The nanostructure of the main binding phase of the hydrated cements, the calcium silicate hydrates (C-S-H), and their structural changes due to aqueous carbonation have been characterized using TEM, nitrogen physisorption, and SAXS. Synthetic C-S-H has been used for this purpose. Two different morphologies were identified, similar to the high density and low density C-S-H types. When submitting the sample to a CO(2) flux, the low density phase was completely carbonated. The carbonation by-products, calcium carbonate, and silica gel were also identified and characterized. The precipitation of the silica gel increased the specific surface area from 95 to 132 m(2)/g, and its structure, formed by particles of similar to 5 nm typical radius, was observed by small angle X-ray scattering. In addition, the resistance of the high density C-S-H to carbonation is reported, and the passivating effect of the precipitated calcium carbonate is also discussed. Finally, the results have been compared with carbonation features observed in Portland cement carbonated experimentally at downhole conditions.