Geopolymers have great potential for the development of sustainable building materials due the utilization of industrial by-products and waste materials for their production. The introduction of natural fibers to geopolymer matrices has shown interesting results for producing alternative building materials with improved mechanical properties. This research explores the use of linen (flax) fibers as reinforcement of clay brick powder based-geopolymers and fly-ash based geopolymers. Linen fibers were obtained from the Institute of Natural Fibres and Medicinal Plants while clay brick powder were obtained from construction waste in Lima (Peru) and fly ash were extracted from Skawina (Poland). The influence of the addition of linen fibers on the mechanical properties of geopolymer matrix was studied by unconfined compression tests and three-point bending tests. The results show that addition of flax fibers significantly enhanced both compressive and flexural strengths of fly ash and clay brick powder based geopolymers. © Published under licence by IOP Publishing Ltd.