Résumé
The use of plant-based fibers in cementitious materials has gained increasing attention due to their low environmental impact and potential to enhance mechanical performance. However, challenges such as high-water absorption and poor durability in alkaline environments limit their widespread application. This study investigates the combined effects of fiber surface treatments (sodium bicarbonate and a Chrysofuge water-repellent) and matrix modification (Ordinary Portland Cement CEM I replaced by slag-blended CEM III) on the performance of flax-fiber-reinforced mortars. The results show that fiber treatments significantly influence both fresh and mechanical properties. The Chrysofuge treatment improves workability by reducing fiber hydrophilicity, while SEM observations and mechanical results suggest that NaHCO3 treatment improves fiber–matrix adhesion, contributing to enhanced flexural strength and toughness. The mechanical performance strongly depends on the compatibility between fiber treatment and cement matrix. In addition, the incorporation of flax fibers improves post-cracking behavior and reduces the brittleness of the composite.