Recycling phosphogypsum to produce artificial lightweight aggregates via stirring and pelleting all-in-one technique: Technical assessment and performance optimization

Abstract

Currently, production of artificial lightweight aggregates (ALAs) is becoming one of the most promising approaches for solid wastes recycling via pelletization technique. In this study, an innovative stirring and pelleting all-in-one technique (S-pelleting) was employed to prepare the ALAs by using phosphogypsum (PG). The effects of different pelleting techniques on properties of the ALAs were investigated. Furthermore, the NaHCO₃, Al₂(SO₄)₃+CaCO₃ (Al-Ca) and foamed particles (FP) were introduced as foaming agents to optimize the performance of the ALAs. The specific strength was used as the main assessment index for performance of the aggregates. The results indicated that the specific strength of the aggregates was improved to 10.96 MPa⋅cm³/g by S-pelleting, which was 60.7 % and 35.6 % higher than that of one-step pelleting or double-step pelleting technique. Compared to NaHCO₃, the Al-Ca was more conducive to the specific strength development of the aggregates, and the FP addition could further improve the specific strength of the ALAs. When the concentration of Al-Ca was 7% and the mass fraction of FP was 10%, the ALAs with high specific strength of 9.6 MPa⋅cm³/g were obtained in this study. Moreover, the mineral composition, microstructure and pore structure of the ALAs were analyzed and the results reveled that the Al-Ca could promote to form more ettringite and reduce the proportion of harmful pores in the aggregates, which was responsible for the improvement in specific strength. The FP acted as a lightweight skeleton supported inside the aggregates and the well bonded ITZ between FP and the matrix could further improve the specific strength of the ALAs. Finally, the durability and environmental impact of the ALAs were evaluated acceptable. Overall, this study not only provided a green approach for PG recycling, but also presented a new technical scheme for more efficient production of ALAs.