A novel elevated-temperature heat-absorbing aggregate for enhancing the high-temperature performance of ultra-high performance concrete

Ultra-high performance concrete (UHPC) is susceptible to spalling and degradation of mechanical properties at high temperatures. In this study, a novel elevated-temperature heat-absorbing aggregate (EHA) was developed, and its effects on the mechanical properties, spalling behavior, and internal vapor pressure at high temperature were thoroughly investigated. Experimental results indicate that the incorporation of EHA effectively mitigated spalling of UHPC and significantly reduced the degradation of compressive strength after exposure to 800 °C. The residual strength could be maintained at 48.8% of the room-temperature value, which was 1.7–2.0 times higher than conventional UHPC. The endothermic function of EHA could absorb heat and reduce the temperature rise of UHPC during heating, thereby mitigating the formation of microcracks, and the molten phase of EHA could also seal pores when penetrating into the matrix, thereby mitigating thermal degradation of UHPC. The inherent porosity of EHA passively promoted permeability at high temperatures, which facilitated vapor transport and enhanced spalling resistance.