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In spite of its wide use, the properties of the CaO/CaCO3 system are still far from being understood, especially the mechanisms underlying the transition between both phases. To investigate this issue, important in practice for storage devices, we employ first-principles atomic-scale simulations of the solid–gas interactions between CO2 and CaO, by considering insertion of CO2 within the subsurface, in configurations characteristic of calcite surface nucleation. Comparing the (001) and (111) surfaces demonstrates that nucleation should be strongly surface-selective, with a sharp preference for (111), and an important role of the CO2 arrangement in the surface layer. To interpret these results, especially the important structural instabilities detected, we investigate the elastic properties of coherent CaO/CaCO3 layered systems. This reveals the special role played by the (111) interface orientation and confirms the behavior detected at the atomic scale. From our study, we propose a mechanism for calcite nucleation as well as a plausible explanation for the degradation of performances crippling the CaO-based devices used for CO2 storage.