The Impact of Microwave Treatment on the Andesite Rock Mechanical Properties

Alternate rock-breaking techniques to drilling and blasting could include using microwaves to break up rock masses. Drilling for blasting has a significant issue with the cost of drill bit replacement in a short time (weekly). The microwave-breaking technology could be a solution, but research is continuing. This research clarifies the proposed methods through a case study of PT. ATC is an andesite quarry that employs the drill-and-blast technique to break rock. This experiment replicates the previous study’s conclusion that rock strength decreases following microwave treatment on two rock strength tests involving the Point Load Index (PLI) and Schmidt Hammer (SH). The dynamic test (PUNDIT) result indicates that the p-wave velocity of microwaved samples is greater than that of non-microwaved samples (NMW). As a consequence of evaporation and mineral transformation within the rock, the density of the rock decreases, and the porosity increases.


Introduction
Andesite is typical igneous rock used in construction, ornaments, wall accessories, and raw material for sculpting and medical purposes [1].PT.Ansar Terang Crushindo (ATC) is one of mining companies that mainly extracts andesite for construction and dimension stone.It is located in Pangkalan, 50 Kota regency, West Sumatera, Indonesia (Figure 1).In general, the strength of andesite ranges from low to extremely high strength [4] or varies between 16.4 to 273 MPa [1]- [3], but for construction, it has high strength.These rocks can be scattered using a breaker or drill and blast method [5], [6].Manual rock breaking techniques such as using breaker is not an effective method in hard rock material because it can yield high operating cost.The typical andesite quarry mining in the observed area is categorized as small to medium scale.Therefore, the high expenses will be strictly considered.Otherwise, the profit will take long time to be achieved.Using drill and blasting techniques could also be costly, especially in changing the drill bit in the drilling stage.Based on the company's financial record, it found that the technique charges the company high monthly operating costs of around 2.6 million IDR or 1,745 USD equivalent.The same issue was also discovered in the Tunnel Boring Machines (TBMs) activities with significant bit wear and maintenance [7].New methods other than blast and drill techniques that proposed less expense could sustain the future of quarry mining.The proposed method is by using microwave breaking techniques.Utilizing microwave energy to break the rock mass is not a new technique.In the 1960s, the microwave rock shattering technique was introduced [8], [9].It is expected to be an alternative to drill and blast techniques.This method has been used in many industries such as food [10], [11], health-related [12], and mineral processing [13], [14] industries.Because of its high effectiveness and lack of secondary pollutants, microwave technology is a viable technique for breaking rock [15], [16].The potential of these techniques still needs more investigation, especially in industrial applications.This research aims to observe the effect of microwave on the andesite rock strength from rock mechanical testing such as PLI and Schmidt Hammer (SH), as well as the dynamic and physical properties of the rock.

Sample preparation
The sample is andesite rock that had been taken from PT. ATC, an andesite quarry mining company.A rock boulder from drill and blast activities was taken and prepared in the laboratories to be assessed in the form of cylindrical (NX size) for Point Load Index (PLI), Schmidt Hammer (SH), and dynamic tests (PUNDIT).While the irregular cylinder for physical properties and one block size (15 x 15 x10) cm 3 for SH.The testing procedure is based on the ISRM, and ASTM standard methods [17]- [20].The samples were divided into two treatments i.e., without microwaving (Code A) and with microwaving (Code B) as shown in figure 2. The used microwave is commercial standard for cooking, with a power ranging from low to high with maximum power output have an inside temperature approximately 185°C.The microwave machine has power output at 700 watts.

Protocol of experiment and instrumentation
Samples encounter two testing conditions, including original condition without microwaving treatment and with microwaving treatment.In the original condition, the cylindrical sample was tested in Non-Destructive testing (NDT) using a dynamic test (PUNDIT), followed by a Schmidt hammer (SH).In certain conditions, the Schmidt hammer can also have a small, destructive impact on the sample, especially when the sample has been microwaved.Later, the destructive tests had been used such as Point Load Index (PLI) to obtain the rock strength (σc).PLI was used rather than the Uniaxial Compressive Strength (UCS) test because several sample conditions after microwaving treatment already had tiny flaking.Therefore, PLI can manage all samples in all conditions and yield their rock strength.In the Schmidt hammer test the cylindrical and block samples were compared before and after microwaving.A complete flowchart of the research scheme is shown below.

Result and discussion
Experimental action had been done in order to achieve the research aim.In general, the sample had been evaluated using several tools such as PLI and SH to observe the rock strength; physical rock properties; and dynamic test using PUNDIT.More importantly these testing will be conducted in microwaving and without microwaving treatment excluding the physical rock properties test that only used the original rock samples.

Physical Properties of andesite
Table 1 shown the physical information of the andesite rock samples from the observed area.Three basic important information had been shown such as density, spesific gravity and the porosity of the rock.These parameters did not have significant difference in all samples.Therefore, the results are well representative.The typical density of andesite rock in this area had the average value of around 2.5 g/cm 3 .In general, it could be about 2.7 g/cm 3 [5].The result seems parallel to the worldwide andesite range value that ranges from around 2.3 to 2.7 g/cm 3 [2], [3].

Ranges of Rock Strength from different samples and testing
The experiment result reveals that the rock strength of cylindrical samples from SH test were lower (6-9 MPa) than the SH block type and PLI cylindrical type (43-114 MPa) either before or after microwaving treatment (Figure 5).PLI cylindrical result seems realistic because the value is within the literature range between 16.4 to 273 MPa [1]- [3].Moreover, the low value of cylindrical SH test is still normal due to the repetitive impact on the penetration area.That repetitive impact could weaken the sample's inherent properties, such as porosity and micro fissure [21].

Schmidt Hammer (SH)
There two type of samples that experience SH test involving cylindrical and block type.The cylindrical have a NX size with diameter of 10.8 to 11 cm, with the sample mat is made of brick.The SH test was conducted on the top and bottom surface and the curved side of the cylinder (Figure 6A,  B).It revealed that the rebound number (Rn) is larger in the central position than on the cylindrical boundaries (curved side, bottom, and top surface of cylinder).It found similar with earlier observation [22], [23].
On the test, the rebound number (Rn) occasionally drop down from 10 to zero indicating that the sample was not in the appropriate spot during the penetration procedure (between the base and the sample as if there is a gap) or in the position as shown 6A.From the test result, it was observed that the Rn and Rock Strength have a positive correlation in both sample types.For the cylindrical sample, the rock strength can be predicted using a linear equation = 0.62 − 4,1 with R 2 = 0.99.In the same way, the rock strength of block sample can be estimated using = 2,9 − 82,85 with R 2 = 0,99 (Figure 8).The figure also confirms that increasing microwaving treatment (duration and temperature) contributes to decreasing the rock's strength.Apart from the cylindrical samples, block type sample was also tested to find out the size effect of the samples on the Rn value.It found that the Rn value of a block sample is much larger than the cylindrical samples, which differs by approximately 46 points on average (Figure 8).The field assessment found that the sample block type was more stable in receiving SH punch than the cylindrical sample.The result of block sample (Cluster B) in Figure 8 paraller with the literature range [1]- [3].Meaning that block type sample (Cluster A) seems more reliable in comparison with cylindrical sample (Claster B) using SH test.This is logical because the cylinder's effected area is less than that of the block type.

Point Load Index (PLI)
Another tools to predict the rock strength is by using PLI test.It is a much popular test because of its mobility, quick result, and low prices compared to Uniaxial Compressive Strength (UCS) test.
Especially, PLI is more recommended to be used in weak rocks because UCS test in weak rock is challenging [24].In the experimental situation, it was found that the two cylindrical samples were broken inside the microwave (Figure 10).With the broken samples, using UCS cannot be conducted because the sample did not fulfill the testing standard whether ASTM or ISRM.Alternatively, PLI had taken a place.Those three samples before and after microwaved had tested using this method.It reveals that the microwaving treatment consistently decreases the rock strength which around 50% on average (Figure 9).The trend is similar as in Figure 5, and 7.

Dynamic Test (PUNDIT)
The dynamic test used PUNDIT (Portable Ultrasonic Non-destructive Digital Indicating Tester).This method is commonly used in civil engineering to monitor material quality.It can predict the compressive strength [25], [26], material homogeneity [27], [28], and crack estimation [29].In the experiment, six samples involve three samples with microwaving and the rest without microwaving treatment had been tested.Microwaved (MW) samples received maximum power output (HIGH) with temperatures around 185°C to 219°C within 30 minutes.After 30 min, it was observed that two samples were broken in the microwave box (S1-B, S2-B).Sample S3-B was still solid (Figure 10.The broken rock sample reveals a positive result of the expectation of using the microwave equipment.It has indirectly been shown that microwave have a great potential an effective rock-breaking technique.The breaking phenomenon is caused by the evaporation and transformation of quartz inside the rock [30].Therefore, small crack initiation appears and develop to weaken the rock mass.The evaporation effect is clearly confirmed from table 2, where the original weight sample decreased after microwave treatment, such as observed in the sample S3-B where the samples weight decreased from 0.695 kg to 0.685 kg.Unfortunately, the broken rock cannot be measured because it has scattered all over the place.From PUNDIT test, it was found that p-wave velocity in the original condition is higher than in the microwaved conditions (Figure 11).This happened due to the alteration of the internal properties of the rock sample, such as density, porosity, an indication of microfractures, rock texture, and mineral composition [3], [31].The natural fracture intensity, including microfractures, can be quantified using p-wave velocity [32].This internal shift in the specimens reduces the velocity of the acoustic wave and decline the rock strength [33], [34] (Figure 12).P-wave velocity and rock strength are positively correlated, meaning the weaker the rock, the minor its p-wave velocity value.The relationship can be estimated using the equation = 0.0818Vp -57.031 with (R 2 =0,74).

Conclusion
Andesite rock is one of the solid rocks that can be used for material construction.Its strength ranges from 17 to 273 MPa (weak to very strong).One of the companies mine the andesite is PT.ATC that uses drill and blast method to break the rock masses.Even though the drill and blast method are more effective than a manual method like a breaker, it found that the cost of changing the drill bit is still expensive.Looking at this phenomenon and the idea of microwave treatment as a new tool in rock breaking led this research to clarify the proposal.
Fourth test methods i.e., PLI, SH and dynamic test (PUNDIT), and physical test properties is delivered in this research.It found that microwaving techniques can reduce the rock's strength.Reducing rock strength can change the breaking method from drill and blast to manual breaking using breaker machine.This good indication to tackle cost problem of changing drill bit in blasting method.The decreasing of rock strength through microwaving techniques is because of the alteration of internal properties of rock, involving density because of the inherent water evaporated, porosity, and microfractures that might increase, as well as the rock textures and its mineral compositions.
A comparative figure of the rock strength result from PLI and SH shows that SH testing requests accuracy and caution.Otherwise, the result will have great discrepancies with similar rock type value from literature.Alternatively, block type specimens can be an excellent type of sample to be used in SH testing due to their wide penetration area.
In the future, using a microwave to fully change the drill and blast technique is still far from applicability due to the efficiency of the microwave performance.The microwave tool is expected to decrease the rock strength during truly short periods, for example, in a second.At this time, microwaves can change the established method.Otherwise, the cost still becomes a challenging issue for this breaking technique.

Figure 2 .
Figure 2. Original samples Condition after final preparation activities (the cylindrical sample is NX size)

Figure 5 .
Figure 5. Range of averaged rock strength (MPa) from Schmidt hammer (SH), Point Load Index (PLI)before and after microwave treatment, also a comparison to the literature sources (Max and Min condition[1]-[3] )

Figure 7 .
Figure 7. Correlation between Rn and Rock Strength (converted from Rn) of cylindrical samples (Left) and Block samples (Right) with and without microwaving treatment

Figure 9 .
Figure 9.Comparison of the effect of Microwave treatment on Rock Strength (PLI)

2 )Figure 10 .
Figure 10.Condition of the samples before (1) and after (2) microwaving for 30 min with temperature output set to maximum-high (3).PUNDIT test on the microwaved samples (4)

Figure 11 .Figure 12 .
Figure 11.The influence of the microwave on the intact rock in a dynamic test (PUNDIT)

Table 1 .
Shows the result of the physical properties of the sample.

Table 2 .
Weight of sample before and after giving microwave treatment