Experimental study of the thermal cracking mechanism of limestones during burning

Spontaneous combustion of coal seams will cause different degrees of structural damage to surrounding rocks at different high temperatures, significantly altering their physical and mechanical properties and engineering geological effects. To delve into the structural evolution patterns of rocks during burning, this study investigated the limestones from the floor of a coal mine in Xuzhou. It conducted thermal cracking experiments under different temperatures, exploring the thermal cracking patterns of rocks on multiple scales. Furthermore, this study revealed the genetic mechanism of thermal cracking by combining numerical simulations and microstructure tests. Key findings are as follows: (1) The thermal cracking temperature thresholds ranged from 300℃ to 400℃ for relatively weak structural planes like calcite-filled fissures and bedding and around 500℃ for intact rock structures. Below 600℃, rock samples exhibited relatively simple thermal cracking morphologies dominated by a few narrow linear cracks. Above 700℃, arc cracks with significantly increased widths and lengths emerged, displaying distinct colors on both sides. Besides, some secondary cracks connected to the arc cracks were observed in the white parts near the arc cracks. At 800℃, the rock samples all turned white, with structures completely broken and appearing in a dispersed form. (2) This study aimed to account for the genetic mechanism of thermal cracking of limestones from the perspective of microstructure and thermal stress. Findings indicate that the thermal cracking of rock samples primarily occurred as the temperature dropped in the experiments, with the maximum thermal stress differences primarily distributed on the approximately annular zones near the rock sample surfaces. Below 500℃, the thermal cracking was principally because the thermal stress was concentrated in the original defects of rock samples and exceeded the tensile strength of some weak structures. Above 500℃, the thermal cracking was jointly governed by thermal stress and mineral decomposition. In this case, the maximum thermal stress generally exceeded the tensile strength of limestones during temperature fluctuations. Furthermore, as minerals such as magnesite, muscovite, and dolomite gradually decomposed, the gradually increasing internal defects created favorable conditions for stress concentration, exacerbating thermal cracking. As the temperature reached 800℃, rock samples manifested a high degree of thermal cracking. This was accompanied by the rapid decomposition of dolomite and calcite, producing CaO. Part of CaO was exposed to the air during cooling, generating Ca(OH)₂. Therefore, thermal cracking and the decomposition of predominant minerals jointly led to the structural fragmentation of rock samples. The results of this study lay the foundation for analyzing the evolutionary mechanisms of rock mass fracture and geological structure.