The chloride resistance of blended cementitious materials can be improved by incorporating a high volume of supplementary cementitious materials (SCMs), thereby improving the durability and service life of concrete structures. However, their mechanical properties decrease significantly, especially at early age. To improve the early-age mechanical properties and chloride resistance, the type and addition of cementitious materials in the fine, mid-size, and coarse fractions were optimized based on particle close-packing and pore fillability during hydration. The blended cement pastes with greater chloride binding capacity and pore tortuosity are obtained due to the pore refinement of fine particles and efficient hydration of both clinker and SCMs. In addition, a blended cementitious material with 3-day compressive strength of 27.3 MPa and 28-day chloride diffusion coefficient of 0.36×10-12 m2/s was prepared using only 59% clinker with a size range of 4-55 μm. The results provide a deeper insight in the microstructural optimization and durability improvement of cement-based materials.