Targeted separation of adhered mortar for high-quality recycled aggregate recycling based on salt-frost separation technology: the role of sustained ice pressure

Recycled aggregate (RA) is often limited in high-performance structural applications due to its weak adhered mortar (AM). This study proposes a novel salt-frost separation technique based on sustained ice pressure to efficiently and selectively remove AM to realize high-quality recycled aggregates. The salt-frost environment provides an external reservoir of unfrozen liquid, enabling continuous liquid transport at low temperatures. This progress limits the reduction of internal hydraulic pressure, leading to sustained high ice pressure that enhances destructive effects. Due to its sensitivity to permeability and penetration distance, the liquid transport causes negligible negative effects on natural aggregate with low water absorption and permeability. After two cycles at 1.5% concentration, AM content decreases from 35.43% to 3.08%, significantly reducing the water absorption of RA to a level comparable to that of natural aggregate. It is noted that the chloride ions introduced into the aggregate are far below the threshold for affecting the concrete durability. Economically, the technique is highly viable, while offering superior RA. Using modified RA in UHPC restores 95% of the compressive and flexural strength and enhances RA’s mechanical contribution during fracture, confirming the obvious quality improvement of RA and the possibility of use in UHPC. This study offers a novel technology and theoretical basis for producing high-quality RA, promoting the sustainable development of concrete materials.