Mechanical Behavior and Modeling of Flax Fiber-Reinforced Geopolymers in Comparison with Other Natural Fiber Composites

Highlights: What are the main findings? Flax fiber as reinforcement in metakaolin-based geopolymers sustained compressive strength, while improving ductility and post-peak response under flexural and indirect tensile loading. Stress–strain-based concrete damaged plasticity (CDP) modeling reproduced the key experimental trends, with the fracture-energy-based model formulation providing a closer representation of post-peak behavior. What are the implications of the main findings? Flax fibers can contribute to enhanced damage tolerance and toughness of geopolymer composites at the material scale. The combined experimental–numerical approach supports trend-level calibration of the CDP material model and can provide reference information for further sustainable studies, with additional validation required before structural applications. The rising environmental concerns over cement-based construction materials have led to the development of sustainable alternatives. Among these, geopolymers represent a promising class of low-carbon binders offering environmental benefits and competitive mechanical properties; however, their intrinsic brittleness limits their tensile and post-cracking performance. This study investigates the adoption of flax fibers as natural reinforcement to enhance ductility and post-peak behavior of metakaolin-based geopolymers. The performance of metakaolin-based geopolymers with flax fibers (MKFLAX) was experimentally evaluated in terms of strength, stiffness, toughness, and failure behavior. The addition of flax fibers enhanced ductility, toughness, and post-peak load-carrying capacity while slightly improving stiffness due to the bridging of cracks and the fiber pull-out mechanism. In comparison with the available literature on sisal, flax, and jute fibers, flax fibers showed improved performance due to the better dispersion within the matrix and higher tensile modulus. These findings highlight that flax fiber-reinforced metakaolin geopolymers show enhanced post-cracking behavior at the laboratory scale and could be of interest for sustainable cementitious materials, subject to further validation at the structural scale. Furthermore, a nonlinear finite element model was adopted based on damage mechanics to simulate the damage localization, stress–strain response and post-peak behavior of geopolymer composites. The numerical results showed a reasonable agreement with the experimental trends, particularly in the elastic and early softening phases. The findings are limited to the studied material system, fiber content, and small-scale samples and should be viewed as trend-level observations rather than generalized performance claims.