The spatial distribution of CaCO3 polymorphs formed during the experimental carbonation of water saturated Portland-cement cores (30 mm in diameter), with supercritical CO2 at 90 °C and 30 MPa, has been investigated using Raman microspectrometry on polished sample sections and X-ray microdiffraction. The three calcium carbonate polymorphs (calcite, aragonite and vaterite) were clearly distinguished using both techniques and their distribution along the main CO2 diffusion direction could be mapped at the millimetre scale using a dynamic line-scanning Raman mapping tool. The calcium carbonate 2D distribution clearly shows that vaterite, the least stable of the three CaCO3 polymorphs, is mostly located in a 500 μm wide ring ahead of the carbonation zone. This feature indicates that vaterite is the first CaCO3 polymorph to crystallize within the cement sample in the course of the carbonation process. The presence of a vaterite front indicates that local mineral–solution equilibration can be slower than species transport, even above ambient conditions, and that kinetics cannot be ignored in the cement carbonation process. By using calcite and vaterite precipitation kinetic data from the literature and assuming water–mineral kinetics based on the Transition State Theory, the vaterite front inferred from Raman mapping is reproduced with a purely diffusive 1D transport code.

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