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) Ca2(+) and F- to generate CaF2 precipitation; (2) hydration products (C-S-H gel and calixarenes) immobilized F- by physical adsorption and chemical bonding, where the alkaline component of the RM (Na2O) further promotes the formation of sodium hexafluoroaluminate (Na3AlF6) 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 CaF2 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.

Magnesium phosphate cement (MPC), renowned for its rapid hardening, low water demand, low-temperature hydration capability, and excellent wear resistance, is an ideal construction material for the extreme lunar environment, characterized by high vacuum, low gravity, and severe temperature fluctuations. In this study, by-product B-MgO from lithium extraction in salt lakes was utilized to develop four types of phosphate cement systems: ammonium magnesium phosphate cement (MAPC), sodium magnesium phosphate cement (MSPC), calcium magnesium phosphate cement (MCPC), and potassium magnesium phosphate cement (MKPC). Through a comparative analysis of the physical and mechanical properties of these systems at varying calcination temperatures of MgO, MKPC was identified as the most suitable for lunar construction. Further investigations examined the influence of the water-to-binder ratio (W/B) and the mass ratio of raw materials (M/P) on MKPC performance, alongside a detailed analysis of its phase composition and microstructure. The results revealed that the optimal MKPC performance is achieved at an MgO calcination temperature of 1000 degrees C, an M/P ratio of 1:1 to 2:1, and a W/B ratio of 0.2 to 0.25. Additionally, MKPC was employed as a cementitious material to produce MKPC-simulated lunar regolith concrete with regolith contents of 30 %, 53 %, and 70 %. The fabricated concrete met the required mechanical properties and 3D printability standards under lunar environmental conditions. Even at high regolith content, the concrete maintained satisfactory mechanical performance. These findings provide an efficient and reliable material solution for lunar infrastructure construction. (c) 2024 Published by Elsevier B.V. on behalf of COSPAR.

The rational disposal and resource utilization of municipal solid waste incineration bottom ash (MSWI-BA) is an urgent problem to be solved. This study explores the impact of MSWI-BA and its finely ground powder (MSWIBAP) as fine aggregates and solidifying agent components in pre-mixed fluidized solidified soil (PM-FSS) by conducting tests on the unconfined compressive strength and volume stability. Additionally, it analyzes the composition and microstructure properties of hydration products using techniques such as XRD, TG-DSC, MIP, and FTIR. The results demonstrated that the PM-FSS incorporating MSWI-BA and MSWI-BAP exhibited a dense microstructure and excellent mechanical properties, with the main hydration products being Aft, C-(A)-S-H gel, square crystal, etc. The volume deformation of PM-FSS with MSWI-BA and MSWI-BAP increased, but it did not affect the development of its mechanical strength. MSWI-BA can be used as a solidifying agent component and fine aggregate for the preparation of PM-FSS, achieving its resource utilization.

To address the challenges posed by the significant quantity of ammonia-alkali white mud, this study explores the preparation of fluid solidified soil using ammonia-alkali white mud, mineral powder, and fly ash. The findings reveal that ammonia-alkali white mud primarily comprises sulfate, carbonate, and soluble chloride salt, with an alkaline solution and a well-developed pore structure. Optimal fluid solidified soil formulation, comprising 30% white mud, 30% salt mud, 25% mineral powder, 10% fly ash, and 5% calcium oxide, yields a slurry fluidity of 176 mm and a compressive strength of 3.98 MPa at 28 days. Microscopic analysis highlights AFt and C-S-H gel as the principal hydration products of fluid solidified soil. The fine particles of calcium carbonate in ammonia-alkali white mud fill the structural pores and intertwine with the hydration products, facilitating the formation of a dense structure, which constitutes the primary source of strength in fluid solidified soil. Furthermore, the heavy metal content of the solidified soil aligns with the first type of land use requirements outlined in the GB 36600-2018 standard, and the toxicity of the leaching solution adheres to the emission concentration limit stipulated by GB 8978-1996.