This study examines the rheological characteristics of cementitious paste, focusing on the impact of the colloidal phase and solid volume fraction on its non-Newtonian properties, including yield stress, structural build-up and shear-dependent viscosity. Utilizing Monte-Carlo particle-based modelling and Computational Fluid Dynamics (CFD) in combination with experimental results, it explores the time-dependent flow properties and structural build-up in cementitious paste due to colloidal cohesive interparticle forces, and the effects of cement hydration and external shear on flow behavior. Addressing current gaps in modeling the time-dependent viscoelastic behavior of blended cementitious materials with supplementary cementitious materials (SCMs), it emphasizes the need for predictive rheological models for modern sustainable concretes, facilitating the optimization of concrete mix designs for enhanced workability and placeability.