As a non-homogeneous material, concrete has a large number of microcracks and even macro-defects on its surface and inside. Cracks in concrete dams can impair the integrity of the dam, change its stress state, and shorten its service life. In practical engineering, most concrete structures are subjected to dynamic loads such as earthquakes and dynamic water pressure. Dynamic loading has uncertainty and complexity, which has an important impact on the load-carrying capacity and durability of concrete structures. Research on the crack resistance of concrete is conducive to delaying and mitigating the cracking of concrete as well as taking measures to stop the cracking of cracked concrete, thus improving the durability and safety of concrete structures.

Dam concrete is usually fully-graded concrete, subject to various conditions usually uses wet-screening concrete to carry out experimental research, but its test results do not fully reflect the mechanical properties of fully-graded concrete due to the sieve out of the large aggregate. It is thus necessary to carry out the fracture test of fully-graded concrete. In addition, many concrete high dams in China are constructed in the southwest and northwest earthquake areas, and the existing research on the fracture under dynamic loading is seriously lack. This paper was to carry out tests during rapid loading to obtain the dynamic fracture parameters of fully-graded concrete.

Fully-graded concrete specimens were prepared using the same mix ratio as the Baihetan Arch Dam. The 15MN dynamic and static material was used and an axial tensile fracture test was carried out. The specimens had a diameter of 450 mm and a height of 450 mm. The central part of the concrete specimen was cut by a cutting machine to form a ring prefabricated crack with a depth of 45 mm and a ratio of the length of the prefabricated crack to the length of the ligament of 0.2.

To compare the fracture properties of fully-graded concrete under static and dynamic loading, a strain rate of 10^-6 s^-1 was set as the quasi-static condition. According to the range of seismic characteristic strain rate, a strain rate of 10^-3 s^-1 was set as the dynamic working condition. The test was controlled via displacement loading, and the change process of load and loading point displacement was recorded during the test. At the cracks, one trans-seam displacement extensometer was arranged at every 90° degree along the ring, with a total of four extensometers at a distance of 50 mm, and the direction of the displacement extensometers was parallel to the length of the specimen, which was used to determine the change curve of the crack opening displacement with the load in the loading process. Laser displacement gauges were arranged adjacent to each of the displacement gauges at a distance of 350 mm to measure the overall deformation of the specimen. The fracture toughness was calculated via the experimental and numerical data, where the numerical method was an extended finite element method, and the eccentricity phenomenon that occurred during the test was also taken into account.

All the specimens are fractured along the prefabricated crack surface. For the specimens under quasi-static loading, the fracture surface is rough and exposed more coarse aggregate, and the interface between aggregate and matrix stripping mostly appears on the fracture surface, and the fracture of aggregate is rare. In the specimens under dynamic loading, the fracture surface is slightly flat, and in addition to the interface between the aggregate and matrix stripping, a number of coarse aggregates are pulled off in the section. During the slow increase in load, the cracks tend to expand along the weak surface, i.e., the aggregate-matrix interface. Under dynamic loading, the cracks do not have time to develop along the weakest part but expand along the shortest path, leading to damage to the specimen.

The increase in the strength of concrete specimens under dynamic loading is because the process of extension along the prefabricated cracks runs directly through the coarse aggregate, without having time to pass through the weak part of the cement mortar-coarse aggregate bond. In essence, the inertial forces and inhomogeneity of the concrete material itself lead to an increase in the strength of the specimens under dynamic loading. Under quasi-static loading, the rate of strain energy release and aggregation in concrete specimens is slower, and crack expansion occurs along the weak part of the material. In contrast, under dynamic loading, the rate of strain energy release and aggregation in concrete specimens accelerates and cracks may propagate through the stronger parts of the material, leading to an increase in strength.

The peak load, energy absorption capacity, fracture energy, and fracture toughness of fully-graded concrete specimens under dynamic loading were increased by 58.51%, 145.75%, 124.48% and 47.71%, respectively, compared to static loading, while the characteristic length was reduced by 11.67%. The numerical simulation calculated the fracture toughness taking into account the eccentricity phenomenon during the test, which was increased by 71.98% for quasi-static loading and by 60.35% for dynamic loading, compared to the uniform loading method without taking into account the eccentricity phenomenon.

The existing alkali-activated repair materials cannot adapt to the corrosion repair of concrete in marine engineering with high humidity and high salt environments. In this paper, a polyacrylic acid lotion (PAA) modified alkali-activated slag repair material with high strength, low shrinkage and high durability was developed by adding PAA to regulate the flexibility of the consolidation body, and its performance formation mechanism was analyzed. The results indicate that the mechanical properties of alkali-activated slag repair materials are positively correlated with the content of sodium silicate added when the modulus of sodium silicate is greater than 1.2. The optimal activating ratio for slag repair materials is 1.2 modulus and 15% content of sodium silicate. PAA filling in the gaps of amorphous cementitious substances in alkali-activated slag repair materials significantly enhances its mechanical properties, bonding properties, volume stability, and sulphate resistance.

A powder used was cement (i.e., P·II 52.5 grade Portland cement) mixed with slag (i.e., a granulated blast furnace slag) with a hydraulic coefficient of 2.18, an activity coefficient of 0.25, an alkalinity coefficient of 1.11, and a quality coefficient of 2.07. Alkali activator used was sodium silicate (with a modulus of n=3.3 and a Baume degree of 40) and sodium hydroxide(granular sodium hydroxide with a purity of ≥98%). The organic lotion was a polyacrylic lotion with a molecular weight of 3 000-5 000 and a solid content of ≥30%. The aggregate was river sand with the maximum particle size of of 2.5 mm, the bulk density of 1.6 g/cm³, and the apparent density of 2.5 g/cm³. Water used was tap water.

Sodium hydroxide weighed was fully dissolved in sodium silicate under stirring magnetically. The solution was colourless and transparent, and there were no suspended sodium hydroxide particles in the solution. Subsequently, a beaker with the solution was covered with a thin film to prevent the evaporation of water due to the dissolution and heat release of sodium hydroxide. The solution was cooled to room temperature before use. The prepared solution was poured into a mixing pot, and mixed with the powder under stirring. After curing until the specified age, the relevant tests were conducted.

The physical and mechanical properties of alkali activated slag repair materials were determined in accordance with the provisions of GB/T50081—2019 "Standard Test Methods for Physical and Mechanical Properties of Concrete". The bond strength was measured via bond bending tests. According to GB/T 50082—2009 "Standard Test Methods for Long term Performance and Durability of Ordinary Concrete", the shrinkage rate of alkali activated slag repair materials was measured by a contact method. According to GB/T 50082—2009 "Standard Test Methods for Long term Performance and Durability of Ordinary Concrete", the sulphate attack resistance of the alkali activated slag repair material was determined, and the corrosion resistance coefficient (K) was used as an indicator to measure the sulphate attack resistance of the sample. The phase composition of alkali activated slag repair materials was analyzed by a model D8ADVANCE X-ray diffractometer. The main functional group composition and chemical bond types in alkali activated slag repair materials were characterized by a model TENSOR27 Fourier infrared spectrometer. The microstructure of alkali activated slag repair materials was characterized by a model ZEISS Gemini 500 scanning electron microscope.

Increasing the modulus of sodium silicate can improve the strength of alkali activated slag repair slurry. Adding 15% sodium silicate with a modulus of 1.2 is an optimal excitation approach for slag repair materials. When the content of PAA is 1%, the flexural strength and compressive strength of PAA modified alkali activated slag repair slurry are increased by 7% and 14%, respectively, compared to alkali activated slag repair slurry, reaching 9.90 MPa and 57.87 MPa. High-temperature curing at 50 ℃ further improves the strength of alkali activated slag repair mortar, and the strength improvement of PAA modified alkali activated slag repair mortar is greater than that of unmodified mortar. After PAA modification, the bonding strength of alkali activated slag repair mortar is improved, and its resistance to sulphate attack is enhanced. The bonding strength of PAA modified alkali activated slag repair mortar is greater than 2 MPa, and the corrosion resistance coefficient is greater than 0.85, meeting the requirements of JC/T 2381—2016 for the bonding strength and corrosion resistance coefficient of repair mortar. The shrinkage rate of alkali activated slag repair mortar at all ages increases with the increase of sodium silicate modulus. The PAA modification further reduces the shrinkage rate of alkali activated slag repair mortar. When the PAA content is 1%, the shrinkage rate of alkali activated slag repair mortar is reduced by 54%.

The early flexural strength development of alkali activated slag repair slurry was slower, while the early compressive strength development was faster. Adding a small amount of (10%) sodium silicate could stimulate the activity of the slag, slightly improving the strength of the slurry, and significantly increasing the strength at 15% and 20% dosages. The addition of PAA did not change the product composition in the alkali activated slag repair material, but filled in the gap of amorphous cementitious material. Before PAA modification, the cross-section of the alkali activated slag repair material was smooth and flat. After PAA modification, the cracks on the both sides of the alkali activated slag repair material were connected by fibrous filaments, further enhancing the strength of the material.

Cementitious materials are widely used in construction due to their wide range of raw material sources and easy construction. However, the characteristics of brittleness and easy cracking restrict their application in ductile structures. Fiber toughening is an effective approach to enhance the toughness of cementitious materials. The damage of cementitious materials is a gradual, multi-scale occurrence of the process. The first micrometer-scale cracks emerge, gradually expand and become larger with the increase of the load, forming the microscopic cracks, following convergence, developing macroscopic cracks, and ultimately leading to the material failure. Therefore, the toughening and crack-resisting effect of single-scale fibers are difficult to meet the multi-scale cracking process of cementitious composites. To correspond to this process, the hybrid fiber is an effective method, and the synergistic effect between different types of fibers can achieve the effect of "1+1>2". There exist a few studies on the static flexural performance of hybrid fiber-toughing cementitious composites (HFTCC), but there is paucity of studies on the dynamic flexural behavior of HFTCC. To understand the hybrid effect of steel fiber and polyethylene fiber (PE fiber) on the static and dynamic flexural properties of cementitious composites, the static and dynamic flexural performances of HFTCC were investigated via three point bending test and drop weight impact test. In addition, the hybrid effect of steel fiber/PE fiber and the corresponding toughening mechanism were also analyzed.

The three-point bending tests were carried out based on a reference (GB/T 17671—2021). An electro-hydraulic servo universal testing machine (INSTRON 1342) was used to carry out the three-point bending test, at a loading rate of 0.2 mm/min. Load transducers and displacement sensor were used to record the load and mid-span deflection (δ) changes during the test, and the loading was stopped when the δ value reached 5 mm.

The impact flexural properties were determined by a model INSTRON-CEAST 9340 drop weight impact testing machine. The required velocities were obtained by releasing the impactor from a predetermined height. The mass of the impact hammer head used in this test was 4.08 kg, the set impact speeds were 3 m/s and 4 m/s, and the impact force was collected by a high-speed data acquisition system (within 22 kN) at a data acquisition frequency of 0.5 MHz.

The failure pattern of specimens indicates that fibers can effectively improve the dynamic flexural properties of specimens due to the fiber bridging and crack-blocking effect. Compared with PE fibers, steel fibers have a higher stiffness and a better ductility, which can resist the deformation of the specimen and limit the degree of opening of the main crack, thus improving the flexural properties of the specimen.

Comprehensive analysis of load-displacement curve of HFTCC specimens shows that the static flexural performance of HFTCC is positively correlated to the PE fiber content, and the dynamic flexural performance demonstrates a positive correlation with the steel fiber content. The specimen with 1.5% (in volume fraction) PE fiber and 0.5% steel fiber exhibits the optimal static flexural peak stress, while the specimen with 1.5% steel fiber and 0.5% PE fiber presents the optimal dynamic flexural energy absorption. Under static flexure, the peak strength and energy absorption of HFTCC are increased by 26.5%-31.7% and 14.8%-56.8%, respectively, compared to the specimen with 2.0% steel fiber content.

The dynamic increase factor (DIF) and energy absorption of HFTCC exhibit a significant strain rate effect. Under dynamic loading, the toughening effect of PE fibers is weaker than that of steel fibers, and the incorporation of a large number of PE fibers is not conducive to the enhancement of the dynamic energy absorption capacity of the specimen, which is related to the change in the failure mode of PE fibers. A small amount of PE fibers instead of steel fibers plays a toughening effect, which can exert the enhancement of the specimen dynamic energy absorption capacity.

Synergistic effect analysis reveals that the steel fiber plays the dual effect of enhancement and toughening on the higher rate of dynamic flexure when the steel-PE fiber system with a high dosage of steel fiber due to the high tensile strength, stiffness, and ductility of the steel fiber, effectively enhancing the energy absorption capacity of the HFTCC under the lager deformation.

The schematic diagram of HFTCC physical model was established via the analysis of SEM images. The steel-PE fibers effectively prevent the development of micro-cracks and the formation of macro-cracks, and the steel-PE fibers have a positive effect on the improvement of the mechanical properties and impact resistance of the cementitious materials during the static and dynamic loading process. Also, the micro-morphological analysis of HFTCC indicates that the steel-PE fibers form a three-dimensional mesh structure to have the effect of fiber toughening and crack-resisting on the matrix.

1) The steel-PE hybrid fibers improved the static and dynamic flexural properties of HFTCC, in which the 0.5% (in volume fraction) steel-1.5% PE hybrid fiber HFTCC exhibited the optimal static peak bending and tensile stresses, while the 1.5% steel-0.5% PE hybrid fiber HFTCC had the optimal energy absorption under the dynamic loads. The DIF and energy absorption of HFTCC had significant strain rate effects.

2) The steel-PE fibers in HFTCC showed a synergistic effect. For the peak load, static flexure mainly showed a positive synergistic effect, and the contribution of PE fibers to the peak stress of HFTCC was weaker than that of steel fibers under the dynamic impact loads. The specimen with 0.5% steel-1.5% PE hybrid fiber under the static flexure effectively improved the energy absorption of HFTCC, with the maximum hybrid coefficient S of 1.239, while the impact dynamic loading under the impact dynamic loading of the specimen with 1.5% steel-0.5% PE hybrid fiber effectively enhanced the energy absorption of HFTCC, with the maximum S of 1.086.

3) The toughening mechanism of HFTCC mainly originated from the synergistic crack-blocking and toughening effect between steel and PE fibers. Especially, the micro-fine steel fibers and PE fibers could fully utilized their respective mechanical properties at their appropriate dosages, thus effectively improving the static and dynamic flexural performances of HFTCC.

Conventional concrete is a porous heterogeneous quasi-brittle material, which has some problems such as poor toughness, low flexural tensile strength and poor durability, and cannot meet the needs of modern construction for high strength, high crack resistance and high durability of concrete materials. The existing methods used to toughen concrete are fiber reinforcement and matrix toughening. Among them, fiber is an extrinsic toughening. The toughness of cement matrix can be improved with polymers and nanomaterials. In addition, compared with mixing polymers directly, the modified method of in-situ polymerization of organic monomers can improve the flexural strength of cement pastes. However, the effect and mechanism of this method on toughening concrete are still unclear. Therefore, the effect of in-situ polymerization of acrylamide (AM) monomer and wollastonite (CS) whisker on the mechanical properties, hydration heat release, phase composition and microstructure of concrete were investigated to provide an idea for toughening concrete matrix.

Raw materials were P·Ⅱ 52.5 Portland cement, quartz sand with the fineness modulus of 2.6-2.9 and basalt with the particle sizes of 5-16 mm. The composite modified materials used were all adscititious, in which an organic monomer was acrylamide (AM), an initiator was ammonium persulphate (APS), and a crosslinking agent was N,N'-methylenebisacrylamide (MBA). The inorganic modified material was wollastonite (CS) whisker, and its micromorphology was dense and short rod fibrous. The water-cement ratios (W/C) of concrete and cement paste were 0.47 and 0.40, respectively. The dosages of CS were 1% (in mass fraction), 2% and 4% by the mass of cement. The dosages of AM were 3% and 5% by the mass of cement, the dosages of APS were 3% and 5% by the mass of AM, and the dosage of MBA was 0.1% by the mass of AM.

The compressive strength, flexural stress-strain curve and fracture energy of concrete specimens (100 mm×100 mm×100 mm and 100 mm×100 mm×400 mm) were tested after curing under standard curing condition (20 ℃, 95% RH) for 3, 7 d and 28 d. The hydration exothermic curve of the cement paste in the early 72 h was recorded at 20 ℃ by a model TAM-Air micro-calorimeter (TA Instruments Co., USA). After the cement paste was cured for 3, 7 d and 28 d, the phase composition was qualitatively and quantitatively analyzed by a model D8 ADVANCE X-ray diffractometer (Bruker Co., Germany). The hydration degree of cement was determined by a model TG-209F3 thermal gravimetric analyzer (Netzsch Co., Germany), and the functional groups were characterized by a model Nicolet IS10 Fouri infrared spectrometer (Thermo Scientific Co., USA). The microstructure was observed by a model Quanta 3D FEG scanning electron microscope (Fei Co., USA).

The effect of single-doped CS on the flexural strength of concrete is not significant, but it can improve the compressive strength to a certain extent at 3 d and 7 d in the early hydration stage. Adding AM has a negative effect on the bending resistance of concrete at each stage. The addition of APS and MBA can make AM crosslinked and polymerized in cement hydrated matrix, which can significantly improve the peak flexural stress and fracture energy. However, the film forming characteristics of AM polymerization lead to a poor compatibility with cement matrix, directly affecting the hydration process of cement and reducing the compressive strength. Under the synergistic effect of CS and AM polymerization induced by APS, the maximum flexural stress and fracture energy of concrete at 28 d are increased by 31.5% and 91.4%, respectively, compared with control group. Note that the compressive strength of concrete at 28 d remains unchanged.

Hydration heat results show that the main exothermic processes of AM monomer polymerization and cement hydration are not synchronized. AM polymerization can immediately release a lot of heat and delay the hydration reaction of cement, while the addition of CS has little effect on the hydration process of cement. Neither CS nor AM polymerization can change the types of cement hydration products, but CS can participate in cement hydration reaction and increase calcium hydroxide (CH) content. Under the action of initiator, AM monomer polymerization can promote ettringite (AFt) formation and reduce CH content. The addition of CS can fill the void in the hydrated paste, and promote the hydration reaction of cement, thus reducing the porosity of concrete. However, AM monomer polymerization can form an organic plasticizing zone, strengthen the bonding force between hydration products such as CH, and interact with needle-shaped AFt, effectively forming an organic-inorganic interpenetration network structure.

AM in-situ polymerization and CS could improve the toughness of concrete. The 28-d flexural stress peak and fracture energy were increased by 31.5% and 91.4%, respectively, and the compressive strength remained unchanged. CS whiskers could promote cement hydration, exert filling effect and reduce the porosity. AM polymerization could build a plasticized zone in cement pastes and weaken stress concentration in concrete. The synergistic effect enhanced the bonding force between the hydration products, formed an organic-inorganic interpenetrating network structure, optimized the structure of concrete across scales, and provided an effective technology for toughening concrete matrix.

In the marine environment, reinforced concrete components are extremely vulnerable to corrosion by harmful ions in seawater, resulting in a serious damage. The impressed current cathodic protection method is one of the effective methods for corrosion protection of reinforced concrete structures. Conventional anode materials such as stainless-steel mesh, magnesium mesh and zinc alloy materials can effectively prevent the corrosion of passivated steel bars, but metal anode materials have some problems such as difficult laying operations, high-cost, susceptibility to corrosion and poor long-term durability. Thus, non-metallic composites, especially conductive cementitious materials can be used as promising anode materials. Conductive cementitious materials are simple to prepare, low-cost, adaptable to different concrete surfaces, and have a possibility of being reused. In this paper, a high-strength, low-permeability cement-based anode material was proposed due to its surface protection and conductive anode functions. Two kinds of cement-based anode materials were prepared in the laboratory, which were high-strength conductive mortar materials mixed with carbon fibers (CF) and carbon nanofibers (CNF) as well as conductive mortar materials embedded with carbon fiber net cloth, respectively. The mechanical properties, resistivity, and impermeability of the two anode materials were tested, and the evolution of the properties under the ICCP accelerated electric field was investigated.

The cementitious material of the mortar matrix was composed of P.O 42.5 cement and silica fume, the particle sizes for two types of quartz sand were 0.11-0.21 mm and 0.21-0.38 mm, a high-performance polycarboxylate superplasticizer was used, and a water-binder ratio is 0.26. In the carbon fiber conductive mortar, carbon fibers (in volume fractions of 0%, 1.0%, 1.5%, 2.0%) and carbon nanofibers (in volume fractions of 0%, 0.6%, 0.9%, 1.2%) were mixed. The single layer carbon fiber net cloth with a mesh size of 5 mm and was placed vertically on the surface of the specimen. The anode materials were cured at 90 ℃ for 72 h.

The anode materials were subjected to the flexural and compressive strength tests (GB/T 17671—2021). Electrical resistivity tests (two-electrode method) were conducted on anode materials cured for different durations. Also, gas permeability tests and one-sided water absorption tests (EN 13057) were carried out to assess the permeability resistance of the anode materials. The performance degradation of the anode materials under the influence of electrical current was tested through cathodic protection acceleration experiments. The designed current density for external electrical current was 20 mA/m². Acceleration tests were conducted by increasing the current density to 100 times (i.e., 2 000 mA/m²) and 150 times (i.e., 3 000 mA/m²). The corrosion status of the rebar was determined by a model ASTM C876 standard test method to assess the effectiveness of the anode materials in the cathodic protection test. The microstructure of the anode materials after ICCP acceleration test at an acceleration voltage of 30.0 kV was characterized by a model VEGA3 TESCAN scanning electron microscope.

For carbon fiber conductive mortar, the mixing of CF with CNF improves the mechanical properties and conductive properties of the carbon fiber conductive mortar matrix. The compressive strength and flexural strength of mortar with 1.0% CF + 0.6% CNF are increased by 28% and 66%, respectively. There is a binding effect between CF and matrix, which can significantly improve the bending strength and increase the toughness of the material, CNF has a nucleation effect and a filling effect, which refines the pore structure inside the material, and the bridging effect of CNF improves the mechanical properties of the material. The resistivity of the mortar fiber-doped with 2.0% CF + 1.2% CNF is 160 Ω·cm, which is as low as nearly one ten-thousandth of that of the mortar fiber-undoped, this is a result of the formation of a conductive network inside the mortar specimen formed via the interconnection of CF and CNF.

For carbon fiber conductive mortar with embedded carbon fiber net cloth, the introduction of carbon fiber net cloth improves the mechanical properties of the material. This is due to the fixed distribution of a layer of carbon fiber net cloth in the matrix, which constrains the cracking process of the material. Carbon fiber mesh cloth is woven from carbon fibers. The current conduction path between the fibers is more complete. The introduction of carbon fiber net cloth, CF, and CNF improves the conductivity of the composite mortar materials.

Carbon fiber conductive mortar and carbon fiber conductive mortar with embedded carbon fiber net cloth both have superior impermeability properties. Intrinsic gas permeability K_v and capillary water absorption rate S are as low as one-tenth of ordinary concrete, indicating that the prepared anode material has a superior physical protection function.

The cathodic protection accelerated test proves the effectiveness of the anode material. The mechanical properties and electrical conductivity of the anode material after energized slightly deteriorates. The compressive and flexural strengths are decreased by only 14.5% and 12.1%, and the resistivity is increased by 18.1%. The anode impermeability of carbon fiber conductive mortar decreases. The impermeability of a conductive mortar embedded with carbon fiber mesh cloth and carbon fiber as an anode material does not change significantly because carbon fiber mesh cloth has a main conductive effect to protect the integrity of the substrate.

The cement-based anode materials both had the superior mechanical properties, i.e., the compressive and flexural strength of 103.0 MPa and 14.6 MPa, respectively, the resistivity of 160 Ω·cm, the gas permeability and capillary water absorption rate of only 1/10 of the ordinary concrete. The concrete structure had a positive physical protection effect. After the ICCP acceleration, the mechanical properties and electrical conductivity of the two materials only produced a slight deterioration phenomenon, in which the anode material with carbon fiber net cloth performed more consistently. It could be suitable for cathodic protection of concrete with applied current. The cement-based anode materials can be also used for electrochemical chloride extraction of concrete components under high chloride salt environment conditions instead of the conventional anode materials.

Thermodynamic modelling has a robustness in studying the phase evolution and durability performance of AAMs induced by chemical reactions. Firstly, thermodynamic modelling can predict the reaction products assemblage and pore solution composition of AAMs. Secondly, thermodynamic modelling can calculate the phase evolution of AAMs under the action of aggressive media, and then study the deteriation mechanism of AAMs. Finally, thermodynamic modelling can be combined with other numerical simulation techniques to investigate AAMs. At present, however, there are still some issues that need to be further studied as follows:

1) The incomplete thermodynamic database for alkali-activated low-Ca system is an important reason for the limited thermodynamic modelling studies on alkali activated low and medium calcium systems. It is expected that a thermodynamic model describing the N-A-S-H gel can be established by ab-initio calculations and molecular dynamics simulations with the development of atomic- and molecular-scale simulation techniques.

2) It is generally assumed that the amorphous phases in precursors are dissolved synchronously in current thermodynamic modelling of AAMs. However, the heterogeneous distribution of composition and structure of precursor makes this assumption in doubt. The non-uniformity of the dissolution of amorphous phases in precursor is an issue to be further considered in future thermodynamic modelling studies.

3) The phase evolution of AAMs is actually a process coupling thermodynamics and kinetics. However, most of the thermodynamic modelling studies only focus on the phase assemblage in the equilibrium state, ignoring the kinetic issues before reaching the equilibrium. Considering the kinetic parameters (i.e., dissolution rate and reaction rate, etc.) in thermodynamic modelling should be a focus of current and future thermodynamic modelling studies.

4) The phase evolution, microstructure damage and ions transport are three inter-dependent aspects for studying the durability performance of AAMs. However, the current thermodynamic modelling studies mainly focus on the phase evolution under the chemical attacks, while ignoring the interaction between the phase evolution, microstructure damage and ions transport. In future studies, it is necessary to consider the interaction and establish a chemical-damage-transport model to numerically analyze the durability performance of AAMs.

The orientation of steel fibres has an impact on the mechanical properties of ultra-high performance concrete (UHPC). It is difficult for the conventional experimental methods to maintain the same fibre orientation and distribution characteristics even under identical conditions, and there is a lack of mesoscale model that can simultaneously capture the complex mechanisms of crack evolution and fibre-mortar interaction in UHPC. This impedes the quantitative analysis and simulation validation. This paper thus conducted direct tensile tests of UHPC in an external magnetic field to achieve a certain fibre orientation, and the distribution density of fibre inclination angles was obtained via image analysis. The influence of fibre orientation on the fracture processes and stress-strain curves was investigated by a digital image correlation (DIC) technique. Also, a 3D microscale numerical model of UHPC was established to explicitly consider fibre orientation and distribution, single fibre pullout force-slip relations based on inclination angles, and discrete cohesive cracking of mortar. The model was validated by the experiments, the complex 3D fracture process of UHPC was quantified, and the influences of mesoscale factors (i.e., fibre distribution and orientation) on the strain hardening characteristics, crack evolution and final morphology, and fibre bridging effects were investigated.

In the experiments, steel fibre orientation was controlled through an external magnetic field, which was created by direct current in a coil loop. The dog-bone specimens were vibrated in the magnetic field during the sample preparation. As a result, the fibres were subjected to the electromagnetic force and underwent a directional distribution, i.e., along the magnetic field direction. Three fibre orientations were a) parallel to the tensile direction, b) perpendicular to the tensile direction, and c) no magnetic field applied. This resulted in three groups of specimens termed aligned-UHPC (AUHPC), perpendicular-UHPC (PUHPC), and UHPC, respectively. A tensile testing device for dog-bone specimens was used to investigate the fibre orientation-dependent mechanical properties of UHPC for subsequent validation of simulations. The loading process was recorded by a high-speed camera, providing data for the fracture process analysis based on the DIC technique. In addition, the cut-off sections were analyzed on specimens AUHPC, UHPC, and PUHPC to obtain the orientation distribution characteristics of fibres for later simulations.

In the mesoscale simulation, cohesive elements were pre-inserted in the mortar to simulate the energy dissipation in the fracture process zone and the opening/closing of discrete cracks. Different fibre orientation distributions in specimens AUHPC, UHPC, and PUHPC were generated by a random algorithm. The fibre-mortar interaction was equivalently simulated through fibre constitutive laws, which were curve-fitted from the pullout force-slip relations extracted from single fibre pullout tests for various inclination angles. The mesoscale model was validated via single fibre pullout tests and direct tensile tests. In addition, the mesoscale influencing factors on the complex multiple cracking and fibre bridging mechanisms were also quantified.

The experimental results reveal that the specimen AUHPC exhibits the most pronounced strain-hardening behavior and maximum peak stress, which are 2.3 times and 1.4 times greater than those of specimens PUHPC and UHPC, respectively. The first cracking stress of specimen AUHPC is also the maximum, while there is no obvious change in the elastic modulus. The increase in peak stress can be explained by the fracture process using DIC: when fibres are more aligned with the tensile direction, there are more microcracks in the specimen. This is because when the fibre orientation follows the tensile direction, the fibre bridging effect is the more intense, thus having the maximum crack resistance. When the fibre orientation is perpendicular to the tensile direction, the fibre bridging effect is the minium. The final crack patterns of specimens AUHPC and UHPC are complex and tortuous, while the crack paths in specimen PUHPC are smoother and mostly perpendicular to the tensile direction. The distinct crack patterns also indicate the differences in the stress-strain curves. Moreover, from the cur-off sections of specimen AUHPC determined by optical microscopy, the fibres after the test have obvious pullout scratching on the fibre surfaces with spalled mortar clinging to them. In addition, there is no obvious necking at the fibre ends, indicating that the fibre failure is pullout rather than yield fracture. The mesoscale mechanisms (i.e., interfacial failure, mortar detachment, and fibre pullout) indicate a much energy dissipation, which improves the material ductility.

The mesoscale simulations predict the fracture patterns and stress-strain curves, which are in reasonable agreement with the experimental results. Based on the analysis of fibre stress evolution, this work can quantify the process of microcrack initiation, multiple cracking, strain hardening, localized major cracks or macrocracks, and gradual closure of other microcracks with increasing the displacement. The stresses of fibres far away from the localization zones decrease, while the stresses of fibres crossing the major cracks continue to increase (i.e., 2 000 MPa) due to fibre bridging. Afterwards, the specimen gradually enters the softening stage until complete failure. The specimen AUHPC exhibits three macrocracks, while specimens UHPC and PUHPC have only one macrocrack each. Besides, the quantities of microcracks in specimen AUHPC are much larger than those in specimen PUHPC, and the fibre average stress level in specimen AUHPC is much higher. This is because when the fibre is aligned with the tensile direction, more stresses can be transferred into the mortar, causing re-cracking, which is a repeated process, leading to multiple cracking in specimen AUHPC. As a result, the stresses of bridging fibres consistently increase. On the contrary, the efficiency of stress transfer for the fibres perpendicular to the tensile direction is the lowest, with the minimum fibre average stress of 94 MPa in specimen PUHPC, while AUHPC and UHPC have 328 MPa and 188 MPa, respectively. It is evident that the fibre orientation has an impact on the fibre utilization level. The simulated stress-strain curves demonstrate that specimen AUHPC has the most pronounced strain hardening and the maximum ductility, while specimen PUHPC is relatively brittle and shows no strain-hardening characteristics. This indicates that aligning the fibres with the tensile direction is an effective way to enhance the strength and ductility of specimen UHPC.

When the fibre orientation was consistent with the principal stress direction, the fibre had a bridging effect on cracks. Therefore, specimen AUHPC had more microcracks, more tortuous paths, higher peak stress, more pronounced strain hardening, and more energy dissipation. specimen PUHPC had only one single, much smoother crack and showed no strain hardening. In addition, specimen AUHPC had the maximum fibre utilization level with the maximum fibre average stress, thus having the most intense bridging effect. The fibre average stress in specimen PUHPC was the minimum with insufficient ductility and significant brittleness. Therefore, optimizing the fibre orientation could be an effective way to improve the strength and ductility of specimen UHPC. Although the fibre-mortar interfaces were not simulated directly, the proposed mesoscale model with rigid embedment of fibres and equivalent fibre stress-strain constitutive relation could prove computationally efficient in simulating the bridging effects from massive random fibres. Therefore, the model could be used for the optimization of material parameters such as fibre dimensions, shape, distribution and volume fraction.

Steel corrosion is particularly severe in marine environments with a high concentration of chloride ions, which causes huge economic losses and also hinders engineering construction in coastal areas. Coating treatment is one of the common methods to protect steel from corrosion. Conventional thicker organic coatings are prone to local breakage, reduced mechanical interlocking force between steel and concrete, and being easy to deteriorate. Since the anions and cations of layered double hydroxides (LDH) both are tunable, this designability gives them a variety of potentials for metal corrosion protection, such as physical barrier, chlorine fixation, corrosion inhibition and chemical self-repair. In this paper, Mg/Al-NO₃^--LDH and Zn/Al-NO₃^--LDH films were grown in-situ on the surface of steel Q235 by an electrodeposition-hydrothermal method, and their adaptability in a simulated high-alkaline concrete environment and corrosion inhibition performance in NaCl solution were investigated.

The in-situ growth of LDH film on steel surface includes electrodeposition and hydrothermal treatment. In the electrodeposition, a three-electrode system was used, with a steel sheet as a working electrode, an Ag/AgCl electrode as a reference electrode, and a platinum sheet as a counter electrode. The electrolyte solution consisted of 50 mL of 0.045 mol/L Mg(NO₃)₂·6H₂O and 0.015 mol/L Al(NO₃)₃·9H₂O, and the steel sheet was immersed in the electrolyte solution to stabilize it for 500 s, and then a potential of -1.5 V was applied for 300 s. The hydrothermal treatment used a hydrothermal solution consisting of 50 mL of 0.06 mol/L Mg(NO₃)₂·6H₂O and 0.02 mol/L Al(NO₃)₃·9H₂O. NH₃·H₂O was added dropwise to adjust the pH value of the solution to 9. Subsequently, the electrodeposited steel sheet was vertically placed into a Teflon®-lined autoclave containing the hydrothermal solution, and reacted in a homogeneous reactor at 90 ℃ for 18 h. After the reaction was completed and cooled down to room temperature, the sheet with the film was removed and named M₁. Similarly, the obtained sample was named Z₁ when replacing Mg(NO₃)₂·6H₂O with Zn(NO₃)₂·6H₂O. To examine the adaptability of the two samples in a high-alkaline concrete environment, the samples M₁ and Z₁ were immersed in saturated Ca(OH)₂ solution for 24 h and then taken out, resulting in samples named M₂ and Z₂, respectively.

The crystal structures of the different samples were characterized using a model D8 Advance X-ray diffractometer (Bruker Co., Germany, λ=0.154 06 nm). Sample's functional groups and chemical bonds were detected using a model Spotlight 200 Fourier transform infrared spectrometer (PerkinElmer Co., USA). The surface morphology and chemical composition of the samples were determined by a model APREO S high-resolution scanning electron microscope with an X-ray energy spectrometer (Thermo scientific Co., The Netherlands). The impedance and corrosion resistance of the four samples in 3.5% (mass fraction) NaCl solution were analyzed via electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization in a ParStat 4000 electrochemical workstation (AMETEK Co., USA).

The electrodeposition and hydrothermal method was employed to in-situ grow a dense and uniform Mg/Al-NO₃^--LDH film and a lamellar porous Zn/Al-NO₃^--LDH film on the surface of steel Q235. After immersing these two LDH films in a saturated Ca(OH)₂ solution for 1 d, the lamellar structure of Mg/Al-LDH remains unchanged, and the interlayer anions are partially exchanged from NO₃^- to OH^- due to the slightly higher affinity of OH^- in the solution. Also, the lamellar structure of Zn/Al-LDH undergoes a chemical reaction due to the presence of a large amount of Ca²⁺ and OH^-, resulting in the generated portion of Ca/Al-NO₃^-/OH^--LDH and amorphous hydroxides remaining on the carbon steel surface. The results above indicate that Zn/Al-LDH is difficult to be stabilized in highly alkaline concrete environments since its constituents, i.e., Zn(OH)₂ and Al(OH)₃, both are amphoteric hydroxides. In contrast, Mg(OH)₂ is an alkaline hydroxide, allowing Mg/Al-LDH to maintain a structural stability in high alkaline environments.

The EIS results show that in a 3.5% (in mass fraction) NaCl solution, the impedance of the Zn/Al-NO₃^--LDH film is only 3.18 Ω/cm², whereas the Mg/Al-NO₃^--LDH film exhibits a greater impedance of 2 722 Ω/cm², which is attributed to the dense structure of the Mg/Al-NO₃^--LDH film. The results of dynamic potentiodynamic polarization tests indicate that the corrosion protection efficiency of the Zn/Al-NO₃^--LDH film and the Mg/Al-NO₃^--LDH film for carbon steel is 63.86% and 97.43%, respectively. Clearly, the Mg/Al-NO₃^--LDH film is more effective in protecting carbon steel against chloride ion erosion due to its superior physical barrier and the double protection effect of "fixing chlorine".

Zn/Al-NO₃^--LDH and Mg/Al-NO₃^--LDH films with a high crystallinity were grown in-situ on the surface of steel Q235 by an electrodeposition-hydrothermal method for corrosion protection. The microscopy showed that the Zn/Al-NO₃^--LDH films were lamellar and porous, and the Mg/Al-NO₃^--LDH films were dense and uniform. The electrochemical results indicated that in 3.5% (in mass fraction) NaCl solution, the corrosion protection efficiency of Zn/Al-NO₃^--LDH film on carbon steel substrate was 63.86%, while the Mg/Al-NO₃^--LDH film had a higher impedance and a lower corrosion current density, and its corrosion protection efficiency was 97.43%, effectively protecting the carbon steel against chloride ion erosion. In a high-alkaline saturated Ca(OH)₂ solution, the Zn/Al-NO₃^--LDH film exhibited an instability, leading to the formation of partially Ca/Al-NO₃^-/OH^--LDH and amorphous hydroxides. In contrast, the Mg/Al-NO₃^--LDH film remained stable, and the anion exchange resulted in the formation of Mg/Al-NO₃^-/OH^--LDH film.

Hydrated salt phase change energy storage materials enable sustainable storage and release of thermal energy through the phase transition of crystalline water. This low-cost, environmentally friendly material has a high heat storage density. However, the significant heat loss and liquid phase leakage after multiple cycles become persistent challenges, restricting the large-scale application of the original hydrated salt phase change material. Consequently, the preparation of shape-stabilized composite phase change materials by infiltrating hydrated salt phase change materials into porous support structures is one of the investigated methods to address the encapsulation challenges of hydrated salt phase change materials. In this paper, carbon aerogels with a high adsorption capacity were prepared with melamine foam as a precursor. The carbon aerogels were used to encapsulate hydrated salt phase change materials.

Carbon aerogel was prepared via multistage temperature carbonization with melamine foam as a precursor. A composite eutectic salt material (EHS) of Na₂SO₄·10H₂O-Na₂HPO₄·12H₂O with 0.2% glucose carbon nanoparticles was added as the phase change matrix. Shape-stabilized melamine carbon aerogel/eutectic salt composite phase change materials were prepared via vacuum melt impregnation.

The apparent morphology of the prepared aerogels was determined by scanning electron microscopy (SEM). The composition of the aerogels was analyzed by X-ray diffractometry (XRD), and the interactions between the MF and the eutectic hydrate salts in the composite phase change materials were investigated by the Fourier infrared spectrometry (IR). The thermal conductivities of the samples at room temperature were determined by the Hot Disk thermal constant analysis. Also, the latent heat of the phase change material was analyzed by differential scanning calorimetry (DSC), and the thermal stabilities of the samples were assessed by the simultaneous thermal analysis.

Melamine foam before carbonization has large pore and smooth surface. The size of pore decreases with increasing carbonization temperature. Meanwhile, the elemental of carbon aerogels mainly includes C, N and O. The surface functional group disappears as the carbonization temperature rises. The XRD patterns and IR spectra of the carbon aerogel reveal the presence of numerous defects in conjunction with the carbon material, indicating a high degree of graphitization. Moreover, the graphitization degree shows a gradual increase with increasing carbonization temperature. The specific surface areas of carbon aerogels MF, MF6, MF7 and MF8 measured by BET method are 1.4, 107.9, 485.1 m²·g^-1 and 313.4 m²·g^-1, respectively. The DFT analysis of the pore size distribution shows that carbon aerogel MF has a number of pore sizes larger than 50 nm, and the volume of carbon aerogel MF7 decreases significantly. Also, a large number of micropores and a part of the mesoporous structure appear. The destruction of melamine formaldehyde structure at high temperatures leads to the reduction of the original micropores and the generation of new pores, which is consistent with the SEM results.

The loadings of carbon aerogels MF and MF7 reach 116 and 122 times after 5 000 solid-liquid cycles, and the loss of carbon aerogel MF7 is less, indicating its better cycling stability and high potential for application. The initial eutectic salts lose completely at 75 ℃. The eutectic salts after encapsulation by carbon aerogels has a better thermal stability. All of them lose water completely at 145 ℃. The thermal conductivities of composite phase change materials PCM6, PCM7 and PCM8 prepared are 1.11, 1.24 W·m^-1·K^-1 and 1.37 W·m^-1·K^-1, respectively. The phase transition latent heat value of PCM is the maximum, but after multiple solid-liquid cycles, its enthalpy of melting is decreased by 15.0% and crystallization enthalpy is decreased by 16.3%, leading to a poor cycling stability. Also, the enthalpy of melting of PCM7 is 278.3 J·g^-1 and the enthalpy of crystallization is 227.0 J·g^-1, which are only decreased by 4.5% and 2.6% after solid-liquid cycling, resepctively. PCM7 has a superior cycling stability.

Carbon aerogel of a three-dimensional network structure with high loading, high elasticity and good ductility was prepared via multistage carbonization with melamine foam as a precursor, and carbon aerogel eutectic salt composite phase change materials were prepared.

The carbonization of carbon aerogel MF at different temperatures had an effect on the surface elements, functional groups, morphology, and the degree of graphitization of the pore structure. The carbon aerogel prepared at 700 ℃ had an advantage over the encapsulated eutectic salt phase change materials.

Carbon aerogel MF had a high loading for EHS, but its leakage prevention, thermal conductivity and cycling stability were poor. Carbon aerogel MF7 had a better pore structure and a needle fiber surface, making it maintain 122 times of its own mass adsorption after 5 000 solid-liquid cycles.

The enthalpies of melting and crystallization of PCM7 were 278.3 J·g^-1 and 227.0 J·g^-1, and the enthalpies were decreased by 4.5% and 2.6%, respectively, after 5 000 solid-liquid cycles. PCM7 had a good cycling stability and a great application potential.

Cerium is the most abundant rare-earth element and is widely used in industry as catalyst, magnet, phosphor, polishing powder and ceramic colorant. Cerium occurs naturally in the form of Ce³⁺. Ce⁴⁺ is easily extracted and separated from cerium fluocarbonate. Therefore, Ce³⁺ is often converted to Ce⁴⁺ during the reprocessing stage in mineral extraction and metallurgical engineering. Ce⁴⁺ is likely to accumulate in the environment, food chain and organisms due to its extensive development and application, causing serious damage to the environment and biological systems. It is thus necessary to detect Ce⁴⁺. AA can be also called vitamin C as one of the most important vitamins to maintain the normal activities of the human body. Previous studies indicate that the level of AA in the human body is closely related to many diseases, lack of AA may suffer from the following diseases like mouth ulcer, hair loss, Alzheimer's disease, scurvy and so on. A simple and healthy way to consume AA is to eat more fruits and vegetables that contain AA. AA concentration is an important index to determine fruit freshness and nutritional quality. It is thus of great significance to check the AA of fruits and vegetables correctly and reliably to prevent diseases and evaluate the freshness of vegetables. The development of a simple, reliable and highly selective AA assay is crucial for analytical applications and clinical disease diagnosis, becoming a key topic in current chemistry research.

In the past few decades, a series of methods for the detection of Ce⁴⁺ or AA are reported, and a fluorescence method has attracted recent attention due to its advantages such as simple instrument, low cost and low sample consumption. However, such fluorescence analysis methods still have some technical challenges and inevitable shortcomings. For instance, the detection results with single emission fluorescence method are often interfered by some factors unrelated to the detection environment, such as background signal, fluorescence substance concentration, and surrounding environment. To solve this problem, a fluorescence analysis strategy, namely ratio fluorescence analysis, was proposed. The ratio fluorescence analysis method realized the self-calibration of the analysis signal through the double fluorescence signal, easily obtaining the more accurate detection results rather than the fluorescence method with single emission signal. It provides an effective way for the analysis and detection of target objects.

According to the method described in the literature, 0.25 g thiourea and 0.25 g citric acid were dissolved in 20 mL ultra-pure water, treated with ultrasound for 30 min, then placed at 80 ℃ for 8 h, centrifuged to collect the supernatant, and dialyzed twice with 1 000 Da dialysis bag. The solution obtained was N, S-CQDs solution.

BSA-AuNCs were prepared according to the method reported in the literature. After 50 mg/mL BSA solution and 10 mmol/L HAuCl₄ solution were mixed under stirring for 2 min, 1 mL NaOH (1 mol/L) was added drop by drop and stirred at 100 ℃ for 7 min. After dialysis in ultra-pure water with 1 000 Da bag for 12 h, BSA-AUNCS solution was obtained.

BSA-AuNCs and N, S-CQDs were mixed at a volume ratio of 1∶2 and stored at 4 ℃ for the coming use. 5 mL N, S-CQDs@BSA-AuNCs as a working solution was mixed with Ce⁴⁺ solution at different concentrations. Deionized water was added into the mixture for a fixed volume to 10 mL, and stabilized for a period of time. Afterwards, the fluorescence test was performed at the excitation wavelength of 345 nm.

5 mL N, S-CQDs@BSA-AuNCs solution was firstly mixed with 100 μL 1 mmol/L Ce⁴⁺, and then mixed with AA solution at different concentrations at a constant volume of 10 mL under stirring for 2 min. The fluorescence test was performed at the excitation wavelength of 345 nm after stability for a period of time.

In this work, nitrogen and sulfur doped carbon quantum dots (N, S-CQDs) and gold nanoclusters (BSA-AuNCs) are combined to form a N, S-CQDs@BSA-AuNCs fluorescence sensor for the continuous determination of Ce⁴⁺ ion and AA. Bsa-auncs were synthesized with bovine serum protein (BSA) as a stabilizer. The BSA-AuNCs have superior fluorescence properties, good biocompatibility, high quantum yield, excellent colloidal stability and ultra-small size. It is indicated that BSA-AuNCs selectively recognizes Ce⁴⁺ ions based on a fluorescence enhancement mechanism, and an "open" NIR fluorescence sensor platform for detecting Ce⁴⁺ is established. The Ce⁴⁺ mediated BSA-AuNCs "off" NIR fluorescence sensing strategy is developed to detect AA via adding AA to the NIR fluorescence sensor platform of N, S-CQDs@BSA-AuNCs system.

The N, S-CQDs@BSA-AuNCs ratio fluorescence sensor was constructed by N, S-CQDs and BSA-AuNCs for the continuous determination of Ce⁴⁺ and AA. When Ce⁴⁺ was added to the system, BSA-AuNCs were oxidized by Ce⁴⁺ and aggregated into coarser particles with an enhanced fluorescence. Therefore, an "open" NIR fluorescence sensing platform for Ce⁴⁺ detection was constructed. The fluorescence intensity ratio of I_F429/I_F692 showed a linear detection relationship with the concentration of Ce⁴⁺, and the detection limit was 0.12 µmol/L. Subsequently, a Ce⁴⁺ mediated NIR fluorescence sensing strategy based on N, S-CQDs@BSA-AuNCs "off" was developed to detect AA, with a detection limit of 0.19 µmol/L via adding the NIR fluorescence intensity of AA quenching N and S-CQDs@BSA-AuNCs/Ce⁴⁺ system. The ratio fluorescence sensor was applied to the detection of Ce⁴⁺ concentration in actual tap water samples, AA concentration in commercial peach juice and vitamin C tablets.