Sodium carbonate is a well-known material for being used as a good sorbent of carbon dioxide from the atmosphere, and as such has gained increasing importance in environmental protection. In order to enhance its sorption ability, mechanochemical activation of sodium carbonate was carried out in this study while the occurred changes after the activation and the relaxation time in a controlled environment were monitored. Activation was performed in a vibro-mill from 1 to 28 min. Based on the results of specific surface area, obtained by BET, the samples activated for 2 and 7 min were chosen and placed in a moist carbon dioxide atmosphere for 96 h, a period called the relaxation time. Differential thermal and thermogravimetric analyses were applied with the aim of determining the changes that occurred on the activated samples during the relaxation period. The decomposition temperature change of activated Na2CO3 samples, as well as mass loss and conversion level of Na2CO3 to NaHCO3 were monitored depending on activation and relaxation time periods. The conversion reactions of sodium carbonate to sodium bicarbonate under the specified environment of carbon dioxide and moisture were also studied, and the thermodynamic data such are changes in enthalpy, entropy, and Gibbs energy at the temperature range of 0–160 °C are given.