Thermal-Induced Chemical Changes in Pyroclastic Rocks From Niğde (Türkiye): Implications for Historical Building Materials

Abstract Rocks used in historical buildings exhibit a wide range of chemical compositions due to the diversity of volcanic activities and the complexity of their formation processes. The structural integrity and chemical form of these rocks can be significantly deformed when exposed to various environmental factors (wind, rain, earthquakes, fires, etc.). In particular, fires can have destructive effects on the mineral composition of these rocks. When building stones used in historical structures are exposed to high temperatures, chemical changes occur. These chemical changes can cause immediate damage to the structures and also reduce their resistance to environmental factors. To pre-emptively identify the chemical changes occurring in structures after a fire and to enhance the fire resistance of the rocks, preliminary laboratory studies are necessary. In this context, pyroclastic rocks collected from the Niğde region (Turkey) were subjected to thermal treatment at 450 °C, 650 °C, and 1000 °C based on the chemical changes observed through thermogravimetric analysis (TGA). Phase changes in the samples (as-received, 450, 650, and 1000 °C) were detected using X-ray diffraction analysis, microstructural changes were examined with scanning electron microscopic (SEM) analysis, and chemical changes were identified using SEM-energy-dispersive X-ray spectroscopy (EDS) and X-ray fluorescence (XRF) analyses. Significant changes were observed in the chemical composition, mineralogical structure, and texture of the rocks when subjected to thermal treatment. TGA analysis showed that chemical changes occurred at 450 °C and 650 °C. It has been determined that silicon and iron-containing minerals undergo phase changes depending on the increase in temperature (transformation of jarosite into Opal-CT and magnetite into hematite).