Investigation of the microstructural characteristics of heated sandstone by micro-computed tomography technique

Heritage buildings always pose challenges due to experiencing high temperatures and pressure over time. Sandstone is one of the common sedimentary rock types used for these buildings. Therefore, it is very important to understand the microstructural variations of rocks associated with these constructions along with the mechanical variations. In this study, the microstructural and mechanical alteration of selected types of sandstones is investigated after it is heated from room temperature to 800 degrees C. Micro X-ray computed tomography (mu XCT), X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetry (TG) and derivative thermogravimetry (DTG) techniques were used to identify the physical, chemical, mineralogical and microstructural changes of sandstone after different heat treatments. The mechanical alteration of the heated rock specimens was also studied using the point load index (PLI). The main changes in microstructure were observed when the sandstone's temperature was greater than 400 degrees C. The total porosity measured by mu XCT of sandstone increased by more than 70% at 800 degrees C compared to its porosity at room temperature. When the temperature increases, the open porosity increases while the closed porosity decreases. Noticeable changes in rock mineralogy were identified at temperatures exceeding 400 degrees C, which can be attributed to the phase transition of quartz, decomposition of feldspar and dehydroxylation of kaolinite. The TG and DTG analyses and point load index were in good agreement concerning these changes in rock mineralogy. Microstructural variation is one of the main reasons for the discrepancy in threshold values between different sandstones.