Synergy effects of phosphogypsum on the hydration mechanism and mechanical properties of alkali-activated slag in polysilicon sludge solidification

Polysilicon sludge (PSS), due to its high organic impurity content, poses significant challenges in solidification using traditional cementitious materials, failing to meet engineering standards. As such, alternative solidification methods are urgently needed to enhance PSS resource utilization. This study explores the use of phosphogypsum (PG) to improve the alkali activation process in the solidification of PSS with slag. Given that calcium sulfate in PG promotes alkali activation, this research investigates the synergistic effects of PG, PSS, and slag as precursors in developing an eco-friendly, resource-efficient solidification approach. Comprehensive characterization techniques, including X-ray diffraction, thermogravimetric analysis, mercury intrusion porosimetry, scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, and solid-state magic-angle spinning nuclear magnetic resonance were employed to assess the influence of PG and PSS dosages on the hydration behavior, mechanical properties, and microstructure of the alkali-activated system. Results demonstrated that the addition of 5 wt% PG significantly improved slag dissolution promoted the formation of hydration products such as hydrated calcium aluminate, reduced sample porosity, and refined the pore size distribution. These effects mitigated the inhibitory impact of PSS on the alkali activation process. However, higher dosages of PG and PSS reduced the system's pH, which in turn hindered early slag solubility and compromised the early strength of the solidified product. In addition, the ternary alkali-activated system developed in this study effectively immobilized heavy metals present in both PSS and PG, thus enabling the synergistic utilization of these waste materials while enhancing environmental compatibility. The findings provide valuable theoretical insights into the use of alkali-activated systems for solidifying waste materials and promote the development of resource-efficient, sustainable waste management strategies. This study paves the way for future research into optimizing the composition of PG, PSS, and slag mixtures to further enhance mechanical performance, long-term stability, and environmental safety.