The CEMHYD3D computer model has been modified to consider the influence of limestone substitutions, allowing a priori prediction of the effects of various limestone substitutions on achieved degree of hydration, microstructure, and strength development. Both the chemical and fine filler effects of limestone on cement hydration have been addressed. The revised model provides good agreement with experimental results. Limestone substitutions are projected to be particularly advantageous in lower w/s (< 0.4) mortars and concretes, where the cement clinker being replaced may only be serving the function of a relatively expensive filler material. In these systems, up to 20 % of the cement could potentially be substituted by limestone (or other filler materials) to economize on the usage of portland cement clinker and to reduce the energy and the deleterious emissions associated with its production.