The results presented in this paper illustrate the importance of the physical microstructure (particle-size distributions and phase fractions and distributions) of the initial cement powder in influencing the hydration kinetics of cement paste. This emphasizes the necessity of an accurate and quantitative characterization of the starting materials to develop a realistic hydration model. The development of a three-dimensional computer model utilizing realistic starting microstructures has been shown to provide quantitative predictions of the effects of cement composition, water-to-cement (w/c) ratio, and curing environment on resultant hydration and physical properties, such as heat release and mortar cube compressive strength. In general, for the two cements examined in this study, once a calibration is performed for one cement and w/c ratio, the predictive ability of the model for other systems is excellent, suggesting that the model may be applicable in designing new materials. Future efforts will concentrate on the incorporation of mineral admixtures, such as silica fume and fly ash, and the addition of one or more computer modules to model the early time (induction period) hydration behavior, so that the length of the induction period can be predicted as well.