The construction sector is a major contributor to CO₂ emissions, largely due to clinker-intensive cement. This study explores low-carbon cementitious matrices by incorporating recycled cement (RC), calcined clays (CC), and limestone filler (LF), particularly in LC3 formulations, to develop eco-efficient modular concrete. An extensive experimental program assessed fresh-state properties, mechanical performance, and durability. Results demonstrated that LC3 + RC mixtures achieved up to 80% clinker reduction while maintaining strength and enhancing durability, proving their potential for sustainable construction.