Effect of granulated phosphorus slag on physical, mechanical and microstructural characteristics of Class F fly ash based geopolymer

Granulated phosphorus slag (GPS) is an industrial byproduct generated from the yellow phosphorus manufacturing process through the electric furnace method, which not only occupies enormous fields but also causes serious environmental issues. This study investigated the effect of GPS on physical, mechanical and microstructural characteristics of Class F fly ash (FFA) based geopolymer. Samples synthesized from FFA with different GPS content (0%, 10%, 20%, 30%, 40% and 50%) were produced by reacting with a mixture of NaOH and Na₂SiO₃ solution. Meanwhile, other variables including different NaOH concentrations (2.5 M, 5 M, 7.5 M and 10 M), various curing temperatures (ambient 20℃ and elevated 60℃) and curing times (3, 7, 14 and 28 days) were also considered. The setting time and compressive strength of geopolymer paste were measured, respectively. The results indicated that the GPS addition decreased the setting time but did not result in the flash setting of FFA based geopolymer paste. Besides, the incorporation of GPS increased the compressive strength of geopolymer samples. Increasing NaOH concentration and curing temperature were beneficial for the strength improvement. Scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) analysis were utilized to identify the microstructure and phase composition of geopolymer samples. The test results showed that after the hydration reaction, the relatively high amount of CaO in the GPS generated calcium silicate hydrate (C-S-H) gel which coexisted with sodium aluminate silicate hydrate (N-A-S-H) gel and thus resulted in a denser and more homogeneous microstructure. This study confirmed the feasibility of utilizing GPS to partially replace FFA, which could not only improve the mechanical property of the derived geopolymer but also realize the synergy between granulated phosphorous slag recycling and geopolymer industry.