Ultra-high-performance concrete from 100% solid waste: Mechanical strength, microstructure and sustainability analysis

To tackle the high resource consumption and carbon emissions of ultra high performance concrete (UHPC), this work developed an innovative all-solid-waste UHPC (ASW-UHPC) using an all-solid-waste cementitious material, fly ash, steel slag aggregate, and waste tire fibers. Experimental results showed that the ASW-UHPC exhibited high mechanical strength, achieving 28-day compressive, flexural, and tensile strengths of 157.9, 20.4, and 9.6 MPa, respectively. Notably, it demonstrated a high early-age compressive strength of 125.7 MPa within 24 h. Microstructural analysis indicated that the high early-age performance was attributable to the rapid hydration of reactive calcium sulfoaluminate (C 4 A 3 Š). Hydration products, including ettringite (AFt) and aluminium hydroxide gel (AH 3 ), optimized the microstructure through pore filling and interfacial transition zone strengthening. Anhydrite dissolution released SO 4 2- ions, stabilizing the hydration environment and effectively preventing metastable AFt from transforming to monosulfoaluminate. Furthermore, those ions provided essential sulphate sources for sustained C 4 A 3 Š hydration, ensuring the continuous formation of AFt and AH 3 , supporting mid- to later-age strength development. However, the low-alkalinity environment limited the chemical activation of fly ash, causing it to function primarily as a physical filler. A sustainability assessment demonstrated that the ASW-UHPC significantly reduced environmental impact and cost, lowering CO₂ emissions by 71 % and production costs by 58 % compared to conventional UHPC. In summary, this work preliminarily demonstrates the technical, economic, and environmental feasibility of producing ASW-UHPC, providing a new pathway for the sustainable development of UHPC. To support the engineering applicability of ASW-UHPC, future research will prioritize the evaluation and improvement of its long-term durability. • Developed the UHPC composed entirely of industrial solid wastes. • Attains 125.7 MPa compressive strength within 24 h without steam curing. • Presents a rapid-early and sustained-strengthening hydration mechanism. • Substantially lowers CO₂ emissions and cost versus conventional UHPC.