Enhancement of the Frost Resistance and Mechanical Property of Recycled Concrete by an Alkali-Excited Sea Urchin-Like Coating

The high water absorption and low mechanical properties of recycled aggregate (RA) exhibit significant challenges for the application of recycled concrete in cold regions. In this paper, a novel alkali-excited sea urchin-like coating process was proposed, which effectively enhanced the interface adhesion between RA and the concrete matrix, thereby improving impermeability and frost resistance. The results indicated that the sea urchin-like coating process could be conveniently performed using a simple mechanical stirring technique. Compared to the control group, the modified recycled concrete exhibited increases of over 10% in both the flexural strength and compressive strength, which exceeded 4 MPa and 39 MPa, even at a 100% RA replacement rate. Furthermore, the modified recycled concrete’s freeze–thaw resistance is significantly enhanced, withstanding 150 freeze–thaw cycles at full RA replacement, which was double that of the control group. The moisture transport model indicated that the alkali-excited coating effectively prevented water transport. The geometric shape of RA and coating thickness also exerted some influence on the moisture transport. The improved material demonstrates potential for use in cement pavements and low-rise building construction.