Lightweight and high ductility cementitious composite was developed as a fire-resistive coating (FR-ECC) with domestic raw materials. The feasibility of FR-ECC as fire-resistive coating was demonstrated. The mechanical properties of FR-ECC with different proportions were investigated via axial tensile test and compressive test. The cooperative deformation performance of FR-ECC and steel member was determined via four-point bending beam test. The results indicate that FR-ECC with three proportions has the characteristics of multi-cracking and strain-hardening with an average tensile strength of 1.30 MPa and an average strain capacity of 1.93%. The results of compressive test show a high compressive deformation capacity of FR-ECC with an average strength of 3.99 MPa. The tensile and compressive properties can be enhanced by increasing the fiber content. According to the results of the four-point bending beam test, FR-ECC can adapt to a larger bending deformation as the coating thickness decreases. When the coating thickness is no more than 10 mm, FR-ECC can produce some fine cracks with the deformation of the steel beam instead of delaminating or falling off. FR-ECC has a capability of cooperative deformation with steel member under a complex stress.

The orientation of steel fibers has an effect on the mechanical properties of steel fiber reinforced concrete (SFRC). In elements or structures such as slabs, floors and pavements, vertical steel fibers are useless but harmful to the structures. In this case, the optimal distribution of steel fibers is two-dimensional (no vertical, but random in the plane) dispersion. In this paper, the steel fibers were aligned using a rotating uniform magnetic field, and the 2D aligned steel fiber reinforced concrete (2D-SFRC) was prepared to enhance the mechanical properties of SFRC and the efficiency of reinforcement of steel fibers. The splitting tensile performance, flexural behavior of prism and circular plate specimens of 2D-SFRC were tested and compared with those of the specimens with randomly distributed steel fiber reinforced concrete (RD-SFRC). Besides, the mechanism of reinforcement of 2D-SFRC was investigated from the perspective of fiber distribution characteristics and acoustic emission signals. The results show that the mechanical properties of 2D-SFRC are significantly improved, compared with RD-SFRC. The fiber distribution statistics and acoustic emission analysis indicate that the increased amount of steel fibers in the fracture surface and enhanced fiber orientation effective factor are the main reasons for the improved mechanical properties of 2D-SFRC. The 2D-SFRC has the characteristics of high performance and low cost, which is of great significance for improving the engineering quality and reduce the materials consumption.

To provide a theoretical reference for reducing the shrinkage and cracking of concrete and promoting the development and application of the shrinkage-reducing polycarboxylate superplasticizer, a shrinkage-reducing polycarboxylate superplasticizer (SR-PCE) with dispersion and shrinkage reducing properties was synthesized via free radical polymerization with isopentenyl polyoxyethylene ether (IPEG), acrylic acid (AA), butyl acrylate (BA) and other monomers. The molecular structure of SR-PCE was characterized by gel permeation chromatography and Fourier transform infrared spectroscopy, and its shrinkage reduction mechanism was analyzed. The results show that the molecular structure of SR-PCE is a designed structure. The optimum shrinkage reducing performance of the synthesized PCE can be obtained at a molar ratio of IPEG∶AA∶BA of 1∶5∶4. The mechanism of SR-PCE with the optimum shrinkage reducing performance mainly is since SR-PCE can further reduce the surface tension of the solution rather than ordinary PCE, SR-PCE can reduce the evaporation rate of the pore solution of cement paste and improve the pore water retention.

The isothermal exposure tests for 270 d at different air pressure (i.e., 101 kPa, 60 kPa, and 20 kPa) and relative humidities (i.e., 43%, and 98%) were carried out for hardened cement pastes. Effect of air pressure on the moisture content and the chemically bound water content in hardened cement pastes was investigated. The moisture transfer characteristics at different low air pressures were analyzed by a proposed numerical model. The results show that a low air pressure promotes the water loss within the hardened cement pastes during the isothermal drying process, resulting in a slight decrease in the chemically bound water content. Under the isothermal wetting condition, the mass of hardened cement paste grows at a logarithmic rule due to the moisture diffusion into the pore network, the decrease of air pressure significantly improve the early-stage mass growth rate and the final mass gain. The numerical approach on the moisture transfer indicates that the increases of water vapor diffusion coefficient, mass transfer coefficient, and intrinsic permeability of hardened cement paste are the main causes of moisture transfer behavior during the isothermal drying and wetting processes.

Acrylamide was selected as a monomer to generate polyacrylamide network in 3D printing mortars via in-situ polymerization, and the effect of in-situ polymerization on the mechanical properties of 3D printing mortars was investigated. The influence mechanism was analyzed by Fourier transform infrared spectroscopy, scanning electron microscopy, mercury intrusion porosimetry, and X-ray computed tomography. The results show that the in-situ polymerization of AM can enhance the fluidity of 3D printing mortars, reduce the generation of macroscopic defects, and improve the printing quality. After AM monomers are polymerized, a "rigid-flexible" composite network structure is formed, in which the organic network and the cement hydration products are intertwined. This structure greatly enhances the flexural strength and relative bonding strength of the 3D printed specimen. The 28 d flexural strength and the relative interfacial bonding strength can be increased by 52.4% and 78.1%, respectively.

To further understand the influence of climate environment on the microstructure of concrete materials in northwestern China, the microstructure characteristics of concrete under different temperature and humidity curing conditions were investigated by a nano-indentation technology. The results show that the lack of curing temperature and humidity leads to the increase of capillary pores and low-density calcium silicate hydrate (C-S-H) gel content in cement mortar. The indentation modulus and hardness of concrete interfacial transition zone (ITZ) under different curing conditions were statistically analyzed. It is indicated that the average thickness of concrete ITZ is increased by 5 μm and 10 μm when the curing conditions are 10 ℃-70% relative humidity (RH) and 3 ℃-50% RH, respectively, compared with that the curing condition is 20 ℃-95% RH.

At present, the compatibility of cement and flash setting admixture often appears in the application of accelerator, and the evaluation of the compatibility of cement and flash setting admixture and its influencing factors becomes a necessity. In this work, the macro-performance and early hydration heat of different cement-accelerator systems were analyzed. The macro-performance and micro-indicators were organically combined to calculate and compare the compatibility of different cement-accelerator systems by a comprehensive method for quantitative evaluation of compatibility, i.e., the multi-factor comprehensive index (f(a) value) evaluation method. The results show that the alkaline flash setting admixture has an optimum compatibility for the Reference cement with the maximum transition phase content, and the maximum f(a) value is 74. Alkali-free flash setting admixture has an optimum compatibility for the Jidong cement with the maximum C₃A and C₃S contents, and the f(a) values are greater than 80.

Calcium silicate hydrate (C-S-H) gel is a gene of cement-based cementitious materials, and its structural changes are crucial to the development of concrete macroscopic properties. To clarify the modification mechanism of cellulose nanocrystals (CNCs) in cement, C-S-H gels were prepared by a co-precipitation method, and the morphological and nucleation effects of CNCs on the microstructure of C-S-H gels were investigated by X-ray diffraction (XRD), transmission electron microscopy, nanoindentation, and nuclear magnetic resonance. The results show that the hydroxyl groups (—OH) on the surface of CNCs can complex Ca²⁺ ions, and then react with SiO₄^2- ions in solution to form C-S-H gels that wraps around the CNCs. The CNCs can provide additional nucleation sites for the precipitation and growth of C-S-H gels, promote the reduction of the polymerization degree and the shortening of the chain length of the C-S-H gel, and increase the content of high-density C-S-H gel.

The massive waste carbon fibers in modern industry need to be recycled and reused. In this work, the effect of recycled carbon fibers (RCF) on the fluidity, mechanical property and electrical conductivity of high-strength cement-based material (HSCBM) was investigated using waste carbon fibers with the HSCBM. The results indicate that the incorporation of RCF reduces the fluidity of HSCBM, while the fluidity can be increased by 7.74% if RCF is modified by NaOH solution and HNO₃ solution. Incorporating unmodified and modified RCF at different water-to-binder ratios can reduce the compressive strength of HSCBM. However, the flexural strength of HSCBM after modification of RCF with NaOH solution and HNO₃ solution is improved, and the flexural strength of HSCBM can be increased by 30.61% and 24.46%, respectively. The RCF/HSCBM has no conductivity when the water-binder ratio is rather low (i.e., 0.20). When the water-binder ratio increases to 0.25, adding RCF can improve the conductivity of HSCBM, but its strength greatly reduces. After RCF is treated with NaOH solution and HNO₃ solution, the conductivity of HSCBM is further improved. The 7 d and 28 d resistivity can be decreased by 81.41%-82.23% and 94.88%-96.15%, respectively, when the RCF amount reaches a percolation threshold (i.e., 0.6%). The research results can provide a new idea for the functional properties of HSCBM and RCF recycling.

The ultra-high performance concrete materials (UHPC) are developed rapidly and are widely used in bridge engineering and other fields. The corresponding advantages involve the reducing dead weight, simplifying reinforcement details, shortening construction period, and improving durability. The comprehensive economic and technical benefits are achieved. This review represented recent development on the UHPC building structures. The characteristics of UHPC structure and the current engineering application were summarized, and the basic design principles and methods were analyzed. Recent studies on the UHPC building structural components and joints were introduced, including lightweight thermal insulation sandwich roof panels, sandwich floor/wall panel-based prefabricated structures, two-way waffle slabs, post-poured beam-column joint in precast reinforced concrete frame, prefabricated frames based on the UHPC connections, etc.. Some methods and benefits of UHPC application in building structures were concluded. In addition, the future research aspects of UHPC building structure were also given to promote the application of UHPC in building structure and improve its comprehensive performance and quality.