Experimental Study On Damage Evolution Of Granite Structure Under Microwave High Temperature Irradiation

Microwave irradiation is considered as a potential and effective method for breaking rock.In order to improve the efficiency of microwave irradiation for breaking granite,a new method named microwave high-temperature irradiation was proposed.Structural evolution of granite irradiated by microwave between 300-800? was assessed through the morphology,mechanical performance and mineral characteristics.Furthermore,the structural difference of granite heated by microwave and traditional method(resistance heating,based on heat transfer)was also studied.Besides,the temperature parameter was optimized for granites of different grain sizes.In this study,the main conclusion can be drawn as follows:(1)Under microwave high-temperature irradiation,a spherical melt cavity with distinct radial cracks formed near the biotite-rich area at 600? and the granite completely disintegrated at 800?.The distinct cracking of traditional heated granite occurred at 700? but no melting was observed at 800?.(2)The scanning electronic microscope(SEM)results of microwave-irradiated granite at 600? indicated that the intergranular cracking dominated and crack propagation was affected by the distribution and shapes of mineral grains and original cracks.The melted minerals at 800? formed dense class substance with many bubbles and pores.(3)The uniaxial compressive strength(UCS)of granite irradiated by microwave declined from 88.17 MPa at 25? to 18.61 MPa at 800?.Between 600-800?,the decrease slowing was mainly due to the partial melt of rock.The UCS of granite heated by traditional method only decreased by approximately 20%at 300 QC and it presented no change between 300-800?.However,the continuous decrease of P-wave velocity with temperature increasing revealed the deterioration of granite structure.(4)XRD results presented that no distinct chemical reaction occurred in granite under the two conditions.The biotite diffraction peak intensity increased between 300-600? and declined with shifting to large angle between 600-800?.Biotite and feldspar melted at 800?.Moreover,the presence of quartz at 800? showed that the maximum temperature was between the melting point of feldspar(1400?)and quartz(1750?).(5)To sum up,the damage of granite between 300-600? under microwave high-temperature irradiation was mainly due to cracking induced by moisture releasing,quartz transition and thermal stress concentration;and 600-800?,it was mainly affected by the melt-flow-solidification of minerals.Furthermore,the melting cavity was considered to be originated from the cracking-intensive feldspar near the biotite-rich area.(6)The granite with coarse-medium grain size exhibited higher degree of damage at the same temperatures than the medium-fine one.However,the mechanical strength of granite specimens can be effectively weakened through controlling the surface temperature to 600?under microwave irradiation.Besides,the mechanical performance of granite irradiated by microwave was linked to the feldspar.Furthermore,the plagioclase was more sensitive to temperature.