Mineral Composition, Pore Structure, and Mechanical Characteristics of Pyroxene Granite Exposed to Heat Treatments

Abstract

In deep geoengineering, including geothermal development, deep mining, and nuclear waste geological disposal, high temperature significantly affects the mineral properties of rocks, thereby changing their porous and mechanical characteristics. This paper experimentally studied the changes in mineral composition, pore structure, and mechanical characteristics of pyroxene granite heated to high temperature (from 25 °C to 1200 °C). The results concluded that (1) the high-temperature effect can be roughly identified as three stages: 25–500 °C, 500–800 °C, 800–1200 °C. (2) Below 500 °C, the maximum diffracted intensities of the essential minerals are comparatively stable and the porous and mechanical characteristics of granite samples change slightly, mainly due to mineral dehydration and uncoordinated thermal expansion; additionally, the failure mechanism of granite is brittle. (3) In 500–800 °C, the diffraction angles of the minerals become wider, pyroxene and quartz undergo phase transitions, and the difference in thermal expansion among minerals reaches a peak; the rock porosity increases rapidly by 1.95 times, and the newly created pores caused by high heat treatment are mainly medium ones with radii between 1 μm and 10 μm; the P-wave velocity and the elastic modulus decrease by 62.5% and 34.6%, respectively, and the peak strain increases greatly by 105.7%, indicating the failure mode changes from brittle to quasi-brittle. (4) In 800–1200 °C, illite and quartz react chemically to produce mullite and the crystal state of the minerals deteriorate dramatically; the porous and mechanical parameters of granite samples all change significantly and the P-wave, the uniaxial compressive strength (UCS), and the elastic modulus decrease by 81.30%, 81.20%, and 92.52%, while the rock porosity and the shear-slip strain increase by 4.10 times and 11.37 times, respectively; the failure mechanism of granite samples transforms from quasi-brittle to plastic, which also was confirmed with scanning electron microscopy (SEM).

The Size Effect and Microstructure Changes of Granite after Heat Treatment

“…is is consistent with the high-temperature exposure of granite reported by Chen et al [41] and Shang et al [43].…”

Effects of Thermal Treatment on Mineral Composition and Pore Structure of Coal

“…As shown in Figure 2, the main mineral composition of three groups of coal 5 Geofluids specimens is basically unchanged; namely, no chemical changes occur after the thermal treatment. The result is consistent with that of granite [20]. As shown in Figure 2(a), the main mineral compositions of JJ coal specimens are kaolinite, calcium phosphate, and clinochlore.…”

“…Mineral Composition after Thermal Treatment. XRD tests are commonly used to investigate the mineral composition of coal and rock materials [20,39,46,53,54]. Therefore, XRD tests were carried out on coal powder after different temperature treatments (25, 50, 75, and 100°C).…”

“…Previous studies have proved that rocks and rock-like materials have significant thermal effects, and the physical and mechanical properties change significantly after thermal treatment. The physical and mechanical properties of granite after temperature treatments have been a research hotspot for underground nuclear waste disposal, and many scholars have analyzed its physical and mechanical properties, such as fracture toughness [10], rockburst proneness [11], uniaxial compressive strength [12,13], elastic modulus [13], longitudinal wave velocity [13], mechanical behavior [14,15], tensile strength [12,16,17], acoustic emission characteristics [18,19], mineral composition [20], pore structure [16,20,21], and permeability [22]. The physical and mechanical properties of sandstone after temperature treatments have also attracted a lot of attention; the properties such as thermal cracking process [23], peak strength [24], mechanical behavior in unloading conditions [25], wave velocity [26,27], porosity [27], triaxial mechanical behavior [28], permeability behavior [28], microstructure [26], pore characteristics [29,30], elastic modulus [29], tensile strength [31], energy evolution [32], microstructure deterioration [33], and morphological properties [34] have been investigated in depth.…”

Thermal jet drilling of granite rock: a numerical 3D finite-element study

“…beyond the α-β phase transformation of quartz), the thermal expansion is assumed constant. This assumption is a compromise between the quartz behaviour, which exhibits a linearly descending trend beyond the α-β phase transformation, and the feldspar behaviour, which increases linearly from the room temperature up to 800°C [24]. When a material loses its integrity in form of crack opening or void creation, the thermal properties should also change.…”