Red mud valorization in stoneware pastes: Technical and environmental assessment

Agrawal V., Paulose R., Arya R., Rajak G., Giri A., Bijanu A., Sanghi S.K., Mishra D., N P., Khare A.K., Parmar V., Khan M.A., Bhisikar A., Srivastava A.K., Thankaraj Salammal S.

Red mud is a solid hazardous alumina industrial waste, which is rich in iron, titanium, aluminum, silicon, calcium, etc. The red mud contains 30-60% of hematite, which is suitable for shielding high energy X- and gamma rays. So, the iron rich red mud was converted into diagnostic X-ray shielding tiles through ceramic route by adding a certain weight percentage of BaSO4 and binders (kaolin clay or sodium hexametaphosphate) with it. The kaolin clay tile possess sufficient impact strength (failure point is 852 mm for 19 mm steel ball) and flexural strength of ~25 N/mm2, which is suitable for wall applications. The 10.3 mm and 14.7 mm thick red mud:BaSO4:kaolin clay tile possess the attenuation equivalent to 2 mm and 2.3 mm lead at 125 kVp and 140 kVp, respectively. No heavy elements were found to leach out except chromium and arsenic from the sintered tiles. However, the leaching of Cr (0.6 ppm) and As (0.015 ppm) was found to be well below the permissible limit. These tiles can be used in the X-ray diagnosis, CT scanner, bone densitometry, and cath labs instead of toxic lead sheet and thereby to protect the operating personnel, public, and environment from radiation hazards.

Mudgal M., Singh A., Chouhan R.K., Acharya A., Srivastava A.K.

World produces annually multimillion of tones of fly ash (FA) and red mud (RM) as industrial wastes. There is a dire need to develop a process that can potentially use these wastes as large stockpiling of these materials can lead a potential hazard to the common people. The FA is rich in silica and alumina, two major ingredients of the aluminosilicate network containing inorganic polymer called geopolymer. The present work demonstrates the geopolymer prepared from fly ash and red mud (FA-RM-geopolymer) with significant high strength of 65 MPa under optimized experimental conditions. Geopolymers were synthesized from the raw material under ambient condition of 25 °C with 28 day of curing. It was observed that after 28 days of curing maximum compressive strength of 65 MPa was achieved in presence of 10 wt percent of red mud. While fly ash based geopolymer (FA-geopolymer) showed 53 MPa compressive strength under similar experimental conditions. X-ray photoelectron spectroscopy (XPS), X-ray diffraction spectroscopy (XRD), particle size analysis and FE-SEM were used to study the structural properties of raw material and the geopolymer. XPS studies revealed that the iron of the RM does not take part in the geopolymerization, as no change was observed for the Fe2p 3/2 peak of the RM and FA-RM-geopolymer. However significant change was observed for the Al2p 3/2 XPS peak. Further it was observed that the presence of RM resulted in increases Si/Al and Na/Al molar ratio compared to FA-geopolymer. SEM studies confirmed the formation of high dense structure for the FA-RM-geopolymer suggesting RM provides aluminum to participate in the geopolymerization and simultaneously acts as filler thereby leads to enhanced and active geopolymerization process on blending of fly ash with red mud. • Fly ash with red mud geopolymer has been studied by XPS, XRD and SEM. • Presence of 10 wt % red mud results in high compressive strength. • XPS studies revealed increase in Si/Al and Na/Al molar ratio in presence of red mud. • Red mud leads to enhanced geopolymerization process.

Özcan M., Birol B., Kaya F.

Titanium dioxide is one of the best semiconducting photocatalysts available for photocatalytic cleaning applications. Especially nano-sized TiO 2 particles deposited on porous substrates can be utilized as a filter for solid and liquid media. On the other hand, red mud and thermal plant fly ash are hazardous wastes that are produced in large quantities. Recycling/reuse of these waste material in a porous ceramic production would be beneficial both for environmental and economical issues. In the present study, a porous substrate was produced from red mud and fly ash with varying ratios and additives of H 3 BO 3 , CaCO 3 , and MgCO 3 for lowering the melting temperature and porosity formation. Sintered ceramics were then coated with nano-sized TiO 2 particles by the sol-gel method. Ultrasonic dispersion of nano-sized TiO 2 nanoparticles was also utilized as an alternative method for impregnation of nanoparticles into the porous structure of the ceramic substrate. Finally, photocatalytic activities and degradation of methylene blue (MB) under UV radiation of substrates were investigated. According to the SEM investigations, the sol-gel method was observed to be a better way of nanoparticle deposition because deposited particles are homogenous throughout the ceramic body. Also, this method provides lower particle sizes than the ones that were deposited by the ultrasonic dispersion method. This results in higher surface area and better photocatalytic activities.

Agrawal S., Dhawan N.

The recovery of valuable metals present in the red mud is investigated through the microwave acid baking–water leaching process. The titanium and scandium dissolution during the leaching stage were optimized using the statistical design of experiments by varying microwave power, exposure time, and acid ratio. Microwave power is the governing factor for heating the mixture during the baking process. The acid ratio and exposure time are influential parameters in the investigated range for metal dissolution. The microwave irradiation synergizes the baking process by generating submicron cracks in the iron(III) sulfate and titanium oxysulfate particles. The SEM-EDS analysis of the baked products revealed that the aluminum(III) sulfate particles have fine morphology, leading to higher aluminum dissolution irrespective of the baking conditions. The linear correlation between the scandium and titanium dissolution suggests that the scandium values are majorly associated with the anatase phase. The microcracks generation in the iron and titanium sulfate particles improves the penetration of water molecules and enhances the scandium dissolution. The optimal baking conditions (600 W, 6 min, 1 mL/g) yielded titanium dissolution of 73.3% and scandium dissolution of 88.4%. The leach residue contains unreacted hematite, anatase, and quartz phases majorly. The material balance indicates that acid baking of 1 kg red mud containing 0.094 kg titanium and 60 g t −1 scandium yielded 0.27 kg residue containing 0.12 kg iron, 0.026 kg titanium, and

Zhou J., Ma S., Chen Y., Ning S., Wei Y., Fujita T.

Recovery of scandium (Sc) from various wastes is generally one strategy to solve potential supply shortage problems of Sc. This work proposes a synergistic process for co-recovery of Sc by using the acid titanium white waste acid (TWWA) to leach the basic red mud, to which the acidity was adjusted with concentrated sulfuric acid when necessary. First, the effect of adjusted acidity of TWWA on Sc leaching from red mud was studied when the acidity of TWWA was adjusted to 5 mol/L H 2 SO 4 , which inhibited the formation of silica gel in the leachate with a Sc recovery of 70%. Then, the volume fraction of 10% P204 (di-(2-ethylhexyl) phosphate)-5% TBP (tributyl phosphate)-85% sulfonated kerosene was used to extract Sc from the leachate under the most suitable conditions. Specifically, at an A/O (volumetric liquid phase over oil phase) ratio of 10 for 30 min at room temperature, over 99% Sc was extracted and almost no Fe and Al were co-extracted. The selective scrubbing of 99% Ti was achieved via 5 mol/L H 2 SO 4 –1% H 2 O 2 at 60 °C with almost no scrubbing towards Sc. A solution of 3 mol/L NaOH was then used as the stripping agent, which resulted in solid Sc enrichment with a high stripping efficiency of 99%. The process generated a final Sc-enrichment product with a purity of 17.4% and a total recovery yield of 68.6%. • A novel process for co-recovery of Sc was proposed by using the acid titanium white waste acid to leach the basic red mud. • The formation of silica gel was inhibited in the Sc leaching step by adjusting the acidity. • 10% P204–5% TBP - 85% sulfonated kerosene was selected to recover Sc with an extraction efficiency over 99%. • 3 mol/L NaOH at 60 °C can effectively strip Sc with a stripping efficiency of about 99%. • Scandium enrichment with a purity of 17.4% and a recovery yield of 68.6% was achieved.

Berta K.M., Kurdi R., Lukács P., Penk M., Somogyi V.

Despite efforts to utilise bauxite residue, the amount of red mud stored in reservoirs is increasing. This paper aims to evaluate the potential of red mud and other sludge waste types as a soil substitute by monitoring plant development. Pot experiments were carried out testing two types of mixtures: dredging sludge from Lake Balaton mixed with garden soil and the sewage sludge and soil blend. These were then treated with red mud (15 and 30% w/w). The plants were under-, while the roots were more developed in the sewage sludge mix than the dredging sludge blend and the control soil. In the sewage amendment, the phosphorous content increased while the calcium content was lower than in the other soil types and the optimum. The metals uptake of the plants was a factor of the red mud quantity. Lead, nickel, titanium and silicon had elevated concentrations parallel to higher red mud content, but only the nickel exceeded the threshold of the Hungarian legislation. Silicon and titanium were beneficial for plant growth, compensating for the potentially toxic effects of lead and nickel. Results suggest that the red mud in a mixture with either sewage sludge or dredging sludge can act as catalysts for the growth rate of test plants, allowing their utilisation as secondary raw materials. • Easy-to-use homogenisation of red mud with waste sludge as soil substitute media. • Red mud treatment of 15% in dredging sludge amended soil supported maximum growth. • Sewage sludge addition resulted in increased root density and plant coverage. • Metal contents on Sinapis alba were within the phytotoxic thresholds. • Red mud with waste sludge is a promising soil substitute medium for reclamation.

Vilarinho I.S., Lopes A.L., Carneiro J., Pinto C., Labrincha J.A., Seabra M.P.

Mill scale (MS) is a iron-rich waste generated in the wire drawing process with high iron content and is still mainly disposed in landfills. The scientific community has been studied its use in other applications such as pigments, concretes, among others. This work aims to study a new added-value application for MS—the development of coloured ceramic pastes. For this purpose, the influence of the added amount (0, 1, 3, 5 and 10 wt.%), the pre-treatment (milling + sieving at 212 μμm), the maximum firing temperature (from 1043 to 1165 °C) and the type of furnace (laboratory/industrial) were analysed on the sample’s characteristics. A dark grey stoneware product was obtained through the incorporation of 10 wt.% of MS and leaching tests conducted at pilot scale with cups confirmed its immobilization in the ceramic matrix. Furthermore, it was proved that the firing temperature can be reduced by about 100 °C without affecting the specimen’s characteristics. This reduction leads to a considerable decrease in the energy consumption upon firing, inducing economic and environmental advantages. Therefore, this work provides a new added-value application for MS and contributes to the reduction of virgin raw materials consumption and development of more sustainable stoneware products.

Agrawal S., Dhawan N.

Red mud is an inevitable and under-utilized byproduct of the aluminium industry. The current study evaluates the effect of mechanical and thermal activation on the acid leaching of red mud. The acid leaching of the pretreated product is followed by acid-base treatment to precipitate the dissolved aluminium values from solution and enrich the residue with Fe and Ti values. Mild acid leaching yielded ~76% aluminium dissolution with 81% silicon removal. The direct acid leaching resulted in dissociation of sodium aluminosilicate; meanwhile, gibbsite phase remains in the residue. The mechanical milling for 1 h enhanced the aluminium dissolution to 89.5% with silicon and iron dissolution of 98.5 and 9.6%, respectively. The silica and iron oxide values were separated before HCl leaching by thermal pretreatment step using microwave heating with NaOH and carbon additives respectively. The sodium silicate and sodium aluminate phases formed during alkali thermal treatment resulted in ~33% silicon and ~ 27% aluminium removal by water washing. Meanwhile, HCl leaching of the water wash residue resulted in 82% aluminium dissolution with 70% recovery and residue containing ~52% Fe2O3, and 28% TiO2 is obtained. The carbothermal reduction yielded iron-rich magnetic concentrate having 48% iron grade with 20% metallic iron, and ~ 83% iron recovery. Leaching of the non-magnetic fraction containing hercynite and fayalite as the major aluminium and silicon bearing phases resulted in 81% aluminium dissolution with overall aluminium recovery of only 37%. Mullite formation is also observed in the precipitate with a high Al/Si molar ratio. The carbothermal route was found as an energy-efficient route, whereas the mechanical milling route yielded maximum Al extraction of ~90% with Fe and Ti content of 41.1% and 16.8% in the residue.

Li X., Zhang Q., Mao S.

• Propose a procedure of RM modifying aggregate to improve mechanical properties significantly. • Compressive and flexural strength of concrete prepared with AMBRM increased by over 10.00 %. • Wettability theory used to reveal formation mechanism of ITZ microscopic structure. • Harmful micropores with >100 nm in the concrete prepared with AMBRM were significantly fewer. In this study, a method is proposed for modifying aggregate with Bayer red mud (RM), and the bond strength and microstructure of the interfacial transition zone (ITZ) in the concrete prepared using the modified aggregate is determined. Compared to concrete prepared using natural basalt aggregate, concrete prepared with RM-modified basalt aggregate aged for 7 and 28 days had a 25.08 % and 21.75 %, respectively, higher compressive strength and a 39.53 % and 15.30 %, respectively, flexural strength. Compared to concrete prepared using natural limestone aggregate, the compressive and flexural strengths of concrete prepared with RM-modified limestone aggregate increased by over 10.00 % and 20 % respectively, after aging of both 7 and 28 days. The RM had a higher wettability to cement paste than basalt and limestone, implying that cement paste on the surface of RM-modified aggregate had a correspondingly stronger microflow and filling capacity. In addition, concrete prepared with the RM-modified aggregate had a low voidage, a compact ITZ structure and strong interfacial adhesion, resulting in considerably enhanced mechanical properties. This study provides novel applications for RM that can be widely used in building materials and waste reduction and a new method for improving the mechanical properties of concrete.

Liang X., Ji Y.

Red mud (RM) is an industrial solid waste with high alkalinity. The preparation of RM as a geopolymer for construction is an effective means of recovery and treatment of RM. However, the durability of the geopolymer containing red mud is an important reason for restricting its promotion. In this paper, chloride ion permeability of geopolymer mortar and cement mortar was measured by two types of electric flux methods. Besides, corrosion potential and corrosion current of steel bars in geopolymer mortar and cement mortar conditioned in artificial climate and natural environment were studied, respectively. The results showed that the chloride ion resistance of the geopolymer was better than that of the cement mortar. The improved ASTM C1202 method was suitable for the evaluation of chloride ion permeability of geopolymer mortar since it could effectively solve the problem of long immersion time. The ASTM C876 Standard was not suitable for judging the corrosion state of steel bars in geopolymer mortar since the corrosion current method can better reflect the corrosion of steel bars in geopolymer than the corrosion potential method. The critical content of chloride ion in the geopolymer mortar (0.5 wt%) that causes the steel to passivate is significantly higher than that of cement mortar (0.3 wt%). Geopolymer mortar exhibited a stronger ability to protect steel bars from corrosion than cement mortar due to its density and resistance to ion penetration.

Zhang D., Chen H., Nie Z., Xia J., Li E., Fan X., Zheng L.

Red mud (RM) is a typical mass quantity of solid waste generated from industrial alumina production and usually consists of valuable metals such as Al and rare earths (REs). This work mainly investigated the extraction of Al and REs from RM via aerobic and anaerobic bi-stage bioleaching by Acidianus manzaensis with the addition of pyrite, by monitoring the morphology, phase and speciation transformations of Al, Si, S and Fe using primarily SEM-EDS, XRD, XPS, and XANES. The results showed that during the aerobic bioleaching stage with increase in mass ratios of pyrite/RM, the extraction rates of Al and REs increased; the highest rates for Al (85.1%), Ce (82.4%), Gd (86.8%), Y (85.3%) and Sc (78.6%) were achieved at pyrite/RM mass ratio 2:1 after 22 d of bioleaching. A slight decrease then occurred because that the complexes formed from jarosites, hematite, diaspore and SiO2-gel can adsorb small amounts of the dissolved elements. Under anaerobic conditions, the additive S0 gradually oxidized to sulfate, and the jarosites gradually dissolved into Fe2+ and disappeared at day 10 with maximal bioleaching rate for Al, Ce, Gd, Y and Sc as 52.5%, 86.3%, 93.7%, 90.2% and 74.9%, indicating that the residues from the first stage were dissolved with the dissimilatory reduction of jarosites by A. manzaensis using S0 as an electron donor. These results suggested a new method for the efficient recovery of valuable metals from RM.

Wang F., Pan H., Xu J.

In this study, by products such as red mud, phosphorus gypsum and fly ash were used as binders, and are compared with Portland cement (PC) in immobilizing Cu, Pb and Zn. Cu, Pb and Zn -doped pastes and mortars were prepared with a metal to binder ratio at 1%. Samples were cured for 7d, 14d and 28d. The unconfined compressive strength (UCS) test, a batch leaching test along with scanning electron microscopy (SEM) and thermogravimetric analysis (TGA) were applied for the testing of Cu, Pb and Zn-doped pastes and mortars. The UCS results show that red mud-phosphorus gypsum treated samples produce higher strength than these treated by red mud-fly ash, or PC. The results of leaching test revealed that the immobilization degree of heavy metals from these pastes depends on the leachate pH of these pastes. With the aid of the cement, red mud-phosphorus gypsum-cement pastes leached less metals compared to that of red mud-phosphorus gypsum pastes. The leachate concentrations of Cu, Pb and Zn from red mud-phosphorus gypsum-cement pastes are 1.5 mg/L, 1 mg/L, and 3 mg/L respectively. They are able to meet the China Ministry of Environment Protection (MEP) regulatory limit. With the increase of the curing time, the unconfined compressive strength and the leaching concentrations of these pastes showed a slightly increasing trend. In addition, SEM and TGA analyses show that the major hydration product is ettringite.

Oprčkal P., Mladenovič A., Zupančič N., Ščančar J., Milačič R., Zalar Serjun V.

Remediation of contaminated soil can be performed by using various techniques, which must be adequately tailored for each specific case. The aim of this research is to critically evaluate the potential use of red mud and paper ash and a combination of the two as immobilization additives for the remediation of contaminated soil from one of the most polluted sites in Slovenia. The proposed procedure involves the preparation of geotechnical composites made from contaminated soil and mixed with 25 wt% of immobilization additives and an optimal quantity of water to achieve consistency, at which maximum compaction according to the Proctor Compaction Test procedure can be achieved. The results reveal a positive, time-dependent trend for the immobilization of potentially toxic elements in the composite with paper ash, because of the formation of the new hydration products with potentially toxic elements. In a composite containing only red mud, potentially toxic elements were immobilized by sorption mechanisms with no general time-dependent trends. The composite with a combination of additives demonstrates the remediation characteristics of both red mud and paper ash. Using this approach excavated contaminated soil, red mud and paper ash can be successfully recycled in the proposed composites, which can be beneficially used in situ for rehabilitation of contaminated sites. Nevertheless, mobilization of some potentially toxic elements at high pHs may represent a limiting factor and has to be taken into the consideration when a combination of red mud and paper ash is used as immobilization additive. • Sustainable remediation of contaminated soil by an immobilization method. • Characterization of red mud and paper ash as an immobilization additives. • Preparation of chemically inert geotechnical composites for fill or embankments. • Environmental assessment and mechano-physical characteristics of composites. • Description of the main immobilization mechanisms in the obtained composites.

Wang W., Sun K., Liu H.

Ceramic floor tiles with various surface morphologies of mullite were prepared from red mud and kaolin, α-Al2O3, ρ-Al2O3, γ-Al2O3 and Al(OH)3 as alumina sources. AlF3 and V2O5 were used as catalyst to promote the formation of mullite crystals at low temperature. Influence of Al2O3 crystal types and sintering temperature on microstructure and development of mullite, and performance of ceramic floor tiles were investigated. Results indicate that both the abilities of α-Al2O3, ρ-Al2O3, γ-Al2O3 and Al(OH)3 reacting with SiO2 and the reactivities of them increase successively, therefore the amount of generated needle-like mullite rise in that order. The better dispersion of acicular mullite throughout the ceramic matrix could significantly improve the properties of the ceramics. The ceramic floor tile with Al(OH)3 sintered at 1230 ℃ was found to possess the optimal performance. Under this condition, bulk density, flexural strength, porosity and water absorption of the ceramic floor tile were 1.83 g/cm3, 185.46 MPa, 18.56% and 7.37% respectively, meeting the requirements of building material. The results of this study will improve our understanding of recycling red mud for the fabrication of ceramic materials.

Liu S., Zeng J.

Colored coatings with thermally activated red mud (RM) as an electrolyte additive were applied to 5005 aluminum alloy using the plasma-electrolytic-oxidation (PEO) technique. Effects of the RM on the PEO coating composition, structure, and properties were investigated. The pure PEO coating was white and consisted of γ-Al2O3 and α-Al2O3. The RM PEO coatings were colored, and consisted of α-Al2O3 and γ-Al2O3, plus CaTiO3, CaCO3, SiO2, Fe2O3, and mCaO·nAl2O3·xFe2O3·ySiO2, which were formed at high temperatures. Surface porosities and hardnesses for most of the RM PEO coatings were lower, but their thicknesses and coating/substrate bonding strengths were higher (and increased with increasing coating thickness), and their structures were denser than those of the pure PEO coating. After 30 min erosive wear, mass loss and wear scars for most of the RM PEO coatings were lower than those of the pure PEO coating. This indicates good wear resistance. Electrochemical corrosion tests showed that the RM PEO coating corrosion resistances were better than that of the pure PEO coating, especially for heat treatment at 300 °C.

Khanna R., Konyukhov Y., Zinoveev D., Li K., Maslennikov N., Burmistrov I., Kargin J., Kravchenko M., Mukherjee P.S.

The present study developed a novel approach for transforming red mud (RM) into soft magnetic materials (SMMs) for applications in advanced electrical devices in the form of Fe-Si and Fe-Si-Al alloys. A total of ten blends were prepared based on two RMs, three iron oxide additives (Fe2O3, black and red mill scales), alumina and carbonaceous reductants in a range of proportions. Carbothermic reduction of the blends was carried out in a vertical Tamman resistance furnace at 1600–1650 °C for 30 min in an argon atmosphere; synthetic graphite was used as a reductant. Reaction products were characterized using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray fluorescence (XRF) and X-ray diffraction (XRD). Significant amounts of Fe-rich metallic droplets/regions of different grain sizes (0.5 to 500 μm) were produced in these studies. The formation of Fe-Si alloys with Si contents from 3.9 to 6.7 wt.% was achieved in 8 out of 10 blends; the optimal levels of Si for SMMs ranged from 3.2 to 6.5 wt.%. There was clear evidence for the formation of Fe-Si-Al (up to 1.8 wt.% Al) alloys in 4 out of 10 blends. In addition to lowering operating challenges associated with RM processing, blending of RMs with iron oxide additives and alumina presents a novel recycling approach for converting RMs into valuable SMMs for possible emerging applications in renewable energy, storage, electrical vehicles and other fields. Along with reducing RM stockpiles across the globe, this approach is expected to improve resource efficiency, mitigating environmental impacts while generating economic benefits.

Zhao S., Chen Y., Yang J., Ding N.

Paz-Gómez D.C., Vilarinho I.S., Carvalheiras J., Pérez-Moreno S.M., Seabra M.P., Labrincha J.A., Bolívar J.P.

AbstractFour industrial wastes, namely, tionite (T), iron grit (IG), electroplating sludge (ES), and mill scale (MS), are typically disposed of in controlled hazardous landfills because of their toxic content, posing potential harm to human health and to the environment. At the same time, the chemical composition of these wastes, specifically the nature and content of transition metals, makes them potentially attractive for reuse in pigments manufacturing. This work details the study of these residues for producing coloured perovskites to be tested as inorganic pigments. The residues were mixed, in different proportions, and subsequently calcined to produce the required structures. The colouring potential was then assessed in a ceramic paste and in a transparent glaze. Leaching tests were carried out to verify the effective immobilisation of the hazardous species. Dark pigments were successfully obtained from the mixtures of T: ES: Co3O4, T: MS and T: IG. The crystalline phases present in T: ES: Co3O4 are nickel–chromium iron oxide spinel–Fe1.5Cr0.5NiO4 (without Co) or trevorite–Fe2NiO4 (with Co), titanium nickel oxide–TiNiO3 and titanite–CaTiSiO5. The mixtures T: MS and T: IG presented hematite (Fe2O3) and pseudobrookite (Fe2TiO5). Leaching tests confirmed the non-hazardous or inert character of the synthesized pigments. Products showed brownish or greyish hues, depending on the pigment added. T:75ES_1100, T:73ES:2Co_1100, T:75MS_1000, T:75MS_1100 and T:75IG_1000 pigments can effectively and safely be used to colour ceramic paste replacing partially or totally the commercial pigments.

Cui W., Liu J., Duan W., Xie M., Li X., Dong X.

Red mud, as the primary byproduct of processing bauxite into aluminum, raises environmental concerns due to its significant alkalinity and substantial heavy metal content. Addressing the limitations of traditional methods, this study proposes an innovative solution to utilize red mud, steel slag, fly ash, and phosphogypsum together to produce a new type of low-carbon, environmentally friendly cementitious material. Utilizing advanced techniques such as X-ray diffraction, thermogravimetric analysis, Fourier-transform infrared spectroscopy, mercury intrusion porosimetry, and scanning electron microscopy with energy-dispersive spectroscopy, this study conducted a comprehensive evaluation of cementitious materials, revealing their synergistic mechanisms. Furthermore, artificial neural networks and genetic algorithms were employed to optimize the mix ratio of the quaternary binder, aiming to predict and enhance the material's mechanical strength. Research shows that quaternary cementitious materials outperform traditional binary and ternary materials in compressive strength, hydration properties, microstructure, and environmental performance. Particularly, the increased content of calcium-aluminate-silicate-hydrate and sodium-calcium-silicate-aluminate-hydrate gels and a denser matrix structure highlight the synergistic effect in the hydration reaction of the quaternary system. Through mix proportion optimization, a significant enhancement in compressive strength up to 21.4 MPa is achieved. Environmental assessment shows that the composite material effectively solidifies Na+, with non-renewable resource and energy consumption reduced by 84.7% and 83.8%, respectively, compared to traditional cement, highlighting its environmental and energy-saving advantages. This approach addresses red mud storage, produces green building materials that meet standards, and fulfills resource and environmental goals.

Yang J., Xiao H., He X., Su Y., Zeng J., Li W., Li Y., Qi H.

Sintering red mud (SRM) is an alkaline and aqueous solid waste from bauxite sintering for alumina production. The challenges in the effective utilization of SRM are strong alkalinity, aqueous state and leaching toxicity of the heavy metals. Herein, a rapid wet grinding carbonation technique of simultaneous injection of CO2 in wet grinding process was proposed. The rapid carbonation of alkaline metal ions and the efficient solidification of heavy metal ions were achieved in the wet grinding carbonation. After 40 min of short-term carbonation, the CO2 uptake and carbonation degree of SRM reached 30.25 g CO2/100 g SRM and 79.31 % respectively. The high-efficiency CO2 uptake of SRM was attributed to the continuous stripping of the attached surface CaCO3 layer by wet grinding, the constantly exposed new surface area after refinement and the largely increased Ca2+ concentration, which was 12.5 times the initial value at 10 min. Furthermore, the simultaneous carbonation realized the solidification of dissolved chromium ion (Cr(III)/Cr(VI)), which was transformed into CaCrO4, Ca3Cr2(OH)12 or Cr(OH)n(CO3)n(3n-3)- complexes under the weak acid environment of CO2. After 40 min of wet grinding carbonation, the Cr concentration was notably decreased by 85.71 %, which was lower than the limit of groundwater standard requirement.