Understanding the interplay between the micro-mechanical properties and the microstructure of cementitious materials can provide a basis for intelligently designing concrete with enhanced properties. In this study, the nanoindentation technique is used to measure the micro-mechanical properties of single phases through discrete nanoindentation and the properties over large areas of hardened pastes through grid nanoindentation. Comparisons are made between pastes with water-to-cementitious materials (w/cm) ratio by mass of 0.3, 0.4, and 0.5 and the slag content of 0, 50, and 70 % for w/cm = 0.3 paste, to illustrate their differences on the phase distribution and the micro-mechanical properties. Significant portion of composite phase is found in slag-blended paste. A three-phase model is used to determine the volume fraction of the unhydrated phase included in the composite. A nanoindentation-based methodology is proposed to calculate the degree of hydration of Portland cement and slag-blended cement pastes. The results are then compared with the degree of hydration quantified by using thermo-gravimetric analysis and the backscattered electron image analysis methods.