Experimental investigation of utilizing waste flue gas desulfurized gypsum as backfill materials

Flue gas desulfurized gypsum (FGDG) is a waste material produced in the power stations during the flue gas desulfurization process. In China, a small amount of FGDG is used as cement additives or for gypsum boards, but there still exist a mass of FGDG due to the lack of reusing methods. Also, the waste fly ash (WFA) is another waste material from the power plant. To alleviate the accompanying environmental issues, large-volume utilizations such as in civil engineering applications are usually adopted to consume these materials. However, these materials cannot achieve high strength due to the lack of amorphous content. In this study, combined with WFA, a large volume of waste FGDG was investigated for the utilizations as backfill materials. With using two types of activators, sodium hydroxide and water glass, in small doses, mixtures with various ratios of FGDG and WFA were included to investigate the mechanical, chemical, microscopic, and softening properties at different curing periods. Then the field experiments including the dynamic cone penetration test and the load plate test were conducted to evaluate the bearing capacity, settlement and compaction effect of the compacted mixtures. The experimental results showed that the NaOH activated mixture (FGDG: WFA = 50:50) could acquire acceptable mechanical properties for backfill applications, which can serve as a practical option for the large-volume utilization of FGDG and WFA.