The cement industry significantly contributes to greenhouse gas emissions, and geopolymer concrete has been proposed as a sustainable alternative; however, its thermal behavior under fire exposure requires detailed investigation. In this research, the thermal gradient of geopolymer concrete containing different fibers was evaluated. Five mix designs were prepared: control (without fibers), containing 0.5% steel fibers, 0.5% shape memory alloy fibers, 1% glass fibers, and a hybrid combination (0.25% steel + 1% glass fibers). Specimens were heated in an electric furnace at a rate of 5°C/min to temperatures of 150, 400, 600 and 900°C. Temperatures were recorded every 10 minutes at three points (inside the furnace, on the surface, and at a depth of 7.5 cm). The thermal gradient was calculated using the equation TG = (T0 - T7.5) / d. The results showed that the control specimen reached 150, 400, 600 and 900°C in 35, 210, 250 and 310 minutes, respectively. The maximum thermal gradient (19.33 °C/cm) was recorded at 900°C in the first 60 minutes. The specimen containing shape memory alloy fibers exhibited the best performance, reducing the thermal gradient by 17.7% and creating a 20-minute delay in core heating. An exponential regression model (R² = 0.97) was proposed for predicting the thermal gradient. The ISO 834 standard showed better agreement with experimental data compared to ASTM E119. The ACI 216 code underestimated the thermal gradient of geopolymer concrete by up to 30%.

SEM micrographs at 600°C revealed extensive microcracks and a needle-like morphology, justifying the increase in the thermal gradient. Furthermore, XRD analysis indicated the complete degradation of C-S-H phases and an increase in crystallinity at this temperature.