Characterisation and properties of metahalloysite-based geopolymer composites with micro/nano-Fe ₃ O ₄ contents: effect of the mixing procedure

In this study, the effects of micro- and nano-sized iron powders, as well as the mixing procedure, on the fresh and hardened properties of metahalloysite-based geopolymer composites were investigated. Iron powders were incorporated at dosages of 2, 4, and 6 wt% of the binder, and two distinct mixing procedures were adopted for specimen preparation. In the first procedure, metahalloysite was dry-blended with the iron powder prior to the addition of the alkaline activator. In the second procedure, the iron powder was first dispersed in the alkaline solution, after which metahalloysite was gradually introduced into the mixture. Afterwards, the resulting paste samples were subjected to mechanical properties evaluation. Furthermore, both the raw materials and the synthesised binders were characterised in terms of phase composition and microstructure using Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The results indicated that compressive strengths of 55 and 58 MPa were achieved with the addition of 2 wt% nano-sized and 6 wt% micro-sized iron powder, respectively, when the first mixing procedure was applied. In contrast, for the second procedure, compressive strengths of 55 and 56 MPa were obtained at the corresponding iron powder contents. Overall, the findings suggest that nano-sized iron powder can effectively enhance the mechanical performance of geopolymer composites. The relatively high strength observed in specimens incorporating micro-sized iron powder is attributed to a filler effect that promotes pore refinement and results in a more compact, denser microstructure. Conversely, pre-dissolution of iron powder in the alkaline solution prior to the addition of metahalloysite was found to hinder the geopolymerisation reaction and the development of a well-connected geopolymer network, resulting in reduced compressive strength.