Macro- and micro-mechanical response and damage mechanism of sandstone under high-temperature conditions

Abstract

The thermal effects of coal combustion considerably influence the physical and chemical properties, structural characteristics, and stability of rocks, posing a serious threat to the safety of coal mining operations. In this study, the impacts of temperature on the physical and chemical characteristics (i.e., mineral phase, microstructure, and mechanical strength) of sandstone were investigated by employing experimental methods, including microstructural analysis, uniaxial acoustic emission (AE), and nuclear magnetic resonance (NMR). The results indicate that temperature alters the mineral phase and the pore characteristics, and these two factors jointly affect the mechanical properties of sandstone. The influence of temperature on the mechanical strength of sandstone is categorized into low-temperature strengthening and high-temperature damage, with a threshold temperature identified at 600 °C. The low-temperature strengthening effect encompasses both pore strengthening and mineral phase strengthening, while the high-temperature damage effect primarily results from pore damage. As the experimental temperature rises, both the number of AE events and the AE energy transition from a surge in the post-peak failure stage to a stepwise increase during the loading process. This transition implies that the failure mode of the sandstone sample evolves from brittle failure to tensile failure.

1. Introduction

During coal combustion, an enormous quantity of heat will be released [1–3]. Besides, the combined effects from the structural stress arising from the formation of a combustion cavity and the thermal stress induced by the elevated temperature will result in surrounding rock damage [4–6]. As a consequence, the mineral phase and microscopic structure of the surrounding rock will experience notable alterations. This process gives rise to a multitude of pores and cracks within the rock mass, which severely threatens its stability [7] and compromises engineering safety. Therefore, investigating the failure mechanisms of thermally damaged rocks is crucial for maintaining safety in underground coal mining [8].

The type, structure, and pore characteristics of rocks are the primary factors influencing their bearing capacity and susceptibility to fracture failure [9]. The influence of temperature on the conventional physical and chemical properties of rocks is well established and acknowledged by experts in related disciplines [10–12]. As the experimental temperature rises, the physical and mechanical parameters of rocks, such as weight, Poisson’s ratio, elastic modulus, and wave velocity, exhibit a decrease, whereas their porosity and permeability demonstrate an increase. These findings lay a solid foundation for research on high-temperature rock mechanics. Pathiranagei et al. [13] explored the engineering properties of four types of rock including sandstone and basalt under high-temperature conditions, and obtained the critical temperature for the strength change of each type. Deng et al. [14] found that high-temperature heating changes minerals within the rock and thereby induces a decrease in its uniaxial compressive strength. Moslehy et al. [15] reported that rocks composed of fine particles are more susceptible to temperature, and rock strength is inversely proportional to heat treatment temperature. Zhao et al. [16,17] reached the conclusion that the microstructures and mineral crystals of rocks also transform with the change in temperature, which eventually causes changes in their damage characteristics and strength during the load-bearing process. These changes have a profound impact on the physical and mechanical properties of rocks.