Development of a novel SUHPC based on PC-CAC-C$ system: Rheology, early-age strength, and hydration kinetics

Achieving rapid setting, early-age strength and volumetric stability is critical for shotcrete used in repair applications. In this study, a novel sprayed ultra-high-performance concrete (SUHPC) incorporating a Portland cement–calcium aluminate cement–gypsum (PC–CAC–C$) ternary binder system is formulated to tackle challenges in rheology, shrinkage, and early-age strength. A series of SUHPC mixtures with varying C$/CAC ratios (0.25–1.0) and the addition of a 2 wt% alkali‑free accelerator were systematically investigated. QXRD, TGA, calorimetry, rheology, and SEM reveal that a C$/CAC ratio of 0.5 maximizes early ettringite formation, conferring a fourfold increase in thixotropy over the cast UHPC and achieving a 3 h compressive strength of 15.1 MPa. A stage-dependent synergy between the alkali-free accelerator and the PC–CAC–C$ system is identified: at early ages, the accelerator significantly enhances ettringite formation and accelerates the hydration of C₃S and CA by introducing Al(OH)₄^- and SO₄^2-; at later stages, it promotes strätlingite formation, mitigating CAC conversion of ternary system in SUHPC. Meanwhile, the ternary system provides intrinsic micro-expansion that offsets both accelerator-induced shrinkage and autogenous shrinkage of SUHPC, ensuring dimensional stability. This novel SUHPC achieves rapid setting, enhanced thixotropy, early strength, and micro-expansion, making it a promising candidate for large-scale shotcrete applications.