Search Results (245)

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. Full article

In order to study the cement–industrial waste-based synergistic curing of silt soil, orthogonal design tests were used to prepare a new curing agent using cement, fly ash, blast furnace slag, and phosphogypsum as curing materials. In order to evaluate the cement–industrial waste-cured soils, unconfined compressive strength tests, fluidity tests, wet and dry cycle tests, and electron microscope scanning tests were carried out. The mechanical properties and microstructure of the cement–industrial slag were revealed and used to analyze the curing mechanism. The results showed that, among the cement–industrial wastes, cement and blast furnace slag had a significant effect on the unconfined compressive strength of the specimens, and the optimal ratio for early strength was cement–fly ash–slag–phosphogypsum = 1:0.11:0.44:0.06; the optimal ratio for late strength was cement–fly ash–slag–phosphogypsum = 1:0.44:0.44:0.06. In the case of a 140% water content, the 28d compressive strengths of curing agent Ratios I and II were 550.3 kPa and 586.5 kPa, respectively. When a polycarboxylic acid water-reducing agent was mixed at 6.4%, the mobilities of curing agent Ratios I and II increased by 32.1% and 35.8%, and the 28d compressive strengths were 504.1 kPa and 548.8 kPa, respectively. When calcium chloride was incorporated at 1.5%, the early strength of the cured soil increased by 33% and 29.1% compared to that of the unadulterated case year on year, and the mobility was almost unchanged. From microanalysis, it was found that the cement–industrial waste produced the expansion hydration products calcium alumina (AFt) and calcium silicate (C-S-H) during the hydration process. The results of this study provide a certain basis and reference value for the use of marine soft soil as a fluid filling material. Full article

Modified phosphogypsum (MPG) is a new type of solid waste, which could show unique mechanical properties in complex stress conditions. In this study, the effects of different loading rates (0.05, 0.1, 0.5, and 1 MPa/s) on the mechanical properties and acoustic emission (AE) characteristics of modified phosphogypsum were systematically studied through uniaxial compression tests combined with AE technology. The results showed that (1) the peak strength and elastic modulus of MPG increased as a power function of the loading rate, while the peak strain gradually decreased. (2) The cumulative event count of AE decreased as a power function with an increasing loading rate. Compared to the lowest loading rate, the cumulative event count was reduced by nearly two orders of magnitude. (3) An increase in the loading rate resulted in greater large-scale macroscopic failure in MPG specimens, along with an increased proportion of low-frequency AE signals and tensile cracks. (4) The b-value of AE decreased with an increasing loading rate, suggesting that microcrack-dominated small-scale damage prevailed at low loading rates, whereas large-scale damage became more pronounced at high loading rates. The abrupt drop in the b-value served as a precursor signal for macroscopic failure. This study presents an innovative methodology combining variable loading rates with AE technology to investigate the mechanical response of MPG, and the findings reveal the influence of the loading rate on the mechanical properties and AE characteristics of MPG, providing a theoretical basis for its engineering application under different loading environments. Full article

Phosphogypsum leachate significantly accelerates carbonate rock dissolution in karst regions. The dissolution mechanism of phosphogypsum leachate associated with carbonate rock interaction and the corresponding numerical simulation need further study. In this study, 3D digital core imaging was used to scan undisturbed carbonate rock specimens from phosphogypsum landfill sites, and corresponding 3D structural models were constructed. We carried out indoor dissolution experiments in which we used Scanning Electron Microscopy as well as Energy Dispersive Spectrometer to observe changes in the surface micromorphology and elemental content of the rock specimens under different dissolution conditions. A reactive numerical model was developed based on the 3D structural model obtained from 3D digital core imaging, and numerical simulation studies were conducted. The dissolution reaction between phosphogypsum leachate and carbonate rocks exhibited an initial rapid phase followed by gradual stabilization. The pH of the leachate showed an exponential negative correlation with the dissolution amount per unit area of the rock specimens, while a power-law negative correlation was observed between pH and chemical dissolution rates. The numerical model effectively reproduced the reactant concentration states observed in experiments, confirming its capability to simulate reaction processes within rock specimens. Simulation results demonstrated that preferential flow through fracture channels led to higher reactant concentrations near fractures due to incomplete reactions, whereas lower concentrations occurred in sub-fracture regions. As the fracture aperture increased, the concentration disparity between these regions became more pronounced, with higher concentration of reactants at the outlet. Full article

Waste from livestock farms contains various pathogens, including eggs and larvae of helminths—pathogens of parasitic diseases harmful to animals and humans. One of the methods for their effective processing to obtain biofertilizer and biofuel is anaerobic digestion, which requires further improvement to completely suppress the viability of pathogenic microorganisms in mesophilic conditions. To this end, the use of anaerobic digestion under sulfate reduction conditions to suppress pathogens using biogenic hydrogen sulfide is promising. Consequently, this study aims to study the effect of a sulfur-containing additive such as phosphogypsum on the disinfection of pig manure during anaerobic digestion. Egg mortality was already found to increase significantly compared to the control (80% and more), even at a minimum concentration of phosphogypsum (5%), on the fifth day of the experiment. At the same time, the maximum effect (100% mortality of Trichuris suis eggs) was recorded at a 10% concentration of phosphogypsum, starting from the 10th day of the study. Our experiment showed that changes in anaerobic digestion conditions using phosphogypsum could positively affect digestate disinfection. However, further research is needed to optimize the conditions of the process for an effective combination of disinfection with the production of environmentally safe organic fertilizers and high-quality biogas with a high level of methane. Full article

To address the high carbon emissions and resource dependency associated with conventional ordinary Portland cement (OPC) production, this study systematically investigated the preparation processes, hydration mechanisms, and chemical properties of high-belite calcium sulfoaluminate (HBCSA) and calcium sulfoaluminate (CSA) cements based from industrial solid wastes. The results demonstrate that substituting natural raw materials (e.g., limestone and gypsum) with industrial solid wastes—including fly ash, phosphogypsum, steel slag, and red mud—not only reduces raw material costs but also mitigates land occupation and pollution caused by waste accumulation. Under optimized calcination regimes, clinkers containing key mineral phases (C₄A₃S⁻ and C₂S) were successfully synthesized. Hydration products, such as ettringite (AFt), aluminum hydroxide (AH₃), and C-S-H gel, were identified, where AFt crystals form a three-dimensional framework through disordered growth, whereas AH₃ and C-S-H fill the matrix to create a dense interfacial transition zone (ITZ), thereby increasing the mechanical strength. The incorporation of steel slag and granulated blast furnace slag was found to increase the setting time, with low reactivity contributing to reduced strength development in the hardened paste. In contrast, Solid-waste gypsum did not significantly differ from natural gypsum in stabilizing ettringite (AFt). Furthermore, this study clarified key roles of components in HBCSA/CSA systems; Fe₂O₃ serves as a flux but substitutes some Al₂O₃, reducing C₄A₃S⁻ content. CaSO₄ retards hydration while stabilizing strength via sustained AFt formation. CaCO₃ provides nucleation sites and CaO but risks AFt expansion, degrading strength. These insights enable optimized clinker designs balancing reactivity, stability, and strength. Full article

This study presents the geotechnical evaluation of phosphogypsum, a byproduct of phosphate fertilizer production. The objective is to assess the suitability of phosphogypsum or its mixtures with natural materials as a technically viable and environmentally responsible backfill material for the restoration of closed and abandoned quarries. This study adds to the scarce existing literature on the use of phosphogypsum for quarry reclamation and further investigates the behavior of phosphogypsum mixtures incorporating clay and marble dust. A comprehensive experimental program was conducted to evaluate typical geotechnical properties, i.e., grain size distribution, Atterberg limits, compaction characteristics, permeability, compressibility, and shear strength. The results indicate that phosphogypsum is fine grained, low in plasticity, and exhibits relatively high permeability and compressibility, which limits its application as a deep fill material. The addition of clay increased the liquid and plastic limits but had a limited positive effect on strength and compressibility. In contrast, mixtures with marble dust improved particle gradation, reduced permeability, and enhanced compaction behavior without significantly increasing plasticity or settlements. Notably, the most promising mixture of phosphogypsum with a modest proportion of marble dust demonstrates improved shear strength and reduced hydraulic conductivity, making it suitable for use in the upper layers of quarry fills. Full article

Utilizing supplementary cementitious materials is an effective way to fabricate low-carbon cement-based materials. In this paper, the composite mortars with good properties were prepared by mixing them with basic oxygen furnace slag (BOFS), Bayer red mud (BRM), and phosphogypsum (PG). The influences of the replacement amounts of BRM and PG on the mechanical properties, hydration characteristic, chloride corrosion resistance, and microstructure of the materials were investigated. The results showed that simply adding 10 wt% BRM slightly modified the properties of the composite mortars. With the increase in PG, the mechanical strength and corrosion resistance coefficient K_C of the mortars first increased and then decreased, in contrast to the chloride migration coefficient D_RCM and electric flux Q. Among the samples, sample S3, with 6 wt% BRM and 4 wt% PG, had the best properties, a flexural strength of 6.6 MPa, and a compressive strength of 43.5 MPa at a curing age of 28 d. And the values of D_RCM and Q of the sample, respectively, decreased by 44.06% and 22.83% compared with the control sample, along with the value of K_C corroded after 120 d increasing by 16.33%. The microstructure analysis indicated that the alkali activation of BRM promoted the generation of lamellar portlandite and reticular and granular C-S-H gel. The free aluminum in BRM could dissolve into C-S-H gel to induce the generation of C-A-S-H gel. Furthermore, the generated amount of ettringite increased by adding PG. The aforementioned improvement in mechanical properties is primarily attributed to BRM promoting the hydration of the composite mortars and inducing the transformation of the C-S-H gel into C-A-S-H gel, and PG promoting the generation of ettringite. Moreover, the filling effects of BRM and PG decreased the porosity and number of harmful pores. It increased the compactness of the microstructure to endow the composite mortars with excellent chloride corrosion resistance. Full article

Red mud (RM), a highly alkaline waste from alumina production, poses severe environmental threats due to massive stockpiling (>350 million tons in China) and groundwater contamination. This study evaluates three scalable strategies to repurpose RM: iron recovery via magnetic separation, sintered brick production using RM–fly ash–granulated blast furnace slag (6:1:3 ratio), and non-calcined cementitious binders combining RM and phosphogypsum (PG). Industrial-scale iron extraction achieved 23.85% recovery of iron concentrate (58% Fe₂O₃ grade) and consumed 3.6 million tons/year of RM, generating CNY 31 million annual profit. Sintered bricks exhibited 10–15 MPa compressive strength, meeting ASTM C62-23 standard while reducing material costs by 30%. The RM–PG binder achieved 40 MPa compressive strength at 28 days without cement or calcination, leveraging RM’s alkalinity (21.95% Na₂O) and PG’s sulfate activation. Collectively, these approaches reduced landfill reliance by 50% and CO₂ emissions by 35%–40% compared to conventional practices. The results demonstrate RM’s potential as a secondary resource, offering economically viable and environmentally sustainable pathways for the alumina industry. Full article

The accumulation of silty soils and industrial solid waste not only results in a significant waste of land resources but also causes environmental pollution. Phosphogypsum and cement are commonly utilized as binding agents for the solidification of silt in engineering applications. However, the use of PG and cement alone may lead to issues such as insufficient strength, crack formation, and poor durability. Therefore, this research considered and employed a two-phase stabilization method using phosphogypsum and cement to solidify silt. Additionally, to further enhance the durability of the stabilized silt, polypropylene fiber (PP) and sodium sulfate (Na₂SO₄, NS) were incorporated. The effects of two-phase phosphogypsum and the proportion of hemihydrate phosphogypsum (BHPG) in the two-phase phosphogypsum on the strength characteristics of the stabilized silt were investigated through unconfined compressive strength tests and durability tests. The results show that when the content of two-phase phosphogypsum is 5%, and the proportion of BHPG in the two-phase phosphogypsum is 20%, the 28-day unconfined compressive strength of the stabilized silt reaches 1.42 MPa, and the deformation modulus is 95.5 MPa. After incorporating sodium sulfate (NS), the water and frost resistance of the stabilized silt significantly improved. The microstructural analysis shows that NS promotes the formation of ettringite. Furthermore, an excessively high proportion of hemihydrate phosphogypsum (BHPG) in the two-phase phosphogypsum content can lead to dihydrate phosphogypsum (2HPG) not being encapsulated by hydration products, which results in a less dense structure of the solidified silt and a decline in performance. Full article

The article investigates the characteristics of the phase composition and structure of supersulfated cement (SSC) during hardening using X-ray, electron microscopy, and ultrasonic analysis methods. The influence of different types of activators, hardening accelerators, and superplasticizers on the type and morphology of the newly formed phases during SSC hardening was studied. The effect of a polycarboxylate-type superplasticizer and calcium chloride on the standard consistency and setting times of SSC was experimentally determined. It was established that the introduction of the superplasticizer reduces the standard consistency by 10–16%. Experimental data showed higher effectiveness of phosphogypsum as a sulfate activator compared to gypsum stone. The strength increase of SSC at 7 days reached up to 35%, and at 28 days, up to 15%. Based on the kinetics of ultrasonic wave propagation during SSC hardening, the main stages of structure formation and the influence of cement composition on these stages were determined. The experimental results demonstrate the effect of SSC composition on its standard consistency, setting time, and mechanical properties. The impact of the type of activator and admixtures on the change in SSC strength during storage was investigated. It was found that the addition of a polycarboxylate-type superplasticizer significantly reduces the strength loss of SSC during long-term storage. Using mathematical modeling, experimentally obtained statistical models of strength were developed, which allow for the quantitative evaluation of individual and combined effects, as well as the determination of optimal SSC compositions. Full article

Phosphogypsum, a byproduct of phosphate fertilizer production, poses significant environmental challenges due to its large volume, hazardous composition, and radioactivity. Conventional disposal methods, such as stockpiling and landfilling, contribute to soil and water contamination and present risks to human health. This article explores the potential of integrating phosphogypsum into a circular economy framework, focusing on reducing environmental impacts and extracting value from this industrial waste. A detailed assessment of phosphogypsum’s chemical composition, including trace metals and radionuclides, is essential for effective management. This review paper examines safe handling, storage, and disposal strategies to minimize environmental risks. Additionally, innovative reuse applications are investigated, such as incorporating phosphogypsum into construction materials like cement, plasterboard, and concrete and its use in agriculture as a soil amendment or for land reclamation. The recovery of critical elements, particularly rare earth elements (REEs), highlights its potential to reduce waste and contribute to meeting the growing demand for strategic resources. Despite its promise, challenges remain, including chemical variability and the presence of radioactive components. This article identifies the technological and regulatory steps required to enable safe, large-scale reuse of phosphogypsum, emphasizing its role in advancing sustainable resource management within a circular economy. Full article

River sludge usually contains a high content of organic matter, leading to its low strength or difficult solidification in its solidification/stabilization (S/S) treatment projects. This study selected river sludge with medium and high content of organic matter for the S/S treatment using modified curing agent (GCP) and cement (P.O). Effects of humus and curing agent on the S/S process of river sludge were investigated via analyzing physical properties, changes in organic matter, microstructure, and mineral compositions of the solidified sludge. The results showed that the increase rate of compressive strength of the solidified sludge was influenced by the content of organic matter and composition of the curing agent. The presence of humus inhibited the hydration reaction and reduced the increase rate of compressive strength of solidified sludge. Slag and phosphogypsum in GCP promoted the hydration reaction, significantly enhancing the compressive strength of the solidified sludge to 2242.24 KPa. The water content of the solidified sludge was influenced by the environmental conditions and curing agent, which could reflect the level of hydration reaction in the solidified sludge. The pH of the solidified sludge was directly affected by the humus in the sludge, with a decreasing trend during the S/S process. Decomposition of the humus in the sludge released H⁺, which reacted with OH⁻ produced by the hydration reaction via neutralization reaction. The pH of the solidified sludge was lowered, and the hydration reaction was inhibited, hindering the decrease in the water content of the solidified sludge. Therefore, the hydration reaction has an antagonistic effect on the decomposition of the humus. Microstructure analysis (SEM) confirmed that GCP could effectively solidify the organic-rich river sludge. This study provides a theoretical basis for the S/S treatment of organic-rich river sludge. Full article

This study examines the potential of phosphogypsum—a by-product of the phosphoric acid production process—as a low-cost and sustainable adsorbent for the removal of crystal violet dye from aqueous solutions. Phosphogypsum was characterized using X-ray fluorescence, X-ray diffraction, particle size distribution, and zeta potential measurements, revealing that it is primarily composed of di-hydrate calcium sulfate, with a negatively charged surface in the pH range from 1.8 to 8.2 and a mean particle size of 12.2 microns. Experiments were conducted to evaluate the effects of pH, adsorbent dose, contact time, and temperature on its adsorption ability. The results indicated that the adsorption capacity increased with the pH up to a value of 5, while higher initial dye concentrations enhanced the uptake capacity but reduced the removal efficiency. The adsorption process was well described by the Langmuir isotherm, suggesting chemisorption as the dominant mechanism, while the pseudo-second-order kinetic model indicated that adsorption primarily occurred on the exterior surface. The thermodynamic analysis revealed that the process was exothermic and spontaneous at 20 °C and 30 °C, with a decrease in favorability at higher temperatures. The adsorbent demonstrated reusability, with a removal efficiency of 71% after five regeneration cycles. Furthermore, phosphogypsum was successfully applied to treat real textile effluent, achieving significant reductions in both biochemical oxygen demand (71%) and dye content (87%). These findings highlight the potential of phosphogypsum as an effective and eco-friendly adsorbent for wastewater treatment, contributing to waste valorization and environmental sustainability. Full article

Dispersive soils are highly erodible and prone to segregation in water, posing significant risks to the soil and structural stability. Traditional stabilization methods using cement and lime are effective but raise sustainability concerns due to the high carbon emissions. This study explored the utilization of phosphogypsum (PG), a by-product of the fertilizer industry, as a sustainable alternative to improve dispersive soils. PG was evaluated both individually and in combination with lime, focusing on its effects on the plasticity, swell, consolidation, compaction, and unconfined compressive strength (UCS) characteristics. Soil samples were treated with varying proportions of lime (2–10%) and PG (2–10%). The results demonstrated that combining 4% lime with 8% PG significantly enhanced the properties of dispersive soil, reducing the swell pressure from 115 kN/m² to 72 kN/m² and the swell potential by 67%. The UCS increased by 320% after 7 days of curing, while the coefficient of consolidation improved 2.74 times and the compression index decreased by a factor of 8.55. Regression analysis was conducted and validated for UCS prediction. Utilizing PG not only improves the soil stability, but also offers a sustainable solution by recycling industrial waste and reducing the dependence on conventional materials. These findings underscore the potential of PG as an eco-friendly soil stabilizer for dispersive soils. Full article