Cementitious composites incorporating slag were investigated by multiple techniques, including scanning electron microscopy with backscattered electron and energy-dispersive X-ray spectroscopy detectors, thermogravimetric analysis, instrumented nanoindentation and ultrasonic measurements. The physical and mechanical properties of individual phases in cementitious composites with and without slag at different water to binder ratios were studied and are reported here. Subsequently, based on the identified phase properties, the macroscopic elastic moduli were calculated through a micromechanical method and verified with the experimental data. The results of this study show that the physical and mechanical properties of hydration products in cementitious composites incorporating slag are quite different from those of pure cement pastes. To explain the differences and improve scientific understanding of the influence of slag on the properties of hydration products, the composition, microstructures and mechanical properties were linked together for cementitious composites incorporating slag through this study.