Slag, fly ash and silica fume were used as reactive aluminosilicate materials to prepare high-strength geopolymer. The influence of silica fume on the setting and hardening of geopolymer was particularly investigated based on the optimization of solid raw materials and activator compositions by a simplex centroid design method. The results show that silica fume can be used as a filler in the packing void between slag and fly ash particles, thus increasing the liquid film thickness due to the release of activator from the voids. This reduces the thixotropy of the paste as well as yield stress and plastic viscosity, thereby improving the flowability of geopolymer. A rapid dissolution of silica fume in a highly-alkaline activator increases the concentration of silicates and decreases alkalinity, leading to deceleration of the reactions of slag and fly ash and the formation of C-A-S-H gels with the prolongation of setting, the retardation of acceleration period, and the decrease of peak reaction heat rate and cumulative reaction heat. This affects the early reaction products and compressive strength development of geopolymer. The compressive strength of the resultant geopolymer reaches higher than 100 MPa with a good workability and reasonable setting time when slag content is more than 50%, fly ash is less than 30% and silica fume is less than 30%, indicating a possibility of producing geopolymer-based UHPC.

The development behavior of shrinkage restraint stress on ultra-high performance concrete (UHPC) surface of steel bridge deck was investigated by theoretical method and restrained shrinkage experiment. The results show that steel bridge deck plays a leading role in restraining shrinkage deformation of UHPC and its restrained degree reaches to 0.41. An exponential growth trend occurs for reinforcement ratio regarding the restrained degree. Also, the restrained degree is linearly correlated to the generated stress in UHPC. The increment of the generated stress induced by reinforcement decreases as reinforcement ratio increases. In addition, a shrinkage value below 300×10^-6 and feature of strain harideing for UHPC applying on bridge deck surface for cracking control were also proposed. A dense reinforcement (≥3%) can have a greater cracking risk in UHPC and equivalent stiffness in structure.

In order to reduce or compensate the autogenous shrinkage of ultra-high performance concrete (UHPC), effects of expansive agent, super-absorbent polymer (SAP), expansive agent (EA) and SAP on the slump flow, setting time, strength and autogenous shrinkage of UHPC were investigated. The results show that EA can reduce the slump flow and shorten the setting time of UHPC, the addition of SAP increases the slump flow and the setting time of UHPC with EA. The EA addition of 4% can slightly increase the compressive and flexural strength of UHPC, the EA addition of 8% can decrease the compressive and flexural strength of UHPC, and the addition of SAP increases the compressive and flexural strength of UHPC with the EA addition of 8%. The autogenous shrinkage of UHPC is reduced by 34.4% at the EA addition of 4%, and the autogenous shrinkage of UHPC is compensated completely with a volume instability at the EA addition of 8%. The SAP addition can further reduce the autogenous shrinkage of UHPC at the EA addition of 4% and increase the volume stability of UHPC at the EA addition of 8%. It is thus indicated that the combined addition of EA and SAP could be an ideal approach to reduce the autogenous shrinkage of UHPC.

To improve the workability of ultra-high performance concrete (UHPC), the effect of superplasticizers (i.e., carboxylic-superplasticizer and phosphonic-superplasticsizer) on the rheological properties of cement/silica fume pastes with a low water-binder ratio was investigated, and the adsorption behavior of the superplasticizers in paste, the packing density and the viscosity of interstitial solution were analyzed. The results show that the superplasticizers have a preferential adsorption behavior on different particle surfaces, and carboxylic-superplasticizer has a superior adsorption ability on cement particle surfaces, and phosphonic-superplasticsizer has a stronger adsorption ability on silica fume particle surfaces. It is indicated that for a cement paste with the similar flow spread, a lower viscosity of the interstitial solutions is the main reason for a lower viscosity of paste with carboxylic-superplasticizer rather than that with phosphonic-superplasticsizer. However, the well- dispersion of silica fume particles with phosphonic-superplasticizer can result in an increased packing density, thus leading to a lower viscosity of cement-silica paste, compared to that of paste with carboxylic-superplasticizer.

A rehydration test of ultra-high performance concrete (UHPC) matrix was conducted to measure the chemically bound water contents, specimen expansion ratios, and compressive strengths of hardened cement pastes at different rehydration time, and the rehydration model was proposed based on the Krstulović-Dabić hydration dynamics of cement and microstructure information of cement hydration. The mechanism regarding the rehydration effect on the water stability was discussed, and the microscopic appearance change with pore structure change was analyzed based on the proposed model. The results show that the bound water content of rehydration increases during the early period but decreases during the late period as the water-cement ratio decreases from 0.30 to 0.15. The specimen expansion ratio increases as the water-cement ratio decreases during the rehydration process. The compressive strength of hardened cement pastes at different water-cement ratios has an alternating tendency of increasing and decreasing with the increase of rehydration time. The simulated data by the model are in good agreement with the experimental results, indicating that the proposed model can accurately simulate the rehydration process of UHPC matrix. The increased scope of compressive strength of hardened cement paste with the water-cement ratio of 0.30 is greater due to its higher hydration rate of cement. During the early period of rehydration, a hydration rate of cement becomes greater, and the rehydration products gradually fill in the initial pores of hardened cement paste. However, during the late period of rehydration, a hydration rate of cement is lower, and the rehydration products cause some microcracks of hardened cement paste due to its volume expansion, resulting in the degradation of properties of UHPC matrix.

The bending properties of ultra-high performance concrete (UHPC) at ambient and low temperatures (i.e., 20 ℃ and-20 ℃) were investigated. The results show that the strength and elastic modulus of UHPC increase with decreasing the temperature, and the strength increases more significantly than the elastic modulus. Compared to the corresponding results at 20 ℃, the cubic compressive strength, the prism compressive strength, the splitting tensile strength, the flexural cracking strength, the flexural limit strength and the elastic modulus of UHPC at -20 ℃ are increased by 7.3%, 8.0%, 10.8%, 13.7%, 9.0% and 4.1%, respectively. The flexural deformability and ductility of UHPC decrease with decreasing the temperature. Compared to the corresponding results at 20 ℃, even though the initial crack strain of UHPC at -20 ℃ is increased by 6.3%, resulting from that the flexural cracking strength of UHPC increases more significantly than its elastic modulus, and the peak strain, limit strain, residual strain and ductility index of UHPC at -20 ℃ are decreased by 2.9%, 8.2%, 5.5% and 6.4%, respectively.

The flexural strength of ultra-high performance concrete prism specimens with three kinds of strength grades, two types of steel fibers and four groups of fiber contents was determined to investigate the effects of strength grade, steel fiber and fiber content on the flexural strength and its size effect of ultra-high performance concrete. The results show that the size effect of the flexural strength becomes more obvious as the strength grade increases. The scale effect of the flexural strength of the specimen with a strength grade of R160 is 1.26 times greater than that of the specimen with a strength grade of R120. The content of steel fiber has an effect on the size effect of the flexural strength. The larger the content of steel fiber is, the more pronounced the size effect will be. The flexural strength of specimens with straight fiber and hook fiber and strength grade of R120 at a fiber content 3% (volume fraction) is 71% and 78% greater than that of the specimens without steel fiber reinforcement, respectively. The size conversion factor for the flexural strength of ultra-high performance concrete with different strength grades was given. The calculation formulas for the size effect law of the flexural strength of ultra-high performance concrete were proposed.

Effects of coarse aggregate content and particle size on mechanical properties of ultra-high performance concrete (UHPC) with river sand as a coarse aggregate replacing quartz sand were investigated. The results show that the cube compressive strength and axial compressive strength of UHPC matrix are obtained at the coarse aggregate filling content of less than 400 kg/m³. The cube compressive strength and axial compressive strength decrease when the filling content of coarse aggregate further increases. The increase of particle size of coarse aggregate has a strengthening effect on the cube compressive strength and axial compressive strength of UHPC matrix, while the particle size of coarse aggregate has little effect on the UHPC flexural and splitting tensile strength. The UHPC initial crack strength, peak strength, initial crack deflection, peak deflection and energy dissipation decrease slowly with the increase of coarse aggregate filling content. The strength and energy dissipation in each stage decrease with the increase of particle size of coarse aggregate, while the initial crack deflection and peak deflection have a slow increasing trend with the increase of particle size of coarse aggregate, and the toughness index decreases gradually with the increase of aggregate filling content and particle size.

Fine and short steel fiber was orientally distributed in the paste to prepare a ultra-high performance concrete (UHPC). The compressive strength, flexural strength, flexural tensile strength, and the interfacial bonding strength between steel fiber and UHPC matrix of the directionally distributed steel fiber UHPC (D-UHPC) and the disorderly distributed steel fiber UHPC (L-UHPC) at different volume contents of steel fiber were analyzed. The results demonstrate that the compressive strength of L-UHPC firstly increases and then decreases, and the compressive strength of D-UHPC continues to increase as the volume content of steel fiber increases. The flexural strength of both types of UHPCs improves with the increase of the volume content of steel fiber. At the same volume content of steel fiber, the flexural strength of D-UHPC is greater than that of L-UHPC. In the tensile fracture of the specimen, the crack width produced by D-UHPC is narrower than that of L-UHPC, and D-UHPC has more small cracks by dispersing and absorbing loads. The initial cracking deflection, ultimate tensile deflection, initial cracking strength, ultimate tensile strength, and toughness index of D-UHPC all increase, compared to that of L-UHPC at the same volume content of steel fiber. Increasing the embedding angle of the steel fiber decreases the peak value of the pull-out load and increases the deflection corresponding to the peak value of the load, thus indicating that the direction of the steel fiber affects the mechanical properties of UHPC.

A whole coral sand concrete was prepared via mixing coral gravel, coral sand with seawater, cement and water reducer. The coral damping ratio with the deformation and failure process of concrete evolution curve was investigated by upper limit stress, lower limit stress at different loading frequencies and sine wave cycle and unloading tests. The load waveform parameters on the coral damping characteristics of concrete law and the mechanism were also analyzed. The results show that the damping ratio has the characteristics of initial attenuation, steady state and acceleration. The average damping ratio of coral concrete is 3.9%-5.9%, which is greater than that of ordinary concrete. Increasing the upper limit stress, decreasing the lower limit stress or increasing the loading frequency will increase the damping ratio of coral concrete, and the damping ratio is the most sensitive to the change of the upper limit stress rather than the change of the lower limit stress, and the lowest sensitive to the change of frequency.

Effects of scoured staghorn coral, light coral reef and heavy coral reef as coarse aggregates on the mechanical properties of coral concrete were investigated. The results show that staghorn aggregates have the most significant effect on the compressive strength of coral concrete cubes and prisms rather than heavy coral reef and lightweight coral reef aggregates. The compressive strength of coral concrete at 90 d is positively correlated to that at 28 d. The compressive strength of prisms increases linearly with the compressive strength of the cube, regardless of the aggregate type. The peak strain of coral concrete prisms increases logarithmically with the increase of peak stress, and the stress-strain curve equation and the compressive failure morphology of the prisms are not affected by coral aggregate type.

Air-void structure characteristics, water penetration resistance and freeze-thaw resistance of hardened shotcrete with different spraying methods were evaluated. Also, the micro-characteristics of freeze-thaw damage zone in shotcrete were analyzed. The results show that circle-like and polygon-like air bubbles with the diameter of < 40 μm in shotcrete prepared with wet-mix spraying increase, which behave less independently with adjacent air bubbles, thus leading to a decreased later mechanical strength. However, shotcrete prepared with dry-mix spraying has poor mechanical strength and water penetration resistance, compared to that with wet-mix spraying, due to its high proportion of air bubbles with the diameter of > 280 μm. For 150-175 freeze-thaw times in a quick freezing method, the compressive strength loss ratio of shotcrete reaches nearly 40%-48%, which is worse than that of ordinary concrete with the similar water-cement ratio, but better than that with the similar 28-d compressive strength. The less independent air bubbles in wet-mix shotcrete connect each other due to the hydrostatic pressure under a freezing heave effect, thus leading to the microstructure damage in shotcrete with a poor freeze-thaw resistance.

Wire beam electrode (WBE) was used to investigate the corrosion behavior of rebar for mortar with cracks of 0.1-0.3 mm, and the distribution of galvanic current and EIS on WBE were tested. Also, a reinforced concrete with the similar cracks was also exposed to marine splash zone for 700 d, the Nyquist figures were examined and the corrosion current density evolution was calculated. The results indicate that the passivation time of rebar protected by non-cracking concrete is 10-17 d, and the existence of cracks in concrete accelerates the oxygen migration and promotes the passivation of rebar. The passivation and corrosion of rebar for concrete is an uneven process from pit to surface. The cathode zone is formed near the cracks firstly because oxygen can migrate from cracks to rebar surface. Chloride ions transport from cracks to rebar surface, forming the massive anode zones. For the transverse crack size of less than 0.1 mm, the micro-cracks of concrete will be self-healing and decrease oxygen supply when concrete is exposed to marine tidal and splash zone for a long term, and the anode zones near cracks will transfer to cathode zones and delay corrosion of rebar. For the transverse crack size of more than 0.3 mm, the corrosion of rebar in crack zones will occur quickly and extend to both sides of crack, and the corrosion rate of rebar in concrete is 0.484 μm/a when it exposed to splash zone.

A castable high-performance carbonatable concrete (HPCC) was designed based on the optimized packing design of the material system. In HPCC, a steel slag powder was the main binder, a small amount of hydraulic ordinary Portland cement was used to provide the mould-stripping strength and silica fume was added to improve the compactness of the system. Effect of carbonation curing on the mechanical properties and microstructure of HPCC was investigated. The results show that the compressive strength of HPCC after 1-d seal curing is 5.2 MPa, and the compressive strength of HPCC after pre-drying and 24-h carbonation curing reaches almost 90 MPa. A proper pre-drying is beneficial to the carbonation reaction, and the optimal moisture loss rate is 74.4%. Calcium carbonate crystals formed due to carbonation reaction are mainly calcite with a trace amount of aragonite. This leads to a further densification of the matrix, which accounts for the high strength of HPCC. In addition, the soundness of HPCC is also improved after carbonation.

In order to clarify the relationship between the permeability of cement-based material and its pore structure, several mortars at different water to cement ratios and curing temperatures were investigated through the measurements of their pore structures by the low-field nuclear magnetic resonance (LF-NMR) and mercury intrusion porosimetry (MIP). Their permeability coefficients to water were also measured based on the Darcy law. The results show that the pore size distribution of water-saturated mortar is obviously different from that at totally dry state, whose critical diameter is larger by roughly 1 order of magnitude. This great gap is rooted to the essential water permeability for hydrated calcium silicate (C-S-H) and thus cement-based material, making their nanoscale pore structure highly depend on water contents. Furthermore, the total porosity and critical pore size both remarkably increase after curing in hot water of 80 ℃ for 56 d, which therefore increases the water permeability. These effects are brought by the chemical aging of C-S-H gels, which harms the durability performance of cement-based material. When investigating the evolution of pore structure through common MIP, the effect of drying preconditioning on pore structure should be taken into account because it is much greater than the effect of chemical aging at elevated temperature. If treating all the pores as bundles of capillary tubes, the classical Kozeny-Carman theory is able to predict the water permeability of mortars according to the pore structure achieved through LF-NMR at water-saturated state. The relative prediction error of the Kozeny-Carman theory with a linear coefficient of 1.09 is as low as within [-42.7%, 71.1%], which is similar to the experimental error of extremely low water permeability.

Immersion solutions at different salt ratios and concentrations were prepared based on the seawater salt composition in Shenzhen ports, China. The effect of immersion solution on the deterioration of the mortar and the chloride diffusion coefficient were investigated. The results show that the influence of sulfate ions on the diffusion of chloride ions is related to the salt concentration in multi-ion solutions. The diffusion of chloride ions is inhibited by high-concentrated sulfate and magnesium ions in the early stage and promoted in the later stage. This phenomenon is not consistent with the effect of multi-ion interactions in the marine environment. The low concentration of sulfate and magnesium ions in seawater does not inhibit and even slightly promote the diffusion of chloride ions in the mortar.

The influence of adding crystalline silica and silica fume on the mineral composition and physical-mechanical properties of the set-cement was investigated. The results show that optimizing the particle size distribution of cement dry blend can improve the 7-d compressive strength. Increasing the total silica addition can enhance the 30-d compressive strength. At 30-d curing, two optimized systems have an average compressive strength of greater than 40 MPa and a water permeability of lower than 2×10^-6 μm². The physical, mechanical, mineral and microstructual property evolution of the two systems within 30 d were analyzed. It is indicated that there are little changes in the mineral compositions and their crystallinity during 7-30 d. However, apparent microstructure coarsening occurs, leading to the decline in compressive strength and modulus of elasticity as well as the almost linear increase in water permeability of the set cement as a function of time.

A series of cationic sacrificial agents (CSA) were synthesized via free radical copolymerization of unsaturated quaternary ammonium salt monomer and unsaturated nonionic (anion) monomer. The effect of sacrificial agent on the dispersion of polycarboxylate superplasticizer in cement mixed with montmorillonite (MMT) was evaluated via the fluidity of cement paste. Also, the effects of monomer polymerization ratio, quaternary ammonium salt monomer type and nonionic (anionic) monomer type on the action effect of the sacrificial agent were investigated by organic carbon analysis, thermogravimetry, X-ray diffraction, and scanning electron microscopy. Trimethyl allyl ammonium bromide-acrylamide (ATMA-AM 2∶1) was determined as an optimum structure, and its action mechanism was also analyzed. The fluidity of PCE in cement paste mixed with MMT increases from 156 mm to 225 mm at 1.0% of ATMA-AM (2∶1). The adsorption capacity of PCE on MMT and the holes on the surface of MMT all reduce, thus effectively preventing the polyether side chain of PCE molecule from entering the interlayer of MMT and enhancing MMT effect on the dispersion property of PCE.

The hardened cement mortar samples with different water-cement ratios were prepared and subjected to salt freezing experiment (CDF freeze-thaw). The microscopic images and three-dimensional pore structure of hardened cement mortar with different freeze-thaw cycles were investigated by X-ray tomography. The evolution process and mechanism of meso-structure in freezing-thawing cycle were analyzed. The variation law of pore parameters of hardened cement mortar under the action of CDF freeze-thaw cycle and the influence of water-cement ratio on the freeze-resistance of mortar sample were investigated. The results show that the freezing-thawing microcracks develop along the porous interface between cement aggregate and fine aggregate, and the reduction of water-cement ratio can reduce the interfacial porosity, thus improving the frost resistance of cement-based materials. The cement mortar with a water-cement ratio of 0.30 has a stronger frost resistance. The freeze-thaw damage evolution and crushing process of cement mortar can be characterized by fractal dimension curve.

Improving the dispersion of polycarboxylate superplasticizer (PCE) in cementitious materials containing clays has attracted recent research attentions. Using novel technologies to investigate the interaction between PCE and clay has made a significant breakthrough in recent years. A variety of approaches to improve the dispersibility of PCE in clay-containing cement paste were also emerged. This review summarized recent work on the improvement of clay tolerance of PCE, including the interaction mechanism between PCE and montmorillonite, the preparation of clay-tolerance PCEs, and clay-tolerance sacrificial agents. In addition, the future research direction in this aspect was also discussed.

Ultra-long MnOOH nanowire was prepared by a simple hydrothermal process using KMnO₄ and ammonium acetate without additional template agents. MnOOH sample was characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, amperometry, constant current charge/discharge characterization and electrochemical impedance spectroscopy. The results show that the length of MnOOH nanowires is more than 10 μm, and the diameter is approximately 20 nm. The specific capacitance is 285 F/g at current density of 1 A/g in 1.0 mol/L Na₂SO₄ electrolyte, and it can still keep a good cycling stability after 4 000 cycles. The post-capacitance retention rate reaches 96.2%. The MnOOH nanowire as an electrode material has an excellent electron transmission channel, exhibiting a superior ultracapacitor performance, which can have broad application prospects as an electrode material for supercapacitors.