Multi-Nanoparticle Synergy in Ultra-High-Performance Concrete: Optimizing Strength, Durability, and Shrinkage Using RSM

Abstract

Ultra-High-Performance Concrete (UHPC) is emerging as a next-generation construction material due to its exceptional mechanical and durability properties. However, issues related to early-age shrinkage, limited workability, and cost-effective performance enhancement persist. To address these limitations, this study investigates the synergistic incorporation of nano-silica (NS), nano-alumina (NA), and carbon nanotubes (CNTs) into UHPC, a combination rarely explored in a unified optimization framework. The objective is to optimize these nanoparticles for enhanced compressive and flexural strength, reduced chloride permeability, and minimized shrinkage. A central composite design within a Response Surface Methodology (RSM) framework was employed to design 20 experiments with varying nanoparticle dosages. Mechanical tests, durability assessments, and advanced microstructural techniques (SEM, XRD, FTIR, TGA, TEM) were used for comprehensive evaluation. Results revealed that the optimized nanoparticle blend (1.6% NS, 1.1% NA, 1.4% CNTs) increased compressive strength by 20.4%, reduced RCPT by 50.6%, and decreased shrinkage by 25.4% compared to control. TEM and EDS images confirmed CNT bridging and NS/NA densification at the ITZ. However, excessive CNTs led to agglomeration and reduced benefits, highlighting the need for controlled dispersion. These findings demonstrate that nanoparticle synergy, when properly tuned, can redefine UHPC performance. Future research should address long-term durability under environmental loading and conduct life cycle assessments for sustainable large-scale applications.