The effect of high temperature on the physical and mechanical properties of ignimbrites used as building material

One of the factors that causes significant changes in the physical and mechanical properties of rocks is high temperature. This effect results in permanent damage to the rocks, leading to their deterioration and consequently a reduction in their strength. During a fire, temperatures can reach very high values over time. Experiments conducted considering the post-fire condition are typically performed on samples that are first heated and then cooled, evaluating the effects of temperature. However, studies investigating the temperature-dependent physical and mechanical properties of certain rock types used solely as building stones in our country are quite limited. This study aims to assess the changes in physical and strength properties and long-term durability of ignimbrites used as building stones when exposed to high temperatures, and to determine the threshold stress values of crack progression stages. For this purpose, ignimbrite samples with different colors and textures produced as building stones in the Cappadocia Region (7 different ignimbrite building stones) were collected. These samples were subjected to various temperatures ranging from 0°C (not exposed to thermal effects) to 1000°C and then slowly cooled to room temperature. The physical and mechanical properties of all samples were determined and evaluated. During the strength tests, the Acoustic Emission (AE) technique was used to determine the threshold stress values associated with crack progression stages due to stress increase. Additionally, Scanning Electron Microscope (SEM) examinations were conducted on selected ignimbrite samples to observe mineral changes and crack development related to temperature. According to the results obtained from the study, although there were minor fluctuations, the bulk density decreased with increasing temperature, while the specific gravity did not show significant changes with temperature. Specifically, at 800°C, the uniaxial compressive strength of each sample decreased compared to 600°C, but increased again at 1000°C. The elastic modulus values obtained from the stress-strain curves were found to be consistent with the behaviors observed in the uniaxial compressive strength tests. The changes in the initial crack initiation and crack coalescence threshold stress levels with temperature, determined using the Acoustic Emission technique, were found to be parallel to the changes in UCS values, with these levels being approximately 18% and 55% of the UCS values, respectively. SEM images revealed that at 1000°C, there was no change in the minerals of the samples, but melting in the ash matrix occurred, which is thought to affect the rock's strength indirectly by causing weakening. At 800°C, crack development was observed, while at 1000°C, cracks deepened and spalling occurred in the ash matrix.