**1. Introduction**

Different soil drying methods can have various effects on the structure and, thus, on the geomechanical and filtration features (thermally induced thermo-hydro-mechanical behavior of saturated or unsaturated soils [1–3]). The rapid change in conditions can affects the soil fabric [4] and therefore, it can weaken the soil grains or solid material, leading to cracking, fracturing, and crushing. Furthermore, with cohesive soils, the repeated drying procedure may cause bond degradation or cementation. Moreover, temperature variations can initiate mineralogical changes. All these phenomena can also occur in situ, although on a different time scale and markedly smaller temperature ranges (generally to approximately 40 ◦C [4,5]).

Many experimental studies have explored rapid microwave drying [6,7] and the influence of multiple wet–dry cycles (such as those that occurred during laboratory drying) on the physical and mechanical properties of soil or its mixtures with various additives (e.g., with cement, liquid modifiers, bentonite, etc.), which include thermally modified swelling parameters [8,9], hydraulic conductivity/permeability coefficients [8,10,11], shear strength [12,13], durability, stiffness, and the void ratio [12], Atterberg limits [8], and even intensification of landslides [11,14,15]. Listed soil features which may be (but do not have to be) affected by the impact of thermal changes are strongly related to changes in the macrostructure (crack intensity factor, length, and cracks opening), the microstructure (e.g., porosity and density), and the geochemical composition (mineralogy and chemical composition) of the soils [10]. Nevertheless, reducing the drying time of soils has many

**Citation:** Kaczmarek, Ł.; Jastrz ˛ebska, M.; Wejrzanowski, T. The Impact of Microwave Drying on the Structure of Exemplary Soils—Insights Using X-ray Microtomography. *Materials* **2022**, *15*, 5891. https://doi.org/ 10.3390/ma15175891

Academic Editor: Tamas Varga

Received: 2 July 2022 Accepted: 20 August 2022 Published: 26 August 2022

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implications, which make the procedure of accelerated drying of soils in the laboratory and their potential field applications important research issues.

Therefore, in this pilot study, the focus is on the influence of rapid changes in water content (because of microwave drying) on the structure of selected soil (specifically, medium sand and hydrophobic silty clay). The impact of temperature on soil structure is a complex phenomenon and requires a multifaceted approach, especially due to the variety of existing microwave drying procedures. In order to evaluate the effects of microwave drying, comparative drying in a conventional oven and air drying were performed. The reference test was the air-dry state of a sample that was not exposed to high-temperature drying. Non-invasive and non-destructive X-ray computed microtomography (XμCT) was used to determine structural changes.
