Influence of microstructure on the mechanical strength of alkali-activated slags

Enhanced Landfill Mining is a novel integrated waste management concept seeking the recovery of valuable resources from landfills sites. In this concept, the valorization of each stream of materials is critical, contributing to both environmental sustainability and economic viability. In particular, significant volumes of Fe-Si-Ca rich slags are expected to be generated in the Refuse Derived Fuel gasification/vitrification processes and their use as inorganic polymers (IP) precursors has been identified as a foreseeable and promising valorization route. Aiming to contribute to the above, the present work aspires to identify the effect of different processing parameters on the synthesis of such IP binders. Several properties on fresh and hard pastes were examined, including setting time, viscosity, shear behaviour, yield stress, microstructure and physical properties. A Design of Experiments (DoE) methodology was used to identify synergistic and/or competitive effects. For curing at room temperature, the compressive strength exceeded 100MPa, whereas the most crucial processing parameters were found to be the solid-to-liquid ratio and the K2O/SiO2 ratio; the influence of KOH molarity was mediocre. Considering that the produced IP binders incorporated a high content of a Fe-Si-Ca rich slag (86.7 wt% of solid content), this work demonstrates the feasibility of producing high strength IPs with only minor levels of virgin raw-materials. Moreover, the results give insights on the impact that different synthesis parameters have on the properties of Fe-Si-Ca-rich IP binders.