Influence Mechanism of Temperature Paths on Hardness of Sandstone

Understanding the characteristics of rock hardness variation under different temperature paths is of great significance to expand the deep rock-breaking capacity. In this study, high temperature treatments with four temperature paths of slow heating-natural cooling (S-S), fast heating-natural cooling (F-S), slow heating-water cooling (S-F), and fast heating-water cooling (F-F) were conducted on sandstone specimens in the temperature range of 25-900 & DEG;C. Leeb hardness and indentation hardness tests were conducted on the rocks after heat treatment, and the influence of temperature paths on rock hardness and its mechanism was studied and discussed. In addition, chromaticity and wave velocity tests were conducted to analyze the correlation between the hardness of thermally-damaged rock and chromaticity and P-wave velocity. The results showed that, with increasing temperature, the Leeb hardness and indentation hardness of sandstone under S-S and F-S paths increased first (25-300 & DEG;C), then decreased (300-700 & DEG;C) and then increased (700-900 & DEG;C), while the Leeb hardness and indentation hardness under S-F and F-F paths decreased gradually. For natural cooling treatment, sandstone hardness under F-S path was slightly higher than that under S-S path before 500 & DEG;C, and under S-S path it was higher when the temperature reached 500 & DEG;C and hotter. Compared to natural cooling, water cooling resulted in more severe damage to the heat-treated sandstone, manifested in lower hardness values. Combined with scanning electron microscopy, it was found that different degrees of crack propagation caused by thermal stress generated by the temperature gradient in rocks under different temperature paths was the main reason for different degrees of rock damage. There was no significant correlation between rock chromaticity and hardness parameters, while a good linear correlation was shown between P-wave wave velocity and hardness parameters, indicating that the hardness of thermally-damaged rocks can be predicted by P-wave velocity to some extent.