The incorporation of waste plant particles in a cementitious matrix has led to obtain high-performance materials with good economic and ecological benefits. In this paper, a characterization of the main hydric and mechanical properties of a new material composed by flax shives was performed. The aim is to study the impact of the flax shives content on its hydric and mechanical behavior. Moreover, a comparison between the behavior of this material and those of hemp concrete was carried out. This allows, on the one hand, to apprehend better the hydric behavior of the different applications of this material, and on the other hand, to promote new economic perspectives for these products. The results showed that this material has a good hydric behavior. The formulation with a high flax shives content showed a moisture buffer value (MBV index) of 2.80, which allowed it to be classified as an excellent moisture regulator. The comparison of the properties of flax concrete with those of standardized hemp concrete showed that both materials are relatively identical with advantageous and satisfactory properties.