The effect of class F fly ash on the geopolymerization and compressive strength of lightweight aggregates made from alkali-activated mine tailings

Abstract

Lightweight aggregates (LWAs) produced using alkaline activation of aluminosilicate-rich mine tailings (MTs) are viable substitutes for natural lightweight aggregates used in construction and building industry. However, most pure MTs do not contain adequate quantities of amorphous aluminosilicates to make geopolymerization feasible without adjusting the Si:Al ratio through the incorporation of amorphous aluminosilicates or other aluminum/silicon source materials. A comprehensive exploration of the production of LWAs using MTs in conjunction with reactive aluminosilicates has not yet been thoroughly investigated. In this study, class F fly ash (FA) was used as an amorphous supplement to adjust the Si:Al ratio, leading to improved geopolymerization of LWAs. The mixtures of MTs and FA were pelletized using a disk granulator machine by spraying 10 M NaOH with a liquid-to-solid ratio (L/S) of 0.25. The effects of different concentrations of FA (10%, 15%, 20%, 25%, and 30%) on the chemical, physical, mechanical, mineralogical, and morphological features of the resulting LWAs were examined. The results indicate that increasing the FA content of LWAs decreased their water absorption and porosity while increasing their bulk density and compressive strength without affecting their specific gravity. The scanning electron microscope (SEM-EDS) images show that by increasing the FA content, especially at 20% and upward, homogeneous geopolymerization was achieved in the specimens and the Si:Al ratio in LWAs with 20% or more FA was greater than three, indicating a greater likelihood of geopolymer backbone formation. The results of x-ray diffraction spectrometry (XRD) and Fourier-transform infrared spectroscopy (FTIR) showed that the alkali activation process resulted in the disappearance of the gypsum phase originally available in the MTs and the appearance of sodalite (sodium carbonate) in the LWAs. Future research should utilize the Life Cycle Assessment (LCA) methodology to evaluate environmental impact and should explore advanced additives/modifiers for improved mechanical characteristics and manufacturability to increase potential construction applications.