IntroductionPolyimide film is one of the most commonly used organic film materials for spacecrafts. In the low-Earth orbit (LEO) environment, the polyimide film applied to the surface of spacecraft will be eroded by atomic oxygen. Generally, performance failure will occur in about one year due to the erosion effect , affecting the service life of spacecrafts. The siloxane coating prepared by plasma-enhanced chemical vapor deposition (PECVD) method has good atomic oxygen resistance, which can reduce the atomic oxygen erosion rate of polyimide film to less than one-thirtieth. At the same time, the coating also has good light transmittance, flexibility and bendability. In recent years, there have been many studies on the effects of electrode spacing, power, pressure and other parameters on the properties of siloxane coatings prepared by PECVD at home and abroad, but the effect of oxygen flow rate on the atomic oxygen resistance of siloxane coatings has not been reported. This work focuses on the deposition rate, cross-sectional morphology, composition and structure of siloxane coatings prepared by PECVD under different oxygen flow rates. The atomic oxygen effect test was carried out on the coatings, and the mass loss data of different samples were successfully collected. The oxygen flow rate parameters of coatings with excellent atomic oxygen resistance has been obtained, which provides support for the research of atomic oxygen resistant coatings.MethodsThe coating samples were prepared by the PECVD method using a self-developed continuous roll-to-roll PECVD coating equipment. The substrate material for coating was 50 m thick DuPont Kapton HN polyimide film. The reactant gases were Alfa Aesar hexamethyldisiloxane monomer (HMDSO) and oxygen (O2). With a fixed HMDSO flow rate of 25 mL/min, the O2 flow rate was adjusted to 4, 8, 12, 16, 20, and 24 mL/min respectively, while maintaining a deposition pressure of 8 Pa and a reaction power of 400 W. This setup was used to investigate the deposition rate, structure and composition of deposited coatings under different precursor ratios. The coatings were subjected to atomic oxygen irradiation tests using a microwave atomic oxygen generator. The atomic oxygen flux was calibrated using DuPont Kapton HN polyimide film. The composition of the coatings was analyzed using the INVENIO R Fourier Transform Infrared (FTIR) spectrometer from Bruker Optics, Germany. In this test, the siloxane coating was deposited on the surface of a nearly mid-infrared transparent potassium bromide (KBr) single crystal to reduce the influence of the substrate on the test results. The surface and cross-sectional morphology of the coatings were examined using a Zeiss Sigma 500 scanning electron microscope from Germany, and the thickness of the coatings was also determined.Results and discussionUnder low oxygen flow rates, the deposition rate of the coating was significantly higher than that under high oxygen flow rates. This was attributed to the fact that at lower oxygen flow rates, most of the precursor reactants contribute to the formation of the coating, resulting in a higher deposition rate. Conversely, at higher oxygen flow rates, the probability of reactions between HMDSO and O2 increases, leading to an increase in the amount of gaseous products generated and a corresponding decrease in the deposition rate. All the siloxane coating samples exhibited a fairly dense cross-sectional morphology with no observable crystallization. When the oxygen flow rate was 8, 12, or 16 mL/min, the coating cross-sections were smooth and free of texture. At 4 mL/min, however, local damage was observed in the coating cross-section, possibly related to an incomplete formation of the composite network structure. At 20 and 24 mL/min, the coating interfaces remained dense but exhibited a textured structure, indicating the formation of local atomic ordered arrangements within the coating and the generation of structural stress. The FTIR spectra of the coatings revealed that as the oxygen flow rate increased, the intensities of rocking and stretching vibration peaks (both symmetric and asymmetric)belonging to the Si-O bond increased, indicating an increase in the content of silicon oxides in the coating. Concurrently, the intensities of the methyl bending vibration peaks in Si—(CH3)2, Si—(CH3)3, and Si—CH3, as well as the C—H stretching vibration peak, decreased with increasing oxygen flow rate, suggesting a reduction in the methyl content of the coating. The stretching vibration of the O—H bond became more pronounced, indicating that higher oxygen concentrations favored the combination of H and O elements. Atomic oxygen exposure tests on siloxane coatings prepared under different oxygen flow rates showed that as the oxygen flow rate increased from 4 mL/min to 20 mL/min, the mass loss of the coatings decreased exponentially. However, when the oxygen flow rate reached 24 mL/min, the mass loss increased sharply. Coatings prepared at lower oxygen flow rates exhibited smooth surface morphologies without cracks, holes, or other defects. In contrast, the siloxane coating prepared at 24 mL/min exhibited a large number of cracks, with the coating tilting at the crack sites. These severe cracks provided channels for atomic oxygen erosion, leading to a higher atomic oxygen erosion rate for the sample.ConclusionsDuring the deposition process of siloxane coatings, the deposition rate is highest when the oxygen flow rate is 8 mL/min, and then it decreases as the oxygen flow rate increases. The cross-section of the siloxane coatings grown under low oxygen flow conditions does not exhibit visible grains or grain boundaries. In contrast, the cross-section of siloxane coatings grown under higher oxygen flow conditions indicates the occurance of textures, although they haven’t displayed typical grain morphologies, which may be related to the stress within the coatings. The increase in oxygen flow rate can facilitate the oxidation of CH3 in the gas phase during PECVD and the formation of siloxane coatings. As the oxygen flow rate increases, the mass loss of the coating under atomic oxygen exposure decreases at first and then increases. At lower oxygen flow rates, insufficient oxidation of HMDSO results in the presence of more methyl and hydroxyl groups in the coating. These methyl and hydroxyl groups are easily oxidized by atomic oxygen to produce volatile gases, leading to higher mass loss. However, at higher oxygen flow rates, cracks appear in the coating, causing an increase in mass loss. This study demonstrates that a suitable oxygen flow rate is crucial in optimizing the performance of siloxane coatings by promoting the reactions of HDMSO and forming a dense microstructure,

IntroductionZnO inorganic white paint is composed of ZnO pigment and a potassium silicate binder as a type of thermal control coating of spacecraft. It is commonly used for spacecraft antennas, cabin sections, and radiator surfaces. In general, the solar absorptance of ZnO inorganic white paint is lower than that of other thermal control coatings. ZnO mainly exists in the form of wurtzite structure, and its zinc oxygen ratio often deviates from stoichiometry, generating intrinsic defects internally. Oxygen deficient compounds are generally generated due to the easy escape of oxygen in zinc oxide. During the in-orbit service of spacecraft, ZnO inorganic white paint is subjected to space environmental effects, such as electrons, protons, ultraviolet (UV), and atomic oxygen (AO), resulting in its performance degradation and affecting the stable service of spacecraft in orbit. It is thus necessary to investigate the degradation and damage mechanism of ZnO inorganic white paint to support the optimization of material properties.MethodsZnO pigment sample was a circular flake powder, which was pressed into a grinding tool with an inner diameter of 25 mm using a machine. The pressure parameter is 10 kN and the pressure time is 5 min. A circular sample with potassium silicate-coated zinc oxide (KSZ) inorganic white paint with a diameter of 30 mm, composed of K2SiO3 binder and ZnO pigment was prepared on an aluminum substrate by a spray coating process.Space environmental irradiation experiments were conducted on ZnO pigments and KSZ coatings by environmental irradiation and low orbit AO equipment. The electron energy of ZnO pigment irradiation experiment is 50 keV, with a fluence of 1×1016 cm-2, a proton energy is 50 keV, with a fluence 6×1015 p/cm2 and the UV dose is 5000 ESH. The AO fluence is 1×1021 atoms/cm2. The electron and proton energies of KSZ irradiation experiment are the same as those in the ZnO pigment experiment, with fluence of 2.25×1016 e/cm2 and 3.15×1016 p/cm2, respectively. The UV dose is 5000 ESH. The AO fluence is 2.0×1022 atoms/cm2.The spectrum reflectance of ZnO pigment and KSZ at wavelengths of 300-2200 nm was determined by a model Lambda 950 UV-Vis-NIR spectrophotometer (Perkin Elmer Co., USA), and the solar absorptance of KSZ was calculated. The fluorescence spectra of ZnO pigment were characterized by a model RF-5301PC spectrometer (SHIMADZU Co., Japan). The excitation source was a xenon lamp with an 220 nm wavelength. The electron spin resonance on ZnO pigments at a microwave frequency of 9.435 GHz was measured by a model JES-FE3AX electron spin resonance spectrometer (JEOL Co., Japan).Results and discussionAfter environment irradiation experiment, the spectral reflectance coefficient of ZnO pigment increases sharply and reaches its maximum value in a wavelength range of 360-500 nm. The spectral reflectance coefficient decreases with increasing wavelength. The peak position of the optical absorption band of ZnO pigment is at 425 nm. Compared to other environments, the absorption peak changes most severely under proton irradiation.Under electron, proton, and UV irradiation, the inherent VZn2− in ZnO pigments undergoes ionization, generating single ionized zinc vacancies (V'Zn−). AO causes a very small portion of the inherent VZn2− in ZnO pigments to be converted into V'Zn− due to its intense electron affinity and oxidizing properties, while the majority of VZn2− is converted into VZn0.The initial value of the solar absorptance of KSZ coating is 0.12. After the electron, proton, and UV combined irradiation experiment, the solar absorptance of the coating increases with the increase of environmental test fluence. The solar absorptance of the coating increases to 0.27 at electron fluence of 2.25×1016 e/cm2, proton fluence of 3.15×1016 p/cm2, and UV dose of 5000 ESH. The solar absorptance of KSZ coating increases within 2000 ESH UV irradiation, and then the solar absorptance stabilizes at 0.15. The KSZ solar absorptance is increased by 0.02 after 2.0×1022 atoms/cm2 AO irradiation.ConclusionsAfter space environment irradiation experiment, the spectral reflectance coefficient of ZnO pigment decreased, mainly concentrated in the visible and near-infrared regions, and the peak position of the optical absorption band was at 425 nm. The ionization of the inherent VZn2− in ZnO pigments occurred, resulting in the formation V'Zn−, which was the main mechanism for the degradation of the optical properties of ZnO pigments under electron, proton, and UV irradiation. AO caused a very small portion of the inherent VZn2− in ZnO pigments to be converted into V'Zn− due to its intense electron affinity and oxidizing properties, and the majority was converted into VZn0. After the combined experiments of electron, proton, and UV (i.e., electron fluence of 2.25×1016 e/cm2, proton fluence of 3.15×1016 p/cm2, UV dose of 5000 ESH), the solar absorptance of KSZ coating increased from 0.12 to 0.27. After 5000 ESH UV irradiation, the KSZ solar absorptance reached to 0.15. After AO irradiation of 2.0×1022 atoms/cm2, the KSZ solar absorptance was increased by 0.02. Compared to UV and AO, the electron and proton irradiation had a greater impact on the optical properties of KSZ inorganic white paint.

IntroductionA long-term service capacity of Portland cement is important for cement mineral composition manipulating, concrete mix proportion design, and building maintenance. In 2017, beam cores were sampled on the Hugee building as one of the first reinforced concrete structures in China built in 1916. Macro- and micro-properties of concrete core samples were tested, i.e., mechanical properties, anti-permeability properties, microscopic morphology, chemical composition, mineral composition and thermal stability properties. Meanwhile, concrete mix proportion parameters were disclosed by a reverse analyze method. In this paper, the macro- and micro-properties of concrete after a century of service and the evolution law of concrete properties were investigated. The results show that although the water-cement ratio of the concrete used in the construction of the Hugee building is relatively large, the mechanical properties and anti-permeability properties of the concrete are satisfying after a century of service. In addition to conventional C-S-H gel, CH crystals, AFt crystals, and AFm crystals, the concrete hydration products also contain a large amount of calcium carbonate crystals (i.e., calcite, aragonite and vaterite), indicating that the concrete is gradually carbonized during its service. The results of SEM and quantitative XRD analysis show that after a century of service, the concrete still contains unhydrated Belite clinker minerals (hydration degree of 69.4%), which provides a material basis for the continuous improvement of concrete strength, showing that concrete prepared with Portland cement has a service capacity of more than 100 years.MethodsChemical composition, mineral composition, microscopic morphology, mechanical properties, anti-penetration ability of century-long service Hugee building concrete were systematically measured. Especially, the mix proportion of Hugee concrete was analyzed by a reverse analysis method in the absence of historical records of the material and mix proportion for the Hugee concrete.Core specimen obtained from Hugee building was first dismembered to obtain its coarse aggregate, fine aggregate and cement paste (i.e., unhydrated clinker mineral and hydration products). The particle size distribution of coarse aggregate and fine aggregate were analyzed via sieving. The chemical composition, mineral composition and microscopic morphology of cement paste were analyzed by X-ray fluorescence (XRF), X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). In addition, the hydration degree of the Portland cement clinker minerals in the Hugee concrete was also quantitively calculated based on the analysis by quantitative X-ray diffraction (QXRD) and thermal gravimetric-differential thermal analysis (TG-DTA).Results and discussionThe results of reverse concrete mix proportion analysis show that the water to cement ratio (W/C) of the Hugee concrete is 0.69, and the bulk volume ration of cement:sand:aggregate is 1.0:2.1:4.0, which is well in line with the concrete preparation guidelines in 1920s. Although a high W/C value in the Hugee concrete restricts its strength and density according to the Abrams W/C-strength law, after a century of service, the compressive strength of the Hugee concrete is 23.4 MPa and the passed charge of the Hugee concrete is 936 Coulomb, which is satisfactory for a centenarian concrete.The results by SEM and QXRD show that after century-long service, the Hugee concrete is severely carbonated. The carbonation products are calcite, aragonite and vaterite, resulting from the carbonation of CH crystal, C-S-H gel and Aft crystal from cement hydration products. The hazard of the carbonation is the consumption of cement hydration product, and the benefit of the carbonation is the densification effect of concrete, preventing the penetration of harmful liquid into concrete. The guiding value of the carbonation measurement is that the W/C ratio can be lowered by chemical admixture to optimize the pore structure in concrete and enhance its anti-penetrating ability, which is important for guaranteeing concrete durability. Also, after century-long service, cement clinker minerals like alite, celite and ferrite are completely hydrated. However, partial of cement clinker mineral like belite still exists, which guarantees the continuous strength growth and micro-crack healing ability of concrete. Based on the results by TG-DTA, the hydration degree of belite calculated is 69.4% after a century-long service. The results of cement clinker hydration degree analysis show that high Belite cement is of unique value for guaranteeing concrete long-life service.ConclusionsThe reverse concrete mix proportion analysis method promoted could well reveal concrete mix proportion starting from scratch, which is important for understanding concrete mechanical, anti-penetration and micro-properties. The century-long service Hugee concrete is of well mechanical strength and anti-penetration ability, resulting from well preparation and casting at its birth age. After century-long service, the hydration products in the Hugee concrete include CH crystal, C-S-H gel, AFt crystal, AFm crystal, gypsum crystal and types of calcium carbonate crystals. The calcium carbonate crystals are resulted from the carbonation of Portland cement hydration products. After century-long service, there is still 30.6% of belite mineral to be un-hydrated, which is a material basis for the continuous improvement of concrete strength, manifesting that concrete prepared with Portland cement has a service capacity of more than 100 years.

IntroductionCompared with the conventional moulded concrete process, the layer-by-layer stacking process produces a large number of interlayer interfaces in 3D printed concrete (i.e., 3DPC), leading to a low flexural and tensile strength of 3DPC and a poor structural integrity of 3D printed concrete structure, which restricts the development and large-scale application of concrete 3D printing technology. Reinforcement is considered as one of the most effective measures to improve the flexural and tensile properties of 3D printed structures and enhance the structural integrity. Various reinforcement techniques for 3DPC, such as fiber reinforcement and transverse reinforcement, are developed, but all of them fail to effectively reinforce the interlayers. A longitudinal reinforcing approach involving short-cut straight and n-shaped bars was firstly proposed in 2018, and some work showed that longitudinal reinforcing method of n-shaped bar could enhance the mechanical properties of 3DPC.In this paper, the influences of the penetration degree of the n-shaped bars and the overlapping conditions on the flexural properties of 3DPC were investigated, and the damage process of 3DPC was analyzed via crack displacement and section morphology. The different reinforcing methods were compared.MethodsPortland cement P·I 42.5 according to Chinese standard and natural river sand with a fineness modulus of 3.4 as a fine aggregate were used. Hydroxypropyl methylcellulose (HPMC) with an apparent viscosity of 198 Pa·s was selected as a viscosity enhancing agent and a bauxite-based powdered accelerator with a fineness (80 m square-hole sieve residue) of 9% was used to adjust the thixotropy and buildability of 3DPC. A polycarboxylic acid-based high-performance superplasticizer with a solid content of 39% was used to adjust the flowability of the printed materials. In this work, a ratio of the layer number penetrated by n-shaped bar (LP) to the total layer number (LP) was defined as a penetration degree (P) including overlapped penetration degree Povr and nonoverlapped penetration degree Pnon. Eight types of n-shaped bar layouts that meet the constraint requirements of the formula were investigated.To ensure the accuracy of the n-shaped bar overlapping and positioning, a manual reinforcing method was used to accurately locate and reinforce the n-shaped bars. This process was repeated for ten layers of concrete, resulting in the completion of 3DPC specimens reinforced by n-shaped bars. For the unreinforced specimens, the preparation was completed via directly printing ten layers in a row.After printing, the specimens with the sizes of 40 mm×200 mm×180 mm were placed in an indoor environment for 24 h and then transferred to a standard curing room for 28 d. In the curing period, the specimens underwent a three-point bending test using a CMT-300 universal pressure testing machine. The span between the specimen supports was 160 mm, and the loading was controlled using mid-span displacement mode at a loading speed of 0.01 mm/min. The flexural strength of each group with three specimens was obtained via calculating the average value.Results and discussionWhen Pnon=0.1, the addition of n-shaped bars fails to strengthen the 3DPC, and reduces the flexural strength of 3DPC. When Pnon=1.0, the maximum flexural strength reaches 5.88 MPa, which is only slightly greater than that of the unreinforced 3DPC. Under the condition of nonoverlapped, the n-shaped bar is not effectively connected at the interface, and the n-bar has little enhanced effect.Povr=0, Compared to the unreinforced 3DPC as Povr=0, the flexural strength of reinforced 3DPC increases when Povr=0.2, with an increase of 100%. As the penetration degree increases, the flexural strength increases by 128% when Povr=0.3 and by 152% when Povr=0.9. The flexural strength is highly improved by an overlapped n-shaped bar. The damage mode of overlapped n-shaped bar is mainly a ductile failure with obvious strain hardening characteristics.The influence weights of ft0, P and of flexural strength of reinforced 3DPC are 0.896 2, 0.836 0 and 0.766 3, respectively. Based on the analysis of these influence weights, the advantages and disadvantages of the two reinforcement methods, i.e., the rivet method and chopped bar method, are evaluated. To ensure a fair comparison, each parameter is normalized to the same order of magnitude and unit, and transformed into equivalent flexural strength. The chopped bar and n-shaped bar (overlapped) methods both have the similar reinforcing effects, with equivalent flexural strengths of 27.74 MPa and 26.25 MPa, respectively, based on the proposed equivalent flexural strength formula.ConclusionsThe interface defects at the shoulder and bottom of the n-shaped bar were increased when the n-shaped bar was nonoverlapped, having little effect on the flexural strength. All the reinforced 3DPC exhibited a brittle damage. When the n-bar was overlapped, the flexural strength of reinforced 3DPC increased with the penetration degree, but there was a saturation value for the reinforcement of flexural strength by penetration degree, and there was a most economical penetration degree in the actual project. The damage mode of 3DPC reinforced by n-shaped bar under overlapped condition was a ductile failure with strain hardening characteristics. Based on the gray entropy system theory, the flexural strength of reinforced 3DPC (ft) of the influence weight was obtained in a decreasing order, i.e., matrix flexural strength (ft0)>penetration degree (P)>reinforcing ratio (). To improve the flexural strength of current longitudinal reinforcement methods, the chopped bar and overlapped n-shaped bar were proven to be more effective rather than the rivet and nonoverlapped n-shaped bar techniques. It was essential to carefully choose an appropriate reinforcing method corresponding to the specific requirements of the printing project.

IntroductionThe existing cement clinker systems have some challenges. The production process of calcium sulfoaluminate (CSA) clinker, which primarily contains mineral C&#x2084A3$, requires a significant amount of high-quality bauxite resources, thus leading to a higher production cost. Compared to CSA, a high belite calcium sulfoaluminate (BCSA) cement reduces the demand for high-grade bauxite during production, but has a slower strength development over time. To ensure the synergistic development of both early age performance and strength development in clinker systems, calcium sulfoaluminate-modified Portland cement (designated as S.M.P.) is further developed. However, alite (C3S) remains a dominant mineral, resulting in relatively high carbon emissions.For the compositional and performance characteristics of S.M.P. and BCSA clinker, this study proposed a new system for preparing a low-calcium calcium sulfoaluminate-modified Portland cement clinker with belite (C2S) as a dominant mineral (i.e., C3S/C2S-C&#x2084A3S-C&#x2084AF-CaSO&#x2084).This study also utilized minerals such as C3S and C2S from Portland cement clinker as crystal seed additives. The effect of adding Portland cement clinker on the sintering process and the mineral composition structure of the low-calcium calcium sulfoaluminate-modified Portland cement clinker was investigated. In addition, the underlying mechanisms were explored to provide a theoretical and technical guidance for the design and development of low-calcium cement clinkers.MethodsThe designed mineral composition of the calcium sulfoaluminate-modified low-calcium Portland cement clinker was 45%-50% (in mass, the same below) of C2S, 5%-10% of C3S, 25% of C&#x2084A3$, 10% of C&#x2084AF, and 10% of f-CaSO&#x2084. Calcium sulfoaluminate-modified low-calcium Portland raw meals were prepared with industrial raw materials. Portland cement clinker was used as a crystal seed at different incorporation contents of 0%, 1%, 3%, 5%, 10%, and 12%.The mineral composition and microstructure of calcium sulfoaluminate-modified low-calcium Portland cement clinker were determined by in-situ high-temperature X-ray diffraction (XRD), linear shrinkage measurement, free lime titration, scanning electron microscopy (SEM) and thermal analysis.Results and DiscussionThe linear shrinkage of the samples generally increases gradually with the increase in clinker addition percentage from 0% to 10%. The linear shrinkage remains relatively unchanged at >12% of clinker additions. The XRD patterns of the synthesized clinker samples within the 20° to 51° range reveal that the diffraction peak intensity of -C2S gradually increases, while the intensity of ′-C2S and C4A3$ peaks decreases, and no significant diffraction peaks for C3S appear as the clinker crystal seed content increases.The analysis by rietveld whole-pattern fitting indicates that the addition of Portland cement clinker seeds does not lead to a significant increase in C3S content. There is a notable increase in C2S content and a gradual decrease in C4A3$ content, accompanied by a relative increase in the iron phase content.The high-temperature in-situ XRD patterns indicate that at 950 ℃, an original C3S diffraction peak disappears, instead of relatively weak C2S diffraction peaks at approximately 27.5° and 38.0°. This indicates that C3S decomposes into C2S and CaO at 950 ℃.According to the analysis by backscattered electron-energy dispersive spectroscopy (BSE-EDS), the solid solubility of aluminum, sulfur and iron in C2S increases with increasing the addition of clinker crystal seeds, thus favoring the stability of -C2S. Furthermore, the presence of sulfur impedes a reaction between C2S and CaO, thus inhibiting the formation of C3S. An increase in the iron phase content and the Al/Fe ratio within this phase, coupled with enhanced Al solubility in C2S, reduces the amount of Al available for sulfate reaction.ConclusionsThe incorporation of Portand cement clinker seeds facilitated the formation of liquid phases, favoring the liquid-phase sintering of clinker and enhancing the densification of the microstructure of the synthesized clinker samples. This incorporation also increased the grain sizes of C2S and C4A3$. The introduction of clinker seeds lowered the decomposition temperature of CaCO3, promoting the formation of C2S and C4AF. Consequently, the contents of C2S and the iron phase increased sulfur-aluminous modified low-calcium Portand cement clinker. However, the decomposition of C3S in clinker crystal seeds at lower temperatures prevented an effective promotion of C3S formation alongside C4A3$.The content of the iron phase and the Al/Fe ratio increased with increasing the addition of clinker crystal seeds. Increased Al solubility in C2S led to a reduction in the Al available for sulfate reactions, thereby decreasing the formation of C4A3$. This effect was particularly pronounced when the clinker crystal seed content reached 10%, resulting in a significant reduction in C4A3$ content.The inclusion of Portland cement clinker seeds led to a decrease in the content of highly reactive ′-C2S and c-C4A3$ in the clinker. As the clinker crystal seed content increased, this reduction becomes dominant. This was primarily due to an increase in the calcium-to-silicon ratio in C2S and an increased dissolution of Al, Fe, and S in C2S, stabilizing -C2S but destabilizing ′-C2S, and was not conducive to C3S formation. Concurrently, a decreased Fe dissolution in C4A3$ favored the stabilization of orthorhombic C4A3$ rather than cubic C4A3$.

IntroductionIn the context of marine water fluctuation environment, the transport of chloride ions in concrete involves two mechanisms, i.e., diffusion due to concentration gradients and capillary adsorption due to moisture gradients. Most of the existing studies focus on diffusion and horizontal capillary adsorption of chloride ion transport patterns in water fluctuation environments, while neglecting vertical capillary adsorption. A few studies on vertical capillary adsorption are in static water level environments, which are not considered for water level changes. Vertical capillary adsorption exists in concrete under oceanic water level fluctuations, which may have an impact on the chloride ion transport process. It is thus necessary to carry out a research on the effect of vertical capillary adsorption on the chloride ion transport process under the environment of marine water level change.MethodsTo investigate the chloride ion transport patterns within concrete considering vertical capillary absorption in water level fluctuation environments, this study utilized a homemade marine tidal cycle simulation device. Two sets of comparative experiments were arranged, i.e., one accounting for vertical capillary absorption and another not. Concrete samples were ground in an active grinding machine at intervals of 1mm per layer within the 0-5 mm range from the diffusion surface, and at intervals of 2 mm per layer within the 7-21 mm range. The chloride ion content in the powdered samples in concrete for both sets of experiments was measured by a model ZDJ-5B-5/4B/4A conductometric titrator. The location of the most severe chloride ion erosion in concrete under water level fluctuation conditions was identified, and the effect of vertical capillary absorption on chloride ion concentration distribution, surface chloride ion concentration, and diffusion coefficients in concrete were evaluated.Results and discussionBased on the results of the chloride ion concentration distribution in concrete, the maximum chloride ion concentration in the capillary group appears in the area above tidal zone, while it in the control group appears in the area where the dry time accounted for 13/14 of the cycle. Comparing with the control group, the capillary group has higher chloride ion concentrations above the tidal zone but lower chloride ion concentrations in the tidal zone. This is due to the vertical capillary absorption on concrete. In the drying phase of the wet-dry cycle, vertical capillary absorption causes the concrete to absorb moisture from the bottom, maintaining a certain level of humidity. This reduces the intensity of capillary absorption during subsequent wetting, thus lowering the surface chloride ion concentration in the tidal zone. Also, vertical capillary absorption promotes the transfer of chloride ions from the tidal zone to the area above tidal zone, resulting in higher chloride ion concentrations above the tidal zone and lower chloride ion concentrations in the tidal zone.In the tidal zone, the diffusion coefficients for chloride ions in the capillary test group are lower than those in the control group. For instance, after 200 d exposure, the diffusion coefficient for the capillary group is 2.11×10-12 m2/s when the dry time ratio is 13/14, compared to the diffusion coefficient of 2.33×10-12 m2/s for the control group. The surface chloride ion concentration (Cs) for the capillary group is 1.63% (in mass fraction), while that for the control group is 1.80% (in mass fraction). The surface chloride ion concentration and the diffusion coefficient in the capillary group both are approximately 10% lower than those in the control group.In the area above the tidal zone, vertical capillary absorption significantly increases both the diffusion coefficient (Da) and the surface chloride ion concentration (Cs). For instance, in the 0-50 mm segment above the highest water level, the surface chloride ion concentration and diffusion coefficient in the capillary group both are higher than those in the control group throughout the entire exposure period.Based on Fick's second law, an empirical model for chloride ion transport in concrete considering the impact of vertical capillary absorption is established, and the predicted data by the model are in reasonable agreement with the experimental results.ConclusionsVertical capillary absorption increased the chloride ion concentration above the tidal zone and reduced the chloride ion concentration in the tidal zone. The location with the maximum chloride ion concentration was within the 0-50 mm range above the highest water level due to the influence of vertical capillary absorption. In the tidal zone, vertical capillary absorption reduced the surface chloride ion concentration and the diffusion coefficient of chloride ions, leading to a less pronounced variation in chloride ion diffusion coefficient with wet-dry ratio. In the area above the tidal, vertical capillary absorption increased the surface chloride ion concentration and the diffusion coefficient of chloride ions within the 0-100 mm range above the highest water level. The effect of vertical capillary absorption diminished with increasing elevation, and the most significant impact occurred within the 0-50 mm range above the highest water level. A time-variable model for chloride ion transport considering vertical capillary absorption in marine water level fluctuation environments was established. The predicted data by the model were within ±15% deviation, compared to the measured results, validating the rationality and accuracy of the model.

IntroductionThe micro-structure evolution and macro-property development of cement-based composites are affected by the wall effect. For the cement paste, the formation of the interfacial transition zone (ITZ) is related to wall effect, i.e., the lower density of cement particles near aggregate induces the formation of ITZ. In addition, the aggregate density is low near the formwork during the concrete placement and molding, which results in a difference of properties between the wall effect zone and the material interior. Characterizing the influences of aggregate or cement particle shape and size-polydispersity on the thickness of wall effect layer and further on the volume fraction distribution of particles is important for controlling the performance of cement-based composite. Compared with theoretical and numerical simulation methods, the experimental method, such as concrete section cutting is time-consuming and labor-intensive. With the development of computer technology, it provided a technical basis for the study of wall effects in concrete through numerical simulation. In general, cement-based composite can be regarded as a particle packing system. However, most of the existing numerical and theoretical studies employs simple particle, such as circles, ellipses and spheres to model the aggregate or cement particles, which makes it difficult to accurately analyze the role of particle shape and size-polydispersity on the wall effect. It is thus of great significance to construct a complex particle packing system and develop a micro-structure characterization algorithm applicable to the particle packing system to investigate the influence of the particle shape and size-polydispersity on the wall effect.MethodsThis work used variable shaped ovoidal particles to construct aggregate and cement particles. A contact detection algorithm for ovoidal particles was proposed based on a geometric potential concept, and ovoidal particle packing systems were generated to simulate the packing state of aggregate and cement particles. Moreover, the cross-section analysis algorithm applicable to the ovoid particle packing system was proposed. Based on the cross-section analysis algorithm and the stereology unbiased estimation method, the influences of particle shape and size-polydispersity on the thickness of the wall effect layer were analyzed and a quantitative relationship between the thickness of the wall effect layer and particle shape and size-polydispersity was established. A prediction formula was proposed to determine the particle volume fraction distribution at different section planes considering the wall effect, and the accuracy of the prediction formula was verified via comparing it with the results of the existing studies. Finally, the modulus of elasticity of concrete at different locations was calculated by the quantitative model of wall effect layer thickness and particle volume fraction distribution based on the differential effective medium theory.Results and discussionsFor monoshaped-monosized particle packing system, the relationship between volume fraction of particle (VV) and distance from boundary plane to cross-section plane (h) is obtained. The results show that the VV-h curves for different shaped particle packing systems are basically similar, indicating that the influence of particle shape on the thickness of wall effect layer TP can be ignored.For monoshaped-binarysized particle packing systems, the VV-h curves move in the direction of increasing distance as the proportion of larger size particles increases. The results indicate that the thickness of wall effect layer TP enlarges with increasing the proportion of larger size particles. For monoshaped-polysized particle packing system, the thickness of wall effect layer TP enlarges with the increase of average equivalent diameter of particles.Based on the obtained results, the prediction formulas are proposed to determine the wall effect layer thickness and particle volume fraction distribution, and the reliability of prediction formulas is verified via comparing the predicted data with the results from this work and other publications.The relationship between elastic modulus EMT and distance h is similar to that between volume fraction of particle (VV) and distance h. The variation of EMT-h curves can be divided into the increasing and stabilizing parts. The results indicate that the elastic modulus EMT is small near the boundary plane, which is induced at a low particle volume fraction.ConclusionsThis work could provide a guidance for the optimization of the microfine structure of concrete and the regulation of macroscopic properties, and the following conclusions were summarized: 1) The influence of particle shape on the wall effect was not dominant; 2) The thickness of wall effect layer TP increased with enlarging the average diameter of aggregate, and the thickness of wall effect layer TP decreased with enlarging the aggregate volume fraction; 3) The relationship between elastic modulus EMT and distance h was similar to that between volume fraction of particle (VV) and distance h.

IntroductionGeopolymer is a green cementitious material produced via the activation of silica-aluminate-rich precursor materials in an alkaline environment. It has outstanding mechanical qualities and durability. However, the different raw materials and complicated reaction process of geopolymers result in some parameters affecting mechanical qualities that are difficult to effectively manage and forecast. It is thus necessary to examine the variation rule of mechanical properties in geopolymer concrete (GPC) under various conditions via numerical simulation. Though the majority of existing numerical models are applicable to ordinary Portland cement (OPC), they cannot be directly applicable to GPC due to variations in the chemical characteristics as well as isomorphic model of mortar and interface transition zone (ITZ) in GPC and OPC. The existing studies lack clear criteria for material inhomogeneity in each phase of the model, and the majority of them neglect the impact of interior micropores on the model mechanical characteristics. In this work, an innovative method was used to account for non-homogeneity and determine the mechanical parameters and intrinsic model of geopolymer materials through experiment. In addition, the impacts of aggregate content, model size, and porosity on the mechanical properties of GPC were also investigated by the verified model.MethodsThe mechanical parameters and intrinsic model were determined via a geopolymer mortar test, and the model reliability was verified via a geopolymer concrete test. To prepare the specimens, slag and fly ash were used as precursors, and NaOH and water glass were used as alkaline activators. A coarse aggregate was a continuously graded gravel with a particle size range of 5-20 mm. A fine aggregate was a standard sand with a fineness modulus of 2.5. In this proportion, the alkali equivalent (in mass proportion of Na2O to precursor) was 4%, the water-cement ratio was 0.50, the modulus of alkali solution (SiO2/Na2O) was 1.2, and the mortar was kept in the same proportion of mortar phase in the concrete.At the simulation level, the Mesh-Placement-Identification-Assignment (MPIA) procedure was utilized to create a 3D GPC meso-scale model, and the model fundamental conditions were consistent with the experiments. It was assumed that the mechanical characteristics of each phase material inside the concrete could follow the Weibull distribution, and the mortar phase element non-homogeneous parameters could be solved using an equivalent unit approach. Also, initial flaws were introduced into the model, and multi-scale analysis was utilized to assess their impact on mechanical characteristics because the mixing process could form air bubbles in the concrete. After the model was constructed, the material constitutive model was determined via mortar experimental curve fitting, and the mortar test results and test algorithms were used to estimate the material mechanical characteristics. The model was imported into ABAQUS to set the boundary and loading conditions before being compared to the GPC test results obtained after the simulation. The results indicated that the damage patterns and mechanical characteristics of the test and simulation results could be nearly compatible, and the model was regarded as reliable.Results and discussionBased on the validated model, GPC models with different aggregate volume fractions of 30%, 40%, and 50% are simulated under uniaxial compression. The simulation findings reveal that the compressive strength of GPC increases as the aggregate volume percent increases, as does the energy absorbed for damage. This is because the aggregate plays a role in preventing crack development, so as the aggregate content increases in a specific range, the crack morphology inside the model becomes more complex, the energy required for damage increases, thus improving the macroscopic performance of the mechanical properties.Under uniaxial compression, GPC cubes with the side lengths of 70, 100 mm, and 150 mm are simulated, respectively. According to the simulation results, the compressive strength of GPC descends when the specimen volume grows. Compared to OPC, GPC displays the same pronounced size effect phenomena and aligns well with the Baant theoretical formulation.The GPC models with internal porosities of 1.9%, 4.3%, and 5.9% are simulated under uniaxial compression, having the compressive strengths of 49.10, 47.90 MPa, and 46.19 MPa, respectively. The mechanical characteristics of the geopolymer concrete decrease slightly as the model inside porosity increases. This is since the introduction of pores in this study takes into account both porosity and pore size distribution, and the number of small-sized pores increases as the porosity increases. The small pores have less influence on the mechanical properties, and the results show that the model mechanical properties are insensitive to the change of porosity.ConclusionsA non-homogeneous meso-scale model of GPC was proposed, and the simulation results were similar to the experimental result. Also, an innovative approach to describe the non-homogeneity of GPC was proposed. The equivalent unit method was used to determine the non-homogeneity parameter, and the initial microporous defects were characterized via multiscale modeling and folding coefficients. The energy required to destroy GPC and the compressive strength increased with increasing aggregate content. As the model size increased, the mechanical characteristics of GPC decreased, aligning with Baant's theory. The mechanical characteristics of geopolymer concrete diminished slightly as initial microporosity increased.

IntroductionThe rapid adoption of 3D printed cement-based materials (3DPC) in construction industry, particularly in sustainable buildings, highlights a critical importance of understanding its shrinkage characteristics. Different from conventional concrete, 3DPC is characterized due to the absence of coarse aggregates, the lack of templates support during printing, and immediately exposure to environmental conditions, which make it more susceptible to shrinkage-induced cracking. Shrinkage in cement-based materials leads to internal stresses and potential durability issues, affecting a long-term structural integrity. It is thus to investigate the shrinkage behavior of 3DPC. In particular, the effect of material compositions (i.e., water-binder ratio (W/B), sand-binder ratio (S/B), and supplementary cementitious materials such as fly ash (FA) and ground granulated blast furnace slag (GGBS)) on the shrinkage (total shrinkage, autogenous shrinkage, and drying shrinkage) is not fully clarified. This study was to investigate the shrinkage characteristics of 3DPC and compare it with other cement-based materials (i.e., UHPC and cast mortar).MethodsTo investigate the impact of material composition on 3DPC shrinkage, a series of laboratory experiments were conducted by a standardized contact shrinkage testing method. Four key factors evaluated were W/B, S/B, FA replacement ratio, and GGBS replacement ratio. These material variables were selected based on their known influence on the microstructure and shrinkage behavior of traditional cement-based materials.Total shrinkage (TS), autogenous shrinkage (AS), and drying shrinkage (DS) were measured at specific intervals to capture early-age shrinkage behavior in 14 d. TS was calculated as a sum of AS and DS to represent the overall dimensional changes of the material for over time. The degree of influence of each material factor on the shrinkage was determined by grey relational analysis (GRA). GRA is a statistical method that enables the quantification of the relationship between multiple variables in complex systems, making it particularly suitable for determining the relative significance of material compositions in the context of 3DPC shrinkage.For comparative purposes, the shrinkage characteristics of conventional cast mortar and UHPC were also analyzed under identical conditions, enabling a comprehensive evaluation of the shrinkage performance of 3DPC.Result and discussionThe results reveal the distinct shrinkage patterns based on the material composition of 3DPC. Increasing the W/B can significantly reduce AS. However, the influence of W/B on DS is less than that on AS. Also, the S/B has a measurable effect on drying shrinkage. After an initial curing period of 3 d, a higher S/B ratio leads to a marked reduction in DS likely due to the presence of fine sand particles, which restricts the movement of water within the matrix and limit capillary stresses.Besides, the replacement of cement with FA has a beneficial effect on shrinkage reduction. The 14 d AS and DS both are significantly lower in 30% FA-modified 3DPC, compared to the control sample. Also, the incorporation of FA reduces the 14 d autogenous shrinkage and drying shrinkage of 3DPC in the same proportion. In contrast, the use of GGBS has a more complex influence on shrinkage. While GGBS replacement increases AS after 2 d, it reduced drying shrinkage before 5 d. The TS of 3DPC firstly decreases and then increases with the increase of GGBS dosage, and the TS of 3DPC is the minimum value when the dosage of GGBS is 10%.The grey relational analysis shows the factors influencing 3DPC shrinkage in an order of significance, i.e., W/B, S/B, GGBS and FA. This highlights a predominant role of water content in governing both autogenous and drying shrinkage in 3DPC systems, which is consistent with those in other cement-based materials. However, the relative importance of sand and supplementary materials indicates that a further optimization of the mix design can lead to an improved shrinkage performance. Comparing the shrinkage behavior of 3DPC with that of UHPC and cast mortar, the shrinkage magnitudes of 3DPC are generally higher. These results indicate a necessity of tailored shrinkage control strategies in 3DPC, especially in large-scale applications where cracking risk is a major concern.ConclusionsThis study provided a comprehensive evaluation of the shrinkage behavior of 3D printed cementitious materials, especially the influence of material compositions such as W/B, S/B, FA, and GGBS. Increasing the W/B of 3DPC significantly reduced AS more than DS. The DS decreased with increasing S/B after 3 d. The replacement of FA proportionately decreased the 14 d AS and DS of 3DPC, whereas the replacement of GGBS replacement ratios increased AS after 2 d and DS before 5 d. The influence of material composition on 3DPC shrinkage was ranked in a decreasing order of W/B, S/B, GGBS, and FA. Compared to other cement-based materials like UHPC and cast mortar, 3DPC exhibited a higher shrinkage, indicating the need for more focused research and innovation in shrinkage reduction techniques for 3DPC. These findings contributed to a deeper understanding of the material behavior and provided a foundation for a future work on optimizing 3DPC for large-scale and crack-resistant applications.

IntroductionTitanium dioxide (TiO2) is used as a photoelectric conversion material due to its non-toxicity, good photothermal stability and low cost. However, TiO2 has a wide electron bandgap, and its photo-generated electrons and holes are easy to recombine. The photoelectric performance of TiO2 is often improved via increasing the specific surface area, speeding up the axial transport of electrons. In this paper, titanium dioxide inverse opal photonic crystals (TiO2 IOPCs) were selected as photoelectric conversion materials due to the advantages of large specific surface area and promoted light absorption. TiO2 IOPCs were prepared by a template method and a sol-gel method. Also, TiO2 IOPCs was modified with amorphous titanium dioxide (A-TiO2).MethodsSelf-assembled opal photonic crystals on fluorine-doped tin oxide (FTO) were used as templates. The precursor liquid prepared with isopropyl titanate, ethanol, acetyl acetone, hydrochloric acid and deionized water was filled into the gap of opal photonic crystal. After the sol in the opal crystal gap was dried and gelled, the electrode was calcined at 450 ℃ to remove the opal photonic crystals template to obtain TiO2 IOPCs. Afterwards, amorphous titanium dioxide modified titanium dioxide inverse opal photonic crystals (A-TiO2/TiO2 IOPCs) were prepared via placing TiO2 IOPCs electrodes into TiCl4 aqueous solution at a low temperature without stirring.The phase composition, elemental composition, energy level structure and morphology of the electrode were analyzed by X-ray diffraction (XRD), X-ray electron spectroscopy (XPS), ultraviolet visible spectrophotometry (UV-VIS) and scanning electron microscopy (SEM). A three-electrode system was used to detect the photocurrent in the dark, and the LED with 390-410 nm was used as an excitation light source. The current-time (I-T) curve was recorded when the light source was switched.Results and DiscussionThe XRD patterns show that amorphous titanium dioxide modified anatase titanium dioxide electrode can be obtained. Based on the XPS analysis, the prepared material is TiO2. The valence band value of the electrode is determined by the XPS valence band spectra. The indirect band gap of the electrode material is determined according to the UV-VIS analysis. The conduction band value of the electrode can be obtained by calculation. The SEM images indicate that A layer of amorphous titanium dioxide is deposited on the orderly arranged porous skeleton of TiO2 IOPCs, which retains a porous structure and further increases the specific surface area. In the photocurrent test, TiO2 IOPCs photocurrent reaches 100 nA, and the photocurrent of A-TiO2/TiO2 IOPCs reaches 600 nA, indicating that its photocurrent is increased by 6 times. The conduction band value of A-TiO2 is more negative than that of anatase TiO2 IOPCs. The photogenerated electrons of A-TiO2 can be transferred to the conduction band of anatase TiO2 IOPCs. The valence band value of anatase TiO2 IOPCs is more positive than that of A-TiO2, and the photogenerated holes can be transferred from the anatase TiO2 IOPCs valence band to A-TiO2 valence band, thereby reducing the recombination of photogenerated electrons and holes. In addition, the light absorption area increases due to the porous structure of A-TiO2/TiO2 IOPCs, thus increasing the photocurrent.ConclusionsTiO2 IOPCs were prepared by a template method and a sol-gel method, and then amorphous TiO2 was deposited on TiO2 IOPCs skeleton at a low temperature to prepare an amorphous titanium dioxide modified titanium dioxide inverse opal photonic crystal electrode. The porous structure of the electrode increased the light absorption area, and the modification of amorphous titanium dioxide reduced the recombination of photogenerated electrons and holes, and the photocurrent of the electrode was increased by 4 times. The results indicated that this method could be simple and feasible, which could obtain the superior photoelectric performance of the electrode, thus providing an effective approach for the development of green energy.

IntroductionCadmium magnesium telluride (CdMgTe) crystal is an ideal material for room-temperature radiation detection. However, there are still many defects in as-grown crystals, which degrade the detector performance. At present, a single annealing method is unable to achieve the effect of both eliminating defects and maintaining or increasing resistivity. The multi-step combined annealing method can improve the crystal quality and the detector performance. There are a few studies on the annealing of CdMgTe crystals. In this work, a multi-step combined annealing method was used, i.e., the CdMgTe crystals grown under Te-rich condition were annealed in Cd atmosphere and Te atmosphere. The effect of annealing temperature on the defects, mechanics, photoelectric properties and detector performance of CdMgTe crystals was investigated.MethodsHigh purity (7N) cadmium (Cd) and tellurium (Te) were selected as raw materials. The selected slices for annealing were derived from In-doped Cd0.95Mg0.05Te ingot grown by a modified vertical Bridgman method under Te rich condition, with the size of 5 mm×5 mm×2 mm. The annealing in Cd atmosphere was mainly divided into two steps, i.e., high and low temperature annealing. In the high temperature annealing step, the temperatures of the slices were 873, 923 K and 973 K, respectively. The corresponding temperatures of Cd source were 901, 946 K and 986 K, respectively. The annealing time was 120 h. After high temperature annealing, the temperature of the slices and Cd source reduced to 623 K, and the annealing time was also 120 h. After annealing in Cd atmosphere, it continued annealing in Te atmosphere. The annealing temperature of both slices and Te source was selected to 773 K, and the annealing time was also 120 h.The CdMgTe crystals before and after annealing were characterized by infrared transmission microscopy, scanning electron microscopy, ultraviolet-visible-near infrared (NIR) spectroscopy, infrared transmittance spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Vickers indentation analysis and current-voltage (I-V) measurement. Au/CdMgTe/Au planar detectors were fabricated by an evaporation method, and the energy spectrum was collected in ORTEC test system. The radiation source was 241Am particle source with a non-collimated energy of 5.48 MeV.Results and discussionThe band gap of the as-grown crystal is 1.510 eV, and the maximum band gap is increased by 0.021 eV after annealing. After annealing in Cd atmosphere, Te inclusions in the crystals are greatly reduced. At the annealing temperature of 973 K, Te inclusions can be completely eliminated. The IR transmittance and resistivity are reduced. The density of Te inclusions has little change after annealing in Te atmosphere. However, the IR transmittance and resistivity improve and are better than those of the as-grown crystals. The optimum IR transmittance and resistivity are 63% and 2.41×1010 ·cm, respectively. After multi-step combined annealing, there is no extra oxidation on the crystal surface, and the microhardness is increased by 16%. The Raman spectra show that the crystal quality is improved.Au/CdMgTe/Au planar room temperature radiation detectors are fabricated with the crystals obtained under the optimum annealing conditions. The time-of-flight (TOF) measurements show a significant increase in electron mobility, up to 526.86 cm2/(V·s). The optimum energy resolution and electron mobility lifetime product of the detector are 15.6% and 1.89×10-4 cm2/V, respectively, using 241Am particles with an energy of 5.48 MeV as a radiation source.ConclusionsCdMgTe crystals were annealed by a multi-step combined method. After annealing, there was no additional oxidation on the crystal surface, and Te inclusions decreased significantly. The band gap, microhardness, IR transmittance and resistivity of the crystals were all improved, indicating the improvement of crystal quality. The optimum annealing conditions were in Cd atmosphere at high (i.e., 923 K) and low (623 K) temperatures for 120 h, and then in Te atmosphere at a low (i.e., 623 K) temperature for 120 h. The optimum performance of CdMgTe detector was electron mobility (e) of 526.86 cm2/(V·s), the energy resolution of 15.6%, and the mobility lifetime product ()e of 1.89×10-4 cm2/V.

IntroductionMgO-CaO refractories exhibit superior characteristics such as high refractoriness, good thermodynamic stability, and superior slag resistance. They perform particularly well in steel purification and improving steel quality, playing an important role in high-temperature industries. However, utilizing used MgO-CaO refractories becomes a challenge as the use of these refractories expands. Effective recycling of used MgO-CaO refractories can mitigate environmental pollution, conserve valuable land resources, and lower energy costs and carbon emissions during raw material production. The high-value utilization of used MgO-CaO refractories is thus critical for sustainable development.The recycling of used MgO-CaO refractories has attracted recent attention. Some researchers regenerated MgO-CaO refractories using used MgO-CaO bricks, reporting that the regenerated bricks exhibited comparable physical properties to the original ones and showed an enhanced resistance to low-basicity slag. However, the effect of used brick incorporation on the slag resistance of MgO-CaO refractories are still lack, and the underlying mechanism remains unclear.The existing research on the slag-erosion mechanism of MgO-CaO refractories mainly involves static and dynamic slag resistance experiments or on-site industrial tests to analyze the original bricks. However, the random distribution of aggregates complicates the analysis of the effect of used refractory content on slag resistance, making it difficult to elucidate its impact on the refractory. In this study, six groups of MgO-CaO refractory matrix samples with different used brick contents (i.e., 0%-50%, in mass fraction) were prepared via mixing MgO-CaO aggregate, MgO-CaO powder, MgO powder, and used MgO-CaO bricks, and crushed into fine powders. This approach eliminated the influence of aggregate distribution and focused on the matrix portion of MgO-CaO refractories. The slag-erosion behavior of the matrix with different used brick contents was analyzed to clarify the impact mechanism of used brick incorporation on the slag resistance of MgO-CaO refractories.MethodsMgO-CaO aggregate (bulk density: 3.18 g/cm3, apparent porosity: 4.96%-6.62%), used brick (bulk density: 3.04 g/cm3, apparent porosity: 7.34%-7.84%), MgO-CaO powder, MgO powder, and high-silica slag, which were provided by ShanXi Luweibao Taigang Refractories Co., Ltd., China, were used as raw materials. After batching, each aggregate was separately crushed in a vibratory crusher for 1 min to obtain a mixed powder with a particle size of 200 mesh (75 m). The mixed powder was then placed in a mixing barrel and homogenized with 200 mesh MgO-CaO powder and MgO powder for 30 min. Subsequently, the homogenized powder was placed into a mold and pressed into &#x03D536 mm cylindrical samples under 100 MPa. The green samples were sintered at 1600 ℃ for 3 h, followed by furnace-cooling. Finally, the high-silica slag-erosion test was conducted by a sessile drop method. High-silica slag was pressed into cubic blocks (2 g) with the sizes of 10 mm×10 mm×10 mm and placed at the center of the MgO-CaO matrix samples. The samples and slag were heated to 1600 ℃ for 3 h at a rate of 5 ℃/min, and then cooled within the furnace.Results and discussionAt the addition of 10% used bricks, F10 exhibits the deepest slag penetration depth (i.e., 1059 m), the largest spread area (i.e., 95%), and the smallest contact angle (i.e., 19.5°). The slag penetration depth gradually decreases as the used brick content increases. Microscopically, the interface between each sample and high-silica slag is comprised of a slag layer, a slag-erosion layer, and an original layer. F0 has the thinnest slag-erosion layer (i.e., 214 m), while F10 displays the thickest layer (i.e., 488 m). The slag-erosion layer thickness decreases when the used brick content exceeds 30%. The pore analysis shows that the average pore diameter is the largest (i.e., 31.2 m) in F10 and smaller in F50 (i.e., 24.6 m). The pore diameter is smaller than that of F0 when the used brick content exceeds 40%.According to the thermodynamic calculation and the MgO-CaO-SiO2 ternary phase diagram, the primary reaction products between high-silica slag and MgO-CaO refractories are Ca2SiO5 and Ca2SiO4. The slag-erosion model simulations reveal that the maximum difference in liquid phase content is only 5 g, indicating that minor variations in phase composition are not a main cause of the different erosion behaviors. However, liquid phase content significantly affects the sintering behavior. An appropriate liquid phase content enhances a wettability between solid particles, promoting particle rearrangement and filling voids, forming fine closed pores. Liquid phases encapsulate pores, preventing them from being trapped within crystals and preserving their shape and position, thus leading to a uniform pore structure. F10 has larger and more pores, while F50 exhibits finer pores. As the temperature increases, high-silica slag melts, and smaller pores absorb slag more readily due to the more intense capillary action. In contrast, larger pores with a weaker capillary action accommodate more slag and have a greater surface area, thus increasing slag penetration. The interfacial reaction in F10 is more intense, decreasing the contact angle and promoting further reactions. Compared with MgO, CaO phase exhibits a higher reactivity with SiO2, forming Ca3SiO5 or Ca2SiO4, which are gradually eroded by slag.ConclusionsHigh-silica slag exhibited an erosive effect primarily on CaO in the MgO-CaO refractory matrix. In the slag-erosion model simulations, the introduction of used bricks slightly increased the liquid phase content at 1600 ℃, while the composition and variation of the phases during the slag-erosion process showed minimal differences. This was not main factor contributing to the different erosion behaviors. The liquid phase content significantly affected the sintering behavior of the raw materials. The pore diameter reached its maximum when adding 10% used bricks, and as the content of used bricks increased, the pore sizes in the matrix became more uniform and smaller, enhancing the slag-erosion resistance of the regenerated MgO-CaO refractory matrix. Utilizing MgO-CaO aggregate, MgO-CaO powder, MgO powder, and used bricks in appropriate proportions could effectively enhance the slag-erosion resistance of the regenerated MgO-CaO refractory matrix. This improvement could be further optimized via strictly controlling pore size according to the adjustments of material composition, particle size distribution, forming pressure, and sintering conditions, thereby significantly enhancing the slag-erosion resistance of MgO-CaO refractories.

IntroductionDatong, located in the northern of Shanxi province of China, has an important transportation hub, connecting the Central Plains and Inner Mongolia. Various cultures converge in Datong and form rich aesthetics, reflecting in the famous carving black-glazed wares produced during Jin and Yuan Dynasties in Hunyun kilns located in Hunyuan county, Datong. The carving wares are characterized by gray-white body decorated with carved black glazed patterns. Such a gray-white and black contrast gives the patterns a relief-like decoration. Many researches were devoted to analyze regional features, decorative themes and technology of black-glazed carved wares by conventional archaeological methods. Wang et al. investigated one piece of black-glazed carved ware by micro-Raman spectroscopy and found a large number of -Fe2O3, a few hematite and magnesio-ferrite on the glaze surface, leading to the brown tune of the carved black patterns. However, the lack of the microstructure, chemical and mineral composition of carved black-glazed wares leads to the raw materials and firing protocol of black glazed carving wares, and its relationship to ordinary pure black glazed is still unclear. In this work, carved black-glazed wares and pure black glazed wares of Hunyuan kilns were investigated by a series of analytical techniques. The characteristic of raw materials and firing protocol of black glazed carving ware and its relationship to black glazed ware were discussed.MethodsEleven fragments were excavated from Jiezhuang village, Hunyuan county, Datong, China, which could be dated to Jin and Yuan Dynasties according to the archaeological layer. The fragments were prepared as cross-sections by a cold mounting method. The details were described in elsewhere. The morphology, chemical composition, mineral composition and valence state of colorant elements of glaze and body were measured by a model VXH-7000 optical microscope (OM, KEYENCE Co., Japan), a model XGT-7200 X-ray fluorescence (XRF, Horiba Co., Japan), a model Renishaw-invia micro-Raman spectroscope (micro-RS, UK), a model TD3500 Macro-X-ray diffracometer (macro-XRD, Dadong Co, China) and a model Axis Supra X-ray photoelectron spectroscope (XPS, Shimadzu Co., Japan).Results and discussionThe results by XRF analysis show that the raw materials used for glazes and bodies of carved black-glazed wares and pure black-glazed wares are similar except for the glaze of former, which is much richer in CaO. A higher CaO can enhance the adhesion of the glaze and body. Besides, the bodies are composed of higher TiO2 and lower MnO2 and Fe2O3 rather than the glazes, leading a gray-white and dark black contrast between the bodies and glazes. The glaze surfaces contain massive dendrite -Fe2O3 crystals and a few of hematite, magnetite and magnesioferrite. A few of zircon, anorthite, pseudobrookite and quartz appear in the glaze cross-sections. The bodies contain cristobalite, quartz and mullite. There is no obvious difference in the raw materials of carved black-glazed wares and pure black-glazed wares. The degrees of the glassy polymerization (Ip) of carved black glaze and pure black glaze are similar, i.e., 1.34-2.25, which are consistent with the glaze modifiers/network formers mole ratios, i.e., 0.12-0.2. The similar Ip values and mole ratios indicate that the wares both are prepared at the similar firing temperature. Most of carved-black glazed wares and black glazed wares present the similar ratios of iron bivalence and trivalence, i.e., 2.0-2.6, indicating that the wares both are prepared in a reductive atmosphere. Besides, the Raman spectra of the dendrite -Fe2O3 crystals show an obvious shift to s higher wavenumber possibly due to the lighter ions substituting irons in the crystals to improve their stability. Moreover, the spectra also present different intensity of Raman features, accounting for the different crystal orientation of these crystals. Such a different orientation can make the dendrite crystals react with incident lights in diffractive or reflective function, giving rise to the glaze surface presenting iridescent or silvery effect.ConclusionsThe similar raw materials were used for preparing the carving black-glazed wares and pure black glazed wares expect for the slightly higher CaO in the glazes of carved black glazes. The bodies were composed of higher TiO2 and lower MnO2 and Fe2O3 rather than the glazes, leading to a gray-white and dark black contrast between the bodies and glazes. The wares both were fired at the similar firing temperature and in a reductive atmosphere. The main minerals of the bodies were cristobalite, quartz and mullite. Numerous dendrite crystals in micron-size with different crystal orientation were distributed in the glaze surfaces, which could react with incident light in diffractive and reflective functions, leading to the glaze presenting iridescence or silver. Besides, a few of magnetite, magnesioferrite, zircon, anorthite, pseudobrookite and quartz were detected in the glazes.

C3N5 as a two-dimensional (2D) layered polymer material has great prospects in the field of energy storage due to the excellent light absorption, low electron transfer resistance, and environmental friendliness, etc. However, several drawbacks such as the high charge carrier recombination rate, weak reduction ability and low density of surface reactive sites give rise to a poor photoactivity, restricting the large-scale application. Recent researches focus on the synthesis and the detailed molecular structure of C3N5, as well as the various modification strategies to promote the photocatalytic activity. It is thus necessary to provide a general guidance for designing high-efficient C3N5 catalyst systems based on the existing results.C3N5 is commonly prepared by thermal polymerization method in the presence of organic materials precursors that contains a large amount of nitrogen element like melem hydrazine, 3-Amino-1,2,4-Triazole, 5-Amino-1h-Tetrazol, etc.C3N5 has three structures like triazine-triazole structure, azo structure and terminal triazole structure. The unique structures and bonding types of C3N5 endow it with a promising possibilities for photocatalytic applications.Various modification strategies including morphology control, nitrogen vacancy creation, element doping and heterojunction construction in promoting the photocatalytic activity of C3N5 are discussed. Morphology control is beneficial to improving the specific surface area and enhancing the density of surface reactive site of C3N5. Nitrogen vacancy and element doping favors optimizing the band structure and improving the utilization of solar energy. Heterojunction construction like the Schottky junction, type-Ⅱ, Z-scheme and S-scheme C3N5-based heterojunctions enable the efficient spatial separation of charge carriers and maintain the intense redox capabilities.Summary and prospectsC3N5 as a two-dimensional (2D) layered polymer material has advantages such as the unique structures, larger amount of nitrogen active sites, narrower band structure and excellent chemical stability. It is more favorable to the development prospects of C3N5 in various photocatalytic fields. This review summarizes the synthesis and molecular structure of C3N5, and the modification strategies developed to improve the photocatalytic performance of C3N5 nanomaterials in recent years, including morphology control, nitrogen vacancy creation, element doping and heterojunction construction. However, compared with the abundance of other semiconductor catalyst systems, a research on C3N5 still needs to be further explored in terms of rational preparation, construction of unique composite systems, elucidation of the intrinsic mechanism, and the application areas, etc.In rational preparation, the eco-friendly and cost-effective large-scale preparation of C3N5 nanomaterials is still a challenge. The relationship between the reaction parameters in the thermal polymerization process and the resulting morphology structure is unclear, which hinders an ability to optimise and control the process further. Also, the synthesis of C3N5 inevitably results in the formation of toxic by-products, necessitating the development of supplementary follow-up treatment technology. The optimization of the rational synthesis of C3N5 for environmentally and large-scale preparation is more in line with sustainable development strategies.In the construction of unique composite systems, the precise regulation is a pivotal concern, both in terms of industrial applications and fundamental scientific research. Until now, it becomes a challenge to elucidate the precise conformational relationship between vacancy/elemental sites density and photocatalytic performance, representing a significant obstacle to further performance optimization. The advancement of the precise regulation is instrumental in enhancing the catalytic performance of C3N5-based materials.In the elucidation of the intrinsic mechanism, it is essential to investigate the atomic and electronic structures at the material/interface level in order to clarify the performance of the reaction process. This provides a theoretical foundation for the subsequent design and development of efficient photocatalysts. It is thus necessary to employ advanced characterizations like environmental transmission electron microscopy, aberration scanning transmission electron microscopy, synchrotron radiation and in-situ spectroscopy. The structural configurations for catalysts and the formation of intermediates during photoreaction can be monitored. The fine update is more beneficial to optimizing the properties of the catalyst material.In the context of application areas,, the optimization of selectivity in catalytic reactions represents a crucial challenge for designing high-efficient photocatalytic systems. In the context of energy catalysis, such as methane conversion and CO2/N2 reduction, a detailed understanding of the intrinsic reaction mechanism and kinetic processes elucidated by theoretical simulation and in-situ monitoring is beneficial for the design and development of C3N5 catalysts with a high selectivity and a purpose of improving the yield of the target product. In the context of environmental remediation, the utilization of wastewater and seawater instead of purified water for hydrogen production from water is another way to achieve a sustainable development strategy. The design and development of C3N5-based materials with a high selectivity can facilitate their broader range of applications.

Silicon-carbon composite materials and compounds combine the high lithium storage performances of silicon with the structural excellence of carbon. However, their complex electrochemical behavior and volume changes during charging and discharging are urgent issues to be addressed. As an effective theoretical tool, first-principles calculation plays a crucial role in predicting, revealing the characteristics of electrode materials and understanding electrochemical mechanisms in atomic scale. In this review, the first-principles calculation methods were summarized, and recent research results on first-principles calculations of typical silicon-carbon composite materials and compounds as lithium-ion battery anode materials were represented. The laws of diffusion kinetics, interface reactions, mechanical properties and thermodynamic stabilities of silicon-carbon anode materials for lithium-ion batteries were proposed. Furthermore, the first-principles calculations for silicon-carbon anode materials in lithium-ion batteries were outlooked based on three perspectives like density functional theory (DFT), algorithm integration and experimental integration. The development of novel silicon-carbon anode materials with superior performances were anticipated, thereby contributing to the development of the new energy sector.Summary and prospectsIn this review, the first-principles calculation methods and the research progress of the first-principles calculations on typical silicon-carbon anode materials for lithium-ion batteries were elaborated. The diffusion kinetics, interfacial reactions, mechanical properties and thermodynamic stability laws of silicon-carbon anode materials for lithium-ion batteries were revealed.1) From the perspective of lithium-ion diffusion kinetics, the diffusion mechanism of lithium ions in silicon-carbon anode materials is complex, which is affected by material structure and chemical composition. In silicon/graphite composite materials, lithium ions are prone to first intercalating between graphite layers and then diffusing into silicon, which is regarded as a two-dimensional diffusion. Lithium ions in silicon/carbon nanotube composite materials undergo an one-dimensional diffusion, primarily along the walls or gaps between carbon nanotubes. The two-dimensional structure in silicon/graphene composite materials facilitates a rapid diffusion of lithium ions. Lithium ions in silicon carbide exhibit bulk and interfacial diffusion, which are influenced by charge transfer and electric field effects.2) From the perspective of interface reaction, the formation of the SEI film is crucial for the performance of silicon-carbon anode materials. Meanwhile, cycle life and efficiency of the batteries are affected by stability and uniformity of SEI film. The volume expansion of silicon in silicon/graphite composite materials can damage the adjacent SEI film, leading to the continued generation of new SEI film. In silicon/carbon nanotube composite materials, more active sites for lithium ions are provided for the unique tubular structure and high specific surface area of carbon nanotubes, making the SEI film more stable. Silicon/graphene composite materials have a layered structure that promotes the uniform dispersion of electrolyte on the surface for the composite materials, resulting in a more uniform SEI film. Silicon carbide has a great hardness and a wear resistance, which enhances its ability to resist mechanical stress during charging and discharging, effectively protecting the SEI film.3) From the perspective of mechanical properties, different types of carbon materials have different buffering effects on the volume expansion of silicon. Volume expansion problems of silicon are alleviated because of impurities and defect structure of microcrystalline graphite in silicon/graphite composite materials. In silicon/carbon nanotube composite materials, the flexibility of carbon nanotubes helps to withstand greater structural deformation and enhance the structural stability of silicon. In silicon/graphene composite materials, core-shell structures are more conducive to suppressing the volume expansion of silicon. The decrease in electrochemical performance caused by volume expansion is alleviated by the tight interface bonding between silicon carbide and the matrix material, which also helps to improve the fatigue resistance of materials.4) From the perspective of thermodynamic stability, the thermodynamic stability is assessed through calculations of formation energy, binding energy, reactivity, and changes in free energy. Silicon/graphite composites, benefiting from the layered structure of graphite, exhibit a good thermodynamic stability due to their low formation energy, high binding energy, and low reactivity. Silicon/carbon nanotube composites have a high binding energy due to their unique structure, but their thermodynamic stability is significantly affected by free energy under high-temperature and high-pressure conditions. Silicon/graphene composite materials, leveraging the two-dimensional structure of graphene, have a moderate formation energy and a high binding energy, resulting in a stable thermodynamic performance. Silicon carbide has a high formation energy, and the covalent bonds between silicon and carbon atoms contribute to its high structural stability and good thermodynamic stability.Conventional DFT method struggles to accurately simulate intermolecular forces due to its lack of description of nonlocal electron correlations, resulting in significant errors in calculating material structural parameters, mechanical properties and energetic properties. To overcome this problem, the van der Waals Density Functional method is introduced. The Becke 86 exchange functional is incorporated by a named optB86b method, enabling effective corrections to the Generalized Gradient Approximation. The excessively strong repulsive interaction of the exchange functional at short distances is reduced, and the accuracy of DFT method is enhanced.To enhance the computational efficiency of first-principles calculations and have deeper insights into the lithium storage mechanism of silicon-carbon anode materials in lithium-ion batteries, the first-principles calculations and machine learning, phase-field methods, multiscale simulation techniques are integrated. The data analysis and model optimization are accelerated via machine learning. The evolution of microstructures is captured cross phase-field methods. The macroscopic reaction mechanisms of lithium-ion batteries from a microscopic perspective are revealed based on molecular dynamics and finite element analysis. The precision and efficiency of material development are significantly improved via interdisciplinary integration, expanding the application of silicon-carbon anode materials in the field of new energy.The first-principles calculations and experimental research should be combined due to some factors such as high experimental costs, stringent experimental condition and low data reproducibility. The accuracy of first-principles calculation methods can be validated via comparing computational predictions with experimental results, promoting the coordinated development of theoretical calculations and experimental research. It is expected that this will lead to the development of new silicon-carbon anode materials with a higher specific capacity, a better cycle stability, and a faster charge/discharge rate, facilitating their application in sodium-ion batteries and other fields and bringing greater breakthroughs to the development of the new energy field.

The rapid development of electromagnetic wave technology brings a unprecedented convenience, but leads to some related concerns regarding electromagnetic pollution and associated health risks. Consequently, the development of electromagnetic wave absorption materials becomes crucial for ensuring both survival and safety. Biomass porous carbons (BPCs) can be used as highly promising materials for addressing these challenges due to their unique properties, including ultra-low density, large surface area, abundance of precursors, and exceptional dielectric loss capabilities, showcasing immense potential for electromagnetic wave absorption applications. However, manipulating the absorbing properties of biomass carbon-based composite materials through changes in structure and composition is restricted, and the absorption mechanisms of BPCs materials still remain unclear, due to the absence of a comprehensive framework.In this review, the absorption mechanism of BPCs materials as absorbers is explored, including impedance matching, interface polarization, multiple reflection and scattering, conductivity of network structure, pore size control to optimize absorption performance, and unique absorption mechanism of magnetic media/ biomass-derived porous carbon materials. Also, several puzzles and unsolved problems of the absorption performance are emphasized, and a further research is needed to solve these uncertainties and deepen the understanding of the electromagnetic wave absorption capacity of BPCs.In addition, this review proposes a promising theoretical method to enhance the electromagnetic absorption properties of BPCs materials, providing valuable insights and directions for future research work in this field. Despite the considerable progress made in both experimental and theoretical aspects of BPCs materials, some challenges persist. These include difficulties in synthesizing highly conductive networks, challenges in precisely controlling pore parameters, and issues related to the aggregation and oxidation of magnetic nanoparticles in magnetic BPC composites, all of which require further exploration and resolution.Summary and ProspectsThe rapid development of electromagnetic wave technology leads to significant conveniences and also concerns electromagnetic pollution and associated health risks. Consequently, the development of electromagnetic wave absorption materials becomes crucial for ensuring survival and safety. Biomass porous carbons (BPCs) can be used as promising materials in this regard due to their unique properties such as ultra-low density, large surface area, abundance of precursors, and excellent dielectric loss capabilities, highlighting their enormous potential for electromagnetic wave absorption applications. However, despite a significant progress in manipulating the absorbing properties of biomass carbon-based composite materials, there is a fragmented understanding of the absorption mechanisms of BPCs materials, necessitating a more comprehensive framework. This review provides a thorough examination of the absorption mechanisms of BPCs materials, encompassing impedance matching, interface polarization, multiple reflections and scattering, the conductivity of network structures, control of pore size for optimizing absorption performance, and the distinctive absorption mechanisms of magnetic media/biomass-derived porous carbon materials. Furthermore, this review gives several challenges for a further exploration. One significant challenge is related to the contribution of conductive networks to the absorption performance of BPCs materials, with some aspects such as difficulties in preparation and poor stability. The precise control of pore parameters and exploration of suitable specific surface areas for optimal interface polarization effects remain major challenges in material preparation methods. Moreover, the aggregation and oxidation of magnetic nanoparticles in magnetic BPC composites affect their absorption performance, necessitating a further research in this area. Although this review is based on theoretical summaries derived from laboratory results and does not delve into the synthesis processes of the materials, it offers a comprehensive elucidation of the absorption mechanisms of BPC-based absorbing materials. This review strives to guide future research and applications via providing insights into cost-effective and high-performance BPC-based absorption materials. Some studies need to contribute to the development of efficient and versatile absorption materials via addressing these challenges and advancing the understanding of BPCs' electromagnetic wave absorption capabilities, ultimately mitigating electromagnetic pollution and ensuring public health and safety.

Engineered cementitious composites (ECC) are widely used in masonry structure reinforcement, bridge deck connection and structural repair due to the strain hardening and multi-crack cracking characteristics, which can overcome some shortcomings as poor ductility and weak crack control ability from ordinary concrete. The existing mechanism analysis for concrete fracture mechanics is mainly based on the micro- and macro-experimental results. Although a research can obtain experimental data, the experimental process and result analysis are time-consuming and labor-intensive, and there is a huge deviation between the experimental results and the intended target. Computer simulation provides an effective research tool for accurately analyzing the working mechanism of cement-based composites. Computer simulation can accurately predict and deeply analyze the experiment results due to the advantages of high efficiency, visualization and low cost, which helps to better understand the achievement criteria of special properties for cement-based composites. The related research on the nano-, meso-, and macro-scopic cracking characteristics during the tensile process of ECC by computer simulation technologies in different scales becomes popular. However, there are a few systematic work for the characteristics, differences and correlations of different scale simulation methods on ECC.In the nano-scale simulation of ECC, the molecular dynamics (MD) can analyze the influence mechanism of unmodified fiber and modified fiber on the bonding performance at fiber-matrix interface during ECC deformation from the molecular or atomic level. MD can be used to establish the fiber/C-S-H interface molecular model, which can visually display the fiber-matrix interface bonding, and reveal that the interface bonding between unmodified/modified fibers, and C-S-H gel can be achieved via forming polymer, electrostatic interaction, chemical bond, hydrogen bond and the van der Waals force.In the mesoscopic scale simulation of ECC, the numerical (finite element) simulation can analyze the influence of macroscopic and mesoscopic factors such as fiber, matrix and their interface on the tensile strain hardening behavior of ECC. Firstly, the mesoscopic scale model-the lattice discrete particle model can analyze the influences fiber type and dosage. Secondly, crack propagation mode I can simulate the influence of matrix characteristics such as matrix internal pore structure and matrix fracture toughness. A numerical (finite element) model is established to simulate the single fiber pullout process to analyze the influence of fiber-matrix interface parameters on the tensile properties of ECC. The ECC tensile finite element model is proposed to analyze the influence of sample shape and size on the tensile properties of ECC.In the macroscale simulation of ECC, there are a few studies on the multi-crack cracking characteristics of ECC from peridynamic (PD) simulation. The fiber-matrix interaction using PD is simulated to investigate the discontinuous cracking process of ECC, or predict the maximum tensile strain of fiber reinforced cementitious composites, indicating the feasibility of PD simulation in this aspect. In addition, this review introduces how to establish an artificial neural network (ANN) prediction model to achieve concrete mix ratio design, predict the correlation between relevant parameters and mechanical properties, and extend the ANN model to the prediction of ECC micro-mechanical properties indicators.Summary and prospectsThis review represents the research status of four types of simulation technologies according to nano-, meso-, and macro-scale in tensile cracking behavior of ECC. In the nanoscale simulation of ECC, MD can be used to analyze the bonding performance of fiber-matrix interface by virtue of its visualization advantage for molecular/atomic interaction. Using MD model in the molecular or atomic level for investigation of the interface interaction between unmodified/modified fibers and C-S-H gel is enhanced via forming hydrogen bonds, the van der Waals forces, etc., or filling nanopores and bridging nanocracks with nanomaterials, thus improving the tensile properties of ECC. However, the MD simulation on the fiber-matrix interface performance after ECC cracking and the crack propagation behavior is lack. For the mesoscopic scale simulation of ECC, the numerical (finite element) model based on meso-mechanics shows that the uneven dispersion of fibers can reduce the ultimate tensile strain and ultimate tensile strength of ECC, and show that the tensile strain and tensile strength of ECC decrease with the increase of sample width and thickness. However, there is a lack on numerical (finite element) simulation for the tensile strain of ECC from the perspective of the close packing of mortar matrix particles. In the macroscale simulation of ECC, PD can be used to solve the discontinuous cracking problem of micro-/macro-mechanics of cement-based materials. However, the stress transfer between fiber and matrix during ECC multi-cracks cracking and the prediction of crack propagation path after loading are rarely involved. The ANN model can predict the basic mechanical property indexes of ECC and the stress-strain curve of FRC, and optimize the concrete mix proportion. When it is combined with a genetic algorithm to predict the compressive strength, slump and interfacial bond strength of concrete, the prediction accuracy becomes greater. However, the ANN prediction model on ECC micro-mechanical indicators is not reported. In addition, it is also crucial for the improvement of the prediction accuracy and efficiency of computer models to explore how to achieve two-path intelligent optimization of input parameters and prediction targets.

The freeze-thaw resistance of concrete is a crucial factor determining its service life in cold regions. This review represents recent research progress on concrete freeze-thaw resistance based on freeze-thaw damage mechanisms, influencing factors, damage models and service life prediction, as well as measures to enhance freeze-thaw resistance.The freeze-thaw damage mechanisms of concrete mainly are based on powers hydraulic pressure theory, osmotic pressure theory, the Setzer's micro-ice lens theory, Scherer and Bresme's crystallization pressure theory, and the salt frost theory. These theories provide insights into the damage mechanisms of concrete during freeze-thaw cycles from different perspectives. However, a unified theory is not yet established, and a further research is thus needed.Some factors influencing the freeze-thaw resistance of concrete can be classified as direct and indirect factors. The direct factors primarily encompass pore structure characteristics (such as porosity, pore size distribution, air void spacing factor, etc.) and degree of saturation, with the air void spacing factor being the decisive element affecting concrete's performance in resisting freeze-thaw cycles. The indirect factors can be classified into two categories, i.e., material composition and environmental conditions. In terms of material composition, the type of aggregates, water to cement ratio, mineral admixtures, and various chemical admixtures play important roles. Using lightweight aggregates or reducing the water to cement ratio can enhance the freeze-thaw resistance. The addition of mineral admixtures like silica fume and fly ash can optimize concrete pore structure and improve the freeze-thaw resistance. Chemical admixtures such as air-entraining agents and water reducers can enhance concrete freeze-thaw resistance from different aspects. The application of fiber materials and nanomaterials has a potential in improving concrete freeze-thaw performance, primarily via enhancing its mechanical properties and microstructure. The environmental conditions like freeze-thaw temperature, curing conditions, and freeze-thaw rate are key factors affecting concrete freeze-thaw resistance. Lower temperatures and rapid freeze-thaw cycles often exacerbate concrete damage, while appropriate curing conditions can significantly improve its freeze-thaw durability. These environmental factors interact with material composition factors to collectively determine concrete performance in cold environments.Recent work propose various freeze-thaw damage models to describe the performance evolution of concrete under freeze-thaw cycles. These primarily include models based on cumulative damage, probability distribution models (such as the Weibull distribution), and multi-scale numerical simulation models. These models provide a theoretical basis for predicting the service life of concrete structures. However, some limitations remain in parameter determination and practical application.To enhance concrete freeze-thaw resistance, some improved techniques are proposed. These mainly include: 1) adding chemical admixtures such as air-entraining agents, superabsorbent polymers, and antifreeze agents; 2) incorporating mineral admixtures like silica fume, fly ash, metakaolin, rice husk ash, and ground granulated blast furnace slag; 3) using fiber materials such as steel fibers and polypropylene fibers; 4) adding nanomaterials like nano-silica, nano-alumina, and carbon nanotubes; and 5) applying surface coating treatments, such as using hydrophobic materials like silane. These techniques primarily aim to improve concrete freeze-thaw resistance via enhancing its pore structure, reducing internal free water, and strengthening its internal structure.Summary and ProspectsThis review represents recent research status on concrete freeze-thaw damage, encompassing key mechanisms, influential factors, damage models, and remedial measures, thereby establishing a basis for future investigations. However, the existing research still faces some challenges, from microscale freeze-thaw damage mechanisms to macroscale performance evaluation, with some aspects that need to be resolved. A future research needs to make breakthroughs in multiple directions, including an in-depth exploration of microscopic mechanisms, the development of new materials, the optimization of damage models, the study of composite environmental effects, and the improvement of evaluation standards and test methods. These efforts will drive concrete technology towards more sustainable and intelligent directions, enhancing the service life of concrete structures in cold regions, while meeting growing engineering demands and environmental requirement. The research can focus on the following directions, i.e., in-depth investigation of freeze-thaw damage mechanisms and their interactions at the microscale; development of new frost-resistant materials, such as novel supplementary cementitious materials, chemical admixtures, and nanomaterials; optimization of multi-scale damage models to improve their prediction accuracy and applicability; study of concrete durability under coupled environments; improving evaluation standards, test methods, and design specifications for concrete frost resistance. These research directions will help to enhance the service life of concrete structures in cold regions, promote the development of concrete technology towards more sustainable and intelligent directions, thereby meeting the growing engineering demands and environmental requirements, while providing more reliable theoretical foundations and technical support for the frost-resistant design and service life prediction of concrete structures.

Calcium silicate (aluminate) hydrate (C-(A-)S-H) as a predominant hydration product formed in Portland cementitious materials plays a pivotal role in determining the mechanical strength, durability, and creep resistance of Portland cementitious materials. The complex and intricate micro-/nano-structures of C-(A-)S-H become a challenge in deciphering its characteristics of micro-/nano-structures. Furthermore, the sensitivity of C-(A-)S-H to service environments exacerbates the complexity of micro-/nano-structures analysis. Throughout the operational lifespan of Portland cementitious materials, fluctuations in temperature, mechanical stresses, water flow dynamics, and attack of ions (i.e., Na+, Mg2+, Cl- and SO42-) lead to continuous and dynamic changes in the micro-/nano-structures of C-(A-)S-H. Recent work on the methods for characterizing C-(A-)S-H micro-/nano-structures, computational simulations of C-(A-)S-H, and the integration of machine learning techniques are carried out to clarify the micro-/nano-structures of C-(A-)S-H. These advancements facilitate iterative improvements in micro-/nano-structure models, enabling more precise predictions and control of macroscopic properties in micro-/nano-scale in Portland cementitious materials, accurately forecasting the evolution of C-(A-)S-H micro-/nano-structures under ions attack conditions, having a significant promise for further refining micro-/nano-structure models. Such advancements can provide theoretical insights and offer practical avenues and strategies for optimizing the performance and durability of Portland cementitious materials through informed materials design and engineering strategies.It is necessary to explore the advanced characterization of C-(A-)S-H micro-/nano-structures and clarify the intricate development of models depicting micro-/nano-structures of C-(A-)S-H, micro-/nano-structures evolution under ions attack environments and the specific mechanisms by ions (i.e., Na+, Mg2+, Cl- and SO42-) interact. A C-(A-)S-H micro-/nano-structure model is developed via utilizing the advanced techniques such as small molecule capping, time-of-flight secondary ion mass spectrometry (TOF-SIMS), molecular simulations and machine learning. The overarching goal remains to provide theoretical foundations for optimizing C-(A-)S-H micro-/nano-structure model.In addition, predicting the evolution of C-(A-)S-H micro-/nano-structures under ions attack conditions accurately is a multifaceted endeavor with a potential to advance theoretical understanding and provide practical solutions. The significance of this undertaking is that it paves a way for Portland cementitious materials optimization through intelligent design, thereby creating cementitious materials that are more durable, efficient, and environmentally sustainable to meet the demands of modern construction and other related fields.Summary and ProspectsThis review represents the micro-/nano-structure models of C-(A-)S-H and discusses recent advancements in understanding its evolution under common marine ions attack (i.e., Na+, Mg2+, Cl- and SO42-). This review elucidates the mechanisms by which aluminum-rich supplementary cementitious materials promote the formation of C-A-S-H, highlighting advancements in characterization techniques and the integration of molecular simulation and machine learning. These innovations significantly advance the development of models describing the micro-/nano-structures of C-(A-)S-H.Advancements in characterization techniques supported by cutting-edge technologies such as molecular dynamics simulations and machine learning deepen our understanding of atomic ratios within C-(A-)S-H in relation to its degree of polymerization, average chain length, micro-mechanical properties, and durability. These developments facilitate the establishment of mappings between the micro-/nano-structures of C-(A-)S-H and its properties. Furthermore, this can contribute to advancing sequential studies of silicate (aluminate) tetrahedra on the Dreierketten chains.At present, some aspects merit a further investigation and have a potential to emerge as future research focal points as follows: 1) the micro-/nano-structure model of C-(A-)S-H can be optimized and a framework utilizing first-principles calculations, molecular dynamics simulations, and machine learning techniques needs to be proposed. These aim to predict the mechanical properties and durability of Portland cementitious materials based on the micro-/nano-structure of C-(A-)S-H. It is thus feasible to enhance understanding and predictive capabilities concerning the performance and longevity of Portland cementitious materials at the microstructural level via integrating these advanced computational methodologies. 2) Atomic arrangement analysis of C-(A-)S-H is needed. It is thus feasible to determine the relative abundance of structural units within C-(A-)S-H by nuclear magnetic resonance (NMR). Chemical environment analysis techniques, such as X-ray photoelectron spectroscopy (XPS), enable elemental valence state analysis in C-(A-)S-H. The detailed atomic arrangement analysis of C-(A-)S-H can be obtained, which is crucial for directing the targeted production of Portland cementitious materials. 3) The micro-/nano-structures evolution mechanisms of C-(A-)S-H in marine environments need to be investigated. The micro-/nano-structures evolution mechanisms of C-(A-)S-H under complex ions attack conditions are investigated. The degradation mechanisms of C-(A-)S-H in simulated marine environment need to be further simulated via establishing a database of products under single-factor attack environments and employing thermodynamic simulation techniques, and the simulation results with the micro-/nano-structures of C-(A-)S-H in real marine service environments need to be compared. 4) Low-carbon green building materials are developed. The Ca/Si ratio in C-(A-)S-H can be adjusted based on the optimized models of micro-/nano-structures of C-(A-)S-H, meeting engineering requirements, while optimizing the raw material composition of cement. This approach can reduce CO2 emissions during cement production, thereby promoting the development of low-carbon green building materials.

The corrosion of steel bar is one of the main reasons for the lack of durability of concrete structure. The serious corrosion of steel bars can lead to mechanical strength loss of steel bars, cracking and spalling of concrete, eventually structural deterioration and failure, resulting in serious safety problems and huge economic losses. It is thus of great practical significance to control the corrosion of reinforced concrete structures for their long life and safe service. Steel bar corrosion inhibitor is one of the main measures used to slow down steel bar corrosion because of its advantages of simple construction, economical and effective. However, the problems of long corrosion test cycle and high cost become increasingly prominent. Amino acids, drugs, plant extracts, and green compounds are used as corrosion inhibitors with the continuous development of green organic corrosion inhibitors. Its composition is complex, the mechanism of action is complicated, and it is difficult to screen the effective ingredients. Molecular dynamics (MD) simulation is widely used in the research of corrosion inhibitors, which can explain the mechanism of corrosion inhibitors from the atomic level and help material control and test design. The effect and mechanism of corrosion inhibitor can be elucidated comprehensively via combining the methods of microstructure observation and substance composition tests. The MD simulation effectively solves the problem of long test cycle and high cost as one of the important methods to investigate corrosion inhibitors for steel bars, which has significant advantages and wide application prospects. This review briefly summarized the basic principles, common formulas and application processes of molecular dynamics simulation. The application of molecular dynamics simulation in mechanism research and property screening of steel bar corrosion inhibitors in recent years was represented. In addition, the research and application of MD simulation in corrosion inhibitors were prospected to provide a theoretical support for the development of simulation technology in corrosion inhibitors.The theoretical basis and common formulas of MD simulation are briefly introduced, as well as the environmental factors that need to be set in the application process and the basic simulation process are given. In the application of MD simulation, some appropriate parameters and conditions are set according to the research environment, and then the results can be calculated. The molecular adsorption and diffusion are used to determine the effect of corrosion inhibitors.The application of MD simulation in corrosion inhibitor research in recent years is summarized. In the application of corrosion inhibitor mechanism research, MD simulation is mainly used for a wide variety of organic corrosion inhibitors with a complex mechanism of action. It is reported that the corrosion inhibition mechanism of organic corrosion inhibitors is usually the formation of physical or chemical adsorption films on the surface of steel bars, and the adsorption behavior mainly depends on the physical and chemical properties of the corrosion inhibitor molecules. These properties are related to their functional groups, spatial structure and electron orbital properties, and the relevant calculation model is proposed. Based on the quantum chemistry calculation and molecular dynamics simulation, the adsorption energy of corrosion inhibitor molecules and steel bar surface can be calculated, and the most stable adsorption configuration can be simulated, so as to better explain the corrosion inhibition mechanism. The environmentally friendly, pollution-free and sustainable green corrosion inhibitors become a research hotspot, i.e., waste drugs, plant extracts, vitamins, DNA, green organic compounds, etc., as corrosion inhibitors. However, some of them are complex in structure and contain a variety of active ingredients. Conventional research methods need to carry out a large number of experiments, which consumes manpower and time. The MD simulation can be used to construct the motion behavior of different kinds of molecules on specific surfaces, and select the optimal molecules according to adsorption energy, diffusivity and barrier properties, thus improving the research efficiency of corrosion inhibitors.Summary and prospectsThe MD simulation method can obtain the thermodynamic structure and mechanical properties of complex molecules, analyze the dynamic evolution of the system, and obtain the time-dependent dynamic properties of the system. This can favor exploring the corrosion mechanism more conveniently and intuitively, and improving the research efficiency. The MD simulation becomes an effective tool to investigate the mechanism and development of steel bar corrosion inhibitors and has broad application prospects. However, the existing MD simulation cannot simulate complex chemical reactions, so it is more commonly used in the study of organic corrosion inhibitors, and the simulation method is relatively simple, mostly adsorption of an inhibitor molecule in pure water environment. Some studies show that a variety of ions in concrete environment can affect the effect of inhibitor molecules, and the inhibition effect is affected by molecular concentration and environmental temperature. In future studies, OH- can be added to simulate the alkaline environment, and Ca2+, K+, Na+ and other cations can be introduced according to the real situation when simulating the concrete environment. The interaction between cations and inhibitor molecules can be intense in the simulation of phenolic and metal cation complexation reaction, thus obtaining more accurate and scientific simulation results. The use of MD to simulate the dynamic process of adsorption of multiple corrosion inhibitor molecules to film formation and corrosion barrier needs a further study. The study of the coupling adsorption of various inhibitor molecules on the surface of steel bars can favor expanding the application range of MD simulation and providing the data support for future prediction methods such as machine learning and deep learning.