Pull-out behavior of different fibers in geopolymer mortars: effects of alkaline solution concentration and curing

Reinforcing geopolymer materials with fibers can enhance tensile and flexural strengths and fracture toughness. The bond between fiber and geopolymer matrix is a critical factor that needs to be investigated to optimize the performance of the fiber reinforced composite. In this study, single fiber pull-out tests are conducted on steel and polypropylene fibers embedded in geopolymer matrices; in addition, OPC mortars are tested as control condition. The following parameters are investigated: fiber type (i.e. steel and polypropylene) and shape, concentration of alkali solution in the geopolymer matrix, and curing conditions. Bond-slip performance, failure modes, and slip resisting mechanisms of different matrices and fibers are compared and discussed. The fiber deformation ratio, a novel parameter, is introduced to quantitatively investigate the effect of fiber shape on the mortar performance. In case of steel fibers, the geopolymer-fiber composite performs better for lower fiber deformation ratios, where the full fiber pull-out mechanism can be exploited. For higher deformation ratios, the strong bearing forces developed, combined with the high adhesion strength of the geopolymer-steel fiber interface, lead to more brittle failure mechanisms, such as fiber breakage or matrix failure, as observed in end-deformed and length-deformed steel fibers, respectively.