Control of calcium carbonate morphology by precipitation in compressed and supercritical carbon dioxide media

Carbonation of Hydrous Materials at the Molecular Level: A Time of Flight-Secondary Ion Mass Spectrometry, Raman and Density Functional Theory Study

“…This is also shown by the growth of the Raman peak at 1083 cm -1 of 16 O-calcite over time ( Figure 3) and can be explained by the transformation of the initial metastable carbonates into more stable phases over time. [64][65][66][67][68][69][70][71][72][73][74][75][76][77] Figure 3 shows the disappearance over time of a peak at 495cm -1 that can be related to metastable hydrated phases 55 .…”

“…The intensity of the peak at 152 cm -1 remains unaltered over time, whereas the intensity of peaks at 271, 698 and 1083 cm -1 increase, suggesting progression of the carbonation reaction with the formation of an increasing amounts of 16 O-calcite. The broad peaks at 450 cm -1 disappearing after 10 minutes reaction, suggests the presence of a metastable phase such as ACC (main peak included in the 1067cm -1 peak as previously mentioned).…”

“…29 60,61 ) whereas the atmospheric water and CO2 are the only source of 16 O. Any mixture of the isotope in the resulting carbonate phases provides the most likely source of oxygen in the formation of carbonate ions and therefore the carbonation mechanism.…”

“…at high pH, which does not allow isotope equilibration), the dissolution-and-precipitation mechanism leading to the formation of stable carbonate phases and the precipitation of new carbonate phases richer in 16 O, occurs at lower pH allowing an isotopic equilibrium of carbonates. It is worth noting that in our experiments this equilibration occurred within the initial 24 hours even at 50% RH (a relatively low RH value for carbonation) and this suggests that stage 2 of the carbonation is a fast kinetic pathway.…”

“…Two carbonation mechanisms have been proposed for portlandite at standard conditions, 25˚C and 1atm, based on solid state and a dissolution-carbonation [15][16][17][18][19] processes (the former is highly unlikely whereas the latter has been documented in a number of cases). Moorehead 20 suggested that carbonation of Ca(OH)2 at room temperature in CO2-rich environments is a solid-state reaction, which entails substitution of hydroxyl groups (OH -) with carbonate groups (CO3 2-) in a topotactic transition, leading to a change of the portlandite d{001} spacing.…”