The use of seawater sea-sand to replace freshwater and river sand in concrete preparation not only relieves the consumption of resources in the civil engineering industry, but also saves a lot of transportation costs and time costs for the construction of coastal cities and distant islands in situ. Therefore, the research on the resource utilization of seawater sea-sand has become particularly critical. This paper aimed at seawater sea-sand concrete (SSC). The researches on the characteristics of seawater sea-sand at home and abroad were compared. The mechanical properties of SSC, the durability of SSC under dry-wet cycles and freeze-thaw cycles, and the development of the bonding properties between SSC and fiber reinforced polymer (FRP) bars were analyzed. Finally, the current research results of seawater sea-sand concrete were summarized, and suggestions for future research were made.

To overcome the problem of low early strength of supersulfated cement (SSC), the regulation effects and mechanisms of nano-SiO2(NS) on the hydration hardening performance of SSC were explored. The effects of NS on the mechanical properties, chemical compositions, microstructure, morphology, and pore structure of SSC were studied. The results show that the incorporation of NS significantly improves the mechanical properties of SCC, in which the 3 d compressive strength increases by 32%, the 3 d flexural strength increases by 28%, and the 90 d compressive strength is nearly doubled. Meanwhile, NS significantly densifies the pore structure of SSC, reducing its critical pore diameter from 728 nm to 6.5 nm. The enhancement mechanism of NS is mainly to promote the hydration of slag and increase the formation amount and polymerization degree of C-(A)-S-H gel in the product. This work proves the feasibility of NS to improve the hydration hardening performance of SSC and provides a new idea for the performance enhancement of low-carbon SSC.

In order to prolong the service life of concrete, a 2,6-bis ［N-(carboxyethyl carbonyl) amino］ pyridine (DAP) supramolecular hydrogel driven self-healing cement-based material and hydroxyapatite self-healing technology were developed. In alkaline environment, the DAP supramolecular hydrogel loaded with NH4H2PO4 can realize the releasing of repair agent intelligently. DAP supramolecular hydrogel loaded with repair agent was encapsulated in a hollow glass tube and used to prepare self-healing component, and experiments of healing crack and rebar protection were carried out. The results show that the crack surface with a width of 0.592 mm is repaired after 28 d of healing. The healing product is hydroxyapatite. The self-healing component delays the rebar corrosion. The repair agent is released at the cracks and the generated healing products fill the cracks, thus blocking the intrusion channels of hazardous ions and gas.

The hydration behavior of calcium aluminate cement (CAC) is mainly affected by phase composition, particle size, curing temperature, and additive, etc. Research works revealed that the zeolite-type minerals would affect the hydration behavior of CAC, while the specific mechanism should be further investigated. Therefore, the hydration behavior and the detailed mechanism of CAC containing zeolite at various curing temperatures (20 ℃, 25 ℃, 30 ℃ and 40 ℃) were investigated, through taking XRD, SEM, FTIR, thermal analyses and electrical conductivity tests. The results show that the effect of synthetic zeolite on the hydration behavior of CAC is dominated by the ion concentration at corresponding temperature. At 20 ℃, the low solubility of CAC leads to low ion concentration, meanwhile, the ultra-high specific surface area of synthetic zeolite hinders the saturation of ions, which prolongs the induction period and thus delays the hydration of CAC. In comparison, the pore structure and ultra-high specific surface area of zeolite provide more nucleation sites for hydrates at 25~40 ℃, which facilitate the hydration of CAC. In addition, the introduction of synthetic zeolite effectively eliminates the abnormal setting behavior of CAC cured at 25 ℃.

Incorporating steel fibers into cement-based materials effectively prevent the generation and development of microcracks, thus improve anti-crack performance. The stress intensity factor (SIF) solution of the interaction between the crack and steel fiber in the infinite plate under plane stress condition was obtained through the Eshebly equivalent inclusion theory and the maximum circumferential stress criterion. The maximum stress expression of the crack tip of Ⅰ-Ⅱ combined fracture mode with fiber was derived. Through the maximum stress ratio of crack tip after adding steel fiber, the direction, location and the elastic modulus of steel fiber on toughening effect were analyzed. The results show that, the maximum stress around crack tip is reduce by changing the strain field and SIF. The maximum stress ratio of crack tip reaches the minimum value when the steel fiber is perpendicular to the crack and the toughening is significantly affected after steel fiber aligning. Further more, the calculation result shows that the direction of steel fiber is the major factor on toughening effect and little effect on the elastic modulus of steel fiber.

In this paper, the variation law and deterioration mechanism of fiber cement-based material properties during freeze-thaw cycles were explored. Mercury injection method was used to measure the pore characteristic parameters (total pore volume, total porosity, most probable pore size, critical pore size and average pore size) and the volume percentage (harmless pore, less harmful pore, harmful pore and multi-harmful pore) of fiber cement-based materials without freeze-thaw and after 300 freeze-thaw cycles. And the gray relative correlation degree was used to correlate the frost resistance index of fiber cement-based materials without freeze-thaw and after 300 freeze-thaw cycles with pore characteristic parameters and volume percentage respectively. The study shows that with the increase of the number of freeze-thaw cycles, the mass loss rate of fiber cement-based materials decreases gradually, that is, the mass increases gradually, and the compressive strength, dynamic elastic modulus and ultrasonic wave velocity all decrease to different degrees. The variation law of its internal pore characteristic parameters and volume percentage explain the microscopic reasons for the variation of the frost resistance index. After the freeze-thaw cycles, the relative correlation between the average pore size and pore volume percentage of fiber cement-based materials and the frost resistance index is significantly improved.

The chloride penetration resistance of cement mortar modified by graphene oxide (GO) was studied by rapid electric flux method and long-term chloride salt immersion test. The relationship between electric flux and apparent chloride diffusion coefficient was analyzed. The microstructure of the mortar was tested by scanning electron microscope (SEM). The results show that the incorporation of appropriate amount of GO improves the chloride penetration resistance of cement mortar. When the content is 0.06% (mass fraction), it is the most significant. Compared with the control group without GO, the electric flux and chloride erosion depth of cement mortar are reduced by 34.5% and 27.2%, respectively. After long-term chlorine salt immersion for 60 d and 120 d, the free chloride concentrations at 0~5 mm decrease by 28.4% and 15.3%, respectively. There is a good linear relationship between electric flux and apparent chloride diffusion coefficient. GO effectively regulates the shape of hydration products, reduces internal pores, and makes its microstructure more dense, thereby improving the chloride penetration resistance of cement mortar.

3D printing mortar generally has the problems of weak interlayer interface bonding and low flexural strength. The expansion cement slurry coated on the surface of mortar can produce surface compressive stress and then improve its flexural strength. As an interface agent, coating between layers can simultaneously enhance the interfacial bond strength. In this paper, the coating composed of sulphoaluminate cement and expansive agent was coated on the surface and interlayer of the 3D printing mortar, and the effect of the coating on the mechanical properties of the 3D printing mortar specimens without fiber and with 0.5% (volume fraction) basalt fiber was studied. The experimental results show that the interlayer coating improves the interlayer interfacial bond strength of fiber-free and fiber-doped rapid setting 3D printing mortar by 21.4% and 12.2%, respectively. At the same time, it also improves the compressive strength. The effect of coating only surface on the flexural strength of the 3D printing mortar is similar to that of coating only between layers. At the same time, surface coating and interlayer coating have the most obvious improvement effect on the flexural strength of the 3D printing mortar. Compared with the fiber-free and fiber-doped non-coating specimens, the highest improvement rates are 44.2% and 23.2%, respectively. The effect of coating on the interlayer interfacial bond strength and flexural strength of fiber-free 3D printing specimens is better than that of fiber-doped specimens.

Calcium silicate hydrate (C-S-H) is the most important component of cement hydration products, and is the main cementitious phase of cement-based materials. The interlayer water of C-S-H has a significant effect on its nanostructure and mechanical properties. The structure and mechanical properties of C-S-H with different humidity were studied by molecular dynamics method. The influence of humidity on the structure and elastic parameters of C-S-H as well as the mechanical and deformation properties under tension, compression and shear conditions were analyzed by radial distribution function and concentration distribution of atoms, elastic constants and stress-strain relationships. The results show that the increase of humidity leads more O atoms to accumulate around Si and Ca atoms in C-S-H, and makes the distance between layers larger and the stratification more obvious. At the same time, the increase of humidity also reduces the elastic properties of C-S-H. With the increase of humidity, the mechanical and deformation properties of C-S-H under different stress states reduce. The influence degrees on tensile strength and shear strength are greater than that on compressive strength. In terms of deformation properties, humidity has the greatest influence on deformation properties under tension and has the least influence on deformation properties under compression.

The effects of unitary retarder system and binary complex retarder system on setting time, fluidity and mechanical properties of high strength magnesium phosphate repair mortar were studied. The results show that in unitary retarder system, compared with borax and sodium tripolyphosphate, the retardation effect of sucrose alone is the best. But excessive content of sucrose significantly reduces the fluidity and early mechanical properties of magnesium phosphate repair mortar. In the binary complex retarder system, the retardation effect of borax and sucrose is better than borax and sodium tripolyphosphate. Under 5% (mass fraction) borax and 2.5% (mass fraction) sucrose, the setting time of magnesium phosphate repair mortar reaches 18 min, the fluidity is 230 mm, and the strength of 1.5 h, 3 d and 28 d is 22.6 MPa, 56.8 MPa and 62.4 MPa, respectively. The setting time and early strength of magnesium phosphate repair mortar achieve dynamic balance.

In order to achieve the efficient regulation of the rheological properties of ultra-high performance concrete (UHPC), bentonite was used as an auxiliary cementing material and the performances of the prepared UHPC were evaluated. The fluidity, static yield stress, dynamic yield stress, plastic viscosity and thixotropy of UHPC slurry were studied, and the effect of different bentonite content on the rheological properties of UHPC slurry was systematically analyzed. The results show that: the overall fluidity of UHPC slurry shows a downward trend, and the decline amplitude gradually increases with the increase of bentonite content. When the content of bentonite increases from 0% to 15.0% (mass fraction), the static yield stress, dynamic yield stress and plastic viscosity of UHPC slurry increase significantly, by about 17.05 times, 5.78 times and 1.16 times, respectively. The hysteresis loop area and thixotropy index increase with the increase of bentonite content, and the thixotropy is significantly improved. Meanwhile, UHPC with bentonite still meets the advanced mechanical properties of UHPC.

The bending performance of a concrete beam can be reflected by its macroscopic characteristics of surface crack distribution and evolution during the bending process. The bending behaviors of concrete beams were investigated, and the fractal theory was applied to analyze the development of the surface cracks. Basalt fiber reinforced plastic (BFRP) bars reinforced ultra-high performance concrete with different waste tire steel fiber (WTSF) replacement ratios were used as the concrete matrix. The results show that the distribution of cracks on the surface of concrete beams exhibits fractal characteristics and self-similarity, the fractal dimension is between 0.892 and 1.064. The relationships between the fractal dimension of cracks on the surface of beams and the applied load, WTSF replacement ratio, mid-span deflection and maximum crack width were discussed, and the functional relationships between WTSF replacement ratio and the fractal dimension of the whole beam section and pure bending section under complete failure state were fitted, respectively. It is found that fractal dimension has logarithmic function relationship with applied load, mid-span deflection and maximum crack width. Under the complete failure state, the fractal dimension of cracks on the beam surface increases with the increment of WTSF replacement ratio, but the adverse effect is not obvious on the whole. The results obtained in this study can provide some theoratical basis and practical experiences for the application of ultra-high performance concrete.

To systematically study the effects of fly ash content, silica fume content and curing temperature on the mechanical properties of wet shotcrete, the range and variance analysis of the compressive strength of wet shotcrete were carried out by designing orthogonal tests. The results show that the compressive strength of wet shotcrete increases with the increase of curing age, but the increase rate slows down with the extension of curing ages. The compressive strength of wet shotcrete can be effectively improved by the increase content of silica fume and pulverized coal, but when the content of fly ash exceeds 10% (mass fraction, the same below), the increase of fly ash content has no significant influence on the later compressive strength of concrete. The order of the influence of the three factors on the compressive strength of wet shotcrete is silica fume content＞curing temperature＞fly ash content. The sensitivity of the compressive strength of wet shotcrete to mineral admixtures is positively correlated with the curing temperature. The improvement effect of mineral admixtures on compressive strength of wet shotcrete increases with the increase of curing temperature. With the increase of curing temperature, a large number of flocculent cementitious substances are generated. The hydration products bond more densely, and the compressive strength and bearing performance of concrete are further enhanced. The established multiple nonlinear models can predict the compressive strength of concrete, and wet shotcrete has the best compressive strength when the silica fume content, fly ash content and curing temperature are 15%, 15% and 10 ℃, respectively.

Damage evolution process (DEP) of concrete can be evaluated by acoustic emission (AE) technology. In order to resonably describe the DEP of concrete, AE test of the whole process of concrete damage under uniaxial and eccentric compressions were carried out and the evolution law of AE characteristic parameters (AECPs) in the whole process of concrete damge were analyzed. Moreover, the optimal AECPs were resonable selected, and the relationship between the characteristic points of stress-strain curve and AECPs was revealed. Results show that the AECPs including counts, duration, amplitude, signal strength and average frequency well describe DEP of concrete. Meanwhile, there is a high correlation between characteristic points of AECPs and stress-strain curve during the compression failure of the concrete specimen. The first demarcation point of AECPs corresponds to the starting point of elastic stage of stress-strain curve, the second demarcation point corresponds to the peak stress point of stress-strain curve, and the maximum mutation point of signal strength corresponds to the cracking point of stress-strain curve.

From mesoscale aspect, concrete was considered as a three-phase composite material consisting of aggregate, cement mortar and the interfacial transition zone between aggregate and cement mortar. Based on Python language, a unite-cell model with periodicity was established by using Monte Carlo method. The prediction method of equivalent elastic modulus of concrete was proposed by combining homogenization theory and periodic boundary conditions. The predicted values of the model were compared with the results in existing literature to verify the validity of the present model. On this basis, the influences of unite-cell size, interfacial transition zone thickness, aggregate volume rate and maximum aggregate size on the equivalent elastic modulus of concrete were investigated. The results show that the unite-cell model can effectively predict the elastic properties of concrete composite materials. The error locates in the range of 1.87% to 4.97%, compared with the existing results. The size of concrete unite-cell model is recommended to be 150 mm. In the range of 20% to 40%, the aggregate volume rate has significant effect on the equivalent elastic modulus of concrete. Moreover, the interfacial transition zone thickness and maximum size of aggregate exhibit slight effect on the equivalent elastic modulus of concrete, showing a monotonically decreasing or monotonically increasing effect pattern.

Due to freeze-thaw, dry-wet, salt intrusion and so on, the durability of hydraulic concrete which is of the long-distance water conveyance project in the saline areas of Northwest China has a negative influence, which directly affects the water conveyance stability and the operation safety and the benefit. In this paper, based on the water delivery structure of the Gansu Yinda-Qin project and accelerated indoor test, the changes of durability indexes such as mass loss rate, compressive strength loss rate and relative dynamic modulus of elasticity were studied for two working conditions: dry-wet-freeze-thaw cycle and dry-wet-freeze-thaw-salt invasion cycle. And the improved matter-element extension model was used to analyze the effects of four factors, namely, slag powder, fly ash, air-entraining agent and polypropylene, on the durability of hydraulic concrete. The influencing degree of four factors on the durability of hydraulic concrete was analyzed, and the calculated results were compared with the test results and the actual situations. The results show that in the dry-wet-freeze-thaw condition, the influence of fly ash on the durability of hydraulic concrete is more significant than that of slag powder, air entraining agent and polypropylene, while in the dry-wet-freeze-thaw-salt invasion condition, the influence of polypropylene is the most significant and sensitive. The improved matter-element extension model is used to comprehensively analyze the above four factors, and the influencing degree is polypropylene (3.450 9) > fly ash (2.509 0) > mineral powder (2.447 2) > engraver (2.232 5). According to the size of the characteristic value of the grade variable, the state of the concrete test block after large circulation can be judged, which is consistent with the actual situation. Therefore, the improved matter-element extension model can comprehensively analyze the influencing factors of the durability of hydraulic concrete in the saline areas and has certain practical value.

The chloride ion transport mode and influence mechanism in the coastal dry-wet alternate environment are the main contents of concrete durability research, which has attracted wide attention and discussion of scholars. The calculation model describing the chloride ion transport behavior in concrete is usually composed of partial differential equations, and the solution method is complex. To analyze the chloride ion transport process simply and accurately, this paper proposed a new way to calculate the depth parameter Δx in the modified Fick’s law model. The calculation model of chloride ion transport in the dry-wet alternate region was established by the meso-scale composition of concrete, the ambient temperature field, water migration and chloride ion transport. According to the model, the value of Δx can be accurately obtained. Then the chloride ion transport in different service environments was analyzed. According to the calculation results, the time-varying formula of convection zone depth Δx considering the service time, daily average temperature and drying-wetting time ratio was summarized in this paper. These formulas improve the applicability and accuracy of the modified Fick’s law model for predicting chloride ion transport and distribution in dry-wet alternate environment. The above results can provide a simple and effective calculation method for the durability design of concrete structures in dry-wet alternate environment.

Self-compacting concrete has the characteristics of high fluidity, anti-segregation, and better compactness, which is beneficial for the improvement of engineering quality and the reduction of the project cost. The addition of recycled aggregate into self-compacting concrete realizes the recycling of construction and demolition waste. To ensure the feasibility and safety of self-compacting recycled concrete applied in practical engineering, a large number of the modified studies on the workability and mechanical properties of self-compacting recycled concrete must still be further carried out. Based on the rheological compaction mechanism of self-compacting concrete, the design principle of self-compacting concrete proportion was summarized. The research progress of workability and mechanical properties of self-compacting recycled concrete were further summarized and reviewed. Finally, the problems existing in the current research on the workability and mechanical properties of self-compacting recycled concrete were analyzed, and then the future development of self-compacting recycled concrete was prospected. This research will be helpful for promoting the large-scale application of self-compacting recycled concrete in the practical engineering.

Zeolite was utilized to partially or completely (0%, 50%, 100%, volume fraction) replace river sand and the effect of zeolite on the macro and micro properties of seawater mixing ultra-high performance concrete (UHPC) was studied. The flowability, setting time, autogenous shrinkage and compressive strength of UHPC were tested. Besides, the water absorption and water releasing of zeolite as well as the interfacial transition zone (ITZ) of UHPC were studied by low field NMR and nano-indentation. The results reveal that seawater mixing enhances the hydration of UHPC, so it shortens the setting time, increases the compressive strength and the autogenous shrinkage of UHPC. However, zeolite absorbs chloride ions, which inhibits the accelerated hydration induced by chloride ions, extends the setting time of UHPC and reduces the compressive strength. In addition, the internal curing effect of zeolite effectively improves the autogenous shrinkage of seawater mixing UHPC, and its optimization effect is better than that in freshwater mixing UHPC. The phenomenon can be attributed to the earlier and longer releasing water time of zeolite in seawater mixing UHPC. As the strength of porous zeolite is lower than that of river sand, the early compressive strength of seawater mixing UHPC gradually decreases with the increase of zeolite content. After curing 28 d, because the internal curing effect of zeolite optimizes the interfacial transition zone, the later hardening paste is further compacted.

In order to study the effect of basalt fiber on the chloride ion penetration resistance of recycled concrete, the electric flux test of recycled concrete with five coarse aggregate mass replacement rates (20%, 40%, 50%, 60%, 80%) under the volume content of four kinds of basalt fibers (0%, 0.2%, 0.4%, 0.6%), and ordinary concrete were carried out. The microscopic mechanism of the effect of basalt fiber on the chloride ion penetration resistance of recycled concrete was studied from the perspective of cement hydration and pore structure by fourier transform infrared spectroscopy (FTIR) and mercury injection method (MIP). The results show that the addition of basalt fiber greatly increases the chloride ion penetration resistance recycled concrete. The combination of basalt fiber content of 0.2% and coarse aggregate replacement rate of 50% achieve the best chloride ion penetration resistance, even better than ordinary concrete. Based on FTIR, it is found that basalt fiber improves its chloride ion penetration resistance by changing the polymerization degree of hydration product C-S-H and the formation of CaCO3 of recycled concrete. Based on MIP, it is found that under the best combined dosage, the pore size distribution of recycled concrete is optimized, the porosity is the smallest, and its chloride ion penetration resistance is improved.

In order to investigate the dynamic properties of jute fiber reinforced loess, the natural jute fiber with low price and good performance was added into the loess according to certain mass mixing ratio. The dynamic stress-strain relationship, dynamic modulus of elasticity, damping ratio and other indicators were tested to analyze the dynamic properties of loess with different mass ratios of jute fiber and to study the reinforcement mechanism. The test results show that the dynamic stress increases first and then decreases with the increase of jute fiber content. Under the same strain condition, the dynamic stress reaches the maximum when jute fiber mass ratio reaches 0.3%. The dynamic modulus of elasticity of loess with different jute fiber mass ratios is greater than that of normal loess and decreases with the increase of dynamic strain. The damping ratio effectively reduces by incorporating a certain mass of jute fiber, and the average values of maximum damping ratio decrease compared to minimum damping ratio at different surrounding pressures (100 kPa, 200 kPa, 300 kPa) are 91%, 120%, 100%, respectively. SEM images indicate that the incorporation of excessive fiber causes the fibers to be entangled with each other, resulting in the decrease of dynamic properties and other indicators.

In order to study the effects of nano-SiO2 and polypropylene fiber on the mechanical properties and water stability of coal gangue lime-fly ash mixture, the strength values of each specimen before and after modification were measured by unconfined compressive strength test and splitting test, and the quality difference, strength value and acoustic emission characteristics of the specimens after dry-wet cycle were analyzed. The results show that nano-SiO2 improves the compressive strength of the mixture, and the improvement effect is the best when the mass content is 2.5%. Adding nano-SiO2 and polypropylene fiber at the same time further improve the mechanical properties of the mixture. The mechanical properties of coal gangue lime-fly ash mixture are the best when 2.5% nano-SiO2 and 0.15% (volume fraction) polypropylene fiber were added. It is found that the water stability of coal gangue lime-fly ash mixture mixed with 2.5% nano-SiO2 and 0.10% (volume fraction) polypropylene fiber is the best.

The effect of fly ash content on the performance of seawater and sea sand high performance concrete was studied by carrying out experiments on mechanical properties, chemical shrinkage, pH value, chloride ion concentration, and microstructure. The results show that in order to maintain the stability of reinforcement passive film, the content of fly ash should not be greater than 30% (mass fraction, the same below) under high temperature steam curing and 50% under standard curing. The concentration of free Cl- in seawater and sea sand high performance concrete fluctuates with curing time, increases first and then decreases sharply in the early stage, and increases slowly in the later stage. The Cl- concentration under standard curing is significantly lower than that under high temperature steam curing. Seawater and sea sand high performance concrete has early strength, and its strength decreases with the increase of fly ash content. High temperature steam curing significantly improves the flexural and compressive strength of concrete. The more fly ash is added, the more unhydrated particles remain. High temperature steam curing effectively improves the microstructure and compactness of concrete. When fly ash content is too large or too small, it increases the chemical shrinkage of Portland cement. The chemical shrinkage value of concrete is small when the fly ash content is 30% and 40%.

Molybdenum tailings are difficult to develop and utilize because of their high crystallinity mineral phase and low-grade metal elements. In order to realize the resource utilization of molybdenum tailings, based on the mineralogical characteristics of molybdenum tailings, the effects of heat treatment temperature and time on the phase composition and microstructure were studied, and the thermal activation modification mechanism of molybdenum tailings was developed. Based on the optimal heat treatment temperature, molybdenum tailings based alkali-activated geopolymer (AAG) cementitious material was successfully prepared, and the effect of heat treatment time on its curing time and compressive strength was studied. The results show that heat treatment at 850 ℃ for 2 h promotes the decomposition of muscovite and calcite in molybdenum tailings, and some low crystalline garnet and quartz phases transforme into amorphous aluminosilicate (ferrite), which significantly improve the content of active silicoaluminate in molybdenum tailings. The surface roughness of molybdenum tailings particles increases with the increase of heat treatment time, and the pozzolanic activity is significantly improved. The curing time of AAG prepared by molybdenum tailings after heat treatment at 850 ℃ for 2 h is shortened to 48 h from 168 h, which is significantly lower than that of AAG with untreated molybdenum tailings at room temperature, and the compressive strength after curing for 14 d increases by 2 times, reaching 35.7 MPa. The research results greatly broaden the raw material sources of geopolymer, and provide a feasible modification scheme for the utilization of molybdenum tailings.Key words:

Carbonate activated cementitious materials have the characteristics of low environmental impact, easy operation, and good material properties. Slow early age strength development limits its application for practical engineering. In this study, layered double hydroxides (LDHs) were used as the key material to promote the early-age performance of sodium carbonate activated slag. Combined with material characterization, isothermal calorimetry, XRD, TG-DTG, SEM and mercury intrusion porosimetry (MIP), the effect of LDHs on early age hydration reaction and products were systematically analyzed. The results show that with the increase of LDHs content, the hydration rate of sodium carbonate activated slag increases significantly. LDHs acts as a nanocrystalline nucleus site and facilitates the growth of hydrotalcite and C-S-H gels, which accelerates the formation of main hydration products in the early stage of sodium carbonate activated slag. The pore structure of the sample is improved in the early stage of hydration, and 1 d compressive strength reach up to 22 MPa.

Carbide slag (CS), as a kind of industrial waste slag with high alkalinity has low comprehensive utilization rate. In order to solve the problem of excessive carbide slag, the alkali excitation performance of carbide slag on slag cementitious system was studied by using the strong alkaline of carbide slag. Based on the characteristics that carbide slag carbonization reaction generated calcium carbonate, the influences of different carbonization systems on the performance of alkali-activated slag cementitious system were explored. The results show that CS with large dosage has a good alkali-activated effect on the slag cementitious material, and a large amount of C-(A)-S-H gel is generated. The cementitious material with fly ash and metakaolin has best performance. The mechanical properties of carbide slag-slag composite material can be effectively improved after being treated by different carbonation systems. CO2 gas using as the external carbonation source leads to dense structure on the surface of material matrix, while urea as internal carbonization source makes the internal carbonation of matrix uniform, which improve mechanical properties of the carbide slag-slag composite cementitious material.

Lower early hydration activity is one of the important factors limiting the application of higher content smelting slag in the cementitious materials system. In this study, silicomanganese slag (SM) was activated by sulfate in circulating fluidized bed combustion ash (CFBA), and the hydration process and activity development of SM-CFBA composite supplementary cementitious materials were studied. Results show that the fluidity of mortars prepared by using SM-CFBA composite supplementary cementitious materials decreases significantly with the increase of CFBA. However, early and later activity of SM-CFBA composite supplementary cementitious materials are effectively improved with the increase of CFBA. When the content of CFBA is 10% (mass fraction) in SM-CFBA composite supplementary cementitious materials, 3 d, 7 d, and 28 d strength activity indexes of them reach 61%, 71%, and 95%, respectively, which is higher than those of SM (3 d, 7 d, and 28 d strength activity indexes are 50%, 53%, and 81%, respectively). The incorporation of CFBA stimulates the early hydration of cement and supplementary cementitious materials, and delays the transformation of ettringite into monosulfur-type hydrated calcium sulfoaluminate (AFm) during the hydration process, which allows early hydration of the cementitious materials to form more ettringite.

In order to explore the mechanical properties of lead smelting slag and metakaolin based composite cementitious materials under sodium silicate alkali excitation, the effects of ball milling time of lead smelting slag, alkali equivalent, alkali activator modulus and the mass ratio of metakaolin to lead smelting slag on the mechanical properties of composite cementitious materials were studied by single factor test and orthogonal test. The hydration mechanism of composite cementitious materials was comprehensively analyzed by XRD, SEM and FTIR. The results show that the order of influence on the 28 d compressive strength of composite cementitious materials is as follows: alkali activator modulus, ball milling time of lead smelting slag, alkali equivalent, mass ratio of metakaolin to lead smelting slag. When the ball milling time of lead smelting slag is 4 h, alkali equivalent is 6% (mass fraction), alkali activator modulus is 1.4, and the mass ratio of metakaolin to lead smelting slag is 3∶7, the 28 d compressive strength of composite cementitious materials reaches 56.18 MPa. Metakaolin promotes the hydration of lead smelting slag and generates more gel and silicaluminate crystals with reticular structure to fill matrix pores, which promotes the late strength development of cementitious system.

As an important part of the vanadium slag system, finding out the various characteristics of silicate minerals in the roasting stage will help to better grasp the evolution law of the vanadium slag system during the roasting process, and it is of great significance to guide the improvement of the roasting process. In this paper, the variation characteristics of main silicate minerals in the roasting stage of high calcium vanadium slag were analyzed through X-ray diffraction analyzer, mineral analyzer, scanning electron microscope and energy dispersive spectrometer. The results show that the temperature range of oxidative decomposition of fayalite is 334~850 ℃. At the initial stage of decomposition, amorphous quartz forms inside and outside the particles. With the increase of temperature, a gradually thickened iron oxide edge forms on the outer edge of fayalite particles until the particles are completely decomposed, and its decomposition products are quartz and iron oxide. The temperature range of oxidative decomposition of hedenbergite is 334~850 ℃, and the initial stage of decomposition is also marked by the formation of amorphous quartz inside and outside the particles. As the temperature rises, calcium vanadate ring and iron oxide appears on the outer edge of the hedenbergite particles until the particles are completely decomposed. And its decomposition products are mainly quartz, calcium vanadate and iron oxide. The reaction temperature range of dicalcium silicate is 640~850 ℃, and the initial stage of the reaction is marked by the presence of a calcium vanadate ring on the outer edge of the particles. With the increase of temperature, dicalcium silicate is gradually decomposed, and its decomposition products are mainly calcium vanadate, quartz and iron oxide.

To improve the water resistance of magnesium oxychloride cement (MOC) and transform the solid waste-titanium extraction tailings (TETS) powder into resources, the TETS powder was added into MOC as an admixture. The effect of TETS powder on the setting time, compressive strength, dissolution rate of chloride ion, phase composition, microstructure and pore structure was analyzed by Vicat, electronic servo testing machine, ion chromatograph, X-ray diffractometer, scanning electron microscope and mercury intrusion porosimetry, etc. The results show that untreated TETS increases the total porosity and the large pores (>100 nm in diameter) content of the MOC matrix, resulting in a decrease in the compressive strength and water resistance of MOC. TETS powder treated by grinding can improve the dispersion of MgO particles in the MOC system, promote the growth and development of 5Mg(OH)2·MgCl2·8H2O crystals, and reduce the total porosity of MOC, resulting in an increase in the compressive strength and water resistance of MOC. The 28 d compressive strength and 28 d strength retention coefficient of MOC containing 20% TETS powder treated by grinding (by weight of light-burned magnesia) reach to 102.4 MPa and 0.88, respectively. Meanwhile, the concentration of chloride ion in soaking liquid decreases to 11.2 mmol/L.

The adsorption performance and mechanism of converter steel slag for Ni2+ in aqueous solution were discussed from the aspects of steel slag dosage, adsorption time and acid conditions, and the effect of Cu2+ on the adsorption of Ni2+by steel slag was also discussed. The results show that when 100 mL Ni2+ solution with a concentration of 50 mg·L-1 is treated with 200 mesh (0.074 mm) 0.15 g steel slag at 25 ℃ for 30 min, the adsorption capacity of Ni2+ is 33.290 mg·g-1 (adsorption rate is 99.88%). The adsorption process of Ni2+ by steel slag conforms to the pseudo-second-order kinetic equation and Freundlich isothermal model, and the correlation coefficient R2 is 0.984 and 0.994, respectively. The adsorption of Ni2+ and Cu2+ by steel slag belongs to competitive adsorption, and the adsorption capacity of Cu2+ by steel slag is better than that of Ni2+. When the pH value is very low, the adsorption effect of steel slag on Ni2+ and Cu2+ is poor. The adsorption process of steel slag on Ni2+ is mainly chemical adsorption, accompanied by physical adsorption, and with the decrease of adsorption sites on the steel slag surface, the physical adsorption of steel slag on Ni2+ will gradually weaken. The research may provide some guidance to the treatment of industrial wastewater containing Ni2+ and Cu2+.

Limited by high chloride ion migration, lots of low-grade FGD gypsum cannot be effectively used in gypsum products. The influence and mechanism of Q phase (Ca20Al26Mg3Si3O68) on the migration of chloride ion in FGD gypsum were studied. The results show that the hydrates of gypsum-Q phase in which 10%~30% (mass fraction) of Q phase is blended are mainly calcium sulfate dihydrate, ettringite and gibbsite. The chloride ion immobilization capacity and absolute dry compressive strength of the FGD gypsum sample are greatly improved. Adding 30% Q phase achieves the best performance improvement compared with FGD gypsum. At this time, the chloride ion immobilization capacity reaches 19.55% and the absolute dry compressive strength of the gypsum increases 51.5%. Combining XRD, SEM and thermal analysis, the mechanism of ettringite and gibbsite in the FGD gypsum-Q phase hydrates on the immobilization of chloride ion and the improvement of the mechanical properties of the gypsum are explored.

Calcium sulfate whiskers were prepared by hydrothermal synthesis method with desulfurized gypsum as raw material. The effects of acidification pretreatment, slurry concentration and reaction temperature on the preparation of calcium sulfate whiskers were studied by X-ray diffractometer, scanning electron microscope, energy spectrometer, thermal gravimetric analyzer and other test methods. The experimental results show that the acidification pretreatment effectively removes CaCO3 impurities in raw materials. As the slurry concentration and reaction temperature increase, the aspect ratio of whisker increases first and then decreases. When the slurry concentration is 3%(mass fraction), the pH value is 6.5, the reaction temperature is 120 ℃ and the reaction time is 2 h, calcium sulfate whiskers achieve the best average length of 161.2 μm, and the best average aspect ratio of 46.06.

To explore the feasibility of waste ceramic as aggregate in concrete engineering, the effects of ceramic coarse and fine aggregates on the mechanical properties of concrete were discussed through laboratory tests. The effects of different types and contents of ceramic aggregates on the compressive strength, flexural strength and elastic modulus of concrete were studied. A three-dimensional prediction model of concrete mechanical properties considering the content and type of waste ceramic aggregates was established. The results show that the internal curing effect of ceramic coarse aggregate increases the elastic modulus and compressive strength of concrete by 10.42%~12.28% on average. And the addition of ceramic fine aggregate reduces the elastic modulus and compressive strength of concrete by 7.57%~13.88% on average. The addition of ceramic coarse and fine aggregates reduces the flexural strength of concrete by 7.24%~10.52% on average. Besides, the similarities and differences between the mechanical properties of waste ceramic aggregate concrete and domestic and foreign specifications were obtained based on laboratory tests and domestic and foreign specifications. The established three-dimensional prediction model is suitable for the prediction of mechanical properties of waste ceramic aggregate concrete, which improves the accuracy of its two-dimensional model to a certain extent. The results provide some reference for the design and preparation of waste ceramic aggregate concrete, and provide some new ideas for the development of green concrete.

Silicon nitride ceramics were sintered by pressureless sintering with α-Si3N4 powder as raw material, and Y2O3 combined with MgAl2O4 as sintering aids. The effects of sintering temperature, holding time, content of sintering aids and composition ratio on the densification and mechanical properties of the silicon nitride ceramics were studied. The results show that silicon nitride ceramics can be fully densified by the addition of Y2O3 and MgAl2O4 sintering aids. Using Y2O3 and MgAl2O4 as sintering aids system, when the sintering temperature is 1 600 ℃, the holding time is 4 h, the content of sintering aids is 12.5% (mass fraction), and the mass ratio of Y2O3 and MgAl2O4 is 1∶1, the pressureless sintered silicon nitride ceramics achieve the best comprehensive properties with apparent porosity of 0.21%, the relative density of 98.10%, the bending strength of 598 MPa and Vickers hardness of 15.55 GPa.

The agglomeration state of nano-powders affects the microstructure of ceramic nano composites, and hence its optical and mechanical properties. In this paper, Y2O3-MgO nano-powder was synthesized by sol-gel method. Y2O3-MgO nano composites with excellent optical and mechanical properties were prepared by using hot pressing sintering (HP). The effects of the molar ratio of metal ions to citric acid (m/c) in the precursor on the agglomeration state of nano-powders and the optical and mechanical properties of the nano composites were studied. For the molar ratio of metal ions to citric acid of 0.75, the agglomeration state of the powder is the lowest. Y2O3-MgO nano composites prepared by hot-pressing sintering have an uniform phase domain, an average grain size of 140 nm, and an in-line transmittance of 80% within 3~6 μm wavelength range. Vickers hardness, fracture toughness and bending strength are 10.90 GPa, 2.21 MPa·m-1/2 and 226 MPa, respectively.

Defects, such as crystal vacancy and impurity oxygen, introduced during preparing processes of the ceramic seriously damage the thermal conductivity of silicon nitride. To explore the effect of defects on the material, various silicon nitride models with different defect states were designed and studied by molecular dynamics simulation method with considering the ratio, distribution, and position of defects, as well as the temperature. Simulation results show that the thermal conductivity of silicon nitride decreases with the increase of defects and the temperature. Typically, the thermal conductivity of silicon nitride decreases obviously in cases those defects are concentrated into a block that lies across the flux pathway in the material. The position of the defects present in the crystal lattice of silicon nitride also shows a significant effect on the thermal conductivity. Such variation of defect ratio and temperature on thermal conductivity is verified by calculating the phonon density of states of the silicon nitride models. The research results provide important guidance for the preparation of silicon nitride ceramics with high thermal conductivity.

Steel structures are widely used in the field of engineering building because of its high toughness, high strength, easy processing and other advantages. However, steel structures will lose their mechanical properties under high-temperature environments, causing structural failure. Therefore, fire protection becomes crucial to the safety of steel structures. In this study, using metakaolin, slag and hydrophobic-treated expanded perlite as main raw materials, and potassium water glass with modulus of 1.5 as activator, a non-expanded potassium-based a geopolymer fire resistance coating was prepared. The fire resistance performance of coating was characterized by the big panel method under 1 200 ℃. At the same time, the mechanical properties, apparent morphology, phase composition, and microstructure evolution of the geopolymers before and after heat treatment at room temperature, 1 000 ℃ and 1 100 ℃ were characterized and analyzed, and the ceramicization process of geopolymers at high temperature was explored. The results show that the geopolymer coating has great fire resistance properties. The temperature on back of the steel plate is lower than 160 ℃ after exposure to 1 200 ℃ for 2 hours. The compressive strength of coating increases to 30.80 MPa after heat treatment at 1 100 ℃ for 2 h, which is 5.8 times of that under room temperature. The amorphous geopolymer phases of fire retardant coating starts ceramization at 800 ℃ and the main crystalline phases after ceramization at 1 100 ℃ are anorthite, leucite and mullite.

SiO2 aerogel thermal insulation composites have been widely used in aerospace, petrochemical and other thermal insulation fields, and their application can be significantly expanded through hydrophobic modification. In order to maintain good hydrophobic properties of SiO2 aerogel thermal insulation composites at high temperature, polysiloxane modified silicate coatings were used to modify the surface hydrophobicity of SiO2 aerogel thermal insulation composites by brushing. The effect of coating thickness on crack expansion was then investigated, as well as the failure mechanism of hydrophobic properties of the coating at high temperature and the wear resistance of the composite before and after brushing coating modification. The results show that the surface of the coating is crack-free and the contact angle reaches (113±2)° when the coating thickness is about 13 μm. The contact angle still is maintained at 105° after heat treatment at 450 ℃ for 1 800 s, showing good thermal stability. Meanwhile, the coating significantly improves the wear resistance of composites.

In this study, the nano-Fe3O4/saponite composites were prepared by hydrothermal synthesis and coprecipitation method, and used to remove 2,4-dichlorophenol (2,4-DCP) from water. The phase composition, structure, morphology and elemental composition of nano-Fe3O4/saponite composites were characterized by X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray fluorescence (XRF). The removal efficiency of 2,4-DCP by nano-Fe3O4/saponite composites under solution with different 2,4-DCP concentration, reaction time and pH were investigated. The results show that the in-situ growth of nano-Fe3O4 on the surface of saponite is more evenly dispersed, which effectively reduces the agglomeration of Fe3O4. The maximum removal of 2,4-DCP by nano-Fe3O4/saponite composites is 178.46 mg/g when the pH value is 6, and the removal of 2,4-DCP reaches saturation when the reaction time is 5 min. The removal kinetics of 2,4-DCP by nano-Fe3O4/saponite composites accords with the pseudo-second-order kinetic model. On this basis, liquid chromatography-mass spectrometry (LC-MS) analysis shows that 2,4-DCP is degraded into chlorophenol and other intermediate products. The research results provide a new, environmentally-friendly and efficient environmental functional material for the degradation of organic pollutants in water.

The thermal insulation material fabricated by sodium silicate has the advantages of lightweight and low thermal conductivity, however, its poor water resistance needs to be improved. In this paper, the thermal insulation materials were fabricated by an intermediate-temperature sintering method with boric acid as a modifier, and the effects of boric acid dosage on the microstructure and properties of the materials were investigated. The results show that boric acid can adjust the frame structure of Si-O tetrahedron and effectively reduce hydroxyl groups in the thermal insulation material. In addition, it improves the softening coefficient of the thermal insulation material, and inhibits the leaching of silicate ions and sodium ions in the solution, which improves the water resistance of the material. When boric acid content is 1.00% (mass fraction), the softening coefficient increases from 0.519 to 0.701, increasing by 35%. At the same time, boric acid can make the pore size distribution of the thermal insulation material more uniform, and improve its compressive strength. But the thermal conductivity and apparent density of the thermal insulation material will increase slightly. The thermal conductivity, apparent density and compressive strength of the thermal insulation material modified by 0.75% boric acid are 0.052 W/(m·K), 128 kg/m3 and 0.442 MPa, respectively. And it satisfies the performance requirements of inorganic thermal insulation materials.

To alleviate the shortage of petroleum asphalt and explore the feasibility of microalgae oil for asphalt modification and the long-term performance of modified asphalt, microalgae oil was obtained by degradation, centrifugation and extraction, and modified asphalt was prepared. The content of microalgae oil was determined by testing the ductility, softening point and viscosity of modified asphalt under different microalgae oil content. Through high and low temperature rheological test and mixture road performance test, the performance changes of microalgae oil modified asphalt after rolling thin film oven test (RTFOT) short-term aging, pressurized aging vessel (PAV) long-term aging and UV aging were analyzed and compared with SBS modified asphalt. The molecular structure and composition of microalgae oil modified asphalt were analyzed by infrared spectroscopy. The results show that when the content of microalgae oil is 30% (mass fraction of admixture), the ductility of modified asphalt reaches the maximum, and the softening point and viscosity meet the requirements of modified asphalt. After RTFOT short-term aging, the performance difference between microalgae oil modified asphalt and SBS modified asphalt is not significant. The resistances of PAV long-term aging and UV aging of microalgae oil modified asphalt are better than SBS modified asphalt, especially UV aging resistance. Infrared analysis shows that the two modified asphalts contain similar components such as a vinyl double bond, aromatic C-H, methyl and methylene, but the contents of aromatic C-H and telescopic C-C are different. Compared with SBS modified asphalt, microalgae oil modified asphalt increases amide unsaturated group and carboxyl group, which is conducive to the formation of network molecular structure of modified asphalt.