Influence of Polypropylene Fibres on Energy Dissipation Mechanisms and Thermo-Chemical Degradation of Cement Mortars Subjected to High Temperatures

This article is a continuation of research conducted by the authors on the effects of fire on cementitious composites and presents findings of an investigation into cement mortars that incorporate monofilament (I) and multifilament (F) polypropylene fibres following exposure to temperatures between 100 and 600 °C. Research was undertaken to examine the effect of adding fibre on the mechanical performance, microstructural characteristics, and thermochemical degradation behaviour of the mortars under conditions representative of high-temperature exposure during fires in energy infrastructure facilities. The scope of the research comprises establishing the modulus of elasticity using dog-bone-shaped specimens, as well as flexural and compressive strength tests performed on prisms measuring 4 × 4 × 16 cm and on 10 × 10 × 10 cm cubes to determine the strength class of the mortars. Microstructural analyses complemented the mechanical testing, performed with the use of scanning electron microscopy (SEM); this made it possible to assess temperature-induced changes in the cement matrix. The results have demonstrated that polypropylene fibres had a significant influence on the degradation behaviour of mortars subjected to elevated temperatures, particularly those between 200 and 400 °C, where fibre melting promoted the formation of additional pore channels. This phenomenon promotes the dissipation of internal energy associated with boiling water vapour contained in the capillary pores, as well as water released during the dehydration of cement hydration products, thereby limiting rapid pressure build-up and reducing the risk of explosive spalling. Moreover, the observed microstructural changes were associated with progressive decomposition of C–S–H gels and other thermo-chemical processes occurring within the cement matrix. The results confirm that polypropylene fibres act as a passive mechanism for the dissipation of thermal and mechanical energy in cement mortars, which has a positive effect on their performance under high-temperature conditions. The study provides new experimental data of significance for the design of cement-based materials with enhanced resistance to thermal exposure in energy-sector facilities.