SEM/X-ray images of cements in the Virtual Cement and Concrete Testing Laboratory were analyzed and used in simulations to investigate the effect of spatial distribution of clinker phases within particles on hydration and resulting properties. Percentages of the clinker phases actually consumed in the total hydrated clinker at any time during hydration ('active phase content's) are different than those measured on the unreacted powder ('overall phase content's), which may weaken predictions of the overall rate of hydration and property (heat and strength) development especially at early ages, and when hydration is incomplete. A demonstration in 2D of how 'active phase content's differ from the measured 'overall phase content's at different degrees of hydration, is provided as well as one in 3D, using a hydration model, for one of the cements. The influence of phase distribution on hydration and property development is discussed by comparing four different cases/microstructures hydrated under the same model conditions. For the same phase contents and fineness, differences in distribution have a significant effect on early-age properties but a lesser effect at later ages. (C) 2012 Elsevier Ltd. All rights reserved.