We present a micro‐Raman study on the hydration and carbonation of the main silicate phases of Portland cement, i.e. monoclinic dicalcium silicate (C₂S) and monoclinic tricalcium silicate (C₃S). We investigate the reaction products and the loss of crystallinity induced by hydration on these two compounds. In the CO₂‐contaminated pastes we find that calcite, aragonite, and vaterite are inhomogeneously formed. We study sample cross sections to evaluate the maximum depth at which CaCO₃ is formed. We find that carbonation is limited to the first 500–1000 µm from the surface in the C₃S pastes, while in C₂S pastes CaCO₃ is formed well beyond this depth. Our results show the great potential of Raman spectroscopy in the study of the chemistry of cements. Copyright © 2006 John Wiley & Sons, Ltd.