materials Article Structure and Strength of Artificial Soils Containing Monomineral Clay Fractions Grzegorz Jozefaciuk * , Kamil Skic , Agnieszka Adamczuk , Patrycja Boguta and Krzysztof Lamorski   Citation: Jozefaciuk, G.; Skic, K.; Adamczuk, A.; Boguta, P.; Lamorski, K. Structure and Strength of Artificial Soils Containing Monomineral Clay Fractions. Materials 2021, 14, 4688. https://doi.org/10.3390/ma14164688 Academic Editor: Tamas Varga Received: 22 July 2021 Accepted: 17 August 2021 Published: 19 August 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Institute of Agrophysics, Polish Academy of Sciences, 20-290 Lublin, Poland; k.skic@ipan.lublin.pl (K.S.); a.adamczuk@ipan.lublin.pl (A.A.); p.boguta@ipan.lublin.pl (P.B.); k.lamorski@ipan.lublin.pl (K.L.) * Correspondence: jozefaci@ipan.lublin.pl; Tel.: +48-817-445-061 Abstract: Structure and strength are responsible for soil physical properties. This paper determines in a uniaxial compression test the strength of artificial soils containing different proportions of various clay-size minerals (cementing agents) and silt-size feldspar/quartz (skeletal particles). A novel empirical model relating the maximum stress and the Young’s modulus to the mineral content basing on the Langmuir-type curve was proposed. By using mercury intrusion porosimetry (MIP), bulk density (BD), and scanning electron microscopy (SEM), structural parameters influencing the strength of the soils were estimated and related to mechanical parameters. Size and shape of particles are considered as primary factors responsible for soil strength. In our experiments, the soil strength depended primarily on the location of fine particles in respect to silt grains and then, on a mineral particle size. The surface fractal dimension of mineral particles played a role of a shape parameter governing soil strength. Soils containing minerals of higher surface fractal dimensions (rougher surfaces) were more mechanically resistant. The two latter findings appear to be recognized herein for the first time. Keywords: mesostructure; mercury porosimetry; particle shape; particle dimension; clay minerals 1. Introduction Soil structure is a unique and important feature governing the development of plants, absorption of nutrients, growth of roots [1], aeration and water transport [2], resistance to erosion [3], and many others features. The strength and durability of soil structure are key factors influencing a number of soil performance characteristics, such as stability of landfill liners, specific barriers for nuclear waste disposal, foundations for buildings, roads and embankments [4–7], soil bearing capacity [8,9], settlement [10], deformation by machines [11], the velocity of seismic waves [12] and many others. Soil structure and strength are governed primarily by the soil components. Among them, organic matter, considered as the most important, has probably been studied most intensively [13,14]. A number of reports concern its effect on soil compressibility, void ratio, liquid and plastic limits [15–17], specific gravity, bulk density [15,16] and compressive strength [18]. The strong impact of iron oxides, alumina and silica on soil structure and strength was reported as well [19–21]. In general, organic matter, sesquioxides and silica decrease soil strength by increasing soil porosity and reducing bulk density. Clay minerals, the next important factors influencing soil structure, usually increase soil strength. The extent of this effect depends on different volumetric compressibility, swelling and shrinking potential, plastic and liquid limits, density and porosity of particular minerals [22,23]. Sharma et al. [4] have reported an increase in the specific gravity, dry density, unconfined compression strength (UCS) and the Young’s modulus with an increase in bentonite proportion in bentonite-sand mixtures. Tiwari and Ajmera [24] revealed that the compression index for montmorillonite dominated soils was higher than for kaolinite or illite dominated soils. They have noticed that the intrinsic compression line was unique for each dominating clay mineral. In studies of multicomponent systems Materials 2021, 14, 4688. https://doi.org/10.3390/ma14164688 https://www.mdpi.com/journal/materials