Towards a carbon-neutral society, in the concrete sector, in addition to considering the use of large amounts of additions to reduce the amount of cement, CO2 fixation technologies using carbonation reactions are also being considered. Considering a sustainable society, the use of recycled aggregates is also an important topic, as is the production of concrete using CO2-captured aggregates. Furthermore, if the CO2 absorption of the constructed structure can be quantified, it is possible to link this to the carbon footprint of the constructing structures. Therefore, in this study, an attempt was made to quantify the CO2 absorption of cement replaced with ground granulated blast furnace slag in units of cement paste. Here, the relationship between the degree of hydration and the amount of CO2 absorption was investigated, assuming that the CO2 absorption of hydration products in the hardened cement paste is large. As a result, it was found that hydration progressed with extended curing, and that the amount of CO2 absorbed was greater when hydration progressed, and a high correlation was observed between the amount of CaO that became hydrate as a result of hydration progression and the amount of CO2 absorbed. By applying this, it was confirmed that the CO2 absorption in the paste section in recycled aggregate can be increased.