**1. Introduction**

Coal plays an essential role in the energy supply of China and many countries in the world. Coal gangue is produced during coal mining, accounting for almost 10–25% of total coal extraction [1–3]. A large number of coal gangues are piled up around mines. With continuous coal mining, more and more coal gangues are piled up, but the total utilization rate is low [4]. On the one hand, the accumulation of coal gangue occupies many valuable land resources and pollutes the soil, groundwater, and air in the accumulation place. On the other hand, a large amount of coal gangue accumulation may cause spontaneous combustion endangering the safety of the public and property. Therefore, there is an acute need to tackle this underutilization of coal gangue from a sustainability perspective [5–8].

As a suitable filling material, coal gangue was mainly used for filling subsidence areas and land reclamation earlier in many countries. In order to improve the utilization rate of coal gangue, researchers in some European countries started investigating the application of coal gangue in different fields, such as preparation of chemical materials,

**Citation:** Guan, J.; Lu, M.; Yao, X.; Wang, Q.; Wang, D.; Yang, B.; Liu, H. An Experimental Study of the Road Performance of Cement Stabilized Coal Gangue. *Crystals* **2021**, *11*, 993. https://doi.org/10.3390/ cryst11080993

Academic Editors: Peter Taylor, Yifeng Ling, Chuanqing Fu and Peng Zhang

Received: 28 July 2021 Accepted: 19 August 2021 Published: 20 August 2021

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**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/).

mining minerals, filling subgrade, and preparation of building materials [9–11]. In recent years, Li, Dong, and others [12–15] have analyzed the mechanical properties, from micro and macro perspectives, of cement mortar with calcined coal gangue replacing the cement content and have analyzed the specific factors affecting its properties. Many experts and scholars [16–20] have carried out a large number of tests to study the mechanical properties of coal gangue concrete and have analyzed the factors affecting its durability. Coal gangue material has been widely used in many civil engineering fields, such as for highway subgrade, a foundation cushion in construction engineering, etc. Many countries have taken the lead in applying coal gangue to engineering examples, such as the road network in northern France, the road network in the Ruhr region of Germany, and the railway stations of Gloucester and Croydon in Britain [21]. The application of coal gangue in highway engineering in China is gradually increasing with the advancement in transport infrastructure [22–24]. Many research findings show that the strength of coal gangue as a subgrade filler can fully meet the requirements of subgrade design [25–28]. At present, coal gangue material is mainly used as a subgrade filler and cushion, and the research into its use as a highway base is relatively scarce.

With the development of traffic infrastructure, the use of coal gangue in road engineering in China is progressively increasing. Relevant research and engineering applications show that the mechanical properties of coal gangue as a subgrade filler can fully meet the requirements of subgrade design [29,30]. He et al. [31] carried out screening tests, compaction tests, consolidation tests, permeability tests, bearing ratio tests, and direct shear tests to study the influence of soil on the engineering mechanical properties of coal gangue. The results indicated that the strength of soil gangue used as subgrade filler can fully meet the subgrade design requirements. Wu et al. [32] studied the strength and deformation characteristics of a coal gangue subgrade filler under different confining pressures, different gradations, and different compactness through large-scale triaxial tests with the method of artificial grading. The test results showed that the compactness of the coal gangue subgrade filler should not be less than 93%. Di et al. [33] conducted a preliminary study on the engineering properties of coal gangue through a compaction test and triaxial strength test. They systematically analyzed the variation model of shear strength parameters, maximum dry density, and optimum moisture content of coal gangue with coarse-grained material content. Geng et al. [34] carried out unidirectional frost heaving tests on common filling materials and coal gangue for high-speed railway subgrade under open system and closed system conditions. The test results showed that it was feasible to use coal gangue as a high-speed railway subgrade filler in permafrost regions. Zhou et al. [35] used lime fly ash stabilized coal gangue as a pavement base material and designed 15 different ratios. The test results confirmed that its strength meets the requirements of the expressway and first-class highway for the base and subbase. However, its freeze-thaw resistance is insufficient, so it is necessary to add cement to improve the durability of stabilized coal gangue. Therefore, the utilization and applicability of coal gangue are limited due to the constraints mentioned above. The utilization rate is not high, which necessitates exploring methods for improvement. The research into the frost resistance and drying shrinkage performance of coal gangue material in the base of high-grade highways is still sparse. The research into the application of coal gangue material in the base of high-grade highways is scarce, which does not allow the full utilization of coal gangue.

Given the existing problems in coal gangue applications as a highway base, this paper takes the coal gangue material produced in the Hebi area of Henan Province as its research subject. The optimum gradation of stabilized coal gangue was determined by using the power function model *y* = *ax<sup>b</sup>* [36], and cement was used to stabilize the coal gangue. Subsequently, through a compaction test, an unconfined compressive strength test, a freeze–thaw test, a splitting test, and a drying shrinkage test, the mechanical properties and durability of cement stabilized gangue were analyzed. The objective was to provide a basis, guide, and reference for the application of coal gangue materials in a high-grade highway base.
