An experimental investigation was undertaken on the influence of different surface treatments of flax fibres on mortar properties in both fresh and hardened states. Three types of coating were explored for surface modification atmospheric-pressure plasma, mineral coating made of a grout of cement and ground granulated blast-furnace slag, and polymer coating based on linseed oil. Water absorption tests were performed to evaluate the performance of the coatings. Water absorption kinetics and/or water retention capacity were drastically modified by the plasma treatment at atmospheric-pressure and by the linseed oil treatment. The workability of the pastes incorporating fibres coated with the cement grout or linseed oil was greatly improved compared to the raw fibres. The presence of fibres significantly increases the rate of entrapped air, which seems to depend on the nature of the coating. In the hardened state, regardless of the nature of the fibre coating, the flexural strength increases beyond 90 days of curing. Up to 320 days of curing, the compressive strengths of the different composites are similar to that of the control mortar, except for the composite with linseed oil coated-fibres which proves to be the most porous. The presence of flax fibres also enhances the mortar toughness and prevent from desiccation cracking. Nevertheless, none of the treatments experimented in this work makes it possible to preserve the beneficial effects of fibres on the mechanical behaviour of composites in the long term. © 2018 Elsevier Ltd