Microstructural characteristics of hydrated triclinic tricalcium silicate [C3S(t)], monoclinic tricalcium silicate (alite), and type I portland cement at ages between time zero and 28 days were observed using electron microscopy. Image analysis was used to follow the development of calcium hydroxide (Ca(OH)(2)) and calcium silicate hydrate at micro- and meso-length scales. In all cases, easily recognizable islands of Ca(OH)(2) were noted to form, though the morphology and rate of formation differed for the three cement types. The formation of these islands was found to influence hydration by hindering the reaction of unhydrated cement particles that are embedded within their largely Ca(OH)(2)-bearing matrix. Such may be important features to include and validate in developing microstructural and other forms of computational models. The extent to which such microstructural features influence the local rates of reaction seems to depend, at least in part, upon the crystallography of the C3S.