Effect of fiber reinforcement on the thermal resistance of ternary geopolymer mortars

This study investigates the thermal performance of ternary geopolymer mortars reinforced with micro steel (MS) and polypropylene (PP) fibers under elevated temperatures (200 °C, 400 °C, and 600 °C). The binders used include fly ash (FA), ground granulated blast furnace slag (GGBS), and palm oil fuel ash (POFA), with fiber dosages of 0%, 0.5%, 1%, and 1.5% by volume. A total of 84 specimens were prepared and tested to evaluate the influence of fiber content and temperature. Key properties assessed were residual compressive strength, mass loss, ultrasonic pulse velocity (UPV), and dynamic modulus of elasticity (Ed). Results indicate that MS fibers demonstrated improved thermal resistance and post-heating durability due to their effective crack-bridging ability and stability at elevated temperatures. At 600 °C, the 1.5% MS mix exhibited only 3.21% mass loss, 43% strength loss, and 51% UPV reduction, outperforming the control mix (10.2%, 90%, and 66%, respectively). The same mix also retained the highest Ed (5.21 GPa), while the 1.5% PP mix recorded the lowest (1.83 GPa). At 200 °C, MS fibers reduced thermal degradation effects by 50–80%, with similar benefits noted at 400 °C. While PP fibers helped reduce spalling at lower temperatures, their mechanical performance declined under high heat. An empirical exponential model (R2 = 0.97) effectively captured the stiffness degradation trend, supporting the use of MS fibers in fire-resistant, sustainable geopolymer composites.