Search Results (62)

Red mud, fly ash, ground granulated blast furnace slag, and carbide slag are industrial byproducts posing significant environmental challenges. The synthesis of geopolymers represents a promising approach for their sustainable valorization. This study investigated the strength development mechanisms and microstructural evolution of Red Mud–Class F Fly Ash-Based Geopolymer under co-incorporation of ground granulated blast furnace slag and carbide slag through compressive strength tests, X-Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), and Scanning Electron Microscopy–Energy Dispersive Spectrometer (SEM-EDS). Key findings include the following: (1) single incorporation of ground granulated blast furnace slag achieved a 60-day compressive strength of 11.6 MPa—6.4× higher than carbide slag-only systems (1.8 MPa); (2) hybrid systems (50% ground granulated blast furnace slag/50% carbide slag) reached 8.8 MPa, demonstrating a strength peak at balanced ground granulated blast furnace slag/carbide slag ratios; (3) the multi-source geopolymer systems were dominated by monomeric gels (C-A-H, C-S-H, C-A-S-H), crystalline phases (ettringite and hydrocalumite), and poly-aluminosilicate chains ((-Si-O-Al-Si-O-)n); (4) elevated Ca levels (>40 weight percent in ground granulated blast furnace slag/carbide slag) favored C-S-H formation, while optimal Si/Al ratios (1.5–2.5) promoted gel polycondensation into long-chain polymers (e.g., Si-O-Al-O), consolidating the matrix. These results resolve the critical limitation of low strength (≤3.1 MPa) in ambient-cured red mud–fly ash geopolymers reported previously, enabling scalable utilization of red mud (46.44% Fe₂O₃) and carbide slag (92.43% CaO) while advancing circular economy paradigms in construction materials. Full article

This study developed a sustainable high-strength coal gangue backfill material for underground mining applications using coal gangue, fly ash, and cement as primary raw materials, with red mud (RM) as an alternative alkali activator. The mechanical properties of the backfill material were systematically optimized by adjusting coal gangue particle size and alkali activator dosage. The optimized formulation (coal gangue/fly ash/cement = 5:4:1, 3–6 mm coal gangue particle size, 5% RM, which named BF-6-5RM) achieved superior compressive strengths of 8.23 MPa (7 days) and 10.5 MPa (28 days), significantly exceeding conventional backfill requirements and outperforming a CaO-activated reference system (coal gangue/fly ash/cement = 5:4:1, 3–6 mm coal gangue particle size, 2% CaO, which named BF-6-2CaO). Microstructural and physicochemical analyses revealed that both formulations produced calcium silicate hydrate gels (C-S-H gels) and ettringite (AFt) as key hydration products, though BF-6-5RM exhibited a denser microstructure with well-developed ettringite networks and no detectable portlandite (CH), explaining its enhanced early-age strength. Environmental assessments confirmed effective heavy metal immobilization via encapsulation, adsorption, precipitation and substitution, except for arsenic (As), which exceeded Class III groundwater thresholds (DZ/T 0290-2015) due to elevated raw material content, displaying “surface wash-off, diffusion and depletion” leaching behavior. The findings confirm that red mud-based alkali activation is a viable technology for underground backfilling, provided it is coupled with arsenic control strategies like chemical stabilization or the selection of low-arsenic raw materials. This approach not only enables the resource utilization of hazardous industrial waste but also facilitates the production of backfill materials that combine both mechanical strength and environmental compatibility, thereby delivering dual economic and ecological benefits for sustainable mining practices. Full article

In high-concentration tailings backfilling, the stability of the backfill largely depends on the slurry’s self-weighted siltation behavior. Red mud—the strongly alkaline by-product of the Bayer process and a mixture of coarse and fine particles—requires a clear understanding of its self-weight settling–consolidation mechanisms to ensure safe and efficient backfilling. In this study, red mud slurry was selected as the research object, and a multi-scale approach combining sedimentation column tests and microstructural image analysis was employed to reveal the intrinsic relationships among the sediment layer height, sedimentation rate, and pore structure changes during self-weighted siltation. The results show that the flocculated structure of red mud slurry exhibits distinct stratification during self-weighted siltation, including a clarified layer, a structural transition layer, and a dense sediment layer. During the siltation process, the sediment layer height, sedimentation rate, and floc structure of red mud evolve nonlinearly. The addition of flocculants significantly enhances the formation rate of flocculated structures but increases the porosity of the sediment body. At the macroscopic level, this results in a shortened self-weighted siltation time and increased final sediment layer height. To describe the regulatory effect of flocculants on red mud floc structure, a macro–micro predictive model for the sediment layer height was established by introducing a structural enhancement coefficient, considering the effect of flocculants. The model achieved a prediction error within 16%. These findings provide theoretical support for structural control technologies and process optimization of high-concentration fine tailings backfilling, thereby contributing to the sustainable utilization of red mud and the development of environmentally responsible backfilling practices. 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

Bayer red mud (RM)-based geopolymers are economical and ecofriendly alternatives to cement because of their superior performance. This study investigated alkali-activated cementitious materials by combining RM, fly ash (FA) and slag, and the mixtures were used to produce ecofriendly composites. The influence of the Si/Al molar ratio (3.30–3.79) on the initial properties (setting time and flowability) and hardened properties (compressive strength, drying shrinkage and water permeability) of the composite materials was studied. The Na₂O content was fixed at 4 wt%, and the thermal activation temperature was 800 °C. The phase evolution and geopolymerization mechanism of the effect of the initial Si/Al molar ratio on the material properties was investigated by FTIR, XRD, TG–DTG and SEM–EDS. The results of M1.2Si333 indicated that the compressive strength of the blends can reach 33.5 MPa at 28 days, with a drying shrinkage rate of 1.20%. Compressive strength decreases, while drying shrinkage increases with a higher initial Si/Al ratio. Microstructural analyses revealed that a low Si/Al ratio and alkali activator modulus enhance the dissolution of precursors to form C–(A)–S–H gels, which increase the compressive strength. The results promoted the application of RM-based geopolymer-engineered cementitious composite and enhanced the resource efficiency of the bauxite residue. Full article

The massive stockpiles of Bayer-process red mud (BRM) severely compromise soil integrity, necessitating the urgent development of efficient large-scale utilization strategies. BRM contains large amounts of calcium, silicon, and aluminum. Theoretically, water glass and flue gas desulfurization gypsum (FGD) can increase the active substances in BRM, making it a cementitious raw material capable of replacing cement. This study pioneers a novel activation strategy utilizing water glass–FGD synergism to amplify the BRM reactivity, enabling an increased dosage in construction materials through enhanced pozzolanic activity. They were blended into the cement at different ratios to prepare a grouting material (BF-C) for fissure sealing in mine rock strata. The hydration mechanism of BF-C was analyzed from a micro perspective by XRD, FTIR, ICP-OES, and SEM-EDS, and combined with the Ca/(Si + Al) ratio to reveal its hydration synergy. The results showed that the 3 d and 28 d strength of 70% BRM-FGD reached 8.94 MPa and 13.71 MPa, respectively. At this ratio, the hydration synergy of BF-C was the strongest. The addition of water glass and FGD can directly modulate the Ca/(Si + Al) ratio of the system to an optimal value of 0.94, which promotes the formation of early hydration products. C-S-H gel, calcite, and C(N)-A-S-H are the main hydration products of BF-C. C-S-H gels are encapsulated on cancrinite, and their three-dimensional network structures are dense. Meanwhile, C(N)-A-S-H crystals are interspersed between C-S-H gels, making the structure more stable. This achievement introduces an innovative method for the large-scale utilization of Bayer red mud, providing an effective solution in grouting technology using solid waste as raw material. Full article

The use of reduction leaching in the production of alumina from bauxite by the Bayer process in order to decrease the amount of waste (bauxite residue) by adding elemental iron or aluminum, as well as Fe²⁺ salts and organic compounds in the stage of high-pressure leaching, requires the purchase of relatively expensive reagents in large quantities. The aim of this study was to investigate the possibility of the use of electrolytically reduced bauxite residue (BR) as a substitute for these reagents. Reduced BR was obtained from Al-goethite containing BR using a bulk cathode in alkaline suspension. The degree of deoxidation of Fe³⁺ compounds was 55% after 2 h of electrolysis with a current yield of more than 73%. The addition of reduced BR according to the shrinking core model leads to a change in the limiting stage of the high-pressure boehmitic bauxite leaching from a surface chemical reaction to internal diffusion. The activation energy decreased from 32.9 to 17.2 kJ/mol by adding reduced red mud. It was also shown that the addition of reduced BR increased the rate of thickening of the slurry after leaching by a factor of 1.5 and decreased the Na₂O losses by 15% without the addition of lime. The solid residue was examined by means of X-ray diffraction analysis and scanning electron microscopy to confirm the presence of magnetite and elemental iron. A preliminary techno-economic analysis was carried out to assess the applicability of the proposed process. Full article

This study provides a comprehensive evaluation of red mud as a sustainable material for road base construction, particularly in combination with bottom ash. Red mud, a by-product of the Bayer process used in alumina extraction, is known for its high alkalinity and heavy metal content. For this reason, this waste material causes environmental challenges. Red mud sourced from the Eti Aluminum Factory in Seydişehir, Konya (Turkey), was stabilized with bottom ash. Then, these waste materials were tested through a number of experiments, such as in relation to their Atterberg limits, compaction characteristics, unconfined compressive strength (UCS), California bearing ratio (CBR), and microstructure through a scanning electron microscopy (SEM) analysis. The results highlight that the UCS of stabilized red mud samples significantly improved with the addition of bottom ash and longer curing periods. Specifically, the UCS values increased from 0.5 MPa to 2.5 MPa after 28 days of curing. Moreover, RM specimens stabilized with 25% bottom ash achieved a CBR value of 146.64% after 28 days, far exceeding Turkey’s road fill material requirement, which mandates a minimum unsoaked CBR value of 15%. These findings indicate that red mud stabilized with bottom ash not only meets but exceeds the structural requirements for road base materials. This approach provides a sustainable solution for the environmental management of red mud while contributing to infrastructure development. Through the recycling of these industrial by-products, this study presents a viable method to reduce waste and support economic and environmental sustainability in road construction projects. Full article

As the most widely used building material, cement has attracted the attention of scholars because of its large carbon emission. To alleviate the problems of carbon emission and limited resource use caused by cement production, this study focuses on the performance of mortar after carbonization curing by regulating the composition of ternary binders. Testing involved mechanical parameters, carbon shrinkage, water absorption, hydration product, microstructure, adsorption of carbon dioxide, calcium carbonate content, and carbonization degree of mortar, as well as comparisons with the effect of calcium carbide slag and sintered red mud. We carried out several studies which demonstrated that carbonization curing and adjusting the content of calcium carbide slag and sintered red mud were beneficial to improve the mechanical properties, peak load displacement, slope, elastic energy, plastic energy, carbon shrinkage, carbon dioxide adsorption, calcium carbonate content, and carbonization degree of mortar, while the addition of calcium carbide slag and sintered red mud increased the water absorption of mortar, and the greater the dosage, the greater the water absorption. Meanwhile, adding 25%–50% calcium carbide slag and sintered red mud still had negative effects on the mechanical properties of mortar. But carbonation curing and the addition of calcium carbide slag and sintered red mud could promote the hydration reaction and consume calcium hydroxide formed by hydration to form calcium carbonate. When the dosage was 50%, the carbon dioxide adsorption capacity, calcium carbonate content, and carbonization degree of calcium carbide slag mortar were higher than those of sintered red mud mortar, which increased by 29.56%, 102.73%, and 28.84%, respectively. By comparison, calcium carbide slag and sintered red mud still showed superior carbon sequestration capacity, which was higher than fly ash and Bayer red mud. From the experiment, we came to realize that adjusting the composition of cementitious materials could realize the carbon sequestration of cement-based materials and promote the road toward low-carbon sustainable development of cement. Full article

In times of increasing demand for resources, processing various waste materials is becoming more economically and ecologically viable. Red mud is a waste material that originates from the bauxite process, also known as the Bayer process. Red mud, due to its high alkalinity and heavy metal content, is often stored in landfills, which can lead to accidents such as those in Brazil or Hungary, especially if the storage takes place above ground. Red mud contains not only iron and aluminum residues but also other economically valuable metals such as manganese, titanium, cadmium, or cobalt. Currently, only 4 million tons of the annual production of 150 million tons are utilized in various industries, which is a relatively small amount. Typically, only the iron content is further processed, leaving other potential resources untapped. Chitin has a high binding capacity for various trivalent and divalent metal ions, making it a suitable material for separating red mud into its components. It has been demonstrated that chitin can effectively remove aluminum, barium, cadmium, cobalt, copper, manganese, iron, nickel, lead, strontium, and various lanthanides from a red mud-like sludge. The elements bound to chitin can be easily removed using wet chemistry. Biologically compatible substances are predominantly used in this process, with few exceptions. The removal of elements from red sludge or other mining wastewater using chitin is a viable alternative to traditional mining methods. Full article

Because of the strong alkalinity of red mud, it is difficult to recycle, and the long-term accumulation of red mud causes environmental pollution. The study shows that the solidification characteristics of bayer red mud (RM) under the action of Ca(OH)₂ (CH) are obvious. The mechanical properties of Bayer RM paste with different amounts of CH at different ages were tested. The strength of RMCH gradually increases with the increase in CH content and age, reaching a turning point in strength at 26.4% content of CH, with the highest strength at 28 days, reaching 2.73 MPa. The solidification products were characterized by XRD, FTIR, TG-DTG, and SEM-EDS. The results show that under the action of CH, the main solidification products of RM are C-(A)-S-H, hemicarboaluminate, and monocarboaluminate. In the solidification process, hydroxysodalite and faujasite-Na react with CH to generate C-S-H, Al(OH)₄⁻, and Na⁺, then react to generate hemicarboaluminate, monocarboaluminate and C-(A)-S-H, among which hemicarboaluminate is transformed into monocarboaluminate in the presence of calcite, and further monocarboaluminate decomposes to generate calcite. It provides a basis for the study of the interaction mechanism between a single substance and RM and provides a research basis for the sustainable utilization of red mud. Full article

Aluminum is produced from its primary bauxite ore through the Bayer process. Although Al is important nowadays in the development of humanity, its production leads to the generation of a huge amount of waste, called red mud. Globally, the estimation of the stock of red mud is about 4 billion tons, with about 10 million tons located in Turkey. The presence of rare-earth elements (REEs) in crucial materials such as red mud makes it a major source of these elements. A number of methods have been developed for treating red mud, which are employed globally to recover valuable products. The application of a suitable method for REE extraction from red mud is a way to overcome the supply risk, contributing to reducing the environmental issues linked to red mud pollution. The current review summarizes the research on red mud processing and examines the viability of recovering REEs from red mud sustainably, utilizing hydrometallurgy and biohydrometallurgy. Full article

The online measurement of the aluminum–silicon ratio of red mud in the dissolution stage of the Bayer alumina production process is difficult to achieve. The offline assay method has a high cost and strong time delay. Soft sensors are an effective and economical method to solve such problems. In this paper, a hybrid model (TPRF model) based on a tree-structured Parzen estimator (TPE) optimized random forest (RF) algorithm is proposed to measure the Al–Si ratio of red mud. The probability distribution of the hyperparameters of the random forest model is estimated by combining the TPE optimization algorithm with the random forest algorithm. According to this probability distribution, the hyperparameters of the random forest algorithm are adjusted in the parameter search space to obtain the best combination of hyperparameters. We established a TPRF soft sensing model based on the optimal combination of hyperparameters. The results show that the best performance of the TPRF model is a mean absolute percentage error (MAPE) of 0.0015, a root-mean-square error (RMSE) of 0.00378, a mean absolute error (MAE) of 0.00162, and a goodness of fit ($R^2$) of 0.9893. The goodness of fit improved by 93.2% compared to the linear model, 39.1% compared to the SVR model, about 21.2% compared to the GRU model, and 5.5% compared to the RF model. This level of performance is demonstrated to be better than traditional soft sensors. Full article

The basic oxygen steelmaking process is based on the CaO-FeO-SiO₂ ternary slag system, characterized by a high melting point and low lime dissolution rate, often becoming one of the key factors limiting the efficiency of the converter. The bulk solid waste red mud, produced by the Bayer alumina process and rich in Fe₂O₃/Al₂O₃/Na₂O, significantly reduces the melting point of the steelmaking slag system and enhances the efficiency of lime dissolution. This study utilized red mud as the main raw material to prepare a fluoride-free flux. An in situ online observation system was used to measure the melting point of the flux and the dissolution rate of lime in the flux. The results indicate that the melting point of the red mud-based flux is below 1200 °C, and under the same conditions, the lime dissolution rate is 10 to 15 times higher than when this flux is not used. Experiments in a 10 kg induction furnace show that using this flux, the dephosphorization rate under conditions without oxygen blowing is close to 40%, far higher than the rate achieved using CaF₂. Under oxygen-blowing conditions, the dephosphorization rate using the red mud-based flux is comparable to that of CaF₂, and significantly higher than without any flux, especially under high [C] content conditions. The data show that the red mud-based flux has the potential to be widely used as a fluoride-free flux in the steelmaking process. Full article

Bauxite residue (BR), an alkaline industrial waste, is a major byproduct of the alumina production process known as the Bayer process. The Bayer process generates a byproduct known as bauxite residue (red mud). This residue was leached with sulfuric acid in a pilot plant of Mytilineos S.A to recover scandium. Scandium was selectively recovered in pilot-scale experiments using ion exchange. This process generated a raffinate solution containing various dissolved impurities such as aluminum, sodium, calcium, iron, and mainly sulfate ions. The regeneration of the raffinate solution can reduce the cost of the process and minimize the use of H₂SO₄. The potential of raffinate recycling as a technology for reducing the usage of H₂SO₄ in the leaching process was evaluated by neutralizing bauxite residue with a raffinate solution before the leaching step. This study aimed to investigate the feasibility of using a raffinate solution for the neutralization of BR, enabling its reuse and improving the process’s environmental sustainability. The neutralization process decreases the pH value of BR pulp with 50% w/v pulp density from 11 to 6. Experimental investigations were carried out to assess the leaching behavior of bauxite residue compared to neutralized bauxite residue (NBR) using sulfuric acid. The obtained results were compared to evaluate the effectiveness of NBR as a substitute for bauxite residue in the leaching process. The consumption of acid during the leaching of neutralized BR was three times less than the BR leaching. An X-ray diffraction (XRD) analysis of BR and NBR was conducted to determine the mineralogical phases of the materials. The results of the study provide valuable insights into potential ways to reduce the cost of the BR leaching process, while also improving its environmental impact by recycling valuable materials. Full article