Microcapsules, as a new type of material, have attracted extensive attention due to their designability, easy dispersion in the matrix, ability to prolong the durability of the core material, and self-healing intelligence. This review provided a detailed introduction to the design and preparation of self-healing microcapsules, and summarized the physical and chemical repair mechanisms of microcapsules. The research progress on self-healing microcapsules in concrete was represented. The challenges and future development directions of microcapsules in core-shell preparation, cement-matrix relationship, and repair effect evaluation were analyzed. This review could provide a reference for the future production and application of microcapsules in concrete.

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.

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.

Pouring fresh mortar on the substrate is a common repair and reinforcement measure. The water exchange occurs when the moisture degree of the new mortar and the old mortar is different, thus changing the mixing ratio of the repair material, and affecting the hydration process, microscopic characteristics, permeability and bonding performance between the repair material and the old mortar. In this paper, the water exchange process among fresh ordinary mortar, strain hardening cementitious composites (SHCC), and old mortar with different moisture degrees (i.e., 0%, 30%, 70% and 100%) was investigated by a weighing method. The influences of the type of fresh repair mortar and the initial moisture degree of the old mortar on the rate of water exchange and the final water-binder ratio of the repair mortar were analyzed. The effect of the water exchange on the bond strength between the repair mortar and the old mortar, the microstructure of the repair mortar and the water absorption properties was investigated via the mechanical tests, low field nuclear magnetic resonance and weighing method. The results show that there is a good linear relationship between the mass of water absorbed per unit area of the unsaturated matrix and the square root of time in the early stage of water exchange. The shear strength between SHCC or ordinary mortar and substrate is maximum at 0% moisture degree of the substrate. The shear strength between SHCC as a repair material and the substrate is greater than that of ordinary mortar. The porosity, micropores volume fraction and capillary absorption coefficient of ordinary mortar gradually increase and the volume fraction of mesopores and large voids gradually decrease as the moisture of old mortar increases. The volume fraction of SHCC porosity, mesopores and large voids gradually decrease as the moisture degree of old mortar increases, and the volume fraction of micropores and capillary absorption coefficient change slightly.

The performance of polycarboxylate (PCE) superplasticizer in concrete with machine-made sand greatly depends on the adsorption behavior on stone powder, which is concerned with the lithology and specific surface area. The workability can be improved via enhancing the adsorption of PCE on stone powder. In this regard, the rheological behavior of cement paste and cement/microcline paste with conventional PCE superplasticizer and phosphate-containing PCE superplasticizer was tested. The incorporation of phosphate group can enhance the adsorption of PCE molecule on the surface of microcline, thus improving the packing density of cement/microcline mixture. The average particle separation distance increases. Therefore, the dispersing performance of PCE superplasticizer in the mixture is obviously improved. The apparent viscosity of paste is reduced. The performance of phosphate-containing superplasticizer is even better when incorporating high content of microcline.

The force transferring between fiber and matrix is closely related to the interfacial bonding properties of fiber-resin during the loading process of fiber reinforced polymer (FRP) bars. However, its bonding properties are degraded by the diffusion of chloride ions in the pore solution of seawater sea-sand concrete into the interfacial zone of fiber-resin. In this paper, a micro-debonding device was independently designed and combined with the digital image correlation (DIC) technique to track the degradation process of basalt/glass fiber-resin bonding property during the corrosion in chloride solution for 28 d. The results show that the bonding property between glass fiber and resin is better than that of basalt fiber. The interface bonding force of basalt/glass fibers is decreased by 73.9% and 71.8% after the corrosion of 28 d in NaCl solution. Matrix cracking and fiber pulling out are the main failure modes during the pull-out process. The formation of the hydrogen bond is dominated due to the bonding mode between resin and fiber matrix. However, the number of hydrogen bond atoms is reduced by the interaction between the chloride ions and the atoms at the matrix interface, thus accelerating the debonding between the fiber and resin.

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.

Cement-supplementary cementitious materials (SCM)/graphene oxide (GO) combine a lower carbon footprint with ductility enhancement. Aluminum phase released during the SCM hydration occurs in various forms in C-A-S-H/GO. The effects of bridging Al(Q²_b) and interlayer Alinter on the mechanical properties of C-A-S-H/GO were investigated based on the reactive molecular dynamics. It is indicated that Alinter with less than Al(Q²_b) redistributes the stress during the fracture of the system, activating load sharing between the layers, and thus triggering a "recovery-reinforcement" mechanism to multiply the mechanical properties. The "recovery-reinforcement" can be explained via analyzing Al's contribution to the reorganization of the interfacial bonding network and the chemical bonding evolution during fracture. The results show that the targeted regulation of Al chemical environment in cement-SCM/GO preparation is a key issue to break the ductility bottleneck.

Cement-based materials are highly hydrophilic, and aggressive substances in service environment often penetrate into the cement-based materials through the transmission of water, resulting in corrosion of steel bars, carbonization of concrete, and other hazards. In this paper, two types of microbe cells were applied to improve the hydrophobic properties of the mortar specimens. The results show that the contact angle of the mortar specimens increases to 122.7° and 112.1° after the addition of Starmerella bombicola CCRC22302 and Bacillus sphaericus LMG22257, respectively. The diffusion rate and absorption rate of water on the fracture surface are obviously slowed down. The capillary water absorption is decreased by 39% in the groups added with 2.5% (in mass) Starmerella bombicola by mass of cement. The bacterial cell improves the hydrophobicity through the combined action of the rough structure and its hydrophobicity. However, it is indicated that the flexural and compressive strengths of the mortar specimens are significantly decreased, thus becoming a challenge for the practical application.