Including Class F Fly Ash to Improve the Geopolymerization Effects and the Compressive Strength of Mine Tailings–Based Geopolymer

Mine tailings (MTs) that are rich in aluminosilicates can be reused to create geopolymer via alkaline activation as an alternative to portland cement concrete for construction and building applications. However, pure MTs may lack sufficient aluminum or amorphous aluminosilicates to facilitate geopolymerization that competes with traditional concrete. It is essential, therefore, to add supplemental aluminum source materials and amorphous aluminosilicates to adjust the $\mathrm{Si}:\mathrm{Al}$ ratio to change the geopolymeric cell structures of MTs. The study presented in this paper utilized Class F fly ash (FA) as the amorphous supplement for better reactivity or aluminum source to alter the $\mathrm{Si}:\mathrm{Al}$ ratio that can be reacted through alkali activation to facilitate better geopolymerization. First, a series of laboratory tests was conducted to obtain the characterizations of the FA. Then geopolymer samples using different amounts of FA were produced by activating the $\mathrm{MTs}+\mathrm{FA}$ mixtures with 10 M NaOH solutions at a moisture ratio of 16% cured for 7 days with a slightly elevated temperature. Finally, uniaxial compression tests (UCTs) were applied to evaluate the influence of the addition of different amounts of FA (5%, 10%, 15%, and 20%) on the compressive behaviors. In addition, microscopic insights with different FA additions were evaluated regarding morphology, chemical bonds, and mineralogy. Results show that adding FA supplements based on the initial moisture content and curing conditions increased the mechanical behavior of the geopolymer specimens, followed by a decrease due to insufficient water for a chemical reaction. However, due to the water loss during the curing process, improvement in mechanical properties began to decrease when the FA was increased to 20%.