Research efforts on the concrete industry currently focus on the reduction of Portland cement (PC). Alternatives to reduce PC content in concrete such as the use of particle packing models (PPMs) and blended systems of PC and limestone fillers (LF) have been proven to be effective. However, the combined response of PPMs and LF at high levels of PC replacement still needs further investigation, particularly in the fresh state. In this work, the concept of Inter-particle Separation (IPS) is employed as a criterion to understand the behaviour of cementitious pastes made of PC and designed through continuous PPMs incorporating high LF dosages. Results suggest that IPS is a valuable tool to predict the fresh state behaviour of highly packed systems incorporating LF at low PC content. Moreover, the concept of interparticle separation among PC particles (IPScement) is proposed to describe setting behaviour and compressive strength of cementitious systems.