Gas-solid carbonation experiments were carried out by using an infrared microscope coupled to a reaction cell. The hydroxide ions (OH) consumption and the production of molecular water (H2O) and carbonate (CO32-) vibration bands were directly monitored as a function of time. Herein, we demonstrated that the gas-solid carbonation of calcium hydroxide (or portlandite) was exclusively activated by initial adsorbed water-molecules (water activity approximate to 0.6 in the lab room) at low temperature (30 degrees C) and low CO2 pressure (0.5-1.5 bar). We assume that carbonation reaction was then rapidly autocatalysed by the water production and followed by a passivation step due to the formation of a dense layer of carbonate around the reacting particles of portlandite. The fast carbonation and passivation steps were satisfactory fitted by using a kinetic pseudo-second-order model. Moreover, the infrared measurements provided complementary insights with relevance to the reaction mechanism of gas-solid carbonation of calcium hydroxide. Herein, the formation of metastable aragonite was identified and a hydrated calcium carbonate was suspected during carbonation process. On the other hand, when initial adsorbed water onto reacting particles was removed by in situ vacuum drying (P < 10(-5) mbar, T = 110 degrees C) prior to injection of CO2 in the reaction cell (water activity approximate to 0), the carbonation of calcium hydroxide particles was no more detected by infrared spectroscopy at low temperature (30 degrees C). However, there was evidence for a very limited carbonation reaction at higher temperature (300 degrees C) and low CO2 pressure (<1 bar). Another carbonation mechanism is required to explain this observation, for example the migration of oxygen atoms from the solid towards adsorbed CO2. (C) 2010 Elsevier B.V. All rights reserved.