Compared with conventional construction methods in terms of the building structure, 3D-printing concrete (3DPC) technology has the advantages of formwork-free, low labor costs, and less construction waste. The integration of digital design and automatic construction improvs the construction efficiency. 3DPC technology based on extrusion manufacturing is applied in various scenarios such as bridges, landscape sketch, and buildings. However, the stacking-up method also causes some issues like layered appearance, mechanical anisotropy, and high requirements of constructability. At present, the mechanical properties of 3DPC are investigated experimentally, however, the researches on the mechanical properties of 3DPC mainly focus on the static properties of materials, and a few studies involve the dynamic mechanical properties of 3DPC. For concrete structures, they bear static loads, and inevitably face dynamic load (i.e., earthquakes). As a typical dynamic load, the strain rate of concrete under earthquakes ranges from 10^-4 s^-1 to 10^-2 s^-1, causing a catastrophic damage to engineering structures. It is essential to understand the dynamic mechanical properties of 3DPC for engineering application. In this paper, the dynamic mechanical properties of 3DPC in three orthogonal directions were investigated by uniaxial compression tests, and the influences of strain rate and different directions on the compressive strength, elastic modulus, peak strain, and anisotropy of 3DPC were analyzed.

A 42.5 fast hardening early strength sulfoaluminate cement and silica fume were used as cementitious materials. A local river sand was used as a fine aggregate. A polycarboxylate based water reducing agents with a water reducing rate of > 30% and a tartaric acid retarder were used as additives. A water/cement ratio of 3DPC was 0.29. A concrete printing system was applied to print blocks with dimensions of 220 mm×240 mm×400 mm at an extrusion speed of 1 L/min and a printing speed of 30 mm/s. The printed filament had a width of 30 mm and a height of 10 mm. The moving direction of printing nozzle on the horizontal plane is defined as x direction, where y direction is perpendicular to x direction, and z direction is the gravity direction and perpendicular to the x-y plane. After 28-d standard curing, cylinder specimens were obtained via taking cores from three directions of printed block, and then they were cut into Φ 100 mm×200 mm specimens for compressive strength tests. An electro-hydraulic servo universal testing machine was used to applied dynamic loads. The applied strain rates were 10^-5, 10^-4, 10^-3.5 s^-1, and 10^-3 s^-1.

When the weak interface is parallel to the loading direction, 3DPC is prone to failure along the interface due to the development of microcracks at the weak interface. At strain rates of 10^-5 s^-1 and 10^-4 s^-1, vertical cracks develop along the inter-layer and inter-strip interfaces in the y and z directions. When the strain rate increases to 10^-3.5 s^-1 and 10^-3 s^-1, a diagonal crack appear in the y direction, while cracks in the z direction still appear in the inter-strip interfaces. As the strain rate increases, the deformation rate cannot meet the demand for dissipated energy, leading to an increase of damage in the matrix. Therefore, cracks appear in all three directions in the matrix at higher strain rates.

The dynamic peak stress in each direction has a certain degree of strain rate dependence. The dynamic compressive strength in all directions significantly increases as the strain rate increases. The compressive strength in the y direction is more affected by strain rate, compared to that in the x and z directions. At the same strain rate, the compressive strength in the x direction (F_x) is larger than that of cast concrete (F_C) in the z (F_z) and y (F_y) directions, indicating that 3DPC has significant anisotropic dynamic compressive strengths.

During the elastic stage, pores and defects in concrete are compacted. Their deformation and quantity have a significant impact on the elastic modulus of concrete. When the specimens in the z and y directions are compressed, there are more interfaces perpendicular to the compression direction, and the static elastic modulus in these two directions are smaller. As a result, the growth factors of elastic modulus in these two directions are more sensitive to the strain rate. However, as the strain rate increases, the peak strains in the x, y and z directions stabilize at 0.002.

At the strain rates from 10^-5 s^-1 to 10^-3.5 s^-1, the anisotropy index of 3DPC decreases rapidly with the increase of strain rate. Because more defects are compacted at high strain rates, the differences in material properties in various directions decrease gradually, that is, the anisotropy of printed concrete decreases gradually.

The failure of 3DPC originated from defects at the layers or strips interface parallel to the loading direction, which was the weak part of the printed concrete. Under dynamic loading, the compressive strength of 3DPC exhibited a strain rate-dependence in all the directions with F_x>F_C>F_z>F_y, and the strain rate effect in the y-direction was dominant. The dynamic increasing factors of compressive strength and elastic modulus increased linearly with the logarithm of strain rate ratio. When the loading strain rate increased from 10^-5 s^-1 to 10^-3 s^-1, the dynamic strengths of 3DPC in the x, y, and z directions were increased by 33.47%, 49.88% and 32.90%, respectively, compared to the strength at a strain rate of 10^-5 s^-1. The elastic modulus of printed concrete in the x, y, and z directions was increased by 15.63%, 40.53% and 40.68%, respectively. However, the dynamic strength and elastic modulus of cast concrete were only increased by 31.56% and 13.60%. When the applied strain rate increased from 10^-5 s^-1 to 10^-3 s^-1, the anisotropy coefficient decreased from 5.69 to 2.96. The anisotropy of 3D printed concrete decreased with the increase of strain rate, but the change range was small at a high strain rate.

There are still some problems in HVPE AlN that need to be solved in the future.

The most promising application of HVPE AlN is a large-area free-standing substrate, but the prerequisite for its realization is the development of effective lift-off technology. It is a long-standing challenge and a technical barrier for HVPE AlN application. A fundamental technological tactics to solve the lift-off problem is an idea regarding substrate technology, such as using patterned substrates with a large depth ratio, using template or buffer layers made of two-dimensional materials, or using the both.

Little work on HVPE AlN epilayers with a diameter of 2 inches, a thickness of 100 μm and a TD density of 10⁶ cm^-2 has been reported yet due to the limitations of heteroepitaxy and growth temperatures below 1 400 ℃. The coupled temperature and flow fields as well as the position of the gas-solid interface can affect the uniformity and crystal quality of the AlN epilayers. A problem is an intense stress field caused by heteroepitaxy, which varies with the size of the epilayer. HVPE AlN epilayers with a thickness of several microns can crack, and those with a thickness of several tens of microns can cause a substrate fracture. A solution to the stress problem also can be innovative substrate technologies such as patterned substrates and two-dimensional material buffer layers.

In addition, achieving n-type and p-type doping in HVPE AlN with a high carrier concentration and a high mobility remains a challenge. Specific issues such as nonpolar surface epitaxy and parasitic reactions also need further studies.

Ferroelectric materials are widely used in detection, conversion, storage and information processing. The development of energy-saving, environmental protection, multi-functional, miniaturized lead-free multi-functional ceramics becomes inevitable. However, lead-free ferroelectric ceramics as dielectric ceramic capacitors with a low energy storage density and a poor energy storage efficiency seriously hinder their practical application. The development of ceramic capacitors with a high energy storage density and a high energy storage efficiency thus greatly expands the applications of ferroelectric ceramics in the field of energy storage. Doping rare-earth elements is a simple, direct and effective modification method. Rare-earth elements with special electronic structure and stable luminescence properties are expected to give new luminescent properties and improve the electrical properties of ferroelectric ceramics. Ba_0.85Ca_0.15Ti_0.90Zr_0.10O₃ (BCTZ) lead-free piezoelectric ceramics have a small dielectric loss and superior electrical characteristics. Rare-earth element Er³⁺ is widely used in laser and lighting due to its excellent green luminous intensity. In this paper, the effect of Er³⁺ doping amount on the microstructure, ferroelectric, energy storage and photoluminescence properties of BCTZ ceramics and their internal relations was investigated to broaden the applications in photoelectric materials.

Er³⁺ doped Ba_0.85Ca_0.15Ti_0.90Zr_0.10O₃ (BCTZ:x%Er³⁺, x=0.0, 0.2, 0.4, 0.6, 0.8 and 1.0, in mole fraction) ceramics were prepared by a high-temperature solid-state reaction method. CaCO₃ (99.9%), ZrO₂ (99.9%), TiO₂ (99.9%), BaCO₃ (99.9%), Er₂O₃ (99.9%) were used as raw materials. The raw materials were firstly mixed and ground in a ball mill for 12 h, and then pre-burned at 1 100 ℃ for 6 h after drying The pre-burned materials were naturally cooled to room temperature, granulated and pressed. The pressed samples were sintered at 1 300 ℃ for 6 h. The crystal structure of ceramic samples was analyzed by a model D8 ADVANCE X-ray diffractometer (XRD, Germany). The surface morphology of the samples was characterized by a model JEM-7800F scanning electron microscope (SEM, JEOL Co., Japan). The electrical properties of ceramics were tested by a model TF Analyzer 3000E (Germany), and the both sides of the ceramics were plated with silver electrodes before testing. The luminescent properties of ceramics were determined by a model FS5 fluorescent spectrometer (Edinburgh Co., UK).

Rare-earth Er³⁺ doped BCTZ ceramics all have a well-crystallization with a single perovskite structure and little impurities. The results show that the mean grain size of the ceramics decreases monotonically with the increase of Er³⁺ doping concentration, from 7.59 μm to 3.82 μm, which is decreased by 49.67%.

All the ceramics exhibit superior ferroelectric properties. The ferroelectric domains are overturned and the density are different because of the lattice distortion and grain size change caused by the addition of different contents of Er³⁺, resulting in unstable polarization states. Therefore, the Er³⁺-doped BCTZ ceramics have a larger ΔP(P_max-P_r) rather than pure BCTZ ceramic. The released energy density (W_rec) and energy storage efficiency (η) of the ceramics are increased by 29.81% and 122.79%, respectively.

The worse the lattice symmetry, the better the photoluminescent (PL) performance. At a great doping amount of Er³⁺, Er³⁺ replace the A-position Ba²⁺ and Ca²⁺ positions, gradually occupying the gap position or replacing the B-position ions, resulting in more lattice distortion, reducing the lattice symmetry, and significantly enhancing the photoluminescent intensity. When the doping amount is 0.8% (in mole), PL strength reaches the maximum value. As the doping amount is greater than 0.8%, a cross relaxation phenomenon occurs due to the increase of luminescent ions and the decrease of luminescent ion spacing, resulting in fluorescence quenching and a decreased luminous intensity. The relative change in PL strength of BCTZ:0.8%Er³⁺ ceramic is enhanced by 173%, compared with BCTZ:0.2%Er³⁺ ceramic.

The mean grain size of the lead-free multifunctional ferroelectric ceramics decreased monotonously, and the microstructure became more compact and uniform with the increase of Er³⁺ doping concentration. All the ceramics had the superior ferroelectric properties. Er³⁺ doping could destroy the long-range order in BCTZ structure and the interaction between electric domains, so that the released energy density (W_rec) and energy storage efficiency (η) of the ceramics were increased by 29.81% and 122.79%, respectively, as Er³⁺ doping concentration increased. In addition, BCTZ:x%Er³⁺ ceramics exhibited an intense green emission at 548 nm under excitation of 487 nm near-ultraviolet light, and the luminous intensity was relatively adjustable up to 173.09%. The rich physical and photoelectric properties of BCTZ:x%Er³⁺ ceramics could lay a foundation for the development of new multi-functional materials with energy saving, environmental protection and low energy consumption.

The development of high-performance fuel electrode materials suitable for reversible operation is crucial for the application of reversible solid oxide cells (RSOCs). The fuel electrode should have excellent catalytic activity such as hydrogen oxidation reaction and hydrogen evolution reaction in the solid oxide fuel cells/solid oxide electrolysis cells (SOFC/SOEC) dual mode. Conventional Ni-YSZ fuel electrode materials are susceptible to sulfur poisoning when using fuel gas containing sulfur impurities and carbon buildup when using hydrocarbon fuel. La_xSr_1-xTiO₃ perovskite materials have a superior resistance to coking and sulfur poisoning, which is widely used in RSOCs fuel electrodes in recent years. However, the poor ionic conduction and the non-ideal catalytic ability of fuel gas affect the further application of La_xSr_1-xTiO₃ fuel electrodes. It is thus necessary for the improvement of the catalytic activity and the electrochemical performance of the fuel electrodes to optimize the microstructure and surface modification. In this paper, La_0.2Sr_0.8TiO_3-δ (LST)/YSZ-based fiber fuel electrode by Ce_0.9M_0.1O_2-δ (M=Fe,Co,Ni) impregnation modification was prepared, and the effect of impregnation on the electrochemical properties of fiber fuel electrode in SOFC/SOEC dual mode was investigated.

La_0.2Sr_0.8TiO_3-δ (LST)/YSZ composite fibers were directly prepared by an electrospinning method. In the preparation, stoichiometric amounts of La(NO₃)₃·6H₂O, Sr(NO₃)₂ and C₁₆H₃₆O₄Ti were weighed and dissolved with N, n-dimethylformamide (DMF) as a solvent and polyvinylpyrrolidone (PVP) as binder and one-dimensional template. The YSZ suspension was added to the LST precursor solution to obtain the LST/YSZ electrospinning solution for electrospinning. The high catalytic activity of Ce_0.9M_0.1O_2-δ (M=Fe, Co, Ni, CMO) was loaded on LST/YSZ composite fiber skeleton by an ion impregnation method to prepare the fiber fuel electrode. The electrolyte-supported single cells (CMO@LST/YSZ||YSZ||YSZ||GDC||LSCF/GDC) were fabricated to evaluate the electrochemical performance of the fiber fuel electrode.

The phase composition of the prepared composite fiber and the fuel electrode after immersion and calcination was determined by X-ray diffraction (XRD). The composite fiber morphology and the cross-section morphology of the tested fuel electrode were characterized by scanning electron microscopy (SEM). The two-phase distribution of the composite fiber was determined by high resolution transmission electron microscopy (HRTEM). The electrochemical performance of the fiber fuel electrodes in SOFC/SOEC dual-mode was tested via assembling a single cell. The impedance data were analyzed via the distribution of relaxation time (DRT), and the relationship between the microstructure and the electrochemical process was analyzed.

The electrospinning solutions with stable spinning are prepared via adjusting the LST:YSZ mass ratio. The LST:YSZ with a mass ratio of 6∶4 shows a criss-cross fiber morphology and smooth surface with uniform diameters (i.e., 150-200 nm) through the electrospinning and calcination. The composite fibers consist of the LST skeleton with YSZ particles dispersed and embedded, which is prepared directly by electrospinning, thus forming a three-dimensional network of mixed conductor channels with electronic channels as the main path and ionic channels interspersed. The LST:YSZ composite fiber was used to prepare a fuel electrode skeleton, and CMO precursor solution is prepared to impregnate and modify the fuel electrode. Compared with the fiber morphology, the morphology of fiber fuel electrode is smaller in length and diameter, showing a short rod-like morphology. The surface of the fiber is evenly covered by the impregnated CMO nanoparticles (i.e., 50-80 nm), still retaining abundant and interconnected pores. The three-dimensional network fiber electrode is constructed. The impregnate uniformly wraps on the fiber surface to strengthen the interfacial bonding between the fuel electrode and the YSZ transitional layer.

The different impregnation modification has an effect on the catalytic activity of the fuel electrode in SOFC/SOEC dual mode. The fuel electrode using Ce_0.9Ni_0.1O_2-δ as an impregnate has more Ni nanoparticles exsolution after the testing, effectively increasing the length of the three-phase interface and the active sites for the catalytic reaction. Based on the comprehensive analysis of EIS and DRT, the modified fiber fuel electrode can significantly affect the interfacial charge transfer process corresponding to the middle- and low- frequency. Ni nanoparticles exsolution can adsorb/dissociate hydrogen, and have a tendency to form an activated complex with water molecules, so that the fiber fuel electrode has great hydrogen oxidation capacity and hydrogen reduction capacity.

LST/YSZ composite fiber with rich connected pores was directly prepared by an electrospinning technique. This fiber had a three-dimensional network hybrid conductor pathway with LST skeleton as the main electronic channel and YSZ particles as ion channel interspersed. The interface bond between the fuel electrode and the electrolyte layer was further enhanced via modifying the fuel electrode skeleton with CMO nanoparticles impregnation. The results showed that the fuel electrode with Ce_0.9Ni_0.1O_2-δ impregnation had a high hydrogen oxidation and reduction capacity. Under the combined action of the impregnation and the precipitation of Ni nanoparticles, the single cell had the maximum power density of 342 mW/cm² at 850 ℃, 3% (in volume fraction) H₂O/H₂, and the current density of 0.313 A/cm² under 46.8% H₂O/H₂ for electrolysis at 1.3 V.

As one of the most extreme environments, the cryogenic temperature can make materials expose extremely complex and unpredictable potential intrinsic properties. Conducting the relevant research is to reveal the unknown attributes of materials in extreme environments, and to explore their potential applications in the future. This becomes a frontier topic and breakthrough point of scientific barriers in materials science research. The main conclusions are as follows: 1) In cryogenic environments, the mechanical properties of ordinary concrete can deteriorate. Therefore, the cryogenic temperature resistance of concrete structures can be greatly improved via increasing the ratio of stirrups, using prestressed steel bars, adding fibers, and filling high-strength steel pipes with UHPC. 2) The concrete strength is mainly related to its strength grade, temperature, water content, and porosity. As the temperature decreases, the compressive strength, flexural strength, and tensile strength of concrete firstly increase and then decrease, while the bonding strength shows a linear increasing trend. The critical temperature corresponding to the mechanical failure limit of concrete is different. 3) After cryogenic freeze-thaw cycles, the freezing, freeze-thaw and migration of pore water in concrete can destroy the pore structure. The uncoordinated thermal deformation, loose aggregate structure and high water absorption induce cracking in concrete interface transition zone. The C-S-H gel can undergo depolymerization fracture or interlayer ion bonding recombination, leading to interlayer pore collapse and shrinkage cracking. 4) At cryogenic temperature, the strength increase of C-S-H gel and the phase transition of pore water are the internal factors that enhance the concrete performance. During the cold freeze-thaw cycle, based on the volume expansion caused by pore water freezing and its thermodynamic process, the theories of hydrostatic pressure, osmotic pressure, crystallization pressure, micro-ice lens, glue spall and unsaturated poroelasticity are proposed. Although the relevant theories can explain a freeze-thaw failure at low temperatures, they are not fully applicable to explain the failure mechanism of concrete in cryogenic environments.

The existing research are conducted in the related fields, but the preliminary conclusions are only drawn on the macroscopic properties, microstructure, and failure mechanism of concrete. However, the research and application of concrete materials in extreme environments involve multiple levels of physical and chemical reactions, coupled with a superposition effect of multiple external factors, which presents complex and variable characteristics. This has certain technical limitations for human-being to utilize the existing cognition to adapt or modify nature. There are still some key issues that need to be solved in this field, i.e., 1) For ordinary concrete, its extreme temperature-variation resistance is inferior due to the limitation of material properties, and the optimization technology and improvement effect are not effective. Therefore, there is an urgent need to develop new concrete materials with an excellent resistance to cryogenic temperatures and long-term service, and deeply analyze their performance evolution and enhancement mechanism in extreme environments. 2) The existing research on cryogenic concrete mainly focuses on macroscopic mechanical and thermal properties, while there is little research on the in-situ evolution of concrete microstructure characteristics at cryogenic temperatures. Meanwhile, the relevant failure theory is only proposed for ordinary freeze-thaw cycles in a small temperature range, and is not applicable to the deterioration mechanism of concrete in complex and extreme cryogenic environments. 3) From the perspective of technical methods, the temperature-variation effect in cryogenic environments has great challenges to the performance testing of concrete and how to coordinate its thermal coupling effect. Simultaneously, it is particularly momentous to achieve in-situ monitoring of concrete performance in extreme environments. That is because many conventional testing equipment, techniques, and methods for macroscopic or microscopic properties of concrete will no longer be applicable. 4) In cryogenic environments, the macroscopic properties and microstructure of concrete exhibit enhancement and degradation effects, and these effects show different dominant trends in different temperature ranges. It is thus necessary for further research to make clear the dominant effects of the reinforcement or deterioration behavior of cryogenic concrete in different temperature ranges and the internal causes of temperature-performance transition.

Additive manufacturing has attracted recent attention for glass fabrication due to its advantages to manufacture parts with complex geometries. In 3D printing, various techniques such as fused deposition modeling, selective laser melting, direct ink writing, and stereolithography are developed for glass fabrication. For its high printing accuracy, fast speed, and excellent surface quality, stereolithography shows a great potential in fabricating fused quartz glass with intricate structures and optical lenses. This paper was to enhance the optical performance of fused quartz glass including optical absorption, photoluminescence and light scattering properties via infusing metal ions into the glass. We investigated the metal salt solution immersion, debinding and sintering processes based on liquid crystal display 3D printing technology. We fabricated a fused silica glass doped with Cr³⁺ and Co²⁺ metal ions.

A homogeneous solution of photopolymer was prepared via mixing 60% 2-hydroxyethyl methacrylate, 10% triethylene glycol diacrylate, and 30% phenoxyethanol. 2,4,6-trimethylbenzoyldiphenyl phosphine oxide, Sudan orange G, and diethyl phthalate were added to the solution and dispersed for 5 min. Silica nanoparticles of 53% (in mass fraction) were then added and dispersed by a dissolver for 10 min. The photosensitive paste was cured using an LCD 3D printer. Subsequently, the printed parts were heated at 600 ℃ for 2 h, and then soaked in a metal ions solution for 1 h. After being air-dried at 50 ℃ for 1 h, the doped debinding part was sintered at 1 280 ℃ for 2 h. Metal ion-doped glass in the furnace can be obtained after cooling to room temperature.

After debinding and sintering, the printed fused quartz glass parts are densified. The removal of organic binder from the debinding parts causes a contraction of the parts. The micropores disappear after the sintering process, resulting in dense parts that are colorless and transparent. Furthermore, a high concentration of the doped mental ions leads to an extensive crystallization and reduces the transparency of the samples.

Based on the Raman spectra of parts doped with Cr³⁺ and Co²⁺ and commercial glass KN7980, four characteristic peaks of fused silica glass appear, confirming that the finished parts are fused silica glass. In the Raman spectra of the fused silica, the characteristic peaks at 896 cm^-1 and 985 cm^-1 doped with Cr³⁺ correspond to the polymerized chromate species and the Cr—O stretching vibration of discrete chromate species (CrO₄^2-), respectively. This indicates that chromium oxide crystallization occurs in fused silica glass doped with a certain concentration of Cr³⁺. The XRD patterns show that Cr³⁺ induces a crystal precipitation in the fused silica glass, for characteristic peaks of quartz at 2θ of 26.43° and cristobalite at 2θ of 63.56° and 67.91° in the glass doped with Cr³⁺. The characteristic diffraction peaks in the glass doped with Co²⁺ appearing at 2θ of 32.2°, 47.7° and 57.4° are consistent with those of CoO.

The absorption spectra of the glass doped with 2.042 μmol/cm³ concentration of Cr³⁺ reveal that the bands at 471 nm and 660 nm are due to the spin-allowed transition A_2g(F)→⁴T_1g(F) and ⁴A_2g(F)→⁴T_2g(F), respectively. Consequently, Cr³⁺-doped fused silica glass can absorb blue light at 471 nm and red light at 660 nm. The absorption spectra of the parts doped with 12.249 μmol/cm³ concentration of Co²⁺ indicate that the bands at 597 nm and 662 nm result from the spin-allowed transition ⁴T_1g(F)→⁴T_1g(P) and ⁴T_1g(F)→⁴A_2g(F), respectively. Co²⁺-doped fused silica glass shows absorption of blue light at 597 nm and red light at 662 nm. Doping the glass with metal ions imparts unique optical and structural properties to the glass, so that chromium- and cobalt-containing glasses have applications in industries, such as Integrated optics, filters, fiber lasers and near-infrared laser materials.

LCD 3D printing technology was used to produce optical components of fused silica glass doped with Cr³⁺ and Co²⁺ metal ions, having the superior properties, compared with commercial glass KN7980. The Cr³⁺-doped green fused silica glass had a great filtering ability for blue light (471 nm) and red light (660 nm). The Co²⁺-doped blue fused silica glass had a great filtering ability for yellow light (597 nm) and red light (662 nm). The filtering performance of multilayered silica glass could be deliberately tailored to enhance its performance since the filtering ability of silica glass was correlated to the metal ion doping concentration.

The utilization of seawater and sea sand for concrete production can effectively relieve the environmental pressure caused by the consumption of freshwater and river sand resources. Previous studies demonstrate that the mechanical properties of seawater sea sand concrete are consistent with those of normal concrete, which can meet the construction needs. However, the multiple element ions in seawater alternate the components and microstructure of hydration products via participating in the hydration reaction, affecting its durability performance. Mineral admixtures can be used to mitigate the possible adverse effects of multiple element ions on cementitious materials. The existing research on the interaction between inorganic ions and the internal microenvironment of cementitious materials mainly focus on the effect of single ions, and the influence mechanism of multiple element ions lacks systematic investigations. Meanwhile, the influence mechanism of mineral admixtures on the performances of seawater sea sand concrete Is still unclear. In this paper, the pore solution composition and the mineral phases of the hydration products were characterized to reveal the evolution mechanism of the internal microenvironment under the coupling effect of multiple element ions in seawater and mineral admixtures.

A cementitious system was designed via the centroid simplex optimization of Portland cement, fly ash and silica fume. The artificial seawater was prepared according to the standard (ASTM D1141-98) at a pH value of 8.29. Sea sand from Jiaozhou Bay, China, was used as fine aggregates with a fineness modulus of 2.65. The polycarboxylic acid high-performance water reducer with a water reduction rate of 30% was used to adjust the flowability of fresh stage cement paste, (Shanxi Feike New Material Science and Technology Co. Ltd., China). The specimens with a water/binder ratio of 0.35 and a sand/binder ratio of 1.0 were then prepared for characterization, and the water-reducing agent dosage was adjusted to maintain the same fluidity among different sample groups.

After curing for 3, 7 d and 28 d, the ion concentration in pore solution obtained from a pressure filtration method was characterized by inductively coupled plasma emission spectrometry, and its pH value was measured by a pH meter. The phase composition of powder samples after soaking in isopropanol and further drying was characterized by a model MiniFlex 600 X-ray diffractometer. In addition, the thermal behavior of the powdered samples aged at 28 d was determined by thermogravimetric analyzer/derivative thermogravimeter. The compressive strength of cubic mortar specimens with the sizes of 70.7 mm×70.7 mm×70.7 mm aged for 3, 7 d and 28 d was measured.

The compressive strength of seawater sea sand mortar specimens is higher than that of freshwater river sand mortar specimens aged at 28 d. This can be attributed to the pore filling effect by hydration products and the formation of more compact C-S-H gels in the presence of the multiple element ions.

The multiple ions in seawater also increase the pH value of the pore solution. The pore solution alkalinity can be further regulated via mixing mineral admixtures. At the same dosage, silica fume significantly outperforms that of fly ash in reducing the alkalinity of the seawater cement paste pore solution. Silica fume can nearly consume the calcium hydroxide component at a dosage of 30%, and the pH value of the pore solution decreases to 11.69 at 28 d. Also, silica fume can generate more hydrated calcium silicate gels with a low Ca/Si ratio to enhance its binding ability with sodium and potassium ions, and the enhancement effect is better than that of fly ash. The active aluminum phase in fly ash can promote the generation of aluminum phase products (i.e., AFt and Friedel's salt), and enhance the effective solidification of sulfate and chloride ions. The chloride ion solidification effect of fly ash is better than that of silica fume, and the chloride ion concentration of pore solution can reduce to 0.27 mol/L at the dosage of fly ash of 30%.

The results showed that the multiple element ions in seawater promoted the hydration of tricalcium silicate and dicalcium silicate in cement, increased the alkalinity of seawater cement pate pore solution and accelerated the pozzolanic reaction between mineral admixtures and calcium hydroxide. Silica fume significantly outperformed fly ash in reducing the alkalinity of the pore solution. The effective solidification of sodium and potassium ions was achieved through the generation of more hydrated calcium silicate gel products with silica fume. Fly ash could promote the generation of aluminum phase products (i.e., AFt and Friedel's salt) and enhanced the solidification effect of chloride ions, and its enhancement effect was better than that of silica fume. This study could serve as a solid base for the alkalinity regulation and performance design of seawater sea sand concrete.

Steel slag is a solid waste produced from steelmaking, which has a potential cementitious activity. However, its cementitious activity is much lower than that of Portland cement. Mechanical grinding, acid activation, alkali activation, high-temperature curing, modification/reconstruction and organic activation methods are commonly used to improve the hydraulic activity of steel slag. Triethanolamine is commonly used as an organic admixture to improve cementitious activity of steel slag. The existing research on the hydration of TEA on steel slag still focuses on a steel slag-cement composite system, in which the proportion of cement is higher and the hydration rate is faster, so the results obtained are related to cement hydration, and it is difficult to characterize the effect of TEA on steel slag. It is thus necessary to investigate the hydration of pure steel slag in the presence of triethanolamine. In this paper, the hydration characteristics of pure steel slag under the complexation of TEA were characterized by compressive strength, heat of hydration, thermogravimetry, phase analysis, complexation ability and dissolution characteristics as well.

The compressive strength of hardened steel slag paste was tested. In this work, the hydration characteristics of steel slag were investigated at different TEA concentrations, and the concentrations selected were 0, 500, 1 000 mg/L and 2 000 mg/L. A mass ratio of TEA solution to steel slag was kept at 0.35. The size of test block was 20 mm×20 mm×20 mm, and the curing age was 3, 7 d and 28 d. In the process of compressive strength testing, the test entrance force was set to be 200 N, and the pressure was applied by force control, and the pressure rate was 100 N/s. After the compressive strength was tested, the specimens were smashed (<5 mm) and the center portion of the specimens was immersed in an isopropanol solution to terminate hydration, and the terminated hydrated steel slag pieces were dried in a vacuum drying oven at 40 ℃. After drying, steel slag pieces were ground and sieved through a 200 mesh (74 μm) sieve. A portion of steel slag powder was taken for thermogravimetric analysis and X-ray diffraction (XRD).

The hydration heat of steel slag paste was determined using calorimetry, and the exothermic process of hydration was recorded from the addition of water or TEA solution for 72 h. The TEA solution concentration and its ratio to steel slag both were consistent with the tests of compressive strength. The complexation ability of TEA with different metal ions was determined by a conductivity method. The complexation ability of TEA with metal ions was characterized via continuously adding TEA to a solution of metal ions at the same concentration and recording the conductivity data. The conductivity method was also used to characterize the dissolution process of steel slag in the presence or absence of TEA. The conductivity of solution without steel slag was the initial value, which was measured from the contact of steel slag with water and recorded continuously for 25 min.

The compressive strength of hardened steel slag paste with TEA is higher than that of the blank group without TEA at each age. This indicates that TEA can simultaneously increase the early and late compressive strength of hardened steel slag paste, and this strengthening effect increases with the increase of TEA concentration. When TEA is in steel slag paste, the diffraction peaks of monocarboaluminate (Mc) appear, and the diffraction peaks of C₂F gradually become weaker. This indicates that TEA can promote the formation of Mc, and the content of CaCO₃ is fixed at different TEA concentrations, so it can be inferred that TEA promotes the formation of Mc via promoting the hydration of aluminum phases. Meanwhile, TEA promotes the hydration of C₂F, and [Fe] in C₂F can participate in the formation of Mc as a substitutional ion of [Al] to form (). The heat of hydration and thermogravimetric tests further indicate that TEA can promote the hydration of aluminate and ferrate phases. The hydration degree is enhanced by the formation of a large amount of Mc. The increase in hydration degree ultimately leads to an increase in compressive strength.

The promotion of Mc formation by TEA is related to its complexation. When the concentration of TEA is less than or equal to 0.05 mol/L, i.e., the molar ratio of TEA to metal ions is less than or equal to 1, the complexation ability of TEA on Fe³⁺ is stronger than that of Al³⁺ and Ca²⁺. When the concentration of TEA is greater than or equal to 0.05 mol/L, i.e., the molar ratio of TEA to metal ions is greater than or equal to 1, the complexation ability of TEA on Al³⁺ is stronger than that of Fe³⁺ and Ca²⁺. TEA promotes the dissolution of mineral phase in steel slag through complexation. The free-moving metal ions and the complexed metal ions in liquid phase of steel slag can simultaneously participate in the precipitation reaction of hydration product, producing a large amount of Mc, and thus improving the early and late hydration of steel slag paste, which in turn manifests itself as an increase in the compressive strength.

The complexation ability between TEA and metal ions and effect of TEA complexation on dissolution and hydration characteristics of pure steel slag were investigated. TEA significantly increased the early and late compressive strength of hardened steel slag paste. The main reason for the increase in compressive strength of hardened steel slag paste was that TEA improved the hydration degree of steel slag paste in early and late stages. TEA promoted the hydration of aluminate phases (C₃A, C₁₂A₇) and ferrate phase (C₂F), and through the interaction with carbonate, a large number of () were formed, and the formation of such hydration products was the main reason for the improvement of the hydration degree and the compressive strength of steel slag paste. The large formation of Mc was related to the complexation and solubilization effect of TEA. TEA could complex with Ca²⁺, Al³⁺ and Fe³⁺, and the complexation of TEA promoted dissolution of steel slag and formation of metal ions that could participate in the precipitation reaction, thus promoting the hydration of steel slag paste.

Water shortage and pollution become serious due to the improvement of economic and the acceleration of industrialization. Water bodies contain a large number of pollutants, affecting the environment and human-being health. Heterogeneous catalyst activation of peroxymonosulfate (PMS) is an effective degradation technology for organic pollutants, but SO₄^-· is difficult to effectively activate due to the high redox potential. It is thus necessary to develop catalytic materials that can activate PMS efficiently. The metal organic frame materials based on Prussian blue have the typical physical and chemical characteristics of metal-organic framework (MOFs) materials, adjustable and controllable chemical composition, high specific surface area and porosity, and diversified structure and function, etc., which become the hot materials in various fields. The conversion of Prussian blue analogue (PBA) into nitrogen-doped carbon materials with a high specific surface area, a high porosity and a high nitrogen content through high-temperature calcination or inert gas carbonization is of great significance for improving the catalytic performance of the catalyst. In this paper, a nitrogen-doped carbon nanosheet supported FeMnO catalyst was prepared by a high-temperature carbonization method. Its catalytic degradation performance was analyzed. The effects of different systems, pH value of solution, Rhodamine B (RhB) initial mass concentration, PMS mass concentration and catalyst mass concentration on the RhB degradation were investigated. In addition, the possible mechanism of RhB degradation was also proposed.

Fe-Mn PBA and colloides were prepared by a co-precipitation method. After grinding, the catalyst was heated in nitrogen atmosphere at 750 ℃ at 2 ℃/min for 2 h to synthesize transition bimetal oxide (i.e., Fe_0.099Mn_0.9010/FeMn₂O₄) N-doped carbon nanosheets. Compared with FeMnO and graphite C, the degradation properties of FeMnO@C in different systems and under different factors were characterized by ultraviolet-visible spectroscopy (UV-Vis). The structure and morphology were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FT-IR).

Heterogeneous catalyst activation of PMS can inhibit the dissolution of metal ions and it is not easy to cause secondary pollution. An efficient nitrogen-doped carbon catalyst FeMnO@C was obtained based on Fe-Mn PBA, which has a high specific surface area and provides a high density of active sites for catalytic reactions, and can effectively activate PMS to rapidly degrade organic pollutants. FeMnO@C has good magnetic properties, which can be conducive to recycling. The interaction between different metals in the double transition metal materials makes the metals difficult to be dissolved, greatly reducing the concentration of toxic metal ions in the system. The redox cycle between Mn and Fe ions on the surface of FeMnO@C catalyst and a series of free radicals generated after the self-decomposition of PMS make FeMnO@C have a superior catalytic degradation performance. The effects of PMS amount, initial concentration of RhB, catalyst amount and initial pH value of RhB solution on the catalytic performance were analyzed. The optimal reaction conditions were PMS concentration of 0.15 g/L, concentration of RhB solution of 20 mg/L, catalyst concentration of 0.1 g/L and pH value of 7. The possible degradation mechanism was proposed based on capture experiments and EPR spectra.

A transition bimetallic oxide catalyst supported by nitrogen-doped carbon nanosheets was synthesized by a high-temperature carbonization method, which could be used as an efficient PMS activated heterogeneous catalyst to degrade the target pollutant RhB. In the PMS activation system, the FeMnO@C catalyst could degrade 95.5% of RhB within 10 min. The redox cycling between Mn and Fe ions on the surface of the FeMnO@C catalyst and a series of free radicals generated after the self-decomposition of PMS could make the FeMnO@C have a superior catalytic degradation performance. The capture experiments showed that ¹O₂ was the main contributing radical. In addition, FeMnO@C catalyst also had a magnetic property and a good cyclic stability. 84.7% RhB removal rate could be obtained after five consecutive cycles. The results indicated that FeMnO@C had a good magnetic property, which could be easy to be recycled, having a promising potential in treating organic wastewater.

Sulfate attack and chloride-induced corrosion are recognized as crucial factors leading to the deterioration and failure of concrete structures. Chloride ingress triggers steel corrosion, while sulfate attack alters the microstructure of concrete matrix. When the concrete is subjected to a poorly mineralized or acid solution, calcium leaching happens and has also a negative effect on concrete durability. In harsh natural environments, concrete is vulnerable to the combined sulfate-chloride attacks, often accompanied by calcium leaching. The mechanism of coupled ionic attack differs substantially from that of individual attacks. This paper was to propose a numerical model on the interaction between sulfate attack, chloride attack and calcium leaching. The ionic transport patterns and hydration product distributions under coupled degradation were quantitively analyzed by the proposed model, thereby providing new insights into the deterioration mechanisms in concrete under the simultaneous sulfate-chloride attack and calcium leaching.

A comprehensive transport model was established based on the accelerating effect of aggressive ions on the calcium solid-liquid equilibrium curve. The impact of calcium leaching on the ionic binding effect was also taken into accounts. The chemical reactions between multiple ionic species and cementitious matrix were incorporated based on the transport model. A reaction source term intuitively reflecting the desorption effect of sulfate ions on chloride ions was also introduced. A transport-chemo model was further developed. Based on the volumetric changes induced by expansive hydration products and calcium leaching coupled with the damage effect caused by expansive cracking in the matrix, a time-dependent diffusivity model was established. Finally, an integrated coupled degradation model was proposed, encompassing transport, chemical reactions, and time-dependent diffusivity. The proposed model was validated through third-party experiments according to sulfate concentration distribution and chloride ion concentration distribution. This model is capable to predict ionic transport and product distribution based on given initial material parameters and external ion concentrations. Some indexes such as ionic penetration depth, time-varying diffusion coefficient and product content could be applied to monitor the deterioration process of concrete.

Under combined sulfate-chloride attack, there are two distinct regions in the concrete, i.e., a sulfate-rich zone and a chloride-rich zone. The diffusion of free sulfate ions is restricted, so that the increase in total sulfate content peak at a concrete depth of 3 mm is attributed to the precipitation of ettringite (AFt). As an intermediate product of AFt, the content of newly produced gypsum remains low. In contrast, chloride ions progressively penetrate deeper, resulting in the accumulation of chlorine hydrate products such as Friedel's salt, in which a peak gradually moves inward. The production of Friedel's salt falls between AFt and gypsum in terms of concentration.

The concrete surface is primarily affected via calcium leaching, leading to a more than fourfold increase in the diffusion coefficient. As the influence of calcium leaching diminishes with increasing concrete depth, there is a rapid reduction in porosity. A distinct trough in the porosity curve emerges at approximately 3 mm, precisely aligning with the sulfate-rich zone. The mutual counteraction of internal pore filling effect and damage effect due to expansive cracks results in the time-dependent diffusion coefficient close to the initial value.

Simultaneous exposure to sulfate and chloride attacks may provide a short-term mitigation of either sulfate or chloride attack individually. Chloride ions have a more significant inhibitory effect on sulfate attack under these combined conditions. For calcium leaching, sulfate attack can become more severe and concentration peaks of ettringite, and gypsum appear near the surface. Neglecting calcium leaching inhibits the precipitation of ettringite, thus causing the peak content of ettringite to decrease from 201 mol/m³ to 23 mol/m³, which is only 1/10 of that for calcium leaching.

Increasing external chloride concentrations amplify a suppressive impact on sulfate attack. Apart from the environment factor, a higher initial calcium aluminate content in concrete corresponds to a more pronounced sulfate attack in concrete matrix. The concentration of hydrate products increases linearly with the extension of time.

The proposed model had the interactions among various factors and provided a rational prediction of the coupled degradation process. Concrete had distinct sulfate and chloride zones. The sulfate penetration depth remained with the deposition of AFt near the surface. In contrast, the chloride penetration depth gradually increased with a broader distribution of generated Friedel's salt. The trough of porosity distribution corresponded to sulfate-rich zone at depth of 3 mm, in which the diffusion coefficient was approximate to the initial value. The pore filling effect was offset by the damage effect caused by expansive cracks. Combined attack could mitigate sulfate or chloride attack in the short term. Chloride ions had a more pronounced inhibitory effect on sulfate attack. Calcium leaching led to less ettringite generation but a wider distribution in concrete. Based on the proposed model, a parameter analysis of environmental or material parameters was performed, thus having insights to service life prediction of concrete under chloride-sulfate attack.