Physical and Thermomechanical Performance of Kevlar and Carbon–Kevlar‐Reinforced Geopolymer Composites at Elevated Temperatures

Fiber‐reinforced geopolymer composites (GPCs) are garnering heightened interest owing to their exceptional thermal stability and mechanical properties relative to traditional cement‐based systems. Kevlar and carbon–Kevlar hybrids are notably advantageous due to their exceptional tensile strength, low density, and ability to maintain structural integrity at extreme temperatures. This study examines the mechanical and physical properties of GPCs reinforced with Kevlar fiber (KF) and hybrid carbon–Kevlar fiber (CKF) under elevated temperatures. Geopolymers are typically brittle at high temperatures, but fiber reinforcement enhances their thermal and mechanical resilience. Samples with 1%, 1.5%, and 2% fiber content were heated to 200°C, 400°C, 600°C, and 800°C. Mechanical properties such as compressive and flexural strength, along with physical properties like color, cracks, and voids, were evaluated, and thermal conductivity was measured. For KF‐reinforced composites (KF‐GPCs), the highest compressive (14.6 MPa) and flexural strengths (10.02 MPa) were observed at 200°C with 1% fiber content. Strength decreased as the fiber content increased and temperatures rose, with partial recovery at 800°C, presumably owing to microstructural densification and phase transitions that briefly improved load‐bearing capability, although ductility remained markedly diminished. In CKF‐reinforced composites (CKF‐GPCs), compressive and flexural strength increased with fiber content, peaking at 200°C with 2% fiber (10.86 MPa). However, CKF‐GPCs showed increased brittleness and limited strain at 800°C. Thermal conductivity dropped significantly for both composites at 800°C (54.55% for KF‐GPC and 59.23% for CKF‐GPC). SEM analysis revealed stronger fiber–matrix bonding and lower porosity in KF‐GPCs compared to CKF‐GPCs, which exhibited higher porosity and fiber pullout.