Dissolution kinetics of silicon, aluminum, and calcium in red mud, steel slag, and fly ash under alkaline and sulfate environments

The global cement industry contributes approximately 8% of carbon dioxide emissions. Utilizing low-carbon wastes such as red mud, steel slag, and fly ash to replace cement is of great significance for carbon neutrality. To address the issue of their low reactivity in geotechnical engineering applications, this study proposes an alkaline-sulfate synergistic activation method. Techniques such as backscattered electron spectroscopy and X-ray diffraction were employed to reveal the microstructural characteristics and reaction kinetics. Experimental results indicate that the concentration of alkaline solution, temperature, dissolution time, solid-to-liquid ratio, and sulfate concentration significantly affect the dissolution of Si, Al, and Ca from the waste materials. Kinetic analysis confirms that the dissolution process follows the internal diffusion mechanism of the shrinking-core model. Among the elements, Si has a lower activation energy under alkaline conditions, resulting in higher dissolution reactivity, while Ca has a higher activation energy, making its dissolution rate more temperature-sensitive. Molecular dynamics simulations show that in the NaOH-Na2SO4 system, Si and Al exist in the form of SiO32− and Al(OH)4-, respectively. This study provides a theoretical foundation for the development of low-carbon geotechnical materials and supports the low-carbon transformation of the construction industry.