Issue 50

Q. Hu et alii, Frattura ed Integrità Strutturale, 50 (2019) 638-648; DOI: 10.3221/IGF-ESIS.50.54 646 Figure 8 : FWHM vs. temperature of K-feldspar at 2θ = 27.4 ° and plagioclase at 2θ = 28.0 °: (a) microwave irradiation group and (b) resistance-heating group. C ONCLUSIONS comparative study is presented for investigating the deterioration difference of granite between directly heated by microwave irradiation and indirectly heated by heat transfer. Danfeng granite specimens were heated up to 400- 1000 °C and then kept for 15 min by using the microwave irradiation and resistance-heating treatment, respectively. From the experimental investigations, the following conclusions can be drawn: (1) UCS results detect that there is a similar variation in two groups between 400 °C and 900 °C, which is initial strengthening (less than 500 °C), subsequent weakening (500-600 °C) and final stabilizing (600-900 °C). In this range of temperature, the group A is lower than group B by approximately 20 MPa and 10 MPa respectively. The maximum reduction in group A is about 20 % around 600-700 °C, whereas about 10 % is displayed in group B at the same temperature. When temperature reached 1000 °C, group A presents a second reduction accompanied by partial melting while the group B is almost unchanged. (2) From the TG/DSC and XRD results, the strengthening below 500 °C is perhaps related to the transition of iron minerals at 300 °C. SEM and XRD results reveal that the feldspar and biotite melted and formed the deficient porous glass substance, resulting in the second decline at 1000 °C. Furthermore, temperature-controlled microwave irradiation was found to lead a similar and irreversible variation on feldspar FWHM, which is consistent with the corresponding UCS data. Especially plagioclase is more sensitive to microwave irradiation than K-feldspar, which means the FWHM of plagioclase can be used as a guide to pre-estimate the deterioration of granite irradiated by microwaves before carrying out mechanical performance testing. (3) Microwave irradiation is beneficial to ensure the stability of surrounding rocks by reducing the strengthening due to localized transition plasticity and further promotes the deterioration of rock structure in weakening stage. To meet the practical engineering, authors proposed employing microwave to irradiate the vulnerable positions (surface edge and cleavage) and kept the whole rock mass being heated up to 600 °C. Besides, the non-uniform heating and structural variability are detected which significantly affect the deterioration of granite exposed to heating treatments. Further research will be focused on quantizing these influences. A CKNOWLEDGEMENTS his research was supported by the National Natural Science Foundation of China (21677118). The authors would like to express sincere gratitude to Lanjie Hou of Key Laboratory of Solid Waste Treatment and Resource Recycle, Ministry of Education for the help in rock mineralogical analysis. R EFERENCES [1] Hassani, F., P. M. Nekoovaght, and N. Gharib. (2016). The influence of microwave irradiation on rocks for microwave- assisted underground excavation. J. Rock. Mech. Geotech. Eng. 8(1), pp. 1-15. DOI: 10.1016/j.jrmge.2015.10.004. A T