Issue 50

Q. Hu et alii, Frattura ed Integrità Strutturale, 50 (2019) 638-648; DOI: 10.3221/IGF-ESIS.50.54 639 presented a second decline at 1000 °C. Compared to heat transfer, microwave irradiation can reduce the strengthening due to localized transition plasticity and further promotes the deterioration of rock structure in weakening stage. TG/DSC results indicate that the strengthening is related to the iron mineral transition. The formation of porous glass substance which is mainly composed of feldspar and biotite. Furthermore, temperature-controlled microwave irradiation induced the variation of feldspar FWHM, which is consistent with the corresponding UCS data, especially the plagioclase. In practical application, microwaves can be used to irradiate the vulnerable positions (surface edge and cleavage) and kept the whole rock mass around 600 °C. K EYWORDS . Granite; Thermal damage; Microwave irradiation; Heat transfer treatment, Frattura ed Integrità Strutturale, 50 (2019) 638-648. Received: 15.03.2019 Accepted: 02.09.2019 Published: 01.10.2019 Copyright: © 2019 This is an open access article under the terms of the CC-BY 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. I NTRODUCTION n recent years, due to the demand of deep geological applications, microwave-assisting mechanical method is profoundly discussed for more efficient excavation and lower energy consumption [1,2,3]. It means that rock is pre- irradiated by microwaves and subsequently excavated by mechanical tools. Actually, only a part of rock is concentrated exposed to microwave irradiation in practical application, so called the directly irradiated zone [4,5]. Artificially cracks within the directly irradiated zone are induced to weaken the integrity of rock structure and reduce the level of excavation tools wear and maintenance. However, most of internal rocks are indirectly heated by the irradiated zone through heat transfer instead of directly irradiated by microwaves. Subjected to the different heating mechanisms, the damage of whole rock mass is apparently heterogeneous. Investigating the deterioration of rock mass under the two conditions can prevent the serious accident due to hastily excavation. Rocks responding to microwave irradiation relies on the rock-forming minerals absorbing the electromagnetic energy and converting into heat energy [6,7]. Rock-forming minerals exist different heating rates due to the otherness of dielectric properties [8,9,10]. Lu [10] further identified that most rock-forming minerals were weak microwave absorbers, e.g. quartz and feldspar, and the good microwave absorption of enstatite and biotite were linked to the contribution of ferrum. Consequently, thermal gradient appeared and leaded to selective cracking along the boundaries of good microwave absorbers [11,12,13]. In addition, both high microwave power level and long exposure time can induce the higher temperature and more serious damage by generating more heat [5,14]. Researchers normally employed resistance-heating furnace to simulate the rock at different high temperature conditions. Yang [15] declared that the failure mechanism of granite over 400 °C was attributed to the coalescence of intergranular and transgranular cracks in feldspar and quartz gains. Zhang [16] and Zhang [17] found that 200 °C and 500 °C were the temperature thresholds of the limestone microstructure deterioration. Sirdesai [18] demonstrated that the phase transition of quartz induced the sharply decrease of sandstone mechanical performances over 573 °C. Besides, mineral dehydration as temperature increased were also declared to weaken the rock structure [19-22]. Almost all literatures mentioned the deterioration of rock with temperature increasing is significantly related to the crystal mineral characteristics’ changing at specific temperatures induced by either microwave irradiation or heat transfer. Granite is one of most common hard rock in underground excavation. The deterioration of granite in a broad range of temperature by using resistance-heating treatments have been widely investigated. However, there are few references considering the structural deterioration difference of granite for the temperature greater than 400 °C exposed to microwave irradiation and heat transfer. This study will qualitatively describe the difference of granite structural deterioration over 400 °C treated by microwave irradiation and heat transfer on the basis of mineralogical variation and furthermore assess the appropriate irradiating condition in practical engineering applications. Therefore, the uniaxial compressive strength (UCS) testing was employed in order to obtain the forces acquired for destroying the specimens, and the thermogravimetric/differential scanning calorimetry analysis (TG/DSC), Scanning electronic microscope (SEM) and X-ray diffraction (XRD) were used for characterizing the variation of crystal minerals in granite. I