Degradation mechanisms of natural fiber in the matrix of cement composites

If pulp fiber–cement composites are to be used for exterior applications, the effect of cyclical wet/dry exposure must be known. In this research program the effects of three fiber treatments—beating, bleaching, and drying—were investigated to identify those that may minimize effects of environmental aging and degradation during wet/dry cycling. After 25 wet/dry cycles, all composites showed significant losses in first crack strength, peak strength, and post-cracking toughness. The majority of losses in mechanical properties occurred within the first 5 wet/dry cycles, though ductile fiber failure was still observed by scanning electron microscopy (SEM). A three-part progressive degradation mechanism during wet/dry cycling is proposed: (1) initial fiber–cement debonding, (2) reprecipitation of hydration products within the void space at the former fiber–cement interface, and (3) fiber embrittlement due to fiber cell wall mineralization. Unbeaten fiber–cement composites exhibited greater peak strength and post-cracking toughness, prior to cycling, while no significant differences were seen after 25 cycles. The effects of fiber beating varied prior to and after cycling. Unbleached fiber–cement composites exhibited the slowest progression of degradation during cycling. The initial drying state appeared to have no effect on composite performance after 25 wet/dry cycles.

Durability of kraft pulp fiber–cement composites to wet/dry cycling

Fracture energy of natural fibre reinforced concrete

Supplementary cementitious materials for mitigating degradation of kraft pulp fiber-cement composites

Geopolymer has been of great research interest as a material for sustainable development. As ordinary Portland cement, however, geopolymer exhibits brittle behavior with low tensile strength, ductility, and fracture toughness. This paper investigates the reinforcement of fly ash-based geopolymer with alkali-pretreated sweet sorghum fiber. The sweet sorghum fiber comes from the bagasse (residue), a waste after the juice is extracted from sweet sorghum stalks for ethanol production. Specifically, the unit weight of fly ash-based geopolymer specimens containing different contents of sweet sorghum fibers was measured. Unconfined compression, splitting tensile, and flexural tests were conducted to investigate the effect of incorporated sweet sorghum fiber on the mechanical properties of fly ash-based geopolymer. Scanning electron microscopy imaging was also performed to study the microstructure of the sweet sorghum fiber–geopolymer composite. The results indicate that the unit weight of the sweet sorghum fiber–geopolymer composite decreases with higher fiber content. Although the inclusion of sweet sorghum fiber slightly decreases the unconfined compressive strength, the splitting tensile, and flexural strengths as well as the post-peak toughness increase with the fiber content up to 2 % and then start to decrease. The splitting tensile tests also clearly show the transition from the brittle failure of the plain geopolymer specimen to the “ductile” failure of the geopolymer specimen containing sweet sorghum fiber.

Hans-Erik Gram

Papers

Durability of alkali-sensitive fibres in concrete