Issue 50

Q. Hu et alii, Frattura ed Integrità Strutturale, 50 (2019) 638-648; DOI: 10.3221/IGF-ESIS.50.54 644 SEM SEM provides a convenient path to observe the fracture surface of original and treated specimens after UCS testing, as given in the Fig. 6. The number and dimension of cracks significantly increased with temperature increasing. Those were also found the internal cracks of biotite and a radially oriented fracture which originated from the biotite (see in Fig.6 (g)). Besides, the shape of fracturing edge at low temperatures exhibited sharp. However, the sharpness declined with temperature increasing. In addition, specimens at high temperatures were destroyed accompanying with remarkable powdering during the UCS testing. These might be due to the numerous interlaced micro-cracks observed nearby the fracturing edge, seeing in Fig. 6 (f). Moreover, Fig. 6 (d) indicated that molten minerals formed the glass substance with numerous pores and bubbles. It is apparent that the strength of glass structure is far less than the original crystal framework, inducing the distinct decrease of UCS in group A at 1000 °C. Figure 6 : SEM images of fracture surface after uniaxial compressive strength testing XRD The XRD analysis enables obtaining of mineral phases variation before and after the two heating treatments (Fig. 7). The mineral composition exhibited unchangeableness after both heating treatments, but the variation of mineral crystal structures can be detected. The phase of quartz in two groups exhibit unchangeableness even at 1000 °C. However, diffraction peaks of both biotite and feldspar in group A vanished at 1000 °C due to amorphization. The powders were obtained from molten areas and mixed uniformly, which could eliminate the variability of components. Therefore, the glass substance observed in Fig. 6 (d) was mainly composed of feldspar and biotite. It also can be inferred that the maximum temperature was below the melting point of quartz. Besides, it also found in group A that the diffraction peak intensity of