Phosphogypsum (PG) is produced in large quantities, and its main resource utilization is in the construction sector. This study investigates the feasibility of using PG to manufacture phosphogypsum composite cement-based permeable bricks (PGCPB), focusing on the effects of aggregate size distribution, water-to-binder ratio, and slag powder (SP) content on their mechanical and durability properties and assesses the potential risks related to heavy metal content in PGCPB. The results indicate that the highest 3-day compressive strength of PGCPB is 21.1 MPa at a water-cement ratio of 0.26. The maximum 3-day compressive strength of 25.78 MPa is achieved when the fine-to-coarse aggregate ratio is 3:2. At 14 days, SEM observations reveal that incorporating 20% SP leads to an optimal crystalline microstructure and a denser matrix, corresponding to flexural and compressive strengths of 4.47 MPa and 15.25 MPa, respectively. The 14-day flexural and compressive strengths of the cementing material are 4.47 MPa and 15.25 MPa, respectively, when the SP content is 20%. With an increase in PG proportion, the 28-day compressive strength of PGCPB declines, the water permeability coefficient first rises and then falls, and its frost resistance progressively deteriorates. When PG content is 20-30%, PGCPB meets the JC/T 945-2005 permeability standard and reduces carbon emissions by 22.91% compared to conventional cement-based bricks. Environmental risk assessments confirm that PGCPB poses no risk to either soil ecology or human health, making it a safe and eco-friendly material for pavement applications.

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.

The problem of chemical soil pollution after military actions on the territory of Ukraine is becoming quite urgent in terms of ecological risks. The aim of the article was to establish the level of ecological safety of soils after the application of biosorption technology and to substantiate its ecological and economic feasibility. Within the scope of the study, three scenarios were set to evaluate the level of ecological risk under the condition of actual complex contamination of soils with five heavy metals (Zn, Cu, Ni, Pb, and Cd) - Scenario 1 and in the case of biosorption technology application for soil protection - Scenarios 2 and 3. Scenarios 2 and 3 differed in the type of substrate for anaerobic digestion (chicken manure and sewage sludge, respectively) compatible with phosphogypsum to obtain a biocomposite. Innovative approach for ecological risk assessment was improved based on the Bayes' theorem and developed set of qualitative and quantitative parameters. Based on the theoretical substantiation of the complex formation indicator and the fluorescent properties of digestate organic matter, the efficiency of heavy metal immobilisation in the soil was evaluated, which contributed to the reduction of ecological risk from moderate to low level for both scenarios. The results of the risk assessment based on Bayes' theorem showed a decrease in the level of risk from high to medium. Ecological and economic efficiency was assessed according to methodology of ecological damage after hostilities. The economically effective technology developed can be recommended for the comprehensive soil restoration scheme due to the obtained results.

To enhance the applicability of multiple solid waste road base materials in seasonally frozen soil areas and reduce the negative impact of red mud (RM) on the environment owing to its strong alkalinity, this paper utilizes untreated bayer method RM, fly ash (FA), and phosphogypsum (PG) as raw materials for preparing the road base materials. The mechanical properties, leaching characteristics, and Freeze-thaw (F-T) resistance of the materials from different solid waste systems were investigated through F-T cycle tests, unconfined compressive strength (UCS) tests, and leaching tests. The hydration, sodium solidification, and F-T deterioration mechanisms were revealed using an X-ray diffractometer and a scanning electron microscope. Results indicated that when the mix ratio of RM: FA: PG: cement was 64:28:2:6 (RFP2), the specimen exhibited the best F-T resistance. After 10 F-T cycles, the compressive strength retention rate (BDR) of the specimen was 91.43 %, and the Na+ leaching concentration was 390 mg/L, which still met the Chinese standard. The main hydration products of the material include C-S-H gel and ettringite crystals. These crystals and gels are intertwined and connected to form a dense mesh structure, which improves the material's F-T resistance and sodium solidification effect. The F-T cycle results in the expansion of cracks within the material, which leads to the destruction of the adhesion of the cementitious products, thus causing a deterioration of the strength of the specimen and the reduction of the sodium solidification effect.

Phosphogypsum (PG), phosphate sludge (PS), and sewage sludge (SS) are regarded by-products produced in huge amounts. However, PG, PS and SS are no longer considered as waste, but as valued resources in accordance with the circular economy's rules. Their management provides a serious environmental problem. In order to assess the impacts of SS, PS, and PG on soil physico-chemical parameters (pH, EC, OM, nutrients, and heavy metals) in response to diverse experimental settings, the purpose of the current study was to conduct a meta-analysis on previously published results. The VOSviewer program was used to construct bibliometric maps using the VOS mapping and grouping techniques. The findings indicated that there were statistically significant changes (P < 0.05) in electrical conductivity (EC), organic matter (OM), and pH in connection to the different by-products employed. The application of SS considerably elevated pH by 46.15% compared to the control. Furthermore, a beneficial effect on P and K was detected, regardless of the by-product used. Moreover, Cd, Pb, and Ni concentrations in SS treatments had a substantial reduction of 30.46%, 30.70%, and 18.07%, respectively. Cd, Pb, and Cu concentrations in PG treatments revealed a substantial decrease of 47.71%, 36.14%, and 46.01%, respectively. Based on the acquired data, PG, PS, and SS need to be regularly monitored and regulated. This study serves as an early investigation for the construction of a new approach to restore damaged land on mine sites by employing phosphate industry by-products and sludge for revegetation objectives.

Phosphogypsum (PG), an industrial solid waste produced from the wet phosphoric acid process, has seriously damaged the ecological environment. Its comprehensive utilization rate needs to be improved urgently. In this paper, the chemical enhancement effect of solid waste PG on expansive soil, known as engineering cancer, was investigated through systematic macroscopic and microscopic experiments. The positive and negative environmental impacts of the PG modifier were also comprehensively analyzed. Laboratory soil test results show that PG mixed with expansive soil can change the consistency limit of expansive soil, effectively increase the soil strength by 2-3 times and reduce the expansion of expansive soil to 62%. Therefore, it can be considered to be applied to the improvement of expansive soil roadbed. However, when the dosage is too high, it may be affected by the dissolution of PG, and the improvement effect is relatively decreased. The optimal dosage of PG is 15%. XRD, XRF, SEM and MIP microcosmic tests show that the mineral composition, element content and porosity of the expansive soil have changed after the addition of PG. Its microstructure is much tighter. Through TCLP test, the environmental effects of heavy metals caused by resource utilization of PG modified expansive soil were evaluated. In this study, only Cr element exceeded 2.6% slightly when the content of PG was 25%. The analysis found that the engineering properties of expansive soil were effectively improved, resulting in the effective solidification of heavy metals in PG.

Microbial induced carbonate precipitation (MICP) is gaining recognition for enhancing the mechanical properties of construction materials. This study aims to explore the potential of using phosphogypsum (PG), a solid waste mainly composed of CaSO42H(2)O, as both a sufficient calcium source for MICP based bio-cement and an aggregate for mine backfill applications. First, the interaction between MICP bacteria and the PG was assessed by monitoring pH, electrical conductivity, and Ca2 + and SO42- levels. Results indicated that bacteria maintained robust urease activity in the PG environment, leading to CO32- production. These ions, combined with the Ca2+ naturally present in PG to form CaCO3 precipitation, which acted as a binding agent for PG backfill. Further testing of the bio-cemented PG backfill showed excellent fluidity which is suitable for efficient pipeline transportation in underground mining. After a 7-day curing period, the backfill exhibited an unconfined compressive strength (UCS) of 947 kPa, meeting the standards for mining backfill applications. Additionally, the environmental impact of the bio-cemented PG backfill was notable. Unlike traditional cement-based backfills with high pH levels (>11), the leachate from the bio-cemented PG backfill maintained a neutral pH (7.16), highlighting its eco-friendly nature. This positions the bio-cemented PG backfill as a sustainable solution for the construction and mining industries.

The effective utilization of phosphogypsum (PG) and industrial waste soil is of paramount importance in the real world. The combination of phosphogypsum and soil in a single mixture can simultaneously utilize both materials. In this study, a novel green road material was developed according to the concept of synergistic utilization of multiple solid wastes, which is based on conventional cement stabilized soil. The GGBS was employed to gradually replace cement to stabilize PG-soil mixtures. The curing effect of GGBS replacing cement and the modification effect of PG on stabilized soil were evaluated in three aspects: mechanical properties, water stability, and environmental performance. This evaluation was conducted using the unconfined compressive strength (UCS), softening coefficient, and ionic concentration of heavy and trace metals. Furthermore, microscopic characterization techniques, including a pH meter, UV-visible spectrophotometer, FTIR, XRD, SEM, and EDS, were used to perform further analyses of the curing mechanism. The objective was to enhance the UCS of stabilized soil by incorporating an optimal amount of PG, avoiding the necessity for a complex and costly pretreatment process for PG. The UCS reached approximately 8 MPa in 7 days without immersion in water curing and 4 MPa in 7 days with 1 day immersion in water curing. Despite the decline in water stability resulting from the incorporation of PG, the stabilized soil exhibits superior mechanical properties compared to the majority of studies on the application of PG to stabilized soils. The monitoring of contaminant ions in the stabilized soil over a period of 28 days demonstrated compliance with EPA requirements, indicating that PG-based stabilized soil does not negatively impact the surrounding environment in the presence of water. Additionally, the optimal ratio of GGBS to cement is 1:1. Meanwhile, excessively high or low cement content has a detrimental impact on the properties of stabilized soil. Lastly, the practical engineering application of this novel green road material was achieved, and its mechanical properties and economic benefit were demonstrated to be superior to those of conventional cement stabilized soil. The study of PG in stabilized soil was transformed into the utilization of realworld projects without the necessity for a complex pretreatment process for PG. Concurrently, the replacement of GGBS for cement results in a reduction in both carbon emissions and economic costs, due to an enhanced utilization of solid waste. Additionally, it offers a more detailed analysis of the curing mechanisms in stabilized soils with respect to strength, water stability, and harmful ions.

Featured Application Incorporating phosphogypsum and sediment into subgrade material for pavement construction offers a promising solution for waste management and resource conservation while potentially enhancing the performance and cost-effectiveness of pavement systems.Abstract (1) Background: The construction industry continuously seeks sustainable alternatives to traditional materials for subgrade material in pavement construction, aiming to mitigate environmental impact while maintaining performance standards. This study investigates the feasibility of incorporating phosphogypsum (PG) and contaminated sediment into subgrade materials, focusing on their physico-chemical and physico-mechanical properties. (2) Methods: The physico-chemical and physico-mechanical properties, performance, and mechanisms of solidified sediment with phosphogypsum (3% and 5% of phosphogypsum in mixture) were studied using long-term leaching tests (ANS 16.1), uniaxial compressive strength (UCS), California Bearing Ratio (CBR), X-ray fluorescence (XRF), and thermogravimetric analysis (TGA). (3) Results: Based on the pseudo-total metal content (Cr, Ni, Cu, Zn, As, Cd, Pb), the sediment is classified as third- and fourth-class, indicating it is polluted and requires treatment before disposal in the environment. To assess the long-term behavior of the sediment treated with phosphogypsum (S/S), a semi-dynamic ANS 16.1 leaching test was performed. The results showed that the metals exhibit moderate mobility, with average diffusion coefficients (De) ranging from 10-8 cm2/s for Zn (in both mixtures) to 10-12 cm2/s for Cr (in mixture F-3). The leaching index (LX) values for both mixtures were above 9 for most metals, confirming their suitability for controlled use. Granulometric analysis indicated a predominance of fine particles, which enhances the material's plasticity and mechanical properties. Atterberg consistency tests showed that increasing phosphogypsum content improved both the Liquid Limit and Plastic Index. However, UCS tests indicated that neither the 3% nor 5% phosphogypsum mixtures met the minimum strength requirements for subgrade material. On the other hand, CBR values demonstrated promising performance, with 12.5% for the 3% phosphogypsum mixture and 22.9% for the 5% phosphogypsum mixture. Overall, phosphogypsum positively influenced the strength development of the sediment-PG mixtures, as confirmed by XRF and TGA analyses. (4) Conclusions: Environmental considerations, such as leachability of contaminants, were investigated to ensure the sustainability of the proposed subgrade materials. Leaching tests indicated minimal pollutant release, suggesting the potential for safe utilization of PG and sediment in subgrade material. This study provides valuable insights into the physico-chemical and physico-mechanical properties of pavement mixes incorporating PG and sediment, supporting the feasibility of using these alternative materials in sustainable subgrade material for pavement construction and offering a viable solution to mitigate waste generation while enhancing pavement performance.

The utilization of industrial waste as a construction material has become imperative in the era of sustainable engineering. In this context, the present study delves into the performance and behavior of two commonly discarded waste materials: phosphogypsum (PG) and red mud (RM), when employed in subgrade soil applications. PG is highly acidic, with a pH of 2.83, while RM is highly alkaline, boasting a pH of 10.12. Consequently, RM is introduced in varying proportions to neutralize the acidity of PG, and the engineering characteristics of the resulting mixture are thoroughly investigated. The study encompasses the evaluation of fundamental properties such as specific gravity and Atterberg's limits, alongside engineering properties including unconfined compressive strength (UCS) and California bearing ratio (CBR) tests. These assessments aim to determine the suitability of the PG-RM blend for use as a construction material. Additionally, the study demonstrates that the addition of RM to PG effectively mitigates its acidity through neutralization. Results from the UCS tests highlight that the highest strength is achieved with a 40% replacement of PG, after which the UCS value decreases. Furthermore, a 40% replacement of PG with RM yields a higher CBR value of 9.13%. This comprehensive study instills confidence in the use of PG blended with RM as a subgrade material in road construction projects, contributing to sustainable engineering practices.