Thermomechanical behavior of gypsum-GPPS composites: A multiscale investigation of temperature and formulation effects

Plaster coatings are widely used as protective coatings in buildings, offering essential passive properties such as fire resistance, thermal insulation, humidity regulation, and soundproofing. From the perspective of plastic waste valorization, this study evaluates the influence of incorporating general-purpose expanded polystyrene (GPPS) aggregates on the thermomechanical and microstructural properties of plaster-GPPS composites before and after exposure to temperatures simulating a fire (150–300°C). Ten formulations were developed with varying incorporation rates (0%, 5%, 10%, and 15% by mass) and three distinct particle sizes (δ ≤ 1 mm; 1 mm < δ ≤ 1.25 mm; 1.6 mm < δ ≤ 3 mm). At room temperature, the results show that increasing GPPS content significantly improves thermal performance, at the cost of a slight reduction in mechanical properties due to decreased workability. The optimal formulation, consisting of 15% intermediate-sized particles, reduces thermal density, conductivity, and diffusivity by 20.7%, 42.5%, and 53.6%, respectively, while maintaining a compressive strength of 6.64 MPa, compliant with EN 13279. After thermal exposure, GPPS degradation leads to a significant deterioration in performance: compressive strength drops by 66% to 82% at 150°C and by 34% to 65% at 300°C, while density decreases by 25.8% and 29.5% at these same temperatures due to differential thermal expansion and the formation of microcracks. This study thus highlights the trade-off between the thermal benefits and the mechanical limitations associated with the addition of GPPS, providing useful data for its application in passive fire protection.