Utilizing recycled concrete powder (RCP) in ultra-high performance concrete (UHPC) by taking a constrained dense suspension packing (DSP) theory: Towards ensured mechanical properties, lower shrinkage and enhancing sustainability

Ultra-high performance concrete (UHPC) delivers outstanding strength and durability, yet suffers from excessive shrinkage, high cement consumption, and environmental impact. To address these limitations, this study proposes a sustainable and performance-guaranteed solution by incorporating recycled concrete powder (RCP) as a partial cement replacement to produce low-shrinkage and eco-efficient UHPC. A key innovation is the development of a constrained dense suspension packing (DSP) theory, which goes beyond traditional physical packing by optimizing the synergistic interaction among RCP, silica fume (SF), and sand gradation to tailor both workability and hydration behavior. This design approach enhanced flowability by 108 % and improved compressive strength by up to 87.3 % at 30 % RCP content, while maintaining performance losses within acceptable limits, no more than 22.1 % reduction in flowability and 16.6 % in strength, thereby satisfying the requirements of GB/T 31387–2015 . It was also found that across the full RCP replacement range of 10–50 %, autogenous and chemical shrinkage progressively decreased with increasing dosage, reaching maximum reductions of 58.2 % and 68.1 % at 50 % replacement, primarily due to the dilution effect and internal curing capability of RCP. Microstructural analysis confirmed that higher RCP levels suppressed hydration and increased porosity, yet delayed internal humidity loss and stabilized shrinkage development. Importantly, a life cycle assessment (LCA) verified that the proposed RCP-UHPC substantially reduces energy consumption and environmental impact. This study establishes a practical and applicable approach for integrating relatively inert recycled construction and demolition waste powders into UHPC, ensuring dimensional stability and structural performance while improving the environmental friendliness. • Recycled concrete powder (RCP) is used as a partial cement replacement in UHPC. • Dense Suspension Packing (DSP) theory is optimized with three constraints for mix design. • Constrained DSP ensures flowability and strength of RCP-UHPC at RCP dosages ≤ 30 %, meeting regulatory standards. • RCP reduces autogenous shrinkage through dilution effects and internal curing.