Experimental Study of Geopolymer Synthesized with Class F Fly Ash and Low-Calcium Slag

Abstract

This paper presents an experimental study on geopolymer synthesized with class F fly ash (FA) and low-calcium slag (SG). Geopolymer specimens were produced using FA and SG at different relative amounts (FA/SG=0/100, 25/75, 50/50, 75/25, and 100/0), NaOH solution at different concentrations (7.5, 10, and 15 M), various curing times (1, 2, 4, 7, 14, and 28 days) and curing temperatures [25 (ambient), 45, 60, 75, and 90°C]. The unit weight and uniaxial compressive strength (UCS) of the geopolymer specimens were measured. Scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDX) and X-ray diffraction (XRD) were also performed to characterize the microstructure and phase composition of the geopolymer specimens. The results indicate that the incorporation of SG not only increases the UCS of the geopolymer specimens but also decreases the initial water content and, thus, the NaOH consumption at the same NaOH concentration required for geopolymer production. In addition, the inclusion of SG increases the unit weight of the geopolymer specimens, simply because SG has a much greater specific gravity than FA. The results also show that the strength of the FA/SG-based geopolymer develops rapidly, with a major portion of the UCS (approximately 70%) gained within only 2 days and no obvious strength gain after 7 days. The optimum curing temperature (the curing temperature at which the maximum UCS is obtained) at a FA/SG ratio of 50/50 is around 75°C. This research contributes to the knowledge of geopolymers produced from a combination of source materials and promotes the reuse of wastes through geopolymerization.