Preparation and property studies of ferric sulfoaluminate cement based on Bayer red mud and phosphogypsum

The Bayer red mud (RM) and phosphogypsum (PG) accumulation have caused significant environmental contamination. However, practical and effective resource utilization technologies are still lacking currently. This work aims to develop ferric sulfoaluminate cement (FSAC) employing low-cost materials including Bayer red mud, phosphogypsum, and other materials. This method effectively improves the utilization rate of Bayer red mud and phosphogypsum. Under the premise of ensuring the performance of FSAC, the utilization rate of solid waste can reach up to 48.56%. The effects of different red mud dosages on cement mineral formation, workability, and mechanical properties are investigated. Then, untreated phosphogypsum is adopted as a retarder for FSAC, and the hydration process, working properties, mechanical properties, types of hydration products, and morphology of FSAC are explored. The results suggest that the crystal transformation of Ye’elemite \(\left({\mathrm C}_4{\mathrm A}_3\;\overline{\mathrm S}\right)\) $\left({\mathrm{C}}_4,{\mathrm{A}}_3,,\overline{\mathrm{S}}\right)$  is promoted with the increase of Bayer red mud content. Cubic crystal system Ye’elemite \(\left({\mathrm C}_4{\mathrm A}_3\;\overline{\mathrm S}-\mathrm c\right)\) $\left({\mathrm{C}}_4,{\mathrm{A}}_3,,\overline{\mathrm{S}},-,\mathrm{c}\right)$  with higher hydration activity is generated, which increases the early strength of cement but greatly reduces the setting time, hindering the later strength growth. Untreated phosphogypsum can effectively delay the early hydration process of FSAC, prolong the setting time of cement, and increase the strength of FSAC in the later stage. When the dosage of Bayer red mud and phosphogypsum is 17.64% and 9.21%, respectively, with phosphogypsum dosage of 20%, the prepared FSAC has satisfactory mechanical properties, and the 3-day and 90-day compressive strengths are 34.6 MPa and 57.1 MPa, respectively. In addition, the study of heavy metal leaching indicates that the FSAC prepared by Bayer red mud, phosphogypsum, and other raw materials will generate no environment pollution, and the solidification of heavy metal elements in the cement slurry is superior.