Composition-properties-performance relationships of alkali-activated materials and geopolymers over extended silicon-aluminum-sodium-calcium ratios in adsorption applications

Abstract

Composition of alkali-activated materials (AAMs) and geopolymers affects their material properties and performance in adsorption applications, yet their correlations remain largely unexplored. In this study, AAMs/geopolymers were synthesized by systematically varying their compositions in the ranges Si1Al1Na1–Si20Al1Na1 (i.e., Ca-free geopolymers) and Si1Al1Na1Ca2–Si20Al1Na1Ca21 (i.e., Ca-containing AAMs). The material properties (e.g., connectedness of the aluminosilicate structure, specific surface area, pore volume, average pore size and zeta potential) of AAMs/geopolymers were correlated with adsorption performance for cations with different aqueous radii: methylene blue (MB), rhodamine 6G (R6G), and ammonium (NH4+). In the Ca-free geopolymers, the adsorption of MB and R6G increased with increasing the Si/Al molar ratio and strongly correlated with specific surface area, whereas NH4+ adsorption showed an opposite trend, correlating positively with the Al/Si ratio and zeta potential but negatively with specific surface area. Adding Ca to the systems caused the adsorption amounts for MB, R6G, and NH4+ reaching a minimum at Si5Al1Na1Ca6 composition while lower or higher Ca content increased adsorption. The results of this study could be a valuable reference for tailoring the future AAM/geopolymer compositions over wide Si:Al:Na:Ca ratios to obtain specific material properties in high-end applications, such as wastewater treatment.