Issue 50

L. He et alii, Frattura ed Integrità Strutturale, 50 (2019) 649-657; DOI: 10.3221/IGF-ESIS.50.55 654 the dark area, these results demonstrate that biotite is distributed in the dark area. This is consistent with the conclusions of XRF. Mechanical performance The average peak stress of the specimens before and after microwave radiation obtained from the UCS test is shown in Fig. 7. The strength decreases significantly from 122.41 MPa (25 °C) to 45.49 MPa (600 °C), which is 37% of the virgin granite value. In addition, the samples’ strength at 700 °C and 800 ℃ could not be tested, and the strength was considered small, depending on the damage. It can be deduced from Fig. 7 that the strength exhibits a relatively slow decline below 500 °C. This could be attributed to the damage of the mineral crystal for losing the crystal water and structural water, which is consistent with the results of TG-DSC [28,29]. However, a sharp reduction was observed above 500 °C, which was mainly owing to the schistosity cracking and internal melting in the dark area. Furthermore, the structural changes in the granite caused by the quartz phase transition at 573 °C [9] and the thermal expansion mismatch between different minerals [32] also contributed to a reduction in the curve. Figure 7 : Average peak stress of the analysed specimens, before and after microwave radiation. D ISCUSSION he structural failure mechanism of microwave-irradiated Qingyuan granite was summarised, and the schematic diagram is shown in Fig. 8. Combined with the above analysis, the structure of the dark area was considered the main reason behind the structural failure of microwave-irradiated Qingyuan granite. Qingyuan granite exhibits an obvious schistosity structure. According to the results in Section 2.1, most of biotite is distributed in the dark area, while the light area is mainly composed of feldspar and quartz. Such distribution of minerals induced rapid heating of the dark area upon exposure to microwave radiation. Lu et al . [7] demonstrated that a 2.5-fold volumetric expansion of biotite occurred after microwave radiation, and stress was concentrated at the boundary of the biotite crystal [32]. Ali et al . [19] considered that cracks are mostly radially oriented tensile cracks that developed from the absorbent mineral boundary (such as biotite). The colour of granite became shallower with increasing temperature, which was attributed to the irreversible dehydration of Fe-rich minerals [31], which would also induce cracking. Therefore, cracking initially appeared around the microwave-absorbing phase in the dark area. Furthermore, it could be observed and inferred from Figs. 4d to 4f that most of the macro-cracks in the dark area were parallel to the schistosity trend above 600 °C. This could be also attributed to the schistosity structure. Microwave radiation would weaken the connection among minerals within granite [33]. Considering that schistosity is a compressive structural plane, the authors inferred that the mineral connection in the direction perpendicular to the schistosity plane is relatively weak. Therefore, micro-cracks would develop preferentially in the direction parallel to the schistosity trend and converge T