The pH Reduction of the Hydrated Cement Paste by the Supercritical Carbon Dioxide

Recently, the use of recycled aggregate from waste concrete is increasing in the construction industry. However, its utilization has been limited due to its high alkalinity, which mostly came from the unremoved cement paste particles that were attached at the surface of recycled aggregate. Various efforts has been made to reduce its alkalinity by using Carbon Dioxide(CO2), but currently available methods that uses CO2 generate the problem with pH recovery. Therefore this study proposes an eco-friendly treatment method to reduce the pH of waste mortar using supercritical carbon dioxide. Considering the fact that supercritical Carbon Dioxide (scCO2) can provide more rapid carbonation of cement paste than by normal CO2, scCO2 was utilized in this work. The reaction between scCO2 and hydrated cement paste has been systematically evaluated. According to the results, it was found that powder type showed higher carbonation compared to that of cube specimens. It seems the carbonation by scCO2 has occurred only at the surface of the specimen, and therefore still showed some amount of Ca(OH)2 calcium aluminates after reaction with scCO2. With powder type specimen, all Ca(OH)2 was converted into CaCO3. Moreover, additional calcium that came from both calcium aluminate hydrates and calcium silicate hydrates reacted with scCO2 to form CaCO3. After carbonation with scCO2, the powder type specimen did not show pH recovery, but cube specimens did show due to the presence of portlandite.