Effect of thermal activation on the pozzolanic activity of coal gasification fine slag: Focus on the evolution mechanism of the short-range ordered structure Si-O-Si/Al

The application of coal gasification fine slag (CGFS) as supplementary cementitious materials (SCMs) could achieve the large-scale utilization. Thermal activation is an efficient and feasible method to remove residual carbon from CGFS and thereby enhance its pozzolanic activity. However, the migration mechanism of active Si-Al components in CGFS during thermal activation process and its effect on pozzolanic activity remain unclear. In this study, the thermal activation mechanism and the subsequent hydration behavior of CGFS was revealed based on the evolution of its short-range ordered structures. Results show that with increasing activation temperature, the residual carbon removal of CGFS reaches 93.63% at 600 °C. The decrease in polymerization degree of CGFS leads to the disruption of dominant short-range ordered structures Si-O-Si and Si-O-Al, thereby enhancing the mobility of [SiO 4 ] 4- and [AlO 4 ] 5- structural units. These units eventually react with Ca²⁺ to crystallize as anorthite. Activated CGFS exhibits an increasing content of non-bridging oxygen in its amorphous phase, improving its the dissolution ratio in the NaOH solution for the enhanced pore connectivity of the product layer. The dissolution kinetics of activated CGFS follows the product layer diffusion model, with the apparent reaction rate constant k α of 2.38–6.89 × 10 −5 /h. At an activation temperature of 600 °C, the 28-day strength activity index reaches 103%, which meets the requirements of the building industry standard JG/T 486–2015.