The building and construction sector is one of the sectors most concerned by the challenges of sustainable development. Indeed, building (construction) represents about 40% of CO2 emissions in developed countries, 37% of energy consumption and 40% of waste produced. In this perspective, the development of building materials and devices that emit less CO2, consume less non-renewable resources and are as widely recyclable as possible has been the subject of numerous studies. This work relates to the treatment of flax fibers with citric acid in order to improve the behavior of a cementitious mortar. The effect of the treatment on the kinetics and the water absorption rate of the fibers was first evaluated, then its influence on the properties of the fibrous mortar was studied. The treatment reduced the setting time of the cementitious mixtures and significantly increased the flexural strength compared to the raw fiber composites. SEM and wettability test confirmed the treatment efficiency to reduce the hydrophilic nature of the treated fibers.