Design of an iron-flake-modified ER@EC microcapsule for ultrasound-triggered self-healing in cementitious materials

Ultrasound demonstrates significant potential for triggering microcapsules in cementitious materials. However, currently reported microcapsules with effective ultrasound responsiveness are limited to urea-formaldehyde (UF) ones. The UF-based microcapsules face critical limitations, including formaldehyde emissions and synthesis instability, with no viable alternatives specifically engineered for ultrasound triggering. To bridge this gap, we designed an innovative iron-flake-modified epoxy resin (ER)@ethyl cellulose (EC) microcapsule. Fundamental and functional characteristics of microcapsules were analyzed through SEM, FTIR, UV–VIS, and nano-indentation. Rheological properties of fresh cement pastes incorporating microcapsules were examined. Strength and durability repair tests were conducted to quantify the ultrasonic triggering efficiency. Additionally, the healing effectiveness was also evaluated through impermeability tests and MIP. Results showed that the incorporation of micro iron flakes successfully endowed the microcapsules with ultrasound responsiveness, along with enhanced sustained-release performance, micro-mechanical properties, and hydrophobicity. The novel microcapsules also demonstrated superior rheological regulation compared to unmodified ones. Moreover, ultrasonic triggering brought a severalfold improvement in both strength and durability repair of self-healing specimens compared to mechanical triggering. Impermeability tests and MIP further confirmed the superior efficiency of ultrasonic triggering on the iron-flake-modified ER@EC microcapsules.