**1. Introduction**

The accumulation of end-of-life tires has become a worldwide problem with dramatic consequences for sustainability and with a trend to worsen due to the increasing demand of cars, with 1.4 billion tires produced annually across the world, which is equivalent to an amount of 17,000,000 tons/year of tires used [1]. In 2017 in the United States, 287.3 million tires were disposed of, corresponding to 4.7 million tons [2]. In Colombia, an estimated 61,000 tons of tires are annually disposed of [3], from which 18,861 tons are regenerated yearly in the capital city, Bogota [4]. In this city, 15,880 tons of tires were processed and reutilized between 2016 and 2018 [5].

Due to the high stresses in car tires under normal conditions, they are fabricated from rubber matrix reinforced by textile fibers and steel wires. Combining textile fibers and steel wires make tires resistant to biodegradation and give them high strength properties [6]. All these materials can be used after the tire's life cycle. Sienkiewicz et al. [1] summarized five procedures to handle discarded tires: pyrolysis, retreading, product recycling, energy recovery, and material recycling. Retreading deals with the substitution of used parts with new ones [7], although there are serious safety concerns for the vehicle when this is used [8]. Energy process is one of the most common applications for utilizing the used tires as energy source [1]: with a calorific capacity value of 32 MJ/kg, used tires compete with fuel [9]. Pyrolysis is a chemical process that gives flammable gas as byproduct of the tire waste transformation, in addition to carbon black (CBp), liquid fuel, pyrolytic oil, and pyrolysis char [10]. Product recycling involves using the discarded tires in different components,

**Citation:** Jiménez, J.E.; Fontes Vieira, C.M.; Colorado, H.A. Composite Soil Made of Rubber Fibers from Waste Tires, Blended Sugar Cane Molasses, and Kaolin Clay. *Sustainability* **2022**, *14*, 2239. https://doi.org/10.3390/ su14042239

Academic Editor: Syed Minhaj Saleem Kazmi

Received: 18 January 2022 Accepted: 11 February 2022 Published: 16 February 2022

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**Copyright:** © 2022 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/).

where recycling aims to obtain the original raw materials via processing [1]. Most of these strategies are not massively implemented in many countries representing a significant part of the world, mostly because of the costs involved in the process and other factors including absence of clear environmental laws [11].

In construction materials, perhaps the most known use for waste tires is its use in asphalt pavements to improve the concrete performance [11], although, waste tires are used in multiple applications. One of them is in low-scale retaining structures and architectural furnishing elements [12,13]. Two byproducts of discarded tires have been evaluated in concretes in order to know the mechanical properties: recycled tire polymer fibers (RTPF) and recycled rubber [14–16]. Moreover, steel fibers were evaluated as a potential reinforcement for concrete [17,18]. In another approach, the recycled tire rubber particles were combined with soil containing clay and sand as a potential new material [19–21]. Other composite materials used in construction have been studied and insulation properties were identified [22].

On the other hand, molasses are an organic byproduct of the repeated crystallization of sugar. It is a viscous syrup, with a dark brown color. The main elements in molasses are carbon and oxygen, while others in minor contents are calcium, sulfur, magnesium, chlorine, and potassium. Molasses are commonly used in the agricultural sector to control parasites [23], improve pastures [24], and to create nutritional supplements for cattle [25]. Molasses have also been used in different areas such as an additive for cement and concrete [26], during the cement production [27], stabilizer for expansive clays [28], and as a carbon source for bioremediation of contaminated soils [29].

Kaolin is a white fine-grained clay soil with kaolinite as its main constituent as hydrous aluminum silicate, composed of alumina and silica alternately stacked [30]. Kaolin has commonly been used in specific applications in paper, ceramics, refractories, plastics, rubber, adhesives, and paint industries [30]. Recently, kaolin has been used in additive manufacturing as well [31,32]. Furthermore, it was extensively used as construction material, and in its calcinated form, it has been used in concrete, for improving properties such as strength and durability [33]. Kaolin is also a very important source of alternative cements, geopolymers [34].

Several authors have addressed the use of waste materials from different origins to improve the ground, obtaining considerable improvement in the engineering properties of soils. Plastic wastes have been used to counteract traction stresses in the soil mass [35]. Ground plastic bottles have been used as reinforcement material when mixed with different soil types [36], waste fique natural fibers [37], and ashes from different processes were mixed with soils in order to develop cementing chemical reactions [38].

The present research studied an environmentally friendly alternative to the disposal of RTPF with the addition of molasses. Molasses were used as cementing material and as a matrix for the dispersion of the fibers. Characterization via X-ray diffraction (XRD), scanning electron microscopy (SEM), and shear strength, was used to evaluate the ground improvement. Finally, microorganisms that grew in the soil samples are shown.
