Effects of alkali-treated kenaf fiber on environmentally friendly geopolymer-kenaf composites: Black liquid as the regenerated activator of the geopolymer

[Display omitted] •Activator with black liquid did not show negative effects on the geopolymer.•Good interfacial compatibility showed between CaCl2-treated fiber and geopolymer.•The toughness of composites was dependent on fiber pretreated types and contents.•The ductile failure mode of composite was improved due to the fiber bridging.•Composite prepared by regenerated activator was environmentally friendly. Interfacial compatibility enhanced by alkali-treated fiber is the key to the geopolymer-fiber composites (GFCs). However, applications of natural fibers in the GFCs are greatly limited by the black liquid pollution and weak compatibility between geopolymer and natural fibers after alkali-treatment. In this study, the improved compatibility, flexural strength (increased by 69.1%), toughness (increased by 473%), and ductile failure mode of the GFCs were obtained by CaCl2 solution (1 mol/L) immerged into the alkali-treated kenaf fiber and black liquid recycled as a regenerated activator. This outcome could be served as a basis for the low-cost and eco-friendly preparations of the GFCs.