Microscopic characterization of microcrack development in marble after cyclic treatment with high temperature

Crack density of rocks is greatly affected by high temperature treatment and the induced thermal damage influences the strength and deformation characteristics of the rock. A good understanding of thermal cracking behavior is useful for geological evaluation of engineering structures associated with high temperature problems. This study investigates the characteristics of thermally-induced microcracks in a fine-grained dolomitic marble with different degrees of thermal damage using an optical microscope. Different degrees of thermal damage were first generated by treating the rock specimen with different heating and cooling cycles. Optical microscopy was then used to characterize the microcrack type and statistically examine the width, length, and anisotropy of thermally-induced microcracks. The results reveal that most of the generated microcracks induced by cyclic high temperature treatment are grain boundary microcracks. The width and length of microcracks significantly increases with an increasing number of heating and cooling cycles. It is also found that both grain boundary microcracks and intra-grain microcracks do not show predominant direction after thermal treatment. Finally, a quantitative relation is established to correlate the mechanical behavior of rocks (i.e., strength and modulus) with the crack density. The proposed relation is useful in understanding how the microstructure affects the properties of rocks after treatment with high temperature.