Permeability Assessment of Ultrahigh-Performance Concrete with High Dosages of Rice Husk Ash

This study investigates the influence of high rice husk ash (RHA) dosages (up to 40%) and particle sizes (5–100 μm) on the permeability, hydration, and microstructural properties of ultrahigh-performance concrete (UHPC). Experimental assessments included water absorption, chloride migration coefficient, and electrical flux measurements, supported by microstructural characterization using mercury intrusion porosimetry (MIP), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). Results demonstrated that fine RHA particles (≤5 μm) significantly enhance UHPC impermeability by reducing porosity, increasing gel pores, and promoting secondary hydration reactions. At 40% replacement, fine RHA achieved water absorption levels comparable to those of the control sample and reduced chloride migration by 17.2%, demonstrating its ability to maintain or improve performance at high dosages. In contrast, coarse RHA particles (≥50 μm) increased porosity by 45% and compromised durability, leading to higher chloride penetration. Furthermore, fine RHA improved hydration efficiency by consuming calcium hydroxide (CH) and generating additional calcium silicate hydrate (C─ S─ H) gel, whereas coarse particles exhibited lower pozzolanic activity and left unreacted residues, adversely affecting performance. The findings provide critical insights for optimizing RHA incorporation in UHPC, balancing performance, cost, and sustainability.