Calcium sulfate hemihydrate is widely used in the construction and industrial fields, and its application effect largely depends on its dispersibility. Early studies mostly used anionic polymer dispersants to improve its performance. Later, with the application of polycarboxylate superplasticizers (PCE) in the cement and concrete fields, the exploration of its application in half-water gypsum began. Research shows that the charge characteristics and side chain structure of PCE have a significant impact on its adsorption behavior and dispersibility. Introducing functional groups using strong complexation ability of Ca²⁺ can improve its dispersibility. However, the mechanism of the influence of PCE molecules' adsorption behavior and dispersibility in calcium sulfate hemihydrate paste with high SO₄^2- concentration and weak acidity is still unclear. Therefore, three types of PCE containing carboxyl groups, sulfonic acid groups and phosphate ester groups were synthesized. Through various means, the dispersibility and interface interaction mechanism of these PCE in the calcium sulfate hemihydrate system were systematically studied, providing a theoretical basis for the development of high-performance PCE molecules.

Using MAA, SMAS and HEMAP as the main chains, PCEs containing carboxylic acid groups (PCE-C), sulfonic acid groups (PCE-S) and phosphate groups (PCE-P) were synthesized. The synthesized water reducers were subjected to tests such as PCE molecular structure characterization, gypsum paste test, hardened gypsum performance test, simulated gypsum pore solution test, quantum chemical calculation and molecular dynamics simulation.

Through comprehensive analysis of ¹H NMR spectrum and FTIR spectrum, it can be known that carboxylic acid groups, sulfonic acid groups and phosphate groups were successfully introduced into the three PCEs respectively. GPC showed that the conversion rate of the monomers was above 90%. The performance test of gypsum paste showed that the three water-reducing agents all had dispersing effects on gypsum, among which the dispersing effect and retention effect of PCE-S were the best. After introducing Na₂SO₄ (SS), the influence on the other two water-reducing agents was significant, while PCE-S still showed good fluidity. The water-reducing agents prolonged the setting time of gypsum, while the introduction of SS reduced the setting time of gypsum to a certain extent. PCE-S exhibited better crystal growth regulation ability and had a relatively small impact on the strength of the gypsum hardened paste. The introduction of SS led to a slight decrease in the absolute values of the zeta potential of PCE-S and PCE-P, indicating high adsorption stability in the presence of sodium sulfate. In the adsorption capacity test, PCE-S showed a higher saturated adsorption capacity, and it was least disturbed by SS. The introduction of SS significantly increased the particle size of PCE and broadened the particle size distribution, weakening the electrostatic stability between molecules. The pH titration test showed that PCE-S and PCE-P had higher charge density in the gypsum system. The static charge density distribution of gypsum crystal surfaces showed that the sulfonic acid groups induced obvious electron cloud rearrangement and structural reconstruction on the gypsum surface, and through the "cooperative adsorption effect" induced by surface reconstruction, its macroscopic adsorption capacity and dispersion effect could be significantly enhanced.

The type of PCEs adsorption groups significantly affects its dispersion performance in the calcium sulfate hemihydrate system. PCE-S shows the best improvement in fluidity and resistance to sulfate interference, followed by PCE-P and PCE-C, with the poorest performance. The degree of ionization and apparent charge density of PCE in the calcium sulfate hemihydrate environment play an important regulatory role. PCE-S and PCE-P almost completely ionize at the experimental pH, for both having a higher apparent charge density and enhancing electrostatic adsorption with Ca²⁺ and the degree of molecular unfolding, thereby improving its dispersion stability. The sulfonic acid groups achieve enhanced "cooperative adsorption effect" by inducing significant electronic rearrangement and crystal plane reconstruction on the (001) surface of calcium sulfate hemihydrate, thereby improving its performance in macroscopic adsorption capacity and dispersion properties. Additionally, PCE-S possesses a higher affinity for Ca²⁺, evidenced its stronger complexation capacity and superior competitive adsorption against SO₄^2-. Its increased steric hindrance and high surface coverage further lead to enhanced dispersion efficiency.

Silane-impregnation is one of the most used surface waterproofing treatments for concrete structures to improve the durability, but long-term engineering observations find that silane-impregnation will deteriorate during the service life of concrete structures. The deterioration of silane treatment undermines its effectiveness in protecting the concrete structures and improving their durability. However, the deterioration mechanism of silane-impregnated treatment has rarely been studied. To fill this gap, This paper studied the deterioration mechanism of surface hydrophobicity of silane-impregnated cement-based materials through laboratory accelerated tests. The accelerated tests include drying-wetting cycles by pure water and ultraviolet (UV) irradiation. Then, the silane-impregnated materials are characterized by infrared spectroscopy, pyrolysis gas chromatography-mass spectrometry, thermal analysis system, and dynamic water vapor desorption. The aim of this study is to provide a research basis for quantifying the usage life of surface silane impregnation treatment used in engineering.

Laboratory accelerated deterioration tests were conducted to study the deterioration mechanism of surface hydrophobicity of silane-impregnated cement-based materials. The accelerated tests include drying-wetting cycles by pure water and ultraviolet (UV) irradiation. For drying-wetting cycle tests, an experimental design of 4 h wetting time and 8 h drying time was adopted. The irradiance of ultraviolet (UV) irradiation tests is irradiance, and the irradiation cycle is 30 d. After accelerated deterioration tests, the silane-impregnated cementitious materials are characterized by infrared spectroscopy, pyrolysis gas chromatography-mass spectrometry, thermal analysis system, and dynamic water vapor desorption.

Infrared spectroscopy was conducted to study the silane molecular structural changes during accelerated deterioration tests; pyrolysis gas chromatography-mass spectrometry and thermal analysis system was conducted to characterize the silane content in cementitious materials; and dynamic water vapor desorption was conducted to characterize the hydrophobic performance of impregnated cementitious materials.

Under the action of drying-wetting cycles: 1) infrared spectroscopy results of impregnated cementitious materials showed that the relative content of different alkyl groups had not changed, indicating that the molecular structure of silane was stable; 2) Both pyrolysis chromatography-mass spectrometry and thermal analysis system results showed that the silane content in impregnated cementitious materials was decreasing, combining with the analysis of infrared spectroscopy results, silane in impregnated cementitious materials was most likely underwent hydrolysis reaction, and the hydrolyzed silane continuously detached from impregnated particles under the action of drying-wetting cycles; 3) In addition, the water vapor adsorption capacity of cement paste powders, the water sorptivity and contact angle of mortar specimens had all increased.

Under the action of UVB-313 ultraviolet radiation: 1) the silane impregnated on the surface of cement-based material particles underwent chemical bond breakage. Based on the wavelength of the ultraviolet radiation, it is inferred that this likely caused the breaking and recombination of tertiary carbon bonds. Infrared spectroscopy, gas chromatography-mass spectrometry, thermal analysis system, and water vapor adsorption capacity results all indicate this aging process; 2) Due to the direct impact of ultraviolet irradiation on the surface of impregnated materials, the water sorptivity and contact angle of the impregnated mortar specimens had changed significantly. However, it should be noted that this significant aging effect depends on the UV wavelength and irradiation energy of UVB-313 lamps using in the laboratory. In the natural environment, the irradiance of sunlight is much lower than that of UVB-313. Therefore, the accelerated UV testing results are primarily suitable for mechanistic research and cannot not be directly applied to the aging behavior of silane impregnations in natural environment.

The stability of cement-based materials impregnated with high-purity silane (isobutyl triethoxysilane) was confirmed through drying-wetting cycles and ultraviolet irradiation. The stability is attributed to the condensation network between silane monomers and the condensation reaction with the surface of cement-based materials (Si—O—Si bonds); The wavelength distribution of natural light and the barrier effect of cement-based materials make the deterioration caused by ultraviolet irradiation less likely to occur in natural environment. The deterioration caused by hydrolysis of silane should be more related to the effect of environmental water. Based on the study of the drying-wetting cycles results and the mechanism of silane hydrolysis, it can be reasonably inferred that the hydrophobicity degradation of cement-based materials surface (layer) impregnated with silane in natural environments is related to the hydrolysis of silane and the migration of hydrolyzed silane.

For the four different cement pastes used in this study, OPC showed the greatest decrease in water vapor adsorption capacity after silane impregnation, indicating that the silane impregnation effect of OPC is better than that of OFA, OSG, and OFS. Combined with the chemical reaction mechanism of silane impregnation, alkaline conditions can catalyze the impregnation reaction of silane. However, pozzolanic reaction, occurred in the three cementitious materials mixed with fly ash or slag, consumes the alkali reserve of the pore solution. Therefore, the alkalinity of the pore solution of OPC is the highest, resulting in the best silane impregnation effect.

Strain Hardening Cementitious Composite (SHCC) is one kind of engineered materials designed based on the principles of mesoscale and fracture mechanics. It exhibits a tensile strain capacity of up to 3%-8%, which are approximately 300~800 times greater than ordinary concrete. SHCC has been successfully used in the rehabilitation of dams and bridge decks in Japan owing to its excellent mechanical properties. These structures withstand not only static loads but also potential dynamic impact loads during service. To accelerate the application of SHCC in seismic-resistant structures, blast-resistant structures and other critical infrastructure, it is essential to have knowledge of its dynamic mechanical properties and failure mechanisms. Therefore, this study characterized the mechanical behavior and failure modes of SHCC under high strain rate loading using Split Hopkinson Pressure Bar (SHPB) testing, with high-speed digital image correlation (DIC) capturing full-field strain evolution on specimen surfaces during impact tests. X-ray computed tomography (X-CT) technology was used to observe the internal damage of SHCC after high strain rate loading. The influences of strain rate on the dynamic peak stress, energy absorption capacity, dynamic growth factor, full-field dynamic strain distribution, internal damage form and microstructure of SHCC were investigated.

This study adopted Ordinary Portland cement 42.5 (GB175—2023, with an average particle size of 27 μm and the particles of 3-30 μm account for about 70%) and Class I fly ash (GB1596—2017, with a particle size less than 10 μm account for more than 80%) as cementitious materials. The quartz sands with a maximum particle size of 0.3 mm were used as fine aggregates. SHCC samples were prepared using PVA fiber produced by Kuraray, Japan. Meanwhile, the polycarboxylate superplasticizer (provided by Nanjing Subote New Materials Co., Ltd.) was added to enhance the workability, ensuring uniform fiber dispersion of the slurry.

After mixing, the slurry was cast into molds, followed by vibration, and demolded after a 24 h curing period under standard laboratory conditions. Specimens were then cured under standard conditions until testing age. Static compression tests were employed using 70.7 mm cube specimens. Moreover, dynamic properties were evaluated using a SHPB system with ϕ48×25 mm cylindrical specimens. During impact loading, high-speed industrial camera was used to captured specimen deformation for subsequent VIC-2D digital image correlation analysis. The damaged SHCC samples after high strain rate loading were scanned using X-ray computed tomography and observed with scanning electron microscope (SEM).

The dynamic compressive stress-strain curve of SHCC presents typical three-stage characteristics under high strain rate loading: linear elastic deformation stage, nonlinear strain hardening stage and damage softening stage. Dynamic compressive stress-strain curves of SHCC exhibit strong dependence on strain rate. At higher strain rates, the dynamic compressive response of SHCC exhibits significant strain hardening, primarily governed by fiber bridging effects and secondarily enhanced by compaction of micro-defects. The dynamic peak stress of SHCC increases significantly with strain rate, demonstrating a pronounced strain rate sensitivity. Simultaneously, the dynamic peak stress of SHCC increases substantially at high strain rates. This enhancement primarily arises from the significantly shorter stress wave propagation time compared to the characteristic relaxation time of internal defects under high strain rate loading. Consequently, stress waves preferentially propagate through high-strength regions (e.g., matrix grains and fiber reinforcements), inducing localized stress concentrations. The energy absorption capacity of SHCC increases with the increase of strain rate. When the strain rate exceeds 180 s^-1, E_d shows an approximate exponential growth trend, reaching 546.12 kJ/m³ at 188 s^-1, with a relative increase of about 106.42%, demonstrating exponential growth behavior and significant strain rate strengthening. This energy absorption surge primarily stems from synergistic multi-scale damage mechanisms. At identical strain rates, the dynamic increase factor for stress demonstrates both a higher mean value and greater variability than that for energy, reflecting its stronger dependence on strain rate effects. This indicates greater strain rate sensitivity in dynamic peak stress than energy absorption capacity, confirming the dynamic peak stress is more susceptible to strain rate.

The main conclusions of this paper are summarized as following. SHCC exhibits strain rate sensitivity of dynamic mechanical properties, with both peak stress and energy absorption capacity increasing significantly under high strain rate loading. The sensitivity of dynamic stress to strain rate is significantly greater than that energy absorption capacity. DIC analysis shows that the vertical expansion of the specimen is substantially more significant than the lateral compression. X-CT analysis reveals significantly denser crack distribution near specimen edges compared to the central region. Moreover, multiple cracks appear at the same crack point, which is an essential reason for the excellent dynamic mechanical properties of SHCC.

The predominant characteristics of high-altitude climates include low air pressure, low humidity, and large diurnal temperature fluctuations. In practical engineering scenarios in high-altitude regions, many pier surface cracks only appear several months after erection, and cyclic thermal stress is identified as the main trigger for such cracking. The thermal stress in concrete structures has been investigated for decades but remains incompletely understood. Structural engineers typically regard concrete as an isotropic material and calculate the thermal stress using code-specified coefficients of thermal expansion (CTEs) along with temperature conditions and constraints. Because the CTE of hardened cement paste is more than twice that of many aggregates, reducing the CTE of coarse aggregates can further exacerbate the thermal deformation incompatibility between the coarse aggregate and mortar matrix. In this paper, a comprehensive thermal-elastic mechanics model for pier concrete was developed to analysis the temperature-induced surface stress.

A series of mechanical and thermophysical tests were conducted on the diorite aggregate, ITZ cement paste, and mortar, and concrete. A test pier was constructed on open ground near the Yarlung Zangbo River at an altitude of 3800 m. The pier had a diameter of 1.8 m and height of 2 m. Temperature sensors were embedded in the cross-section at a height of 1 m, positioned along the south-north and east-west directions. The embedding depths (distances from the pier surface) were 0, 1.5, 3, 4.5, 6, 7.5, 9, 12, 15, 20, 25, 30, 35, 40, 50, 60, 70, 80 cm, and 90 cm.

A model of the bridge pier concrete for surface-level analysis was constructed. The model consists of a concrete unit formed as a sphere-shell-shell composite, including the aggregate, ITZ, and mortar layers, embedded in the surface layer of a bridge pier. Outside this unit, the pier concrete was treated as an isotropic, homogeneous elastic material. The real-time internal temperature fields of high-altitude concrete bridge piers, measured on-site, were incorporated into the model. By applying thermoelastic mechanics theory and finite element solutions for plane strain problems, the three-dimensional thermal stresses on the surface layer of high-altitude bridge piers were analyzed.

During the experimental period, the lowest and highest temperatures on the bridge pier in the high-altitude region were 9.6 ℃ and 42.6 ℃, respectively. These occurred before sunrise and sunset on sunny days, respectively, corresponding to the local maximum temperature gradients during the surface heating and cooling stages, as well as the maximum temperature difference between the surface and center during these stages. The thermal stress on the pier concrete surface was obtained by superimposing the stresses caused by the uneven distribution of the internal temperature field and those caused by the incompatible thermal deformation among the different components in the surface concrete

Before the erection of the upper structures, the absolute values of the tangential and vertical stresses were the same; therefore, only one curve was observed. From 22:00 to 8:00, the pier concrete surface was in tension, whereas from 11:00 to 22:00, the pier concrete surface was in compression. The surface of the pier concrete was subjected to biaxial forces of equal magnitude with a maximum compressive stress of 12.52 MPa and maximum tensile stress of 2.15 MPa, respectively at 18:00 and 8:00. According to the fatigue equation, the concrete was predicted to crack after 21 d of temperature cycling. Moreover, if humidity-induced stress is added on top of this, the tensile stress may approach or even exceed the concrete's tensile strength, thereby posing a significant risk of cracking.

After the erection of upper structures, the tangential and vertical stresses no longer coincide because the upper structures have been erected. The curve of the tangential stress is unchanged, whereas the curve of the vertical stress is translated downwards by 1.57 MPa due to the structural deadweight. Therefore, the maximum tangential compressive stress remained 12.52 MPa, whereas the maximum vertical compressive stress increased to 14.09 MPa. Additionally, the maximum tangential tensile stress was 2.15 MPa, and the maximum vertical tensile stress was 0.58 MPa. According to Appendix C of GB/T 50010 and the fatigue equation, stresses are unlikely to cause cracking of the pier concrete surface.

Although a higher CTE of the coarse aggregate slightly increased the maximum compressive stress, the differences among the three groups of concrete were minimal and could be ignored. Specifically, the maximum compressive stresses on the pier concrete surface were 12.54, 12.45 MPa, and 12.56 MPa when using diorite, limestone, and basalt, respectively. By contrast, a lower CTE of the coarse aggregate results in a greater maximum tensile stress on the pier concrete surface. For example, when using limestone, which has a low CTE, the maximum tensile stress on the pier concrete surface is 2.28 MPa, compared to 2.17 MPa when using diorite and 2.14 MPa when using basalt.

The finite element simulation results indicated that the maximum compressive stress on the pier concrete surface was 11.72 MPa, whereas the maximum tensile stress was 2.10 MPa. These results are approximately consistent with the theoretical calculations. This consistency provides mutual verification.

Surface cracking in pier concrete occurs predominantly before the erection of upper structures. Under sunny conditions, the orthogonal decomposition of the superficial stress revealed that the maximum compressive stress during the day was approximately 12.52 MPa, whereas the maximum tensile stress was approximately 2.15 MPa. This tensile stress approached the tensile strength of the C35 concrete under biaxial tension. The risk of cracking increased significantly when humidity-induced stress was considered. After the erection of upper structures, the maximum tangential tensile stress on the pier surface remained at 2.15 MPa while the maximum vertical stress decreased to 0.58 MPa, both of which are well below the tensile strength of C35 concrete under biaxial tension. Although the use of coarse aggregates with a lower coefficient of thermal expansion reduced the tensile stress induced by temperature gradients, it increased the stress owing to material deformation incompatibility, leading to a slight increase in the maximum tensile stress on the pier concrete surface.

As the depth of resource exploitation in our country increases, power shock disasters occur frequently in underground engineering, which will severely restrict the rapid development of the economy. In the process of strengthening the foundation with fly ash concrete, the strengthened body not only bears static loads but also dynamic loads, forming initial defects such as joints and cracks inside. The development, extension, and penetration of cracks under dynamic loads will further reduce the stability of the strengthened structure. Therefore, it is urgent to deeply study the mechanical properties of fly ash concrete with cracks under dynamic loads.

Dynamic impact tests were carried out with the split Hopkinson pressure bar (SHPB) test system on the samples with different fracture angles. Digital image correlation system (DIC) was used to monitor the deformation of surface strain. Characteristics of the samples with different fracture angles after dynamic impact were compared and analyzed including the dynamic strength, energy dissipation and fractal fracture.

The results show that with the increase of the crack angle, the peak stress-strain presents a typical V-shaped pattern, reaching the minimum value at 30°. With the increase of fracture angle, the initial damage zone also moved from the fracture tip to the middle of the fracture, and then the damage range gradually expanded until the formation of macro failure. The failure modes are tensile failure (0°), shear failure (30°), tension-shear mixed failure (45° and 60°) and collapse failure (90°), respectively. The dissipated energy of the sample firstly increase and then decrease with the increase of the fracture angle, reaching the maximum value at 30°. The process of energy accumulation and transformation in fractured rock mass is characterized by the strain energy density. With the increase of the crack angle of the sample, the fractal value of the sample first increases and then decreases, and reaches a maximum value at 30°, indicating that a higher degree of sample breakage associated with dissipates relatively large energy. Based on fracture mechanics, the analytical solution of stress around the crack is given, and the variation of crack initiation angle, stress distribution and energy dissipation under different crack angles is theoretically explained, which is in good agreement with the experimental measurements.

This paper uses the SHPB and DIC systems to conduct dynamic impact tests on specimens with cracks at different angles, and compares and analyzes the dynamic strength, energy dissipation, and fracture fractal characteristics of specimens with different crack angles after dynamic impact, and gives the theoretical results of the stress around the crack. To provide some theoretical support for the deformation control of cracked concrete mixed with fly ash under dynamic action.

Ultra-High-Performance Concrete (UHPC) demonstrates substantial potential in civil engineering applications owing to its superior mechanical characteristics and enhanced durability. The material's exceptional workability and structural adaptability make it particularly effective for thin-layer strengthening of aging concrete structures. Pumping technology has become prevalent in construction operations due to operational efficiency and site adaptability. Existing investigations have documented performance variations in conventional concrete materials under diverse pumping conditions. These studies indicate material-specific response patterns, with post-pumping mechanical properties exhibiting notable disparities across concrete categories. However, the mechanical behavior evolution of UHPC under pumping conditions remains insufficiently explored, particularly given its distinct material composition compared to conventional concrete. This study conducts a controlled experimental investigation to evaluate the mechanical performance variation of UHPC subjected to three transport-casting conditions: non-transported baseline, screw pump transportation, and piston pump transportation. Through systematic comparison of pre- and post-pumping mechanical parameters, the research aims to clarify the pumping-induced performance modifications in UHPC.

With transport methods (direct casting without transport, screw pump transport, piston pump transport), casting directions (random casting, parallel casting, perpendicular casting; Here parallel casting denotes casting along the long axis of prismatic specimens, while perpendicular casting denotes casting perpendicular to this axis), and steel fiber volume fractions (0%, 1%, 2%) as test parameters, 11 specimen groups were fabricated. Each group comprised 4 prismatic specimens for flexural strength testing and 4 cubic specimens (cut from the prisms) for cubic compressive strength testing. During flexural testing, electrical resistance strain gauges with a 20 mm gauge length were bonded to the tensile face of specimens to quantify longitudinal strain development within the pure bending zone prior to crack initiation. High-precision displacement transducers were vertically aligned at mid-span to quantify flexural deflection, complemented by dual auxiliary sensors longitudinally mounted on opposing sides of the specimen's tensile surface to monitor elongation within the pure bending zone. Following mechanical testing, prismatic specimens were longitudinally divided into four sections. Each exposed surface was digitally imaged using high-resolution photographic equipment. Subsequent computational analysis quantified the angular deviation between steel fiber orientations and corresponding cross-sectional normals through dedicated image processing algorithms.

Screw pump transport casting significantly influenced fiber orientation in specimens. Compared with directly cast specimens, at a 1% fiber content, the fiber orientation angle of screw-pumped parallel-cast specimens decreased by 9.8° across the full section, with the orientation coefficient increasing from 0.454 to 0.607; at 2% fiber content, the orientation angle decreased by 10.6°, and the coefficient increased from 0.461 to 0.637. Which indicates that screw pump transport with parallel casting significantly enhances fiber orientation in the mixture, with similar orientation effects across different fiber contents. Piston pump transport had no significant impact on fiber orientation. Compared to direct casting, at 1% fiber content, the piston-pumped parallel and perpendicular casting methods reduced fiber orientation angles by 1.9° and 0.2°, respectively, and increased the orientation coefficients by 5.7% and 0.4%, indicating no obvious fiber orientation occurred during piston pump transport. A significant wall effect was observed in fiber distribution across specimen sections: fiber orientation coefficients at the edges were higher than those in the middle for all specimens, with notable differences across casting methods. The difference in orientation coefficients between edge and middle sections reached 30%-35% for directly cast specimens; this was reduced to 5%-10% for screw-pumped parallel-cast specimens but expanded to 35%-45% for perpendicular-cast specimens; piston-pumped specimens exhibited edge-middle differences of 15%-20%. Compression tests showed that UHPC compressive strength decreased by 2.6%-8% after transport, though the reduction was not significantly affected by pumping method, casting direction, or fiber content—primarily attributed to disturbance of the UHPC pore structure during pumping. Flexural tests revealed that at 1% and 2% steel fiber contents, screw pump transport with parallel casting increased flexural strength by 19.9% and 32.1%, respectively, compared to non-pumped direct casting. Flexural strength of piston-pumped specimens remained similar to direct casting, with differences within 6%.

Based on the test results, the recommended values for the steel fiber orientation coefficient in UHPC under different transport and casting methods are as follows: 0.5 for UHPC with direct casting or piston pump transport-casting, 0.62 for UHPC with screw pump parallel casting, and 0.38 for UHPC with screw pump perpendicular casting. When screw pumps are employed, material placement direction should align with the structure's principal stress direction. No directional constraints are required for piston pump applications. The flexural strength of UHPC under different transport methods and casting directions was predicted using the proposed fiber orientation coefficients, and the predicted results closedly matched the experimental measurements.

Shanxi Province, as one of the regions with the largest accumulation of coal gangue in China, faces an urgent demand for efficient resource utilisation of bulk solid waste to promote regional sustainable development. The rapidly developing 3D printing construction technology, with its precise control over material distribution and the advantage of forming complex components without molds, provides an innovative path for the large-scale consumption of solid waste. Existing research focuses on the application of coal gangue as fine aggregate substitute in printing mortar, while studies regarding its utilisation as coarse aggregate in 3D printed concrete remain unreported. It is vital to reveal the micro/mesoscale mechanisms governing the printability and fracture mechanics behaviour of such concrete material, in order to develop low-carbon coal gangue-based concrete materials and advance their engineering applications. This study innovatively uses coal gangue as a coarse aggregate and combines it with 3D printing technology to produce a novel 3D printed coal gangue concrete (3DP-CC), offering a data foundation and experimental reference for the resourceful and intelligent application of coal gangue.

In the experiment, 3DP-CC specimens were prepared with coal gangue contents of 0%, 10%, 20%, and 40% by mass. Firstly, the printing properties were evaluated, including flowability, buildability, extrudability, and printable time. Flowability was measured using a concrete slump cone in accordance with the JG/T 248 standard. Buildability was assessed through cylindrical slump and layer stacking tests. Extrudability was determined by evaluating the continuity and uniformity of the extruded filaments, with measurements of their width and thickness. Printable time was recorded as the duration over which the mixture could be extruded smoothly and consistently. Secondly, the internal microstructures of the 3DP-CC were examined using high-resolution X-ray CT scanning technology, allowing for the analysis of pore and aggregate characteristics. Finally, uniaxial compression and three-point bending tests were performed to determine the compressive and flexural strengths. The anisotropy index λ was introduced to quantify the directional variation in mechanical strength, followed by an analysis of the underlying mechanisms by which printing direction and coal gangue content influence mechanical properties. Failure modes were examined to highlight the critical roles of weak interlayer and interstrip interfaces, thereby providing further insight into the microscopic mechanisms governing the mechanical anisotropy of 3DP-CC.

The test results indicate that both the compressive and flexural strengths of 3DP-CC exhibit a trend of first increasing and then decreasing with rising coal gangue content, accompanied by notable mechanical anisotropy. At coal gangue contents of 10% and 20%, strength improved significantly while anisotropy was reduced. In contrast, a content of 40% led to a decrease in strength and an increase in anisotropy. These findings suggest that an appropriate amount of coal gangue coarse aggregate can provide skeletal support, thereby enhancing the mechanical performance of concrete and mitigating anisotropic effects. However, when the coal gangue content is too high, the low strength and high brittleness of the gangue itself, combined with an increase in medium and large pores, result in diminished overall strength and greater anisotropy.

Coal gangue content also has a pronounced effect on the pore structure of 3DP-CC, with total porosity initially decreasing and then increasing as the gangue content rises. At coal gangue contents of 10% and 20%, the total porosity of the specimens is 1.8% and 2.1%, respectively—both lower than the 3.9% observed in the control 3DPM. However, the total porosity increases to 3.4% in the 3DP-CC-40 specimens. From a microstructural perspective, the morphology of coal gangue aggregates plays a significant role in this change. The aggregates exhibit rough surfaces and angular shapes, which lead to loose packing and reduced efficiency in filling medium and large pores. Consequently, the number of pores with equivalent diameters greater than 1 mm increases in 3DP-CC-40 specimens. Furthermore, the water absorption capacity of coal gangue affects the internal pore structure of the concrete. As the aggregates absorb water, the moisture content in the surrounding matrix decreases, resulting in increased matrix porosity.

In terms of failure modes, the compressive behaviour of 3DP-CC is strongly influenced by the loading direction. When loading is applied on the x-face, the failure pattern on the y-side face shows cracks initiating at both edges, propagating inward, and then extending vertically downward. These cracks often penetrate the interlayer interfaces, leading to a relatively brittle failure mode. In contrast, loading on the y-face or z-face produces a distinctive "π"-shaped crack pattern on the x-face, accompanied by spalling damage on both sides, indicating that the weak interlayer and interstrip interfaces play a critical role in crack development. The failure mode under three-point bending is also dependent on the loading direction. When the x-face serves as the loading surface, all specimens—regardless of coal gangue content—exhibit through-thickness, straight cracks and undergo brittle fracture into two parts. When the y-face is loaded, through-thickness cracks are absent in specimens with 0% coal gangue but present in those with higher contents. For loading on the z-face, only specimens with 40% coal gangue exhibit through-thickness cracks, while others show no visible cracking on the loading surface. These observations suggest that specimens loaded on the x-face exhibit greater brittleness, with cracks more readily propagating along interstrip interfaces.

The coal gangue content significantly influences the performance of 3D printed concrete (3DP-CC). Optimal mechanical properties—both compressive and flexural strength—are achieved at a coal gangue content of 10~20%, attributed to the skeletal support provided by the gangue particles and the associated reduction in porosity. However, at a content of 40%, both strengths significantly decrease due to the inherent weakness and brittleness of coal gangue, its tendency to induce cracking, and the increase in macroporosity. The pore structure exhibits a non-linear response to coal gangue content: total porosity decreases when the content is below 20%, but rises sharply at 40% as a result of insufficient fine aggregate filling, leading to the formation of numerous large pores. 3D printing introduces directional porosity, with the upper layers exhibiting slightly higher porosity. In addition, pronounced mechanical anisotropy is observed, as quantified by the anisotropy index. Increasing the coal gangue content tends to reduce the anisotropy of compressive strength. In contrast, the anisotropy of flexural strength exhibits a non-monotonic trend: it initially decreases and then increases, reaching a maximum at 40% gangue content. Failure is predominantly governed by weak interlayer and interstrip interfaces, along which cracks consistently propagate under both compressive and flexural loading. Coal gangue aggregates influence crack trajectories—cracks either bypass or penetrate the aggregates depending on local strength fluctuations. These findings offer valuable insights for the design of sustainable 3D printed concrete, emphasizing the importance of optimizing coal gangue content and microstructures to balance strength, anisotropy, and resource utilisation.

In recent years, the combination of construction 3D printing technology and ultra-high-performance concrete (3DP-UHPC) leverages the rapid construction and controllable shaping advantages of 3D printing along with the ultra-high strength and high toughness advantages of UHPC. Existing research on 3DP-UHPC, constrained by current printing technologies, has primarily focused on shorter steel fibres (6 mm and 10 mm), whereas the steel fibre length commonly used in engineering UHPC is 13 mm. Additionally, research on the micro- and meso-structural characteristics of 3DP-UHPC and the resulting anisotropic mechanical properties and damage-fracture mechanisms is relatively scarce. Therefore, revealing the multiscale effects of anisotropic compressive mechanical behaviour in 3DP-UHPC is of vital importance. This study employs 13 mm steel fibres to investigate the influence of different fibre volume fractions (0%, 0.5%, 1.0%, 2.0%) on the compressive performance of 3DP-UHPC. By combining digital image correlation technology (DIC) and X-ray computed tomography (CT) methods, we aim to clarify the coupling mechanism between fibre orientation and interlayer/interstrip interfacial defects, providing critical insights for material optimisation and engineering deployment of 3DP-UHPC.

In the experiment, 3DP-UHPC specimens with steel fibre volume fractions of 0%, 0.5%, 1.0%, and 2.0% were prepared. First, a particle packing-based mix design was carried out. By testing the particle size distribution of the raw materials and applying the modified Andreasen-Andersen model, the optimal packing proportions of the mortar matrix components were determined. Subsequently, in accordance with the GB/T 2419—2005 standard, a flow table test was conducted using an electric jolting apparatus to evaluate the printability of mixtures with varying fibre volume fractions. Furthermore, high-resolution X-CT was employed to acquire the internal structure of the specimens, from which microstructural parameters such as pore morphology and steel fibre orientation were extracted. Finally, uniaxial compression tests were performed using DIC to measure the compressive strength and to capture, in real time, the processes of crack initiation, propagation, and coalescence, thereby revealing the evolution characteristics of the failure modes. In this process, the synergistic effects between interlayer interfacial defects and fibre orientation were further analysed, and their coupling mechanisms during damage evolution were investigated, thereby providing deeper insights into the microstructural mechanisms governing the anisotropic mechanical behaviour of 3DP-UHPC.

The test results indicate that in the 3DP-UHPC specimens, a total of 37 irregularly shaped large pores were observed at interlayer and interstrip interfaces, with equivalent diameters ranging from 2000 μm to 3200 μm. These accounted for only 0.174% of the total pore volume, while the remaining 92.195% consisted of nearly spherical or ellipsoidal pores, resembling the typical pore morphology of cast UHPC. Notably, the maximum (3200 μm) and average (264 μm) equivalent pore diameters in the printed specimens were both smaller than those reported for cast UHPC (4500 μm and 700 μm), suggesting that the extrusion process during printing effectively compacts the material and facilitates air removal. Therefore, the total porosity of 3DP-UHPC (2.24%) was lower than that of cast UHPC (3.45%-5.90%).

Steel fibres within 3DP-UHPC exhibit a clear tendency to align along the printing direction (x-axis), with more pronounced orientation in the x-z plane than in the x-y plane. Specifically, 65.2% of fibres were aligned at θ = 75°-90° in the x-z plane, compared to 32.8% at φ = 75°-90° in the x-y plane. This anisotropic distribution stems from the filament geometry, where the height (17 mm) is smaller than the width (36 mm), creating spatial constraints that favour alignment in the vertical cross-section. In total, 3,324 fibres (52%) were oriented within the 60°-90° range, reflecting a strong directional preference induced by the layer-by-layer deposition process.

The compressive strength of 3DP-UHPC exhibits directional anisotropy, influenced by layer interfaces, fibre volume fractions, and fibre orientation. At fibre volume fractions of 0.5%, 1.0%, and 2.0%, compressive strength consistently followed the order x < y <z, with the weakest-to-strongest directional differences being 2.95%, 7.82%, and 19.50%, respectively. These values are all below 20% and notably lower than those reported in earlier studies. The achieved reduction in anisotropy is attributed to the optimisation of printing parameters in this study, particularly adjustments to printing speed and path, which improved inter-filament contact, enhanced pore filling, and mitigated the adverse effects of interfacial defects.

3DP-UHPC outperforms cast UHPC in compressive strength across all principal directions (x, y, and z) at fibre volume fractions of 0%, 0.5%, and 1.0%. This advantage is due in part to the absence of fibre sedimentation, water bleeding, and entrapped air defects that may occur during vibration-based casting. The extrusion-based fabrication in 3D printing mitigates these issues, resulting in improved uniformity and mechanical properties. At a fibre volume fraction of 2.0%, the printed specimens exceeded cast specimens in compressive strength along the z-axis by 10%, while showing slightly lower values in the x- and y-directions.

Increasing the steel fibre volume fraction enhances the compressive strength of both 3DP and cast UHPC. For the cast specimens, an increase in fibre content from 0% to 2.0% led to a 49.9% improvement in compressive strength. In the 3DP-UHPC specimens, the same increase led to strength enhancements of 16.8%, 11.0%, and 37.7% in the x-, y-, and z-directions, respectively, indicating that the reinforcing effect of fibres is directionally dependent. Even at a modest fibre volume fraction of 0.5%, compressive strength stabilised around 133 MPa across all directions, indicating a beneficial role in mechanical consistency. In contrast, fibre-free specimens showed brittle failure dominated by interfacial defects, resulting in greater variability in strength measurements.

The 3D printing process significantly influences the microstructural characteristics and mechanical performance of 3DP-UHPC. Although certain interlayer and interstrip voids are inevitably introduced during the 3D printing process, the extrusion-induced "degassing" effect helps lower the overall porosity, resulting in a denser microstructure in 3DP-UHPC compared to conventionally cast UHPC. Under the combined effects of nozzle extrusion and printing path guidance, steel fibres tend to align along the printing direction, in contrast to the random distribution observed in cast specimens. This directional alignment improves the crack-bridging efficiency of fibres. The compressive failure behaviour exhibits pronounced anisotropy: under loading in the x-direction, cracks preferentially propagate along interlayer interfaces, while in the z-direction, fibre alignment enhances bridging capacity and delays crack growth. The coupled effects of fibre orientation and interfacial quality are primary contributors to this anisotropy. Enhancing the extrusion-interlocking mechanism between adjacent filaments can improve interfacial densification, thereby mitigating mechanical anisotropy. The steel fibre volume fraction also plays a critical role in both strength and stability: increasing the fibre content significantly enhances compressive strength, particularly under loading in the z-direction; in contrast, lower fibre volume fractions, although less effective in improving strength, contribute to greater performance uniformity and structural consistency. This study further elucidates the multiscale mechanisms through which 13 mm steel fibres influence compressive anisotropy in 3DP-UHPC. It identifies key microstructural features, such as pore distribution and fibre orientation, and preliminarily establishes their correlation with macroscopic mechanical behaviour, providing valuable insights for optimising the structural performance of 3DP-UHPC.

Polypropylene (PP) separators are widely used in lithium-ion battery (LIBs) separators due to their excellent mechanical properties, high resistance of electrolyte corrosion, superior ionic permeability, easy processability, and low cost. However, PP separators have poor affinity with electrolytes and lack the function of inhibiting lithium dendrite growth, which limits the performance of LIBs. In this paper, polypropylene/polydopamine (PP-PDA_x) composite separator was rapidly prepared by solution polymerization under the catalytic action of sodium periodate (SP), and its properties were systematically studied. The results show that when the modification time is 4 h and the deposition amount of PDA nanoparticles is 0.075 mg·cm^-2, that is, the PP-PDA₄ separator has the maximum electrolyte absorption rate of 324%, the contact angle drops to 9°, and it also exhibits the best electrochemical performance with an ionic conductivity of 0.51 mS·cm^-1. When a LiFePO₄/PP-PDA₄ separator/Li coin-type battery is assembled, the discharge capacity can still remain at 113.9 mAh·g^-1 after 100 cycles at 1C. In addition, the PP-PDA₄ separator shows a remarkable ability to inhibit lithium dendrite growth.

1.2 g of Tris was dissolved in deionized water, and adding hydrochloric acid solution to adjust pH to 8.5 dissolved a certain amount of dopamine hydrochloride in the prepared Tris buffer solution. The polypropylene separator was rolled into a column and immersed in the solution for 5 min to make the surface of the polypropylene septum was evenly wetted on separator. Added Sodium periodate to the solution and fully steep for 2, 4 h, and 6 h. After that, PP-PDA_x separators were be obtained after drying.

In this paper, the optimal sample is PP-PDA₄ that the polydopamine deposition is 0.075 mg·cm^-2, PP separator as a control group. PP-PDA₄ electrolyte contact angle decreased 82% to 9°, the tensile strength increased from 101 MPa to 110 MPa. And tested the separators electrochemical performance, the ionic conductivity increased from 0.26 mS·cm^-1 to 0.51 mS·cm^-1. Introduced positive and negative electrode materials to assemble the full cell, and the discharge specific capacity of PP-PDA₄ was the highest, which was up to 128.2 mAh·g^-1, and tested the rate performance of the full cell in the range of voltage from 2.5 V to 4.2 V, and the current density from 0.2 C to 2C. the discharge capacity of PP-PDA₄ is increased by 9%, 17%, 30%, and 9% compared with PP at 0.5, 1, 2 C, and 0.2 C. Finally, assembled Li/separator/Li batteries to evaluated the ability of PP-PDA₄ to inhibit lithium dendrites, and PP-PDA₄ could work stably for 400 h and maintain at a lower voltage polarization. Meanwhile, the SEM photographs of the lithium film after cycling showed that the lithium deposition was more uniform.

In summary, PP-PDA_x separator were synthesized rapidly via solution polymerization catalyzed by SP. This strategy exploits the intrinsic properties of PDA to enhance the affinity between the separator and the electrolyte, thereby markedly improving electrochemical performance and concurrently inhibiting lithium dendrite formation. Under the catalytic effect of SP, the optimum modification time is only 4 h. This treatment not only improves wettability but also significantly boosts electrochemical performance, without compromising the separator's integrity.

Rechargeable lithium-ion batteries (LIBs) are critical energy storage devices widely used in smartphones, electric vehicles, and hybrid vehicles due to their high efficiency and versatility. Spinel LiMn₂O₄ stands out as a promising cathode material owing to its eco-friendliness, safety, and cost-effectiveness. However, its practical application is limited by rapid capacity fading during cycling, especially under high current densities or elevated temperatures (55 ℃), caused by Jahn-Teller (J-T) distortion, Mn³⁺ dissolution, and oxygen defects. To address these issues, doping LiMn₂O₄ with elements like Al³⁺, B³⁺, Ni²⁺, or Mg²⁺ is a common strategy to stabilize Mn's average oxidation state (+3.5) and suppress J-T distortion. Concurrently, crystal facet engineering optimizes performance: truncated octahedral particles with dominant (111) facets (low surface energy, dense Mn packing) minimize Mn dissolution, while (100)/(110) facets enhance Li⁺ mobility for improved rate capability. Combining doping and facet control has emerged as a key research focus. For instance, B-doped LiMn_1.96B_0.06O₄ exhibited 110.3 mA·h·g^-1 at 5.0 C with 51.8% capacity retention after 2000 cycles, outperforming undoped LiMn₂O₄. Ni—B co-doping further refined morphology and boosted retention to 70.6%. Similarly, Mg—Al co-doped LiMg_0.03Al_0.10Mn_1.87O₄ retained 76.2% capacity after 1000 cycles at 10.0 C, while Zn—Al doping achieved 70%-75% retention over 2000 cycles. These studies highlight how doping regulates crystal growth, favoring truncated octahedral morphologies with balanced stability and kinetics. Building on this, our work synthesizes LiB_0.08Al_yMn_1.92-yO₄ (y=0.01, 0.02, 0.04, 0.06, 0.08) via solid-state combustion to study Al doping's impact. Results reveal that Al content modulates facet growth: low doping levels yield two-facet (111/100) truncated octahedrons, while higher doping introduces (110) facets, ultimately leading to quasi-spherical particles. This controlled facet producing strategy is designed to optimize structural stability and electrochemical performance for advanced LIBs.

LiB_0.08Mn_1.92O₄ and LiB_0.08Al_yMn_1.92-yO₄ (y=0.01, 0.02, 0.04, 0.06, 0.08) were synthesized by stoichiometric mixing of Li₂CO₃, H₃BO₃, MnCO₃, and Al(OH)(CH₃COO)₂ (total 30 g) with 1.5 g citric acid as a combustion agent. The mixture was ball-milled in ethanol for 10 h, dried, and calcined at 500 ℃ for 1 h. After grinding, it was sintered at 650 ℃ for 6 h. Products were labeled as B_0.08 and B_0.08Al_y. Cathode slurry was prepared by blending active material, carbon black, and PVDF (8∶1∶1) with NMP solvent, followed by 30 min ball milling. The slurry was coated onto Al foil, dried at 80 ℃ for 4 h, and cut into 14 mm discs (active material loading: about 1.0 mg/cm²). CR2032 coin cells were assembled in an Ar-filled glovebox using Li metal anode, LiPF₆ electrolyte (1.0 M in EC/DMC/EMC), and Celgard 2320 separator. Material phases were analyzed via XRD (Cu K_α, 10°-80°). Morphology and microstructure were examined by FE-SEM and TEM. Surface composition and valence states were determined via XPS. Electrochemical tests (Land CT2001A) included cycling (25/55 ℃, 3.0-4.5 V) and rate performance. CV and EIS were conducted using a CHI604D workstation.

XRD analysis confirmed that B_0.08 and B—Al co-doped samples crystallized in the spinel LiMn₂O₄ structure (). B—Al co-doping enhanced crystallinity, with diffraction peaks intensifying until y(Al)≥0.06, beyond which crystallinity declined. A lattice contraction was observed via (400) peak shifts, attributed to smaller B³⁺/Al³⁺(0.0400/0.0535 nm) replacing Mn³⁺(0.0645 nm) and shorter B/Al—O bonds, suppressing J-T distortion. SEM revealed that B—Al co-doping refined particle uniformity and morphology: B_0.08Al_0.02 exhibited truncated octahedral single-crystals with high (111) and minor (100)/(110) facets, while higher Al content (y≥0.04) led to spherical polyhedrons due to reduced facet growth rates. TEM/SAED confirmed improved crystallinity for B_0.08Al_0.02, with distinct lattice fringes (0.476 nm for (111)) and ordered diffraction patterns. XPS verified B³⁺/Al³⁺ incorporation and a higher Mn⁴⁺ ratio (B_0.08Al_0.02∶ Mn³⁺/Mn⁴⁺=47.29%/52.71%), stabilizing the Mn—O framework. Electrochemical tests demonstrated superior performance for B_0.08Al_0.02∶ At 5.0 C, it retained 83.9% capacity after 500 cycles (vs. 71.3% for B_0.08) due to suppressed Mn dissolution and enhanced Li⁺ transport from truncated octahedral morphology. High-rate testing (15.0 C, 20.0 C) showed B_0.08Al_0.02 retained 77.2% capacity after 500 cycles, outperforming B_0.08 (58.2%). Elevated-temperature (55 ℃) cycling revealed B_0.08Al_0.02 maintained 70% capacity retention (200 cycles), attributed to reduced side reactions and structural stability. CV and EIS confirmed faster Li⁺ diffusion (D_Li⁺: 3.14×10^-11 cm²·s^-1) and lower charge-transfer resistance for B_0.08Al_0.02. Kinetic analysis showed a lower activation energy (24.76 kJ/mol vs. 28.53 kJ/mol for B_0.08), indicating an enhanced reaction kinetics.

B—Al co-doped LiB_0.08Al_yMn_1.92-yO₄ (y=0.01, 0.02, 0.04, 0.06, 0.08) cathode materials were synthesized via solid-phase combustion. B—Al co-doping enhanced crystal growth and facet evolution: particles transitioned from truncated octahedrons with two facets (111, 100) to those incorporating (111), (100), and (110) facets as Al content increased. At y(Al)≥0.04, spherical-like polyhedrons dominated. The optimized LiB_0.08Al_0.02Mn_1.90O₄ (B_0.08Al_0.02) exhibited superior crystallinity and well-defined truncated octahedral single-crystals. B_0.08Al_0.02 demonstrated exceptional rate capability and cycling stability. At 1.0 C, 5.0 C, 10.0 C, 15.0 C, 20.0 C, and 55 ℃/5.0 C, initial discharge capacities reached 114.7, 102.8, 86.3, 82.6, 75.3, and 107.4 mA·h·g^-1, respectively. After 500 cycles, capacity retention rates were 83.9% (5.0 C), 81.9% (10 C), 90.3% (15.0 C), 77.2% (20.0 C), and 62.0% (55 ℃/5.0 C), outperforming other samples. CV and EIS analyses demonstrated superior electrochemical properties for the B_0.08Al_0.02, including a higher Li⁺ diffusion coefficient (3.14×10^-11 cm²·s^-1), a lower apparent activation energy (24.76 kJ/mol), and a reduced charge-transfer resistance (114.2 Ω). These results indicate enhanced Li⁺ transport kinetics and improved structural stability.