Semi-dry natural carbonation at different relative humidities: Degree of carbonation and reaction kinetics of calcium hydrates in cement paste

This study investigated semi-dry carbonation at different relative humidities (RH) under atmospheric CO2 concentrations to determine the effect of RH on the degree of carbonation (DoC) and reaction rates. The carbonation kinetics of each calcium-containing hydrate exhibited an initial rapid chemical-reaction-limited stage, followed by a significantly slower stage. DoC values plateaued after 200 days of carbonation, reaching 78 % at 95 % RH and 34 % at 33 % RH, aligning with EN 16757 values for sheltered outdoor and indoor environments, respectively. When the samples reached a stable DoC at a given RH, further carbonation occurred upon exposure to higher RH, implying that the DoC was governed by the highest RH to which the samples had been exposed. The phase assemblage was also affected, approaching thermodynamic equilibrium at higher RH but deviating at lower RH due to the formation of local equilibria and the presence of metastable phases. •Final degree of carbonation (DoC) values at various RHs are proposed.•DoC is governed by the highest RH to which the samples has been exposed.•Reaction kinetics exhibit an initial rapid step followed by a slower phase.•High continuity of pore solution allows calcium ions to diffuse-out.