The carbonation of two hydrated ordinary portland cements of alkali content 1.03 percent or 0.43 percent Na_2O equivalent and hydrated calcium aluminate cement (0.1 percent Na_2O equivalent) was studied in a semi-dynamic atmosphere of 100 percent CO_2, and 65 percent relative humidity at 20 +- 1 deg C for a period of 100 days. The changes of the microstructure before and during the carbonation were characterized by x-ray diffraction, mercury intrusion porosimetry, and scanning electron microscopy. The kinetics of the process was evaluated from the total CaCO_3 content by means of thermogravimetric analysis. The changes of the mechanical flexural strength were also studied. The pore solution was collected and analyzed before and after different periods of time. The results were compared with those obtained under natural carbonation conditions. The results showed that the alkali content of cement does not influence the kinetics of the process when the carbonation is accelerated. In the case of natural carbonation, an induction period is produced in the ordinary portland cement of low alkali content and calcium aluminate cement. The carbonation rate of calcium aluminate cement is the slowest for accelerated and natural carbonation.
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