Low-vacuum tunnels are the core infrastructure of ultra-high-speed low-vacuum magnetic levitation transportation systems. The internal environment of these tunnels is characterized by extremely low pressure, alternating airflows, and very low humidity. Based on a comprehensive review of existing research, concrete is considered a highly competitive material for low-vacuum tunnels due to its cost-effectiveness and technological maturity. However, studies have shown that under conditions of extremely low pressure and fluctuating atmospheric pressure, concrete experiences rapid moisture loss, resulting in significant changes to its pore structure and leading to the deterioration of its mechanical properties and long-term performance. Additionally, the high-velocity airflow caused by alternating pressure in low-vacuum tunnels can induce aerodynamic fatigue damage to the internal and external surfaces of the concrete, generating severe dust that poses a significant threat to the safe operation of high-speed trains. To address these challenges and ensure the reliable performance of concrete in low-vacuum environments, protective measures or the use of high air-tightness concrete is necessary. Coating materials have been identified as an effective solution to enhance the service safety of concrete in vacuum tunnels.

The cement used was P·I 42.5 Portland cement produced by Fushun Cement Co., Ltd. Fine aggregates consisted of natural river sand with a fineness modulus of 2.8. Coarse aggregates were crushed limestone with continuous gradation, sized between 5 mm and 20 mm, with a mud content of 0.4%. Water used was municipal tap water. The water-reducing agent was a non-air-entraining polycarboxylate- based superplasticizer. The coating system was a two-component fluorocarbon material jointly developed by Beijing Jiaotong University, consisting of a solvent-free epoxy penetrating primer and a fluorocarbon resin topcoat. The primary film-forming material was fluorocarbon resin, with hexamethylene diisocyanate as the hardener, and the pigment-to-binder ratio of the coating was 0.4.

To better understand the effect of coating materials on concrete performance, the coating was applied during the early hydration stage of concrete. After casting, the specimens were stored at (20 ± 2) ℃ for one day before demolding, after which the primer coat was applied. Following two days of air drying, the topcoat was applied, and the specimens were further air-dried for two days. The compressive and flexural strengths of the concrete were tested using specimens sized 300 mm × 100 mm × 100 mm and 400 mm ×100 mm × 100 mm, respectively. To evaluate the impact of surface coating on concrete mass loss, specimens of 40 mm × 40 mm × 40 mm were prepared. Adhesion strength tests were conducted to examine the effects of low-vacuum environments on the coating's adhesion. Hardened cement paste samples with the same water-to-cement ratio as C50 concrete (0.37) were also prepared, cured to the specified age, cut into cylinders with heights of 20 mm and 10 mm, and embedded in epoxy resin for analysis. Hydration was terminated by immersing these samples in isopropanol.

The surface coating's effects on C50 concrete were further analyzed using scanning electron microscopy (SEM) and backscattered electron (BSE) analysis. Atmospheric curing was conducted in a standard curing chamber at (20 ± 2) ℃ and ≥95% relative humidity. A U-shaped vacuum chamber with a pressure range of 1000-2000 Pa and a temperature consistent with the local outdoor environment (-5 ℃ to 20 ℃) provided the low-vacuum exposure environment.

The application of surface coatings significantly reduced the mass loss of concrete under low-vacuum conditions. Compared to uncoated concrete, coated specimens exhibited a noticeably lower mass loss rate, demonstrating that the surface coating effectively suppressed the loss of free water within the concrete.

The surface coating markedly improved the flexural strength of concrete under both atmospheric and low-vacuum conditions. Under atmospheric curing, the flexural strength of coated specimens at 14, 28 d, and 60 d increased by 28.2%, 23.9%, and 36.2%, respectively, compared to uncoated specimens. For coated specimens exposed to a low-vacuum environment after six days of atmospheric curing, flexural strength at 14, 28 d, and 60 d increased by 33.3%, 25.0%, and 15.4%, respectively, compared to uncoated specimens under the same conditions. Furthermore, coated specimens exposed to low vacuum after 28 d of atmospheric curing showed a 47.5% increase in flexural strength compared to uncoated specimens.

In contrast, the compressive strength of coated specimens was generally lower than that of uncoated specimens under both atmospheric and low-vacuum exposure. This reduction is attributed to the adverse effects of low vacuum on the compressive strength of coated specimens. Additionally, the application of surface coatings reduces friction between the concrete specimens and the testing machine, diminishing end effects, which may also contribute to the lower compressive strength observed.

Microstructural analysis using SEM revealed similar hydration products in both coated and uncoated specimens, including ettringite and C-S-H. However, uncoated specimens exhibited more needle-like ettringite and network-like C-S-H, whereas coated specimens showed fewer needle-like structures and more plate-like C-S-H. BSE analysis indicated an 18.1% reduction in porosity for coated specimens under low-vacuum conditions compared to uncoated ones, reflecting improved internal density. Nevertheless, the unhydrated cement content in coated specimens was 7.5% higher than in uncoated specimens, suggesting partial hydration inhibition by the coating.

The adhesion strength of the coating increased with time under both atmospheric and vacuum conditions. Vacuum exposure slightly enhanced the coating's adhesion strength, indicating that low-vacuum environments promote better bonding between the coating layer and the concrete surface over time.

The surface coating effectively mitigated free water loss in low-vacuum environments, resulting in significantly reduced mass loss rates for coated concrete compared to uncoated specimens. Coated specimens demonstrated notable improvements in flexural strength under both atmospheric and low-vacuum conditions, although compressive strength was slightly reduced. Early-age exposure to low vacuum negatively impacted both compressive and flexural strength, regardless of coating protection. Microstructural analysis showed that the coating reduced porosity and enhanced internal density, mitigating the detrimental effects of low-vacuum environments on concrete's internal structure. Over time, the adhesion strength of the coating increased under both atmospheric and vacuum conditions, with slight improvements observed following vacuum exposure.

Ultra-high performance concrete (UHPC) has garnered widespread attention in the field of civil engineering due to its exceptional mechanical properties, durability, and workability. These superior properties are primarily attributed to its dense microstructure and unique material composition, which typically includes cement, silica fume, quartz powder, and superplasticizers, along with reinforcing materials like steel fibers. The remarkable performance of UHPC not only meets the demands for high-performance materials in modern engineering but also contributes to reducing the life-cycle costs of structures by minimizing section sizes and extending service life.

However, the development and application of UHPC face several challenges. The complexity of the preparation process, stringent requirements for raw materials, and the high sensitivity of UHPC properties to mix design variations increase both production costs and technical difficulties. Traditional experimental design methods struggle to efficiently optimize UHPC mixtures, and issues such as shrinkage and autogenous cracking require precise design solutions. These challenges make the composition design and performance prediction of UHPC a focal point in the field. With advancements in computer technology and data science, machine learning (ML) offers new tools and approaches for UHPC research and development. Therefore, this study aims to promote the application of ML technology in the UHPC field, thereby advancing the intelligent development of advanced construction materials.

The study introduced a comprehensive ML-based framework for UHPC performance prediction and precise design. The framework began with data cleaning, where the multiple imputation by chained equation (MICE) method with a predictive mean matching (PMM) kernel was employed to impute missing data in the UHPC database. Subsequently, the isolation forest algorithm was applied to identify and eliminate outlier data, thereby improving the quality of the dataset. The cleaned and optimized dataset was then used to train an XGBoost model, optimized via Bayesian hyperparameter tuning, to accurately predict various UHPC properties. Two AI-driven approaches for UHPC mix design were also proposed: one that combined the MAA model with ML predictions for multi-performance optimization, and another that integrated the ML model with genetic algorithm (GA) for multi-objective optimization.

The application of MICE combined with PMM resulted in a substantial improvement in the accuracy of the imputed data, as evidenced by the reduced average deviation between imputed and original data values. This enhanced imputation process allowed for a more reliable dataset, which directly contributed to the performance of the XGBoost prediction models. Outlier detection via the isolation forest algorithm effectively removed data points that exhibited significant deviation from the norm, particularly in compressive strength and density measurements. The refined dataset led to more accurate and consistent predictions across all performance metrics. After hyperparameter optimization, the XGBoost model demonstrated exceptional predictive capabilities, with notable improvements in performance metrics. The predictive accuracy of the model was further validated against experimental data, confirming its effectiveness in real-world applications. These results highlight the potential of ML techniques in advancing the field of UHPC research. The integration of MICE and PMM into the data preprocessing pipeline ensured more accurate and reliable datasets, which were crucial for developing robust predictive models. The success of the XGBoost model, particularly after Bayesian optimization, underscores the importance of hyperparameter tuning in enhancing model performance.

Furthermore, the study proposes two ML-assisted design strategies for UHPC: 1) a combined physical packing theory and ML prediction model for initial mixture design, and 2) a multi-objective optimization approach using a metaheuristic algorithm for fine-tuning UHPC compositions. These strategies provide a framework for the intelligent and efficient design of UHPC materials, aligning with the growing demand for high-performance, sustainable construction materials.

This research demonstrates the significant potential of ML in predicting and optimizing the performance of UHPC. By addressing data challenges and enhancing prediction models, the study provides valuable insights into the application of ML in UHPC design. The proposed ML-assisted design strategies offer a practical approach to developing UHPC with tailored properties, paving the way for more sustainable and efficient construction practices.

Adsorption is one of the most effective heavy metal ion treatment techniques available. Common heavy metal ion adsorbents include activated carbon, chitosan, cellulose and natural zeolite. Geopolymers, as a new type of adsorbent material, show great potential and value for application in the field of zinc-containing heavy metal wastewater treatment. Blast furnace slag is a type of bulk solid waste in the metallurgical industry, and the current high value utilisation rate is low. The use of blast furnace slag to prepare geopolymer adsorbent materials to treat heavy metal wastewater is one of the effective ways to realise the comprehensive utilisation of blast furnace slag in a high value way, which can achieve the purpose of 'waste for waste'. At present, the related research mainly focuses on the adsorption mechanism and adsorption effect of geopolymers on heavy metal ions, while less attention has been paid to the desorption of heavy metal ions after adsorption and the recycling of geopolymers, and only a few scholars have carried out preliminary explorations by using inorganic acid, EDTA-2Na, and NaCl solution as desorbing agents. In this study, the geopolymer adsorbent material was prepared from blast furnace slag to investigate its adsorption performance and mechanism of action on zinc Zn²⁺, and nitric acid was used as a resolving agent to investigate the desorption effect of Zn²⁺ and the recycling performance of geopolymer adsorbent, which is of great guiding significance for the high value-added utilisation of metallurgical solid wastes and the treatment of zinc-containing heavy metal ion wastewater.

Determination of major oxides and their contents in blast furnace slag using XRF. Particle size distribution of finely ground blast furnace slag was analysed using a laser particle size meter. Analysis of the crystalline phase structure of blast furnace slag and geopolymers using XRD. The specific surface area of the geopolymer and its pore size were characterised using BET. The concentration of Zn²⁺ in solution was detected by ICP. SEM-EDS was used to analyse the crystalline structure, micro-morphology and compositional distribution of the geopolymer before and after adsorption. Kinetic analysis of geopolymers using quasi-primary and quasi-secondary kinetic models. Adsorption isotherms were simulated for geopolymers using the Langmuir adsorption isotherm model and the Freundlich adsorption isotherm model.

The surface and interior of geopolymers based on blast furnace slag (BFSGP) contains a large number of pore structures, which are favourable for the migration of Zn²⁺ in the solution, and at the same time increase the adsorption sites, thus effectively improving the adsorption capacity and adsorption efficiency of BFSGP. The N₂ adsorption and desorption isothermal curves and pore size distribution curves of BFSGP indicate that BFSGP is a typical mesoporous structure. The adsorption of Zn²⁺ by BFSGP increased significantly with the increase in the initial ion concentration of the solution. The adsorption process of BFSGP followed the quasi-secondary kinetic model and the Langmuir model, thus indicating that the adsorption process of BFSGP was dominated by chemisorption, and its adsorption process was strongly influenced by the chemical reaction between BFSGP and Zn²⁺, and at the same time conformed to the characteristics of the monomolecular layer adsorption.

In the desorption experiments, within the first 20 min, nitric acid rapidly enters into the pores of the adsorbent and acts to break the chemical bond between Zn²⁺ and BFSGP, and the desorption rate is fast. The desorption rate decreases as the readily desorbed Zn²⁺ decreases. Despite the loss of some active adsorption sites due to chemical bonding and Al³⁺ leaching during the cyclic adsorption-desorption process, BFSGP still maintains good adsorption performance for Zn²⁺, which provides a new idea for the recycling of BFSGP as well as for the recovery of geopolymer-loaded heavy metals by the pickling method.

The adsorption of Zn²⁺ by BFSGP increased with the initial concentration of the solution. When the initial ion concentration was 200 mg/L, the adsorption amount and adsorption rate of Zn²⁺ by BFSGP were 196.14 mg/g and 98.07%, respectively. The adsorption of Zn²⁺ by BFSGP is in accordance with the quasi-secondary kinetic model and Langmuir model and is a chemical monolayer adsorption. BFSGP showed better adsorption performance in cyclic adsorption-desorption experiments, the first desorption rate of nitric acid was 45.52%, and after two cyclic adsorption-desorption cycles, the adsorption rate of BFSGP on Zn²⁺ was still up to 56.83%.

Abnormal strain and structural cracks are common precursors to engineering accidents in concrete structures. However, traditional strain and crack detection methods exhibit a lag, and discovering structural abnormalities in a timely manner incurs high labor costs. Consequently, strain self-sensing cement-based materials have garnered widespread attention from researchers due to their intelligent and automated characteristics. Nevertheless, current cement-based strain self-sensing materials lack exploration in conduction mechanisms, often failing to establish a good correlation between changes in electrical resistance and system strain. Therefore, in this paper, nickel powder, which exhibits excellent durability and conductivity, is selected as the conductive component for the preparation of cement-based strain self-sensing materials. The study explores its conduction mechanism and various electromechanical constitutive models under different states, providing an important reference for correlating the electromechanical relationship of strain self-sensing materials.

The cement used is Swan Brand P·O 42.5 ordinary Portland cement, and the water reducer is polycarboxylate superplasticizer with a water reduction rate of 18%-29%. The nickel powder adopts micrometer-grade high-purity ultrafine conductive nickel powder, with a Ni purity of 99.999% and a loose bulk density of 1.40-1.68 g/cm³. The execution standard is GB/T 7160—2008.

Mix cement, nickel powder, water, and admixtures, then stir and disperse them before pouring the mixture into molds. Place the molds into a magnetic field coil and let them sit for a day before demolding and curing. During preparation, mix rapidly for 5 min to form Mixture 1. Then add nickel powder (with volume fractions of 0%, 5%, 10%, 15%, 17%, 20%, and 23%) to the cement and mix rapidly until the color of the mixture is uniform and no longer changes. This indicates that the nickel powder is fully dispersed and evenly distributed, forming Mixture 2. Pour Mixture 1 into Mixture 2 and mix rapidly for another 5 min. In this step, the water reducer increases fluidity while allowing re-dispersion of agglomerated cement and nickel powder particles, forming a nickel powder cement slurry called Mixture 3. Place Mixture 3 in a vacuum drying oven, adjust the temperature to 20 degrees Celsius, and vacuum for 15 min to eliminate bubbles and fully compact the mixture. Pour the defoamed Mixture 3 into molds with dimensions of 20 mm×20 mm×40 mm and 20 mm×20 mm×80 mm, and seal with plastic wrap to reduce water loss. Prepare two sets of test pieces for each volume fraction, allowing them to form under magnetic field strengths of 0 Gs (blank control) and 200 Gs for about 24 h. Then let them sit at room temperature for 2 d before demolding. Cure them in a 60 ℃ steaming box for 3 d, then cure under standard conditions for 4 d. Finally, dry them in a vacuum drying oven at 60 ℃ for about 24 h until the weight no longer decreases.

With the increase of the volume fraction of nickel powder, the resistivity changes can be roughly divided into three stages, namely, the stage of slow resistivity decrease, the stage of rapid resistivity decrease, and the stage of resistivity decrease again but at a slow rate. The rapid resistivity decrease stage of nickel powder cement-based materials conforms to the percolation theory, and the percolation threshold is between 15% and 17%. In this paper, the percolation threshold is taken as 15%. Near the percolation threshold, the conduction mechanism accords with the tunneling effect theory. For specimens formed under the action of a 200 Gs magnetic field, their resistivity in the direction along the magnetic field will decrease, while their resistivity in the direction against the magnetic field will increase. The measured maximum ratio between resistivity in the direction against the magnetic field and resistivity in the direction along the magnetic field is close to 4. Based on the tunneling effect and percolation theory, this paper derives an equation representing the change of resistance with strain, namely, the electromechanical constitutive model, and expresses this relationship as a polynomial function of strain. The parameters of the polynomial are determined using piezoresistive experiments. A four-point bending test is used to obtain piezoresistive curves of specimens with a 15% doping level formed under no magnetic field and under a 200 Gs magnetic field when subjected to pure bending loads. Based on the bending resistance variation of specimens with and without a magnetic field, this paper proposes an electromechanical constitutive equation representing the bending resistance variation rate and strain value. Subsequently, a supplementary equation for resistance variation after cracking of pure bending specimens is proposed, forming the basic theory for monitoring strain and cracks in bending members.

The specific conclusions are as follows: The optimal mixing ratio for nickel powder cement is approximately 15%. Around this ratio, the conductive behavior of the material conforms to the tunneling effect theory. Through magnetic field treatment, the resistivity of the material decreases in the direction of the magnetic field and increases in the perpendicular direction. When this material is compressed, its resistance first decreases and then increases. During the elastic stage, the relationship between resistance and strain is nearly linear, with a maximum decrease rate exceeding 60%. The material treated with a magnetic field exhibits a smoother performance under compression. When the material is bent, its resistance also decreases initially and then increases, but the decrease rate is very small, only about 1%. The material treated with a magnetic field demonstrates a more stable resistance change when bent. Based on these experimental results, we have derived the electromechanical constitutive relationship of the material under uniaxial compression and pure bending loads.

Recent advances in molecular dynamics (MD) have significantly expanded the study of bentonite buffering performance in deep geological repositories, extending the research scale from the micrometer to the nanometer level. This has provided crucial data and theoretical insights into material behavior, cross-scale modeling, and optimization. The accuracy of MD simulations largely depends on the appropriate selection of force fields and initial structures, with the development of clay-specific force fields (e.g., ClayFF) and databases providing a solid foundation. However, parameter adjustments for specific clay types are necessary to ensure computational precision. MD simulations have uncovered the molecular mechanisms behind montmorillonite expansion under various environmental conditions, challenging traditional models that simplified montmorillonite as a "parallel capacitor" and offering potential for refining theoretical models. These studies also link interlayer configurations to overall mechanical properties, supporting more accurate predictions of radionuclide migration and informing buffer material development. Additionally, the introduction of granular and discrete element methods has overcome the scale limitations of traditional MD, enabling more effective mesoscopic modeling of bentonite. Such models offer significant advantages in simulating aggregate expansion, permeability, and deformation, improving the accuracy of particle interaction and soil-water behavior. However, a unified modeling framework is still a key area of focus in current research.

Looking ahead, future research should prioritize the development of new force fields that can simulate chemical processes, the creation of microstructural models to describe complex hydro-mechanical behaviors, and the integration of MD with multi-scale numerical platforms to predict long-term buffer performance. This integrated approach will provide important scientific support and technical guidance for the construction of geological repositories.

Both domestic and international scholars have conducted extensive research on the adhesive performance between organic adhesives and cement-based materials from a multi-scale perspective. Significant progress has been made in understanding adhesion mechanisms and enhancement techniques. However, research on the driving forces of adhesion at the micro-nano scale remains relatively underdeveloped, especially when compared to the adhesion mechanisms observed with metal materials. Furthermore, there are still several pressing issues regarding the environmental adaptability of adhesives and cement-based materials, which demand higher standards for their long-term reliability in practical applications. Although researchers have gained a deeper understanding of the mechanism of mechanical interlocking and clarified the corresponding intermolecular interactions, there is still a need to further analyze the individual contributions of mechanical interlocking and intermolecular interactions to overall adhesion. From a thermodynamically driven molecular adhesion mechanism perspective, optimizing the molecular structure of adhesives remains an area worth exploring. Existing studies typically link macroscopic adhesion strength directly with intermolecular interactions at the nano-scale, such as hydrogen bonds, while overlooking the important role of the mesoscopic mechanical properties of the adhesive interface. Therefore, it is essential to develop a method for characterizing the mesoscopic mechanical properties of the adhesive interface, taking into account the phase composition characteristics of cement-based materials. Furthermore, a more thorough understanding of the quantitative relationship between these properties, macroscopic adhesion strength, and nano-scale chemical composition is needed. The degradation mechanisms of interfacial adhesion of organic adhesives under coupled working conditions remain unclear. How factors such as vibration loads, temperature, and moisture coupling accelerate debonding of organic adhesives from cement-based materials requires further investigation. In addition, the time-dependent damage behavior of interface adhesion under service conditions should be explored in greater detail. This research is crucial for identifying key parameters that can enhance the durability of concrete structures, thereby providing technical support for ensuring the long service life of infrastructure.

The South China Sea islands and reefs have emerged as key regions with far-reaching implications in multiple domains, including national defense, maritime trade, resource exploration, and ecological conservation. The rapid expansion of infrastructure in this area has made concrete structures the predominant choice due to their versatility and strength. However, the local environment presents an array of formidable challenges. The durability of these structures is thus under constant threat, with potential consequences for the integrity and functionality of the entire infrastructure network. Understanding and enhancing the durability of concrete structures in such a harsh environment is not only essential for the immediate success of ongoing projects but also for the long-term sustainable development and strategic positioning of the South China Sea islands and reefs.

To comprehensively study the durability and service life of reinforced concrete structures in the South China Sea islands and reefs environment, a long-term exposure test station was established in this area. This test station was designed to closely simulate the actual environmental conditions of the South China Sea islands and reefs, providing reliable test conditions for studying the durability of concrete structures.

In the experiment, different types of concrete specimens and components were prepared. These included specimens with different corrosion inhibitors and different concrete strength grades. Electrochemical tests were carried out to monitor the corrosion status of reinforcing bars. The natural corrosion potential and polarization resistance of the reinforcing bars were measured using advanced electrochemical testing equipment. Mechanical property tests were also performed to evaluate the mechanical performance of the concrete structures. Compressive strength, flexural strength, and tensile strength tests were conducted on the concrete specimens and components. Chloride ion content analysis was carried out to understand the penetration and distribution of chloride ions in the concrete. The ChaDuraLife model was used for service life prediction. This model takes into account various factors such as chloride ion diffusion, concrete strength, and environmental conditions to predict the service life of the concrete structures.

The experimental results showed that different corrosion inhibitors had different effects on the corrosion of reinforcing bars in concrete. The SBT-KLJ(Ⅳ) hydrophobic pore-filling agent (HPA) exhibited excellent anti-corrosion performance. After 1227 d of corrosion, the natural corrosion potential of the steel bars in the concrete specimens with HPA shifted positively, and the polarization resistance continuously increased during the erosion period from 365 d to 730 d. In the chloride ion erosion environment of the islands and reefs, adding HPA reduced the chloride ion content on the concrete surface by approximately 59.9% compared to concrete without the corrosion inhibitor.

The mechanical property tests of the concrete components after on-site exposure experiments indicated that the durability of concrete structures with different strength grades varied significantly. C80 concrete showed better resistance to seawater erosion and mechanical properties than C50 concrete. The service life prediction results based on the ChaDuraLife model demonstrated that adding a rust inhibitor or increasing the concrete strength grade could effectively extend the service life of the reinforced concrete structure in the splash zone environment of the South China Sea islands and reefs. The HPA rust inhibitor was particularly effective, and under certain conditions, it could extend the service life of the concrete structure to over 50 years.

In conclusion, the research findings have several important implications. The HPA corrosion inhibitor has demonstrated excellent long-term stability and anti-corrosion performance in the South China Sea islands and reefs environment. Its ability to enhance the electrochemical stability of reinforcing bars and reduce chloride ion permeability makes it a preferred choice for marine engineering applications. The significant improvement in the durability of concrete structures with HPA inhibitor has been clearly established through experimental and modeling results. The differences in durability among different strength grades of concrete highlight the importance of appropriate material selection for the specific demands of the South China Sea islands and reefs infrastructure. C80 concrete's superior performance in terms of chloride ion resistance and mechanical properties indicates its suitability for critical structures where enhanced durability is required. The comparison with the national standard emphasizes the need for further research and innovation to develop more effective strategies for improving the durability of reinforced concrete in this harsh environment. This could involve exploring new admixtures, optimizing concrete mix designs, or enhancing construction and maintenance practices. The data and insights obtained from this study provide a solid foundation for future research and engineering applications, guiding the design and construction of more durable concrete structures in the South China Sea islands and reefs. Ultimately, these efforts will contribute to the long-term safety and functionality of the infrastructure in this strategically important region.

In application, different types of concrete admixtures are usually mixed to solve different problems. The varying adsorption capacities of organic molecules or polymer additives on particle surface of different mineral phase result in "competitive" or "preferential" behaviors, leading to intricate interference or synergistic effects among the additives. Competitive adsorption occurs among retarders, viscosity modifying admixtures (VMA), and superplasticizers, especially when the surface coverage is relatively high. The investigation of competitive adsorption relies on the quantification of each type of admixture based on the special characteristic signal of each admixture (e.g. signal by light scattering or gel permeation chromatography due to the clear difference of solution size between VMA, polycarboxylate superplasticizer (PCE), and retarder molecules; signal of phosphorus in some P-containing retarders). Labeling of PCE molecule by chromophores is an effective strategy. However, the low signal intensity of the labeled PCE in the few former reports limited the universality. Herein the paper reported the synthesis of a labeled PCE by the grafting of naphthylamine onto backbone as chromophore. The mixture of naphthalene-ring-labeled PCE and conventional PCE could be quantified based on UV absorption spectra and total organic carbon (TOC) method. The effect of adding two PCEs with different side chain lengths on the fluidity of cement paste was studied.

Jiangsu Helin P·Ⅱ 52.5 Portland cement with density of 3.06 g/cm³ and BET specific surface area of 0.83 m²/g was used. The labeled PCE (PCE-A) was prepared by the simultaneously grafting naphthylamine (99%, Aladdin reagent) and polyetheramine (monofunctional, primary amine, with a terminal methyl group, from Zhejiang Lukean Chemical Co., Ltd) to polyacrylic acid (weight-averaged molecular weight M_w 2000) in the presence of 1% (in mass) H₂SO₄ under vacuum at 130 ℃. The molar ratio of chain unit as confirmed by ¹HNMR spectra was acrylic acid/naphthylamine/polyetheramine = 4.73/0.70/1.00. PCE-B (poly(methacrylic acid) grafted with poly(ethylene glycol) monomethyl ether (MPEG, M_w 1000), molar ratio of methacrylic acid to MPEG was 3/1) was supplied by Jiangsu Sobute New Materials Co., Ltd.

The UV absorption spectra was recorded between 200-400 nm. All the cement pastes were prepared with water to cement ratio of 0.18. The flow spread of cement paste (GB/T 8077—2012) with the addition of only PCE-A, only PCE-B, and both PCEs was measured, respectively. The total adsorption amount of PCE was measured by TOC method. The adsorption amount of PCE-B when both PCEs were added, was calculated based on UV absorption spectra. The standard curve (UV absorbance at 305 nm against TOC concentration) was obtained by the supernatant of cement paste when only PCE-B was added. The amount of ettringite (AFt) was calculated based on total heat absorbed between (40-105 ℃) in differential scanning calorimetry (DSC) as compared with synthetic AFt. The morphology of AFt was investigated by scanning electron microscope (SEM).

The main UV absorption band of PCE-A (naphthene ring) located between 250 nm and 320 nm, which enabled the differentiation with the background from the pore solution. The quantification by the UV method was confirmed by the supernatant of cement paste with only PCE-B. An excellent linear relationship (correlation coefficient of over 0.99) was found between absorbance at 295, 305 nm and TOC concentration.

When only one type of PCE was added, the flow spread of cement paste achieved a maximum value at ~6 mg/g, then slight decrease could be observed at higher dosage. The maximum flow spread of cement paste with PCE-A (lower adsorption affinity, but higher saturated adsorption amount) is much larger than PCE-B, indicated a stronger steric hindrance, due to the longer side chain. When both PCE-A and PCE-B were added, at PCE-A of 2 mg/g and 4 mg/g, the flow spread of cement paste underwent a gradual increase and then decrease (especially after PCE-B of 4 mg/g) with the increase of dosage of PCE-B. At PCE-A of 6 mg/g, lower flow spread would be found at higher dosage of PCE-B. Regardless of PCE-A dosage, the adsorption amount of PCE-A would be lower at higher PCE-B dosage. The total adsorption amount always became lower at higher dosage of PCE-B, except for the condition of PCE-A 2 mg/g and PCE-B lower than 4 mg/g. The behind reason was, PCE-B could occupy the adsorption site for PCE-A. At fixed PCE-A dosage, the total surface coverage always increased with the addition of PCE-B. However, at high PCE-A dosage, the replacement of surface-adsorbed PCE-A would reduce the steric hindrance.

The increase of PCE dosage induced lower amount of AFt with finer morphology. The size of AFt particle with PCE-B is smaller than PCE-A, due to the even higher adsorption affinity for the inhibition of particle growth. When both PCEs were added, high amount of PCE-B would promote the transformation of AFt particles from rod-like to fine needle-like. The aspect ratio of AFt increased from ~10.6 (PCE-B 2 mg/g) to ~14.1 (PCE-B 6 mg/g). The increase of aspect ratio would result in worse particle packing and therefore lower flow spread.

A PCE labeled with naphthalene ring (UV chromophore) was prepared based on the grafting reaction of naphthylamine, which enabled the quantification of solution concentration in cement paste by absorbance at ~300 nm and therefore the investigation of competitive adsorption with another PCE. At extremely low water to cement ratio (0.18), as the dosage of short side chain PCE increases, the adsorption amount gradually increases, while the adsorption amount of long side chain PCE gradually decreases. At high dosage of PCE with long side chain, with the increase of dosage of short side chain PCE, part of the sites that could have been occupied by long side chain PCE are occupied by short side chain PCE. In addition, the morphology of ettringite (AFt) changes from rod-like to finer needle-shaped, showing a larger aspect ratio, which will deteriorate the packing behavior. The fluidity of the cement paste is therefore reduced.

Selective Catalytic Reduction (SCR) technology, due to its high efficiency and applicability, has become a core technology for NO_x emission reduction in industries such as steel, cement, and glass. As these industries increasingly rely on SCR technology, the recycling and high-value utilization of spent catalysts have become an urgent issue that needs to be addressed. When spent catalysts are difficult to regenerate, the recovery of valuable components becomes the preferred option.

Thermal treatment methods dominate the recycling process due to their high extraction rates but also bring about issues of high energy consumption and environmental pollution. In contrast, hydrometallurgical processes are favored for their simplicity, low cost, and high single-metal extraction rates, yet their development is limited by the low extraction rates of leaching agents for multiple valuable elements and the leaching of many impurities. Therefore, the integration of multiple processes and the development of high-value-added products represent the future direction for SCR catalyst recycling.

In the recycling process of SCR catalysts, the key lies in the efficient extraction of vanadium (V), tungsten (W), and titanium (Ti) while reducing energy consumption and environmental harm. Both acid and alkali methods can effectively recover over 90% of V, but given the toxicity of V, the use of a reducing acid leaching method is recommended to prioritize vanadium extraction, facilitating the preparation of high-purity V₂O₅. The recovery of W and Ti is more challenging, especially for W, whose leaching rate is limited by the choice and concentration of solvents, with low-concentration alkali leaching rates typically around 70%. Ti is only soluble in concentrated alkali solutions, and due to the high proportion of titanium in spent catalysts, its extraction would consume a large amount of reagents, leading to high costs. Thus, the most reasonable method is to use alkali to leach W while controlling the alkali concentration to retain as much Ti as possible in the residue. The alkaline leachate contains high levels of silicon and aluminum impurities, which need to be purified before the recovery of products. Additionally, the presence of heavy metals in spent catalysts requires targeted adjustments to the recycling process to enhance recovery rates and purity.

Currently, research on the recycling and utilization of spent vanadium-titanium catalysts focuses on the impact of operating conditions and reaction mechanisms on recovery rates, with insufficient evaluation of energy consumption and environmental impacts during the recycling process. Future studies could employ life cycle assessment and material flow analysis to evaluate the environmental impacts and resource efficiency of spent catalyst recycling. Moreover, the development of new and efficient leaching agents, optimization of thermal and hydrometallurgical process conditions, and exploration of high-value utilization pathways for spent catalysts, such as the preparation of new materials or as raw materials for other industrial processes, will be key to advancing SCR catalyst recycling technology.

Photothermal catalysis, based on the photothermal conversion to drive the reaction, shows large potentials in various applications like CO₂ reduction, organic pollutant degradation, organic synthesis, and hydrogen production via water splitting is the synergy of photochemical and thermochemical effects. This review summarizes the mechanisms of photothermal catalysis, including the direct process by infrared radiation absorption and the indirect process via non-radiative carrier relaxation. Moreover, the indirect way could be further divided into plasma conversion and non-plasma conversion. Based on the contributions of photocatalysis and thermocatalysis, photothermal catalysis is classified into three modes: Photo-assisted thermal catalysis, thermal-assisted photocatalysis, and photothermal co-catalysis. Metals, semiconductors, carbon-based materials and metal-organic frameworks are common photothermal materials. Various modification strategies are developed to promote the catalytic reactivity of photothermal materials, including improving solar energy utilization, accelerating the separation rate of high-energy charge carriers, and strengthening the thermal management. Though photothermal catalysis shows great advantages in comparison with the traditional thermocatalysis and common photocatalysis, it still faces a series of challenges especially in large-scale applications. Several research directions could be considered to the development and application of photothermal catalysis in the future.

The development of efficient, cost-effective and robust photothermal materials is the central theme in photothermal catalysis. At present, common photothermal materials focus on metals with LSPR effect, semiconductors and carbon-based materials. Emerging two-dimensional materials such as phosphorene, borene, MXene are capable of harnessing both visible and infrared light and also exhibit high photothermal conversion efficiency. These materials hold great potential as light absorbers in photothermal catalytic systems. Additionally, covalent organic framework materials with large conjugated systems also provide a new avenue for the development of high-efficiency photothermal catalysts.

The mechanism of photothermal catalysis need to be further clarified. The understanding of the insight of reaction could guide the construction of photothermal catalytic system. At present, the key research focuses on the synergestic effect between high-energy carriers and photo-derived thermal energy. More details about the exact energy conversions like light to charge carriers or non-radiative relaxation could be figure out. Furthermore, it is pivotal to distinguish the contribution of photocatalysis and thermocatalysis in photothermal catalytic reaction. The rational design of experiments, specific characterizations like in-situ techniques-including in-situ Raman, XPS, AFM, etc. and first principle calculations might provide the possible solutions to discern the catalytic mechanism.

To address the limitations of photothermal catalysts, it is highly desirable to explore simple and effective modification strategies to enhance the absorption and conversion of full-spectrum solar energy. Rational structural modulation of photothermal catalysts is essential to ensure efficient separation of the generated high-energy carriers and then apply to the subsequent reaction. Additionally, reducing the heat loss during photothermal catalytic processes is also a critical research priority. This requires balancing the energy utilization and heat dissipation while optimizing reaction pathways to achieve energy-efficient and sustainable catalytic systems.

The application of photothermal catalysis still requires further investigation, particularly in CO₂ reduction and C-C coupling to generate high-value C₂₊ products with high selectivity, which are the critical approaches to achieve the carbon neutrality.