Raman spectroscopy has been used to study the hydration kinetics of tricalcium silicate (C3S). The progress of the reaction was followed by two different methods: the first one is based on measurement of the intensity of characteristic C3S Raman bands compared with that of an internal standard Raman peak (TiO2); the second one consists in measuring the relative intensity of Raman peaks associated with C3S and calcium hydroxide. These two concordant data sets show that a change in the hydration mechanism occurs at about 13 hours, and enable the kinetic parameters associated with the two hydration processes to be derived. ADVANCED CEME~Cr BASEr) MATERIALS 1995, 2, 15--20 KEY WORDS: Hydration kinetics, Raman spectroscopy, Tricalcium silicate i n his pioneering work, Bensted showed that Raman spectroscopy is of great value to quan-__ __ titatively investigate the hydration behavior of calcium silicates in situ, in particular, tricalcium silicate (C3S; in the following, C -= CaO, S -= SiO 2, H ~-H20 ) . The experimental set-up is particularly convenient: no particular preparation of the sample is needed, and the hydration rate can be determined using a single sample over the whole aging period studied. In Bensted's work, the degree of C3 S hydration was calculated from the decrease in intensity of the principal C3S Raman peak (no internal standard was used) . A +10% experimental accuracy was obtained for approximately 10 minute scanning times over the spectrum range of this peak . In this article, we show that using an up-to-date Raman spectrometer equipped with adequate software allows us to determine the C3S degree of hydration with an improved accuracy for scanning times an order of magnitude lower and a scanned spectrum range an order of mag-