•We examined durability of NaOH treated hemp fibre fly ash alkali-activated mortars.•We tested mortars’ properties prior and after 10 wet/dry cycles.•NaOH fibres treatment separates fibre bundles and increases their roughness.•Treated fibres increase physical properties of mortars prior and after the cycles.•Treated fibres increase mechanical properties of mortars prior and after the cycles. The conducted study investigates the influence of sodium hydroxide-treated hemp fibres on fibre reinforced fly ash-based alkali-activated mortars and their durability under wet/dry cycles, by examinating mortars’ physical and mechanical properties. This is the first study where the natural fibre reinforced alkali-activated materials under wet/dry cycles are examinated. The sodium hydroxide treatment of hemp fibres leads to separation of fibre bundles and increases the fibres’ surface roughness. This leads to the mortar’s porosity and water absorption decrease, the mortar’s compressive and flexural strength increase and importantly energy absorption capacity increase, all prior and after wet/dry cycles.