Transmission Electron Microscopy (TEM), Scanning Transmission Electron Microscopy (STEM) and Chemical Analysis investigations lead to a fine characterization of Reactive Powder Concretes elaborated under different conditions as pressure application during setting and post-set heat treatment. An abrasive thinning method followed by ionic etching allowed for the preparation of 100 mm thick specimens with wide observation surface areas while still avoiding any water or CO2 contact which may cause their alteration. Silica fume distribution and reactivity versus curing processes are studied. The Si diffusion interfacial zone between hydrated products and silica fume, clinker particles or crushed quartz is measured in different curing cases. The Ca/Si ratio spatial distribution in hydrated products and its evolution with the curing processes are then analyzed and shown to be strongly microheterogeneous.