Effect of temperature on compression and fracture behaviour of ultra-high performance geopolymer concrete

This study investigated the residual mechanical properties of steel fibre-reinforced geopolymer ultrahigh performance concrete (GUHPC) after high-temperature exposure. The specimens were heated to 200°C, 400°C, 600°C, 800°C and 1000°C followed by air cooling to room temperature. Subsequently, quasi-static compressive and three-point bending tests were conducted with the aid of Digital Image Correlation (DIC) device to analyse the compressive and flexural properties, including compressive strength, bending strength, Young's modulus (46.1GPa at 25°C), and fracture toughness (5.12 MPa∙m^0.5 at 25°C). The experimental results revealed that the GUHPC specimens subjected to different degrees of thermal damage at high temperatures ranging from 200-1000 °C exhibited good thermal spalling resistance. Finally, X-ray diffraction (XRD) was employed for microstructural analysis to further validate the high-temperature changes mechanism of GUHPC. The results indicated that with increasing temperature, the compressive and flexural strength of GUHPC gradually decreased, specimens retained around 50% of both the compressive (from 138.6 MPa to 71.2 MPa) and flexural strength (from 16.1 MPa to 7.9 MPa) after exposure to 600°C, but over it the performance deteriorated rapidly. XRD analysis revealed that the phase changes formed under hightemperature conditions led to the expansion of pore structures, further damaging the dense structure of GUHPC, thereby reducing its strength.