Enhancing seawater sea-sand reactive powder concrete using ternary admixtures and fibres

The utilization of seawater and sea sand in a reactive powder concrete offers a sustainable alternative for marine infrastructure. However, chloride-induced corrosion and autogenous shrinkage remain critical challenges. This study systematically addresses these issues through a dual strategy: optimizing a ternary cementitious system (fly ash, metakaolin, and slag) and incorporating functional fibre. The effects of different factors on the properties of seawater sand reactive powder concrete (RPC)were investigated by designing an orthogonal test to test the pH value, Cl− concentration, mechanical properties, fluidity, and chemical shrinkage. Orthogonal experiments reveal that fly ash plays a dominant role in chloride immobilization, reducing Cl− concentration by 5.9% at 11% dosage via Friedel’s salt formation. Slag enhances flexural strength by 21.1% at 11% content, while metakaolin significantly improves early-age microstructural densification, albeit at the cost of reduced workability. Fibre hybridization further elevates mechanical performance: 0.2% polypropylene fibre increases the 3-day flexural strength to 22–23 MPa through effective crack bridging, and 0.2% basalt fibre maximizes compressive strength by enhancing interfacial compatibility in saline conditions. A 0.3% carbon fibre exhibits minimal impact on fluidity due to its hydrophobic nature. Chemically, the synergistic pozzolanic reactions convert free chlorides into stable phases, reducing pore solution pH by 1.9% and decreasing chloride permeability by over 20%. These results demonstrate a scientifically robust approach to designing durable, high-performance sea-sand seawater reactive powder concrete (SSRPC), with significant implications for resource-efficient and corrosion-resistant marine construction.