The hydration process of Ca3SiO5 (C3S) cement is investigated, and accelerating effects of adding colloidal silica (CS) are established. CS accelerates dissolution of the C3S phase, and renders a more rapid formation of the calcium-silicate-hydrate (C-S-H) binding phase. The role of water is demonstrated. Water evaporation correlates with the appearance of a sharp IR band at 3650 cm(-1) signifying non-hydrogen bonded OH groups. Incomplete hydration results in Ca(OH)(2) precipitation. Suppression of water evaporation causes the 3650 cm(-1) band to vanish. The Ca2+ ions become incorporated in the C-S-H network, the formation of which is accelerated by CS. (C) 2004 Elsevier B.V. All rights reserved.