The brick-carved cultural relics is a precious cultural resource. The uniqueness of brick carving causes a high requirement for the technique and materials of in-situ repair. The difficulty is to preserve the original features of brick carving while restoring its microstructure. The inorganic reinforcing agents are beneficial for maintaining the physical properties of the historical material and have been considered the optimal choice for restoring the cohesive strength of the historical material in recent years. The restoration material prepared by basic magnesium sulfate cement (BMSC) has the advantages of high tensile strength, high resistance to weather, high viscosity, early strength, and low cost. It has a good effect in reinforcing the weathered sandstone of the cultural relics. The hydraulic lime is widely used due to its waterproof and breathable properties. But it is very expensive. The Silicon-based repair material has good permeability. But it has poor stability in the environment with the changing humidity. Three types of strengthening agents are selected as the alternative options for the restoration of brick carving based on the need for the brick repair and in-situ protection of the ancient mortar during the process of the restoration of brick carving.

Three strengthening agents are Ca-based hydraulic lime, Si-based silica sol, and Mg-based BMSC. Three steps are implemented in order to select the most effective strengthening agent in the restoration of the cultural relics of brick carving. That is the preparation and research of performances of three kinds of imitation bricks, infiltration reinforcement of the ancient material, and the verification of practical engineering. The powder of green bricks is used as the main material for replica bricks. The hydraulic lime, silica sol, and BMSC-based reinforcing agent are firstly mixed with the powder of green brick to form replica bricks, which are the Ca-bricks, Si-bricks and Mg-bricks. The multiple experiments were conducted to investigate the compatibility of replica bricks. The studied properties include the composition of crystal phase, compressive strength, porosity characteristics, and the hydraulic performance of the replica bricks, and so on. WW/T 0065—2015 “Code for Investigation of Stone Cultural Relics Protection Engineering” is referenced. Secondly, the adhesive properties and infiltration reinforcement of the strengthening agents were taken into consideration to evaluate the feasibility of application of these material to the restoration of the brick-carving cultural relics. Finally, the verification of the referred BMSC-based strengthening agent used in actual engineering was conducted because the laboratory environment is different from the complex environment where the cultural relics are located.

Ca-based and Si-based reinforcing agents only serve to adhere the powder of green brick. The BMSC-based reinforcement agent continues to hydrate after being added to the powder of green brick to form the replica bricks. The main hydration product is the 5⸱1⸱7 strength phase. So the BMSC-based reinforcing agent has the strongest adhesion among three types of reinforcing agents. The Mg-brick has the best performance among three kinds of replica bricks, which is similar to the green brick. But Mg-brick has the slight hydrophobicity. The properties of Ca-bricks and Si-bricks are close to the ancient bricks. The bending strength of the masonry of bonded Mg-bricks is 10 times that of the Si-bricks. Only the BMSC-based strengthening agent has no carbonization and has good aging resistance. The infiltration solution with a content of 30% BMSC-based reinforcement agent and a moisture content of 44%–50% has the best effect of reinforcement, maintaining the original physical properties of brick-carved cultural relics while restoring the original cohesion. The referred BMSC - based reinforcing agent can penetrate and bond the slag of brick carving of 9.5 times the mass of agent.

The BMSC-based reinforcing agent demonstrates higher penetration enhancement ability than reinforcing agents based on hydraulic lime or silica sol. The volume stability of the referred BMSC-based strengthening agent meets the requirements. It also can be found that the repair effect of the referred BMSC-based strengthening agent is good after two years. In summary, the optimal ratio of BMSC - based reinforcing agent shows good repair effect at low cost. The Mg-brick and BMSC - based reinforcing agent can be used for the restoration of brick carving.

The South China Sea islands and reefs have emerged as key regions with far-reaching implications in multiple domains, including national defense, maritime trade, resource exploration, and ecological conservation. The rapid expansion of infrastructure in this area has made concrete structures the predominant choice due to their versatility and strength. However, the local environment presents an array of formidable challenges. The durability of these structures is thus under constant threat, with potential consequences for the integrity and functionality of the entire infrastructure network. Understanding and enhancing the durability of concrete structures in such a harsh environment is not only essential for the immediate success of ongoing projects but also for the long-term sustainable development and strategic positioning of the South China Sea islands and reefs.

To comprehensively study the durability and service life of reinforced concrete structures in the South China Sea islands and reefs environment, a long-term exposure test station was established in this area. This test station was designed to closely simulate the actual environmental conditions of the South China Sea islands and reefs, providing reliable test conditions for studying the durability of concrete structures.

In the experiment, different types of concrete specimens and components were prepared. These included specimens with different corrosion inhibitors and different concrete strength grades. Electrochemical tests were carried out to monitor the corrosion status of reinforcing bars. The natural corrosion potential and polarization resistance of the reinforcing bars were measured using advanced electrochemical testing equipment. Mechanical property tests were also performed to evaluate the mechanical performance of the concrete structures. Compressive strength, flexural strength, and tensile strength tests were conducted on the concrete specimens and components. Chloride ion content analysis was carried out to understand the penetration and distribution of chloride ions in the concrete. The ChaDuraLife model was used for service life prediction. This model takes into account various factors such as chloride ion diffusion, concrete strength, and environmental conditions to predict the service life of the concrete structures.

The experimental results showed that different corrosion inhibitors had different effects on the corrosion of reinforcing bars in concrete. The SBT-KLJ(Ⅳ) hydrophobic pore-filling agent (HPA) exhibited excellent anti-corrosion performance. After 1227 d of corrosion, the natural corrosion potential of the steel bars in the concrete specimens with HPA shifted positively, and the polarization resistance continuously increased during the erosion period from 365 d to 730 d. In the chloride ion erosion environment of the islands and reefs, adding HPA reduced the chloride ion content on the concrete surface by approximately 59.9% compared to concrete without the corrosion inhibitor.

The mechanical property tests of the concrete components after on-site exposure experiments indicated that the durability of concrete structures with different strength grades varied significantly. C80 concrete showed better resistance to seawater erosion and mechanical properties than C50 concrete. The service life prediction results based on the ChaDuraLife model demonstrated that adding a rust inhibitor or increasing the concrete strength grade could effectively extend the service life of the reinforced concrete structure in the splash zone environment of the South China Sea islands and reefs. The HPA rust inhibitor was particularly effective, and under certain conditions, it could extend the service life of the concrete structure to over 50 years.

In conclusion, the research findings have several important implications. The HPA corrosion inhibitor has demonstrated excellent long-term stability and anti-corrosion performance in the South China Sea islands and reefs environment. Its ability to enhance the electrochemical stability of reinforcing bars and reduce chloride ion permeability makes it a preferred choice for marine engineering applications. The significant improvement in the durability of concrete structures with HPA inhibitor has been clearly established through experimental and modeling results. The differences in durability among different strength grades of concrete highlight the importance of appropriate material selection for the specific demands of the South China Sea islands and reefs infrastructure. C80 concrete’s superior performance in terms of chloride ion resistance and mechanical properties indicates its suitability for critical structures where enhanced durability is required. The comparison with the national standard emphasizes the need for further research and innovation to develop more effective strategies for improving the durability of reinforced concrete in this harsh environment. This could involve exploring new admixtures, optimizing concrete mix designs, or enhancing construction and maintenance practices. The data and insights obtained from this study provide a solid foundation for future research and engineering applications, guiding the design and construction of more durable concrete structures in the South China Sea islands and reefs. Ultimately, these efforts will contribute to the long-term safety and functionality of the infrastructure in this strategically important region.