Optimization of microstructure and mechanical performance of clay-rich sand-washing slurry-based geopolymers

Abstract

As a byproduct of sand-washing process, sand-washing slurry (SWS) showed a potential geopolymer precursor due to the abundant clay minerals. This study investigated the impact of calcination temperature of SWS and SiO₂/Na₂O ratio of alkaline solution on the microstructure and mechanical properties of SWS-based geopolymers. The analysis results indicated that at a calcination temperature of 550 °C, some clay minerals in SWS underwent a transition from crystalline to amorphous phases, resulting in increased amounts of amorphous Si and Al, thereby enhancing their alkaline reactivity. However, due to the low content of amorphous phases, the formed geopolymer structure was loose, leading to lower compressive strength. Compared to geopolymer treated at 550 °C, the geopolymer produced under temperatures ranging from 650 °C to 750 °C exhibited denser microstructures and higher compressive strength. The improvement in performance was attributed to more clay minerals undergoing dehydroxylation reactions, resulting in more amorphous Si and Al participating in the polymerization reaction. Nevertheless, upon reaching a calcination temperature of 850 °C, the microstructure of the formed geopolymers became loose with diminished compressive strength due to reduced activity of Si and Al in the amorphous phase.

Furthermore, under a calcination temperature of 750 °C, the amorphous content in geopolymers with SiO₂/Na₂O ratios in the alkaline solution of 0.8 and 1.4 was 62.4 % and 64.5 %, respectively, while decreasing to 54.5 % in geopolymers with a ratio of 2. Geopolymers with a SiO₂/Na₂O ratio of 1.4 exhibited denser microstructures and higher compressive strength, indicating that alkaline solutions with this ratio promoted the formation of more amorphous materials and enhanced geopolymer strength. These findings suggested that calcination around 750 °C improved the microstructure and mechanical properties of SWS-based geopolymers, with the appropriate SiO₂/Na₂O ratio in alkaline solutions facilitating geopolymerization.