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A thorough characterization of starting materials is the precondition for further research, especially for cement, which contains various phases and presents quite a complex material for fundamental scientific investigation. In the paper at hand, the characterization data of the reference cement CEM I 42.5 R used within the priority program 2005 of the German Research Foundation (DFG SPP 2005) are presented from the aspects of chemical and mineralogical compositions as well as physical and chemical properties. The data were collected based on tests conducted by nine research groups involved in this cooperative program. For all data received, the mean values and the corresponding errors were calculated. The results shall be used for the ongoing research within the priority program.

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The knowledge of the aqueous phase composition during the hydration of tricalcium silicate (C 3 S) is a key issue for the understanding of cement hydration. A new in situ method of computing calcium ion concentration from the measurement of the electrical conductivity on paste was coupled to isothermal calorimetry and BET measurements to get new insights on the early hydration of C 3 S. Ion concentrations of the aqueous phase are mainly dependent on the degree of hydration and the water to C 3 S ratio. In the case of C 3 S paste, the calcium and silicon concentrations determined at low degrees of hydration can be related to the equilibrium curve of C-S-H having C/S = 1.27 and named C 1.27 SH y . It is expected that C 1.27 SH y thermodynamically controls the aqueous phase composition at this early stage. Indeed, the formation of C 1.27 SH y is quasi-immediate when C 3 S is in contact with water inducing a very rapid increase of the specific surface area that remains constant during the induction period. At higher degrees of hydration, the aqueous phase composition departs from the C 1.27 SH y equilibrium curve. C 1.27 SH y appears to be a metastable C-S-H that could be related to an intermediate phase previously reported. The quasi-immediate precipitation of C 1.27 SH y on C 3 S surface explains why calcium and silicon concentrations remain low during early hydration even though C 3 S is strongly undersaturated. This also agrees with the control of the end of the induction period by the nucleation and growth of more stable C-S-H. K E Y W O R D Scalcium silicate, hydration, phase diagrams, Portland cement, precursors