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The purpose of this study was to investigate the effects of age on calcium silicate hydrate (C–S–H) present in two cement pastes: one in which the C–S–H at a younger age had a high Ca/Si ratio and another that had a low ratio. The study involved characterising a 20 year old neat ordinary Portland cement (OPC) paste (high Ca/Si) and a blend of the OPC with 90% ground granulated blast furnace slag (GGBS; low Ca/Si); the results are compared with data on the same mixes at 14 months old. The morphology of the outer product (Op) C–S–H in the blend was essentially the same as at 14 months, whereas that in the neat OPC paste appeared to be finer. The mean aluminosilicate chain length of the C–S–H was considerably longer in both pastes than in similar systems at younger ages. The chemical composition of the C–S–H in both pastes was significantly different than at 14 months: in the neat OPC paste, the Ca/Si ratio of the outer product C–S–H had increased with age, whereas that of the inner product C–S–H had decreased; the Ca/Si ratio of Op C–S–H in the blended paste had decreased with age. An entirely tobermorite based nanostructural model can account for the data for the C–S–H in the blend, whereas the data for the neat paste require some jennite or calcium hydroxide based structure; the distribution of the TEM–EDX analyses favours the T/CH model. [ABSTRACT FROM AUTHOR]