# Influence of Coupled Activated Recycled Fine Powder on the Performance of Ultra-High-Performance Concrete

## TL;DR

This study explores using recycled materials to enhance the performance of ultra-high-performance concrete while reducing costs and environmental impact.

## Contribution

The novel approach combines calcined recycled fine powder with a composite activator to improve UHPC properties.

## Key findings

- Adding 15% calcined recycled fine powder increased compressive strength by 23.2%.
- Grinding the powder for 60 minutes maximized its activity.
- A 1.5% activator incorporation rate boosted compressive strength to 162.2 MPa, an 8.8% improvement.

## Abstract

Ultra-High-Performance Concrete (UHPC) is being increasingly utilized in major engineering projects due to its excellent mechanical properties, strong durability, and superior overall performance. Nevertheless, the widespread use of premium cementitious materials leads to high expenses and a substantial environmental impact. In this work, crushed recycled paste was calcined at 600 °C for two hours to produce calcined recycled fine powder (RFP) with varying hydration reactivity. UHPC was produced using the RFP in place of some of the cement. Chemical activation was accomplished by adding a composite activator system made up of Ca(OH)2, Na2SO4, Na2SiO3·9H2O, and K2SO4 in order to further improve the performance of UHPC. Particle size, viscosity, fluidity, mechanical properties, and hydration products were analyzed to establish the best activator type and dosage, as well as the ideal activation procedure for recycled fine powder. By mass replacement of cementitious materials, when 15.0% of the calcined recycled fine powder was added, the compressive strength of UHPC reached 149.1 MPa, a 23.2% increase over reference UHPC without calcined recycled fine powder. The results show that the calcined recycled fine powder ground for 60 min exhibits the highest activity. More hydrated products were formed in UHPC as a result of the addition of Ca(OH)2. The compressive strength peaked at 162.2 MPa at an incorporation rate of 1.5%, which is 8.8% higher than UHPC without an activator.