Strength development and microstructure of sustainable geopolymers made from alkali-activated ground granulated blast-furnace slag, calcium carbide residue, and red mud

Red mud (RM) and calcium carbide residue (CCR) are waste generated from alumina refining and acetylene gas producing, respectively. This study utilized alkali-activated ground granulated blast-furnace slag (GGBS) and wet-basis CCR stabilized RM to prepare geopolymers material. The alkali activator solution was composed of CCR supernatant, sodium hydroxide (NaOH) and sodium silicate (Na2SiO3). CCR supernatant was used to reduce the amount of strong alkali. The effects of alkali activator on chemical composition and microstructure of geopolymers were studied by X-ray diffraction (XRD), thermogravimetric-derivative thermogravimetric (TG-DTG) analysis, and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS). The results show that the utilization of alkali activator improved the compressive strength and especially significantly improved the flexural strength of geopolymers. The strength was negatively correlated with the RM content: the geopolymers containing 30 % RM achieved a maximum 28-day compressive and flexural strength value of 20.3 MPa and 4.3 MPa, respectively. The microstructural analysis revealed that the strength increased due to the formation of calcium aluminate hydrates (C-A-H) and calcium silicate hydrates (C-S-H). The toxic heavy metals (Cr, Ni, Cu, Pb, Sb, and Zn) from RM formed some complex compounds and were combined in the geopolymers. The 28-day leaching experiment showed that the toxic heavy metals levels were below the test limits. In summary, this study confirmed the safety and feasibility of preparing RM-GGBS-CCR based geo-polymers via alkali-activated method.

Automated summary

This study utilized alkali-activated ground granulated blast-furnace slag and wet-basis calcium carbide residue stabilized red mud to prepare geopolymers material. The utilization of alkali activator improved the compressive and flexural strength of geopolymers. The microstructural analysis revealed the formation of calcium aluminate hydrates and calcium silicate hydrates, contributing to the increased strength. The leaching experiment showed that toxic heavy metals levels were below the test limits, confirming the safety and feasibility of the prepared geopolymers.