Improvement of calcium sulfoaluminate cement with carbide slag-modified phosphogypsum: Performance and micromechanisms

Phosphogypsum (PG) is an industrial by-product that is difficult to manage and poses serious environmental threats. Long-term accumulation of PG can potentially pollute land and water due to its acidity and harmful impurities. In this study, the raw PG and carbide slag (CS)-modified PG were used to replace anhydrite in calcium sulfoaluminate (CSA) cement, and the performance and micromechanisms of PG-CSA and CS-PG-CSA were investigated. The engineering performances including setting time, fluidity, compressive strength, and hydration characteristics of PG-CSA and CS-PG-CSA were investigated. Furthermore, the crystalline phases and microstructure evolutions of hydration products were confirmed by hydration heat, X-ray diffraction (XRD), thermogravimetry-differential thermal analysis (TG-DTA) and scanning electron microscopy- energy dispersive spectrometer (SEM-EDS) tests. The results showed that CS could effectively remove the acidity and impurities of PG by converting PO 4 3- and F - into Ca 3 (PO 4 ) 2 and CaF 2 . When 20 % raw PG was mixed with CSA clinker, the strength of PG-CSA reached a maximum of 65.83 MPa, which is 41.81 % higher than that of commercial CSA cement. Compared to commercial CSA cement, PG-CSA exhibits a delayed hydration heat release peak, resulting in a longer setting time and greater fluidity. However, CS effectively increases the strength of PG-CSA, but it shortens the setting time and reduces the fluidity. The early and late hydration products of PG-CSA and CS-PG-CSA are mainly ettringite (AFt) and aluminum hydroxide (AH 3 ), respectively. The results of the environmental assessment unequivocally demonstrate that the developed product surpasses traditional cement in all evaluated metrics, achieving a remarkable reduction in CO 2 emissions of nearly 50 %. The key findings indicate that the use of CS-modified PG in CSA cement can not only improve engineering performance but also promote sustainability and economic efficiency, demonstrating significant potential for widespread application.