The use of powdered limestone is a promising approach to reduce the clinker factor of portland cements. Recent regulatory actions in the United States and Canada have allowed for portland cements to contain up to 15% limestone (mass basis). This action allows for the replacement of cement by limestone through: (1) intergrinding the cement clinker and limestone through the production process or (2) by blending the cement and limestone through the concrete batching process. While both avenues appear feasible, there is a need to compare and contrast the performance features of one approach versus the other. This paper evaluates cement pastes containing interground and blended limestone in terms of their hydration and strength evolution behavior. Experiments and numerical simulations performed within a boundary nucleation and growth (BNG) model indicate that the reaction response of interground cements can be achieved or exceeded by blended systems, depending on the characteristics of the cement and the limestone used, i.e., Type I/II, Type III or blend of Type I/II and Type III. Thus, by adjusting the cement or limestone fineness, blended systems can be proportioned to display strengths which are superior to the interground case at early ages. However, by later ages all binders show similar strengths. The results do suggest that for replacement levels up to 15% (mass-basis), intergrinding or blending are both viable strategies to reduce the clinker factors of portland cements, while maintaining early-age properties similar to pure cement formulations.