The effects of NaOH on tricalcium silicate hydration kinetics were investigated using in situ time-resolved quasielastic neutron scattering. NaOH was confirmed to reduce the induction period. After the induction period the rate of products formation is nearly independent of NaOH concentration. However, NaOH suppresses the quantity of hydration products formed, mainly by decreasing the length of the nucleation and growth period, but also because it may cause morphological changes in the hydration products. These effects are commensurate with NaOH concentration, and explain the previously observed reduction in the early strength of tricalcium silicate pastes hydrated in the presence of NaOH. (c) 2005 Elsevier B.V. All rights reserved.