Influence of Thermal and Ambient Curing on the Properties and the Microstructural Features of Geopolymers from Coal-Based Synthetic Natural Gas Slag

ABSTRACT ERRATUM: When this article first published, the final author's name was incorrectly published. The last author's name should be "G. P. Ong." The author name was updated on August 15, 2025. The publisher apologizes for this error. In this study, coal-based synthetic natural gas slag (CSNGS) was used as a precursor and water glass solution as an activator to prepare geopolymers with the aim of promoting the resource utilization of such industrial waste residue. Compressive strength tests and microstructural analyses, including X-ray diffraction (XRD), scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), Fourier transform infrared spectroscopy, thermogravimetric (TG) analysis, and mercury intrusion porosimetry (MIP), were conducted to investigate the effects of the silicon/aluminum ratio, activator modulus, and curing temperature on the mechanical properties and microstructure of CSNGS geopolymers. The findings indicated that increasing the silicon/aluminum ratio in the reaction system or elevating the curing temperature enhanced the compressive strength of CSNGS geopolymers. Conversely, an increase in the activator modulus resulted in a reduction in specimens strength. The silicon/aluminum ratio and curing temperature exhibited similar influences on the microstructural characteristics of CSNGS geopolymers. Specifically, as the silicon/aluminum ratio increased or the curing temperature elevated, more N-A-S-H gel was formed and the porosity of the matrix was modified. However, with an increase in the activator modulus, the amount of gels in CSNGS geopolymers, the silicon/aluminum ratio of the reaction products, and the weight loss during TG test all decreased, leading to a decline in the strength of CSNGS geopolymers. When the silicon/aluminum ratio was set at 2.5 and the activator modulus was maintained at 1.0. The maximum 28-day compressive strengths of the specimens cured at 80°C for 24 h and cured under ambient temperature were 35.3 MPa and 15.3 MPa, respectively. The CSNGS geopolymers prepared in this study have the potential to be used in the field of prefabricated blocks, and the research results may provide valuable insights into improving the properties of CSNGS geopolymers by adjusting the ratios of key elements in the reaction system.