Issue 50

Q. Hu et alii, Frattura ed Integrità Strutturale, 50 (2019) 638-648; DOI: 10.3221/IGF-ESIS.50.54 641 the uniaxial compressive strength was measured by the data acquisition system. The fragments of each specimen were collected for the subsequent characterization and the testing rig was then cleaned in order to avoid the samples mixing. Characterization TG/DSC analysis was carried on a thermogravimetric and differential scanning calorimetry synchronous analyzer (SDT Q160, TA Instruments) with a heating rate of 10 °C/min under air atmosphere for obtaining temperature-dependent characteristics of the granite. A scanning electronic microscope (EM8000F, KYKY) was employed to observe the microtopography of fragment surface after uniaxial compressive strength testing. The fragments were glued to conductive resin and sprayed with golden powders before testing. The mineral phases before and after heating treatments were confirmed by the powder X-ray diffraction (XRD, RINT) with Cu Kα radiation (λ = 1.54184 Å). The XRD pattern was recorded at 2θ from 3 ° to 80 ° with a scanning rate of 8 °/min. Before TG/DSC and XRD testing, the powders in each group were mixed uniformly with absolute ethanol and milled in the agate mortar, and subsequently dried in drying oven. R ESULTS AND DISCUSSION Morphologies fter heating treatments, specimens were carefully inspected to identify the change of morphologies, as shown in Fig. 2 and Fig. 3. The color in two groups changed from gray-apricot to white-jacinth as temperature increased, but the color change in group A was more distinct at the same temperature. In particular, the further color change of specimen in group A, from white-jacinth to white-brown, was observed for the molten area at 1000 °C. The change in color of rock, which is mainly attributed to the mineralogy variation at specific temperatures, generally represents the thermal damage degree of rock. Thus, it preliminarily indicates that the deterioration of rock region directly irradiated by microwaves is serious than the area heated by thermal transfer at the same temperature. Next, from the macro-cracking initiation, the deterioration of specimen in group A is also more serious than the specimen in group B at the same temperature. Visible macro-cracks appeared when the temperature reached 900 °C in group A but until 1000 °C in group B. These cracks cross over the quartz and feldspar grains and connected together with biotite as the node, probably linked to the thermal mismatch and thermal-induced stress concentration [21,23]. In addition, a detached cleavage plane locating at the cubic edge is observed at 800 °C in group B, which belongs to feldspar considering the absence of cleavage in quartz. Moreover, a fractured cubic edge is found at 900 °C (Fig. 4 (g)), which is also caused by thermal- induced stress concentration. The cracking propagation illustrates that the heat energy required for cleavage plane failure is lower than the consumption for generating macro-cracks at surface. It further reveals that the cleavage and surface edge are the vulnerable positions of rock mass exposed to high temperatures. Figure 2 : Morphologies of specimens before and after microwave irradiation. A