Reaction kinetics and structural analysis of alkali activated Fe–Si–Ca rich materials

This paper describes the outcomes of a study conducted to investigate the influence of solid-to-liquid (S/L), K2O/SiO2 molar ratio and type of activation solution on reaction kinetic, structural development and final mechanical properties of Fe–Si–Ca rich inorganic polymers (IP). IPs were synthesized with alkali hydroxide and alkali hydroxide/silicate type activators to investigate the kinetic and structural impact of soluble silicates on the activating solution. Multiple characterization techniques, including isothermal conduction calorimetry (ICC), X-ray diffraction (XRD) and infrared spectroscopy (FTIR), were used to give insights on the impact of each of the considered synthesis parameters. Solid-to-liquid and K2O/SiO2 ratios were found to be the compositional parameters governing the reaction kinetics, whereas the introduction of silicate species in the activating solution contributed to further densification and strength development. By combining the effect of the studied parameters, IP binders incorporating a high content of a Fe–Si–Ca rich residues (≥92.7 wt% of solid content) were synthesized at room temperature to achieve a compressive strength exceeding 119 MPa. This work contributes with new insights into the reincorporation and upscaling of a vast group as yet unexplored Fe–Si–Ca-rich waste streams into the materials cycle, such as non-ferrous slags and vitrified residues generated during the incineration of municipal solid waste or during the gasification refused-derived fuel. The study demonstrates the feasibility of producing high strength IPs with only minor usage of virgin raw materials and the possibility of using the developed products as an alternative to conventional cementitious binders. Promoting synergetic interactions between proxy industries is crucial to the sustainability of our current production processes and will play a critical role in achieving current environmental targets.