Efforts to tune the bulk physical properties of concrete are hindered by a lack of knowledge related to the atomic-level structure and growth of calcium silicate hydrate phases, which form about 50-60% by volume of cement paste. Here we describe the first synthesis of compositionally uniform calcium silicate hydrate phases with Ca:Si ratios tunable between 1.0 and 2.0. The calcium silicate hydrate synthesized here does not contain a secondary Ca(OH)(2) phase, even in samples with Ca:Si ratios above 1.6, which is unprecedented for synthetic calcium silicate hydrate systems. We then solve the atomic-level three-dimensional structure of these materials using dynamic nuclear polarization enhanced H-1 and Si-29 nuclear magnetic resonance experiments in combination with atomistic simulations and density functional theory chemical shift calculations. We discover that bridging interlayer calcium ions are the defining structural characteristic of single-phase cementitious calcium silicate hydrate, inducing the strong hydrogen bonding that is responsible for stabilizing the structure at high Ca:Si ratios.