The alite phase in Portland cement contains different foreign ions.In this work, the effects of magnesium and zinc on the polymorph, hydration kinetics and hydrate morphology of C3S are investigated.It is found that the presence of the two ions has distinct effects on the hydration kinetics.Zinc increases the maximum heat released.Magnesium increases the hydration peak width.The C-S-H morphology is impacted by the presence of these ions.Longer needles are observed in the presence of zinc and thicker needles are observed in the presence of magnesium.Zinc is incorporated in C-S-H, while magnesium is only incorporated slightly, if at all, but rather seems to inhibit nucleation.