Sustainable concrete for a circular economy: Integrating seawater and ferronickel slag as alternative materials

• SW reduces slump, while FS improves workability up to 25%. • Combination of SW and FS increases concrete density by 1.8%. • FS enhances split tensile and flexural strength at replacement levels. • Leaching analysis shows minimal environmental impact with heavy metals below limits. The growing demand for natural sand and freshwater in concrete production poses serious environmental challenges, including resource depletion and ecological harm. This is the first study to evaluate the synergistic effects of ferronickel slag (FS) and seawater (SW) on both the mechanical and environmental performance of concrete. To support more sustainable construction practices, a circular economy approach is applied by repurposing industrial byproducts and seawater as alternative raw materials. Specifically, FS—a byproduct of nickel refining—replaces natural sand at 25% and 50% levels, while SW substitutes tap water in concrete mixes. Tests were conducted to assess workability, density, mechanical strengths, and environmental safety. The results show that SW accelerates early hydration, reducing workability but enhancing early strength. FS improves long-term strength and workability, particularly at 25% replacement. Leaching tests confirm that the release of trace elements remains within safe regulatory limits. Additionally, this work proposes novel power regression models tailored for FS–SW concrete, offering improved predictive accuracy compared to conventional equations by better capturing the interactions between alternative materials. The findings support the scalable use of SW and FS in construction, reducing reliance on virgin materials while maintaining structural integrity and environmental safety.