Environmental issues have led to new research into the development of eco-friendly materials. For building materials, plant fibre reinforced matrix is one of the rising innovative solutions. In this paper, the benefits of reinforcing a pozzolanic matrix with flax and hemp fibres and the impact of time and environmental conditions are presented. The mechanical behaviour of the composites was studied in a bending test over 1year. Two exposure conditions were studied: controlled conditions and natural outdoor weathering. SEM observation coupled with EDS analysis was used to complete the results with the physicochemical interaction between plant fibres and the mineral matrix.