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.