Compared to the properties of ordinary concrete, the recycled coarse aggregate concrete exhibits the lower strength and poor durability due to many pores and micro-cracks inside the recycled coarse aggregates and the old mortar attached to the aggregate surface. However, the silica nanoparticles with a high pozzolanic activity, a crystalline nucleation effect and a filling effect plays an important role in the application of modified recycled coarse aggregate concrete. Recent work on the properties of recycled coarse aggregate concrete modified by silica nanoparticles was analyzed in terms of workability, mechanical properties, durability and interfacial structure, etc. The modification mechanism of silica nanoparticles to improve the performance of recycled coarse aggregate concrete was further discussed, thus providing some theoretical and technical supports for the application of silica nanoparticles in the performance improvement of recycled coarse aggregate concrete.

Alkali-activated concrete (AAC) prepared without cement with the performance of rapid hardening, high strength and good durability, is recognized as one of novel green building materials with a wide application potential. Chloride ingress into concrete to cause the process of steel corrosion is one of the main reasons to the durability failure of reinforced concrete structures, particularly exposed to marine cyclic drying–wetting zones. It is thus of great significance for the development of durability theory to conduct the chloride ingress resistance analysis of AAC under drying-wetting cycles. Compared with concrete C50, the effects of cyclic drying-wetting time ratio (i.e. 3.0:1.0, 11.0:1.0 and 85.4:1.0) and exposure time (i.e. 30, 90 and 180 d) on the chloride ingress properties of AAC were investigated via a designed automatic experimental set-up of cyclic drying–wetting test. A theoretical model of chloride ingress into AAC under drying-wetting cycles was proposed for numerical calculation. The results show that the chloride content within AAC is obviously less than that in concrete C50. This is because AAC has a smaller porosity and a denser pore microstructure rather than concrete C50, thus exhibiting a better resistance to chloride ingress. The surface chloride concentration and apparent chloride diffusion coefficient of AAC first increase and then decrease as the cyclic drying–wetting time ratio increases. It is indicated that the numerical data of chloride profiles calculated by the proposed model of chloride transport into unsaturated concrete under various conditions of the drying–wetting cycles are in reasonable agreement with the experimental results.

Cement hydration is a key process to determine most properties of cementitious materials. Clarifying the mechanisms of cement hydration intersects both academic and practical interests. Recent research work on Portland cement hydration kinetic models was reviewed. The hydration simulation models, such as GEMS thermodynamic calculation, HymoStruc3D and CEMHY3D and other hydration simulation models and their limitations were discussed. The application of quasi-elastic neutron scattering and nuclear magnetic resonance methods in the evolution of water states and the development of pore structure during cement hydration process were summarized. In addition, this review also gave the future developemt and challenges in this field.

To reveal the law of influence of hybrid fibers on the mechanical properties and deformation behavior of strain hardening cementitious composites (SHCC), the tensile and compressive properties of basalt-polyvinyl alcohol(PVA) fiber strain hardening cementitious composites (SHCC) were investigated at a constant fiber content of 2% (in volume fraction), different content ratios of basalt fiber and PVA fiber (i.e., 3:1, 1:1 and 1:3), and different ratios of fly ash to cement (FA/C) (i.e., 1.2, 1.5 and 2.0). The direct tensile and compressive strengths of hybrid fiber SHCC with different fly ash contents were analyzed. The horizontal and vertical strain evolution and crack development of the sample during compression were discussed via digital image correlation (DIC). The results show that the compressive strength of the mixed fiber specimens is greater than that of the single fiber specimens, indicating that the hybrid fiber can effectively improve the compressive strength. At FA/C ratios of 1.2 and 1.5, the optical fiber type is B0.5P1.5 (i.e., the volume fraction of basalt fiber is 0.5% and PVA fiber is 1.5%). At a FA/C ratio of 2.0, the optical fiber type is B1P1. The optical fiber type is different for different fly ash admixtures, indicating that the fly ash admixture affects the optical fiber type. In the hybrid fiber specimens, basalt fibers mainly play a role in improving the strength, but it is not suitable for the ductility of the material. In contrast, incorporating PVA fibers can effectively enhance the material's ductility, the addition of hybrid fibers can enhance the mechanical properties. The DIC can effectively characterize the strain evolution and crack formation and propagation process of the material, thus providing an effective method for investigating the material damage and crack evolution in the loading process.