Effect of thermal pretreatment on the reactivity of red mud valorized as aluminosilicate precursor for geopolymer production

Thermal pretreatment has proven to be an effective method to improve the reactivity of red mud to be valorized as an aluminosilicate precursor for geopolymer production. This approach offers the dual advantage of recovering caustic residue in red mud and encapsulating heavy metal, thus, achieving the transformation of red mud into a cleaner and value-added construction material. However, the inconsistent properties of red mud, including variations in chemical composition, particle size, and amorphous content pose challenges and can lead to instability in geopolymer as well as the effectiveness of thermal pretreatment, limiting the scalability of red mud valorization. Here in this study, thermal pretreatment was employed on five types of red mud from three refineries by calcining each red mud at 800 ○C for 2 h and its effect on the reactivity of each red mud was evaluated by comparing the performance of the resultant geopolymer. The results demonstrated a significant enhancing effect from thermal pretreatment, particularly noticeable at NaOH concentrations of 4 M and 8 M. This again highlighted the potential of thermal pretreatment in the conversion of red mud into construction material. However, amplified disparities between each thermal pretreated-red mud were also observed. To further address the challenges associated with thermal pretreatment variability, the dissolubility of reactive content from each thermal pretreated-red mud under alkali environments was quantified through alkali leaching tests and correlated to the mechanical properties of its resultant geopolymer. A strong negative correlation was observed between the concentration of Si, Al, and Fe from thermal pretreated-red mud and the mechanical properties of its resultant geopolymer. This correlation has the potential to facilitate quality control measures for thermal pretreated-red mud and its derived geopolymer, enabling better process optimization and consistency in geopolymer production.