Effect of synthesis conditions on mechanical properties and microstructure of coal gasification slag-based geopolymer

Coal gasification slag (CGS) is an industrial solid waste produced during coal gasification, and it is mostly deposited on the land at present, resulting in serious environmental pollution. In order to promote its resource utilization, geopolymer was synthesized from CGS in this study. The influence of important factors containing the water-solid ratio, alkali activator dosage, activator modulus and curing temperature on the mechanical properties and microstructure of geopolymers were investigated. Micromorphology, chemical composition, mineralogy and chemical bonding of the geopolymer were assessed by scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR), respectively. Results indicate that the compressive strength of geopolymer can be improved by reducing the amount of initial water. The optimal mechanical properties of geopolymer are obtained when the alkali activator dosage is 25 % and the activator modulus is 1. The moderate temperature (e.g. 60 °C) is the most favorable to the compressive strength development of geopolymer. Under these optimal conditions, the 90 days compressive strength of the CGS-based geopolymer (CGSG) can reach 58.3 MPa. The Si/Al molar ratio of the geopolymer gels is between 2.0 and 3.1, and the Al/Na molar ratio is around 1.0. These results can provide a reference for recycling of the CGS and production of eco-friendly building materials. •CGSG with high compressive strength was successfully synthesized.•The mechanical properties of CGSG are better when the Si/Al molar ratio of geopolymer gel is high.•A moderate temperature of 60 ℃ is the most favorable for the development of compressive strength of CGSG.