Models for hydration kinetics based on simple spatial considerations show promise in describing the hydration behavior of ordinary portland cements and cements blended with inert fillers. Particularly, the dependence of relative degrees of hydration on w/c appears to be well characterized by a model where the instantaneous hydration rate is linearly dependent on the volume fractions of both the water-filled porosity and the remaining unhydrated cement (and filler). For sealed conditions, a further adjustment of the unhydrated cement volume fraction to reflect the probable remaining "active" cement provided the best fit to the limited available experimental data. The models are easily extended to consider the influence of the replacement of a portion of the cement by relatively inert fillers such as limestone powder. Because the achieved hydration is critically dependent on the starting w/c, the influence of filler replacement on relative hydration rates is equally dependent on the w/c of the original unfilled cement paste. Filler replacement may thus accelerate cement hydration much more for an initial w/c=0.3 cement paste than for a w/c=0.45 one.