Hydrometallurgy has high efficiency and practicability in the large-scale application of recycling the cathode materials of spent lithium-ion battery. However, it consumes a large number of chemical reagents during the leaching process, generating pollutants and arousing attention. Therefore, the key issue is to reduce the negative environmental impact caused by hydrometallurgy. The selection of green and efficient leaching technology is beneficial to reduce the production of "three wastes" pollutants in the hydrometallurgy recycle technology, in which the environment-friendly organic acid has the characteristics of both leaching agent and chelating agent, promising to partly replace the inorganic acid leaching in the future. Using the microorganisms to leaches the spent lithium-ion batteries is an environmentally friendly and cheap method, which can design a targeted leaching process by adjusting its own redox properties. However, its strict cultivation environment and inefficiency have become a major difficulty in industrialization. Compared with the traditional acid leaching technology, deep eutectic solvent technology and electrochemistry technology still do not have the advantage in cost. For different cathode materials, auxiliary leaching methods such as mechanical activation, Fe ion medium and wave assistance can be adopted to reduce the amount of leaching agent, increasing the leaching efficiency and economic benefit. The traditional chemical precipitation and solvent extraction for separating and purifying often reveal a complex process, increasing the burden on the environment. The advanced purification technologies such as ion exchange and membrane separation generate positive effects on the environment, its cost still needs to be considered. Besides, enormous efforts have been paid on recycling the single cathode materials. However, the spent cathode materials collected from the market often consisted of a mixture of different cathode materials, which trend to increase the cost of pretreatment and separation process. Therefore, recycling technology for mixed cathode materials should be studied. For recycling other components used in lithium-ion batteries, such as anode materials, separator, current collector and so on, although it generates lower economic benefits than that of cathode materials, the closed-loop recycle of all components of spent lithium-ion batteries is still important in the future, which realizes high-value and deeply utilization of the whole components of spent lithium-ion batteries.

Direct regeneration technologies for LIB cathode materials have shown significant promise in recent years, offering an environmentally friendly and resource-efficient alternative to traditional recycling methods. However, for these technologies to be successfully implemented on a large scale, several challenges need to be addressed. High processing costs and energy consumption, the need for better control over reaction parameters, and the integration of regeneration techniques into existing recycling infrastructures are the key hurdles that need to be overcome. Additionally, ensuring that regenerated cathode materials maintain their long-term stability and performance is crucial for their commercial viability.

The future of LIB recycling will likely focus on optimizing these direct regeneration methods to improve their scalability, reduce costs, and enhance their overall efficiency. Research should prioritize the development of more cost-effective reagents, energy-efficient processes, and technologies that can be easily integrated into current recycling systems. Additionally, innovations in material science, particularly in the design of more durable and stable cathode materials, will play an important role in enhancing the regeneration process. As these technologies continue to mature, they will contribute to a more sustainable and circular economy for LIBs, reducing the need for new mining activities and minimizing the environmental impact of battery disposal.

Ultimately, the successful development of direct regeneration technologies will help bridge the gap between the growing demand for LIBs and the need for sustainable resource management. By improving the efficiency and reducing the environmental impact of recycling, these technologies have the potential to make a significant contribution to the sustainable development of the battery industry, supporting the long-term goals of reducing carbon emissions and conserving valuable resources.

China is a large country in the photovoltaic (PV) industry, with a vast amount of end-of-life (EOL) PV modules requiring proper management. However, encapsulants (EVA) and fluorine-containing backsheet (TPT) in PV laminates, as high molecular weight organic polymers, impede subsequent PV recycling processes. Pyrolysis is an effective means for recycling end-of-life PV laminates by removing the EVA and TPT. However, in practical pyrolysis processes, EVA and TPT are inevitably co-pyrolysis together. Due to variations in manufacturing methods, the proportion of organic components differs among PV laminate models, potentially affecting their co-pyrolysis behavior—A topic that remains underexplored. This study focuses on investigating the co-pyrolysis characteristics and mechanisms of EVA and TPT at different mixing mass ratios. Kinetic modeling was employed to analyze the heat loss behavior and determine the co-pyrolysis mechanisms.

The EVA hot melt adhesive used in the experiment was sourced from Jiacheng Plastics Co., Ltd (China) with a particle size lower than 100 mesh. The TPT was obtained from Youdisheng Electronic Materials Co., Ltd (China). The TPT was crushed, ground, and sieved to pass through a100 mesh standard sieve, and then dried in an oven at 60 ℃ for 12 h. The mixing mass ratios of EVA and TPT were determined to be 5 ∶ 1, 3.4 ∶ 1.0, and 2 ∶ 1 through literature research and experimental validation.

A thermogravimetric analyzer (TG, STA 449 F5 Jupiter, NETZSCH) was used under an argon atmosphere with a flow rate of 50 mL/min. Samples of (10.0 ± 1.0) mg of EVA, TPT, and different ratios of their mixtures were placed in crucibles. The samples were heated from 50 ℃ to 800 ℃. Based on an extensive literature review, the heating rates of 10, 15, 20 ℃/min, and 25 ℃/min were selected. The synergistic effect was characterized using the difference between the theoretical and actual degrees of pyrolysis (ΔW) to investigate potential pyrolytic interactions during the co-pyrolysis process. The Flynn-Wall-Ozawa (FWO) method and the Kissinger-Akahira-Sunose (KAS) method (Model-free method) were employed to calculate the activation energy, and the Coats-Redfern (CR) method (Model-free method) was chosen to ascertain the pyrolysis reaction mechanism.

The co-pyrolysis weight loss was divided into two stages, the first stage was treated at 300-380 ℃, which was mainly due to the deacetylation of EVA to produce acetic acid. The second stage was at 380-510 ℃, corresponding to the degradation of EVA and TPT long chains. At 800 ℃, the co-pyrolysis of EVA and TPT with the ratio of 2∶1 resulted in a total weight loss of approximately 94.5%, while ratios of 3.4∶1.0 and 5∶1 achieved higher total weight losses of 96.7% and 97.9%, respectively. As the proportion of TPT decreased, the total weight loss ratio of the mixture gradually increased, which may be due to the presence of PET in TPT. The presence of benzene rings in PET caused cross-linking of the product, generating more polyaromatic hydrocarbons and promoting coking. The overall synergistic interaction between EVA and TPT during co-pyrolysis was antagonistic effect. The antagonistic effect was most pronounced when the ratio of EVA to TPT was 2∶1, probably due to the increase in TPT, which made the coking more pronounced. The average activation energy of co-pyrolysis was higher than that of EVA and TPT pyrolysis alone, suggesting that more energy was required for co-pyrolysis, further supporting the antagonistic effect of co-pyrolysis. The average activation energies were 244.7-245.5, 239.6-240.1 kJ/mol and 261.2-262.8 kJ/mol for EVA and TPT mass ratios of 2∶1, 3.4∶1.0 and 5∶1, respectively. In order to further determine the reaction mechanism of the co-pyrolysis, the reaction mechanism of the co-pyrolysis process was explored using the CR method. The co-pyrolysis followed the diffusion controlled D5 model, D3 model, and D6 model for EVA and TPT mass ratios of 2∶1, 3.4∶1.0 and 5∶1, respectively. Changes in the mechanism functions of different ratios of co-pyrolysis further indicate the influence of the mass ratio of EVA and TPT on the interactions.

In this paper, the pyrolysis behaviour, kinetics and reaction mechanisms involved in the co-pyrolysis of EOL PV laminates at three mass ratios (2∶1, 3.4∶1.0 and 5∶1) of EVA and TPT were investigated, and key findings are as follows:

1) The main pyrolysis happens between 300 ℃ to 510 ℃. As the proportion of TPT decreased, total weight loss increased. Specifically, the weight loss rose from 94.5% to 97.9% as the EVA : TPT mass ratio increased from 2∶1 to 5∶1.

2) The degree of synergistic interaction varied with mixing ratio. TPT promoted char formation and reduced overall pyrolysis efficiency, particularly at the 2∶1 ratio, where its proportion was highest. This antagonistic effect was reflected by an average activation energy of 245.1 kJ/mol.

3) Coats-Redfern (CR) model fitting revealed that the co-pyrolysis of EVA and TPT followed diffusion-controlled mechanisms. The reaction models corresponded to D5, D3, and D6 for the 2∶1, 3.4∶1.0, and 5∶1 ratios, respectively.

This work provides mechanistic insights into the co-pyrolysis behavior of key organic components in PV laminates and offers theoretical support for the development of thermal recovery methods for waste PV modules.

Birnessite has exhibited moderate activity in room-temperature oxidation degradation of formaldehyde and is considered as the most promising material for formaldehyde degradation. However, birnessite also exposed insufficient surface active sites to meet the needs for long-term and sufficient degradation of formaldehyde. As a cheap and easily obtainable mineral, goethite not only has a large specific surface area and high oxidation-reduction activity, but also usually appears as nanoscale particles. Hematite can be prepared via calcination of goethite, which can maintain the original needle like structure of goethite, and has a larger specific surface area than that of goethite. It is speculated that using hematite obtained by heat treatment of goethite to support hexagonal birnessite might increase the surface active sites of the catalyst, thereby improving the formaldehyde degradation activity at room temperature. Therefore, in this study, goethite was used as a precursor to prepare hematite with different specific surface areas by calcining method at different temperatures. The structure and physicochemical properties of the catalyst, the activity and mechanism of the catalyst for the oxidation and degradation of low concentration formaldehyde at room temperature were studied.

The synthesis method of K0.6hbir was as follows: Dissolve KMnO₄ (4.00 g), MnCl₂ (8.35 g), and NaOH (2.88 g) in 130, 130 mL, and 140 mL of deionized water, respectively. Firstly, slowly added KMnO₄ solution to (within 5 min) NaOH solution and stirred for 1 h. After that, MnCl₂ solution was added into the above mixture within 35 min and stirred for 2 h, and then aged for 4 h. After wash and centrifuge for 3 times, the generated precipitate was dried at 70 ℃. The synthesis method of hematite and birnessite composite was as follows: Goethite was calcined in a muffle furnace at 200, 250, 300, 400 ℃, and 500 ℃ for 2 h, respectively. The obtained hematite was named as Hem-X, where X represents the calcination temperature. Add the above sample to 140 mL of NaOH solution (0.18 mol/L) and stir vigorously for 30 min. Then, synthesize hematite composite birnessite sample using the method of preparing birnessite. The amount of birnessite in the composite was 30% (mass fraction), and the sample was named as K0.6hbir@Hem-X. The phase and structure of the catalyst was characterized using X-ray diffractometer (XRD, Dandong Haoyuan, DX-2700), Raman spectrometer (RAMAN, LabRAM HR Evolution), high-resolution transmission electron microscopy (TEM, JEM-2100F), respectively. The specific surface area was measured using a BET adsorption desorption instrument (BET, NOVA 3000e). Surface elemental valence states of the sample was characterized by X-ray fluorescence spectroscopy (XPS, ESCALAB250Xi). The in-situ degradation process of formaldehyde was carried out in a Fourier transform infrared spectrometer (FTIR, VERTEX 70, Bruker). The dynamic experiment for catalytic oxidation of formaldehyde was carried out in a quartz tube. 1 mg/L formaldehyde gas can be stably obtained by volatilization of polyformaldehyde under constant temperature and circulating cooling water conditions at 10 ℃. The total gas flow rate was 1 L/min corresponding to a WHSV of 1200 L/(g·h). The formaldehyde concentration before and after the reaction were detected online by a formaldehyde detector (Interscan 4160-19.99 m). CO₂ was detected by gas chromatography (GC-2020) under the formaldehyde concentration of 10 mg/L.

Hematite obtained by calcining goethite at 300 ℃ shows a higher formaldehyde removal rate which can maintain a formaldehyde removal rate at about 90% within 2 h, as compared with the other hematite samples and goethite. Based on the results of specific surface area, the formaldehyde removal ability of hematite was closely related to its specific surface area. The K0.6hbir@Hem catalysts also exhibited the same trend as that of the bulk hematite, and had superior removal performance than hematite. K0.6hbir@Hem-300 had the best catalytic activity and maintained a removal rate of over 95% for formaldehyde within 300 min. In particular, the K0.6hbir@Hem-300 catalyst can maintain almost 100% formaldehyde removal rate in 24 h when the WHSV was 400 L/(g·h), but showed a significant decrease with the increasing of WHSV. The results of in-situ DRFITS and quenching experiments indicated that the 'O₂^- species promoted the transformation of DOM into formate, while the ·OH species had the effect on the completely conversion of the intermediates to CO₂ and H₂O during the oxidation degradation of formaldehyde.

K0.6hbir@Hem-300 had superior formaldehyde oxidation degradation performance, achieving almost 100% formaldehyde (1 mg/L) removal rate within 24 h at 400 L/(g·h) WHSV with no activity loss over 5 cycles. The relative humidity significantly affected the oxidation degradation performance of the composite. The hematite and hexagonal birnessite composite had higher CO₂ yield (55%) than that of the goethite and hexagonal birnessite composite (40%), indicating that the obtained hematite was a more suitable support than the original goethite. The specific surface area of K0.6hbir@Hem-300 (116.5 m²/g) further increased compared with that of Hem-300 (86.7 m²/g), which was beneficial to the adsorption of formaldehyde during the degradation processes. The introduction of hematite increased the contents of Mn (III) and surface active oxygen species over the composite, leading to the enhancement of oxidation degradation performance of formaldehyde. In particular, the ·O₂^- species promoted the transformation of DOM into formate, while the ·OH species had the effect on the completely conversion of the intermediates to CO₂ and H₂O during the oxidation degradation of formaldehyde.

Indium tin oxide (ITO) film is an excellent transparent conductive material widely used in liquid crystal displays (LCDs), electroluminescent displays (EL/OLEDs) and various photovoltaic devices. In particular, heterojunction (HJT) solar cells have shown a broader development potential. In order to improve the photovoltaic conversion efficiency of heterojunction solar cells, transparent conductive films need to be deposited on curved or cylindrical solar collectors. The deposition of ITO conductive films on these non-planar substrates requires specially shaped ITO tubular targets as the deposition source. Therefore, the preparation of high-quality ITO tubular targets is a prerequisite for obtaining ITO films with excellent properties on curved or cylindrical substrates, and the shaping and high densification of ITO tubular targets have been a hot and difficult issue in this field. In this work, based on the theoretical model of binary particle grading and the composite spray granulation-cold isostatic pressing moulding process route, the effects of different grading ratios, moulding pressures, times and processes on the properties of powders, billets and targets were investigated.

In₂O₃ and SnO₂ powders (purity 99.99%, Hunan Ruyang Ruijin Electronic Technology Co., Ltd.) were ball-milled at a mass ratio of 9∶1 to formulate indium tin oxide powders with D₅₀ (the particle size corresponding to the cumulative particle size distribution percentage of the samples reaching 50%) of 50 nm and 323 nm, respectively, and were mixed in accordance with different grade ratios. After mixing, anhydrous ethanol (analytically pure, Xilong Science Co., Ltd.) was added to disperse the powder in a ball milling tank, and the mass ratio of zirconia balls to powder in the slurry was 3∶1, and the slurry was mixed and then milled in a planetary ball mill at 350 r/min for 8 h. The slurry was obtained at an inlet temperature of 190 ℃, with a slurry solids content of 60% (mass fraction) and 1.5% binder content, Spray granulation was carried out at a feed rate of 100 mL/h. The granulated powder was loaded into a cold isostatic pressing mould with an inner diameter of 6 mm and a length of 39 mm, and then fully vibrated and consolidated before bagging and sealing for vacuum treatment and cold isostatic pressing. The total holding time of the composite pressing curve was the same as that of the single-point pressing curve. The hollow tubular blanks were sintered in a tube furnace at a flow rate of 5 L/min in an oxygen atmosphere, at sintering temperature of 1550 ℃ and holding time of 8 h to produce ITO tubular targets.

X-ray diffractometer (XRD, D8 Advance, Bruker) was used to study the phase structure. Scanning electron microscope (SEM, Tecnai-450, FEI) was used to observe the microstructure and elemental distribution with an operating voltage of 8 kV. The particle size distribution of the powders was tested using a laser particle size analyser model LS-609 from Omec. The loose packing density of the powders was tested using a WLD-02 Hall flow meter (funnel method). The density of ITO target was tested by Archimedes method and compared with the theoretical density to calculate the relative density of ITO target. The resistivity was measured by a four-point probe meter (MCP-T700, Mitsubishi chemical).

Based on the tightest stacking theory model, the stacking density of ITO powder is significantly improved by binary particle grading, because a reasonable grading ratio can achieve the fine particles to fill the gaps between the larger particles, which effectively improves the loose loading density of ITO powder.

The study of ITO tubular target prepared by cold isostatic pressing technology shows that the densification process of billet and target by forming pressure presents three-stage characteristics: Stage 1, the loose powder is rapidly rearranged under extrusion, with narrowing particle gaps and increasing bulk density, resulting in a lower resistivity; Stage 2, the resistance of internal friction between the particles increases, and the rate of densification tends to slow down; Stage 3, when the pressure exceeds the critical value (the limit), the particle plastic deformation or fragmentation occurs. deformation stress), the particles undergo plastic deformation or fragmentation, and the pores are further filled, but too high a pressure will form a pressure gradient due to the friction between the billet and the mould, which may, on the contrary, trigger internal slip leading to a decrease in density. The effect of holding time showed that within 5-11 min, extending the holding time significantly increased the relative density, which was attributed to the full displacement of the powder to fill the pores and the deformation of the particles, which increased the contact area and the interaction force; after more than 11 min, the pore filling tended to be saturated, and the density growth was stagnant. The composite pressing process effectively discharges the residual gas inside the billet through staged pressure release, eliminating the gas compression reaction force and achieving better densification results. The study reveals the key mechanism of cold isostatic pressure parameters on the microstructure regulation of ITO targets.

In this work, the effects of different grading ratios, moulding pressures, times and processes on the properties of powders, blanks and targets were investigated using a binary particle grading-spray granulation-cold isostatic pressing process. The results showed that particle grading improved the density of the powder, spray granulation improved the sphericity of the powder, and increasing the moulding pressure and prolonging the holding time improved the density of the billet. The billet with a relative density of 62.52% was prepared by the composite pressing process at a holding pressure of 200 MPa for 9 min, and the ITO tubular target with a relative density of 99.85% and a resistivity of 1.333×10^-4 Ω·cm was obtained by sintering.

Polycarboxylate superplasticizer (PCE) is commonly used in cement-based materials, which has the advantages of low dosage, high water reduction and strong molecular designability, which has been widely concerned by many researchers. PCE adsorbs on the surface of cement particles, and then breaks the cement flocculation structure, playing the effect of dispersing cement particles. However, it is known to all that the capacities of dispersing and dispersing retention of PCE to cement slurry are influenced by many parameters, such as cement components and mixing conditions. The polymer adsorption was shown to be very sensitive to the concentration of divalent ions like sulfate ions, so the sulfate ions of the cement slurry appear to be one of the most important parameters that adversely affect performances. In addition, the molecular structure parameters of PCE have an important influence on the adsorption behavior. Therefore, molecular structure parameters are important factors affecting PCE sensitivity to sulfate.

Although previous studies have found that PCE with ionizable side chains (ISC-PCE) has better dispersion retention than PCE with nonionizable side chains (NSC-PCE), the effect of changes in side chain ionizability on its sensitivity to sulfate (sulfate tolerance) is unclear. Therefore, to more effectively realize the performance control of PCE on cement-based materials, the influence of side chain ionizability on sulfate tolerance of PCE is needed. Based on the previously reported ISC-PCE and NSC-PCE, the sulfate concentration was controlled by adding sodium sulfate, under which the new mixed cement slurry flow and its retention and rheological properties were tested to assess the dispersion. The size of PCE in solution, the amount of adsorbed polymer and zeta potential were combined to assess PCE tolerance to SO₄^2-. The salt added species were changed and the effects of SO₄^2- competitive adsorption and ion concentration on PCE performance were analyzed by rheology. The results can help to understand the effect of PCE on cement dispersion in the presence of ion competition and provide a new direction for opening up new structural PCE with good compatibility.

The fluidities of cement slurries were measured according to the Chinese standard GB/T 8077—2012. A RST-SST rotational rheometer with a paddle rotator of VT40-20 was used (shear rate: 10 s^-1→50 s^-1→10 s^-1). The data was processed with Rheology 3000 software to obtain the yield stress. The hydrodynamic radii (R_h) of the polymers were determined in saline conditions by dynamic light scattering experiments. The adsorbed capacities of the PCEs on cement surfaces, which were calculated from the difference in the concentration of polymers, were measured on a Vario Total Organic Carbon (TOC) analyzer by testing the PCE adsorption after different adsorption times. The zeta potentials of the cement pastes were measured at 25 ℃ on a Malvern Zetasizer Nano ZS90 analyzer. The MD simulations used the Gromacs-4.6.7 software package and a conventional oplsaa force field with RESP charges. The UFF force field was used for the substrate. The equilibrium MD simulation was conducted under the NPT ensemble for a total run time of 20 ns at a relaxed liquid configuration (25 ℃). The solution environment simulations were performed using 3000 water molecules. A relaxation system was selected and the energy was minimized by the steepest descent method with a termination gradient of 100 kJ/(mol·nm) before the relaxation. The system temperature is maintained constant through a Nosé-Hoover thermostat, and periodic boundary conditions were applied to all three dimensions. Long-range electrostatics within a relative tolerance of 1×10^-6 is calculated by the particle mesh Ewald method and a cut-off distance of 1 nm was used as the Ewald interaction and van der Waals interactions. The bond lengths of hydrogen atoms were constrained by the LINCS algorithm. A leap-frog algorithm was used with a time step of 1 fs.

Compared with no salt, the hydrodynamic radius of NSC-PCE and ISC-PCE decreased by 36.9% (to 8.40 nm) and 8.1% (to 15.98 nm), the cement slurry mobility decreased by 21.7% and 4.3%, the adsorption volume decreased by 21.7% and 3.9%, and the yield stress increased by 42.7% and 27.8%, respectively.

The main conclusions of this paper are summarized as following. The improved side chain ionizability of PCE can significantly improve the dispersion stability and sulfate tolerance, and reduce its sensitivity to sulfate competitive adsorption and ionic strength. Based on the above results, sulfate tolerance of both PCE was quantitatively assessed (the sulfate resistance index of ISC-PCE was increased by 32% as compared to that of NSC-PCE) and verified by all-atom molecular dynamics simulations. The present results not only deepen the understanding of the impact of sulfate on PCE performance, but also provide a new direction for the development of new PCE with good compatibility.

Against the backdrop of global dual-carbon goals, the wind power industry is developing rapidly. Facing the environmental pressures brought by the increasing number of retired blades in recent years, there is an urgent need to develop low-cost, high-value, and low-carbon recycling technologies to overcome the challenge. The key scientific and technical issues causing difficulties in the degradation and recycling of retired blades mainly include a lack of technology for blades to serve beyond their designed lifespan, insufficient low-carbon material substitution technologies, and a scarcity of complete degradation and high-value recycling technologies. The key scientific and technical issues that make it difficult to degrade and recycle retired blades mainly include the lack of technology for blades to serve beyond their designed lifespan, insufficient alternatives for low-carbon emission materials, and a scarcity of fully degradable high-value recycling technologies. Mechanical recycling, thermal recycling, chemical recycling, and recyclable blade technologies are the current main recycling technologies. However, to meet the needs of blade life extension, the recycling of critical components, and complete blade recycling, relying on a single recycling technology is not sufficient. It is necessary to develop recycling technologies that can be used in combination for different purposes. In the future, the green, low-carbon, and high-quality development of the wind power industry can be promoted, and the rapid formation of new productive forces in China can be supported, through the development of systems for estimating the remaining life of turbines and blades, expanding the high-value application areas of retired blade components, exploring commercial recycling models for retired blades, developing next-generation blade technologies including recyclable technologies, and researching recycling and reuse technologies for retired wind turbines.

Laser processing has demonstrated transformative potential in diamond machining. However, current technologies face several challenges. Laser polishing of diamond requires further roughness reduction, with sequential multi-laser polishing, pulse burst mode processing, and laser-assisted polishing representing viable next steps. Mitigating thermal damage from long-pulse lasers and improving efficiency of short-pulse lasers are critical for widespread adoption. Future research should integrate short- and long-pulse laser advantages through process design or source selection to achieve high-precision, high-quality, and low-defect machining. In recent years, innovative technologies such as Bessel beam shaping, femtosecond laser direct writing, and laser microjet have further broken through the limitations of traditional processes, providing brand new solutions for the manufacturing of quantum devices, optical components, and high-precision microstructures. In conclusion, while challenges persist, continuous advancements in laser technology promise to address these bottlenecks, enabling scalable diamond machining for next-generation applications.

Red mud (RM) as a highly alkaline solid waste generated during alumina production presents significant environmental challenges due to its massive annual output and stockpiling, including land occupation and pollution. The utilization of RM in combination with other solid wastes or minerals to produce geopolymer cementitious materials becomes one of the effective approaches for its resource recovery. However, Bayer-processed RM exhibits a low Si/Al molar ratio and a poor reactivity, making it difficult to form a stable aluminosilicate polymer structure. Most RM-based geopolymers require mechanical/thermal activation pretreatment or high-temperature curing to enhance early strength. The existing studies typically involve either low RM incorporation ratios or result in a low geopolymer strength at high RM dosages. Based on previous research and the concept of multi-source solid waste synergy, this study was to introduce highly reactive ground granulated blast-furnace slag (GGBS) and calcium-rich carbide slag (CS) into an RM-Class II fly ash (FA) geopolymer system. The influence of GGBS and CS composite dosages on the mechanical properties of the geopolymer was investigated. Furthermore, the microstructure of the geopolymer was analyzed from multiple perspectives to elucidate the synergistic reaction mechanism and strength development under the co-incorporation of GGBS and CS.

In this study, RM, FA, GGBS, and CS were used as raw materials at a RM/FA mass ratio of 1.0/1.0. A mixed alkaline activator solution consisting of NaOH solution (NH) and water glass (WG) was used at an NH/WG mass ratio of 1.0/2.5. The NaOH solution had a molarity of 10 mol/L, and the liquid-to-solid ratio (L/S) was 0.65. Different dosages of GGBS and CS were used as the experimental variables.

The RM, FA, GGBS, and CS were manually mixed for 30 s, followed by blending the solid powder mixture with alkaline activator in a cement paste mixer for 3 min. The resulting slurry was then cast into triple molds with internal dimensions of 70.7 mm × 70.7 mm × 70.7 mm. The specimens were cured at 20 ℃ and 95% relative humidity for 24 h before demolding, after they continued to cure under standard conditions.

The compressive strength tests were conducted at different curing ages of 7, 14, 28, 45 d, and 60 d. The collected fragments taken from the center of the specimens were stored in vials and immersed in anhydrous ethanol to halt further reaction. The chemical composition, microstructure, and mineral phases on the geopolymer's strength were determined by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS). Prior to testing, the samples were dried at 60 ℃ for 24 h and ground for subsequent characterization.

All the specimens exhibit a strength development trend with increasing curing age. Among them, the geopolymer RFGC-1 with CS as sole additive shows the lowest strength, reaching only 1.8 MPa at 60 d. In contrast, RFGC-5 with GGBS as sole additive demonstrates the highest strength among the five formulations, achieving 11.6 MPa at 60 d, indicating that GGBS is more effective than CS in enhancing the strength of RM-Class II FA-based geopolymers.

For the samples RFGC-2, RFGC-3, and RFGC-4 incorporated both GGBS and CS, the 60-d compressive strengths are 5.8, 8.8 MPa, and 6.5 MPa, respectively. The strength initially increases and then decreases with increasing GGBS content and decreasing CS content, indicating that the relative proportions of GGBS and CS must be carefully controlled to optimize the geopolymer strength. At a total additive content of ≤12%, the RM-FA geopolymer with GGBS alone exhibits a superior strength, compared to those with CS alone or GGBS/CS combinations.

The microstructural characterization reveasl that, in addition to the original mineral phases (i.e., hematite (Fe₂O₃) and quartz (SiO₂)), the primary cementitious products in the geopolymers include calcium aluminate hydrate (C-A-H), calcium silicate hydrate (C-S-H), calcium aluminosilicate hydrate (C-A-S-H), ettringite (AFt), aluminosilicate oligomers (—Si—O—Al—Si—O—)_n, and hydrotalcite. For RFGC-1 (CS only), the microstructure is relatively loose, with the limited formation of amorphous C-S-H and insufficient conversion into C-A-S-H or (—Si—O—Al—Si—O—)_n, resulting in a low strength. As the GGBS content increases and CS decreases, intermediate products (C-S-H, C-A-H) further react to form C-A-S-H, AFt, and (—Si—O—Al—Si—O—)_n. The gel phases envelop adjacent particles, forming interconnected coatings and enhancing the aluminosilicate and SiO₄ tetrahedral networks, thereby strengthening the geopolymer matrix.

The incorporation of GGBS alone was more conducive to strength development in the RM-Class II FA based geopolymers, compared to CS alone. In the case of co-incorporating GGBS and CS, the primary cementitious products included C-A-H), C-S-H, C-A-S-H monomers, as well as ettringite, aluminosilicate oligomers (—Si—O—Al—Si—O—)_n, and hydrotalcite, which collectively contributed to the strength of the geopolymers. The addition of GGBS and CS to the RM-Class II FA system modulated the contents of Ca, Si, and Al in the composite system, significantly influencing geopolymer strength development. At a higher content of Ca, C-S-H became the dominant product. Conversely, at a higher content of Si or Al, these components first dissolved in the alkaline environment to form gels such as C-S-H, C-A-H, and C-A-S-H. These gels subsequently underwent dissolution and polycondensation under strong alkaline conditions, forming long-chain polymers like Si—O—Al—O and Si—O—Al—Si—O. For overtime, water was expelled, leading to coagulation and polymerization, ultimately resulting in the formation of dense and stable geopolymer structures.

Conventional modification of high-potassium cement clinker relies on adding SO₃ to reduce K₂O solid solution and boost C₃S content. However, SO₃ causes operational issues and stabilizes low-reactive C₂S-β, compromising strength. In this study, SO₃ was used to stabilize highly reactive C₂S-α'_L as a strength-enhancing phase, while K⁺ could suppress C₃S formation. To harness this "contradictory duality", an optimal C₃S/C₂S-α'_L ratio that could maximize the strength contribution of C₂S-α'_L without significantly sacrificing C₃S was established. This strategy could overcome the limitations of SO₃-based methods and enable an efficient utilization of high-potassium limestone.

Raw meals were formulated from analytical-grade reagents (i.e., CaCO₃, SiO₂, Al₂O₃, Fe₂O₃, K₂CO₃, CaSO₄·2H₂O ≥ 99.0%) with fixed silica modulus (SM=2.2), alumina modulus (IM=1.5), and K₂O content (2%, in mass). The lime saturation factor (KH) was adjusted from 0.84 to 0.92 to modulate C₃S/C₂S ratios. Raw materials were wet ground in a ball mill at 600 r/min for 1.5 h, dried at 105 ℃, pelletized under 100 kN, and then step-calcined (i.e., 950 ℃/1 h → 1450 ℃/30 min). The materials were followed by rapid air-quenching to inhibit C₂S-γ formation. The resulting clinker was dry ground to a fine powder with the sizes of ≤200 mesh, blended with gypsum of 3%, and cast into paste specimens (20 mm×20 mm×20 mm, w/c=0.3) for standard curing at 20 ℃ and 95% RH for 3-d/28-d ages.

The chemical composition was analyzed on borate-pressed pellets with a starch binder of 10% by X-ray fluorescence spectrometry (XRF). The phase quantification was determined by X-ray diffraction (XRD)-Rietveld refinement (Bruker D8 Advance, 5°-80°, 40 kV/20 mA) via a software named TOPAS V4. 2 with March-Dollase texture correction for the key phases. The microstructural evolution was characterized by scanning electron microscopy and energy dispersive spectroscopy (SEM-EDS) (gold-sputtered polished sections, 10-point EDS averaging) and optical microscopy (NH₄Cl-etched surfaces). The hydration kinetics were monitored by isothermal calorimetry (TAM Air, at 20 ℃ for 72 h), while the thermal decomposition behavior was assessed by thermogravimetric analysis (TGA) (RT→550 ℃ at 10 ℃/min under N₂). The amorphous content in hydration products was quantified using corundum internal standard (20%) after ethanol immersion and vacuum drying.

The results demonstrate that elevating the C₃S/ C₂S-α'_L ratio fundamentally restructures phase equilibria and ion distribution. As the ratio increases from 0.78 to 1.80, K⁺ solid solubility in C₂S is decreased by ~42% (quantified via EDS point analysis), destabilizing the metastable α'_L phase and triggering its transformation to C₂S-β, which is evidenced by α'_L content reducti on from 39.27% to 25.08% and intensified twinning striation. Concurrently, K⁺ migration toward interstitial phases increases the conversion from C₃A-o to C₃A-c, correlating with increasing C₃A content (i.e., from 9.97% to 1.02%) and declining C₄AF (i.e., from 8.94% to 7.39%). Based on the microstructural analysis, enlarged C₃S crystals with prominent etch pits emerge at high ratios due to K⁺-enriched liquid phases suppressing nucleation, while promoting disordered growth. This is compounded by elevated C₃A/C₄AF ratios increasing melt viscosity, hindering ion diffusion. The elemental mapping confirms that K⁺ predominantly partitions into C₂S and interstitial phases, with minimal C₃S incorporation (i.e., <0.3%, in mole).

The mechanical performance exhibits dual-phase characteristics. The 3-d strength is increased by ~18 MPa across the ratio gradient, driven by three synergistic factors, i.e., increased C₃A hydration activity, accelerated reaction kinetics from free CaO, and enhanced C₃S surface defects. The 28-d strength is declined by ~12 MPa, mechanistically linking to hydration restructuring. The result of calorimetry reveals sulfate depletion peaks advancing for 1.7 hours at higher ratios due to a rapid gypsum consumption by abundant C₃A. This shifts hydration products from AFt to Hc phases, which is evidenced by the XRD quantification, where AFt content decreases from 5.44% to 1.19% at 3 d, while Hc increases from 3.94% to 6.14%. At 28 d, Hc dominates (i.e., 7.55% at the highest ratio), with AFt nearly depleted (i.e., 0.27%).

The C-S-H gel morphology transitions from needle-like aggregates (i.e., Ca/Si=1.99 at a low ratio) to flaky networks (i.e., Ca/Si=1.26 at a high ratio). This structural shift reduces K⁺ adsorption capacity. The data establishe a Ca/Si ratio as a critical variable governing gel-K⁺ interactions, resolving prior discrepancies.

As the C₃S / C₂S-α'_L ratio decreased, the solid solubility of K⁺ and Al³⁺ in C₂S declined, whereas K⁺ incorporation in interstitial phases increased. This ultimately reduced the C₂S-α'_L / C₂S-β ratio in clinker, while elevating the C₃A-o/C₃A-c ratio. Decreasing the ratio could significantly enlarge C₃S particle size in high-potassium clinker, resulting in less distinct edges and increased etch pits. Although the fundamental morphology of C₂S remained largely unchanged, its surface became rougher with pronounced twinning striations.

The 3-day compressive strength of high-potassium clinker diminished at a lower C₃S/ C₂S-α'_L ratio, whereas the 28-day strength exhibited an inverse trend. As C₂S-α'_L competitiveness weakened, the peak heat flow and cumulative heat release in hydration curves both intensified. However, the sulfate depletion peak could emerge earlier, accompanied by increased hydrocalumite (Hc) phase content, further amplified when C₃S dominated over C₂S-α'_L.

The C-S-H gel morphology transitioned from aggregated needle-like fibers at low C₃S/ C₂S-α'_L ratios to flake-assembled network structures at high C₃S/ C₂S-α'_L ratios.