Effects of hybrid basalt-steel fibers on lightweight geopolymer RC beams exposed to elevated temperatures

Purpose The purpose of this study is to experimentally investigate the structural performance of reinforced lightweight geopolymer concrete (LWGPC) beams incorporating hybrid basalt-steel fibers when exposed to elevated temperatures. The research examines the influence of fiber inclusion, reinforcement bar diameter and thermal exposure on load capacity, stiffness, ductility and failure modes under four-point bending. Scanning electron microscopy (SEM) is used to assess microstructural changes, including fiber integrity and interfacial transition zone (ITZ) behavior. The study aims to advance understanding of LWGPC's fire resistance and mechanical resilience, supporting its application in sustainable and thermally robust structural systems. Design/methodology/approach Sixteen reinforced LWGPC beams, with and without hybrid basalt-steel fibers (0.2% volume), were cast using fly ash, alkali-activated solutions and volcanic tuff aggregates. Beams varied in tensile reinforcement diameters (10, 12 and 14 mm) and were exposed to 25°C, 300°C, 450°C and 600°C. After heating, the specimens were tested under four-point bending to measure ultimate load, midspan deflection, stiffness, ductility and failure modes. SEM was conducted on selected samples to evaluate fiber condition and ITZ changes. Results were analyzed to assess the effects of fiber addition, bar size and temperature on structural performance. Findings Hybrid basalt-steel fibers significantly enhanced LWGPC beam performance, increasing compressive strength, ultimate load, stiffness and ductility by up to 45%, 11%, 31% and 51%, respectively, compared to fiber-free beams. Elevated temperatures reduced load capacity, with the maximum loss at 450°C, but partial recovery occurred at 600°C due to matrix densification. Smaller reinforcement diameters retained strength better under heat, while larger diameters improved stiffness and capacity at ambient conditions. SEM analysis confirmed fiber integrity and revealed temperature-induced widening of the ITZ. Overall, LWGPC with hybrid fibers demonstrated superior mechanical resilience and fire resistance, supporting its use in sustainable, high-performance structures. Originality/value This study provides one of the first comprehensive experimental evaluations of LWGPC beams reinforced with hybrid basalt-steel fibers under elevated temperatures. It uniquely combines structural performance testing with SEM microstructural analysis to link mechanical behavior to fiber integrity and ITZ changes. The use of local volcanic tuff aggregate and hybrid fibers offers a sustainable, thermally resilient alternative to conventional concrete. Findings contribute new insights into optimizing fiber reinforcement and bar sizing for enhanced fire resistance, durability and environmental performance, supporting broader adoption of LWGPC in sustainable infrastructure and fire-prone construction environments.