AbstractParticle-packing theory has been widely applied in the field of composite particle materials, and packing density (Φ) is an important parameter to evaluate the compactness of particle packing. In this paper, the rheological curves of fresh cement–fly ash–slag (PC-FA-SL) paste at 5 and 60 min resting time were tested through rheometer, and related rheological parameters were fitted by the modified Bingham model. The packing density of composite particles was calculated by the compressible packing model (CPM) and verified by the modified Andreasen model. Results show that with the increasing content of fly ash (FA) and blast-furnace slag (SL), both yield stress (τ0) and plastic viscosity (η) decrease significantly when the addition of FA and SL is less than 20% by weight. For the PC-FA-SL ternary system, τ0 and η increase with the development of Φ. The relationship between τ0 and Φ takes on exponential growth (τ0=aΦb+c), while the relationship between η and Φ presents linear growth (η=kΦ+m). Both b and k decrease with resting time developing. Zeta (ζ) potential measurement and hydration heat test were carried out to reveal the relationships between τ0, η, and Φ and rheological evolution of fresh blends.