In this study, the self-healing properties of mortars mixed with a crystalline admixture (CA) and superabsorbent polymer (SAP) were investigated. By conducting uniaxial compressive strength tests on the mortar samples, the effects of the two admixtures and different admixture ratios on the initial compressive strength and strength repair ability at different curing ages of the mortar after pre-cracking were investigated. To verify the results, optical microscopy, scanning electron microscopy, and X-ray diffraction were used for microscopic observation of the cracks and their healing products. The results of this study show that CA, which generates dense substances through chemical reactions, has obvious advantages in the self-healing of microcracks and has a greater effect on the flexural strength of mortar compared with SAP, which can effectively fill wider cracks, reduce the width of cracks through physical expansion, and has a greater impact on the compressive strength of mortar compared with CA. Compared with ordinary mortar, mortar mixed with CA only, and mortar mixed with SAP only, the appropriate amounts of both CA and SAP can effectively combine the advantageous effects of CA and SAP and optimise the self-healing effect of mortar so that its self-healing rate reaches 103%. The self-healing filler, consisting mainly of calcium silicate and calcium carbonate, is generated in cracks and enhances the repair strength of the mortar so that the strength of the mortar reaches 46 MPa.

To elucidate the mechanisms of microstructural changes in ultra-high-performance concrete (UHPC) under microwave exposure, this study characterizes the microstructure at multiple scales using a combination of microscopic experiments and molecular dynamics simulations. The hydration products, pore structure, morphology, and interface transition zone (ITZ) of UHPC specimens were analyzed using mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), and scanning electron microscopy (SEM). Molecular dynamics simulations were employed to investigate the uniaxial tensile behavior, free volume, and radial distribution of calcium silicate hydrate (C-S-H) gel, the primary hydration product. The results indicate that microwave curing significantly reduces the pore volume of specimens, with a daily average reduction of 0.15% in the early stages. This accelerated reduction in porosity effectively diminishes the number of high-risk pores. The hydration products formed under microwave curing exhibit higher density and enhanced internal pore optimization. Simulation findings suggest that the non-thermal effects of microwaves play a more significant role in the structural evolution. The molecular orientation of C-S-H changes after oscillation, leading to more ordered molecular arrangements. Mechanical oscillation also expels free volume from the crystal cells, promoting a more compact overall structure and increasing the tensile strength by up to 1 GPa.

Microbial-induced calcium carbonate precipitation (MICP) is an environmentally sustainable technology for the self-healing of concrete cracks. In this experiment, expanded perlite was used as a bacterial carrier. It can facilitate the adsorption of bacteria and serves as a self-healing agent when incorporated into cement mortar. Two different sizes of silica were simultaneously added to evaluate the combined effects of the self-healing agent and varying silica sizes on the mechanical and self-healing properties of cement mortar. The objective is to determine the optimal dosage for each silica size. The results show that the self-healing agent can improve the density and strength of the structure in the early stages of maintenance and realize the self-healing of microcracks after the cement mortar has cracked. Adding a single particle size of silica at the optimal dosage can effectively improve the strength of cement mortar. Adding two different sizes of silica can optimize the particle grading of silica. Nano-sized silica is beneficial to improving the early compressive strength of cement mortar, and micron-sized silica is beneficial to improving the self-healing efficiency of cement mortar and completely sealing cracks at an early stage. The interaction between silica particles of varying sizes can further enhance the mechanical and self-healing properties of cement mortar. Microscopic observations and tests provide effective support for these conclusions and elucidate the mechanism of interaction between the self-healing agents and silica.

This study investigates using low-temperature microwave radiation to prepare ultra-high-performance concrete (UHPC), aiming to contribute to sustainable and green concrete technology. Immediate microwave curing (IMC) accelerates the early hardening process, but rapid moisture loss challenges prolonged hydration and strength development. The research combines strength tests, X-ray diffraction, differential thermal analysis and thermogravimetric analysis, scanning electron microscopy with energy dispersive X-ray spectroscopy, and COMSOL simulations to understand these mechanisms. The findings reveal that microwaves' unique magnetic effects activate interactions between ions, leading to the formation of atypical phases such as hydrotalcite-like phase (HTlp), calcium silicide (CaSi₂), and thaumasite-like phase (TSAlp). Unlike delayed microwave curing (DMC), IMC produces crystalline phases such as ettringite, HTlp, TSAlp, and gypsum, while calcium hydroxide (CH)—commonly observed in Portland cement hydration—is absent, reflecting distinct hydration pathways. These results suggest that IMC alters the hydration process and microstructure of UHPC. Curing regimes must address moisture retention to ensure sufficient hydration for improved mechanical and microstructural properties to enhance its performance. This study highlights the potential of IMC as a rapid curing method while emphasizing the need for strategies to mitigate its limitations.