## 1 Introduction  
The large-scale development of island engineering projects is underway, driven by the rapid expansion of the marine industry [1]. However, constructing these island projects requires a substantial amount of building materials, and transporting these materials from inland is both time-consuming and expensive [2]. Comprehensive studies have shown that collecting and utilizing coral reef debris does not adversely affect marine ecosystems [3]. This approach, which involves the use of crushed and screened coral reef debris to replace gravel and seawater instead of fresh water, can effectively mitigate the environmental impact of traditional construction materials while also reducing transportation and time costs during construction [4]. To date, many scholars have validated the feasibility of using coral aggregate concrete (CAC) in island engineering projects, with the research focusing on the mechanical properties [5; 6], shrinkage [7], bonding performance [8], and durability [9] of CAC. However, coral aggregate has drawbacks, such as high porosity (15-45%), a high chloride ion content, and low strength (0.6-2.1 MPa) [10; 11]. Under the same water-cement ratio, the strength and durability of CAC are lower than those of Ordinary Portland Cement (OPC) concrete. Moreover, the apparent chloride ion diffusion coefficient (\(D_{\alpha}\)) of CAC is 1 to 8 times higher than that of OPC, and its total free chloride ion content (\(C_{\text{f}}\)) is significantly greater than that of OPC [12]. As a result, the corrosion of reinforcement within CAC structures is more severe than in OPC structures when used in island construction projects. It is also noteworthy that two drawbacks limiting its potential as an environmentally friendly material are that the production of Portland cement consumes a large amount of energy and emits significant quantities of CO\({}_{2}\). According to the statistics, producing one ton of cement generates approximately 10 kg of dust, 2 kg of nitrogen oxides, 1 kg of sulfur dioxide, and other by-products [13]. Therefore, it is imperative to address the issue of reinforcement corrosion within CAC structures in marine environments and to seek more \"green\" alternatives for cementitious materials. Geopolymer, as a novel and sustainable alternative cementitious material, primarily utilizes industrial waste materials, such as slag, fly ash, and metakaolin, making it an ideal substitute for traditional cement [14]. The combination of geopolymer concrete (GPC) with coral aggregate facilitates the formation of the C-S-H gel phase at the mortar matrix-coral aggregate interface, while the naturally dense pores of coral aggregate contribute to a tight interface transition zone with the cementitious material [15; 16]. Thus, the incorporation of a geopolymer not only mitigates the carbon emissions issues associated with Portland cement, but also enhances the split tensile strength, axial compressive strength, and elastic modulus of CAC [17; 18]. To prevent rebar corrosion, scholars have proposed various methods, such as applying surface coatings to concrete, increasing the thickness of the concrete cover, using high-performance concrete to block the penetration paths of corrosive agents, galvanizing the rebar, coating the rebar with epoxy resin, using corrosion inhibitors, and providing cathodic protection for the rebar. However, the aforementioned measures cannot fundamentally solve the problem of steel corrosion, and those measures can only extend the service life of the structure by approximately 20 years [19]. [PERSON] [20] studied the corrosion resistance of duplex stainless-steel rebars (SSRs) used for the Hong Kong Zhuhai Macao Bridge and demonstrated that the critical chloride ion concentration for the SSRs in simulated concrete pore solutions is 3.9 to 3.95 mol/L, which is about 60 times that of a carbon steel rebar.