To address the low early-age strength of ultra-high-performance concrete (UHPC) under standard curing, this study develops an ultra-early-strength (UES)-UHPC. The cementitious materials system of the proposed UES-UHPC is a composite of sulfoaluminate cement (SAC) and ordinary Portland cement (OPC). Calcium oxide (CaO) is incorporated to boost early-age strength. The chemical admixtures primarily include an early-strength accelerator, a retarder, and an early-strength superplasticizer. To enhance the toughness of UES-UHPC, steel fibers (SF) were incorporated. X-ray diffraction (XRD) and scanning electron microscopy (SEM) are used to characterize hydration products and microstructures. The acoustic emission (AE) technique is applied during early-age flexural tests to track internal damage evolution. In addition, the flowability of the fresh UES-UHPC mixture and the mechanical properties of UES-UHPC were measured. This study focuses on the synergistic toughening and ultra-early-age compressive-strength enhancement afforded by the SAC–OPC–CaO–SF system. Results show that the proposed UES-UHPC achieves a compressive strength of up to 96 MPa at 2 h and 179 MPa at 28 days. The system rapidly forms an ettringite (AFt) and calcium–alumino–silicate–hydrate (C–A–S–H) skeleton together with a densified interfacial transition zone (ITZ), which in turn enables SF to bridge cracks effectively. Consequently, under ambient curing the system concurrently delivers ultra-early-age strength and high toughness. Moreover, a higher SF content leads to a larger proportion of shear-dominated cracks, especially near failure.
Ren et al. (2026) studied this question.