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The nanoscale morphology of, and pore water interactions in, calcium silicate hydrate (C-S-H), the active component of cement, remain uncertain. H nuclear magnetic resonance (NMR) can fully characterize the nanoporosity of C-S-H in as-prepared material without the need for damaging sample drying. We use NMR to follow the density of C-S-H in sealed cured pastes as a function of degree of hydration (α) and water to cement ratio. We show clear evidence for C-S-H densification. The C-S-H "solid" density, exclusive of gel pore water, slightly decreases from ρ = 2.73 g/cm at α ≈ 0.4 to 2.65 g/cmat α ≈ 0.9 due to an increase in the number of layers in the nanocrystalline aggregates. In the same range, the C-S-H "bulk" density, including gel water, increases from around 1.8 to 2.1 g/cm. The increase corresponds to a transition from growth of low-density product containing gel pores to higher density product devoid of gel pores. We update Powers' classical model from 1947. In contrast to the single "hydrate" of Powers, NMR differentiates between C-S-H and calcium hydroxide and separates out the interlayer water within the C-S-H. It shows a clear nonlinearity in the growth of the different fractions with α. © 2012 American Chemical Society.