Natural fibers as an alternative to synthetic fibers in the reinforcement of phosphate sludge-based geopolymer mortar

Abstract

Geopolymers are a kind of material of great technological and ecological interest thanks to their high mechanical properties and their ability to be produced from industrial by-products. However, like all cementitious matrices, geopolymers have a fragile character and suffer from low ductility. To overcome this challenge while staying environmentally conscious, the present research focuses on the efficiency of using natural fibers instead of synthetic fibers in the reinforcement of geopolymer mortars based on phosphate industry by-products. Geopolymer mortar composites were prepared by incorporating high contents of pine cone fibers (NF) and glass fibers (GF) into a geopolymer mortar matrix based on treated phosphate sludge, metakaolin, and sand. Pine cone fibers reinforced composites were assessed and compared to glass fibers reinforced composites and control mortar in terms of structural, physical, and mechanical properties. The structural analysis revealed that the prepared mortars consist of a continuous three-dimensional network characteristic of a geopolymer matrix without undesirable interactions between the used fibers and the mortar matrix. Density, porosity, and water absorption studies indicate that pine cone fibers created fewer defects in the geopolymer mortar matrix than glass fibers, and that NF-reinforced composites are more resistant to moisture than GF-reinforced ones by about 2%. The results of the mechanical properties showed that the addition of fibers, whether natural or synthetic, decreases the compressive strength and significantly increases the flexural strength. However, Although the flexural strength rises with the fiber content in the matrix, the fiber content of 50 wt% sand volume fraction was the limit, since beyond this content, the workability and flowability of the composites notably decrease. Overall, when both mechanical properties are considered, as well as the ecological and economic impacts, it can be stated that pine cone fibers could potentially replace synthetic fibers in the reinforcement of phosphate sludge-based-geopolymer mortars.