Experimental study on performance, hydration, and sustainability of coal gasification coarse slag based geopolymers for sustainable environment

The large-scale application of coal gasification technology generates a large amount of coal gasification coarse slag (CGCS) difficult to treat. In this study, low-carbon geopolymers (CGRC) were produced using CGCS and ground granulated blast furnace slag (GGBS) as precursors, whereas red mud (RM) and carbide slag (CS) served as solid waste activators. The effects of the binder/activator, RM/CS and CGCS/GGBS ratios and the curing age on the compressive strength, setting time, heavy metal leaching, XRD, SEM-EDS, TG-DTG, FTIR, and LF NMR of CGCS-based geopolymers were investigated, and the hydration mechanism of CGRC geopolymer was disclosed. The results showed that the binder/activator ratio had a negligible effect on the ultimate compressive strength (UCS), the RM/CS ratio had a noticeable impact on the compressive strength and setting time, and increasing the GGBS content significantly improved the performance of CGCS-based geopolymers. Therefore, the synergistic effect of RM and CS led to the activation of CGCS and GGBS. The main hydration products were N-A-S-H, N/C-A-S-H, C-A-S-H gel, CH, and CaCO3. Furthermore, the decrease of the RM/CS ratio which promoted the gel transformation from metastable N-A-S-H to N/C-A-S-H configuration and then to stable C-A-S-H state, finally forming a denser gel matrix. The amounts of various heavy metals leached from CGCS-based geopolymers were even lower than those required by the national standards; therefore the material met the requirements for environmental protection. Furthermore, all raw materials belong to solid waste, which reduces carbon emissions.