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The C–S–H present in water and alkali activated hardened pastes of neat white Portland cement (WPC) and blends of WPC with 30% fly ash hydrated at 85°C has been characterised. The chemical composition of C–S–H could not be determined by SEM–EDX because of intermixing with other phases; TEM–EDX was necessary. The mean Ca/(Al–Si) atomic ratio of C–S–H was lower in the water activated blend than in the neat cement paste due to the reaction of the fly ash. The fibrillar morphology of outer product C–S–H in the water activated neat cement paste was coarser than that formed at lower temperatures. The mean length of the aluminosilicate anions in the C–S–H was much higher than in C–S–H formed at lower temperatures and was higher in the blended pastes than with neat cement. The composition–structure data are discussed in terms of models for the nanostructure of C–S–H. The lattice parameter a inferred from compositional data for the hydrogarnet phase in the water activated blend was in good agreement with the value measured directly by XRD. [ABSTRACT FROM AUTHOR]