The excellent binding properties of Portland cement rely on the reaction of its main mineral phase, tricalcium silicate (C3S), with water to produce calcium silicate hydrate (C-S-H) and calcium hydroxide (CH).Hydration of C3S exhibits complex kinetics that are not yet fully understood.Solid-State 29Si Nuclear Magnetic Resonance (NMR) has proven to be successful in studying hydration kinetics and in elucidating the poorly crystalline structure of silicate hydrates.However, due to the low natural abundance of 29Si, this method has mostly been applied to samples of which the hydration has been stopped and mainly at high degrees of hydration.In this work, 29Si-enriched C3S has been carefully synthesized and the evolution of C-S-H has been followed in situ by NMR.In this way, unique information concerning hydration mechanisms has been obtained with a time resolution of 30 minutes and up to 28 days.