Development of low-carbon binders from ground granulated blast furnace slag activated by red mud, phosphogypsum, and calcium carbide slag: Mechanical and hydration properties

Geopolymers can replace traditional cement-based materials and support the development of eco-friendly materials. In this study, geopolymers were developed using three underutilised industrial wastes as activators: red mud (RM), calcium carbide slag (CS), and phosphogypsum (PG). Ground granulated blast furnace slag (GGBS) was used as the precursor. The effects of the activator dosage on the fluidity and mechanical properties were systematically investigated. The hydration mechanism of the 28-day geopolymer was comprehensively characterised using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, thermogravimetry and derivative thermogravimetry (TG-DTG), and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS). The results showed that phosphogypsum enhanced both the fluidity and strength. With increasing phosphogypsum content, the compressive strength initially increased. It then decreased and subsequently increased again. This pattern was attributed to the content and morphology of ettringite (AFt) and calcium (alumino)silicate hydrate (C-(A)-S-H) gels. By optimising the mixing ratio, binders with good compressive strengths at 3 days (14.52 MPa), 7 days (19.3 MPa) and 28 days (26.98 MPa) could be prepared, as well as a binder with a maximum compressive strength of 29.43 MPa at 28 days. Moreover, the prepared binders showed good immobilisation of heavy metal ions at different pH values. The proposed low-carbon binder showed potential for manufacturing pavement bricks and concrete blocks, while providing a method for the disposal of these industrial wastes.