Effect of blast furnace slag grades on fly ash based geopolymer waste forms

Ground granulated blast furnace slags (GGBFSs) of grades 80, 100 and 120 were investigated for high waste loading fly ash based geopolymer waste forms. Samples were prepared at a fixed fly ash/GGBFS mass ratio of 5/3, using an activating solution prepared from concentrated Hanford secondary waste (HSW) simulant. The fresh pastes were subjected to isothermal conduction calorimetry and Vicat setting time measurements, and the cured waste forms were characterized by compressive strength test, XRD and SEM/EDS analyses, as well as the TCLP leaching test. The results show that GGBFS of higher grade generated more hydration heat, yet not definitely led to higher compressive strength or shorter setting times, suggesting that the GGBFS grading index established for cement industry may not be simply introduced to geopolymer application. It was also found that the reactivity potential of high grade GGBFS in fly ash based geopolymer might be better exploited at enhanced SiO2/Al2O3 and SiO2/CaO ratios. Except for rhenium, which is not regulated in TCLP, all heavy metals and hazardous elements in the HSW simulant were effectively immobilized by the geopolymer waste forms. However, effect of different GGBFS on heavy metals and hazardous elements fixation depended on different metals and elements.