Effect of high temperature and cooling method on compression and fracture properties of geopolymer-based ultra-high performance concrete

High-temperature damage to concrete is one of the most common hazards to be experienced during its service life. In this study, the mechanical properties of geopolymer-based ultra-high performance concrete (GUHPC) after high temperature exposure using different cooling methods were investigated. The specimens were heated to 200 °C, 400 °C, 600 °C, 800 °C and 1000 °C, followed by air and water cooling. Then the quasi-static tests were conducted with the assistance of a digital image correlation (DIC) device to examine the compressive and flexural strength, elastic modulus, fracture performance, and failure patterns. In addition, the microstructural change of GUHPC after high temperature exposure was analysed utilizing X-ray Diffraction (XRD) and Scanning Electron Microscope (SEM) methods. Test results revealed that GUHPC demonstrated different degrees of thermal damage at high temperatures ranging between 200 °C and 1000 °C but exhibited superior thermal spalling resistance. The reduction in strength, elastic modulus and fracture properties after high temperature exposure can be divided into three stages, i.e., 25–200 °C, 400–600 °C and 800–1000 °C, respectively. These three stages defined the mild, moderate, and severe damage. Compared to air cooling, water cooling caused thermal shock to specimens, resulting in a greater loss of strength. The microstructural analysis indicated that the damage to steel fibres along with the formation of new substances due to phase transition at high temperatures led to variations in the internal structure, consequently affecting the mechanical performance of the studied GUHPC. • Effect of temperature and cooling method on GUHPC performance was thoroughly examined. • New empirical formulas describing GUHPC behaviour upon heating and cooling were proposed. • The mechanism of GUHPC deterioration upon heating and cooling was analysed. • The fracture mechanics of GUHPC at elevated temperatures were investigated.