Synthesis, mechanism and physical properties of porous geopolymer–zeolite composites by in situ hydrothermal reaction for adsorption application

A porous geopolymer–zeolite composite was synthesized using an in situ hydrothermal reaction method for adsorption application, with steel slag serving as the primary ingredient. The impact of various factors on the physical properties of the composites was meticulously studied. These factors include the water glass modulus, silicon-to-aluminum ratio, sodium hydroxide concentration, hydrothermal time, and hydrothermal temperature. To elucidate the conversion mechanism, a comprehensive analysis was conducted utilizing X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR). The findings from these analyses provided valuable insights into how each parameter influences the formation and characteristics of the porous geopolymer–zeolite composites. The results show that under specific conditions, three types of zeolites can be converted from steel slag-based geopolymer, including analcime (ANA), sodalite (SOD) and mordenite (MOR). The type of zeolite formed is influenced by cation concentration, silica/alumina ratio and alkalinity. At a particular sodium concentration, silicon tetrahedra react with aluminum tetrahedra in a 2 : 1 ratio to form ANA structures, and in a 5 : 1 ratio to produce MOR structures. When sodium is abundantly available, the silicon and aluminum tetrahedra will conjoin to form SOD structures at a 1 : 1 ratio. Calcium ions present in steel slag can consume a significant portion of silicon tetrahedra to form C–S–H gel, which in turn modifies the Si/Al ratio and consequently leads to variations in the types of zeolites formed. Under optimized conditions, the resultant zeolites exhibit an impressive overall crystallinity reaching up to 90% and boast specific surface areas as high as 259.60 m² g⁻¹.