The microstructure and composition of hardened cement pastes of a wide range of blends of ground granulated blast-furnace slag with ordinary Portland cement have been studied, using techniques of transmission electron microscopy with microanalysis combined with electron microprobe analysis. Throughout the range, a calcium silicate hydrate gel (C-S-H) is the dominant cementing phase, present in the “inner product” within the space originally occupied by either slag grains or alite or belite grains originating from the Portland cement, or in the “outer product” in the originally water-filled spaces. The morphology and composition of the outer product C-S-H and the composition of inner product C-S-H change with blend composition. Inner product of slag grains contains C-S-H of the same composition as the outer product C-S-H, intimately mixed with a Mg, Al-rich hydroxide phase whose fineness shows considerable variation. Inner product C-S-H of alite or belite does not differ significantly in Ca∶Si ratio from that of slag. The reduction of Ca∶Si ratio of all forms of C-S-H with increasing slag loading may have implications for the pH-buffering capacity of blends of large slag loading.