Properties and mechanism of high-magnesium nickel slag-fly ash based geopolymer activated by phosphoric acid

A series of geopolymers were prepared by using phosphoric acid as activator to stimulate high-magnesium nickel slag (HMNS) and fly ash (FA). The effects of amount of phosphoric acid, the ratio of HMNS:FA and water-binder ratio, on the mechanical properties of HMNS-FA-phosphate based geopolymers were studied through compressive strength testing, X-ray diffractometry (XRD), Fourier transform infrared spectrometer (FT-IR) and scanning electron microscopy (SEM), energy dispersive spectrometer (EDS). The results showed that the most convenient amount of phosphoric acid, the ratio of HMNS:FA and water-binder ratio required for producing HMNS-FA-phosphate based geopolymers with better mechanical properties was that phosphoric acid amount was 25%, the ratio of HMNS:FA was 0.7:0.3 and the water-binder ratio was 0.24. The major phase in HMNS-FA-phosphate based geopolymers was newberyite (MgHPO4·3H2O) with crystal structure and amorphous features. The HMNS-FA-phosphate based geopolymers formed a gel-crystal phase structure, which was more reasonable than the gel-based structure, because it used crystals as the framework and colloids as the bonding medium to increase the density of geopolymers and formed a more compact hardened body. The geopolymerization mechanism of HMNS-FA-phosphate based geopolymer was also discussed. This study shows the potential of using HMNS as value source material for geopolymer production activated by phosphoric acid.