Study On The Heating And Weakening Effect Of Water-bearing Rocks Under Microwave Irradiation

Mechanical rock breaking has been widely applied in various rock engineering for its advantages of high automation,low damage on surrounding rock,high penetration rate,et al.However,When facing with hard and highly abrasive rock,it has the drawback of excessive cutter wear and low efficiency,which will eventually cause unexpected extra construction period and cost.Recently,many scholars have found that sensitive substances in rock like sensitive minerals or water can be rapidly heated by microwave irradiation,thus improving rock breaking efficiency and reducing cutter wear.This method is called 'microwave-assisted rock breaking'.Taking igneous rock and sand rock as object,this paper carried out researches on the mechanism of 'breaking rock by the vaporizing expansion of internal water under microwave heating'.Firstly,by comparing the heating characters of dry and saturated rock,the effect of water in the heating procedure and its influence factor is studied.Meanwhile,the influence of water on rock weakening is analyzed under high power microwave irradiation.The factor of water-bearing rock burst under microwave irradiation is investigated using control variate method.Furthermore,the change of water contend in sand rock is tested under both natural state and a constant temperature.Main conclusions are as follows:(1)The heating character of saturated rock is related to rock voidage and microwave sensitivity.Effect of water on the heating procedure is negligible when the voidage is small,and becomes more and more obvious with the increase of voidage.With the same voidage,Effect of water on the heating procedure is more significant in rocks with lower microwave sensitivity.(2)The heating curve of saturated rock can be divided into 3 periods according to heating rate,which is related to the interaction of supplementary heating and escaping of internal water.In period 1,the rock contains much water as its saturated,in which condition there should be significant supplementary heating and negligible escaping.Thus,the heating rate of saturated rock is higher than that of dry one;In period 2,with the reduce of contained water,the supplementary heating is weakened while the escaping is strengthened due to higher vapor temperature and stress.Thus,the heating rate of saturated rock is lower than that of dry on;In period 3,internal water has totally escape from the rock,heating rate is the same in saturated and dry rocks.The difficulty and rate of vapor escaping are related to the rock permeability.When the permeability is higher,the vapor escapes more easily,which brings an earlier and longer period 2,as well as a sharper fall of heating rate.(3)Among the 5 saturated rocks,3(feldspathic quartz sandstone,lithic arkose,quartz sandstone1#)burst under 2000W microwave irradiation,while quartz sandstone2#and alkali-feldspar granite were undamaged.The intermediate state of the foregoing states is hard to achieve in this paper and can't be studied systematically.Sand rocks with low strength,big voidage,high saturation and high microwave power are favorable conditions to burst.(4)In natural condition,water content of sand rock declines with days and eventually reach a stable state,under which the water content is related to the voidage and permeability of sand rock;The heating effect caused by microwave irradiation has no influence on the rock behind working face.So it is applicable to make sand rock burst under microwave irradiation in engineering.