Impure tricalcium silicate (C 3 S) in portland cement may contain various foreign ions. These ions can stabilize different polymorphs of C 3 S at room temperature and may affect its reactivity. In this paper, the effects of magnesium and zinc on the polymorph type, hydration kinetics, and the hydrate morphology of C 3 S were investigated. The pure C 3 S has the T1 structure while magnesium and zinc stabilize polymorphs M3 and T2/T3, respectively. The two elements have distinct effects on the hydration kinetics. Zinc increases the maximum heat released. Magnesium increases the hydration peak width. The C-S-H morphology is modified, leading to longer needles in the presence of zinc and thicker needles in the presence of magnesium. Zinc is incorporated into C-S-H, while magnesium is only incorporated slightly, if at all, but rather seems to inhibit nucleation. Implementing experimentally measured parameters for C-S-H nucleation and growth in the lic hydration model captured well the observed changes in hydration kinetics. This supports C-S-H nucleation and growth to be rate controlling in the hydration of C 3 S.

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It was recently proposed that the induction period observed during the hydration of tricalcium silicate could be explained by the build-up of ions in solution. Due to the importance of defects in this mechanism, this work describes the effect of different annealing effects on the defect structure and hydration behavior of C 3 S. The impact of annealing on the crystal structure was checked by X-ray diffraction and the defect structure studied by transmission electron microscopy. The hydration kinetics were followed by isothermal calorimetry of pastes. Scanning electron microscopy was used to look at the microstructure formation. It was observed that grinding created a highly deformed layer on the surface of the grains, which disappeared after annealing. The defect structure was closely related to the length of the induction period observed in pastes by calorimetry. There was no observable effect on the morphology of C-S-H during hydration, but the number of calcium hydroxide nuclei was less in pastes from annealed material.

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