Thermal stability and strength degradation of lithium slag geopolymer containing fly ash and silica fume.

This paper investigates the thermal stability of lithium slag geopolymer (LSG) containing fly ash (FA) and silica fume (SF) exposed up to 900˚C. The effects of elevated temperatures on the microstructural evolution were investigated by thermogravimetric analysis (TGA), X-ray Diffraction (XRD), mineral analysis, surface porosity, and compressive strength of LSG modified by optimum incorporation of fly ash and silica fume. The results indicated that elevated temperature exposure significantly affects compressive strength, surface porosity, void size, weight loss, and phase transformation of fly ash incorporated lithium slag geopolymer (LSG FA) and silica fume incorporated geopolymer (LSG SF). The maximum loss in compressive strength of 44.07% was observed in LSG FA compared to 31.50% loss in LSG SF after exposure at 900˚C. The weight loss of LSG SF was higher than that of LSG FA between 500 and 800˚C and 800–1000˚C indicating a higher degree of dehydroxylation, crystal phase transformation, and viscous sintering observed in the former mix. A larger shrinkage cracking was observed in LSG SF by self-desiccation of the aluminosilicate paste matrix by dehydroxylation of mordenite molecules. Therefore, the degradation of compressive strength is governed by the combination of the crystallization of aluminosilicate gel and the formation of voids. • Sintering of LSG attributed the phase transformation, pore formation, and strength variation. • LSG FA has higher thermal stability and resistance to dehydroxylation than LSG SF. • LSG containing silica fume shows less shrinkage and more residual strength post fire exposure at 900°C by silica sintering. • High temperature LSG sintering forms aegirine-augite and plagioclase from mordenite and anorthite. [ABSTRACT FROM AUTHOR]