High-resolution electron microscopy study of calcium silicate hydrate (C-S-H) gels in ordinary portland cement (OPC) and a slag/OPC blend has been performed. Nanocrystalline regions on the scale of similar to 5 nm or less in C-S-H are found in both cement pastes, and they are formed after a curing time as brief as 7 d, A change in the d-spacing of the nanocrystalline regions with time is observed for the first time, which is believed to correspond to the development of C-S-H with time. The nanoheterogeneous nature of C-S-H is demonstrated and correlated to the strong Ca:Si ratio fluctuations that are observed.