Utilization of construction and demolition waste in ultra-high performance concrete: Macro-micro properties and environmental impacts

To further improve the utilization of recycled fine aggregates (RFA) obtained from construction and demolition in ultra-high performance concrete (UHPC), this work aims to adopting the nano-silica (NS) to develop a UHPC incorporating RFA with high strength and low shrinkage. A series of experiment evaluation on the workability, compressive strength, autogenous shrinkage and resistance to chloride penetration of UHPC are conducted. The hydration products, pore structure, and microstructure of UHPC are analyzed using isothermal calorimetry, thermogravimetric analysis, scanning electron microscopy, and mercury intrusion porosimetry, respectively. Results indicate that the addition of RFA decreases the flowability and early compressive strength of UHPC. However, it effectively mitigates autogenous shrinkage and compensates for the later-stage strength loss. NS significantly enhances the early compressive strength and resistance to chloride penetration in UHPC containing RFA. The combined incorporation of 20 % RFA and 3 % NS leads to 65.6 % reduction in the autogenous shrinkage within 7 days, 12.1 % improvement in compressive strength and 16.5 % decrease chloride diffusion coefficient of UHPC specimens at 28 days. Furthermore, the synergistic effects of RFA and NS promote the cement hydration, reduce the porosity and further refine the pore structure, and improve the interface transition zone in UHPC. This provides valuable insights for advancing the development of environmentally sustainable which collaborative utilization of RFA and NS in the manufacture of UHPC with reducing carbon footprint.