The main component of Portland cement is a nano-sized and poorly-ordered phase commonly referred as "Calcium Silicate Hydrate" (CSH). Although the nature of this phase was first discussed almost 100 years ago, its detailed crystal structure is still uncertain, mainly because its X-ray diffraction (XRD) pattern exhibits only a few and broad diffraction maxima. As a consequence, CSH is often assumed to be "X-ray amorphous" and classical XRD pattern refinements cannot be performed. This difficulty has been circumvented by numerous authors by using a wide variety of physical methods taking advantage of magnetic radiations, X-rays, or neutrons (FTIR, NMR, EXAFS, SANS, PDF...) to probe the short-range order around Ca, Si and O. These data contributed to the development of, or supported, the currently held view that CSH are structurally related to the rare minerals tobermorite and/or jennite. Discrimination between these two models using local probes is highly difficult because tobermorite and jennite are both layered minerals and they have similar local order, i.e. layers built up of ribbons of silicon tetrahedra, with wollastonite-like structure, running at the surface of calcium layer. To date, no structural model was unambiguously determined, because no modeling of methods probing the long-range order was performed. We combined electron probe micro-analyses (EMPA), transmission electron microscopy (TEM) and powder XRD to determine the structure of four different nano-crystalline CSH synthetized at laboratory temperatures ranging from room temperature to 110 °C. It is demonstrated that the modeling of XRD patterns using a specifically developed method is able to capture most of the structural details from CSH crystals, such as crystallite size, lattice parameters and interlayer occupancy. The genetic link between CSH and its hypothesized natural equivalents is discussed.