JOURNAL OF RAMAN SPECTROSCOPY J. Raman Spectrosc. 2007; 38: 61–67 Published online 4 October 2006 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/jrs.1599 Hydration and carbonation of monoclinic C 2 S and C 3 S studied by Raman spectroscopy Jordi Ib ´ a˜ nez, 1,2 Llu´ ıs Art ´ us, 2* Ramon Cusc ´ o, 2 ´ Angel L ´ opez, 2 Esperanza Men ´ endez 1 and Mar´ ıa C. Andrade 1 1 Instituto de Ciencias de la Construcci ´ on Eduardo Torroja, Consejo Superior de Investigaciones Cient´ ıficas (CSIC), 28033 Madrid, Spain 2 Institut Jaume Almera, Consell Superior d’Investigacions Cient´ ıfiques (CSIC), 08028 Barcelona, Catalonia, Spain Received 1 March 2006; Accepted 17 June 2006 We present a micro-Raman study on the hydration and carbonation of the main silicate phases of Portland cement, i.e. monoclinic dicalcium silicate (C 2 S) and monoclinic tricalcium silicate (C 3 S). We investigate the reaction products and the loss of crystallinity induced by hydration on these two compounds. In the CO 2 -contaminated pastes we find that calcite, aragonite, and vaterite are inhomogeneously formed. We study sample cross sections to evaluate the maximum depth at which CaCO 3 is formed. We find that carbonation is limited to the first 500–1000 μm from the surface in the C 3 S pastes, while in C 2 S pastes CaCO 3 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. KEYWORDS: cements; hydration; carbonation; tricalcium silicate (C 3 S); dicalcium silicate (C 2 S) INTRODUCTION The properties and structure of concretes based on Portland cement are altered due to carbonation by reaction with atmospheric carbon dioxide. 1 The most dramatic effect of carbonation is the pH reduction in the pore solution of concrete, making steel reinforcements sensitive to corrosion. The expansion caused by rust formation cracks the concrete and leads to its deterioration. Although carbonation reactions also yield beneficial effects on concrete such as an increase in its compressive and tensile strength, they give rise to irreversible shrinkage and superficial cracking of the material. A number of works have been devoted to the study of the carbonation of Portland cement and of its main constituent phases, i.e. tricalcium silicate (C 3 S) and dicalcium silicate (C 2 S). 1–5 It is now well established that the Ca/Si ratio of calcium silicate hydrate (C–S–H), which is the principal product of the hydration reactions of Portland cement and the main cause for its hardening, 1,6 is lowered due to carbonation. Nuclear magnetic resonance (NMR) studies showed that the carbonation of C 3 S and of Portland cement pastes results in the polymerization of C–S–H, leading to the formation of a silica-rich gel. 2,3 L Correspondence to: Llu´ ıs Art ´ us, Institut Jaume Almera (CSIC), c/Llu´ ıs Sol´ e i Sabar´ ıs s/n, 08028 Barcelona, Catalonia, Spain. E-mail: lartus@ija.csic.es Different crystalline phases of calcium carbonate (CaCO 3 ) have been observed by electron microscopy and X-ray diffraction in carbonated Portland cement, in calcium sili- cates and in their pastes. 1–5 In particular, TEM measurements on pastes provided valuable information about the effects of carbonation in the inner product (within the original silicate particles) and in the outer product (within the space origi- nally filled with water). 2 The effects of carbonation seem to depend on the characteristics of the cement paste studied (C 3 S or Portland cement), the hydration conditions, and also the way in which the samples have been contaminated (i.e. in CO 2 -rich atmospheres or in air). 3 The conditions for the formation of the different phases of CaCO 3 in the cement constituents is not at present well understood. 4 In any case, most of the previous studies deal with the carbonation of C 3 S or Portland cement, whereas C 2 S has received compara- tively much less attention. 4,5 In particular, no NMR data on the carbonation of C 2 S pastes are available. Raman spectroscopy is a powerful tool for the character- ization of materials. Several Raman studies about cements and related compounds have been published so far. 7–18 In his pioneering works, Bensted 7,8 showed the potential of Raman spectroscopy in the study of clinker minerals and their hydra- tion products as well as their carbonation. In a later work, Conjeaud and Boyer 9 reported the advantages of using a Raman microprobe to obtain Raman spectra from individual crystals of industrial clinkers. However, the Raman spec- trometers and detection systems available before the 1980s Copyright  2006 John Wiley & Sons, Ltd.