Spalling resistance of SUHPC based on PC-CAC-C$ at elevated temperatures: Role of whiskers and steel fibers

Sprayed ultra-high-performance concrete (SUHPC) holds great promise for structural repair and complex construction due to its exceptional strength and durability. However, conventional Portland cement-based SUHPC is highly susceptible to explosive spalling at elevated temperatures. This study develops a spalling-resistant SUHPC by introducing a Portland cement–calcium aluminate cement–gypsum (PC–CAC–C$) ternary system, reinforced with calcium sulfate whiskers (CSW) and steel fibers. The results reveal that the ternary system retained thermally stable crystalline phases (Al₂O₃, C₁₂A₇, wollastonite, CA, CA₂) up to 1000 °C, conferring superior phase stability compared with the PC-based system. At the microstructural level, the ternary system increased the fraction of capillary porosity. The incorporation of CSW promoted the accumulation of hydration products within the interfacial transition zone (ITZ), which refined the CSW–matrix interface and led to improved flexural performance. Notably, benefiting from the synergistic effect of CSW and steel fibers, the ternary system facilitated the formation of continuous and uniformly distributed vapor-release channels, which effectively suppressed explosive spalling and enabled SUHPC to retain a high residual compressive strength of 45.8 MPa and a flexural strength of 7.4 MPa even after exposure to 1000 °C. This study provides an effective strategy for enhancing the high-temperature resistance of SUHPC, thereby broadening its potential for durable structural repair and fire-resilient applications.