Mg(OH)₂, MDH), Calcium Aluminate Sulfate (3CaO·Al₂O₃·3CaSO₄·32H₂O, CAS), and Magnesium Sulfate Heptahydrate (MgSO₄·7H₂O, ESM) with emphasis on vapor-restricted conditions representative of confined porous systems. Differential scanning calorimetry (DSC) experiments were conducted at three heating rates (2, 10, and 20 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. The thermal analysis indicates that ATH and MDH exhibit predominantly single-step dehydration behavior, while ESM shows a complex multi-step mechanism. Although CAS presents a single dominant thermal peak in the DSC signal, the isoconversional analysis reveals a multi-stage reaction behavior, demonstrating that peak-based interpretation alone may be insufficient for such systems. Kinetic parameters were determined using both model-free (Starink) and model-fitting approaches in accordance with the recommendations of the Kinetics Committee of the International Confederation for Thermal Analysis and Calorimetry (ICTAC). All reactions were consistently described using the Avrami–Erofeev model as an effective phenomenological representation of the conversion behavior. The extracted kinetic triplets were validated through numerical simulations, showing good agreement with experimental conversion and reaction rate data. The resulting kinetic parameters and dehydration enthalpies provide a physically consistent dataset for the description of dehydration processes under restricted vapor exchange. These results support the development of thermochemical models for gypsum-based systems; however, their transferability to full-scale assemblies remains subject to validation under coupled heat- and mass-transfer conditions.

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.