Addressing moisture in building structures is one of the most relevant topics in the construction industry. There is a range of methods for preventing moisture ingress into structures and for the actual removal of moisture from building materials. These methods can be today considered fairly effective, but it is necessary to develop them further and to come up with more economical and, in particular, more environmentally friendly solutions. Moisture problems in building materials are particularly serious because the water contained in them greatly affects their physical and mechanical properties. Currently, there are multiple methods that are used for removing moisture from building materials in the Czech Republic and Europe in general, which meet most of the requirements of today’s building practice.

The basic criteria for their use are the speed and efficiency of drying, the economic demands of the process, the cost of necessary equipment, and the overall environmental friendliness of the method. Commonly used methods for drying building materials are efficient, but the drying time can be rather long, and their applicability and suitability depend on the type of building material.

One approach to drying building materials is through the use of microwave radiation. This method has been known since about 1945, but in construction practice, it is currently still not widely used. This is due to the higher purchase cost of microwave generators and the lack of knowledge of the basic principles of microwave drying, which is also related to health concerns about exposure to this type of radiation. A combination of these reasons, together with low awareness about the method, results in limited interest in the use of microwave radiation in construction practice.

The origins of using microwave radiation date back to the first half of the 20th century, although its existence was predicted as early as 1865 by James Clerk Maxwell [1]. August Žáček, a professor at the Faculty of Science of Charles University in Prague, was among the first to describe the principle of magnetron oscillation in 1924; in a foreign journal, his discovery was published in 1928, and since then, he has been considered the inventor of magnetron [2]. It was not until the end of the war and afterward that the subject of microwave heating came up in earnest, and one of the most important people involved was Percy L. Spencer, who was interested in consumer and commercial microwave ovens. He filed his first patent on the microwave heating of food in October 1945. His associates describe the discovery as a gradual trial-and-error process, with experiments such as making corn pop or eggs explode. Percy Spencer worked for Raytheon, where the main focus was on the invention of the microwave oven, but other companies also researched microwave radiation by just concentrating more on industrial applications; for example, in 1947, a paper was published that addressed process acceleration through microwaves [3]. Later, in the 1960s and 70s, the development of microwave technology advanced so much that the first household microwave ovens began to appear. Today almost every home has a microwave, and the principle of microwave heating is used in many other industries outside the food sector [4]. It has been widely used, for example, in medicine [5], the aforementioned food industry [6], the military [7], as well as within the construction industry [8]. Within this industry, a number of studies have already been carried out confirming the potential use of microwaves for drying building materials [9,10,11,12].

Microwaves are a part of electromagnetic radiation with a frequency between 300 MHz and 300 GHz, corresponding to wavelengths between 1 m and 1 mm. For industrial purposes, more frequencies are permitted, but in construction, the globally used frequency was 2.45 GHz with a corresponding wavelength of 12.2 cm. Microwaves belong to a broad spectrum of electromagnetic waves, which also cover, for example, the visible light range. Their behavior is described by Maxwell’s equations. It holds that at any given point and any given instant, the vectors characterizing magnetic and electric fields are perpendicular to each other, and both are perpendicular to the direction of motion of the wave [8,13].

Microwave radiation causes heating, and the molecules become oriented according to their polarity in the electric field. When microwave radiation comes into contact with water molecules, the electromagnetic energy transforms, and heating occurs. This is followed by the heating of the construction materials [8].

Microwaves fall in the range of electromagnetic waves with a lower frequency than solar radiation, so they do not leave any residual radiation that is harmful to health. The use of microwave devices is completely safe, and any damage to health can only occur from direct exposure from a few cm for at least several minutes, either made intentionally or by the improper handling of the device [8].

Excessively strong microwave radiation poses the highest risk to human health. The use of microwave equipment with radiators emitting such electromagnetic radiation outside poses a serious hazard to people in the area of high microwave energy density. The permitted levels of electromagnetic field strength in the range of 2.45 GHz are established by EU regulations, including Directive 2004/40/EC and Recommendation 1999/510/EC, as well as by country-specific regulations. The regulations typically set the permissible electromagnetic field strength (from 7 V/m to 61 V/m) or watt density (from 0.1 W/m² to 10 W/m²).

The fundamental theory of the electromagnetic field was based on a number of physical laws drawn from experiments and findings linked to names such as Coulomb, Savart, Ampere, and Faraday. Faraday’s work, particularly the discovery of electromagnetic induction, and the work of Maxwell, who developed a model of the electromagnetic field following the discovery of the concept of a displacement current, were of major importance to the development of the theory [14].

The general electromagnetic field, natural or man-made, is non-stationary (there is constant time variability). The variability of the field can often be considered marginal or slow. Following this simplification, the field can be classified into four types: (a) a static field, where all charges are considered to be at rest; (b) a stationary field generated by charges that form stationary currents; (c) a quasi-stationary field, which is a simplification of the general non-stationary field because the so-called displacement current against the free electron currents is disregarded here; and (d) a non-stationary field, i.e., a general electromagnetic field [14].

There are multiple applications for microwave energy within the construction industry. It is used for drying building materials and structures [15], accelerating the solidification of mixtures [16], for the sterilization of biotic pests [17], or even for moisture measurements [18].

The material to be heated is subjected to a high-frequency electromagnetic field, causing the polarization of molecules, conduction, and magnetic processes. The internal energy of the molecules gradually increases, which causes the material to be heated. During perfect microwave heating, the distribution of the intensity of the microwave field is in the area completely even, and heat is generated uniformly across the entire volume of the material, but in a real situation, this is not the case. The presence of waves depends on the design of the equipment used and the type of material inside the heating chamber. The amount of energy absorbed varies with the size, shape, dielectric constant, and permittivity of the material. In addition to these properties of the heated object itself, particularly important are the frequency and intensity of the electromagnetic field. The heat generated in the objects spreads to the surroundings through heat transfer. If microwave energy is applied to the material for too long, it can lead to overheating and damage to the object [16].

One of the advantages of microwave radiation is selective heating. Components were heated according to their ability to absorb electromagnetic energy; therefore, mainly, the most absorbent one was heated, which is usually loosely bound water. This component then heats the other material components, which results in a more in-depth heating of the object. Microwave energy can heat up the material more effectively compared to regular heating from the surface. It was used for the drying of various materials and the acceleration of the hardening of mixtures. Disadvantages to this method include higher energy consumption, possible changes in the mechanical properties of the object, and possible local overheating due to the inconsistency of the microwave field and the inhomogeneity of the heated material. Last but not least, there is also the requirement of proficiency while working with EMW radiation [16,19].

The microwave drying/hardening process can be divided into four stages. The first one is the actual heating of the water molecules in conjunction with the secondary heating of the material. The second stage consists of the evaporation of water contained in the surface layer of the material. During the third stage, the volume of water increases due to the heating of the water. This causes an increase in pressure, which spreads in all directions, including to the surface of the material, forcing the heated water to be expelled to the surface. A gradual cooling process is the last stage. The water on the surface of the material is continuously evaporated. The difference in moisture content at the surface and in the depth of the material results in a difference in the partial pressure, which enables the transport of moisture to the surface. The evaporation of moisture from the surface of the drying material requires a considerable amount of heat. Consequently, the surface of the material and the surrounding air are cooled [16].