Study on the Effect of Slurry Concentration on the Mechanical Properties and Fluoride Immobilization of Red Mud-Based Backfill Under Phosphogypsum Neutralization
Red mud (RM) is a strongly alkaline waste residue produced during alumina production, and its high alkali and fine particle characteristics are prone to cause soil, water, and air pollution. Phosphogypsum (PG), as a by-product of the wet process phosphoric acid industry, poses a significant risk of...
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2025-05-01
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| author | Qinli Zhang Jingjing Yang Bin Liu Daolin Wang Qiusong Chen Yan Feng |
| author_facet | Qinli Zhang Jingjing Yang Bin Liu Daolin Wang Qiusong Chen Yan Feng |
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| description | Red mud (RM) is a strongly alkaline waste residue produced during alumina production, and its high alkali and fine particle characteristics are prone to cause soil, water, and air pollution. Phosphogypsum (PG), as a by-product of the wet process phosphoric acid industry, poses a significant risk of fluorine leaching and threatens the ecological environment and human health due to its high fluorine content and strong acidic properties. In this study, RM-based cemented paste backfill (RCPB) based on the synergistic curing of PG and ordinary Portland cement (OPC) was proposed, aiming to achieve a synergistic enhancement of the material’s mechanical properties and fluorine fixation efficacy by optimizing the slurry concentration (63–69%). Experimental results demonstrated that increasing slurry concentration significantly improved unconfined compressive strength (UCS). The 67% concentration group achieved a UCS of 3.60 MPa after 28 days, while the 63%, 65%, and 69% groups reached 2.50 MPa, 3.20 MPa, and 3.40 MPa, respectively. Fluoride leaching concentrations for all groups were below the Class I groundwater standard (≤1.0 mg/L), with the 67% concentration exhibiting the lowest leaching value (0.6076 mg/L). The dual immobilization mechanism of fluoride ions was revealed by XRD, TGA, and SEM-EDS characterization: (1) Ca<sup>2</sup>⁺ and F<sup>−</sup> to generate CaF<sub>2</sub> precipitation; (2) hydration products (C-S-H gel and calixarenes) immobilized F<sup>−</sup> by physical adsorption and chemical bonding, where the alkaline component of the RM (Na<sub>2</sub>O) further promotes the formation of sodium hexafluoroaluminate (Na<sub>3</sub>AlF<sub>6</sub>) precipitation. The system pH stabilized at 9.0 ± 0.3 after 28 days, mitigating alkalinity risks. High slurry concentrations (67–69%) reduced material porosity by 40–60%, enhancing mechanical performance. It was confirmed that the synergistic effect of RM and PG in the RCPB system could effectively neutralize the alkaline environment and optimize the hydration environment, and, at the same time, form CaF<sub>2</sub> as well as complexes encapsulating and adsorbing fluoride ions, thus significantly reducing the risk of fluorine migration. The aim is to improve the mechanical properties of materials and the fluorine-fixing efficiency by optimizing the slurry concentration (63–69%). The results provide a theoretical basis for the efficient resource utilization of PG and RM and open up a new way for the development of environmentally friendly building materials. |
| format | Article |
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| spelling | doaj-art-8b09dac3f2f541ce92b2787f8b8aa6b42025-08-20T02:33:06ZengMDPI AGApplied Sciences2076-34172025-05-011511604110.3390/app15116041Study on the Effect of Slurry Concentration on the Mechanical Properties and Fluoride Immobilization of Red Mud-Based Backfill Under Phosphogypsum NeutralizationQinli Zhang0Jingjing Yang1Bin Liu2Daolin Wang3Qiusong Chen4Yan Feng5School of Resources and Safety Engineering, Central South University, Changsha 410083, ChinaSchool of Resources and Safety Engineering, Central South University, Changsha 410083, ChinaSchool of Resources and Safety Engineering, Central South University, Changsha 410083, ChinaSchool of Resources and Safety Engineering, Central South University, Changsha 410083, ChinaSchool of Resources and Safety Engineering, University of Science and Technology Beijing, Beijing 100083, ChinaSchool of Resources and Safety Engineering, Central South University, Changsha 410083, ChinaRed mud (RM) is a strongly alkaline waste residue produced during alumina production, and its high alkali and fine particle characteristics are prone to cause soil, water, and air pollution. Phosphogypsum (PG), as a by-product of the wet process phosphoric acid industry, poses a significant risk of fluorine leaching and threatens the ecological environment and human health due to its high fluorine content and strong acidic properties. In this study, RM-based cemented paste backfill (RCPB) based on the synergistic curing of PG and ordinary Portland cement (OPC) was proposed, aiming to achieve a synergistic enhancement of the material’s mechanical properties and fluorine fixation efficacy by optimizing the slurry concentration (63–69%). Experimental results demonstrated that increasing slurry concentration significantly improved unconfined compressive strength (UCS). The 67% concentration group achieved a UCS of 3.60 MPa after 28 days, while the 63%, 65%, and 69% groups reached 2.50 MPa, 3.20 MPa, and 3.40 MPa, respectively. Fluoride leaching concentrations for all groups were below the Class I groundwater standard (≤1.0 mg/L), with the 67% concentration exhibiting the lowest leaching value (0.6076 mg/L). The dual immobilization mechanism of fluoride ions was revealed by XRD, TGA, and SEM-EDS characterization: (1) Ca<sup>2</sup>⁺ and F<sup>−</sup> to generate CaF<sub>2</sub> precipitation; (2) hydration products (C-S-H gel and calixarenes) immobilized F<sup>−</sup> by physical adsorption and chemical bonding, where the alkaline component of the RM (Na<sub>2</sub>O) further promotes the formation of sodium hexafluoroaluminate (Na<sub>3</sub>AlF<sub>6</sub>) precipitation. The system pH stabilized at 9.0 ± 0.3 after 28 days, mitigating alkalinity risks. High slurry concentrations (67–69%) reduced material porosity by 40–60%, enhancing mechanical performance. It was confirmed that the synergistic effect of RM and PG in the RCPB system could effectively neutralize the alkaline environment and optimize the hydration environment, and, at the same time, form CaF<sub>2</sub> as well as complexes encapsulating and adsorbing fluoride ions, thus significantly reducing the risk of fluorine migration. The aim is to improve the mechanical properties of materials and the fluorine-fixing efficiency by optimizing the slurry concentration (63–69%). The results provide a theoretical basis for the efficient resource utilization of PG and RM and open up a new way for the development of environmentally friendly building materials.https://www.mdpi.com/2076-3417/15/11/6041red mudphosphogypsumcemented paste backfillfluorine fixationhydration productsenvironmental safety |
| spellingShingle | Qinli Zhang Jingjing Yang Bin Liu Daolin Wang Qiusong Chen Yan Feng Study on the Effect of Slurry Concentration on the Mechanical Properties and Fluoride Immobilization of Red Mud-Based Backfill Under Phosphogypsum Neutralization Applied Sciences red mud phosphogypsum cemented paste backfill fluorine fixation hydration products environmental safety |
| title | Study on the Effect of Slurry Concentration on the Mechanical Properties and Fluoride Immobilization of Red Mud-Based Backfill Under Phosphogypsum Neutralization |
| title_full | Study on the Effect of Slurry Concentration on the Mechanical Properties and Fluoride Immobilization of Red Mud-Based Backfill Under Phosphogypsum Neutralization |
| title_fullStr | Study on the Effect of Slurry Concentration on the Mechanical Properties and Fluoride Immobilization of Red Mud-Based Backfill Under Phosphogypsum Neutralization |
| title_full_unstemmed | Study on the Effect of Slurry Concentration on the Mechanical Properties and Fluoride Immobilization of Red Mud-Based Backfill Under Phosphogypsum Neutralization |
| title_short | Study on the Effect of Slurry Concentration on the Mechanical Properties and Fluoride Immobilization of Red Mud-Based Backfill Under Phosphogypsum Neutralization |
| title_sort | study on the effect of slurry concentration on the mechanical properties and fluoride immobilization of red mud based backfill under phosphogypsum neutralization |
| topic | red mud phosphogypsum cemented paste backfill fluorine fixation hydration products environmental safety |
| url | https://www.mdpi.com/2076-3417/15/11/6041 |
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