PRACTICAL APPLICATION OF THE MICROWAVE OVEN IN THE GEOTECHNICAL LABORATORY

The soil natural moisture content test is one of the basic and the most common research conducted in geotechnical laboratories around the world. The procedure described in current standards of all countries demands drying of soil in conventional oven (with natural or forced circulation of hot air) for at least 24 hours. The alternative for this time-consuming process is drying the soil specimen by using electromagnetic radiation with the frequency of microwaves. The idea is related to the heating effect of the military microwave radars observed during the World War II (Chung and Ho 2008). This phenomenon was later used in many fields of human activity. In science and engineering the microwaves are used to define moisture content of soil, measure moisture content in different products in agriculture (Hassan and Karmakar 2014; Lu et al. 2009), industry (Boone and Wengert 1998) and civil engineering (e.g. microwave gauge that defines water quantity in aggregate, concrete and during the production of steel or asphalt (Singh et al. 2015; Norambuena-Contrerasa and Garcia 2016)). Moreover the microwaves are used in the process of soil pollution treatment (Shang et al. 2007; Kawala and Atamańczuk 1998).

It is worth noting that geotechnical laboratories use microwave radiation not only to determine soil moisture, but also in routine practice to dry soils for other laboratory tests, e.g. to plot the grading curve by sieve analysis, to determine the maximum bulk density of the soil skeleton in the Proctor test or for the preparation of specimens for resistance tests (in the case of soils with too high initial moisture content that prevents the formation of the specific specimen).

All publications ranging from the oldest (e.g. Creelman and Vaughan 1966), to the most recent (e.g. Jalilian et al. 2017) focus on two basic issues related to microwave drying. The former is the compatibility of moisture measurements between two types of ovens: convection drying performed in conventional ovens and microwave drying carried out in commonly available microwave ovens.

Convection drying is an air-to-air method in which, in the case of ovens used in a geotechnical laboratory, hot dry air is the factor that transfers the heat to the soil and drains the moisture away. The air flows around the specimen in a natural or forced way. In turn, microwave drying is associated with electromagnetic radiation, which induces vibrations of water molecules in the specimen forming an internal source of heat. Despite the different drying mechanisms researchers have always confirmed the consistency of results obtained by two methods. Among others, Routledge and Sabey (1976) and Balscio (1992) showed very high compatibility using the coefficient of determination that is R² > 0.980; which was determined by Gilbert (1991) as R² > 0.994; and by the author R² = 0.999. Moreover the results of literature study proved good correlation between results obtained from both methods as the variation in the majority of results is less than 1% (e.g. Gilbert 1988; Hagerty at al. 1990b; Chung and Ho 2008).

The second issue is the development of the proper microwave drying procedure which is greatly influenced by: soil type, specimen mass, number of specimens dried in a microwave oven and the way they are arranged (inside or around the plate), drying time, microwave power, container material type (containing the tested soil). The problem with developing a clear procedure results from the basic difference in the nature of the drying processes. Conventional drying means that the heat is delivered from outside through the surface of the material which is why the surface has the highest temperature. On the other hand, microwave drying is characterized by the fact that the microwaves penetrate the interior of the material heating up the entire volume from the inside. All the factors mentioned above affect the quality and efficiency of microwave drying. The existence of so many types of soils with diversified structure and mineralogical composition makes it necessary to create extensive database of results which would enable the development of universal procedures that have not yet been created or standarized in the majority of countries.

For this reason, the author of this paper completed a comprehensive research plan for three selected soils (FSa – SP, saGr – GW and siCl – CH) which includes the influence of moisture on the drying process, specimen mass, number of dried specimens and their distribution in the microwave for the duration of drying. The results of the microwave drying were confronted with the results of the conventional method. The choice of soils was determined by the will to compare various grain sizes of soils common in the south of Poland. In addition, siCl (CH) is a swelling soil and as far as author’s knowledge is concerned, such soils have not yet been the subject of research in the determination of moisture by microwaves.

The presented research cycle will be continued in the future in order to determine the impact of microwave drying on the soil structure and further on its mechanical parameters.

The first attempts of defining the soil moisture content by using microwave oven were undertaken in the 1960s by Creelman and Vaughan (1966), Algee, Callaghan, and Creelman (1969), Ryley (1969). Later the research results were published by Routledge and Sabey (1976), Carter and Bentley (1986), Hagerty et al. (1990a, b), Gilbert (1988, 1991), Balscio (1992), Gaspard (2002), Chung and Ho (2006). However, the latest research was published between 2012 and 2017 by Berney, Kyzar, and Oyelami (2012), Daod (2012), Cormick (2015), Kramarenko et al. (2015, 2016), Jalilian, Moghaddam, and Tagizadeh (2017). During over 60 years a number of research were published concerning the guidelines for determining moisture content in terms of soil type, specimen mass, number of specimens dried in a microwave oven at the same time and their distribution (inside or around the perimeter of the plate), drying time, type of the microwave oven, the type of container material. Everyone unanimously confirmed the effectiveness of the laboratory microwave drying and compatibility of its results with the results of standard drying. Among others, Gaspard (2002), who tested 4 different drying devices, proved the effectiveness of a microwave oven regarding the economical aspect of the method (saving time and energy) and its accuracy.

Similarly, Berney et al. (2012) in an extensive report proved the above theory first and then recommended a compatible method for use in situ, i.e. the soil density gauge and the gas stove with pan technique. For the purpose of their research 9 devices with different operating mechanisms were tested.

Eventually they did not recommend the field microwave oven powered by batteries due to the loss of power during drying.

In turn, Gilbert (1988) proposed a special place for automatic microwave drying. His work is extremely valuable due to a number of comments regarding the safe use of microwave for the soil drying. Among others, as one of the few, Gilbert (1988) wrote that “undisturbed, low permeability soils and rock, gravel, shale and even brittle undisturbed clay fragments subject to microwave heating may explode”. The author of the paper also encountered a similar phenomenon and the instructions on how to proceed in this situation will be listed later in the following sections of the paper.

Gaspard (2002) also tested the computer-controlled position (CMWO – Computer Controlled Microwave Oven). He confirmed the effectiveness of the above and Standard Microwave Oven (SMWO) methods but due to high costs he recommended the latter. The author shares the opinion of Gaspard that the economical factor is fundamental for the use of microwave radiation. Time, simplicity and availability of microwave ovens are the most important criteria.

Although the microwave soil drying has been the subject of research for many years, so far only in the United States (ASTM D4643-00; ASTM D4643-08), Australia (AS 1289.2.1.4-2015; AS 1289.0:2014) and France (NF P 94-049-1), standardized guidelines of different level of detail have been published regarding the determination of soil moisture in microwave ovens. In addition, a number of national reports created as a result of research can be found, e.g. in Canada (ATT 15/96) or in Hong Kong (Chung and Ho 2008). However, it is worth noting, that the indicated guidelines do not take into account all variants related to soil heterogeneity and research methods.

The selection of the specific heating power and specimen mass is still generally an arbitrary decision of the researcher. In literature sources, the power of the microwave oven adopted by the authors ranges between 700 W (ASTM D4643-00; Chung and Ho 2008; Gaspard 2002) and 1700 W (Chung and Ho 2008). The majority of researchers chose 800 W. In this respect, the author also suggest the power of 800 W. However, for some soils it is believed that the power should be reduced due to the sintering of the tested specimens. Thus, they share the opinion of Jalilian, Moghaddam and Tagizadeh (2017), who based on a series of tests carried out at different powers: 90-180-360-600 and 900 W in heating cycles of 5 and 10 min on specimens of 50 g, concluded that the optimal system is 600 W and 10 min of drying.

With regard to the mass of the specimen, it is even more difficult to find a specific rules. Generally, each researcher formulates their own guidelines. Some authors agreed that too small amount of specimen should not be used due to the possibility of its overheating (ASTM D4643-00; Chung and Ho 2008; Gołębiewska, Połoński, and Witkowski 2003). In turn, the author of this paper believes that both too small and too large specimens in the so-called compacted mass are not recommended because there is a risk of scorching.

Another controversial issue is the recommended drying of the specimens. Considering the fact that it depends on the size of the specimen (that is different in each guidelines), the recommendations cannot be unambiguously determined. In literature sources it ranges between 2 min (Grymowicz and Jastrzębska 2015) and 50 min (Chung and Ho 2008). In addition, it is still necessary to determine the frequency in which the specimen will be weighed during the entire drying process. In comparison, according to the latest ASTM 2008 guidelines, constant heating is recommended based on the time determined by a nomogram. On the other hand, other scientists, based on their own research, suggested drying at different intervals. Thus, the authors formulated their own guidelines (Appendix B).

It is worth noting that the Cormick’s doctoral thesis (2015) is a good summary and description of previous works related to drying of soil using microwaves. Based on work of other researchers and his own, Cormick (2015) approved soil drying for all soil types except for soils with organic content above 10% and bentonites with moisture content greater than 110%. At the same time, from the various specimen mass (from 10 g – 250 g for fine soil and 300 g – 1000 g for medium and coarse soil) Cormick (2015) chose and suggested using standard specimens with a mass of 150 g. Moreover, while analyzing various methods of drying and weighing of the specimens, Cormick (2015) adopted the principle of non-stop drying for the first 3 min and then mass measuring every minute after each heating cycle preserving the 1 min cooling period before the actual weighing. Based on her own research the author recommends this procedure provided that the first test specimen should be weighted from the very beginning every 30 sec, especially when the specimen is small and moderately wet.

Despite the undoubted advantages of drying in a microwave oven, this method has its limitations mentioned by the researchers. It is limited mostly by the mineralogical composition of the studied soils. Among others, Chung and Ho (2008) do not recommend using soils with a high content of halloysite, mica, montmorillonite, gypsum and other hydrated materials, as well as highly organic soils and soils with solids dissolved in pores in order to avoid falsifying of the final results. In particular a very cautious approach regarding organic soils can be noticed. The majority of researchers speak generally about this issue, e.g. Hagerty et al. (1990a, 1990b). The work of Kramarenko et al. (2015, 2016) dedicated to the microwave drying of organic clays and peat constitutes an exception. The guidelines proposed by Kramarenko et al. are summarized in the Table B1 (Appendix B). It is worth noting that these are the only guidelines referring to the ASTM D4643-00 that imply mixing of soil at control weighting. At the same time, they do not specify how to perform this process without losing the soil. The idea seems to be correct, but its implementation causes many problems (unavoidable losses of material) and therefore the author of the paper does not recommend it.

Based on the above literature review, it is clearly visible that despite nearly 60 years of research, the procedure of microwave drying of the soils still requires a lot of experimental work. First and foremost, further research is essential to improve the number of results. Such necessity derives mostly from the soil diversification, its various mineralogical composition and structural construction.

All these reasons contributed to the author’s research concerning the practical application of microwaves in a geotechnical laboratory. The assumptions of the research cycle and the results obtained will be presented in the further part of the paper.

For our research we selected two non-cohesive loose soils from road excavations (fine sand – FSa and sandy gravel – saGr; selected based on the grain size curve and according to the PN-EN ISO 14688-2:2006 classification; Fig. 1) and one cohesive soil from the Triassic deposit in Patoka near Częstochowa (silty clay – siCl, according to Kowalska and Jastrzębska (2017), based on the PN-EN ISO 14688-2:2006). Due to the divergence of soil markings in the European system and the Unified Soil Classification System (USCS), the following soils were tested: FSa is SP, saGr is GW, and siCl is CH.

Figure 1.

Because of its characteristic red-brown color caused by the presence of iron compounds, siCl (CH) was called a red clay. It should be noted that at this stage of the research, the author’s red clay was in the form of a soil paste which had an impact on the preparation of test specimens. All tested soils came from the southern part of Poland and their basic parameters are shown in the Tables 1 and 2.

For the purpose of the research, a total of 345 moisture determinations were performed including 138 measurements in a conventional oven and 207 in a microwave oven and 2 test measurements for the containers filled only with water.

Due to the multitude of different combinations (variables such as soil moisture content, initial specimen mass, number and placement of specimens in the microwave oven), 3 basic variants for each soil type were adopted throughout the research cycle (Table 3).

Variant 1 (two microwave ovens: MO-A and MO-B and a conventional oven: CO) was characterized by variable soil moisture w ≠ const., constant mass of specimens (m ₀ = const., m _FSa = m _siCl and m _FSa ≠ m _saGr based on the marked PKN-CEN ISO / TS 1: 2009) and constant number of specimens during one drying cycle (n = 1). Moisture of specimens changed from the initial state w ₀ (for non-cohesive soils it was air-dried state w ₀ = 3%; for silty clay – after drying of the soil paste and its grinding – w ₀ = 0%) into the moisture content w _i = w ₀ + 40% = 43% in the interval of 4% and further to w _k = w _i + 60% = 103% in the interval of 5%.

Variant 2 (one microwave: MO-A and traditional oven: CO) was characterized by variable specimen mass m ₀ ≠ const., constant soil moisture (w _p = const. = w p,siCl = 45%) and constant number of specimens during the drying cycle in the microwave oven (n = 1). The initial state of the specimens mass was m ₀ = 25 g and it was further increased two-, three-, four- and fivefold, and according to the PKN-CEN ISO / TS 1: 2009 it was respectively: m ₀ = 25 g, 50 g, 75 g, 100 g and 125 g for fine-grained soils. Due to technical reasons, saGr testing was omitted in this variant. The initial moisture content of the red clay in the form of soil paste was assumed to be constant moisture w _p = 45 %.

Variant 3 (only microwave: MO-A) was characterized by a variable number of specimens during one drying cycle n ≠ const., constant specimen mass m ₀ = const = 25 g and its moisture w _p = const. = 45% – as in the previous variants. In Variant 3, the specimen drying in the traditional oven was omitted due to the lack of influence of the specimen number on the drying time which is normally at least 24 hours. The test was carried out for the number of specimens n = 1, 2, 3 and 5 at the same time distributed in the central part of the microwave or evenly around its circumference. Considering the limited capacity of the device, Variant 3 was carried out only for FSa and siCl specimens. In the case of saGr the required mass for the single specimen by the PKN-CEN ISO / TS 1: 2009 should be so big that more than one container would not fit in the oven.

The following were used to determine the moisture content of soil:

Traditional oven (CO) with a capacity of 2300 W equipped with an internal air circulation and regulation of the drying temperature,

The same heating power – of 800 W was used for all measurements. The soils were dried in ceramic and glass containers. The specimens were prepared and stored and their mass was measured in accordance with the PKNCEN ISO / TS 1: 2009. Following the guidelines the minimum masses of FSa and siCl were m = 25 g and saGr – m = 300 g. Natural moisture of particular soils was w n,FSa = 3%, w n,saGr = 0% and w n,siCl = 45% (for soil paste). Therefore, in the first stage of Variant 1 the moisture content of saGr was w ₀ = 3% ( for FSa as well) and then it was changed according to the guidelines. In turn, the siCl in the form of soil paste was first dried and then ground to a powder, and then its moisture content was increased to w ₀ = 3%. Subsequently, specimens were prepared with adequate moisture content based on the assumptions made.

The test drying was carried out prior to the actual research. Its purpose was to determine the most effective method of taking measurements (i.e. current measurements of specimen mass) and to determine how the frequency of readings affects the drying time.

Two containers filled with 25 g of water were dried in a microwave oven and weighed every 0.5 min and 1 min. On the basis of the drying curves (m = f (t)) it was established that the mass of the specimens will be determined every 30 sec for the first 5 min and then every 1 min until the mass of the specimen stabilizes.

The selected results of particular test series are summarized in Tables A1 and A2 (Appendix A), 4 and 5. Two specimens were dried independently for each moisture (w ₁ , w ₂ ), while Tables A1 and A2 (Appendix A), 4 and 5 show the average moisture w _avg (w _avg = (w ₁ + w ₂ )/2) and the corresponding average drying time tavg (t _avg = (t _w,1 + t _w,2 )/2).

Table A1 (Appendix A) applies to measurements made during microwave oven drying for Variant 1 for specimens with moisture content w ₀ = 3%–103% and mass m ₀ = 25 g (FSa and siCl) and m ₀ = 300 g (saGr).

Table A2 (Appendix A) applies to measurements made during a conventional oven drying as part of Variant 1 for specimens FSa and siCl with moisture content w ₀ = 3%–103% (Table A2; Appendix A) shows selected moisture content w ₀ = 3%, 23%, 43%, 63%, 83% and 103%) and mass m ₀ = 25g and saGr specimens with moisture content w ₀ = 3% – 48% (in Table A2; Appendix A): w ₀ = 3%, 23%, 34%, 43%, 48%,) and mass m ₀ = 300 g.

In turn, Table 4 lists the average drying time tavg in a microwave oven of soils with constant moisture content w _p = 45% (variant 2) but different masses m ₀ = 25 g, 50 g, 75 g, 100 g, 125 g and 300 g for FSa and siCl and only m ₀ = 300 g for saGr. The results for Variant 3, including the number of specimens dried simultaneously in a microwave oven, are presented in Table 5. Also in this case each measurement was performed twice for the number of specimens n = 1,2,3 and 5 with moisture w _p = 45 % and mass m ₀ = 25 g (FSa and siCl).

First of all, the compatibility of both methods in soil moisture determination is presented in Fig. 2. It is visible that the specimens correlate well and the maximum difference in the moisture content calculated by the two methods is 0.84% for the FSa with an average difference of 0.01%, 0.83% for siCl with an average difference of 0.1% and 0.40% for saGr with an average difference of 0.17%. The obtained results are consistent with those provided by the literature sources, e.g. Kumar (1987) showed a correlation between 0.2% and 1.1%; Daod (2012) – 1.2% of which the vast majority of the tested samples (similarly to Gilbert (1988) and Chung and Ho (2008)) showed a difference of less than 1%.

Figure 2.

Figure 3 shows the change in moisture content of specimens over time during conventional oven drying. For the sake of clarity, the graphical interpretation is limited to selected moisture content ranges which at the same time allow to observe the rate of moisture loss during the drying process. The continuous curves were drawn for moisture w ₀ = 3% for all soils and w ₀ = 103% for FSa and siCl and w ₀ = 48% for saGr. It is clearly visible that in the first hour the moisture of FSa decreased by 99% for w ₀ = 3%, by 89% for w ₀ = 48% and by 60% for w ₀ = 103%; the moisture of siCl decreased by 85% for w ₀ = 3%, by 54% for w ₀ = 48% and by 50% for w ₀ = 103%; the moisture of saGr decreased by 70% for w ₀ = 3%, by 20% for w ₀ = 43% and 48%. After less than 4 hours, FSa was completely dried (regardless of the initial moisture), while the moisture of siCl was marginal (w < 0.5%). At the same time, longer drying time with increasing initial moisture appeared to be characteristic for saGr. Finally, after the required 24 hours of conventional drying, all tested soils reached moisture content w = 0%.

Figure 3.

Figure 4 shows changes in selected moisture contents (w ₀ = 7%, 31%, 63% and 93%) for specimens FSa (marked with a solid line and a circle marker) and siCl (marked with a broken line and a triangle) of the same mass m ₀ = 25 g (variant 1) and w ₀ = 45% and mass m ₀ = 300 g (variant 2) for FSa specimens (marked as previously) and saGr (marked with a solid line and a square) during microwave oven drying. The particular curves grouped in pairs for the specific moisture clearly show differences in the drying time depending on the soil type, cohesive (siCl) or non-cohesive (FSa) (variant 1), and depending on the grain size for non-cohesive soils (FSa and saGr) (variant 2). The further part of the work will be devoted to this subject in a more detailed manner.

Figure 4.

In turn, Fig. 5 shows the changes in microwave drying time in the entire moisture content range (w ₀ = 3%–103%) for specimens FSa and siCl with mass m ₀ = 25 g and for specimen of saGr with mass m ₀ = 300 g (variant 1). Additionally, Fig. 5 shows the point characteristic for FSa (SP) at which due to the explosion of specimens with moisture of w ₀ > 23% it was necessary to cover them with perforated foil, which prolonged the drying time due to the reduced possibility of evaporation. The phenomenon of soil explosion will be discussed later in this paper.

Figure 5.

Figure 6 shows the influence of specimen mass (m ₀ = 25 g, 50 g, 75 g, 100 g, 125 g, 300 g) on the drying time in a microwave oven (marked in gray) and conventional oven (marked in black) for soil specimens of FSa and siCl with moisture content w _p = 45% (variant 2). The higher the mass, the longer the drying time, which is a logical consequence. It is a correct rule for the microwave and convection drying methods. For the former t the deeper the material is penetrated, the lower the microwave power is. On the other hand, during convection heating, the increase in specimen mass extends the path of the heat.

Figure 6.

In turn, Fig. 7 shows the influence of the specimen number (n) of FSa and siCl with moisture content w _p = 45% and mass m ₀ = 25 g placed simultaneously in a microwave oven (n = 1, 2, 3 or 5; variant 3) on the microwave drying time. The increase in the number of specimens from n = 1 to n = 5 resulted in an extended drying time of 30% on average for FSa and 75% for siCl. This indicates that the decrease in microwave power associated with more specimens in the microwave oven affects clay soils to a greater extent than sandy soils. The explanation of this phenomenon will be included in the following part of the paper.

Figure 7.

Conducting so many research series helped to formulate valuable practical guidelines that are not mentioned by other researchers. The author noticed all the problems associated with the “exploding” of the soil during the microwave drying and thus the discharge from the container and burning or scorching of cohesive soil. For more information on soil exploding during microwave drying please refer to Gilbert (1988). The phenomenon itself is caused by the rapid evaporation of water inside the soil and the inability of quick steam escape towards the surface and further into the environment. As a result, the excess pressure inside the soil may cause damage to the specimen or, in extreme cases, even explosion. This occurs during the heating of clay soils and hard brittle rock and gravel particles especially if too much drying power is used. In the case of author’s research the explosion occurred during microwave drying of specimen FSa with a moisture contents w ₀ > 23%. Despite the use of containers of various sizes, there was always a loss of material. To prevent the material loss, a perforated protective foil was applied on the top of the container. Unfortunately, this procedure resulted in a significant extension of the drying time, up to 3 times which is clearly shown in Fig. 5. In the case of drying of saGr and siCl there was no need to use a protective foil. Although clay soil at high moisture contents also exploded it was not as intensive as in the previous example and it did not require the use of a cover.

Another interesting phenomenon observed and mentioned, was the occasional incandescent or partial burning of siCl specimens. This was probably associated with its mineralogical composition or with too big specimen mass (uneven heating) or too long drying time. Some minerals (including compounds containing iron – as in the tested siCl), occurring in the soil, subjected to microwave energy might start an exothermic oxidation process causing the soil specimen ignite, burn at very high temperature, and lose volatile material (combustible iron compounds) (Gilbert 1988 after Kuehn, Brandvig, and Jefferson 1986). The moister the specimen, the stronger the phenomenon, thereby increasing its conductivity represented by the relative dielectric constant (Gilbert 1988 after Lundien 1971). In this situation, the only solution is to choose the right frequency (not too low and not too high) and the power (not too high) of microwave radiation. Since the majority of commercial microwave ovens operate with an optimally selected frequency of 2.45 GHz, the right choice of power should be made and it is still the subject of research (eg Jalilian et al. 2017). Undertaking the research subject related to the soil drying in a microwave oven has primarily a huge economic significance in terms of the time essential to obtain results (a maximum of a dozen or so minutes for a microwave oven or 24 hours required for a conventional oven) and in terms of energy consumption. As far as the basic aspect is concerned that is the drying time, it was found (Figs. 3 and 4) that in the microwave oven the cohesive soils (siCl) dry faster than non-cohesive soils (FSa and saGr). This can be explained by the drying mechanism “from the inside” in the case of microwave energy. Furthermore, in cohesive soils water is more related to clay cristals and that is why the heating from the inside and the resulting vibrations of the particles cause faster release of water and faster drying.

For example, in author’s research, this process was 3 times faster for siCl than for the other tested soils with the initial moisture content w ₀ = 27%. In turn, Fig. 4 shows that the drying curves (w ₀ = 45%) for FSa and saGr have a similar shape, however, the loss of moisture content is faster for saGr than for FSa. It can be assumed that if the protective foil for FSa specimens were not used (because of their exploding), both soils would react identically. The time needed to dry the saGr specimen of m ₀ = 300 g was 55% shorter (t_saGr = 11 min) than for specimen of FSa which was t_FSa = 17 min (Fig. 5).

The value of the initial moisture of the specimen (w ₀ ), its mass (m ₀ ) and the number of containers (n) inside the microwave oven significantly affect the final duration of the test. In determination of moisture using the traditional method, these factors do not have such a significant impact on the length of the drying process. This is mainly due to the fact that drying parameters remain unchanged during the drying process. In the conventional oven chamber a constant temperature of 105–110°C is maintained which provides constant amount of energy. An increase in soil temperature occurs gradually from the external surface and the process lasts longer than in the case of microwave radiation which almost at the same time increases the temperature of the whole specimen. Since the absorption of power by the soil specimen subjected to microwave energy is not limited, the increase of this temperature in some cohesive soils exceeds 200°C (Gilbert 1988). Apart from the danger associated with this aspect (e.g. soil explosion, burning or glowing), such reaction to microwave radiation significantly accelerates the process of soil drying. Even at high moisture content (e.g. w ₀ = 103%), the microwave drying time is about 19 times shorter for FSa and approximately 50 times shorter for siCl compared with traditional drying. It is similar in the case of saGr which in a microwave oven is dried about 30 times faster than in a conventional oven despite a larger test specimen (m ₀ = 300 g) (Tables A1 and A2; Appendix A). These facts clearly show that the economical superiority of microwaves over the traditional ovens is undeniable.

To confirm the effectiveness of the microwaves, additional moisture measurements were carried out on the specimens of the red clay (siCl) in the microwave. After the drying cycle, the specimens were placed in a traditional oven and left there for 24 hours. After this time the specimens were weighed again and a minimal increase in their mass was noted which did not exceed 0.4% in relation to the mass obtained after the microwave drying cycle. This difference, although small is most likely caused by the partial absorption of the steam contained in the conventional oven which may suggest that the microwaves “dry more” than the conventional oven. A similar conclusion was drawn by Carter and Bentley (1986), who found that the difference in moisture contents for two different clay soils achieved at full microwave power (600 W) were 1.1%–1.2% higher than the conventional oven results. For lower power (420 W and 300 W) and for the remaining 4 cohesive soils and sand they did not observe a similar phenomenon. Only for two other cases (bentonite for 420 W and sand for 300 W) the soil turned out to be dried more in the microwave oven than in the conventional oven. Also in their research Hagerty et al. (1990a) showed that 60% of the microwave test results showed higher moisture contents than the conventional oven test. The explanation of such results should be sought in the very process of microwave drying. This process involves absorbing microwave energy by the specimen and increasing the temperature of the soil-water mixture to the boiling point of water (the so-called heating stage by conduction), later at a constant temperature of 100°C with additional energy absorption, free water is evaporated (so-called magnetic reaction). Subsequently (so-called displacement) the remaining soil and absorbed water still absorb energy which leads to the increase of temperature (Gilbert 1988). Research conducted by Gilbert (1988) (after Gilbert 1974) on 3 different clay specimens and 1 sand specimen showed that for clay soils the temperature increased to approximately 190–220°C and the moisture determined by microwave method was greater than the one determined by conventional method. However, the sand warmed “only” up to 160°C, and in this case the moisture determined by the conventional method was higher than the moisture determined by the microwave method. Undoubtedly, the power of the microwave device is of great importance in this instance and therefore this aspect still requires further research.

All practical guidelines for soil drying in the microwave oven based on our own research and on the selected literature are summarized in tabular form in the Appendix B (Table B1).

During the series of research carried out, the following observations were made: –

Clay specimens with mass m ₀ = 25 g, stabilized the mass loss after 5 min of drying by microwaves. It is almost 60 times faster than during traditional drying which takes 290 min.