Although Portland cement concrete is the world's most widely used manufactured material, basic questions persist regarding its internal structure and water content, and their effect on concrete behaviour. Here, for the first time without recourse to drying methods, we measure the composition and solid density of the principal binding reaction product of cement hydration, calcium-silicate-hydrate (C-S-H) gel, one of the most complex of all gels. We also quantify a nanoscale calcium hydroxide phase that coexists with C-S-H gel. By combining small-angle neutron and X-ray scattering data, and by exploiting the hydrogen/deuterium neutron isotope effect both in water and methanol, we determine the mean formula and mass density of the nanoscale C-S-H gel particles in hydrating cement. We show that the formula, (CaO)1.7(SiO2)(H2O)1.80, and density, 2.604 Mg m(-3), differ from previous values for C-S-H gel, associated with specific drying conditions. Whereas previous studies have classified water within C-S-H gel by how tightly it is bound, in this study we classify water by its location-with implications for defining the chemically active (C-S-H) surface area within cement, and for predicting concrete properties.