Abstract
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.
Highlights
The properties of building materials exposed to high temperatures are directly related to the magnitude of risk faced by the building in the event of a fire
Various methods were used to compare the meso cracks and macro characteristics of the control and the biochar mixed paste specimens exposed to different temperatures (20, 300, 550 and 900 ◦ C)
The internal pores of biochar promoted the release of vapor pressure, which reduced the generation of cracks
Summary
The properties of building materials exposed to high temperatures are directly related to the magnitude of risk faced by the building in the event of a fire. The main causes of damage to cementitious materials exposed to high temperatures can be categorized into two areas: physical changes, such as cracks caused by vapor pressure; and chemical changes, mainly decomposition of cementitious hydration products that support strength. Previous studies showed that suitable fillers or additives contribute to the performance of cementitious materials exposed to high temperatures and improve the high-temperature resistance of concrete. Irshidat et al [4] investigated the thermal properties of nanoclaymodified mortars, and their results showed that the addition of the nanoclay had a positive effect on the residual strength when exposed to higher temperatures. The creation of a pore network in the material to reduce the pore pressure when exposed to high temperatures was established as an effective method to reduce thermal damage to cementitious materials. Kalifa et al [5] reported that the melting of polypropylene (PP)
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