Phase Transformations and Ion Exchange During Early Age Carbonation

The exchange of ions during the dissolution of Portlandite and precipitation of newly formed carbonate phases, at the beginning of the carbonation reaction has been investigated. Changes in the isotopic composition of carbonates was determined using Time-of-Flight-Secondary Ion Mass Spectrometry and Raman spectroscopy. Samples of pure Ca(18OH)2 were carbonated in humid air (containing almost exclusively 16O) and characterized using Scanning Electron Microscopy and Raman Spectroscopy, aided by Density Functional Theory calculations. Results show that the carbonation process at high pH (i.e. >12) is a two-stage mechanism. The first stage occurs over a short period of time after the Ca(18OH)2 is exposed to air and involves the dissolution of surface Ca2+ ions and hydroxyl 18OH- groups. The latter react directly with the dissolved CO2, leading to the formation of C18O16O22- ions containing 1/3 of 18O. Since this ion is the precursor of the newly formed carbonate phases, a similar oxygen content was found in the precipitates. The 18O:16O ratio of 1:2 in the carbonates at the beginning of the reaction is in agreement with a carbonation model proposed by Létolle and colleagues in 1990 [1] describing a direct reaction of CO2 with the OH- groups produced by the dissociation of water. The second stage of the reaction occurs within 24 hours of exposure to air and involves a rebalance of the oxygen isotopic composition in the carbonate phases with a higher concentration of 16O.