Although carbonation of cement phases is well known, the amount of carbon dioxide combined during the process has been much less investigated. Related to the greenhouse effect much more attention is being paid to the sinks for in order to correctly compute the gases emission during production of materials. In the case of cement a strict calculation should discount the emitted from that fixed by the concrete. This is the aim of present work which is a study of the cement based materials ability to combine . Using Thermogravimetry and Differential Thermal Analysis as well as phenolphthalein indicator, quantities of combined and carbonation depth in several cement pastes and concretes exposed to different environments have been measured. A calculation to estimate the contribution of cement based materials carbonation to the partial re-absorption of the emitted in cement production is proposed.