Abstarct: The concrete industry, accounting for approximately 8% of global carbon dioxide emissions, represents one of the major environmental challenges of the modern era. The use of supplementary cementitious materials, such as calcined clay and limestone calcined clay cement (LC3), has emerged as an effective approach for producing low-carbon concrete. Nevertheless, the fracture behavior of these concretes, particularly in the presence of polymeric fibers, remains insufficiently understood, while most previous studies have been limited to the use of a single fracture mechanics approach. Moreover, the relationships among fracture parameters, post-cracking behavior, and microstructural characteristics have not yet been comprehensively investigated. Accordingly, this study evaluated the fracture behavior of low-carbon concretes incorporating calcined clay, LC3, and polypropylene fibers by integrating three complementary fracture mechanics approaches, namely the work of fracture method (WFM), size effect method (SEM), and two-parameter model (TPM), together with microstructural analysis using field-emission scanning electron microscopy (FESEM), digital image correlation (DIC) for deformation-field measurement, and response surface methodology (RSM) for statistical optimization. Twelve concrete mixtures were prepared and tested, incorporating calcined clay at replacement levels of 10%, 20%, and 30% and LC3 at replacement levels of 35% and 50%, with and without 0.25% polypropylene fibers. The main novelty of this study lies in the development of an integrated, multiscale frxamework for evaluating fracture behavior through the combined application of different fracture mechanics approaches, microstructural analysis, deformation-field measurement, and mixture optimization. The results showed that the mixture containing 20% calcined clay and 0.25% polypropylene fibers exhibited the highest fracture energy (130 N/m) and the greatest ductility, with a critical crack mouth opening displacement of 0.02285 mm, corresponding to an approximately 38% increase in fracture energy compared with the reference mixture. In addition, the mixture containing 35% LC3 and 0.25% polypropylene fibers exhibited the best resistance to stress concentration at the crack tip, with a fracture toughness of 40.39 MPa√mm. In contrast, although the mixture containing 50% LC3 achieved the highest compressive strength (40.75 MPa), its reduced critical crack opening displacement and characteristic length indicated a more brittle response. Microstructural investigations further confirmed improved fiber–matrix bonding and reduced porosity in the optimized mixtures. Overall, the results demonstrate that the selection of an optimum mixture for low-carbon concrete should not be baxsed solely on compressive strength; rather, fracture parameters, energy absorption capacity, and ductility should be considered simultaneously.