Rheological Properties of Ultra-High-Performance Cement Paste Governed by Water/Binder Ratios and Particle Packing

This study investigates rheological aspects, including plastic viscosity, yield stress, and transitions, as the water/binder ratio was increased from 0.16 to 0.2 in ultra-high-performance concrete (UHPC) pastes. Ultra-High-Performance Concrete (UHPC) has its exceptional superiority in terms of strength, durability and dense microstructure, which can be achieved with an optimised paste matrix. The rheological behaviour of UHPC paste plays a vital role in governing the fresh-state workability and homogeneous dispersion of fine particles. UHPC characteristics such as high strength and durability are influenced by the proper control of paste rheology through which adequate flowability and enhanced particle packing are possible. The raw materials of UHPC include Ordinary Portland Cement (OPC), Ground Granulated Blast Furnace Slag (GGBS), and silica fume (SF). Model analyses accompanied the rheological investigations to assess the model's fit. The results showed that the plastic viscosity of UHPC pastes decreased significantly from 13.6 Pa · s to 3.83 Pa · s as the W/B ratio increased from 0.16 to 0.2. A critical transition region was observed at W/B ratios of 0.18 and 0.19, beyond which the reduction in plastic viscosity becomes marginal. Relatively, particle packing was also reduced from 0.653 to 0.6, which highlights the interrelationship between particle structure and lubrication effects. A novel particle-packing-based rheological regime map is proposed that classifies the behaviour of UHPC into particle-dominated, transition, and flow-dominated regimes. This framework provides a concise tool for optimising the mix design of UHPC.