Significance of Paste Content on Workability, Mechanical Performance, and Microstructure of Ultra-High-Performance Concrete

This paper addresses the significance of paste content (Pc) and water-to-binder (w/b) ratio on the performance of ultra-high-performance concrete (UHPC) by considering the flow, degree of hydration (DOH), compressive strength, pore volume, and microstructure. UHPC mixes were prepared without and with steel fibers by varying the Pc and w/b ratio. A modified mix design based on particle packing density was adopted to prepare UHPC. The mix design involves the Puntke, marsh cone, and bulk density tests, with the properties of UHPC evaluated through flow spread and compressive strength tests. The microstructure of UHPC was investigated using X-ray diffraction (XRD), thermogravimetric (TG), field emission scanning electron microscopy (FESEM), and mercury intrusion porosimetry (MIP) analysis. Results indicated that increasing Pc and w/b improved the workability of UHPC. Increasing Pc led to greater paste thickness (Pthick), enhancing lubrication, particle dispersion, and workability while reducing cohesion and viscosity. However, optimizing the Pc is crucial for achieving high packing density and strength because both low and high Pc values may impede the formation of a denser microstructure, reducing compressive strength. Lowering the w/b ratio alone within the typical range used for UHPC production may not necessarily result in higher compressive strength; insufficient compaction at lower w/b ratios can adversely impact the packing density of concrete. The incorporation of steel fibers increases compressive strength by approximately 30%. Microstructure analysis revealed that, at a constant w/b, variations in Pc do not affect DOH or pozzolanic reaction. However, it significantly influences porosity and particle packing.