Behavior of hematite, magnetite, and reduced iron powder in geopolymers: Effects of mechanical properties and reaction mechanism

Geopolymers are green, low-carbon building materials and are potential alternative binders to Portland cement. At present, the selection of precursor materials is gradually expanding to Fe-rich precursors in addition to traditional silicoaluminate materials, such as metakaolin (MK). However, the behavior of Fe in different species during the highly alkaline geopolymerization process remains unclear. In this study, Fe-based geopolymers were prepared by adding hematite (HE), magnetite (MA), and reduced iron powder (RIP) into MK. By adjusting the proportions of these Fe substances' mineral phases, we studied variations in the strength properties of Fe-based geopolymers and the changes in Fe during the geopolymerization reaction. Results showed that as the proportions of the three minerals increased, the 7d compressive strength (CS) of HE and MA geopolymers experienced a noticeable decline, while no considerable change was observed in the CS with an increase in RIP content. The effect of HE, MA, and RIP on CS was elucidated through a series of characterization methods. Under alkaline conditions, HE and MA partially dissolved to form NaFeO2 primarily, influencing the polymerization degree and pore structure of geopolymers, thereby causing considerable changes in CS. Conversely, RIP seemingly acted as an aggregate within the geopolymer matrix, supporting the development of CS after being enveloped by the geopolymer on the particle surface.