Evaluation of Porosity and Pore Diameter of Gypsum Boards at Elevated Temperatures

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.