Tricalcium silicate (Ca 3 SiO 5 ) with a very small particle size of approximately 50 nm has been prepared and hydrated for a very short time (5 min) by two different modes in a paste experiment, using a water/solidratio of 1.20, and by hydration as a suspension employing a water/solid-ratio of 4000. A phase containing uncondensed silicate monomers close to hydrogen atoms (either hydroxyl groups or water molecules) was formed in both experiments. This phase is distinct from anhydrous tricalcium silicate and from the calciumsilicate-hydrate (C-S-H) phase, commonly identified as the hydration product of tricalcium silicate. In the paste experiment, approximately 79% of silicon atoms were present in the hydrated phase containing silicate monomers as determined from 29 Si{ 1 H} CP/MAS NMR. This result is used to show that the hydrated silicate monomers are part of a separate phase and that they cannot be attributed to a hydroxylated surface of tricalcium silicate after contact with water. The phase containing hydrated silicate monomers is metastable with respect to the C-S-H phase since it transforms into the latter in a half saturated calcium hydroxide solution. These data is used to emphasize that the hydration of tricalcium silicate proceeds in two consecutive steps. In the first reaction, an intermediate phase containing hydrated silicate monomers is formed which is subsequently transformed into C-S-H as the final hydration product in the second step. The introduction of an intermediate phase in calculations of the early hydration of tricalcium silicate can explain the presence of the induction period. It is shown that heterogeneous nucleation on appropriate crystal surfaces is able to reduce the length of the induction period and thus to accelerate the reaction of tricalcium silicate with water.

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The rheological properties of cement paste in dependence on variation of cement characteristics (chemical and physical properties) are widely discussed in literature. Only a few studies show how the cement hydration products influence the fluidity of the pastes. Results presented here contribute to a better understanding of cement hydration and fluidity. To induce the formation of certain hydration products we systematically modified the cement composition by adding alkali sulphates. Furthermore results of rheology, phase analysis and microstructural investigations were combined in order to reveal the influence of hydration products on the fluidity of the cement paste. Thereby it is shown how the growth of long prismatic crystals like syngenite decreases paste fluidity. The increase in fluidity during continuous stirring is attributed to microstructural changes.

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