This study explores the development of a chloride-resistant, eco-friendly concrete by partially replacing Portland cement with limestone and calcined clay at varying proportions of 0, 30, 45, 52.5, and 60%. The concrete was produced using seawater, sea sand, and gravel. Macro and micro experiments were conducted to evaluate the effects of different replacement rates on the chloride ion content and compressive strength of LC3 seawater sea sand concrete. The main findings are as follows: the concrete with 30% and 45% replacement showed lower strength than the 0% replacement concrete at 1–7 days. However, by 28 days, the compressive strengths of these mixes had improved to 50 MPa and 45.6 MPa, 13.6% and 3.6% higher than 0% replacement rate, respectively. Beyond 45% replacement, a significant reduction in strength was observed. Additionally, concrete with 30 and 45% replacement showed a notable decrease in free chloride ion content from 1 to 3 days compared to the 0% mix. The higher the replacement rate, the more the free chloride ion content was reduced. These results provide valuable theoretical guidance for developing building materials and structural systems for marine structures.