A novel coal gasification coarse slag-based geopolymer: Influences of physico-chemical coupling activation on its properties, microstructure, and hazardous material immobilization

A low-carbon geopolymer was prepared by using coal gasification coarse slag (CGCS), ground granulated blast-furnace slag (GGBS), soda residue (SR) and phosphogypsum (PG). The properties of CGCS based geopolymers were systematically studied under the combined effects of chemical activation and physical activation, mainly including binder-to-activator ratio, SR/PG ratio, water-to-binder ratio, CGCS grinding time, and curing humidity. Further, the contribution of hydration products to strength were revealed by X-ray diffraction (XRD), scanning electron microscopy (SEM-EDS), thermogravimetric analysis (TG-DTG), Fourier transform infrared spectroscopy (FTIR), and low-field nuclear magnetic resonance (LF NMR). The results showed that for the binder-to-activator ratio of 4:1 and the SR/PG ratio of 1:1, the slump of CGCS based geopolymers was 18.5 mm and the corresponding 28 d UCS was 20.38 MPa. Moreover, the matrix strength could be further enhanced by decreasing the water-to-binder ratio, reducing the grinding time of CGCS, and employing water curing. Therefore, the physico-chemical coupling activation could significantly promote the activation of CGCS and GGBS, with C-(A)-S-H gel and ettringite (AFt) being the primary hydration products. When the SR/PG ratio was 1:1, the synergistic effect of C-(A)-S-H gel and AFt was optimal, and the C-(A)-S-H gel formation compensated for the strength loss due to AFt reduced. The leaching rate of heavy metals from CGCS-based geopolymers was lower than the national standard and met environmental protection requirements. Nevertheless, the leaching concentration of Cl was significantly decreased, but the CGCS-based geopolymers could not meet the requirements used with steel bars.