IntroductionUltra-high temperature ceramics include a series of high melting point materials, such as transition metal carbides, nitrides and borides, especially transition metal carbides with excellent high-temperature mechanical properties, stable physico-chemical properties, corrosion resistance and radiation resistance, showing broad prospects for application in the hypersonic aircraft, rocket engines, fourth-generation nuclear reactor, and other extreme environments. However, the traditional single-component transition metal carbide ceramics have been unable to meet the emerging requirements under extreme environments, there is an urgent need to develop a new high-performance material under ultra-high temperature. In recent years, the concept of multi-component “high entropy” has greatly expanded the scope of material composition design and property optimization. Compared with single-component carbides, multi-component ceramics perform better in terms of overall properties, including hardness, creep resistance, oxidation resistance and radiation resistance. These improvements mainly result from their complex component composition, electronic structure and lattice distortion. At present, the influence of elemental species on the microstructure evolution and mechanical properties of carbide high-entropy ceramics is not reported. In this paper, (TiZrNbTaMe)C (Me=V, Cr, Mo, W) high-entropy ceramics are prepared by hot press sintering, and the effects of Me elemental species on the physical phase, microstructure evolution, and mechanical properties of (TiZrNbTaMe) high-entropy ceramics are investigated.MethodsIn this work, (TiZrNbTaMe)C (Me=V, Cr, Mo, W) high-entropy ceramics with equimolar ratio were prepared by carbothermal reduction-assisted hot pressing using transition metal oxides and carbon black as raw materials. The oxides and carbon black were mixed using a planetary ball mill (Fritsch, model P4, Germany). Carbide powders were obtained by carbothermal reduction under vacuum using a pressureless sintering furnace (WS0404, Ningxia Sincere Co. Ltd., China) with a process of 1500 ℃/1 h. The synthesized carbide powders were loaded into graphite molds, and the ceramic samples were prepared by a two-step hot pressing method (AVS, model 1540, USA). The samples were held at 1850 ℃ for 1 h, and then at 2100 ℃ for 0.5 h under pressure of 30 MPa or 10 Pa. Phase analysis was carried out by X-ray diffractometry (XRD; D/max-B, Rigaku, Japan) using Cu-K rays. Scanning electron microscopy (SEM; Quanta 200FEG, USA) was used to analyze the microstructure and elemental content and distribution. Transmission electron microscopy (TEM; Talos F200X, USA) was used to analyze the microstructure, elemental content and distribution, and grain boundary characteristics of the materials. The relative densities of the samples were measured using image analysis software (Photoshop, Adobe, USA) based on the pores in the SEM photographs. Vickers hardness was measured using a Vickers hardness tester (HVS-30) at a load of 9.8 N with a holding time of 15 s. Fracture toughness was also measured using the indentation method. The interaction parameters between the metal elements were also calculated using DFT calculation.Results and discussionThe high-entropy ceramics are all characterized by an FCC crystal structure. Except for the (TiZrNbTaW)C sample, the porosity of the other three high entropy ceramics is low, and their density exceeds 98% and the element distribution is relatively uniform. Compared with the (TiZrNbTaMo)C sample, the grain sizes of the (TiZrNbTaV)C and (TiZrNbTaCr)C samples are significantly reduced, which are 2.69 m and 5.39 m, respectively. This shows that the addition of V and Cr elements helps to improve the sintering properties of the (TiZrNbTaMe)C system and inhibit grain growth. The element content analysis results of (TiZrNbTaCr)C show that the Cr element segregates significantly at the grain boundaries, while the other four metal elements are distributed more evenly. The segregation of Cr at the grain boundaries may be related to chromium carbides with low melting points. For example, the melting point of Cr3C2 is about 1810 ℃, while the sintering temperature is as high as 2100 ℃, suggesting that chromium carbides may form a liquid phase and aggregate at the grain boundaries during sintering. In addition, the complex composition of high-entropy ceramics may also affect the solid solubility of Cr. The interaction coefficients between Cr and each metal element in transition metal carbides show that the interaction parameter values of Cr and other metal elements are high. It is difficult for Cr to form a solid solution with other metal elements, thus tending to be enriched at the grain boundaries. Therefore, (TixZr0.4–xNb0.2Ta0.2Cr0.2)C ceramics with different Ti and Zr contents (x=0, 0.2, 0.3 and 0.4) were designed and prepared. The effect of Ti content on the solid solubility of Cr in the (TiZrNbTaCr)C sample was investigated. As the x value increases from 0 to 0.4, the solid solubility of Cr in the grains increases from 3.18% to 8.68%. It shows that the increase of Ti content is beneficial to improve the solid solubility of Cr. In addition, for the (TiZrNbTaCr)C sample, the high lattice distortion leads to solid solution strengthening in the grains and the high density of the system increases its hardness, which has the best mechanical properties. Its Vickers hardness and fracture toughness reach 29.8 GPa and 3.71 MPa·m1/2, respectively.ConclusionsThe four (TiZrNbTaMe)C (Me=V, Cr, Mo, W) high-entropy ceramics are all face-centered cubic structures, and the elements are uniformly distributed in the high-entropy ceramic systems except for the (TiZrNbTaCr)C sample. The (TiZrNbTaCr)C sample is also characterized by the presence of a significant Cr segregation at grain boundaries, resulting in a low Cr content inside the grains. The (TiZrNbTaCr)C sample has the best overall mechanical properties, with the Vickers hardness and fracture toughness reaching 29.8 GPa and 3.71 MPa·m1/2, respectively. In-depth studies show that the solid solubility of Cr element in high-entropy carbide ceramics is closely related to the species and content of metal elements. Enhancing the content of Ti element helps to improve the solid solubility of Cr element in high-entropy carbide ceramics, in which the solid solubility of Cr element in the grain of (Ti0.4Nb0.2Ta0.2Cr0.2)C system reaches the maximum value of about 8.68%.

IntroductionIn recent years, the high-voltage and high-power multi-layer ceramic capacitors (MLCCs) have attracted extensive attention with the development of new energy vehicles, 5G communication and high-power pulse technologies. At present, the materials used in the MLCC inner electrode are mainly precious metal (Ag/Pd or Au) and base metal (Ni or Cu). Base metal materials have more cost advantages compared with the precious metal materials. In order to prevent the oxidation of the base metal Ni or Cu at high temperatures, it is necessary to sintering in a low oxygen or reducing atmosphere. In order to obtain MLCC products with excellent performance, it is necessary to strictly control the matching of the sintering atmosphere between the inner electrode and the ceramic body during the sintering process. The crystal structure and electrical properties of the ceramic will be significantly reduced in the reducing atmosphere or low oxygen atmosphere environment, mainly because the precipitation of metal lead will destroy the lattice structure, or produce lattice defects. Therefore, it is of great significance to study the properties of PbLaZrSnTiO3-based (PLZST-based) antiferroelectric ceramics in low oxygen sintering atmosphere. In this paper, PLZST-based antiferroelectric ceramics were sintered in four kinds of low oxygen sintering atmosphere. The crystal structure, microstructure and electrical properties of PLZST-based antiferroelectric ceramics were studied. Moreover, the energy storage properties of all samples were studied.MethodsPb0.95La0.02Sr0.02(Zr0.50Sn0.40Ti0.10)O3 (PLSZST) antiferroelectric ceramics doped with were prepared using the traditional solid-state method. High quality raw materials, namely Pb3O4(95%, Sinopharm), La2O3(99.99%, Acros), ZrO2(99%, Aladdin), SnO2(99.8%, Sinopharm), TiO2 (99.8%, Sinopharm) and SrCO3(99%, Sinopharm), were weighted 500 g according to stoichiometric and ball milling for 12 h with ethyl alcohol as a solvent and dried at 70 ℃. These raw materials were calcined at 850 ℃ for 2 h in a closed crucible. Then, 0.6% BaCO3-B2O3-SiO2-K2CO3(BBSK, in mass) was added to the mixed powder according to the solid weight ratio as the calcination powder. After ball milling for 12 h, the powder was dried. Next, the powder was pressed into pellets at 20 MPa with 5% Polyvinyl Alcohol (PVA, in mass) as the binding agent 2 mm thick and 10 mm in diameter. Finally, the green ceramics with BBSK addition were sintered at 1020 ℃ and low oxygen sintering atmosphere (5.3×10–10, 2.1×10–9, 2.1×10–8, 1.3×10–7 atm) and air for 2 h in closed crucibles filled with the calcination powder.Results and discussionIn order to study the effect of sintering atmosphere on the crystal structure of PLSZST-0.6% BBSK (in mass) antiferroelectric material, XRD were conducted as shown in Fig. 1. It can be seen that all samples have perovskite structure, and a small amount of second phase is generated, which may be pyrocholate phase produced by Pb volatilization at high temperature. From the splitting peak near 44.2°, it can be seen that the intensity and Angle of the two splitting peaks do not change under the condition of low oxygen atmosphere, which indicates that the tetragonal structure of the ceramic body does not change. The peaks of (200) and (002) move to the lower Angle at lower oxygen content, indicating that the low oxygen atmosphere leads to the lattice distortion of the material. Then, the microstructure was studied. As can be seen in the surface micrograph, there are no metal lead or other substances precipitated from the ceramic body as observed in those samples treated at low oxygen sintering atmosphere. It can be seen from the f dielectric temperature spectra that all samples show the same variation characteristics. With the increase of temperature, the dielectric constant first increases to the maximum value and then slowly decreases. This dielectric anomaly peak is caused by the transformation of tetragonal antiferroelectric phase into paraelectric phase. With the increase of frequency, the dielectric constant and the peak value of dielectric anomaly do not change significantly, which indicates that the ceramics are a diffuse phase transition, and does not change with the change of oxygen content, which indicates that the oxygen content does not change the phase transition characteristics of antiferroelectric. With the decrease of oxygen content, the breakdown field strength and maximum polarization strength of the ceramic decrease, resulting in the gradual decrease of the saturated energy storage density and recoverable energy storage density of the ceramic body. The ceramic sintered at 5.3×10–10 atm has a recoverable energy storage density of 2.76 J·cm–3 which is lower than that of the ceramic sintered at air (3.56 J·cm–3), showing a discharge energy density of 2.21 J·cm–3.ConclusionsMulti-layer ceramic capacitors with base metal inner electrodes have been widely concerned because of its lower cost advantages. In order to prevent the oxidation of the base metal inner electrodes (Ni or Cu) at high temperatures, it needs to be sintered in low oxygen or reducing atmosphere. The effect of low oxygen atmosphere on the electrical properties of PLSZST antiferroelectric ceramics was studied. With the decrease of oxygen content, the insulation characteristics and energy storage characteristics of antiferroelectric ceramics were reduced. Under the oxygen content of 5.3×10–10 atm, the ceramic has a recoverable energy storage density of 2.76 J·cm–3, which is lower than that of sintered sample under air (3.56 J·cm–3), has a discharge energy storage density of 2.21 J·cm–3, and has a discharge current of 28.6 A. This work provides understanding of anti-reduction characteristics of PLZST-based antiferroelectric ceramics, and further promotes the development and production of PLZST-based antiferroelectric multilayer ceramic capacitors with internal electrodes in base metals.

IntroductionAs a high-quality light source, the performance of phosphor-converted white light emitting diodes (pc-WLEDs) is closely related to the performance of the selected luminescent materials. The traditional method of combining blue LED chips and yellow phosphors to generate white light has problems of low color rendering index and high color temperature. To address the pressing requirements of lighting and display technologies, the development of novel red phosphors with high efficiency and excellent thermal stability is urgent. Perovskite tellurites have attracted extensive attention due to their elemental diversity and structural tunability. Based on the double perovskite Ca3TeO6, a series of NaLaCaTeO6:xEu3+ phosphors that can be effectively excited by near-ultraviolet light have been designed and synthesized by elemental design. Through a co-substitution strategy of [Na+–La3+] substitution of [Ca2+–Ca2+] ion pair, the problem of low Eu3+ doping concentration in Ca3TeO6:Eu3+ is effectively improved by regulating the matrix structure. Meanwhile, the luminous intensity and thermal stability of the sample were also significantly improved.MethodsSeries of NLCT:xEu3+ (0.05 ≤ x ≤ 1.0) samples were successfully synthesized by a high-temperature solid-state reaction. The stoichiometric amounts of raw materials, including Na2CO3 (105.99 g/mol, 99.9%), CaCO3 (100.09 g/mol, 99.9%), TeO2 (159.6 g/mol, 99.99%), La2O3 (325.84 g/mol, 99.99%) and Eu2O3 (351.93 g/mol, 99.99%) purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., were weighed and grounded into homogeneous powder for 30 min. Afterward, the obtained powder was put into crucibles and heated at 1473 K for 12 h in a muffle furnace. Eventually, the target substances were obtained after cooling down and grinding again. A red LED device was assembled using the NLCT:0.6Eu3+ phosphor with a 395 nm chip. The WLED device was fabricated by connecting tricolor phosphors including BaMgAl10O17:Eu2+ (BAM:Eu2+, blue), (Ba, Sr)2SiO4:Eu2+ (BSS:Eu2+, green), along with the prepared NLCT:0.6Eu3+ (red) with a 395 nm chip.Results and discussionThe results of XRD patterns showed that [Na+–La3+] substituted the [Ca2+–Ca2+] ion pair, which did not affect the structure of the sample. The diffraction peaks of the samples with x = 0.1–0.6 did not show obvious impurity peaks, indicating that the synthesized samples were pure phases. Eu3+ ions tended to occupy the site of La in the matrix rather than Te. DRS results also confirmed that Eu3+ ions were successfully introduced into the NLCT matrix. The dominant absorption peak at 395 nm matches well with InGaN-based LED, and the emission intensity reaches the maximum at x = 0.6. The primary mechanism among Eu3+ ions in NLCT:xEu3+ samples was proved to the dipole-dipole interaction.The luminous intensity of the prepared NLCT:0.6Eu3+ red phosphor is about 1.98 and 3.97 times than that of CT:0.2Eu3+ and Y2O3:Eu3+, confirming the feasibility of the co-substitution strategy. The internal quantum yield and color purity of NLCT:0.6Eu3+ are 67.6% and 99.87%, respectively. The CIE color coordinates of this sample (0.658 1, 0.341 0) are closer to the ideal red point (0.670, 0.330) than that of the purchased commercial red phosphor Y2O3:Eu3+. Compared with CT:0.2Eu3+ (about 77.5%), the thermal stability of NLCT:0.6Eu3+ is significantly improved. When the temperature rises to 420 K, the emission intensity at 616 nm remains 91.7% of that at 300 K. Furthermore, the sample exhibits excellent color stability by the CIE coordinates shifting only slightly from (0.658 2,0.340 8) to (0.646 6,0.351 9) with increasing temperature. The CCT and Ra of the prepared WLED device are 6215 K and 80.6, respectively, and the CIE coordinates (0.317 6, 0.334 0) are located in the white light region.ConclusionsA series of innovative NaLaCaTeO6:xEu3+ phosphors were synthesized using a high temperature solid-state reaction method. Under 395 nm excitation, the NaLaCaTeO6:Eu3+ samples exhibit a strong red emission at 616 nm (5D0→7F2) of Eu3+ ions. Through a co-substitution strategy where [Na+–La3+] replaced the [Ca2+–Ca2+] ion pair, the optimized doping concentration of the NaLaCaTeO6:xEu3+ is elevated to 60% (molar fractior), leading to exceptional color purity and high internal quantum efficiency reaching 99.87% and 67.60%, respectively. Furthermore, compared to the Ca3TeO6:0.2Eu3+ sample pre-substitution, the luminous intensity and thermal stability of NaLaCaTeO6:0.6Eu3+ phosphor witness a notable increase. Even at 420K, the emission intensity remains at 91.7% compared to 300K. The resultant WLED displays a color rendering index Ra of 80.6 and CIE chromaticity coordinates of (0.317 6, 0.334 0), underscoring the broad application potential of NaLaCaTeO6:0.6Eu3+ in solid-state lighting.

With the development of technology and the increasing emphasis on patent protection, anti-counterfeiting technologies are emerging one after another. Among numerous anti-counterfeiting technologies, photoluminescence anti-counterfeiting technology is widely used in the field of anti-counterfeiting due to its excellent optical properties. However, traditional fluorescent powder anti-counterfeiting only changes its color when the UV excitation light source is changed, lacking dynamic changes in the time dimension. Therefore, it is necessary to develop an efficient anti-counterfeiting new luminescent material. Currently, the use of cation substitution and multiple occupied sites has become a popular strategy for tunable emission. Due to the interaction between activators and anions, the energy difference between the energy levels of the luminescent center ion can be altered. Therefore, anion substitution can also be an effective strategy to adjust the photoluminescence performance. This article describes the preparation of Mg2-xNaxAl4Si5O18-xQx:1%Eu2+ (Q=Cl, F) by glass relaxation crystallization method, which introduces Na+ sites and replaces 1O2– with 2F–/2Cl– to achieve the opening of the [Al/Si]6O18 hexagonal ring structure, providing a rich field environment for the central ion Eu2+ to enter the -Mg2Al4Si5O18 structure: Na+ sites, Mg2+ sites, and hexagonal ring channel sites. By regulating the Eu2+ occupation, Euvac2+, EuMg2+ and EuNa2+ structures are formed, achieving blue to yellow light emission.MethodsBased on the composition of 2MgO–2Al2O3–5SiO2, a series of Mg2-xNaxAlSiO18–xQx:1%Eu2+ (Q=Cl, F) fluorescent powders (abbreviated as 1Eu2+, 1Eu2+-xNaCl and 1Eu2+-xNaF) were prepared by introducing NaCl (A.R.), NaF (A.R.), and Eu2O3 (A.R.), and glass crystallization after quenching the reactants in ice water. Doping concentration: NaCl are 0, 1%, 2%, 3%, and 5% molar fraction and NaF are 1%, 2.0%, 2.5%, and 3.0%. Weigh according to the set reactant stoichiometry, and grind the mixture thoroughly in an agate mortar for 30 minutes to ensure even mixing. Transfer it to a graphite crucible, place it in a well furnace, heat it up to 1550 ℃ at a heating rate of 5°/min, and keep it in an argon atmosphere for 30 minutes. Then, pour the molten reactants into ice water for rapid cooling, dry and grind to obtain the precursor glass sample PG. Finally, the precursor glass sample PG will be kept in a reducing atmosphere at 1100 ℃ for 1 hour, and then ground into a crystalline sample after cooling in the furnace.Results and discussionWhen single doped with Eu2+, Eu2+ occupies the Mg2+ and channel sites in the - cordierite structure, forming EuMg2+ and Euvac2+ luminescent structural units. When NaF is introduced, Eu2+ occupies the Na+ site to form a new luminescent structural unit EuNa2+. Introducing NaCl/NaF can enhance the occupation of channel sites by Eu2+. When F–/Cl– replaces O2– and partially opens the hexagonal ring of the channel, the channel sites can accommodate more Eu2+, which makes it easier to explain the increased Euvac2+ structure formed when NaCl/NaF is introduced. When NaCl is introduced, Eu2+ hardly occupies the Na+ sites, while when NaF is introduced, as the amount of NaF introduced increases, Eu2+ occupies more Na+ sites. That is because the stronger electronegativity of O2– than Cl–, replacing O2– with Cl– can introduce more covalences in the lattice,that means Eu2+- Cl– has stronger contradiction and polarization than Eu2+–O2–. Moreover, the ionic radius of Cl– differs significantly from that of O2–, making the formation of Eu2+–Cl– more difficult. Therefore, when Cl– is introduced, Eu2+ does not occupy the Na+ sites. Similarly, when F– is introduced, Eu2+ is more likely to occupy Na+ sites. In addition, the amount of Eu2+ introduced is constant, and when Na+–Cl– is introduced, Eu2+does not occupy the Na+ sites. As the Euvac2+ structure increases, the corresponding generated EuMg2+ structure will decrease. As a ligand, compared to Mg2+–O2–, the introduction of Na+–F– has a smaller steric hindrance and greater bonding ability due to the smaller radius and greater electronegativity of F– (compared to O2–) ions. Eu2+ will preferentially occupy Na+ sites with similar radii and then occupy Mg2+ sites. As the amount of NaF introduced increases, more EuNa2+ structures are formed while the corresponding EuMg2+ structures decrease.ConclusionsThe main conclusions of this paper are summarized as following. This article mainly studies the effect of introducing Na+ sites and Cl–/F– substituted O2– on the occupancy of Eu2+ in the - cordierite structure. Exploring the mechanism of distinguishing subgrain structure evolution for luminescent structural probes. By introducing Na+ sites and anionic Cl–/F– substituted O2– to regulate the selective occupancy of Eu2+, temperature sensitive emission from blue white to yellow light is achieved. Cl–/F– substituted O2– opens the hexagonal ring structure of the channel, which facilitates the occupation of channel sites by Eu2+. The proportion of the formed structure increases from 33.32% to 64.89%. When introducing Na+ sites, the presence of F– increases the proportion of Eu2+ occupying Na+ sites to form structures to 18.63%. This series of fluorescent powders can significantly improve their thermal quenching performance when NaF is introduced, and have superior thermal stability at 150 ℃. This series of fluorescent powders can be widely used in the fields of adjustable luminescence, detection, and anti-counterfeiting.

IntroductionThe extensive use of fossil fuel resources has a serious impact on the environment, and hence energy conservation and CO2 emission reduction have become the focuses of current development. Solid oxide fuel cells (SOFCs) have attracted the research attention because of the outstanding energy conversion efficiency, wide range of fuel choices, no pollution, and high quality waste. H2 is the ideal fuel for SOFCs, but its low volumetric energy density and the requirement of high pressure to be liquefied, making it difficult to store and transport. At ambient temperature and pressure, methanol is liquid and its volumetric energy density is much higher than that of hydrogen, which makes methanol easy to storage and transport as a good hydrogen carrier. Furthermore, methanol has been quite mature production process. The industrial methanol production was hydrogenation of CO and CO2. SOFCs can generate power with direct internal methanol reforming. However, the main problem of direct methanol fed SOFCs is carbon deposition. The formation of carbon deposition not only reduces the cell performance, but affects the anode structure as well. In this study, the anode surface of flat-tube anode supported SOFCs was modified by Barium. The cell performance, durability, and methanol conversion of the unmodified and modified SOFCs were compared and evaluated under methanol atmosphere with a steam/carbon ratio of 0.75.MethodsThe anode support was composed of nickel oxide with 3%Y2O3-stabilized ZrO2 (NiO–3YSZ, in mole), and the anode functional layer was made up of nickel oxide with 8%Y2O3-stabilized ZrO2 (NiO–8YSZ). The electrolyte, barrier layer, and cathode layer were 8YSZ, Gd0.1Ce0.9O2– (GDC), and La0.6Sr0.4Co0.2Fe0.8O3––GDC (LSCF–GDC), respectively. The active cathode area was 60 cm2. The cell is denoted as NiO–3YSZ|NiO–8YSZ|8YSZ|GDC|LSCF–GDC. Using 15.68 g Ba(NO3)2 powder dissolved in 200 mL deionized water, being stirred at 70 ℃ for 2 h, to obtain 0.3 mol/L barium nitrate precursor solution for wet impregnation. After sealing one end of the Ni/YSZ cell with wax, the prepared barium nitrate precursor solution (70 ℃) was added into the fuel channels of through the sampler, and the other end of the cell was sealed with wax. The cell was kept in an oven at 50 ℃ for 24 hours to ensure that the Ba(NO3)2 solution was fully immersed in the anode channels. Then, the cell was dried at 90 ℃ for 1 h to remove the wax, and the cell obtained as above preparation processes was recorded as Ba–Ni /YSZ cell. After the cell was reduced at 750 ℃ by H2, 0.60 L/min (SLM) H2 was used as fuel for the initial cell performance test. When methanol was used as fuel for the test, the methanol solution with a steam/carbon ratio of 0.75 was added to the anode channels, and the temperature of the water steam generator was set at 130 ℃. At this time, the corresponding methanol flow rate was 0.30 g/min, and 5 L/min air was passed into the cathode. Fuel cell testing equipment (SOFC-100W, Wuxi Lead Equipment Co., LTD.) and electro-chemical workstation (VMP3B-20, France BioLogic Co., LTD.) were used to test the electro-chemical performance and durability of the cell under different operating condition. The frequencies of electrochemical impedance spectroscopy (EIS) tests were ranging from 20 mHz to 30 kHz. The AC amplitude of EIS testing was 10 mV.Results and discussionThe scanning electrical microscopy (SEM) with energy dispersive spectrometer analysis proved that Ba element was effectively loaded on the surfaces of Ni/YSZ anode fuel channels. Under H2 fuel, the current voltage (I–V) characteristics of Ni/YSZ and Ba–Ni/YSZ cells at 750 ℃ indicate that the open-circuit voltage (OCV) of Ni/YSZ and Ba–Ni /YSZ cells were 1.12 V and 1.07 V, respectively, which revealed that the cell air tightness was good. At 750 ℃, the maximum power density (Pmax) of Ni/YSZ and Ba–Ni/YSZ cells were 555.66 mW/cm2 and 507.03 mW/cm2, respectively, indicating that Ba element had no obvious influence on the cell output power of flat-tube SOFC fueled by H2. The electro-chemical performance of Ni/YSZ and Ba–Ni/YSZ cells in methanol atmosphere [r(S/C) = 0.75] showed that the power density of Ni/YSZ and Ba–Ni/YSZ cells at 750 ℃ and 0.8V were 326.72mW/cm2 and 452.87mW/cm2, respectively. It can be seen that the power density of Ba-modified cell was significantly improved when fueled by methanol. Under OCV condition, the methanol conversion rates of Ni/YSZ and Ba–Ni /YSZ cell were 89.40% and 92.70%, respectively, because Ba on the surfaces of the fuel channels promoted methanol conversion within the cell. The hydrophilicity of the Ba-modified cell was enhanced, and the water cracking into OH– groups that were adsorbed on the Ni surface was promoted. The durability of Ni/YSZ and Ba–Ni/YSZ cells was tested at 750 ℃ under methanol [r(S/C) = 0.75] and 200 mA/cm2. The results showed that the electro-chemical performance of Ni/YSZ cell deteriorated sharply shortly after 3 h to 4 h, while the voltage of Ba–Ni/YSZ cell kept stable. After running for more than 500 h, the OCV value of the Ba–Ni/YSZ cell remained stable, indicating that the air tightness of cell was good, and the voltage was 0.89 V with a deterioration rate of 0.01 (%)/h. The gas composition analysis of the Ba–Ni/YSZ cell during the durability test showed that the conversion rate of methanol in the modified cell during the test was stable, and the conversion rate was larger than 90%. According to the SEM images of the Ba–Ni/YSZ cell before and after the durability test, the average Ni particle size of anode functional layer of the Ba-modified cell lowered from 0.97 m to 0.92 m. This deduced that the Ni particle lost in the anode function layer of the Ba–Ni/YSZ cell was obvious after the durability test, leading to the performance deterioration of the cell. Raman spectra showed that Ni/YSZ cell has detectable carbon deposition after the durability test, while Ba–Ni/YSZ has no carbon deposition. This showed that Ba content on the surface of the anode support of flat-tube SOFC could effectively inhibit carbon deposition.ConclusionsThe performance and durability of the flat-tube Ba–Ni/YSZ cell were investigated in this study. The Ba–Ni/YSZ cell was prepared by wet impregnation. The power density of the Ba–Ni/YSZ cell with methanol under r(S/C)=0.75 at 750℃ was 452.87 mW·cm–2 at 0.8 V, larger than that of Ni/YSZ cell. The durability of Ba–Ni/YSZ with methanol under r(S/C)=0.75 at 750 ℃ reached 500 h, while the Ni/YSZ cell revealed a sudden performance degradation. The Ni particle agglomeration was detected for the Ba–Ni/YSZ cell after the durability test, causing the performance degradation during the test. No carbon was observed on the surface of the Ba–Ni/YSZ after the durability test.

IntroductionAlkali-activated fly ash cementitious materials are characterized by several advantageous properties, including corrosion resistance, high-temperature resistance, and the ability to solidify heavy metals. Additionally, these materials demonstrate a high utilization rate of fly ash and are produced through an environmentally friendly process. Despite these benefits, a significant limitation is their slow solidification and hardening at room temperature, which restricts their practical engineering applications. Consequently, alkali-seed composite activated solutions were used to prepare alkali-seed composite excitation fly ash (ASAFA) specimens, and their properties and structural evolution were explored. The results indicated that N-A-S-H gel and zeolite (sodalite and chabazite-Na) were the main components of alkali-seed composite activated solution, and their proportion played a crucial role in determining the excitation effect. Through adjustments in the activity of raw materials, reaction temperature and duration, it is possible to increase the total content and proportion of N-A-S-H gel and zeolite in the seed, thereby encouraging accelerated and uniform growth of the reaction products within the ASAFA system, and reducing the proportion of harmful pores in the hardened structure. As a result, the setting and hardening process and compressive strength of ASAFA was promoted.MethodsThe study utilized first-class fly ash (FA1) with a specific surface area of 430 m2/kg as the primary raw material. FA1 was subjected to sorting-roller grinding to achieve specific surface areas of 715 m2/kg and 1014 m2/kg, resulting in the production of FA2 and FA3, respectively. The activity of the three types of fly ash was assessed in accordance with GB/T 1596–2005 "Fly Ash Used in Cement and Concrete." The results indicated that the mechanical activation from the grinding process significantly enhanced the activity of FA2 and FA3 at all ages compared to FA1, with activity values at 28 days reaching 92% and 114%, respectively. An 8 mol/L NaOH solution was employed as the alkali activator for the synthesis process.The alkali-seed composite activated solution was prepared by treating fly ash with the NaOH solution at elevated temperatures. During the preparation, four technological parameters were systematically adjusted: ① Type of fly ash used; ② Mass ratio of fly ash to NaOH solution; ③ Synthesis temperature; And ④ synthesis time. The ASAFA specimen was created using the alkali-seed composite activated solution in conjunction with FA1. For comparative purposes, a reference specimen (GB) was prepared using a mass ratio of 0.5:1.0 of NaOH solution and FA1. Cubic specimens measuring 40 mm × 40 mm × 40 mm were fabricated and cured at a temperature of (20 ± 3) ℃ and 60% relative humidity. Following the initial curing period, the specimens were demoulded and subjected to additional curing for 7 and 28 days prior to the compressive strength testing.Results and discussionIn the preparation of the alkali-seed composite activated solution, a significant amount of sodium hydroxide solution is utilized, with a mass ratio of 100:4 to fly ash. The simultaneous application of heating and stirring notably accelerates the geopolymerization process. After a synthesis duration of 0.5 h, a compound activator is produced, characterized by a zeolite phase that includes sodalite and sodium-chabazite, exhibiting a markedly higher content than that observed in conventional polymerization methods. As the synthesis time is extended to 1 hour, the sodalite content within the composite activator continues to rise, with the zeolite phase predominantly existing at the nanometer scale. However, upon further extending the synthesis time to 2 hours, a slight decrease in sodalite content is observed, accompanied by an increase in the N-A-S-H gel content. This phenomenon may be attributed to the interaction between the zeolite phase and the strong alkali present in the sodium hydroxide solution, which facilitates the dissolution of aluminum ions from the zeolite phase into the alkali solution. Consequently, the N-A-S-H gel is generated through a polymerization reaction, a process further evidenced by the blurring of grain corners of the zeolite phase. Overall, in the sample designated as AD0.5, a greater coexistence of N-A-S-H gels and zeolite is observed. In contrast, AD1 contains only a limited number of unreacted fly ash particles, while the N-A-S-H gel undergoes a secondary reaction, resulting in the formation of smaller zeolite grains. In AD2, the content of zeolite particles decreases slightly, yet the structure becomes denser, and the particle size increases, alongside a rise in the N-A-S-H gel phase content. The seeds present in the alkali-seed composite activator primarily consist of N-A-S-H gel and zeolite crystal phases. These nano-products serve as nucleation sites that facilitate the geopolymerization of fly ash. Additionally, they fill the spaces between fly ash particles, promoting uniform growth of the induced polymerization products throughout the system. Notably, an increased content of crystal seeds in the composite activator, particularly a higher proportion of small zeolite crystal phases, significantly enhances their positive impact on the advancement of geopolymerization.ConclusionsThe alkali-seed composite activated solution primarily consists of N-A-S-H gel and zeolite phases, with their content and proportions being influenced by several factors, including the activity and dosage of fly ash, as well as the synthesis temperature and duration. By optimizing these parameters, it is possible to achieve the final setting of ASAFA pastes within as little as 2 h. Furthermore, these optimized specimens exhibit significant compressive strength, reaching 4.5 MPa at 7 d and 13 MPa at 28 d. In contrast, alkali-activated fly ash paste demonstrates a considerably longer final setting time of nearly 100 h, along with a much lower compressive strength of only 0.2 MPa at 7 d. This comparison underscores the advantages of using alkali-seed compounds in enhancing the performance of ASAFA pastes.

IntroductionThe pore structure of cement-based materials significantly influences their compressive strength. Studies have shown that the nanoscale pore structure significantly changes with variations in water content. Traditional pore measurement methods like mercury intrusion porosimetry and gas adsorption require the drying of specimens, potentially altering or even destroying the pore structure. The accuracy and representativeness of the compressive strength-pore structure relationship model established on this basis are therefore questionable. To accurately describe the important correlation between compressive strength and pore structure of cement-based materials, low-field magnetic resonance relaxation technique is utilized to carry out in-situ, nondestructive testing of saturated cement mortar doped with air-entraining agent. This is then combined with compressive strength measurement results to validate and refine the model describing the relationship between compressive strength and pore structure characteristics.Methods(1) Specimen preparation To avoid the ambiguous effects of ferromagnetic substances when using by low-field nuclear magnetic resonance (LF-NMR) technique, white Portland cement with low Fe2O3 content was used to prepare cement mortars with water-to-cement ratio of 0.4. Cement to sand ratio is 1:2. And air-entraining agents (SJ-2 type) were incorporated. Chemical composition of white cement were 0.50% Fe2O3,64.60% CaO,21.71% SiO2,4.60% Al2O3,2.80% SO3,2.43% MgO,0.48% R2O. The mass ratio of air-entraining agents to cement was 0%, 0.05%, 0.10%, and 0.15%. Considering different mass ratios, they are named as WP-BLK, WP-SJ05, WP-WP-SJ10, and WP-SJ15, respectively.According to the designed proportions, the white cement, sand, air-entraining agents and water were mixed and casted into prisms of size 20 mm×20 mm×50 mm. After demounting at 24 h, they were all cured in saturated lime water at (20±2) ℃ for 180 d. After reaching the specified age, ten specimens were taken from each group of mortar and put into the high-pressure water saturation equipment for 3 d to ensure that the specimens were completely saturated and set aside.(2) Pore structure test The pore structure of saturated mortars were tested by LF-NMR technology. Utilising a 2 MHz NMR analyser manufactured by Limecho Ltd., China, transverse relaxation test was performed through the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence at controlled room temperature about 20 ℃. In detail, the echo time e (s) was adopted as short as 60 s to detect the water confined in nanoscale pores of fast relaxation. Preliminary experimental tests had revealed that echo time e ⩽100 s was short enough to make the contribution of diffusion relaxation negligible due to the controlled low concentration of Fe2O3 in white cement pastes. Moreover, A long sequence of N = 40 000–60 000 echoes generated at time = ne (n=1, 2, · · · , N) (s) was recorded to capture the water stored in large capillary pores and even air voids with long relaxation time. 256–1024 scans with enough long repetition time 5–15 s were averaged. Except for the echo time, the rest of the parameters were flexibly adjusted according to water content of specimen. One can see detailed steps for testing pore structure by low-field magnetic resonance techniques in the relevant literature.(3) compressive strength test The compressive strength test of specimens were carried out by YAW-300 microcomputer automatic cement folding testing machine, which was produced by Jinan Hengruijin Testing Machine Co., Ltd., with a maximum capacity of 100 kN. The size of the compression surface of the specimen is 20 mm×50 mm, and the loading rate is fixed at 0.2 kN/s.Results and discussionLF-NMR technology accurately captures the pore structure characteristics of saturated mortar. Empirical models using total porosity or graded porosity as variables are generally effective, but the empirical parameters obtained from model fitting are not stable enough, limiting the model’s applicability. An improved empirical model, based on the pore size distribution curve and assuming a linear relationship between the weights of different pores on compressive strength and their logarithmic pore sizes, offers a concise form, significantly improves fitting accuracy, and accurately reflects the differences within the specimen groups. Furthermore, building on Griffith’s fracture theory, the theoretical model, which uses total porosity and logarithmic mean radius as independent variables, exhibits complex expressions and low predictive accuracy. If pores with a radius above 10 nm are regarded as defects, their influence can be explained by Griffith’s fracture theory. Meanwhile, pores with a radius of 10 nm or less are regarded as internal pores of the gel, and its effect on the strength of the matrix is approximated using the model description proposed by Powers. The resulting modified Griffith fracture theory model is very high in prediction accuracy, stable, and interpretable, accurately reflecting how minor differences in pore structure lead to small changes in compressive strength.ConclusionsThe incorporation of air-entraining agents mainly increases the capillary pores above 10 nm in radius but also decreases the matrix pores below 10 nm in radius. LF-NMR technology can obtain the full-scale pore structure of cement mortar, aiding in the description of the fundamental correlation between pore structure characteristics and compressive strength.

IntroductionSalt crystallization plays an important role in the physical weathering process of materials, and is generally considered to be one of the main problems affecting the durability of materials, and most of the weathering damage of porous materials is related to salt crystallization. Since the permeation and crystallization of salt solutions are unavoidable. Therefore, it is of great value to explore the crystallization process and mechanism of salt in pores for the study of material durability in the fields of construction engineering, historical building protection engineering, traffic engineering, underground and geotechnical engineering, etc, and is of great significance for improving the disaster prevention and mitigation capabilities of historical buildings and infrastructure, and reasonably predicting the service life of structures. At present, the supersaturated crystallization pressure hypothesis is generally accepted as the cause of salt crystallization failure of porous materials, which holds that the crystallization pressure exerted by crystal on the pore surface is the main cause of the failure of cement-based materials. However, existing study has found that the actual crystallization pressure produced by NaCl evaporation crystallization is about 0.46 MPa, which is much lower than the tensile strength of concrete, and the concrete material still subjected to damage. Therefore, in this paper, the finite element method is used to study the crystallization process of NaCl in the micro pore of hardened cement paste, and the mechanism of crystallization pressure during the crystallization process is revealed. In addition, the effects of pore and crystal geometry, crystallization pressure and filling degree on the maximum tensile stress inside the hardened cement paste are also discussed.MethodsThe micro pore model and NaCl crystal model of hardened cement paste are established respectively, and the NaCl crystal model is horizontally put into the micro pore model of hardened cement paste to obtain the NaCl crystallization process model in the micro pore of hardened cement paste. In order to accurately distinguish the crystallization process of NaCl crystal, a two-dimensional spatial filling degree is introduced. The model of NaCl crystallization process in micro pore of hardened cement paste is established in three steps. The first step is to determine the geometric properties of the crystal. The size change of the crystals in the NaCl crystallization process model is controlled by the filling degree, and the crystals without pore wall confinement are obtained, and the crystals in the pore confined state are obtained by Boolean operation. The second step is to split the model. The NaCl crystallization process model is divided into two parts: the hardened cement paste micro pore model and the NaCl crystal model. The third step is to apply crystallization pressure. The stress distribution diagram of hardened cement paste and NaCl crystal is obtained by applying crystallization pressure to the contact parts between the crystal and the pore wall in the micro pore model and the NaCl crystal model.Results and discussionDuring the crystallization process of NaCl in the micro pore of the hardened cement paste, the tensile stress region is distributed within 2 m of the pore radius, and the maximum tensile stress in the crystallization process is about 3 times of the crystallization pressure, which appears at the apex of the long axis of the pore, while the NaCl crystal is mainly compressed and the tensile stress is small. NaCl crystal do not fail before hardened cement paste. The pore axial ratio, filling degree and crystallization pressure are the main factors influencing the maximum tensile stress in the cement-based material during the crystallization process. The maximum tensile stress of hardened cement paste increases with the increase of pore axial ratio, filling degree and crystallization pressure during the crystallization process, while the crystal axial ratio has no obvious effect on the maximum tensile stress.ConclusionsThere are free crystallization and crystallization restricted stages in the crystallization process of salt in micro pore. The crystallization pressure generated by the crystallization restricted stage is a major factor causing damage to cement-based materials, and the tensile stress concentration caused by crystallization pressure is the direct cause of the damage of hardened cement paste. The crystallization pressure leads to the tensile stress concentration in the hardened cement paste, and the maximum tensile stress is about 3 times of the crystallization pressure, which appears at the apex of the long axis end of the pore. NaCl crystals will not be damaged before hardened cement paste. In addition, the pore axial ratio, filling degree and crystallization pressure are the main factors affecting the maximum tensile stress in the hardened cement paste during the crystallization process.

IntroductionMineral carbon sequestration is a crucial area of research in both environmental protection and industry. While much of the existing research has focused on the direct carbon sequestration of minerals to enhance cementation properties, there is a gap in understanding the interface micro-structure evolution of mineral powder particles during the carbonization process for the use of supplementary cementing materials. This study delves into the investigation of three calcium silicate minerals found in steel slag, examining the single-particle carbonization reaction rate and the micro-evolution process of the reaction interface at a micron scale. The goal is to uncover the differences and mechanisms behind the carbon fixation reaction rates of these three calcium silicate minerals.MethodsThree types of calcium silicate minerals, namely -C2S, -C2S, and C3S, were prepared using the sintering method. A time gradient experiment was conducted on these minerals, and the evolution of multi-scale products was observed and analyzed through various microscopic techniques. Quantitative analysis using XRD and TG-DTG was employed to compare the carbon sequestration rates of the three minerals, and reaction equations were constructed through fitting analysis. The morphology, crystal structure, and growth characteristics of calcium carbonate products during the carbon sequestration reactions of the three minerals were analyzed using SEM, TEM, and crystal size calculations. The correlation between the differences in carbon sequestration rates among the three minerals and the evolution of micro-structure was elucidated.Results and discussion-C2S had the highest carbon sequestration rate, which was 47.40% after 2880 min of carbon sequestration, reaching 92.65% of the theoretical carbon sequestration, and still had an exposed reaction surface at the later stage of the reaction. During the carbon sequestration process of -C2S, the carbon sequestration rate increases and the particle size, grain size and structure distribution of the product calcium carbonate aggregate are relatively stable. C3S has the highest carbon sequestration rate. Due to its high reactivity, C3S stagnates rapidly after 5 min, and then the carbon sequestration rate increases slowly, but the crystal shape of surface calcium carbonate products changes with time.The carbon sequestration reaction processes of -C2S and -C2S conform to BoxLucas1 model. The first 60 min is the rapid carbon sequestration stage of -C2S and -C2S, and the inflection point of reaction rate reduction occurs in 180 min and 360 min respectively, and the reaction plateau begins at 360 min. The carbon sequestration reaction of C3S is consistent with the nuclear shrinkage reaction model, and almost stops after 5 min. The calcite produced by the three minerals tend to grow on the (104) rhomboid plane, and the growth of calcite is controlled by the competition between grain growth and grain size reduction.-C2S and -C2S carbon fixation product calcium carbonate is mainly calcite, C3S carbon fixation product calcium carbonate also contains relatively more aragonite. The growth of calcium carbonate products on the surface of -C2S particles shows a transition from epitaxial growth to mantle growth. The epitaxial growth in the early stage leads to more reaction surfaces and increases the grain size of calcite. The growth of calcium carbonate products on the surface of -C2S particles is from mantle growth to epitaxial growth. The mantle growth in the early stage leads to denser nucleation, and the reaction ability is poor in the later stage, but the grain size development is more stable. On the surface of C3S, calcite and aragonite are mixed, and the composite structure of acicular aragonite and brick calcite hinders the gas-solid contact reaction.ConclusionsThe development rules of carbon sequestration rate of -C2S and -C2S are similar, and both conform to BoxLucas1 model; The calcium carbonate crystals on the surface of the two C2S particles are dominated by calcite, and tend to grow more preferentially on the (104) thermo-dynamically stable surface. The calcium carbonate on the surface of -C2S particles grows from mantle to epitaxial. The epitaxial growth in the early stage leads to a larger reaction contact surface, and it still has reactive activity in the late stage, and the calcite grain size is larger. However, -C2S epitaxial growth to mantle growth, the early mantle growth led to denser nucleation, the late reaction ability is poor, the grain size development is more stable. Due to its high calcium content, C3S has the fastest carbon sequestration reaction at the initial stage, and quickly stops after 5 minutes of reaction, which conforms to the nuclear shrinkage reaction model. C3S presents different product evolution rules: In the early stage of carbon sequestration reaction, acicular aragonite and cubic calcite on the surface of C3S are mixed, and its dense structure causes premature stagnation of carbon sequestration reaction. As the reaction proceeds, aragonite is transformed into calcite.

IntroductionAs the issue of steel corrosion in reinforced concrete structures becomes increasingly severe, enhancing their durability has become crucial. Therefore, the application of highly corrosion resistant steel, such as stainless steel and corrosion resistant alloy steel, is an effective countermeasure. However, the durability of steel in concrete is influenced by numerous factors, and its corrosion mechanisms exhibit long term and complex characteristics. The use of simulated concrete pore solution methods in research simplifies influencing factors and shortens the research cycle, thus being widely applied in academic studies. In modern construction practices, to meet low carbon and environmental requirements, a large amount of mineral admixtures is often added to cement. However, the use of these admixtures can cause fluctuations in the pH value of the concrete pore solution, thereby affecting the stability of the passivation film on the steel surface and increasing the risk of steel corrosion. Therefore, this study aims to investigate the passivation behavior and evolution of Cr10Mo alloy steel in simulated concrete pore solutions with different pH values.MethodsSteel was machined using a controlled lathe to produce rebar electrode discs with a diameter of 16 mm and a thickness of 10 mm. The bottom surface of the rebar was used as the working surface, with a working area of 2.01 cm2. All other non-working surfaces were sealed with epoxy resin. Each experiment utilized three rebar electrode discs as parallel samples. The working surfaces were gradually polished to a mirror finish using 400 #, 800 #, 1200 # and 2000 # silicon carbide sandpaper, then cleaned with deionized water, followed by an alcohol wash to remove grease. After drying, the discs were immediately placed into the standard corrosion cell. This procedure was repeated to polish a total of nine rebar electrode discs.The electrochemical measurements were conducted using a classical three electrode system. The rebar electrode served as the working electrode, the saturated calomel electrode was used as the reference electrode, and a platinum electrode was employed as the auxiliary electrode. All measurements were performed at a laboratory temperature of 20 ℃. The parameters selected for the linear polarization resistance test were as follows: the scanning potential was ±10 mV vs. OCP, and the scanning rate was 10 mV/min. The EIS test parameters were as follows: the perturbation voltage was a sine wave signal with an amplitude of 10 mV, and the scanning frequency range was from 10 mHz to 100 kHz.Results and discussionThe OCP and LPR test results indicate that in the CH simulated solution with the lowest pH value, alloy steel exhibits the lowest corrosion tendency after passivation. In the higher pH LC solution, corrosion tendency slightly increases, while in the highest pH ST solution, it is the highest. The Rp and Icorr values are as follows: CH solution at 1100 k·cm2 and 0.047 A/cm2, LC solution at 806 k·cm2 and 0.064 A/cm2, and ST solution at 534 k·cm2 and 0.097 A/cm2. Thus, the passivation performance ranks as CH > LC > ST, indicating that lower pH conditions lead to a more stable passivation film, effectively preventing further corrosion. EIS and equivalent circuit fitting results show that the CH simulation has the best passivation effect, with its value and Rct value surpassing those of the LC and ST simulated solutions, highlighting its superior passivation performance. Additionally, the electrochemical behavior of the rebar samples in different environments reveals that the integrity and electrochemical protection capability of the passivation film are closely related to the pH value. Comprehensive investigations, including Pourbaix diagrams, Gibbs free energy analysis, and the passivation reaction process of chromium, indicate that the selective transformation of Cr(OH)3 to Cr2O3 or CrO2– 4 in the passivation film influences its stability at lower pH values. Specifically, at lower pH values, Cr(OH)3 tends to convert to Cr2O3, forming a denser passivation film, thereby enhancing the passivation performance of the alloy steel. Conversely, under higher pH conditions, some Cr(OH)3 may convert to the less stable CrO2– 4, leading to a decrease in the stability of the passivation film and weakening its protective effect.ConclusionsRegardless of the pH conditions, alloy steel can spontaneously form a gradient passivation film consisting of an inner layer of Cr2O3 and an outer layer of Fe2O3. As the solution pH decreases, the content of Cr oxides/hydroxides in the passivation film increases, resulting in a denser and more stable film structure. The passivation performance of the alloy steel improves with decreasing pH value, primarily due to the selective transformation of Cr(OH)3 to Cr2O3 or CrO2– 4 in different pH environments. At lower pH values, the Icorr of alloy steel significantly decreases, and Rp significantly increases, indicating better formation and stability of the passivation film. In different pH simulated solutions, the passivation performance of the alloy steel follows the order: pH 12.4(CH) > pH 12.9(LC) > pH 13.5(ST).

IntroductionThe use of calcium carbonate powder as a mineral admixture in cementitious materials can reduce CO2 emissions from clinker calcination and fossil fuel combustion, offering a low-carbon solution for sustainable cement production. Calcium carbonate can effectively fill the pores, refine the pore size, and promote the growth and precipitation of early hydration products of cement. On the other hand, calcium carbonate can react with the aluminum phase in cement to generate calcium carbon-aluminate, improving the mechanical properties of cementitious materials. There are two main crystal types of calcium carbonate in nature, calcite and aragonite. Due to the difference in density, stiffness and crystal structure of calcite and aragonite, calcite and aragonite-type calcium carbonate have different effects on the properties of cementitious materials. This paper elucidated the effects of different crystal types of calcium carbonate on the microstructure and mechanical properties of cement pastes and calculated the various effects of calcium carbonate in cement pastes, which would provide a solid and effective theoretical basis for the application of calcite and aragonite in engineering practice.MethodsThe raw materials used in this study include Portland cement (P·I 42.5), slag, calcite and aragonite. The calcite and aragonite were prepared by wet carbonation and analyzed for composition and purity by XRD and TG. Calcite and aragonite were separately added to the cement–slag composite system to prepare two groups of ternary systems with a water-cement ratio of 0.4. The simplex-centroid mixture design method was used to optimize the relative composition design of the raw materials in the cement pastes. According to the cement pastes composition design, 20 mm×20 mm×20 mm specimens were moulded for testing compressive strength, ϕ20 mm×40 mm specimens were moulded for testing splitting tensile strength, and ϕ25 mm×25 mm specimens were moulded for pore structure analysis.The mechanical properties and pore structure of the specimens were tested after curing for 3, 7, 28 d and 90 d in standard curing conditions (temperature (20±1)℃, relative humidity ≥ 96%). The cement paste powders of corresponding age were taken for XRD and TG tests to analyse the type and content of hydration products. Finally, the nucleation effect, dilution effect, filling effect and chemical effect were quantified based on the contribution of calcite and aragonite to the sample compactness.Results and discussionThe addition of approximately 15% calcite or aragonite enhanced the compressive strength of the cement paste during early hydration. This was because calcite or aragonite promoted cement hydration, improved the early hydration degree of cement, and increased the content of hydration products such as Ca(OH)2 and C-S-H. The compressive strength of the calcite specimen was significantly higher than that of the aragonite specimen at the same dosage. Since the density of calcite was lower than that of aragonite, the filling effect of calcite was higher than that of aragonite, and calcite had a more significant nucleation effect than aragonite, with more hydration products deposited on the surface.After 28 days of hydration, the compressive strength and splitting tensile strength of specimens mixed with calcium carbonate and slag were significantly increased. The synergistic effect of calcium carbonate and slag significantly improved the chemical reaction degree of calcium carbonate, and increased the content of calcium carboaluminate. Meanwhile, the formation of calcium carboaluminate inhibited the transformation of ettringite into calcium monosulfoaluminate, the solid phase volume of hydrated product increased, and the compactness of cement paste increased. Therefore, the mixture of slag and calcium carbonate was conducive to the continuous improvement of the mechanical properties of cement pastes.With the increase of hydration age, the toughening effect of aragonite on cement paste was gradually improved. At 90 d of hydration, the content of aragonite was 20%~30% (in mass), the content of slag was 5%~15%, the contribution of chemical effect to the cement paste compactness was 15%~21%, and the splitting tensile strength of the specimen was increased by 39% compared with that of pure cement specimens. The improvement of the tensile strength of aragonite was mainly related to the chemical effect. The calcium carboaluminate formed by the reaction strengthened the bond between aragonite and cement paste, enhancing the toughening effect of aragonite.ConclusionsThe compressive strength of calcite specimens was found to be higher than that of aragonite specimens at the same calcium carbonate content during the early stage of hydration, primarily due to the increased filling and nucleation effects exhibited by calcite compared to aragonite. Due to the toughening effect of fibrous aragonite in cement pastes, the splitting tensile strength of the aragonite system was significantly higher than that of the calcite system. With the increase of hydration age, the reaction degree of calcium carbonate and aluminum phase in the composite system increased, the chemical effect continued to increase, and the compactness of the matrix increased. The addition of calcite or aragonite and slag was beneficial to the continuous improvement of the mechanical properties of the cement pastes. The increase in calcium carboaluminate content enhanced the toughening effect of aragonite and significantly increased the splitting tensile strength of the mixed specimens.

IntroductionUltra-High-Performance Concrete (UHPC), with ultra-high compressive strength and remarkable durability, holds great promise for constructing lightweight and high-performance structures. The ratio of live load effect to total load effect in the lightweight UHPC structure will be significantly increased, and higher cyclic stress amplitudes resulting in fatigue failure of the structure would occur accordingly. The combined tensile and bending stresses exist in certain components of practical engineering structures, such as UHPC bridge decks in the hogging moment region of the girder. Under overall load, the decks are subjected to a state akin to axial tension due to their limited thickness and considerable distance from the neutral axis of the girder. Yet, under the alternating localized wheel loads, the decks are subjected to cyclic bending stresses. Under the simultaneous application of the overall and local loads, UHPC bridge decks in the hogging moment region of the girder are actually subjected to combined tensile and bending stresses with variable amplitude. Many researchers have investigated the effects of steel fiber types, steel fiber content, and temperature on the bending fatigue properties of UHPC. However, there are limited studies on the axial tensile fatigue properties of UHPC, and investigations on the fatigue properties of UHPC under combined tensile and bending action are not found in the available literature. To determine the fatigue-resistant performance of UHPC with different tensile-bending stress ratios or the eccentricity ratio e/h of applied eccentric tension, an experimental study on the fatigue resistance performance of UHPC under combined tensile and bending action was conducted, and the corresponding fatigue strength was determined.MethodsWith the test parameter of eccentricity ratio e/h (0, 0.1, 0.4, and ∞, e/h=0 refers to axial tension and e/h=∞ to pure bending), 25 specimens with a commonly used steel fiber content of 2% were designed, manufactured, and tested. Among these, 9 specimens were tested for monotonic properties, and 16 specimens were tested for fatigue properties. The axially tensile and eccentrically tensile properties of UHPC were tested by C-shape specimens with corbel at both ends, and pure bending behaviors of UHPC were tested by four-point bending beams with identical cross-sections. The fatigue test was carried out under different eccentricity ratios and stress levels using a specially designed three-hinge truss device converting the tension loads to compression loads. Electrical resistance strain gauges were arranged at the extreme tensile edge of the specimens to measure the tensile strain before cracking. Two linear variable differential transformers (LVDTs) were continuously arranged in the extreme tensile edge of the specimen with a gauge length of 100 mm to measure the average tensile strain after cracking. The progression of crack width on the surface of specimen during fatigue testing was tracked using a hand-held crack detector with an accuracy of 0.01mm. The number of load cycles until fatigue failure under different stress levels was recorded, and further, the effects of eccentricity ratios and stress levels on fatigue life of UHPC were analyzed.Results and discussionIt was found that the development of fatigue tensile strain and crack width on the tensile surface of the specimens with fatigue failure reveals three stages of increasing rapidly with a decrease of strain rate, a steady growth trend with an almost constant rate, and increasing suddenly following a fatigue fracture. The tensile strain of the specimen without fatigue failure converges to a stable value after the rapid development in the first stage and remains unchanged in the second stage. The fatigue strength of UHPC increases with the eccentricity ratio rising, which can be attributed to the effect of stress redistribution between adjacent stripes in the tensile region caused by the increasing strain gradient on the cross-section. Compared with the corresponding strength of axial tensile specimens, for specimens with eccentricity ratios of 0.1, 0.4 and ∞, the initial cracking strength is increased by 10%, 38%, and 51%; the ultimate strength is increased by 14%, 57%, and 134%; while the fatigue strength is increased by 11%, 46%, and 105%, which demonstrate that the increased magnitude of fatigue strength lies between that of initial crack strength and ultimate strength. When the stress level of the specimen is close to its fatigue strength, the cumulative damage of the specimen is obvious, and the residual strength after fatigue is significantly lower than the corresponding static strength. When the stress level of the specimen is significantly lower than its fatigue strength, the cumulative damage of the specimen is slight, and the residual strength after fatigue is only slightly lower than its corresponding static strength.ConclusionsAs eccentricity ratios rises, the fatigue strength f and the fatigue strength ratio f/fe increase, while the fatigue strength ratio f/fp decreases, which means the stress level index Se=max/fe relative to the initial crack strength can more reasonably reflect the variation of fatigue strength of UHPC with eccentricity ratios under combined tension and bending action. For the UHPC with steel fiber content of 2% under combined tension and bending action, a fatigue equation for predicting the fatigue life of UHPC under different eccentricity ratios and various stress levels is proposed.

IntroductionCO2 curing technology has been extensively studied due to its dual benefits of carbon sequestration and enhancement in the properties of cementitious materials. During CO2 curing, the carbonatable binders react with CO2 and form carbonation products, such as calcium carbonate and silica gel, leading to matrix densification and rapid strength development. The development of this technology enables the use of low lime calcium silicate minerals, such as wollastonite (CS), rankinite (C3S2) and -dicalcium silicate (-C2S) with very low hydration reactivity to produce low CO2 footprint binders through CO2 curing. The carbonation activity, carbonation products and mechanical properties of the carbonated matrix of calcium silicate minerals have been widely studied. However, the relationship between the mechanical properties and microstructure of carbonated calcium silicate minerals remains unclear, which potentially limits the application of CO2 cured cement and concrete as well as low-calcium cementitious materials. In this work, the microstructure and mechanical properties of CO2 cured Portland cement, -C2S, -C2S, C3S2 and CS were investigated, and the correspondence between the structure and properties of CO2 cured calcium silicate minerals and the carbonation process was analyzed.MethodsThe raw materials used in this study include Portland cement (PC, PI 42.5), -dicalcium silicate (-C2S), -dicalcium silicate (-C2S), rankinite (C3S2) and wollastonite (CS). Pastes were prepared with a water-to-binder ratio of 0.18 for microstructural and phase analysis, while mortars were prepared with a constant water-to-binder ratio of 0.25 and sand-to-binder ratio of 2 for mechanical and porosity analysis. Specimens were prepared by compaction molding, with a pressure of approximately 10 MPa for pastes and 25 MPa for mortars. After molding and pre-conditioning, the compact specimens were placed in a pressure chamber. The curing chamber was vacuumed to a pressure of around -0.1 MPa, and maintained for 3 min. After that, CO2 gas, with a purity of 99%, was injected and maintained at 0.2 MPa at (20 ± 2) ℃ and RH of (60% ±5%) for 3 d.The carbonation degree of calcium carbonate minerals was determined by a thermal gravimetric analyzer. The phase composition of CO2 cured calcium silicate minerals was analyzed by XRD analysis, and FTIR spectra were obtained with a Thermo-Scientific IS10 FTIR instrument. Moreover, the microstructure and morphology of paste specimens were examined using Phenom LE SEM. The compressive strength of cylindrical mortar specimens ( = 25 mm and h = 25 mm) was tested after CO2 curing. Meanwhile, the pore structure was tested by a MAG-MED proton nuclear magnetic resonance spectroscopy.Results and discussionThe carbonation degree of -C2S, C3S2 and CS exceeded 60%, followed by 50.3% for -C2S, while that of PC was only 29.7%. The lowest carbonation degree of PC was because cement particles release more Ca ions than other minerals during early CO2 curing, rapidly forming a carbonate shell that blocks further carbonation inside the compacts. The carbonation products of calcium silicate minerals were mainly calcium carbonate and silica gel. Calcite was the main crystalline calcium carbonate, along with minor amounts of aragonite and vaterite. The silica gel phases in CO2 cured -C2S, C3S2 and CS showed higher polymerization degree and exhibited a clear demarcation from calcium carbonate, while the silica gel phases in the outer layer of CO2 cured PC particles were intermixed with calcium carbonate. This was related to the high hydration reactivity of PC, which could react with water and form a certain amount of hydration products (i.e. C-S-H and portlandite) in the pores. The carbonation of porous C-S-H results in the intermixing of calcium carbonate with silica gel. Additionally, the synergy of cement hydration and carbonation also facilitates the leaching of Ca2+, thus leading to an evaluated content of amorphous phases in CO2 cured PC.After 3 days of CO2 curing, the compressive strength of -C2S and -C2S mortars exceeded 50 MPa, followed by PC and C3S2, while the strength of CO2 cured CS was only 11.6 MPa. The compressive strength of CO2 cured calcium silicate minerals showed a linear relationship with CO2 uptake and porosity, increasing with a decrease in porosity and an increase in CO2 uptake. With the increase of Ca/Si, the CO2 uptake of CS, C3S2 and -C2S increased, the porosity decreased, and thus the compressive strength increased. However, PC and -C2S were subjected to both hydration and carbonation, which promote the formation of amorphous phases, leading to lower porosity and higher compressive strength. Moreover, the crystal size of calcite, the content of amorphous calcium carbonate and the interfacial properties of calcium carbonate and silica gel can also impact the compressive strength and microstructure evolution.ConclusionsThe carbonation products of calcium silicate minerals were mainly calcium carbonate and silica gel. The carbonation degree of non-hydraulic -C2S, C3S2 and CS was relatively higher, followed by -C2S, while PC demonstrated the lowest carbonation degree. The silica gel phases in CO2 cured -C2S, C3S2 and CS showed higher polymerization degree and exhibited a clear demarcation from calcium carbonate, while the silica gel phases in the outer layer of CO2 cured Portland cement particles were intermixed with calcium carbonate. The compressive strength of CO2 cured calcium silicate minerals showed a linear relationship with CO2 uptake and porosity. With the increase of Ca/Si ratio, the CO2 uptake of CS, C3S2 and -C2S increased, the porosity decreased, and thus the compressive strength increased. Portland cement and -C2S were subjected to both hydration and carbonation, leading to lower porosity and higher compressive strength. The smaller size of calcite crystals and the higher content of amorphous calcium carbonate also contribute to an increased mechanical property of CO2 cured Portland cement.

IntroductionMagnesium sulfide oxide cement (MOSC) is an air-hardening cementing material, which is produced by the reaction of light calcined MgO with a certain concentration of MgSO4 aqueous solution. Light calcined MgO raw materials have large differences in the aspect of MgO activity due to different calcination processes, and the MgO activity has a large impact on the performance of MOSC. For the preparation of MOSC, the increased MgO activity usually accelerates the hydration reaction speed of cement paste, leading to the increased exothermic amplitude of the system and the accelerated growth of Mg(OH)2 crystals in MOSC. As such, the process of condensation and hardening of MOSC is accelerated, which is unfavorable to the growth of the 5·1·7 phase, leading to an inferior compressive strength of MOSC. Therefore, efficiently inhibiting the hydration process of high-activity MgO plays a crucial role in the preparation of high-performance MOSC. In this work, a series of organic acids and high-activity MgO were used to prepare MOSC. The effects of organic acids on the hydration process of high-activity MgO, the setting time, the compressive strength, the phase composition, the micro-morphology, the pore structure, and the exothermic temperature of hydration of MOSC were investigated in detail.MethodsThe high-activity MgO powder from Hebei Magnesium Sheng Chemical Technology Co., Ltd. was used, and the content of active MgO was 85% as determined by the hydration method, with an average particle size of 20.82 m and a specific surface area of 47.45 m2/g. The MgSO4·7H2O was of industrial grade with a purity of 99%, and the citric, tartaric, oxalic, lactic, salicylic, lauric, and stearic acids were all analytically pure, purchased from Tianjin Hengxing Ltd.Hydration experiments of high-activity MgO: The hydration reaction was carried out under the conditions involving the temperatures of 45, 60, 75, 90 ℃, the reaction times of 0.5, 1.0, 1.5 h and 2.0 h, and stirring rate of 300 r/min, with a fixed solid-liquid ratio (mMgO: mH2O) of 1:10. The additions of organic acids were 0.25%, 0.50%, 0.75%, 1.00% and 3.00% at the MgO mass fraction. The pH value of the slurry during the hydration of high-activity MgO was monitored by a pH meter. The white solid obtained by filtration at the end of the experiment was calcined at 500 ℃ for 2 h. Then, the hydration rate of high-activity MgO was calculated.Preparation of MOSC: The raw material ratio for preparing MOSC was fixed to be n(MgO):n(MgSO4):n(H2O)=8:1:16. Organic acid was added at 1%, 2%, 3% and 4% of the mass fraction of high-activity MgO. MgO powder and MgSO4 aqueous solution were mixed together at a rate of 300 r/min at room temperature (25 ℃) and low temperature (ice water bath, 0–5 ℃), respectively. After the raw materials were mixed uniformly, the fluidity and setting time of MOSC paste were determined at room temperature (25 ℃) by referring to the national standards GB/T 8077—2012 "Test Methods for Homogeneity of Concrete Admixtures" and GB/T 1346—2011 "TestMethodsfor Standard Consistency of Cement in terms of Water Consumption, Setting Time, and Stability", respectively. The prepared MOSC slurry was placed in a curing box with constant temperature ((25 ± 2) ℃) and constant humidity ((60% ± 5%) relative humidity). The hydration temperature of the slurry was monitored by a RC-4HC-type temperature sensor for 12 h. The specimens (40 mm×40 mm×40 mm) cured in the curing box until 7 d and 28 d were tested for compressive strength. X-ray diffractometer, scanning electron microscope and mercury injection apparatus were employed to test the physical composition, microstructure and pore structure of the specimens cured for 28 d, respectively.Results and discussionAll the tested organic acids such as citric, tartaric, oxalic, lactic, salicylic, lauric, and stearic acids inhibited the hydration process of high-activity MgO, of which the hydration rate decreased with the increase of organic acid addition. Tartaric acid, citric acid and oxalic acid can effectively stabilize the hydration layer of high-activity MgO and inhibit its hydration process. The inhibition ability of organic acids on the hydration process of high-activity MgO can be summarized as: tartaric acid > citric acid > oxalic acid. The addition of tartaric acid, citric acid and oxalic acid as modifiers in the preparation of MOSC can effectively inhibit the hydration reaction of high-activity MgO in the slurry during the prehydration period. Thus, the generation of Mg(OH)2 was suppressed, and the growth of the 5·1·7 phase was promoted, resulting in the extended setting time and improved compressive strength of MOSC. The low-temperature preparation of MOSC significantly prolonged its solidification time and improved the fluidity of the slurry. Among them, the modification effect of tartaric acid reached the optimal performance with adding 2% (mass fraction to MgO) at room temperature. The addition of tartaric acid could effectively prolong the time required for the early hydration reaction to reach the maximum exothermic temperature, reduce the maximum exothermic temperature, stabilize the hydration layer of MgO and form an organic-magnesium complex to impede the generation of Mg(OH)2,. As a result, the growth of the 5·1·7 phase was promoted, with reducing the most available pore diameter of the product. Finally, the setting time and the compressive strength of the MOSC were significantly improved.ConclusionsTartaric acid could effectively inhibit the hydration process of high-activity MgO. Tartaric acid showed a good effect on suppressing the hydration of MgO, for which the the hydration rate of MgO was reduced from 58.7% to 19.2%. The addition of tartaric acid effectively prolonged the initial and final setting time of the slurry, inhibited the generation of Mg(OH)2, and promoted the formation and growth of the 5·1·7 phase in the preparation of MOSC with high-activity MgO. The compressive strength of the MOSC at 7 d and 28 d were 48.4 MPa and 54.8 MPa, respectively, which were increased by 111.4% and 83.9% compared with that of the MOSC prepared without tartaric acid.

IntroductionSupersulfated cement (SSC) is a low-carbon material made from slag, gypsum, and a small amount of alkali activator. However, the poor carbonation resistance limits its large-scale application. In this study, activated carbon with high CO2 adsorption capacity, as well as activated carbon modified by calcium hydroxide (CH), were doped into SSC (0.5% and 1.0%, in mass) to investigate the effects on the mechanical properties, carbonation depth, phase composition and microscopic morphology before and after the carbonation of SSC. This research aims at providing a effective methond to increase the carbonation resistance of SSC.MethodsP·I 42.5 Portland cement (conforming to GB 175—2023), S95 powder, gypsum and coconut shell-based activated carbon particles (Jiangsu Hartel Carbon Materials Technology Co., Ltd.) were used. The activated carbon powder particles were ball-milled and passed through a 150 m sieve. The activated carbon particles were immersed in saturated CH solution for 48 h and dried to obtain CH-modified activated carbon. The water-cement ratio of both paste and mortar specimens was 0.4. The dosage of activated carbon was 0, 0.5%, and 1.0% of the total mass, respectively. The compressive strength of mortar specimens (40 mm×40 mm×160 mm) were tested after curing for standard cured for 3, 28 d and after carbonation for 1, 3, 7, 14 d. The hydration degree of slag in SSC before carbonation was analyzed by EDTA method. The phase composition of hardened SSC before and after carbonation were measured by combination of a thermal gravimetric analyzer (TGA) and X-ray diffraction (XRD), The pore structure and micro-morphology hardened SSC before and after carbonation was tested by low-field nuclear magnetic resonance (LF-NMR, field emission scanning electron microscopy (ZEISS GeminiSEM 560), respectively.Results and DiscussionActivated carbon significantly improved the carbonation resistance and mechanical properties of SSC. Similar to nanomaterials such as carbon nanotubes, the active functional groups on the surface of activated carbon can provide nucleation sites for the growth of C-(A)-S-H gels, a phenomenon that has been confirmed by SEM results. Combined with the analysis of TG and XRD, it can be seen that this nucleation effect increases the number of C-(A)-S-H gels in the SSC. Meanwhile, according to the LF-NMR, the activated carbon would decrease the porosity and critical pore size of the SSC, thus enhance the compressive strength before and after carbonation.After carbonation, the activated carbon group generated more calcite than the control group. This indicates that activated carbon promotes the formation of C-(A)-S-H gel and possibly increases the proportion of Ca2+ in the gel. This is related to the strong adsorption capacity of activated carbon for Ca2+. Therefore, the mechanism by which activated carbon enhances the carbonation resistance of SSC is as follows: Activated carbon provides nucleation sites for C-(A)-S-H gel, promotes the formation of C-(A)-S-H gel, densifies the pore structure, thereby improving the mechanical properties and carbonation resistance of SSC. Furthermore, the clusters formed by activated carbon and C-(A)-S-H gel contribute to improve the distribution uniformity of the gel within SSC, further enhancing structural stability.Furthermore, activated carbon has a high CO2 adsorption capacity. After the carbonation decomposition of the C-(A)-S-H gel encapsulated on the surface of activated carbon, the exposed activated carbon can absorb the invading CO2. Saturated activated carbon might act as a ‘barrier’ to CO2, preventing CO2 penetration and further enhancing carbonation resistance.The carbonation resistance effect of CH-modified activated carbon is significantly lower than that of activated carbon. The CH distributed in the activated carbon could have dissolved into the liquid phase of the SSC and mainly act as an alkali activator. This leads to an excessive amount of alkali activator in the SSC system, thus reducing the C-(A)-S-H gel content, increasing the AFt content, and making the overall structure looser and more susceptible to carbonation. Therefore, activated carbon is effective in enhancing SSC’s carbonation resistance and can also be used to load other functional components.ConclusionsThis study demonstrates that activated carbon could significantly enhance the performance of SSC, particularly the mechanical properties and carbonation resistance. The results show that activated carbon can provide more nucleation sites for the generation of C-(A)-S-H gels, enhance the Ca/Si of C-(A)-S-H gels, increase the amount of C-(A)-S-H gel, densify the pore structure, and improve the compressive strength by 10.6% at the dosage of 0.5%. However, the enhancement effect diminishes at higher activated carbon content.In contrast, CH-modified activated carbon would lead to increase in the carbonation depth and porosity of SSC, significantly reducing its mechanical properties and carbonation resistance. This could be due to CH-modified activated carbon fails to effectively adsorb and immobilize CO2, instead of raising the alkali activator content in SSC. The excess alkali activator results in a decrease in C-(A)-S-H gel content, an increase in AFt content, and a more porous and easily carbonated structure.This paper demonstrates the significant practical value of activated carbon as a low-cost material. It not only greatly enhances the mechanical properties and carbonation resistance of SSC, but also offers greater functionalization potential due to its porous structure. By loading other components into the pores of activated carbon, activated carbon is expected to provide an effective means for the functionalization of SSCs and even other materials. The multifunctionality of activated carbon not only opens up new paths for optimizing the properties of materials, but also offers the possibility in a wider range of construction materials in the future.

IntroductionIn modern society, the electricity is the foundation of all intelligent facilities. To realize the intelligent development of cementitious materials, electrical conductivity is one of the important directions for the future research of engineering structures and building materials. Of which, ultra-high performance concrete (UHPC) has been widely used in large-span building structures, tunnel and bridge structures, and repair and reinforcement projects, etc. If the conductive properties of UHPC are realized, and its self-sensing characteristics are given to achieve the demand of intelligent monitoring, it is of great theoretical significance and practical value to ensure the service safety of the building structure. It has reached a consensus at present that UHPC is modified by adding conductive materials to reduce the resistivity. However, conductive materials are expensive and difficult to achieve large-scale engineering applications. The use of waste carbon fibers (WCFs) can not only meet the performance requirements, but also reduce the preparation cost, improve the utilization rate of resources, and avoid environmental pollution. But at this stage, there is a lack of research on the influence of WCFs on the electrical conductivity of UHPC, and the relationship between electrical conductivity and load-deflection needs to be explored. Therefore, this paper took the electrical conductivity and bending performance of UHPC as the research point, and analyzed the bending sensitivity and load-deflection relationship of C-UHPC under different WCFs content. And the variation law between stress, strain and resistance was explored.MethodsThe UHPC with conductive properties was prepared by WCFs and steel fiber (SF), and the final mix ratio was determined by combining theoretical analysis and pre-experimental research. The water-binder ratio of C-UHPC was 0.16, and the content of WCFs was 0%, 0.5%, 1.0%, 1.5%, and 2% (in volume), respectively. Due to some differences between WCFs and virgin carbon fibers (VCFs), a comparative study of the two fibers was carried out by SEM, Raman spectroscopy, and contact angle. The compressive strength tests of C-UHPC cured for 1, 3, 7 d, and 28 d were carried out. The conductivity of C-UHPC was tested by four-electrode method (28 d). The bending sensitivity of C-UHPC mainly studied the relationship between stress, strain, and resistance under bending load. During the test, the load was automatically recorded by the stress sensor of the test machine, the deflection was measured by the bottom displacement meter of the specimen, and the resistance was measured by the external resistance box. After loading, the change values of load, deflection, and resistance were collected synchronously. Finally, SEM was used to observe the microscopic morphology of fibers and fiber-matrix interface in C-UHPC.Results and discussionCompared with VCFs, the surface of WCFs was smoother, but the wettability and graphitization degree were relatively poor. Compared with the reference group (WCFs content of 0%), the compressive strength of C-UHPC decreased after the addition of WCFs, which was attributed to the smaller bond force between the interface of WCFs and matrix. In addition, WCFs and SF were easily entangled with each other, and the positive synergism between fibers was weak. The resistivity of C-UHPC decreased with the increase of WCFs content, but when the content of WCFs was greater than or equal to 1%, C-UHPC reached the conductive threshold, that is, the effective lap probability between fibers satisfied the conductive path of electronic transmission. The main conductive pathways of WCFs in C-UHPC included overlapping, connecting cracks, and micropores. When C-UHPC was subjected to bending load, the specimen underwent uncracked, critical cracks, macroscopic cracks, and finally failure. Of which, the macro cracks were mainly axial cracks, which expanded from bottom to top.A three-stage constitutive equation applicable to the C-UHPC bending load-deflection model was established with high prediction accuracy. When C-UHPC was subjected to bending load, the resistance changed, and this phenomenon indicated that C-UHPC had a bending sensitive property. Taking the peak load as the change node, the resistance curve of the pre-peak load decreased slightly, and the resistance curve of the post-peak load increased rapidly. This was because when C-UHPC was subjected to external loading, the internal microcracks and micropores were closed, resulting in a temporary increased and more complete conductive path, and the resistance decreased. After exceeding the peak load, the matrix gradually cracked, and the axial cracks expanded, interrupting most of the conductive paths, resulting in an increase in resistance. The stress sensitivity could be divided into two stages: first, the stress sensitivity decreased significantly first and then tends to be gentle with the increase of stress; second, the stress began to decrease after reaching the peak value, and the sensitivity increases. The Poisson's ratio was introduced to improve the formula for strain sensitivity, and the strain sensitivity perpendicular to the loading direction was obtained. The strain sensitivity decreased first and then increases with the increase of strain.Results and discussionThe addition of WCFs could reduce the compressive strength of C-UHPC, but improved its conductivity. Under bending stress, the load-deflection curve of C-UHPC could be divided into elastic stage, bending hardening stage, and failure stage. After mixing with WCFs, C-UHPC took the peak load as the change node, and the resistance curve of the pre-peak load basically did not fluctuate, and the resistance curve of the post-peak load increased rapidly. When WCFs content was 1%, C-UHPC reached the conductive threshold, and continuing to increase fiber content had no significant effect on the resistivity. The sensitivity of C-UHPC decreased first and then increases with the increase of stress and strain.

IntroductionEttringite plays an important role in the early strength and volume stability of cement. In a high-concentration sodium sulfate solution, Na+ and SO42– will be embedded between the structural layers of AFm to form a sodium-substituted AFm phase(U-phase). Therefore, the stability of ettringite and AFm under high temperatures and alkaline environments requires attention. Ettringite will not only transform into AFm but also form U-phase in a high-temperature and alkaline environment. However, the specific process of the mutual transformation of ettringite and AFm under the combined action of high temperature and alkalinity is still unclear. The phase transition of hydration products is bound to affect the performance of cement-based materials in high-temperature and high-alkalinity environments. Therefore, it is important and necessary to clarify the transformation process of Ettringite to AFm. Based on this, ettringite was first synthesized by adding a small amount of NaOH to the solution system, and the effect of NaOH on the synthesis and transformation of ettringite was explored, especially the effect on the evolution of the above-mentioned synthesized ettringite to AFm in an environment of 105 ℃. A detailed discussion was conducted, which provided new understanding of the stability of ettringite in high-alkali and high-temperature environments and also provided theoretical support for the performance degradation of cement-based materials under such conditions.MethodsThe raw materials Ca(OH)2 and Al2(SO4)3·18H2O were used to synthesize the ettringite sample by solution method, and the alkaline environment was adjusted by NaOH. Samples were taken at high temperature at designed time intervals to explore the transformation process of ettringite and washed with deionized water and anhydrous ethanol in turn, dried in a vacuum drying oven at 40 ℃, and ground into powder. The phase composition of all samples was tested by the D8 ADVANCE X-ray diffractometer, and the structure of the samples was analyzed by TOPAS V6 software. ZEISS Gemini 360 FE-SEMs was used to observe the microscopic morphology of the samples. The thermal stability of the samples was tested by a NETZSCH STA449F3 synchronous thermal analyzer. Based on density functional theory (DFT), the Castep module in Materials Studio was used to optimize the crystal structure and calculate the electronic structure properties.Results and discussionThe addition of NaOH is beneficial to the formation of ettringite and has an inhibitory effect on the formation of dihydrate gypsum. Under certain alkalinity conditions, the formation of ettringite is favorable. With the increase of NaOH addition, the short rod-shaped ettringite decreases and tends to be fine needle-shaped, with a certain curvature parallel to the c-axis. The increase in OH– ion concentration causes [Al(OH)6]3 to form rapidly, resulting in an accelerated nucleation rate of ettringite and a smaller crystal size. In addition, the addition of NaOH reduces the crystal cell parameter c and the crystal cell volume of ettringite. The defect formation energy of Na+ replacing Ca1, Ca2, Al1 and Al2 sites in the ettringite crystal structure is Na@Al1>Na@Al2>Na@Ca1>Na@Ca2 from large to small, it can be concluded that Na@Ca2 is the most stable crystal configuration, which indicates that Na+ is more inclined to replace the Ca2 site. Compared with the N-0 sample without NaOH added, the decomposition temperature of the small-sized ettringite synthesized in the NaOH environment is lower, and the stability of the sample is relatively weak.NaOH has a particularly significant effect on the transformation of ettringite to AFm. The intermediate phase U phase exists in the transformation of ettringite to AFm. The rod-shaped ettringite first split to varying degrees at both ends to form a flaky AFm phase. In addition, there are a small number of small needle-shaped rod-shaped ettringite crystals attached to the surface of AFm and transform from AFm into a thicker hexagonal flaky U phase. , and finally form a stable AFm phase. Na+ and SO42– are co-embedded between the layers of the AFm structure, resulting in a larger interlayer spacing of U-phase compared to AFm.ConclusionsThe ettringite samples synthesized by adding a small amount of NaOH at room temperature have fewer impurities than pure ones, the unit cell parameters are reduced, and Na ions easily replace some Ca2 sites in ettringite. With the increase in NaOH addition, the morphology of the synthesized ettringite changes from thick rods to curved needle rods, and the thermal stability of the ettringite decreases. The ettringite synthesized by adding a small amount of NaOH has an intermediate phase U-phase during the transformation to AFm under a high temperature environment, and all of them are transformed into the AFm phase after being kept at a high temperature of 105 ℃ for 24 h; however, the ettringite samples without NaOH almost do not undergo transformation. The transformation process of ettringite to AFm under NaOH and a high-temperature environment can be divided into three stages. Stage 1 (within 3 h): In this stage, most of the calcium sulfoxide directly transforms into the AFm phase. Stage 2 (4–6 h): Two reactions occur in the system. One is that Na+ and SO42– ions react with AFm to form the intermediate phase U-phase with a larger interlayer spacing. At the same time, Ca2+ and SO42– ions in the liquid phase react with AFm again to form calcium sulfoxide, which then decomposes into the intermediate phase U-phase; Stage 3 (8–24 h): The U-phase begins to gradually transform into the larger AFm stable phase.

It is well known that the compressive stress in surface layer can significantly improve the flexural strength and impact resistance of brittle materials, such as prestressed concrete and tempered glass. For the past century, many prestressed concrete and tempered glass have been manufactured successfully and widely used in various fields. However, little progress has been made for ceramic materials because of the high melting temperature and the intrinsic brittleness of ceramics. How to achieve the preset compressive stress on the ceramic surface is the key issue for prestressed ceramic research. This work introduces two ways for the formation of compressive stress on ceramic surface, namely surface coating method and surface extrusion method. Both advanced ceramics and traditional ceramics can use prestressing reinforcement to significantly improve the strength and durability of ceramic components. For advanced oxide ceramics, prestressing can increase the flexural strength by 40%–50%, while for traditional ceramics, the strength can be doubled. In addition, the structural-functional integration of the alumina ceramic substrate was achieved, i.e. both the strength and thermal conductivity of alumina sample were improved through the mixed coating. Besides, prestress strengthening method was applied to prepare the high-performance and mechanically stable solid electrolyte. Due to the sufficiently high compressive stress in the surface layer of solid electrolyte, cracks and dendrite penetration were restrained. Therefore, a controllable and long-life solid electrolyte was obtained. In general, by introducing compressive stress in the surface layer of ceramics, the fracture energy and impact resistance of ceramics were improved effectively.The main factors affecting surface prestressing by coating method are the ratio of expansion coefficient and the ratio of elastic modulus between the coating and the substrate material, as well as the ratio of cross-sectional area. It was found that, to generate the compressive stress on ceramic surface by coating method, it is required that the coating material has a lower coefficient of thermal expansion (CTE) and similar sintering temperature relative to the substrate. The optimal CTE condition for a pre-stressed ceramic component was described as c/s<0.83. Besides, both theoretical analysis and experimental results show that the compressive stress and crack resistance in the surface layer increase with the increasing ratio of the cross-sectional area of substrate to coating. While the main influencing factors of surface extrusion method are the temperature and time of ion exchange. As the temperature and the time of ion exchange increasing, the effect of prestress reinforcement improved firstly and then remained unchanged.Furthermore, inspired by the design of pre-stressed ceramics, the sintering deformation of Al2O3 components would be controlled by taking advantage of the difference in shrinkage between substrate and coating. By adjusting the composition and the position of coating, the Al2O3 ceramics with different shapes can be fabricated successfully. Besides, according to the deformation level, this method can be used to select the suitable coating raw materials. Generally, the way to fabricate the shape controlled ceramic components by the gradient shrinkage is a novel and effective method, which can be used for the plate and shell products with complex shapes.It is well known that the residual stress is not a constant. It is related to the position, the ratio of CTE of substrate to coating, interface bonding, and so on. The form and the magnitude of residual stress affect the mechanical properties of specimens. Hence, the evaluation of residual stress in prestressed ceramics is very important for their engineering applications. To evaluate the residual stress in prestressed ceramics, indentation deformation was used to illuminate the effect of residual stress on crack propagation. By comparing the length and the expanded direction of crack in ceramics with and without coating, the form of residual stress was cleared. Results show that the compressive stress could hinder the crack extension, while the tensile stress could promote the crack extension. Besides, the residual stress could be calculated via relative method. The formula derivation and experimental results show that the residual stress is determined by the elasticity modulus, the coefficient of thermal expansion, the cross-sectional ratio of the coating to the substrate and the temperature and the temperature. And it decreases with increasing temperature because of the stress relaxation.Summary and prospectsBy introducing preset compressive stress on the ceramic surface through the surface coating method or the surface extrusion method, the flexural strength and impact resistance of pre-stressed ceramics could be improved significantly. The relative research shows that the flexural strength of structural ceramics, architectural ceramics and domestic ceramics prepared by this new method could increase by 50%, 70% and 100%, respectively. The simple and economical pre-stressing design has shown great application prospects in the architectural ceramics, domestic ceramics, structural ceramics and structural–functional integration ceramics. Moreover, there is no limitation of size and shape in the fabrication of pre-stressed ceramic components, which is suitable for production of industrialization.

Piezoelectric ceramics, which enable the mutual conversion of mechanical and electrical energy via the piezoelectric effect, are indispensable for various industries, including robotics, communications, biomedicine engineering, etc. Currently, some piezoelectric ceramics face challenges such as difficulty in sintering and element volatilization. Pressure-assisted sintering (PAS) technology can promote the sintering process with the application of external pressure, thereby effectively engineering the electrical properties of piezoelectric ceramics. In this review, three commonly used pressure-assisted sintering techniques in the preparation of perovskite piezoelectric ceramics are highlighted, i.e., hot pressing sintering, hot isostatic pressing sintering, and spark plasma sintering.Hot pressing sintering, which applies a single-axis pressure during the sintering phase, is particularly adept at grain alignment and the fabrication of textured ceramics with directional property enhancement. Hot isostatic pressing sintering applies pressure uniformly in all directions, ensuring homogenous densification and the precise maintenance of complex structure dimensions. This method is invaluable for the creation of precision components where dimensional integrity is non-negotiable. HIP further enables the sintering of ceramics at reduced temperatures and shortened durations, thereby curtailing energy expenditure and averting excessive grain coarsening. Spark plasma sintering is characterized by its expedited heating rates and abbreviated sintering cycles, facilitated by the passage of pulsed direct current through the powder compact. This approach not only catalyzes swift densification but also promotes the development of fine-grained microstructures by curbing grain expansion. SPS is especially beneficial for crafting ceramics with exceptional strength and electrical properties.The advantages for enhancement of ceramic density, grain size, and defect engineering, texturing, as well as preparation of complex microstructures are demonstrated compared with conventional sintering. This review underscores the substantial benefits of PAS in bolstering ceramic density, a parameter of paramount importance to the mechanical and electrical properties of piezoelectric ceramics. The uniform density attainable through PAS is instrumental in enhancing performance and reliability. Moreover, the controlled sintering milieu in PAS is conducive to the precise manipulation of grain size, a factor with a significant bearing on piezoelectric performance. Fine-grained ceramics yielded via PAS have demonstrated superior functionality when juxtaposed with their coarse-grained analogs. Defect engineering represents another critical arena where PAS exerts a substantial influence. By moderating sintering temperatures and maintaining a controlled atmosphere, PAS minimizes the emergence of defects, such as vacancies and dislocations, which can diminish the efficacy of piezoelectric ceramics. The capacity to regulate defect concentration and type within the material is paramount for optimizing its electrical properties, including dielectric constant and piezoelectric coefficients. Furthermore, PAS has unveiled remarkable potential in the realm of complex microstructure fabrication, indispensable for state-of-the-art applications like high-frequency ultrasound transducers.The review also probes potential future research trajectories for PAS in perovskite piezoelectric ceramics. There is an imperative need for deeper exploration into the fundamental mechanisms governing sintering behavior under pressure-assisted conditions. Gaining insights into these mechanisms is vital for devising more efficient sintering protocols and achieving material property enhancements. Additionally, there is a burgeoning interest in the development of in-situ monitoring techniques, capable of furnishing real-time sintering process feedback, thereby enabling more precise control over the final ceramic properties.Despite its considerable merits, the scalability and economic implications of PAS must be deliberated to facilitate broader industrial implementation. Pressure-assisted sintering has validated its transformative potential in the realm of perovskite piezoelectric ceramic fabrication, offering meticulous control over microstructure and material properties. As investigative endeavors in this domain persist, PAS is poised to assume a central role in the genesis of next-generation piezoelectric materials.Summary and prospectsPressure-assisted sintering effectively promotes ceramic densification by providing additional sintering driving force, offering a feasible method for the fabrication of piezoelectric ceramics with high density and performance. Furthermore, Pressure-assisted sintering allows for flexible control of sintering temperature and time, which enables the preparation of high-density ceramics with varying grain sizes. Additionally, pressure-assisted sintering can combine with sintering atmospheres, electric currents, and other physical fields to engineer the defects. Uniaxial pressure can also introduce texture in some piezoelectric ceramics. Therefore, pressure-assisted sintering is expected to achieve synergistic regulation of density, grain size, defects, and grain orientation in piezoelectric ceramics, thereby comprehensively enhancing the mechanical, dielectric, piezoelectric, and optical properties of the material.However, precisely controlling the process of pressure-assisted sintering is challenging due to limitations in sintering equipment and mold materials. The issue of reproducibility caused by inhomogeneous pressure or current distribution deserves concern. For fundamental scientific research, pressure-assisted sintering can bring novel physical properties to piezoelectric ceramics. The clear demonstration of the sintering behavior of piezoelectric ceramics is the prerequisite for the in-depth understanding of these properties. Therefore, developing in-situ sintering characterization may assist in fully understanding the relationships among pressure, sintering behavior, and physical properties. For practical applications, high cost is the most significant barrier to scaling up pressure-assisted sintering for piezoelectric ceramics. The improvement in equipment and optimization of control systems are in high demand to overcome these technical challenges.

Concrete industries in cold regions, such as Northeast and Northwestern China, are facing serious challenges in achieving targeted performance criteria of concrete during winter construction. The early freezing of concrete during winter construction affects the hydration process of cement, rendering porous structure and increased internal frozen water content, which is detrimental to the development of mechanical properties and durability of concrete. Thus, investigating the performance and microstructure of early-age frozen concrete and proposing protection techniques are of great importance to the design, preparation, and durability improvement of concrete infrastructures in cold areas.There is a large amount of unhydrated free water in the newly mixed concrete, which can be transformed into ice once frozen at low temperatures. This process generates 9% increase in volume, leading to expansion stress on concrete. Since the cement paste at an early age has a low hydration degree, its frost resistance is relatively weak due to its low tensile strength. Once the expansion stress exceeds the ultimate tensile strength of the material, cracks will occur and accelerate the deterioration of concrete. In addition, when the temperature rises above the freezing point of the pore solution, the ice melts and leaves cavities in concrete, resulting in irreversible losses of strength and durability.The properties and microstructure of early-age frozen concrete are affected by factors, such as mixture proportion, water-to-binder ratio, antifreeze admixture type and dosage, freezing onset time, freezing temperature, freezing duration, and pre-curing regime, etc. The water-to-binder ratio is the primary factor influencing the damage degree of early-age frozen concrete because it directly determines the content of freezable water and concrete strength. When the water-to-binder ratio decreases from 0.46 to 0.29, the liquid water contents in the total and capillary pores decrease by 34.0% and 56.8%, respectively. Under a given water-to-binder ratio, freezing onset time is the most significant factor affecting the properties and microstructure of concrete, followed by freezing duration and temperature. With the decrease in freezing temperature, the extension of freezing duration, and the advance of onset freezing time, the mechanical properties of concrete gradually decline. This results in increased total and large pore volumes and decreased gel pore volume. In addition, early freezing lead to decreased polymerization degree and amount of C-S-H.To prevent early-age frost damage, concrete should obtain sufficient strength before it freezes, that is, the critical strength. In the technical standards of concrete for winter construction in some countries, the critical strength value is specified, generally ranging from 3 MPa to 8 MPa. Several techniques have been employed to ensure sufficient strength to alleviate the early-age frost damage and guarantee the safety of concrete structures during cold weather concreting. These include postponing the onset time of freezing with the aid of heating devices, enhancing the early-age strength of concrete using rapid-setting cement, nano-materials, alkali activator, etc., reducing the freeable water content through densifying the microstructure of concrete, and inhibiting the freezing of internal water using antifreeze admixtures. The purpose of postponing the onset time of early freezing is to keep concrete at appropriate temperatures to reach the critical strength as soon as possible. The use of nano-materials, early-strength components, and alkali activators can accelerate the early hydration of cement and improve the early strength of concrete. By reducing the water-to-binder ratio or extending the curing time, the internal structure is densified to reduce the amount of freezable water. In addition, antifreeze admixtures can inhibit the freezing of internal water by reducing the freezing point of the solution of fresh concrete mixture. At present, concrete mixed with antifreeze admixtures combined with heating devices are common means to prevent the early-age frozen damage of concrete.Summary and prospectsConcrete should have good early-age and long-term frost resistance under negative temperature conditions. The pore structure of concrete and the variety and dosage of antifreeze admixtures are critical to the development of the performance and microstructure of early-age frozen concrete. So far, precise mixture design of concrete subjected to early freezing is lacking. Therefore, proposing design methods for durable concrete based on the pore structure and antifreeze optimization is of great significance in promoting its application in cold environments. Besides, current literature mainly focuses on experimental investigation of early-age frozen concrete, the early-age frost damage mechanism is not clear since there are many influencing factors. It is necessary to correlate the macro-performance to the microstructure of early-age frozen concrete utilizing experimental tests and simulations. Also, there is a need to establish multi-scale freeze-thaw damage models, which would help explore the damage mechanism and predict the service life of early-age frozen concrete.