Latest ArticlesIn order to prevent and control the corrosion damage of concrete structure caused by the stray current generated of metro operation, a high-impedance and ultra-high performance concrete (HI-UHPC) with 28-day resistivity of greater than 3 kΩ/m and compressive strength of greater than 120 MPa was prepared for subway engineering. The parameter influences on the alternating current (AC) impedance performance of HI-UHPC were investigated by AC impedance spectroscopy. An equivalent circuit model suitable for HI-UHPC was proposed. The results show that the compressive strength of HI-UHPC can be improved by adding steel fiber and PVA emulsion powder. The high-frequency arc of ordinary cement-based materials appears at 10-h hydration, while that of HI-UHPC appears at 1-h hydration. The addition of steel fiber can form the conductive channels via overlapping steel fiber in HI-UHPC, increase a possibility of free electron transfer and reduce the volume resistance of the matrix. When PVA emulsion powder is added into HI-UHPC, the interface between the matrix and PVA emulsion powder film is formed, changing the pore structure, reducing the effective conductive path in the interior of HI-UHPC, and ultimately improving the impedance performance. The equivalent circuit model of HI-UHPC consists of connected conductive path, disconnected conductive path and insulated path as well. As the ages increase, the hydration products and polymer films gradually form, the free water in the matrix consumes, and the pore structure becomes dense, leading to the blocking of the carrier transport channels in the pores. The resistance of connected conductive channel and the resistance of unconnected conductive channel increase, while the capacitance of unconnected conductive channel and the capacitance of insulating conductive matrix decrease.
With the promotion of the national blue ocean strategy, concrete is widely used in various marine engineering facilities (i.e., cross-sea bridges, sub-sea tunnels, harbour terminals, etc.). However, concrete suffers from physical, chemical and biological synergistic effects during its service, eventually leading to its deterioration. The mechanism of concrete deterioration caused by physical and chemical effects is well known. In the marine environment, however, the mechanism of concrete deterioration caused by organisms, especially macro organisms (i.e., plant type and animal type) is still unclear. Therefore, this review represented recent work on the corrosion mechanism of macroscopic organisms in marine concrete, some factors affecting the attachment behavior of macroscopic organisms, and the related protective characterizations as well for more reasonable, efficient, economic and environmentally friendly corrosion management of marine concrete used.
Reinforced concrete structure in a chloride and sulfate coupling environment is vulnerable to be deteriorated due to the combined chloride-sulfate attack. Based on the attack mechanism of chloride and sulfate, this review analyzed the diffusion-binding process of chloride and sulfate ions before and after damage of concrete caused by sulfate attack. Based on the classical diffusion-reaction model and the chemo-damage-transport theoretical framework, the effects of ion activity, temperature, microstructure change and calcium leaching on the chemo-damage-transport system were represented. The coupled transport thermodynamic model of chloride ions and sulfate ions was introduced from the perspective of thermodynamics. In addition, some existing problems in the current research were also prospected.
The use of natural plant fibers to reinforce cement-based materials has attracted recent attention. Compared with synthetic fibers, natural plant fibers have many advantages, such as green environmental protection, light weight, low cost, renewable, etc., and are widely used in many fields. However, natural plant fibers cannot be directly applied to cement-based materials since they are easy to absorb water and swell and have a poor compatibility with cement matrix. Therefore, a proper pretreatment of natural plant fibers is needed to improve the adverse effects of natural plant fibers on the properties of cement-based composites. This review represented , recent work on the development of different pretreatment methods for natural plant fibers (i.e., water treatment, coating treatment, steam treatment, alkali treatment, acid treatment, silane coupling agent treatment and mixing treatment) based on the structure and material composition of natural plant fibers, as well as the effect of the pretreatment method on the properties of natural plant fiber reinforced cement-based composites.
Cement hydration is a complex chemical and physical reaction process, which directly determines the microstructure and macro-property of hardened cement paste. Since conventional experimental methods are in general time-consuming, laborious and expensive, computer simulation thus becomes popular in understanding the mechanism of cement hydration and microstructure development in recent years. In this review, existing hydration models were introduced, and their advantages and drawbacks were described. Recent work on the cement hydration simulation were summarized, in which the emphasis is on the hydration of non-spherical cement particles and nano-scale simulation of calcium silicate hydrate. Based on the practical application of relevant models, the effect of mineral admixtures on the hydration process and microstructure of cement paste was discussed. The properties of cement-based materials (i.e., agent transport, chloride diffusion and microcrack propagation) were represented as well.
Concrete materials in service are often affected by the coupling action of mechanical and environmental factors. Microcracks inside concrete occur due to the complex stress induced by the combined action of moisture, heat, chemistry and mechanics, promoting the ingress process of water and aggressive ions and further accelerating the durability degradation process of reinforced concrete structures. In order to delay the deterioration of mechanical properties and durability of concrete under severe environments, the self-healing mechanisms of crack within concrete crack through the microbial mineralization technology were investigated. Based on the mechanism of self-healing concrete and regarded the realization mode of microbial healing and its influence on durability of concrete, recent studies on the microbial self-healing of cement-based materials under mineralization were summarized. Finally, the application of different types of microbial-induced calcium carbonate precipitation technology in cement-based materials was represented.
VS4 has an one-dimensional chain structure with the chain spacing of 0.583 nm, which is greater than Na+ ion radius of 0.102 nm. Moreover, its theoretical sodium storage capacity is 1 196 mA·h/g, making it one of the most promising anode materials for sodium storage. However, the volume of the electrode material expands and contracts dramatically during repeated cycling, resulting in collapse of the structure, poor cycling stability and capacity decay, and severely affecting the practical application of VS4. In this paper, VS4 microspheres (approximately 1 μm in diameter) with a high stability were constructed for the unique three-dimensional microsphere structure to alleviate the volume expansion, improve the cycling stability and the electrochemical reaction kinetics. This microsphere structured material as an anode material for sodium ion batteries exhibits a superior rate performance (i.e., 372 mA·h/g@2.0 A/g and 297 mA·h/g@5.0 A/g) and a long-cycle life (i.e., stable cycling for 100 times at 5.0 A/g).
To solve the problems of low electrical conductivity and poor stability of V2O3 as an anode electrode of secondary batteries, sulfur (S) doped V2O3/C nanowires were prepared via electrostatic spinning and sulfuration treatment technology. The structure, chemical composition, and energy storage properties were characterized. The results show that the S doped V2O3/C nanowire as an anode electrode for lithium ion batteries has a discharge specific capacity of 538 mA·h/g at a current density of 0.1 A/g after 50 cycles, and the discharge specific capacity at 2 A/g after 600 cycles can still maintain 288 mA·h/g. The specific capacity of sodium ion batteries with S-doped V2O3/C nanofiber as an anode electrode is 233 mA·h/g at 0.1 A/g after 100 cycles.
Different amounts of LiCoO2 from waste lithium-ion batteries were dissolved in nitrite acid and the recovered Li and Co were used to modify Na0.67Fe0.5Mn0.5O2 cathode by a sol-gel method. Based on the results by X-ray diffraction, Li+/Co3+ ions were doped into Na0.67Fe0.5Mn0.5O2, with the smaller lattice parameters. The results by X-ray photoelectron spectroscopy show that the doping of Li/Co ions leads to the conversion of Mn3+ ions to Mn4+ ions, resulting in the less Jahn teller effect. The results by electrochemical tests indicate that the cycle stability and rate performance of the modified Na0.67Fe0.5Mn0.5O2 improve due to LiCoO2 modification. The sample with x of 0.105 (i.e., Na0.53Li0.105Co0.105(Fe0.5Mn0.5)0.79O2) possesses a capacity retention of 67% after 150 cycles at a current density of 1 C, and a specific capacity is 30 mA·h/g at 5 C.
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