Carbonation of slaked lime, mineral name portlandite, has been studied in air at moderately high temperatures ranging from 200°C to 550°C with variable heating time up to 72 h. Formation of calcite was observed for every run, accompanied with some remaining portlandite at low temperatures and quicklime at high temperatures due to surface-limited reaction between source materials and carbon dioxide included in air as much as 330-340 ppm. The maximum yield of calcite was 73.3 percent in the experimental condition. The 51.1 percent yield reached by 450°C heating in 6 h is regarded to have a high potential of practical applications for preventing the greenhouse effect. The reaction rate resulted were interpreted in terms of kinetics, applying the Jander equation. As a consequence, 125.7 kJ/mol was obtained for the apparent activation energy of reaction by the Arrehnius plots.