Durability Assessment of Fiber-Reinforced Limestone Calcined Clay Cement Composites after Exposure to High Temperatures

Various supplementary cementitious materials (SCMs) with fiber reinforcement have been tested to enhance the performance of concrete exposed to high temperatures. However, despite their numerous advantages, SCM-based concrete can exhibit undesirable behavior in high-temperature environments. Limestone calcined clay cement (LC3) has emerged as a novel SCM that reduces clinker content in production, contributing to a more sustainable future by lowering CO2 emissions. This study explores the use of LC3 combined with hybrid fiber reinforcement—polypropylene fibers (PPF) and steel fibers (SF)—to enhance the elevated temperature resistance and durability of concrete. Four concrete mixes were prepared, replacing ordinary portland cement (OPC) with LC3 at a 20% replacement level, with and without hybrid fibers (1% PPF and 1% SF), and subjected to high-temperature regimes (200°C, 400°C, 600°C, and 800°C). A comprehensive experimental program assessed the material’s response to elevated temperatures, including residual mechanical properties, durability tests, and microstructural analysis (TG and XRD analysis). Durability performance was evaluated based on pore spaces (voids), water sorptivity, and chloride-ion penetration. It was found that LC3-based concrete, combined with hybrid fibers, densifies the microstructure of the mix, resulting in fewer voids (5.2%–8.9%), lower absorption rates (from 47×10−4 to 396×10−4 mm/s1/2), and excellent resistance to chloride ion penetration (941°C), thereby enhancing performance on exposure to high temperatures.