Effect of pre-treatment of flax tows on mechanical properties and microstructure of natural fiber reinforced geopolymer composites

In this work we studied the effect of pre-treatment of low-grade flax product (flax tows, FTs) by cleaning it mechanically from shives and weeds, followed by mercerization with a 5% aqueous NaOH solution and ultrasonic (US) treatment (22 kHz, 500 W) in alkaline medium, on the mechanical properties and microstructure of fly ash-based geopolymer (GP) composites. A series of three-point bending flexural tests was conducted on several types of GP/FTs composites while ATR-FTIR spectroscopy, scanning electron microscopy, and confocal laser scanning microscopy were applied to evaluate morphological and chemical characteristics of FTs and their interface strength with GP matrix. It is shown that combined treatment with alkali and high-intensity ultrasound is an effective way to process and modify the surface of FTs fibers for reinforcing “green” geopolymer composites, providing the best technical and economic effects. •Pre-treated flax tows (FTs) were used to prepare geopolymer (GP) composites.•Extraction of non-cellulose components of flax improves fiber–geopolymer interface.•NaOH/ultrasound treatment showed the greatest mechanical properties composites.•FTs-GP composites lower the cost and maximize the utilization of industrial wastes.