Mechanical and Self-Healing Properties of a High-Volume Fly Ash Ultrahigh-Performance Concrete Incorporating Microcapsules

Cracking in ultrahigh-performance concrete (UHPC) threatens long-term durability, especially under harsh exposure. This study developed a sustainable UHPC incorporating high-volume Class C fly ash and urea-formaldehyde microcapsules containing calcium nitrate to enhance autonomous self-healing. The reference UHPC achieved 136.4 MPa compressive strength at 28 days, whereas mixtures with microcapsules had a modest reduction (6%–19%) but retained strain-hardening behavior under tension. Microcapsules decreased water sorptivity and gas permeability, and precracked specimens exhibited healing efficiencies of as much as 92.6% after conditioning. Scanning electron microscopy confirmed that cracks were sealed by calcium carbonate precipitation. These results demonstrate that microcapsule-assisted UHPC can balance high mechanical performance with improved durability, lower cost, and reduced environmental footprint.