<p>Carbonation refers to the reaction between atmospheric CO₂ and cementitious materials. It is a process that has been studied for decades since it brings the conditions required for the uniform corrosion of steel reinforcements. In recent years, however, carbonation has been recognized for its potential in the field of studies into the decarbonation of the industry (via the chemical fixation of CO₂). The amount of free water in concrete pores is of vital importance for carbonation kinetics: while free water hinders CO₂ diffusion, it is nevertheless essential as a reaction medium. Additionally, water is released into the pores during the carbonation process, which in turn has an impact on the carbonation rate. This study investigates the link between the amount of water retained in the pores (desorption isotherm) and carbonation in a paste consisting exclusively of C–S–H. The findings provide a precise description of the effect of carbonation on water release (via the desorption isotherm).</p>