1:45 PM - 3:15 PM
[SMP27-P04] Gas-solid mineralization process of portlandite[Ca(OH)2]
Keywords:Portlandite, Calcium carbonate, Carbonation, XRD, FT-IR
Portlandite [Ca(OH)2] reacts with atmospheric CO2 and then forms calcium carbonate (CaCO3). Since a large amount of portlandite is included in concrete and mortar, it has high CO2 absorption potential due to its large global production volume. Although it is well known that portlandite carbonation depends on relative humidity (RH), the carbonation mechanism, especially the type of CaCO3 polymorphs is still unknown. To clarify the carbonation process of portlandite under various environments, therefore, two experiments, gas-solid carbonation and saturated carbonated water evaporation experiment, were conducted. The CaCO3 polymorphs and their formation mechanisms were investigated in detail by using synchrotron X-ray diffraction (XRD), infrared absorption spectroscopy (FT-IR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
In the gas-solid carbonation experiment, at RH 0% CaCO3 did not form even after 21 days of CO2 exposure time, which suggests that a humid environment is required for portlandite gas-solid carbonation. At RH 30%, a relatively small amount of calcite was formed after 24 hours of CO2 exposure time, and the amount of calcite increased with increasing humidity. On the other hand, at RH 50-60%, aragonite was formed after 24 hours of CO2 exposure time. At RH 50-80%, a small amount of vaterite was formed. From the results of TEM observation, numerous small aragonite particles were observed on the calcite surface in the sample exposed to CO2 for 24 hours at RH 60%, suggesting that the calcite surface was once dissolved in the wet environment and then aragonite was re-precipitated. The saturated carbonated water evaporation experiments confirmed that not only calcite but also aragonite was formed as the evaporation rate changes.
This study revealed that calcite is formed from portlandite when the humidity is lower and the evaporation rate of water is faster. When the humidity is higher and the evaporation rate of water is slower, both calcite and aragonite are formed. It was clarified that aragonite was formed by the calcite surface dissolution and reprecipitation.
In the gas-solid carbonation experiment, at RH 0% CaCO3 did not form even after 21 days of CO2 exposure time, which suggests that a humid environment is required for portlandite gas-solid carbonation. At RH 30%, a relatively small amount of calcite was formed after 24 hours of CO2 exposure time, and the amount of calcite increased with increasing humidity. On the other hand, at RH 50-60%, aragonite was formed after 24 hours of CO2 exposure time. At RH 50-80%, a small amount of vaterite was formed. From the results of TEM observation, numerous small aragonite particles were observed on the calcite surface in the sample exposed to CO2 for 24 hours at RH 60%, suggesting that the calcite surface was once dissolved in the wet environment and then aragonite was re-precipitated. The saturated carbonated water evaporation experiments confirmed that not only calcite but also aragonite was formed as the evaporation rate changes.
This study revealed that calcite is formed from portlandite when the humidity is lower and the evaporation rate of water is faster. When the humidity is higher and the evaporation rate of water is slower, both calcite and aragonite are formed. It was clarified that aragonite was formed by the calcite surface dissolution and reprecipitation.