Surface reinforcement of recycled aggregates by multi-diameter recycled powder blended cement paste

Surface coating is a simple and effective method to reinforce recycled aggregate (RA), and its efficacy is closely related to the basic property of surface-coated material (SM), such as mechanical strength, water absorption, compactness and permeability. In present work, recycled powder was treated by wet grinding to obtain micron-recycled powder (MRP) and submicron-recycled powder (SRP) for preparing the high-preforming SM. MRP was utilized to partly substitute cement to improve the compressive strength, water absorption, permeability and compactness; on this basis, MRP was slightly replaced with SRP to further enhance these properties. The relationship between coat-paste weight or thickness and SM fluidity was obtained. Microstructure, hardness and elastic modulus around interfacial transition zone were studied by SEM-EDS and nanoindentation, and the crushing value, water absorption of surface-reinforced recycled aggregates (SRA) were also tested, as well as the compressive strength and workability of recycled concrete. Results indicated that the comprehensive property of SM was enhanced significantly by the substitute of cement with MRP and SRP. Surface coating evidently decreased the crushing value from 14.6% to 10.2%, however, the water absorption was hardly changed. Besides, recycled concrete prepared with SRA performed much better than that of RA, regardless of workability or compressive strength. This work would provide guidance for the design of coating material and the prediction of coated paste thickness.