A computer model for representing the hydration and three-dimensional microstructure development of portland cements has been developed. Techniques have been developed for creating three-dimensional starting microstructures which match the particle size distribution, phase volume fractions, and phase surface fractions of a given cement. The model kinetics have been calibrated against experimental measurements using the parabolic dispersion model of Knudsen. For Cements 115 and 116 issued by the Cement and Concrete Reference Laboratory at NIST, it appears that a single simple equation can be used to convert model cycles to real time. After this conversion, the agreements between model and experimental degrees of hydration, heat release, and chemical shrinkage are quite good for the three w/c ratios examined in this study. The model in combination with the gel-space ratio theory of Powers and Brownyard has been successfully applied to predicting the 7 and 28-day compressive strength of ASTM C109 mortar cubes from their 3-day values. While particle size distribution is critical to the successful prediction of cement performance, these results indicate that phase volumes and distributions also have a significant influence on the hydration process, particularly at times exceeding a few days.