Synergistic effects of surface modifier and nanomaterial on eco-friendly UHPC containing crushed tunnel waste as manufactured sand—Towards cleaner production and circular resource utilization

The adoption of manufactured sand (MS) in concrete production offers a dual environmental advantage by conserving natural sand resources and mitigating solid waste disposal burdens, which is an effective way to promote sustainable development. However, the research on MS ultra-high performance concrete (UHPC) remains limited, and MS may lead to poor workability and significant flowability loss of concrete, which hinders its further promotion in UHPC. To address these challenges, this study develops an eco-friendly UHPC by utilizing crushed tuff tunnel waste as MS, modified by an independently developed surface modifier and nano-CaCO 3 . The synergistic effects of these modifications are evaluated through multi-scale macroscopic and microscopic performance characterizations. Results indicate that surface modifiers significantly improve the flowability and workability retention ability of UHPC, with 1 h flowability loss dramatically reduced from 27.1% in the reference group to 7.4%. Although surface modifiers prolong the setting time and delay the hydration peaks, nanomaterials can effectively offset these effects and further enhance the mechanical strengths. Modified sand can also inhibit autogenous shrinkage, optimize pore structure (reduce cumulative porosity at low dosage), and densify the interfacial transition zone (ITZ) without elemental enrichment. Furthermore, the production of MS generates 69% lower CO 2 emissions than conventional quartz sand, and the modified MS UHPC demonstrates 14 kg reduction in CO 2 emissions per cubic meter, 24.8% and 25.0% reductions in costs per cubic meter and unit compressive strength versus the conventional UHPC, showing superior environmental and economic benefits and contributing to circular and sustainable construction practices. • A surface modifier is developed to drastically reduce the 1 h flowability loss from 27.1% to 7.4%. • Synergy between modifier and nano-CaCO 3 enhances strength while offsetting retarding effect. • Modified sand optimizes pore structure and interfacial transition zone without detrimental element enrichment. • Tunnel waste sand cuts CO 2 emission by 69% versus traditional quartz sand, promoting circular economy. • The prepared UHPC shows 25% lower cost per unit strength, offering economic and environmental benefits.