Behavior of Biochar-Modified Cementitious Composites Exposed to High Temperatures

In this study, the effect of biochar on the high temperature resistance of cementitious paste was investigated using multiple experimental methods. The weight loss, cracks, residual compressive strength, and ultrasonic pulse velocity (UPV) of biochar cementitious paste with 2% and 5% biochar exposed to 300, 550 and 900 °C were measured. The products and microstructures of biochar cementitious paste exposed to high temperatures were analyzed by X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis, and scanning electron microscopy. The results showed that the cracks of specimens exposed to high temperatures decreased with increasing biochar content. The addition of 2% and 5% biochar increased the residual compressive strength of the specimens exposed to 300 °C and the relative residual compressive strength at 550 °C. As the exposure temperature increased, the addition of biochar compensated for the decreasing ultrasonic pulse velocity. The addition of biochar contributed to the release of free water and bound water, and reduced the vapor pressure of the specimen. The addition of biochar did not change the types of functional groups and crystalline phases of the products of cementitious materials exposed to high temperatures. Biochar particles were difficult to observe at 900 °C in scanning electron microscopy images. In summary, because biochar has internal pores, it can improve the high-temperature resistance of cement paste.