Growth of Nanosized Calcite through Gas−Solid Carbonation of Nanosized Portlandite under Anisobaric Conditions

Abstract : The gas−solid carbonation of nanosized portlandite was experimentally investigated using a static bed reactor under anisobaric conditions. The effects of initial CO2 pressure (10−40 bar), reaction temperature (30 and 60 °C), and relative humidity were investigated. Three steps of the carbonation process were determined: (1) instantaneous CO2 mineralization during CO2 injection period. From 25 to 40 wt % of initial portlandite grains were transformed into calcite during the CO2 injection period (from 0.9 to 2 min). (2) Fast CO2 mineralization after gas injection period (<5 h) followed by (3) a slow CO2 mineralization until an equilibrium state (<24 h). The results revealed high efficiency from portlandite-to-calcite transformation (>95%). For this case, the mineralization of CO2 does not form a protective carbonate layer around the reacting particles of portlandite as typically observed by other gas−solid carbonation methods. This method could be efficiently performed to produce nanosized calcite. Moreover, the separation of calcite particles from the fluid phase is most simple compared with precipitation methods. A kinetic pseudo-second-order model was satisfactorily used to describe the three CO2 mineralization steps except for the carbonation reaction initiated at 40 bar. In this latter case, a kinetic pseudo-first-order model was satisfactorily used; indicating that the slow CO2 mineralization step appears less significant during the carbonation process.