IntroductionIn coastal engineering, the durability of reinforced concrete structures is often compromised due to the corrosion of steel reinforcements caused by chloride ingress. This issue significantly affects the safety and longevity of these structures. In concrete materials, the interfacial transition zone (ITZ) between the cement matrix and aggregates is a critical point, affecting the pathways and rates of chloride ion diffusion. Although previous studies demonstrate that the incorporation of nanomaterials can enhance the durability of cement-based materials, the uneven dispersion and agglomeration of these nanomaterials restrict their effectiveness. In this paper, a modification method was proposed via coating graphene oxide (GO) onto aminated sand particles to improve the dispersion of GO within cement-based composites, thereby enhancing their durability. This approach could have a promising potential for extending the service life of coastal engineering structures.MethodsIn this study, a GO dispersion was firstly prepared and dispersed by ultrasound. Subsequently, a specific chemical treatment method was employed to coat GO onto standard sand surfaces, creating aminated functionalized sand (GO-MAs). The preparation process of GO-MAs included sand pretreatment, amination, and chemical bonding with GO. In the experiments, cement-based composites with different GO contents (i.e., 0.1%, 0.3%, 0.5% by mass) were prepared with high-performance polycarboxylate superplasticizers to enhance GO dispersion. The chloride ion resistance of these composites was evaluated by rapid chloride migration (RCM) tests and natural chloride diffusion tests. In addition, the mechanisms of GO-coated sand enhancing the microstructural properties of cement-based materials, particularly the ITZ, were analyzed by mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and thermogravimetric analysis (TG/DTG).Results and discussionThe experimental results indicate that the incorporation of GO significantly reduces the chloride ion penetration depth and migration coefficient in cement-based materials. At GO content of 0.3%, the chloride ion penetration depth and diffusion coefficient when using the method of using GO-coated sand can be reduced by 24.8% and 18.5%, respectively. The MIP tests further reveal that the size of harmful pores (i.e., greater than 50 nm) is decreased by 36.8%, indicating that GO-coated sand effectively improves the pore structure of cement-based materials, reduces porosity, and enhances material density. The microstructural analysis results reveal that GO-coated sand primarily enhances the performance of cement-based composites via optimizing the ITZ. The coupling agent forms stable chemical bonds with the sand surface, while the covalent bonding of amide bonds between GO and the coupling agent improves the dispersion of GO and promotes ITZ hydration, reducing defects and cracks, thereby enhancing resistance to chloride ion transport. The microstructural morphology analysis shows that the samples without GO have more and larger pores, while the samples with GO-coated sand exhibit significantly fewer and denser pores. The EDS analysis indicates that GO-coated sand promotes the enrichment of hydration products at the ITZ interface of cement-based materials, enhancing the ITZ density. Based on the XRD and TG/DTG analysis, GO-coated sand promotes the hydration reaction of cement, thus generating more hydration products, especially C-S-H and Ca(OH)₂ crystals. The formation and arrangement of these hydration products play a crucial role in maintaining a high alkaline environment within cement matrix and protecting a passivation film on the steel surface. The qualitative comparative analysis of the crystallographic orientation index of Ca(OH)₂ crystals indicates that GO-coated sand reduces the number of Ca(OH)₂ crystals perpendicular to the interface in the ITZ, optimizing the directional arrangement of Ca(OH)₂ crystals in the ITZ, which is beneficial for strengthening the weak interface and enhancing its density.ConclusionsThe method of using GO-coated sand effectively could enhance the resistance of cement-based materials to chloride ion erosion, and the optimal effect occurred at a GO content of 0.3%. This modification method optimized the ITZ through a “dual-bond effect,” reducing porosity and increasing material density, thereby enhancing the barrier against chloride ions. This research could provide an effective technical approach to improving the durability of cement-based materials and offers important scientific evidence for the protection of concrete structures in coastal engineering. A future research needed to explore the long-term performance of GO-coated sand under different environmental conditions and its application effects in practical engineering, thus providing comprehensive theoretical support and technical guidance for the modification and engineering application of cement-based materials.