Physical and micro-structural characteristics of limestone after high temperature exposure

Temperature is a major factor affecting physical and mechanical rock properties. With increasing temperature, a series of variations enlarge the internal defects within rocks, resulting in physical and mechanical rock property variations. To explore the influence of temperature on the physical and micro-structure of limestone, the weighing test and P-wave velocity test were conducted on limestone after exposure to high temperature to reveal the evolution of the limestone mass and P -wave velocity. XRD, XRF, SEM, and mercury intrusion tests were also carried out to examine the mineral composition and content, micro-fracture morphology, porosity, and fractal dimension. The limestone mass and P-wave velocity decrease with increasing temperature. When T ≤ 400 °C, there is no obvious change in chemical composition and crystal structure; when T = 400–500 °C, the diffraction intensity of partial calcite decreases, and dolomite decomposes gradually; when T > 500 °C, illite decomposes gradually, while dolomite decomposes completely; and the diffraction intensity of calcite is significantly reduced. When T ≤ 200 °C, changes in trace minerals or impurities containing K₂O and Na₂O and the decomposition of illite oxide are dominant; when T = 400–600 °C, illite oxide, trace minerals, or impurities containing K₂O and calcium hydroxide begin to decompose. When T = 600–800 °C, magnesite and dolomite begin to decompose. When T ≤ 200 °C, micro-fracture surfaces change slightly. When T = 200–500 °C, micro-fractures begin to develop and propagate gradually, most of which are intergranular cracks with a small number of transgranular cracks. When T > 500 °C, transgranular cracks occur in samples with locally broken crystals, and the cracking of the crystal structure occurs with increasing pore size. With increasing temperature, the limestone pore fractal dimension decreases gradually, and the higher the temperature, the greater the decrease. 400 °C to 500 °C is the temperature threshold interval that causes the pore structure change. These new pores, resulting from the increasing temperature, are primarily mesopores with a pore diameter of 1.0–10.0 μm. This research provides a scientific basis for the design and construction of rock engineering projects to be subjected to high temperatures, deep geological radioactive nuclear waste disposal sites, deep mines, and the exploitation of geothermal resources.