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.

It is believed that the Moon formed following collision of a large planetesimal with the early Earth. Over the similar to 4 Gyr since this event the Moon has been considerably less processed by geological activity than the Earth, and may provide a better record of processes and conditions in the early Earth-Moon system. There have been many studies of magmatic volatiles such as H, F, Cl, S and C in lunar materials. However, our ability to interpret variable volatile contents in the lunar sample suite is dependent on our understanding of volatile behaviour in lunar systems. This is currently constrained by limited experimental data. Here, we present the first experimental mineral-melt partitioning coefficients for F, Cl and H2O in a model lunar system under appropriately reduced conditions (log fO(2) to IW-2.1, i.e. oxygen fugacity down to 2.1 log units below the Fe-FeO buffer). Data are consistent with structural incorporation of F, Cl and OH - in silicate melt, olivine and pyroxene under conditions of the lunar mantle. Oxygen fugacity has a limited effect on H2O speciation, and partitioning of H2O, F and Cl is instead largely dependent on mineral chemistry and melt structure. Partition coefficients are broadly consistent with a mantle source region for lunar volcanic products that is significantly depleted in F, Cl and H2O, and depleted in Cl relative to F and H2O, compared to the terrestrial mantle. Partitioning data are also used to model volatile redistribution during lunar magma ocean (LMO) crystallisation. The volatile content of lunar mantle cumulates is dependent upon proportion of trapped liquid during LMO solidification. However, differences in mineral-melt partitioning during LMO solidification can result in significant enrichment on F relative to Cl, and F relative to H2O, in cumulate phases relative to original LMO composition. As such, Cl depletion in lunar volcanic products may in part be a result of LMO solidification. Crown Copyright (C) 2020 Published by Elsevier Ltd. All rights reserved.

In the 50 years since the first lunar sample return, the investigation of H2O in the Moon has experienced several stages of developments and paradigms. In the early years since Apollo sample return, only bulk soil and bulk rock samples were analyzed for H2O as well as other volatiles. From 1970 to 2007, it was thought that the Moon is essentially devoid of innate H2O, containing probably less than 1 ppb. New technologies gradually enabled the measurements of H2O in lunar glass beads, soil glass, minerals such as apatite and anorthite, and olivine-hosted melt inclusions. The advancements in measurement techniques led to improved data and new insights. Starting from 2008, significant H2O in deep-sourced lunar rocks has been reported, resulting in a paradigm shift from a bone-dry Moon to a fairly wet Moon, although there is still debate about whether the bulk silicate Moon contains similar to 100 ppm of H2O (similar to that in the Earth's MORB mantle) or only a few ppm H2O. The advances on our knowledge of H2O in the Moon is accompanied by increased understanding of other volatiles in the Moon. Gradually, the degrees of depletion of various volatiles in the Moon relative to the Earth were inferred. Using assessed data from available lunar samples, mostly the melt inclusions, and also bulk rock analyses, it is found that the inferred degrees of depletion for volatile elements in the Moon relative to the Earth do not vary much and are independent of the condensation temperature. It is proposed that an early veneer delivered the volatiles to both the Earth and the Moon, but the Moon received proportionally less of the early veneer planetesimals. In addition to H2O in the interior of the Moon, significant surface H2O in the form of ice in lunar polar regions and structural OH in agglutinate glass in lunar regolith originating from solar wind implantation has also been gradually quantified.

The last liquids of the lunar magma ocean, known as urKREEP, should be highly enriched in volatiles, such as water, fluorine, and chlorine. We find chlorine-rich glasses in two pristine KREEP basalts from the Moon and calculate the volatile contents of the urKREEP component, and use this to estimate the fluorine and chlorine content of the lunar magma ocean. The Cl/Nb and F/Nd of KREEP imply that the lunar magma ocean was depleted in fluorine and chlorine by an order of magnitude compared to the Earth's mantle. The extremely dry nature of most lunar samples is simply a result of partial melting of magma ocean cumulates that had already lost their volatiles to the urKREEP layer. The volatile-rich KREEP component may have helped lower the solidus of high-temperature magma ocean cumulates that were melted to form the Mg-suite rocks of the highlands, and also aided the dissemination of the KREEP signature into the upper crust. The chlorine-rich KREEP glasses also demonstrate that the large chlorine isotope anomaly found in lunar samples is likely an early lunar signature.