Microstructural investigation of lithium slag geopolymer pastes containing silica fume and fly ash as additive chemical modifiers

Abstract

Lithium slag is an industrial by-product obtained after lithium extraction from spodumene ore. The higher concentration of sulfate ions (SO4−2) in the form of gypsum/anhydrite makes it a chemically unviable binder. This research investigates the dilution of the sulphatic component in pore solution by additive incorporation of silica fume and fly ash as chemical modifiers in the lithium slag geopolymer. The setting behavior, detailed microstructural properties, mineral phase quantitative analysis, and compressive strength of lithium slag geopolymer containing fly ash and silica fume were studied. The increasing silica to alumina ratios (Si/Al) by incorporating silica fume in sodium tetraborate added geopolymer resulted in the set retardation after the setting accelerated at Si/Al ratio of 3.5. Similarly, the set retardation was observed for all fly ash replaced lithium slag geopolymers marked by the lower dissolution of SO4−2 ions in pore solution. The fragmented and porous N-(C)-A-S-H gel in lithium slag geopolymer densified by additive incorporation of silica fume and fly ash due to suppressed formation of SO4−2 in pore solution, thus increasing the compressive strength. The main binding zeolite phases quantified in mineral and crystal phase analysis of fly ash replaced geopolymer were mordenite, anorthite, analcime, and calcium chabazite, whereas for silica fume incorporated geopolymer were mordenite, anorthite, analcime, and sodium clinoptilolite. Thus, the lithium slag can be a promising geopolymer precursor along with other supplementary cementitious materials. However, further research is suggested for its chemical viability as a sole geopolymer binder.