This work investigates the acidification of a natural limestone source using citric acid in order to produce porous calcium oxide (CaO) CO2 sorbent, with good stability in high-temperature operation. The CO2 sorption behavior of the proposed material was studied in several adsorption–regeneration cycles under different adsorption conditions (600, 650, and 700 °C), indicating the superior thermal stability and CO2 adsorption capacity of the proposed material compared to untreated limestone. Acidification of natural limestone results in the production of a calcium citrate component, which easily decomposes to high-purity fibrous CaO upon calcination at 850 °C. A novel technique based on a controlled atmosphere during the calcination step (two-step treatment) was developed to improve the activity of the CaO sorbent produced from the acidified precursor. A remarkable improvement in the adsorption activity was found for samples prepared using two-step calcination (initially treated in argon, followed by calcination in air) compared to those produced by one-step calcination. The in situ carbon formed during the primary calcination in an argon atmosphere was found to control thermal sintering and promote the dispersion of large agglomerates during burning off in the secondary calcination step under an air atmosphere. The influence of the primary calcination temperature was studied in detail for the acidified sorbents prepared by either one- or two-step calcination.