To investigate the elevated-temperature performance of Ultra-High-Performance Concrete (UHPC) and Ultra-High-Performance Geopolymer Concrete (UHPGC), a systematic comparative study was conducted at 800 °C. This study examined the effects of the steel fibre geometry (micro and hooked-end) and dosage (1.5% and 2.0%) on mass loss, crack propagation, residual compressive, flexural, and tensile strengths, and chemical evolution following a 24 h pre-drying protocol to mitigate explosive spalling. The results demonstrate that UHPGC exhibits superior thermal stability and residual mechanical performance compared with UHPC after high-temperature exposure. Among all mixtures, the UHPGC mixture reinforced with 2% micro steel fibres (UHPGC-M2) achieved the highest residual compressive strength (30 ± 0.4 MPa, corresponding to 25% strength retention compared with 21% for the equivalent UHPC mixture), the lowest post-exposure crack width (0.08 mm), and the highest tensile strength retention (17.9%). Micro steel fibres were more effective in controlling crack propagation and preserving peak load capacity, whereas hooked-end fibres contributed more significantly to post-peak ductility. Chemical analysis revealed substantial chemical changes in both systems after exposure to 800 °C. However, UHPGC exhibited lower mass loss (4.8%) and greater residual performance. These findings establish micro steel fibre-reinforced UHPGC as a sustainable and high-performance material for fire-resistant structural applications.

The widespread use of concrete in construction is driven by its adaptability, strength, and durability. However, the environmental repercussions associated with conventional concrete made from ordinary Portland cement (OPC) have raised concerns due to resource depletion, energy consumption, and greenhouse gas emissions. This study presents the results of compressive strength tests performed on multiple concrete mixes that utilize eco-friendly geopolymer technology and polypropylene fibres that were subjected to elevated temperatures. The study investigated a total of six concrete mix designs that utilize ground granulated blast furnace slag (GGBFS) and fly ash (FA) as a total replacement for conventional ordinary Portland cement (OPC). Sodium Hydroxide was used as an activator with identical proportions between the mixes. The results showed that the inclusion of 1 kg/m3 of PP fibres had no significant effect on compressive strength. The results reveal a notable increase in compressive strength of over 50% when incorporating GGBFS and PP fibres. Upon exposure to elevated temperatures, geopolymer-based mixtures exhibited significant compressive strength reduction, with certain samples experiencing nearly 70% strength loss. Future research should focus on devising strategies to mitigate these effects and enhance fire resistance for practical applications.