Nanoparticles improve the mechanical properties and durability of cement-based materials. However, owing to the high surface energy and specific surface areas of nanoparticles, the packing characteristic of cementitious particles will be affected. With the action of the electrostatic attraction and Van der Waals force, the cementitious particles are agglomerated into flocculation structures, and the free water is entrapped. Furthermore, as the water consumption of cement-based materials increases, the flowability gets worse, which is thought to be one of the reasons limiting its application in practical engineering. In addition, nanoparticles increase the viscosity and thixotropy of fresh cement-based materials and provide more nucleation sites in cement paste, accelerating the cement hydration process in early hydration. In this paper, the research progress on the rheological properties and structural build-up of cement-based materials with the addition of nanoparticles was reviewed. The applicability of rheological test methods and rheological models was summarized. The variation of rheological parameters of fresh cement-based materials affected by nanoparticles species, contents, dispersion method, superplasticizer, etc., were discussed. Based on the packing density, water film thickness, and flocculation structures, the action mechanism of nanoparticles on the rheological properties of cement-based materials was analyzed. Further research topics on the rheology and structural build-up of nano-modified cement-based materials are suggested as well.

The potential of Fiber Reinforced Polymer (FRP) in the reinforcement of concrete structures has been shown in many studies and practical applications. However, few works have focused systematically on the development of quantitative criteria to measure surface roughness and relate this parameter to the bonding mechanical property. Moreover, some researchers have declared that, if the concrete interface is rougher, the bond performance between FRP and concrete will be increase, However, there is no answer to how rough the surface is. There are limited application standards for engineers to conduct in FRP reinforcement projects. This work evaluated several concrete specimens with three different strengths and six types of interface roughness. A single shear test was conducted to study the influence of surface roughness on the interfacial bonding performance of a carbon fiber-reinforced composite (CFRP)-concrete beam. The results show that, among the six interfaces, a concrete interface with the roughness of 0.44 has the best interfacial bonding performance. An interfacial appearance with the cement mortar almost cleaned away, and almost one fifth of the single coarse aggregate bared will get the best bond performance. Roughness parameters significantly influenced the effective bond length. The effective bond length of the six interfaces experienced an overall decreasing trend as the roughness increased. The bond–slip curves of concrete interfaces with roughness of 0.25–0.44 did not significantly change the rigidity within the brittle region. The rougher the interface was, the shorter the brittle region was. After entering a plasticity stage, the bond–slip curves for the six types of interfaces all declined with different slopes, and the max slip values were 0.04–0.35 mm when debonding failure occurred.

Cementitious material durability is affected by the transport and adsorption of water molecules and ions in the nanopore channels of cement hydration products. Hydrated calcium silicate (C-S-H) accounts for about 70% of the hydration product. It determines the mechanical properties of cementitious materials and their internal transport properties. The molecular dynamics method provides a complementary understanding of experimental and theoretical results. It can further reveal water molecules and ions’ adsorption and transport mechanisms in C-S-H gel pores. This review article provides an overview of the current state of research on the structure of C-S-H gels and the adsorption and transport properties of water molecules and ions within C-S-H gels, as studied through molecular dynamics simulations. This paper summarizes the results of the molecular dynamics-based adsorption transport properties of water molecules and ions in C-S-H gels. The deficiencies in the current study were analyzed, and the fundamental problems to be solved and further research directions were clarified to provide scientific references for revealing the structural properties of C-S-H gels using molecular dynamics.

Mixing cement with seawater will cause the hydration process of cement to be different from that of ordinary cement, which will significantly affect cement’s mechanical properties and durability. This article investigates the effects of chloride ion concentration, curing temperature, and nano-metakaolin content on the evolution process of Friedel’s salts and ettringite (AFt) crystals in cement pastes. The study was conducted using X-ray diffraction (XRD), thermal analysis (TG), scanning electron microscopy (SEM), and mercury-intrusion porosimetry (MIP). The results show that chlorine salt can increase the production of Friedel’s salt and ettringite, and the delayed AFt production increases by up to 27.95% after the addition of chlorine salt, which has an adverse effect on cement-based materials. Increasing the curing temperature and increasing the nano-metakaolin dosage increased the generation of Friedel’s salt and decreased the delayed AFt generation, which resulted in a decrease in the length and diameter of the AFt crystals. After 28 days of high-temperature curing and the addition of nano-metakaolin, Friedel’s salt production increased by 13.40% and 14.34%, respectively, and ettringite production decreased by 9.68% and 7.93%, respectively. Increasing the curing temperature and adding nano-metakaolin can reduce the adverse effect of delayed ettringite increases due to chloride ion binding.

The early-age properties of nano-metakaolin (NMK) cement paste were examined from 15 min to 24 h, contacted between cement and water based on the electrochemical impedance spectroscopy (EIS) method. The effects of a superplasticizer and chloride ions were taken into consideration. The variation of the electrochemical parameters of NMK cement paste with or without the superplasticizer and chloride ions was analyzed. The results demonstrated that the solution resistance and impedance modulus of the cement paste decreased first then increased between 15 min and 12 h after cement contact with water. When the cement contacted with water within about 8 h, the pore solution resistance and impedance modulus were less affected by NMK. When the cement hydration was over about 8 h, the pore solution resistance and impedance modulus of the cement paste were significantly improved by the addition of 1% NMK. The pore solution resistance of the cement paste with 1% NMK was increased by 0.49%, 2.64% and 18.17% as compared with ordinary cement paste when the hydration time was 4 h, 8 h and 12 h, respectively. NMK promoted cement hydration and increased the pore solution resistance and impedance modulus in the cement paste with or without the superplasticizer and chloride ions. The superplasticizer and chloride ions reduced the pore solution resistance and impedance modulus of cement paste with or without NMK.

To improve the ability of concrete structures to resist freeze-thaw damage in cold environments, explore the effect and mechanism of nano-metakaolin (NMK) on frost resistance of concrete. And make up for the deficiencies in the mechanical properties and deformation process of na-no-metakaolin concrete in freeze-thaw environments. Rapid freeze-thaw cycle experiment was car-ried out to detect the deterioration law of concrete. Physical and mechanical properties under freeze-thaw environment was measured. The modification mechanism of nano-metakaolin on con-crete frost resistance from micro and meso scales was analyzed. The effect of freeze-thaw damage on nano-metakaolin concrete was characterized. The influence law of stress strain is established, and the meso-statistical damage constitutive model of nano-metakaolin concrete under freeze-thaw action is established. The results show that: Compared with other nano-clays, adding 5% nano-metakaolin can effectively slow down concrete’s freeze-thaw cracking and crack propagation. After 125 freeze-thaw cycles, the surface crack width of concrete mixed with 5% nano-metakaolin is only 0.1mm. Without freeze-thaw cycles, the compressive strength of concrete mixed with 3% nano-metakaolin is the highest, which is 28.75% higher than that of ordinary concrete; after 125 freeze-thaw cycles, the loss rate of compressive strength of concrete mixed with 5% nano-metakaolin was 12.07%. After 125 freeze-thaw cycles, the peak strain is 0.45 times that of concrete without NMK, and the peak stress is 3 times that of concrete without NMK.