The Influence of the Addition of Multi-Scale Zirconia on the Properties of Ultra-High-Performance Concretes

This study explores an innovative application of ultra-high-performance concrete (UHPC) by partially substituting cement with nano-zirconia (NZ) and micro-zirconia (MZ). A series of experiments were conducted to explore the influence of zirconia particle size (3860 nm and 320 nm) and varying replacement levels (0%, 0.5%, 1%, and 1.5%) on the workability, mechanical behavior, and microstructural characteristics of UHPC, utilizing the particle packing density model as a basis. Findings reveal that replacing 0.5% of the cement with MZ and NZ results in workability and mechanical performance comparable to the control mix. However, at 1.5 wt% MZ and 1.5 wt% NZ substitution levels, flowability declines by 22.01% and 24.71%, respectively, accompanied by a substantial increase in viscosity. The wet packing density of UHPC exhibits a linear rise with increasing zirconia content, with nano-sized particles exerting a more pronounced effect than their micro-sized counterparts. Specifically, at a 0.5% MZ replacement level, the 28-day compressive and flexural strengths show marginal improvements of 1.82% and 4.48%, respectively. The NZ1MZ0.5 mix achieves the highest 28-day compressive strength increase, reaching 9.45%, with an absolute gain of 11.92 MPa. Analyses using XRD, FTIR, and thermogravimetric analysis (TGA) demonstrate that zirconia incorporation has a negligible influence on the hydration process and does not alter the composition of hydration products in N-UHPC. Although zirconia effectively reduces porosity, excessive amounts (1.5%) increase pore size within the cement matrix, ultimately compromising mechanical properties. Based on these findings, the optimal NZ dosage for UHPC, when used in combination with MZ, is determined to be 0.5%.