Nuclear Magnetic Resonance relaxometry (1H NMR) is an established technique for non-invasive and non-destructive characterization of pore size distributions and pore-water interaction in porous media.By applying combined CPMG (Carr-Purcell-Meiboom-Gill) and Quad-Echo pulse sequences, all hydrogen protons in the cement paste can be measured and quantified in their different environments: crystalline phases (Portlandite and ettringite), C-S-H interlayer water, C-S-H gel pore water and capillary pore water.NMR cxperiments are quantitative, simple to carry out and provide highly detailed information concerning microstructure better than other techniques.The signal lifetime, known as the spin-spin relaxation time T2, can be used to calculate pore sizes.Quantification of water in the C-S-H and in the capillary pores led to a complete description of white cement paste, C-S-H density, C-S-H chemical composition and pore sizes.We show how 1H NMR can be used to follow hydrating white cement pastes, with and without addition of silica fume and with variation of the water-to-cement ratio.The results showed that the capillary porosity filled with water rapidly becomes very small (8 nm in size, called interhydrate pores).In parallel, a plateau of the gel pore formation is observed and correlates with the lack of capillary water space.This causes a densification of the bulk C-S-H (including the gel water) over time.The C-S-H solid density and water content (C-S-H layers excluding the gel water) is similar between the different mixes under study.In the same way, the sizes of the C-S-H pores do not vary significantly and the interlayer of the C-S-H is calculated at approximately 1 nm and the C-S-H gel pores at 2.5 nm in size.These observations indicate that there is no fundamental change in the underlying mechanisms of C-S-H precipitation during the hydration.The C-S-H chemical composition is however highly variable and depends on the presence of silica fume and on the water-to-cement ratio.