Carbonation Rates of Dry Ca(OH)₂ Mortars for CO₂ Capture Applications at Ambient Temperatures

The carbonation rates of porous mortars, pellets, and extruded forms of Ca(OH)₂ were determined to investigate their suitability as functional materials for direct air capture. Samples of 4-15 mm thickness and porosities between 0.2 and 0.8 were tested by monitoring the progress of the carbonation fronts on time scales from 1 to 500 h. The evolution of such carbonation fronts was found to obey Fick's diffusion law under all tested conditions. To reach CaCO₃ conversions higher than 0.6, a relative humidity above 50%, preferably between 80 and 100%, was required when using dry, low-grade slaked lime with a surface area of 18 m²/g as CO₂ sorbent. For modest relative humidities of 50%, higher grades of Ca(OH)₂ (i.e., with a surface area approaching 40 m²/g) still allowed carbonation conversions above 0.8. The results confirm the applicability of these commercial solids for the direct air capture of CO₂.