Based on environmental characteristics of reinforced concrete infrastructure in western China, cement mortar specimens were subjected to semi-immersed sulfate corrosion in low humidity and varying temperature. The influences of low humidity and varying temperature on semi-immersion sulfate attack cement-based materials were studied by combination of XRD, SEM, LF-NMR, and conductivity titration of sulfate ions. The results show that under low humidity and varying temperature, the mortar subjected to semi-immersion sulfate attack has a water film zone within 30 mm above the surface of sulfate solution, and the corrosion degree is relatively low. The evaporation zone above the solution surface of 30~60 mm has a higher degree of corrosion and obvious peeling phenomenon. Corrosion of evaporation zone of mortar semi-immersed in Na2SO4 solution is more seriously than that semi-immersed in MgSO4 solution due to their difference in saturation under different temperatures. The expansive salt crystallization within pores above 1 000 nm is prone to the occurrence in evaporation zone, which leads to the micro cracks inside mortar and an increase in the proportion of harmful pores, resulting in physical expansion damage being the main cause of this zone. Chemical corrosion in the immersion zone of mortar is dominant due to the formation of swelling corrosion products such as gypsum and ettringite. As corrosion products accumulated in the pores with pore sizes of 50~1 000 nm, the surface of mortar cracks and the proportion of macroscopic pores with pore sizes above 1 000 nm increases. The addition of ground granulated blast furnace slag in cement increases the sulfate corrosion resistance of the immersion zone of mortar via its secondary hydration effect, and the addition of limestone powder decreases the sulfate corrosion resistance of mortar due to its dilution effect.

In order to address the issue of reduced compressive strength in plastic mortar caused by the hydrophobic nature of plastic, this study utilizes the thermoplasticity of discarded polypropylene (PP) plastic to prepare silica fume@plastic particle composite particles (SF@PP) by welding silica fume on the surface of PP through the thermal welding process and tests the hydrophilicity and volcanic ash activity of the composite particles. The results indicate that the modified plastic through silica fume heat welding improves the hydrophilicity of PP and enhances its volcanic ash activity, promoting the deposition of hydration products at the interface and increasing the compactness of the interface. Consequently, the porosity of the plastic mortar is significantly reduced, and the compressive strength is significantly increased by 26.07% to 31.04%. The results of the study are conducive to the efficient utilization of waste plastics and provide a basis for the preparation of high-performance plastic mortar.

Reactive MgO cement (RMC), which undergoes hardening through the reaction of MgO with water and CO2, is a novel cementitious material that has gained significant attention in recent years. The hydration and carbonation processes of RMC samples are significantly affected by the curing method and water content. This work compared the effects of standard curing ((20±1) ℃, 95% relative humidity), water curing ((20±1) ℃), and carbonation curing ((20±3) ℃, 70% relative humidity, 20% (volume fraction) CO2) on the properties of RMC with water-cement ratio (w/c) of 0.5, 0.6, and 0.7, respectively. The types and content of the phases of the hydration and carbonation reaction products as well as their micro-morphological characteristics were investigated. The results show that the residual content of MgO in samples is 10%~15% (mass fraction) at 3 d and disappears at 14 d under standard curing and water curing. When w/c is 0.6, the compressive strength of paste reaches 7.7 and 3.2 MPa at 14 d under standard curing and water curing, respectively. The strength of paste under carbonation curing is superior to that under standard curing and water curing (3 d compressive strength, 16.19, 0.42, and 0.43 MPa, 14 d compressive strength， 22.34, 7.46 and 7.23 MPa). The quantitative analysis results show that the carbonation product is mainly MgCO3·3H2O. When w/c is 0.6, the content of MgCO3·3H2O is 12.12% and 11.29% higher than that of samples with w/c=0.5 and 0.7. The main reason for the optimal compressive strength could lies in that these needle-rod MgCO3·3H2O are bonded to each other form a dense microstructure.

The study of interfacial transition zone characteristics in manufactured sand mortar is of great significance in enhancing the strength and durability of manufactured sand mortar. The interface microstructure, microzone mechanical strength, and non-uniform deformation of mortar composed of river sand and manufactured sand were examined through scanning electron microscopy (SEM), nanoindentation, and three-dimensional digital image correlation (3D-DIC) techniques. The results indicate that the width of the interfacial transition zone in manufactured sand mortar exceeds that in river sand mortar. Nevertheless, the interface cracks between fine aggregate particles and mortar matrix are smaller in manufactured sand mortar. Notably, microcracks are prone to occurring at the edges of manufactured sand particles. The elastic modulus of the interfacial transition zone in manufactured sand is slightly higher than that in river sand, suggesting that manufactured sand can mitigate the edge effect of aggregates to a certain extent. The non-uniformity of shrinkage deformation is most conspicuous in sandstone manufactured sand mortar, displaying larger internal tensile strains, ultimately leading to a more severe distribution of damage. This implies that an excessive proportion of blade-shaped particles in fine aggregates may heighten the risk of cracking in mortar.

Re-dispersible latex powder is the key material for the preparation of steel structure interface mortar. In this paper, the influence of the content of re-dispersible latex powder on the 28 d compressive strength, flexural strength and tensile bond strength of interfacial mortar was researched. At the same time, combined with scanning electron microscopy, X-ray spectroscopy and other microscopic analysis methods, the bonding mechanism of re-dispersible latex powder to enhance the interfacial mortar was analyzed. The results show that the suitable content of re-dispersible latex powder is 15% (mass fraction， same below) in the preparation of steel structure interface mortar. Under the appropriate dosage, the tensile bond strength of the interface mortar reaches 1.12 MPa, and the compressive-flexural strength ratio is 2.3. The addition of latex powder can effectively improve the flexibility of the mortar. The addition of latex powder changes the pore structure of the mortar, and the porosity becomes larger. The hydration products in interfacial transition zone are mainly polymer film encapsulated hydration products C-S-H gel and Ca(OH)2.

In this paper, the aluminum sulfate based alkali-free liquid accelerating agent was modified by fluorine silicon slag, and the performance index of alkali-free liquid accelerating agent modified by fluorine silicon slag was tested. XRD, TG-DTG and SEM were used to explore the action mechanism of aluminum sulfate base alkali-free liquid accelerating agent modified by fluorine silicon slag on cement hydration process. The results show that with the increase of fluorine silicon slag content, the rapid setting effect and strength of mortar increase first and then decrease. When the alkali-free liquid accelerating agent is modified by fluorine silicon slag, the appropriate content of fluorine silicon slag is 5.0% (mass fraction). At this time, the initial setting time is 1 min 20 s, the final setting time is 5 min 20 s, 1 d compressive strength is 13.8 MPa, and 28 d compressive strength is 52.4 MPa. Calcium silicate hydrate is formed by the reaction of silica in fluorine silicon slag and cement hydration product calcium hydroxide, which is beneficial to improve the early strength of cement. However, the process of cement hydration and hardening process is hindered by the reaction of fluorine and calcium hydroxide to produce a large amount of calcium fluoride precipitation.

Cracking and repair of concrete have always been the focus of attention in engineering field. Superabsorbent polymers (SAP) have a certain crack self-healing function. In this paper, the liquid absorption properties of SAP under different environmental solutions (NaOH, NaCl and distilled water) were tested. The compressive strength recovery rate of cracked specimens, the self-healing rate of different crack widths (0.1， 0.2 mm ) and the crack filling state were used as repair indicators. The repair effect of SAP with different mass fractions (0%, 0.5%, 1.0%) on cracked mortar specimens under different environmental solutions was analyzed. The results show that the liquid absorption rate of SAP in different environmental solutions increases rapidly and then tends to be stable, and the liquid absorption rate of SAP in NaOH environmental solution is the highest. The incorporation of SAP improves the compressive strength recovery rate of cracked mortar specimens under different environmental solutions to a certain extent. Among them, the compressive strength recovery rate of specimens mixed with 0.5% SAP in NaOH environmental solution for 28 d is the highest, which is 17% higher than that of reference group. The incorporation of SAP improves the self-healing properties of mortar specimens with different crack widths in different environmental solutions. With the increase of SAP content, the self-healing properties of mortar specimens with 02 mm crack width repaired in NaOH environmental solution for 28 d is significantly higher than that of distilled water and NaCl solution. The self-healing rate of specimens with 1.0% SAP in NaOH environmental solution is 90%. The incorporation of SAP cooperates with hydration products to seal the crack mouth, so it is beneficial to improve the durability of cement mortar.

There is less research on the mechanical behavior of cement-based materials containing superabsorbent polymers (SAP) under high strain rate. In this paper, the total porosity and macroscopic pore (>100 μm) porosity of cement-based materials containing SAP were investigated by the use of mercury intrusion and graphical techniques. The effects of SAP particle size and extra water on the quasi-static (10-5 s-1) and dynamic (60~110 s-1) mechanical properties of the modified materials were analysed. Based on the study of the crack propagation behavior, the damage behavior of the cement-based materials was analyzed under high strain rates. The results show that the addition of extra water and the increase of SAP particle size improve the total porosity and macroscopic pore porosity of the materials, which reduce the compressive strength and elastic modulus of the specimens under quasi-static and dynamic compression. The addition of extra water and the increase of SAP particle size also enlarge the compressive strength dynamic increase factor and elastic modulus dynamic increase factor of the cement-based materials, which is due to the fact that an increase in total porosity and macroscopic pore porosity induces a change in the crack extension path, an increase in fragmentation, and a delay in the strain response.

The organic isooctyltriethoxysilane (TS) and inorganic nanosilicon dioxide (nano-SiO2) were combined to prepare monolithic superhydrophobic hydraulic concrete by "binary synergistic" biomimetic method, and the effects of TS dosage and curing conditions on the mechanical properties and hydrophobicity properties of concrete were investigated. The microscopic morphology, phase composition and hydrophobic modification mechanism of materials were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and fourier transform infrared spectrometer (FT-IR). The results show that the curing conditions have a large influence on the hydrophobicity of concrete. Under natural curing conditions, when TS dosage reaches 3% (mass fraction), the concrete all exhibits superhydrophobicity, with a maximum contact angle of 155.6°. The incorporation of nanoparticles promotes cement hydration and enhances compressive strength of specimens. The modified hydraulic concrete has good hydrophobicity, and the water absorption rate decreases by 68.8% in 288 h immersion.

In order to deal with the problem that the marine concrete with large dosage of mineral admixture in the actual project faces the harsh marine environment prematurely and is easy to be eroded by chlorine salt, this paper proposed to use magnetized water instead of tap water to prepare marine concrete, studied the influence law of magnetized water on the mechanical properties and durability of marine concrete at different ages, and analyzed the microstructure of marine concrete using the mercury instrusion porosimetry and scanning electron microscope. The study shows that magnetized water improves the workability of marine concrete. Compared with tap water, magnetized water has obvious improvement on the early mechanical properties of marine concrete, in which the compressive strength and splitting tensile strength of C50 marine concrete with a curing age of 7 d increase by 17.9% and 16.5%, respectively. Regardless of different curing ages, the resistance to carbonation and chloride ion penetration of magnetized water marine concrete are better than tap water marine concrete. The degree of reaction of cementitious materials in magnetized water marine concrete is higher, with fewer cracks, and the microstructure between cementitious materials and aggregates is more compact. The overall porosity is lower, with more harmless pores and fewer harmful pores, leading to a denser microstructure. Thus the mechanical properties and durability of marine concrete are improved.

Serpentine shielded concrete is a neutron shielding material that can maintain high crystal water content even under high temperature condition for a long time, but its compactness is low and it lacks boron element that can effectively absorb thermal neutrons. As a source of boron, boric acid is an effective material for producing shielded concrete. However, in cement-based hydration systems, the addition of boric acid can inhibit the cement hydration process, leading to deterioration of material strength and durability. This study induced the formation of high boron content and crystalline water content of boric acid in cement slurry by modifying high concentration boric acid solution. The boron content in the system was increased while the inhibitory effect of boric acid on cement hydration was relieved. The effect of modified high concentration boric acid solution on workability, mechanical properties, high-temperature residual compressive strength, microstructure and shielding performance of serpentinite shielded concrete was studied. The results show that the addition of modified high concentration boric acid solution has no adverse effects on the setting time, mechanical properties at room temperature, and high-temperature residual compressive strength of serpentine shielded concrete. After adding modified high concentration boric acid solution, the high-temperature residual compressive strength at 400 ℃ of serpentinite shielding concrete increases by 43.5%, the half value layer at room temperature decreases by 31.7%, and the half value layer decreases by 21.4% after insulation at 350 ℃ to constant weight, enhancing the shielding ability of neutron rays.

At present, ultra-high performance concrete (UHPC) is widely used for reinforcement of composite members and normal concrete (NC) structures. After reinforcement, the interface between UHPC and NC becomes the most vulnerable part due to the combined effect of interface normal tensile stress and interface tangential shear stress. Reinforcement planting treatment is a common method to connect the interface between old concrete and new concrete. In this study, the shear strength of UHPC-NC specimens with and without reinforcement planting and the effect of reinforcement rate on shear performance were comprehensively analyzed by straight shear test and finite element simulation. The results show that the shear strength of specimens without reinforcement planting is only 20% of that of integrally cast specimens, while the shear strength of the reinforcement planting specimens could reach 40% to 80%. In specimens without reinforcement planting, the damage form is brittle damage. In reinforcement planting specimens, the development of interface cracks is controlled, the interface shear strength is improved, the slip of interface slip is increased, and the interface is ductile failure.

The rapid repair of concrete pavement under severe cold environment requires the repair materials to have high early strength and high bond strength with concrete. Using magnesium phosphate cement (MPC) with high early strength to prepare rapid repair materials is beneficial for achieving rapid repair of concrete pavement under non curing conditions in the winter. The effects of magnesium phosphorus ratio (M/P), boron magnesium ratio (B/M), and water cement ratio (W/C) on bond strength between MPC mortar and ordinary Portland cement concrete (OPC) in severe cold environment were studied, and the interface bonding mechanism were analyzed. The results show that M/P 4~5, B/M 0.02~0.03, and W/C 0.12~0.14 are more conducive to improve bond strength between MPC mortar and OPC in severe cold environment. The 3 h bond strength between MPC mortar and OPC can exceed 2.5 MPa, meeting the requirements for rapid repair of concrete pavement in severe cold environment.

In order to study the compressive properties of desert sand concrete (DSC) after high temperature, desert sand was used to replace medium sand with replacement rate of 0%, 20%, 40%, 60%, 80% and 100% (by mass) to make DSC. The effects of fire temperature and desert sand replacement rate (DSRR) on the rebound value, ultrasonic velocity and compressive strength of DSC were analyzed. The grey prediction strength model of DSC after high temperature was established. Ultrasonic-rebound comprehensive method for measuring strength curve of DSC and formula of fire temperature were deduced. The results show that with the increase of fire temperature, the ignition loss rate of DSC increases gradually, specimen surface colour gradually changes from deep to shallow, and the compressive strength, rebound value, ultrasonic velocity of DSC increase first and then decrease. With the increase of DSRR, the rebound value and compressive strength of DSC increase first and then decrease. When DSRR is equal to 40%, the compressive strength and rebound value of DSC reach the maximum. The NSGM(1,3) model has excellent prediction performance with an average relative error of 8.6%, which can be used to predict the medium and long-term compressive strength of DSC after high temperature. The prediction accuracy of DSC ultrasonic-rebound comprehensive method for measuring strength curve meets the requirements of specification.

To explore the influences of different coarse aggregate replacement ratios on the high-temperature performance of recycled concrete with manufactured sand (RCM), RCM specimens were prepared with varying coarse aggregate replacement ratios (0%, 20%, 40%, 60%, 80%, 100%, mass fraction). The surface morphology, mass loss rate, compressive strength, load-deformation curve, and microscopic morphology of RCM were investigated after exposure to high temperatures. Results indicate that the RCM performs well in high-temperature tests below 600 ℃ when the coarse aggregate replacement ratio is between 40% and 60%. At 200 ℃, the specimens in each group show minimal morphological changes, small mass loss rate, and a slight increase in compressive strength. At 400 ℃, cracks begin to appear on the surface of the specimens, with mass loss rate ranging from 4.35% to 6.47%, and compressive strength starts to decline. At this point, the coarse aggregate replacement ratio has a minor impact. At 600 ℃, significant changes in surface morphology are observed, with mass loss rate increasing to 6.42%~8.70%, and compressive strength eventually decreasing to around 80% of the baseline strength. Specimens with coarse aggregate replacement ratio of 40%~60% perform well under these conditions. At 800 ℃, the surface morphology of specimens in all groups intensifies further. The impact becomes particularly pronounced for coarse aggregate replacement ratio exceeding 60%. Simultaneously, various performance parameters indicate that the concrete has experienced a complete loss of effectiveness at this temperature.

In order to study the corrosion effect of vitamin corrosion inhibitor in reinforced concrete systems，the effects of thiamine (VitB1), nicotinic acid (VitB3), pyridoxine (VitB6) and ascorbic acid (VitC) on steel reinforcement in simulated pore solutions of concrete containing chlorine was investigated using dynamic potential polarization and electrochemical impedance spectroscopy tests. The mechanism of vitamin corrosion inhibition on steel reinforcement was also discussed based on density general function theory with quantum chemical calculations. The results show that VitB6 has the best corrosion inhibition effect in simulated pore solution of concrete containing 0.05 mol/L NaCl because of the strongest bonding stability between its highest occupied molecular orbital and the 3d orbital of iron. All four vitamins have a specific concentration value to achieve the best corrosion inhibition effect IE. At this time, VitB1 and VitB3 are anodic corrosion inhibitors and VitB6 and VitC are mixed corrosion inhibitors. With the increase of vitamin concentration, the IE tends to fluctuate inversely, which is due to the inhibition of cathodic reaction caused by the adsorption of polar groups on steel surface, further aggravating the imbalance between cathode and anode.

Based on the classical stress level-fatigue life equation, a mathematical expression for chloride ion diffusion coefficient was proposed with the number of freeze-thaw cycles as independent variable. Furthermore, a three-dimensional mesoscopic numerical model of chloride ion transport in concrete under freeze-thaw cycles was established to investigate the effects of freeze-thaw cycles, mesoscopic structural characteristics of concrete, and bonding effects on chloride ion transport behavior. The results show that the freeze thaw cycle can promote chloride ion diffusion, and this promotion effect is significant when the number of freeze thaw cycles approaches the limit number of freeze thaw cycles. Furthermore, the mechanism of interfacial transition zone promoting chloride ion diffusion is revealed by simulating the diffusion trajectory of chloride ion in concrete meso-structure. Finally, through the simulation of the long-term diffusion performance of chloride ions, it is found that there is a saturated area of bound chloride ion near the ingress surface. And in the saturated area of bound chloride, the concrete loses the curing ability of free chloride and promotes chloride ion diffusion.

In response to the problem of reduced durability of concrete structures in the northwest region of China under salt frost erosion, this study selected concrete specimens with different polypropylene fiber contents (0, 0.6, 0.9, 1.2, 1.5 kg·m-3) and placed them in clear water, 3%NaCl, and 5%Na2SO4 solutions for freeze-thaw cycle testing. The changes in mass loss rate, dynamic elastic modulus and compressive strength of specimens were analyzed, and standard models were established based on Weibull theory and grey theory to predict the maximum service life of polypropylene fiber reinforced concrete structures. At the same time, SEM was used to analyze the mechanism of polypropylene fiber reinforced concrete. The results show that the damage caused by clear water freeze-thaw conditions to concrete is lower than that caused by salt freeze-thaw erosion, with chloride salt causing the most severe erosion damage to mechanical properties of concrete. The infiltration of polypropylene fibers can effectively slow down the degradation rate of mechanical properties of concrete under freeze-thaw erosion and weaken the impact of external erosion on compressive strength. The optimal effect is achieved when the fiber content reaches 1.2 kg·m-3. The life prediction results of grey prediction model and Weibull model are roughly similar. The grey prediction model can only make large-scale inferences based on data currently contained, while Weibull model has more accurate prediction results. This result can provide theoretical guidance and basis for studying the mechanical properties of concrete and selecting the best model to predict the service life of concrete.

Using manufactured sand, extra fine sand and other materials to prepare chloride resistance concrete is of great significance to reduce production costs and improve the service life of bridges in coastal areas. In this study, the chloride resistance of concrete was improved by optimizing aggregate design and cement paste composition. The effects of aggregate gradation, extra fine sand content, sand ratio, water-binder ratio and cementitious material type on concrete performance were studied. The results show that the chloride diffusion coefficient decreases from 4.0×10-12 m2/s to 2.9×10-12 m2/s when the mass fraction of 8~16 mm gravel increases from 50% to 80%. When 50% (mass fraction) manufactured sand is replaced by extra fine sand, the chloride diffusion coefficient is 3.6×10-12 m2/s, which can still meet the design requirements. With the increase of sand ratio, the chloride diffusion coefficient decreases first and then increases. The water-binder ratio is controlled in the range of 0.36~0.38, and the chloride diffusion coefficient is less than 3.5×10-12 m2/s. The hydration of slag component improves the pore structure of concrete, and forms more AFm and C-S-H, which improves the chloride binding capacity. The chloride diffusion coefficient of concrete with slag cement is 40% lower than that of OPC.

In order to investigate the effects of steel fiber, polyvinyl alcohol (PVA) fiber and mineral powder contnet on permeability resistance of steel-PVA hybrid fiber high performance concrete (HFHPC). The orthogonal experiment was designed to conduct the permeability resistance test of steel-PVA HFHPC, and the test results were analyzed through range analysis, variance analysis and regression analysis. The results show that the influence degree on the impermeability of HFHPC is steel fiber, mineral powder and PVA fiber from high to low. In the range of test level, the optimal horizontal combination of impermeability of concrete specimens is as follows: steel fiber volume fraction of 1.0%, PVA fiber volume fraction of 0.7% and slag powder mass replacement rate 20%. When the steel fiber volume fraction exceeds 1.0%, the impermeability of HFHPC decreases slightly, but it is still higher than that of standard plain concrete. Finally, multiple regression analysis is used to establish the prediction model of HFHPC permeability and steel fiber, PVA fiber and mineral powder.

To reveal cracking resistance of fiber reinforced concrete (FRC), an experimental investigation was conducted on concrete with various types of fibers (straight steel fiber, amorphous alloy straight fiber, and steel fiber+amorphous alloy fiber) and various volume fractions (1.0%, 1.5% and 2.0%) to evaluate early cracking behavior and post-hardened fracture toughness. First, the concrete early cracking feature was quantitatively analyzed by using plate method and image quantitative technique, aiming to evaluate early cracking behavior of FRC. Then, the three-point bending tests were conducted on notched prism specimens, the hardened fracture toughness of FRC was analyzed and assessed based on the double K fracture parameters. The results show that the single steel fiber reinforced concrete and hybrid fiber concrete exhibit the best cracking resistance at early age. As the fiber fraction increases from 1.0% to 2.0%, the unstable fracture toughness of steel fiber, amorphous alloy fiber, and hybrid fiber reinforced concrete increases by 58.9%, 44.3% and 55.5%, respectively. Meanwhile, the fracture energy of hybrid fiber concrete with 2.0% fiber reaches 11.8 times that of normal concrete. It shows that steel and amorphous alloy fibers have a synergistic effect at different periods of concrete hardening process. As a result, steel fiber and amorphous alloy fiber have a synergistic effect at the different periods of the hardening process, the hybrid steel and amorphous alloy fibers can not only prevent the early-age concrete plastic cracking but also effectively control the formulation and propagation of post-hardened tensile cracks, thus, achieving a staged anti-crack purpose. Considering the early cracking properties, post-hardening tensile properties, and fracture properties of concrete, the overall performance of hybrid fiber concrete with the same fiber content shows better performance.