Engineering and microstructural properties of carbon-fiber-reinforced fly-ash-based geopolymer composites

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Alkali-activated fly ash and slag-based geopolymer composites offer sustainable cement alternatives to conventional cement materials. Incorporating fibers mitigates issues such as brittleness, insufficient toughness, and cracking in geopolymers. However, limited systematic research exists on the influence of carbon fiber characteristics on geopolymer performance. In this study, the effects of carbon fibers having varying lengths (6 mm and 12 mm) and volume fractions (0%, 0.2%, 0.4%, 0.6%, 0.8%, and 1.0%) on multiple properties of geopolymers, including workability, water absorption, porosity, mechanical strength, drying shrinkage, uniaxial tensile properties, fracture toughness, and microstructure, are investigated. Results indicate that the incorporation of carbon fibers diminishes the flowability of the geopolymer mix and accelerates its setting time. Parameters such as water absorption, porosity, and compressive and flexural strengths initially decrease and then increase with increasing fiber content. Carbon fibers positively affect drying shrinkage, particularly during the early stages. The fiber bridging effect enhances uniaxial tensile properties, reducing brittleness. Carbon fiber addition improves the fracture toughness of the composite, achieving an optimal volume fraction at 0.6%. At this volume fraction, carbon fibers of 6 mm length exhibit better crack initiation toughness and resistance to instability cracking than fibers of 12 mm length. Moreover, the presence of carbon fibers promotes the formation of hydration products, minimises the occurrence of microcracks, and interacts synergistically with the slurry matrix to enhance overall crack resistance and material toughness. This study provides a valuable reference for the development and application of fiber-reinforced geopolymer materials.