Use of Fly Ash and Volcanic Ash for the Treatment of Recycled Aggregates to Develop Ultra-High-Performance Concrete

In this study, recycled fine aggregates (RFAs) were treated with fly ash and volcanic ash slurries and used in the production of UHPC. Four UHPC mixes were prepared using natural fine aggregates (UHPC-C), untreated RFA (UHPC-UT), fly ash treated RFA (UHPC-FA), and volcanic ash treated RFA (UHPC-VA). Flow, compressive strength, flexural strength, water absorption, and volume of permeable voids were determined for all the UHPC mixes. The control mix (UHPC-C) exhibited the highest compressive strength (138.6 MPa) and superior durability parameters, with minimal water absorption (1.3%) and void content (2.1%). However, UHPC-UT showed a significant reduction in compressive strength (114.5 MPa), coupled with higher water absorption (2.8%) and void content (3.5%), indicating a compromise in both strength and durability. In comparison, UHPC-FA and UHPC-VA demonstrated compressive strengths of 125 MPa and 122.9 MPa, respectively, with improved durability characteristics relative to UHPC-UT. Both mixes exhibited decreased water absorption (1.9–2%) and void content (around 2.4–2.7%) compared to UHPC-UT, highlighting the potential of fly ash and volcanic ash in treating the RFAs.