Influence of microwave treatment on mechanical behaviour of compact basalts under different confining pressures

Gaoming Lu ,Xiating Feng ,*,Yuanhui Li ,Xiwei Zhang

a State Key Laboratory of Geomechanics and Geotechnical Engineering,Institute of Rock and Soil Mechanics,Chinese Academy of Sciences,Wuhan,430071,China

b Key Laboratory of Ministry of Education on Safe Mining of Deep Metal Mines,Northeastern University,Shenyang,110819,China

Keywords:Microwave exposure Temperature distribution Conventional triaxial compression(CTC)Stress-strain curve Elastic constants

A B S T R A C T The realisation of microwave-induced fracturing of hard rocks has potential significance for microwaveassisted mechanical rock fracturing and stress release in deep rock masses.In this context,compact basalts were treated by microwave heating in a multi-mode cavity at a frequency of 2450 MHz,and then,we investigated the mechanical behaviour of basalt samples after microwave treatment under uniaxial compression and conventional triaxial compression (CTC)tests.After microwave exposure,cracks appeared on the surface and inside of the rock sample,and the temperature of the sample’s surface was unevenly distributed.The results show that the conventional triaxial compressive strength(CTCS)of basalt samples decreased linearly with microwave exposure time,and the higher the confining pressure,the smaller the reduction in the strength of basalt samples after microwave treatment.Under uniaxial compression,microwave exposure greatly affected the axial deformation,suggesting that deformation resistance of the samples gradually decreases with increasing microwave exposure time.Under triaxial compression,some microcracks induced by microwave exposure closed due to the effect of confining pressure,resulting in the confining pressure inhibiting any rightward shift of the axial deformation curve.Furthermore,under uniaxial compression,the elastic modulus and Poisson’s ratio of basalts also decreased in a quasi-linear manner with elapsed microwave exposure time.Under triaxial compression,microwave exposure has slight influence on elastic modulus and Poisson’s ratio.After microwave treatment,the changes in rock strength and deformation mainly result from changes in between the mineral structures.Confining pressure results in the closure of microcracks produced by microwave exposure,so that effects of microwave treatment on strength and deformation decrease,thus reducing the influence on elastic constants.The cohesion decreases with increasing microwave exposure time and shows an approximately linear decrease over time.In the basalt samples,new microcracks in various directions generated by microwave exposure can increase the discreteness of test results,while the discreteness of test results caused by microcracks gradually reduces with increasing confining pressure.

1. Introduction

Microwave-induced fracturing of hard rock has been considered as a potential method for rock breaking and drilling(Jerby et al.,2002;Hassani et al.,2016).The technology of microwave-induced fracturing for the surface of hard rocks can assist mechanical rock breaking,prolong the service life of mechanical tools,and improve the efficiency thereof(Lu et al.,2019a,b).For example,the method could solve problems related to the wear of disc cutters in excavation of tunnels in hard rock by tunnel boring machine(TBM)(Hassani et al.,2016).In boreholes in hard rock,microwave-induced fracturing can release stress in rock masses encountered in deep underground engineering operations and avoid geological disasters caused by high stress concentration,for instance,it can predict rockbursts or reduce the risks thereof.

Microwaves are electromagnetic waves with frequencies ranging from 300 MHz to 300 GHz and frequencies for microwave heating of 915 MHz or 2450 MHz were generally used.Microwave heating is different from conventional heating as it does not require heat conduction,while ionic conduction and dipole rotation are the main principles of microwave heating:polar molecules or dipoles in dielectric materials move back and forth rapidly in highfrequency alternating current field and generate thermal effects due to friction (Jin, 2001). The amount of heat produced by dielectric materials in microwave fields is related to the types and dielectric characteristics(dielectric constant and dielectric loss)of materials(Monti et al.,2016).Rocks are heterogeneous materials and generally comprise multiple mineral compositions.Due to the different dielectric characteristics and thermal expansivities of mineral compositions in rocks, different mineral compositions impose distinct thermal effects and thermal expansivities.Thermal stress generated by non-uniform thermal expansion of mineral compositions of rocks under microwave irradiation can result in intergranular and transgranular fractures in rocks(Kingman et al.,1998).When the thermal expansive stress exceeds the ultimate tensile strength of rocks,rocks suffer damage and can be broken,thus completely losing their bearing capacity.When the temperature rise induced by thermal effect exceeds the melting point of rocks,rocks are melted.

In mineral processing,microwave treatment can improve the efficiency of fracturing,screening,and milling of ores and reduce energy consumption(Kingman and Rowson,1998;Kingman et al.,2004).The University of Nottingham(UK)has conducted largescale microwave treatment tests and numerical analysis and attempted to apply this technique to production(Batchelor et al.,2017;Buttress et al.,2017).After microwave exposure,Norwegian ilmenite showed new intergranular and transgranular fractures,and the higher the microwave power and the longer the exposure time,the more the cracks generated(Kingman et al.,1998).Moreover,ore grinding tests were carried out on a variety of ores before and after microwave treatment.Based on the Bond work index,the grindability of ores was investigated.Through testing,it is found that microwave exposure decreases the Bond work index of ores,indicating that microwave treatment can reduce the amount of energy required for breaking ores(Kingman et al.,2000,2004;Vorster et al.,2001;Omran et al.,2015).Grinding tests were carried out on four ores:ilmenite,refractory gold ore,copper ore,and carbonate rock after microwave exposure.The test showed that except for refractory ore,the Bond work indices of the other three ores decrease(Kingman et al.,2000).The point load strength of Swedish lead-zinc ores after microwave exposure in single-mode and multi-mode cavities was tested:the point load strength reduces with increasing microwave exposure time,and the singlemode cavity showed a higher heating efficiency than the multimode cavity(Kingman et al.,2004).According to the simulation results obtained by the finite difference numerical simulation system FLAC2D,areas where minerals(pyrite)sensitive to microwave show high temperature and wide propagation of cracks can be predicted(Whittles et al.,2003).The density of applied microwave power exerts important influence on the reduction in ore strength,and continuous waves have better heating effects than pulsed waves(Jones et al.,2007).Microwave heating combined with infrared temperature measurement can distinguish high-and lowgrade ore fragments and has been proposed as a potential excitation-discrimination technique to facilitate sorting of porphyry copper ores(John et al.,2015;Batchelor et al.,2016a).Dissociation and flotation tests were conducted on copper ores after microwave pre-treatment.The results demonstrated that ores treated by microwaves dissociate more copper minerals and have higher copper recovery rates(Scott et al.,2008;Batchelor et al.,2016b).

In geotechnical and mining engineering,some laboratory tests have been conducted in view of thermo-physical characteristics,strength,and strength-reduction characteristics of wave velocity of rocks treated by microwaves.For example,Hartlieb et al.(2012,2016,2018)studied thermo-physical characteristics and fractureinduced damage mechanisms of different rocks by utilising an industrial microwave oven with a multi-mode cavity at a frequency of 2450 MHz and an open ended waveguide setup.By using a multimode cavity at a frequency of 2450 MHz,Peinsitt et al.(2010)investigated the effects of microwave irradiation on uniaxial compressive strength(UCS),wave velocity,and heating characteristics of dry and water-saturated rocks(basalt,granite,and sandstone).Hassani et al.(2011,2016),Hassani and Nekoovaght(2011,2012)and Nekoovaght et al.(2014)investigated UCS and tensile strength of different rocks by employing a multi-mode cavity at a frequency of 2450 MHz.By comparing test results and numerical data,they studied the influence of microwave exposure distance on heating characteristics of rocks and discussed the promotion of microwave-assisted rock breaking for space-based mining in the future(Nekoovaght et al.,2014;Hassani et al.,2016).By utilising a multi-mode cavity at a frequency of 2450 MHz,Lu et al.(2017)investigated the microwave absorption capacity of common rockforming minerals.

As mentioned above,the existing results mainly focus on the physical characteristics,heating,and wave velocity characteristics of different rocks after microwave treatment.However,there are few studies of the stress-strain curve,elastic constants,cohesion,and internal friction angle of rocks after microwave exposure.By utilising a multi-mode cavity at a frequency of 2450 MHz,this study conducted microwave heating tests on basalt samples under different powers and exposure times.Then,the UCS and conventional triaxial compressive strength(CTCS)tests were carried out on the samples after microwave treatment. Furthermore, we examined the mechanical behaviour of basalts after microwave treatment and analysed the discreteness of the test results.

2. Test materials and methods

2.1. Sample preparation

Basalts are basically volcanic rocks widely distributed in the Earth.Basalts can be generally categorised as compact basalts and porous glassy basalts: the former basalts show a higher UCS(≥300 MPa),while the strength of the latter is lower(＜300 MPa).Compact basalts are highly capable of adsorbing microwaves and are typical of hard rocks.Therefore,it is worthwhile studying the influence of microwave treatment on the mechanical characteristics of compact basalts.

Intact compact basalts without visible macrocracks sampled from Chifeng,Inner Mongolia Autonomous Region of China,were used in this study.Mineral compositions and structures of the basalts were determined through X-ray diffraction(XRD)test and observation with hand-lens.The XRD pattern(Fig.1)demonstrates that the basalts mainly consist of 60% plagioclases,20% pyroxenes,15% olivines,and 5% metallic minerals and other substrates.The observations with a hand-lens are shown in Fig.2,in which the basalts are olivine basalts with blocky and intergranular structures.Plagioclases are of idiomorphic columnar form,while pyroxenes are mainly composed of fine particles distributed in the skeleton of plagioclases.Moreover,olivines are composed of subidiomorphic particles.

The basalt block was cored to obtain standard cylindrical samples with diameter of 50 mm and height of 100 mm for microwave heating,UCS,and CTCS tests.All samples were obtained from the same rock block in the same direction,following the test specification recommended by the International Society for Rock Mechanics and Rock Engineering(ISRM)(Bieniawski and Bernede,1979).After preparation,the samples were dried for 48 h in an electric oven at 110°C and then cooled down to room temperature for measuring wave velocity and rock density prior to microwave heating tests.

Fig.1.XRD pattern of the Chifeng basalt.CPS stands for counts per second.

2.2. Microwave heating test

A CM-06S multi-mode cavity microwave device(Fig.3)(Lu et al.,2019a)was used for the microwave heating test.A continuous microwave generator with a frequency of 2.45 GHz was used and its output power was adjustable within the range from 0 to 6 kW. A WR430 standard rectangular waveguide measuring 109.2 mm×54.6 mm was used for transmission and a multi-mode cavity measuring 490 mm in side length was used in the microwave heater.The operating principle of the device is such that energy from continuous microwave generator is transmitted to the microwave cavity through the rectangular waveguide and adsorbed by rocks.

Fig.3.Picture of the multi-mode cavity microwave device(Lu et al.,2019a).

Three power levels(1 kW,3 kW,and 5 kW)were applied to samples prepared for UCS testing.At applied powers of 1 kW,3 kW,and 5 kW,the standard cylindrical basalt samples were broken and damaged after about 320 s,110 s,and 50 s,respectively.To study the mechanical behaviour of the samples before fracturing,the microwave exposure time needed to be less than the aforementioned breaking times.Therefore,at applied power of 1 kW,the microwave exposure times were set to 0 s,60 s,180 s,and 300 s;at 3 kW,we used 0 s,30 s,60 s,and 90 s;and at 5 kW,they were 0 s,10 s,20 s,and 30 s.The power was set to 5 kW for samples prepared for CTCS testing(Table 1).A coaxial transmission line technique was adopted to measure the dielectric properties of the basalt by using a Keysight E5063A vector network analyser(with a frequency in the range of 0.5-18 GHz).The test results of dielectric constant and loss factor of basalt samples are shown in Fig.4.By using an R500EX-Pro infrared camera(temperature measurement range of-40°C-2000°C and a frame frequency of 30 Hz)produced by NEC Avio Company,Japan,the surface temperature distribution of samples was measured.

2.3. UCS and CTCS tests

Fig.2.Micrographs of basalt facies(50×magnification):(a)Single-polarised light,(b)Cross-polarised light,and(c)Reflected light.

Table 1 Schemes for microwave heating and compression tests on basalts.

Fig.4.Dielectric properties(dielectric constant and loss factor)of the Chifeng basalt in the frequency range of 0.5-18 GHz.

The UCS and CTCS tests were conducted on untreated and microwave-treated basalt samples(cooled from normal temperature of 16°C to room temperature)by using the Rockman207 conventional triaxial test system.By utilising a linear variable differential transformer(LVDT)displacement sensor,the axial and circumferential deformations were measured. In the loading process, the loading mode was changed from load control to circumferential deformation control.The samples were loaded to the plastic deformation stage(about 70% of the sample strength)at a loading rate of 1000 N/s and then loaded to failure at the equivalent circumferential deformation rate,thus the complete stressstrain curve showing post-peak behaviours of the samples can be obtainied.In the CTCS test,the confining pressures were 10 MPa,30 MPa,and 50 MPa.To avoid discreteness of test results as possible,the UCS and CTCS tests were both repeated three times(Table 1)and test results were expressed as mean value(±standard deviation).

3. Test results and analysis

3.1. Heating effect and temperature distribution

Minerals sensitive to microwaves in rocks produce strong thermal effects.Different minerals generate thermal expansion stress under thermal effects due to different coefficients of thermal expansion.Under the effects of thermal stress,transgranular cracks appear in the same mineral particles in rocks,while intergranular cracks are found in between different mineral particles.When the thermal stress exceeds the ultimate strength,rocks are cracked and broken apart.After microwave exposure,crack propagation was displayed on the surface of basalt samples when exposed to microwaves for different durations,as shown in Fig.5.Fewer cracks are visible on the sample surface when exposed for a shorter time(at 5 kW and 10 s).With increasing radiation time,cracks gradually propagate,and the number of cracks increases.The main crack on the curved surface is approximately parallel to the long axis of the cylinder,and it extends to the end face and connects the crack on the end face.With increasing microwave irradiation time(i.e.after 50 s),a dominant fracture appears in the sample.Finally,the sample is cracked and broken apart after an irradiation time of 60 s.

Crack propagation inside the sample is observed by cutting the sample into half along the height of the cylinder.In comparison with the pictures before microwave treatment,it can be found that a series of new transgranular and intergranular cracks is generated in the samples after microwave treatment.Transgranular cracks mainly occur in olivine particles,while intergranular cracks mainly appear between olivine and other particles(Fig.6).For the basalts,pyroxenes can absorb significant amount of microwave energy(Lu et al.,2017).Olivines have a large coefficient of thermal expansion(Ahrens,1995).Therefore,for the microwave-induced fracturing of basalts, pyroxenes provide heat, while olivines offer thermal expansion stress.As microwave exposure time elapses,the temperature increases constantly and the generated thermal stress rises continuously.Consequently,transgranular and intergranular cracks expand continuously,widen,and interconnect,thus forming macrocracks or weak surfaces in the rock mass.

After the samples are exposed to microwave irradiation,the temperature of each sample surface is non-uniform(Fig.7).The basalt samples show different temperature distributions after different exposure times.The highest and average temperatures of the sample surface both increase in a quasi-linear fashion with increasing exposure time(Fig.8).The highest temperature in the sample reaches about 289.6°C(at 5 kW and 50 s),and the samples are broken.After that,the internal temperature exceedes the rock surface temperature,which is consistent with previous results(Lu et al.,2019a).After microwave irradiation for 60 s,samples are broken into fragments,with the highest temperature of 331.7°C.

The cylindrical samples,when exposed to microwave irradiation in a multi-mode cavity,exhibit two high temperature zones inside the cylindrical sample,which depends on the sample size:one inside the top half of the sample,and the other within the bottom half close to the bottom of the setup.This non-uniformity of temperature affects the mechanism of crack generation.The hightemperature region underwent greater thermal expansion,while the region at the lower temperature experienced less thermal expansion.The non-uniformity of temperature leads to the nonuniformity of thermal expansion, which further promotes the fracturing of such samples.

3.2. Deformation characteristics

Fig.9 shows the stress-strain curves of the basalts before microwave treatment and that treated with microwave radiation at three power scenarios under uniaxial compression.The stressstrain curves of the basalts under uniaxial compression show a large discreteness.The uniaxial compression of the basalts undergoes three stages,i.e.a linear elastic stage,a yield stage,and a failure stage.After microwave exposure,the compaction stage is prolonged,while the yield stage shortens.Moreover,the post-peak stress drops rapidly and the residual strength decreases,showing brittle characteristics.

Fig.5.Effects of microwave-induced fracturing of the cylindrical basalt samples with diameter of 50 mm and height of 100 mm.

Fig.6.Crack propagation inside the basalt sample viewed under an ultra-depth of field microscope at 300×magnification after microwave treatment.

Fig.7.Temperature distribution of the cylindrical basalt samples measured by using an infrared camera(room temperature is 16°C,and cross cursor refers to the location of the maximum temperature).

After microwave treatment,the axial deformation curve gradually shifts to the right.With elapsed exposure time,the rightward shift of curve increases,that is,axial compressive deformation increases with microwave exposure time.The axial deformation generated under uniaxial compression mainly includes elastic deformation of the skeleton of mineral particles and slippage and closure of microcracks.After microwave treatment,transgranular and intergranular cracks are found in between mineral particles,resulting in growing microcracks in the samples and increasing pore volumes between mineral particles.Therefore,under uniaxial compression, deformation caused by slippage and closure of microcracks increases,thus the axial deformation increases,and the bearing capacity of the samples decreases.In addition,deformation resistance of the samples gradually decreases with increasing microwave exposure time.

Fig.8.Temperature profile of basalt samples at different microwave exposure times.

Due to the compact structure of basalts,the circumferential deformation is much smaller than the axial one.The circumferential deformation also experiences several stages,i.e.elastic deformation, yield, weakening, and failure stages under uniaxial compression. After microwave treatment, the circumferential deformation curve does not shift as much as the axial deformation curve does.The pre-peak curve is coincident with that of the untreated samples and the post-peak plastic deformation increases.The deformation characteristics of the basalts are closely related to their mineral compositions,internal defects and densities.After microwave treatment,the changes in the deformation characteristics of basalts are likely that microwave irradiation changes the structure of internal mineral particles and increases the number and severity of internal defects in the samples.

The stress-strain curves obtained by the CTCS test are displayed in Fig.10.One can see that with increasing confining pressure,the compaction stage becomes shorter and even disappears,while the pre-peak yield stage becomes more obvious.In other words,the plastic deformation occurs before the peak and the peak strain gradually increases.The point at which the peak strength appears gradually shifts to the right,indicating that the peak strength increases.Moreover,basalts exhibit a gradual transition from brittle to ductile failure,and the rate at which the post-peak stress drops decreases while the residual strength increases.After microwave treatment,the axial deformation curves under triaxial compression gradually shift to the right with increasing microwave exposure time and the post-peak residual bearing capacity decreases.As the confining pressure rises,the rightward shift of the axial deformation curve gradually decreases.The reason is that due to the confining pressure,some microcracks in the samples caused by microwave exposure close.With increasing confining pressure,microcracks close completely and slipping at microcracks is reduced.Therefore,the confining pressure inhibits the rightward shift of the axial deformation curve,i.e.the higher the confining pressure,the smaller the effect of microwave treatment on sample deformation.

3.3. Strength reduction

After treatment, the UCS decreases at all three microwave powers used(1 kW,3 kW,and 5 kW),in conjection with microwave exposure time.The UCS decreases in an approximately linear manner with increasing microwave exposure time.The greater the applied microwave power,the faster the decrease rate of UCS(Lu et al.,2019a).Minerals(pyroxenes)sensitive to microwave in basalts show significant heating effect under microwave irradiation,while significant thermal expansion occurs in minerals(olivines)with high coefficient of thermal expansion. This results in occurrence of transgranular cracks in olivine particles and transgranular cracks between olivines and plagioclases,thus increasing microcracking in the samples and pore volumes between mineral particles.With increase of microwave exposure time,microcracks slip and interconnect to develop into weak surfaces.The presence of weak surfaces affects the strength of the rock.In comparison with samples not treated by microwave radiation,the strength of treated rocks reduces to different extents.The longer the microwave exposure time,or the higher the applied microwave power,the more developed the weak surfaces, and the greater the reduction in rock strength.At a certain microwave power,the rock strength decreases quasi-linearly with increasing microwave exposure time.This demonstrates that the development of microcracks or weak surfaces in rocks after microwave treatment monotonically increases with microwave exposure time.

At applied power of 5 kW,the relationship between CTCS of basalts and microwave exposure time is shown in Fig.11.Under four confining pressures(σ3=0 MPa,10 MPa,30 MPa,and 50 MPa),the CTCS reduces at different rates with increasing microwave exposure time.At 30 s exposure,the CTCS reduces by 26.87% ,7% ,2% ,and 2.98% under the four confining pressures,respectively.It is worth noting that,with the increase in confining pressure,the CTCS of the basalts gradually reduces with exposure time according to slopes of the fitting curves in Fig.11,and discreteness of strength decreases in accordance with the error line plotted for these strength data.This indicates that the confining pressure inhibits reductions in discreteness of the CTCS of rocks subjected to microwave treatment,as confining pressure can cause closure of some microcracks in the samples and increase frictional forces that prevent slippage of microcracks.With increasing confining pressure,microcracks close completely and slippage requires greater forces to overcome friction.Therefore,the confining pressure inhibits the decrease of CTCS.The discreteness of CTCS is mainly caused by heterogeneity of microcracks,while the confining pressure leads to closure of microcracks in the samples and reduction of microcracks that produce slippage.Therefore,the confining pressure can decrease the discreteness of CTCS.When using microwaveinduced fracturing in underground engineering operations,the influences of geological factors,such as in situ stress,should be considered.

As rocks are heterogeneous materials,the differences between the samples can cause discreteness in the test results.Minerals sensitive to microwaves with strong thermal expansivity are randomly distributed in the samples and a large number of microcracks in different directions are produced therein after microwave treatment.Therefore,microwave exposure can further increase the discreteness of the test results:the higher the microwave power and the longer the exposure time,the greater the discreteness of the test results.When significant crack propagation occurs or weak surfaces are developed in samples after microwave exposure,the bearing capacity thereof decreases.The comparison of the results of UCS(Lu et al.,2019a)and CTCS reveals that the confining pressure inhibits discreteness of basalt strength and the strength differences induced by microcracking gradually decrease with increasing confining pressure.

3.4. Elastic constants

Fig.9.Stress-strain curves of the basalt samples exposed to microwave radiation for different times at three applied power levels under uniaxial compression:(a)P=1 kW,(b)P=3 kW,and(c)P=5 kW.

Fig.10.Complete stress-strain curves of the basalt samples exposed to microwave radiation for different durations under different confining pressures in triaxial compression tests:(a)σ3=10 MPa,(b)σ3=30 MPa,and(c)σ3=50 MPa.σ1 is the axial stress and σ3 is the confining pressure.

The elastic modulus and Poisson’s ratio are two important mechanical parameters governing rock deformation.By selecting the linear segment of the stress-strain curves,the average elastic modulus and Poisson’s ratio can be calculated after linear fitting of the data.The calculated results are shown in Figs.9 and 10.The basalts,characterised with compact structure,have an average elastic modulus of 97 GPa obtained by UCS test.After microwave treatment,the elastic modulus(Fig.12)and Poisson’s ratio(Fig.13)of the basalts under uniaxial compression both decrease to some extent at 1 kW,3 kW,and 5 kW.At the three applied powers,the decrease rate in elastic modulus always exceeds that of the Poisson’s ratio(Fig.14),and elastic moduli decrease by 21.72% ,24.83% ,and 22.34% ,while the Poisson’s ratios decrease by 10.84% ,19.23% ,and 22.03% ,respectively.The elastic modulus and Poisson’s ratio have an approximately linear decreasing relationship with microwave exposure time.Based on the slope of the best-fit curves,it is known that the higher the applied microwave power,the greater the reduction rates of elastic modulus and Poisson’s ratio.

After microwave treatment,the decrease in elastic modulus indicates that microwave treatment can reduce the bearing capacity of a rock mass.In the CTCS test,the elastic deformation of rocks is mainly determined by the skeleton of mineral particles.Microwave treatment leads to the appearance of transgranular and intergranular cracks in between mineral particles of rocks.Due to increased interconnection of transgranular and intergranular cracks, new microcracks are formed and the initial cracks propagate,widen,and connect.Therefore,microwave treatment changes the skeleton structure of mineral particles in a rock mass,so that the resistance of rocks to elastic deformation weakens,and the bearing capacity of rocks also tends to decrease.

Fig.11.Reductions in compressive strength of basalts under different confining pressures.

Fig.12.Relationships between elastic modulus of basalts and microwave exposure time under uniaxial compression.

Fig.13.Relationships between Poisson’s ratio of basalts and microwave exposure time under uniaxial compression.

Microwave treatment exerts little influence on the elastic modulus and Poisson’s ratio of basalts under triaxial compression.At confining pressures of 10 MPa and 30 MPa,the elastic modulus decreases slightly with increasing exposure time,while the elastic moduli remain unchanged over different exposure times at a confining pressure of 50 MPa.Under the three confining pressures,the values of Poisson’s ratio show certain discreteness.After microwave treatment,the decrease in elastic modulus and Poisson’s ratio of basalt samples mainly results from increased microcracking.Nevertheless,the confining pressures can close microcracks.Therefore,the higher the confining pressure,the smaller the influence of microwave treatment on deformation,which exerts smaller effects on elastic modulus and Poisson’s ratio.

Fig.14.Changes in elastic modulus and Poisson’s ratio of the basalts under uniaxial compression.

Table 2 CTCS test results of basalt samples at different exposure times.

Fig.15.Relationships between cohesion and internal friction angle of the basalts with microwave exposure time.

3.5. Cohesion and internal friction angle

Cohesion of rocks refers to as the attractive forces between molecules on the surface of adjacent mineral particles. After treatment,the peak strength of the samples at different exposure times increases monotonically with confining pressure,following the Coulomb strength criterion.The Coulomb strength criterion is used to determine cohesion c and internal friction angle φ of basalt samples(Table 2 and Fig.15).After treatment,the cohesion decreases with increasing microwave exposure time and shows a quasi-linear decreasing relationship with exposure time.At microwave power of 5 kW and exposure times of 10 s,20 s,and 30 s,the cohesion is reduced by 14.47% ,13.21% ,and 24.57% ,respectively.In accordance with the slope of the best-fit curves,for every 10 s,the cohesion drops by about 0.4 MPa.After treatment,transgranular and intergranular cracks are found in and between mineral particles in these rock masses,which reduces cementation between mineral particles,thus decreasing the overall cohesion.

4. Conclusions

(1)Microwave treatment exerts influences on the mechanical behaviour of basalt samples under uniaxial compression and CTC. After microwave treatment, CTCS reduces to some extent and confining pressure inhibits the reduction in strength of the basalt samples.

(2)Under uniaxial compression,microwave treatment affects the axial deformation,demonstrating that the stiffness of the samples gradually decreases with increasing microwave exposure time.Under triaxial compression,some microcracks caused by microwave treatment are closed due to the effects of the applied confining pressure, so that the confining pressure inhibits the rightward shift of the axial deformation curve.

(3)Under uniaxial compression,the elastic modulus and Poisson’s ratio of the basalts decrease in a quasi-linear fashion with increasing microwave exposure time,while microwave exposure slightly influences the elastic modulus and Poisson’s ratio under triaxial compression.The changes in rock strength and deformation mainly result from structural changes in between minerals.The confining pressure results in the closure of microcracks generated by microwave treatment,so that the effects of microwave treatment on strength and deformation are reduced,thus decreasing the influences on the elastic constants.Cohesion decreases with increasing microwave exposure time,showing a quasi-linear decreasing trend.

(4)Through microwave treatment, many microwaves are generated in different directions in the samples,which increase the discreteness of the test results.The higher the applied microwave power and the longer the exposure time,the greater the discreteness of the test results.The confining pressure inhibits the discreteness of the test results.The larger the confining pressure,the lower the discreteness of test results caused by microcracking in the samples.

Declaration of Competing Interest

The authors wish to confirm that there are no known conflicts of interests associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.

Acknowledgments

Financial support for this work by the National Natural Science Foundation of China(Grant No.41827806),the China Postdoctoral Science Foundation(Grant No.2018M642958),and the State Key Research and Development Program of China (Grant No.2016YFC0600707)are greatly appreciated.