The scientificity of “recycled aggregate concrete” or “recycled concrete” is discussed. It is suggested that “recycled aggregate concrete” must be used.

The development of cohesionless cracks can be regarded as a sign of macroscopic crack development. An identification model of cohesionless cracks was adpoted with results of crack development by digital image correlation (DIC) method. Three-point bending tests were carried out on fully-graded concrete specimens with different loading rates (0.000 1 mm/s, 0.001 0 mm/s, 0.010 0 mm/s), considering the rate correlation of crack propagation of fully-graded concrete. The results show that for the fully-graded concrete specimens with different loading time intervals, cohesionless cracks will occur throughout the peak load, indicating that the prefabricated cracks have cracked when the peak load is reached. The cracks of the specimen is mainly concentrated after the load is loaded to 50% of the peak load. Under the same conditions, the slower the loading rate is, the earlier the unbonded cracks appear. With the increase of the loading rate, the cracks are thin and long.

Accurate identification and quantification of pores and microcracks in lightweight aggregate concrete (LAC) are important for analyzing salt frost deterioration mechanisms. Therefore, a new DIP method based on morphology principle was proposed, which overcomes the difficulties of traditional methods that can not accurately segment pores and microcracks because of only relying on shape factors, and the porous lightweight aggregate interferes with the analysis of pore and microcrack in concrete matrix. Using this method, the microstructure deterioration of shale ceramsite LAC and normal aggregate concrete (NAC) with same mortar to aggregate ratio and same water to cement ratio was compared and analyzed. With the increase of freeze-thaw cycles, the pore structure damage of the two concretes is manifested as pore expansion and microcrack initiation from the pore wall, leading to the relative dynamic modulus of elasticity (rn) loss. Grey correlation analysis indicates that in addition to the initiation and propagation of microcracks, the high dispersion of LAC pore diameter also contributed to the loss of rn in LAC. Test shows the new DIP method realizes the segmentation and independent analysis of pore and microcrack, and achieves the recognition accuracy to 10.6 μm, which has advantages in quantitative analysis of concrete pore structure and microcracks.

As a promising sustainable building material, alkali-activated concrete (AAC) is subjected to complex stresses. However, current investigations on the triaxial compression performance of AAC have restricted confinement ratios and do not account for specimen strength effects. In this work, conventional triaxial compression tests were conducted for AAC to investigate the damage mode, mechanical properties, constitutive model and failure criterion under confining pressure. The confinement ratio range is 0-1.4. The results show that the damage mode of AAC transforms from brittle to pseudo-ductile as the confining pressure increases. The triaxial compression strength increases significantly with AAC strength, attaining the deformation capacity with lower strength. A triaxial compression constitutive model available for Portland cement concrete (PCC) can substantially overestimate the axial strain of AAC at peak stress. Based on the experimental results, the triaxial compression constitutive model and failure criterion for AAC were proposed.

Ultra-high performance concrete (UHPC) was prepared via mixing water containing different chloride salts (i.e., NaCl, CaCl2 and MgCl2). The electrochemical indexes of the internal reinforcement during passivation period were measured by linear polarization, electrochemical impedance spectroscopy and cyclic polarization. The passivation behavior and evolution of steel bars in UHPC under the coupling action of different chloride salts were investigated. The results show that the reinforcement in UHPC is able to be passivated under three different chloride salt systems. The resistances of UHPC matrix, steel passivation film and steel charge transfer increase continuously with the hydration age. At the same concentration of Cl- ions, the introduction of Ca salts increases the possibility of steel corrosion to a certain extent, while the introduction of Mg salts delays the corrosion process of steel bars to some extent. The pitting corrosion sensitivity of reinforcement increases with the increment of Cl- ions concentration. At a high Cl- ions concentration, the pitting sensitivity of steel bars in Na-Ca system is greater than that in Na-Mg system.

Textile reinforced ultra-high performance concrete (TR-UHPC) utilizes the synergistic reinforcing mechanism of short steel fibers and textile to further improve the mechanical properties and strain hardening effect of ultra-high performance concrete (UHPC). To investigate the flexural properties of TR-UHPC and synergistic effects of textile and steel fibers, four-point bending tests were conducted. A design method for TR-UHPC flexural members based on a tension-stiffening model and a bending model was proposed. The effect of tension-stiffening on the flexural responses of TR-UHPC was analyzed. The results show that the flexural capacity of TR-UHPC specimens increases with the increase of steel fiber dosage. The maximum flexural strength can be obtained at a steel fiber volume fraction of 1.0% or 1.5%, indicating that the content of steel fiber can be effectively reduced. The prediction data by the proposed model are in reasonable agreement with the experimental results. The proposed model can be used to predict the load capacity of TR-UHPC flexural member with given materials, or guide the material design according to the demands of structural performance.

As a typical porous media material, the diffusivities of concrete are closely related to its pore structure. It is necessary to propose a model for predicting the diffusivities based on the time-varying pore structure since the pore structure can continuously change with the hydration process. In this work, the parameters of the pore structure, the moisture distribution in the unsaturated state, and the prediction and validation of the relative diffusion coefficients of cement paste at different saturation levels were analyzed based on the reconstructed cement paste microstructure. It is indicated that the predicted relative diffusion coefficients are related to the extracted pore parameters (i.e., hydration degree α, porosity ρ and dimensionless peak pore size B*), and the relationships between α and time t, ρ and α, and B* and ρ were systematically analyzed, and a diffusivity prediction model containing the time-varying parameters f (α(t), ρ(t), B*(t)) was proposed. The proposed model can fully consider the time-varying process of concrete pore structure affected at different water-cement ratios and hydration time, thus providing a microscopic perspective approach for analyzing the diffusivities of unsaturated concrete from the perspective of time-varying pore structure.

The conventional extension degree cannot fully reflect the construction performance of ultra-high performance concrete (UHPC), and it should be supplemented by the rheological properties. However, the proprietary equipment for the measurement of rheological properties is expensive, which is inconvenient to be directly used on the construction site. Therefore, 18 kinds of mix proportions of UHPC were prepared via changing the water-to-binder ratio and superplasticizer dosage. In this work, the flow and rheological parameters of various mix proportions of UHPC were obtained via testing the extension degree, characteristic time of extension degree (i.e., the time required for extension diameter of 0.7 and 0.8 times), rheological behavior, and mechanical properties after curing of the corresponding mix proportions. Finally, a relationship between fluidity and rheology of fresh UHPC (without steel fibers), which the mechanical properties belong to UHPC (with steel fibers), was established. The results show that the Bingham model can be used to describe the rheological behavior of fresh UHPC. The proposed characteristic time of extension degree index has a relatively high test accuracy. The rheological parameters of fresh UHPC can be obtained from the flowability-rheology relationship established. It is indicated that the calculated data by the proposed method agree well with the experimental results.

Using a new partial calcination strategy, which partially decomposes hydromagnesite into magnesium oxide at a lower temperature for a short time. It also explores the potential of preparing cementitious materials by mixing them with calcined clay. The research showed that in addition to MgO, partially calcined hydromagnesite also contained undissolved amorphous magnesium carbonate, which could enhance the rate of MgO hydrated into Mg(OH)2 and significantly improved the early mechanical properties of the cementitious material, but the development rate of strength in the later stage decreased. Compared with completely calcined hydromagnesite, while the content of amorphous magnesium carbonate accounts for 3.0% of the total mass, the 1 day strength was increased by 167%. Partial calcination strategies can effectively reduce carbon emissions and improve the early strength of cementitious materials.

To study the promotion process of cement hydration by polycarboxylate superplasticizers modified calcium silicate hydrate (C-S-H/PCE) and understand the mechanism of C-S-H/PCE seeds promotion of cement hydration. Acrylic acid-based polycarboxylate superplasticizer (PCE-1) and silane-based polycarboxylate superplasticizer (PCE-2) were used to surface modify the hydrothermal synthesized calcium silicate hydrate (C-S-H/Unmodified) seeds, resulting in the polycarboxylate superplasticizer modified C-S-H seeds (C-S-H/PCE-1 and C-S-H/PCE-2). Subsequently, these three C-S-H seeds were added to the cement paste. The compressive strength, phase composition, and heat release curve of cement paste and the surface tension of C-S-H seeds were tested. The results showed that the surface modification of PCE-1 and PCE-2 resulted in an increase in the surface contact angle of C-S-H seeds from 36.9° to 41.1° and 42.9°, resulting in lattice mismatch of the sedimentary phase (hydration product) on the surface of the substrate material (C-S-H seeds) and inhibiting the epitaxial growth of cement hydration products on the surface of C-S-H seeds. PCE modification reduced the promoting effect of C-S-H seeds on the hydration process of Portland cement. Moreover, the degree of reduction in the hydration promoting effect of C-S-H/PCE-2 on Portland cement is significantly greater than that of C-S-H/PCE-1 crystal nucleus. Tests of the adsorption equilibrium of PCE in the pore solution indicated that the C-S-H/PCE seeds did not directly introduce new crystal nucleation sites but exposed new C-S-H surfaces through the desorption of PCE on the surface of C-S-H/PCE seeds. The desorption degree of PCE on the surface of C-S-H/PCE seeds determined the promoting effect of C-S-H/PCE seeds on cement hydration.

The leaching behavior of R7T7 borosilicate glasses with different waste loading amounts was evaluated via hanging the static immersion method at different temperatures and two leaching systems (i.e., deionized water and simulated North Mountain groundwater) for 365 days. The changes of etched layer phases, morphology, and composition with leaching time were also analyzed by X-ray diffractometry, scanning electron microscopy-X-ray energy spectrometry, and inductively coupled plasma mass spectrometry. The results show that the glass starts to appear in heterogeneous phases when the loading amount of waste exceeds 20%, and its composition is mainly molybdenum-rich materials and noble metals. The results of leaching experiments reveal a positive correlation between the mass loss rate of the glass and the leaching temperature. In the initial leaching phase, the three elements Na, Si, and B were leached at a higher rate in deionized water rather than in simulated Beishan groundwater, while after equilibration the three elements were leached at comparable rates in both leaching agents. The surface of the glass has a variety of crystalline phases generated after 365 days of leaching, mainly in spherical, floral, and prismatic and honeycomb morphologies, consisting mainly of components such as Mg-Si-O and Si-Nd-Gd-O. Based on the analysis above, the leaching process of glass solid is controlled due to the ion diffusion mechanism and glass network dissolution mechanism. The results of these studies can provide the data to support the future safety evaluation of high-level radioactive glass waste in geological disposal processes.

In order to obtain a polycrystalline yttrium iron garnet ferrite (YIG) joint with both structural reliability and electromagnetic function matching, a new Bi2O3-B2O3-NiO-Fe2O3 glass is designed to fabricate YIG ferrite. The wettability of the glass on the surface of YIG ferrite, typical organization, the impacts of brazing temperature to the microstructure of the joint and mechanical properties is investigated. The result shows that the glass have a well wettability on YIG joint. Some Al was incorporated into the glass solution during melting process and replaced Fe3+ in NiFe2O4 and YFe3B4O12. The growth of NiFe2O4 and YFe3B4O12 is promoted due to the smaller radius of Al3+ and smaller lattice distortion. Small NiFe2O4 particles in the seam transform to large blocky organization as the temperature goes up, proving that high temperature has a positive effect on NiFe2O4 aggregation. The glass-YIG joint has a high shear strength and reaches its maximum of 71 MPa at 750 ℃, then the shear strength decreases with increasing temperature, dropping to 47 MPa at 800 ℃.

The feed-to-glass conversion within the cold cap and the mathematical modeling of the process are of great concern for the accurate simulation of the melter for vitrification of high-level liquid waste (HLLW). To figure out the feed-to-glass conversion during the HLLW vitrification, the phase transitions and structural features within the cold cap were investigated. The results show that the cold cap consists of the upper reaction layer (at 100-700 ℃) and the lower foam layer (at 700-1 000 ℃). The dehydration and nitrate decomposition occur in the reaction layer, while the continuous melt as well as the formation and dissolution of intermediate phases appear in the foam layer. The reaction layer can be simplified into a one-dimensional mass transfer model based on the mass conservation equation and experimental results. Subsequently, the kinetic equation of feed-to-glass conversion is proposed to quantitatively describe the three reaction rates of dehydration (at 100-400 ℃), and melting (at 200-300 ℃) and decomposition (at 400-700 ℃) of nitrates as functions of temperature in the reaction layer, providing some crucial parameters for melter simulation.

Silicic acid was used as a modifier to magnesium oxysulfate (MOS) cement to improve the water resistance of MOS cement. The effect of silicic acid on the mechanical properties, volume stability, and water resistance was investigated. The phase composition, microstructure, and ion leaching concentration were investigated. The co-existence of magnesium silicate hydrate (M-S-H) gel with MOS system was also analyzed. The results show that the addition of silicic acid enhances the mechanical properties and improves the volume stability of MOS cement through promoting the formation of 5Mg(OH)2·MgSO4·7H2O crystals and M-S-H gel. When MOS cement containing silicic acid immerses in water, the residual silicic acid continues to react with free Mg2+ and OH- to form M-S-H gel. Meanwhile, 5Mg(OH)2·MgSO4·7H2O crystals and M-S-H gel coexist in MOS cement, resulting in an improvement in the water resistance of MOS cement.

Transition metal sulfide is an active material with high specific capacitance. MnS/Ni3S2 electrode material was hydrothermal prepared on nickel foam by a simple and time-saving method. The effects of hydrothermal reaction time and temperature on the morphology and electrochemical performances of the sample were investigated. The results show that MnS/Ni3S2 electrode material has high specific capacitance and excellent magnification performance, and the retention rate of specific capacitance is 90% after 2 500 cycles. A hybrid supercapacitor device is assembled with MnS/Ni3S2 as the positive electrode and activated carbon (AC) as the negative electrode. MnS/Ni3S2//AC can provide 30.2 Wh·kg-1 energy density when the power density is 700 W·kg-1. The specific capacitance retention is 116.0% after 3 000 cycles. It shows that MnS/Ni3S2 is expected to provide potential application value in supercapacitor.

Nano-catalytic materials have broad application prospects in solving bacterial resistance caused by overuse of antibiotics. However, their catalytic efficiency usually depends on a lower pH value range (i.e., pH=2.0-5.0) rather than on a weakly acidic bacterial microenvironment (i.e., pH=6.0-7.0), leading to an limited catalytic efficiency and an insignificant bactericidal effect. In this paper, MnSiO3 samples were prepared by a simple co-precipitation method. The morphology, structure and catalytic properties of the samples were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and ultraviolet-visible spectroscopy (UV-Vis). The antibacterial performance against E. coli and S. aureus were investigated. The results show that the morphology of amorphous MnSiO3 samples transforms from nano-sized particles to micron-sized blobs in buffer solutions at pH 6.4 and 7.0. MnSiO3 nano-sized particles had an intense catalytic ability at a higher pH value and an excellent antibacterial performance against E. coli and S. aureus. Therefore, the in-situ structure transformation of MnSiO3 presents a promising treatment strategy for bacterial infection.

Compared to the properties of ordinary concrete, the recycled coarse aggregate concrete exhibits the lower strength and poor durability due to many pores and micro-cracks inside the recycled coarse aggregates and the old mortar attached to the aggregate surface. However, the silica nanoparticles with a high pozzolanic activity, a crystalline nucleation effect and a filling effect plays an important role in the application of modified recycled coarse aggregate concrete. Recent work on the properties of recycled coarse aggregate concrete modified by silica nanoparticles was analyzed in terms of workability, mechanical properties, durability and interfacial structure, etc. The modification mechanism of silica nanoparticles to improve the performance of recycled coarse aggregate concrete was further discussed, thus providing some theoretical and technical supports for the application of silica nanoparticles in the performance improvement of recycled coarse aggregate concrete.

The mechanical properties of concrete under the effect of long-term radiation are significantly degraded, affecting the long-term operation of existing nuclear power plants. The state-of-the-art on multi-scale evolution of the volume and mechanical properties of concrete under radiation was reviewed and analyzed. It was indicated that the evolution mechanism of volume and mechanical properties of minerals, aggregates, hardened cement paste, and concrete have been understood, and the multi-scale numerical model to predict the performance degradation of irradiated concrete has been developed. However, it was suggested that the multi-scale model of irradiated concrete based on mineral composition should be developed for reliable prediction of the volume and mechanical property, considering the randomly distributed aggregates at the mesoscale level and the differences in irradiation effects of the components.

The durability evaluation of concrete materials based on the related experiments has a low economic efficiency ratio. The prediction accuracy of the conventional empirical formula for durability is restricted and the proportions of concrete mix cannot be calculated according to the performance. It is thus necessary to develop a novel and efficient material quality control and performance prediction tool. In this review, the process of building machine learning models was stated. The basic working flow and advantages of common algorithms, as well as the durability index prediction algorithms based on machine learning were summarized. Its application effects and development direction were discussed. This review can provide a basis for the in-depth development and application of machine learning technology in the field of concrete.

Dielectric behavior pertains to the polarization of cement-based materials, in which the positive and negative charge centers are separated. Recent work on the dielectric behavior of common Portland cement-based materials in a low frequency regime were carried out, including the dependence of dielectric behavior on microstructures, hydration process, loadings, etc. This review represented the results of dielectric behavior of common Portland cement-based materials over the past decade and introduced the dielectric measurement method and the dependence of polarization on aggregates, water cement ratio, stress/strain, and temperature. It is indicated that some movable ions in pore solution dominate the dielectric behavior, in which the electric dipoles are in series. Aggregates and admixtures affect dielectric behavior due to relevant changes in microstructures. Heating and compression strengthen the dielectric behavior, while cooling and tension weaken the dielectric behavior. In addition, some related future research aspects were also prospected.

The incorporation of tuff powder into cement-based materials as mineral admixtures can reduce cement production and greenhouse gas emissions such as CO2, as well as utilize industrial solid waste. This review represented the mechanism of tuff powder in cement-based materials, and summarized the influence of tuff powder incorporation on the setting time, workability, mechanical properties and durability of cement-based materials. Some aspects for the preparation and use of tuff powder were given, and some problems for the effect of tuff powder on the properties of cement-based materials were proposed. In addition, some aspects for future studies on the properties of cement-based materials incorporating tuff powder were also discussed.

Solvent exchange method is widely used in the drying and hydration termination of cement-based materials. This review represented the influence of solvent exchange on the microstructure of cement paste. The selection principle of exchange parameters (i.e., sample size, liquid/solid ratio, exchange time and removal method) was described. The physical and chemical reactions between common exchange solvents (i.e., isopropanol, acetone, methanol, and ethanol) and cement hydration products (i.e., calcium hydroxide, calcium silicate hydrate and ettringite) were discussed. The pore structure parameters (i.e., porosity and pore size distribution) of cement-based materials were analyzed under the influence of exchange parameters and solvents. In addition, the existing problems and research prospects of the solvent exchange method were also given.

For the regular preparation techniques of ceramics, high-temperature sintering has always been a necessary condition to obtain dense microstructures and good properties. As a recently emerged technique, Cold Sintering Process (CSP) can achieve rapid densification for various ceramic materials at ultra-low temperatures (below 350 ℃) through dissolution-precipitation and other mechanisms. CSP has shown tremendous development space and high research potential by effectively solving the problems existing in conventional high-temperature sintering in terms of energy consumption, microstructural control and co-firing with organics. This review starts from the brief summary on the development history, technologic process and densification mechanisms of CSP. Then the application status of CSP on the preparation of ceramics (including bio-ceramic materials, new energy materials, semiconductor materials, dielectric materials, thermoelectric materials, unstable materials at high temperatures) is described, and its future development is prospected.