Road Performance Evaluation of Unburned Coal Gangue in Cold Regions

Abstract

At present, the cumulative storage of coal gangue in China exceeds 7 billion tons, covering an area of approximately 70 km². The engineering application of unburned coal gangue is mainly utilized as concrete aggregate or cement production after the overfire process. However, it is prone to environmental pollution and has limited consumption. Using unburned coal gangue as roadbed filling not only alleviates the difficulty of land acquisition and soil collection for road constructions in mining areas, but also consumes a large amount of accumulated unburned coal gangue. This study conducts research on the road performance of unburned coal gangue. A series of laboratory tests have been performed to determine the physical and chemical properties of the unburned coal gangue and its performance as a filling material in cold regions. The influence of compaction effort, clay content, and number of freezing–thawing cycles on the mechanical performance of the unburned coal gangue was investigated. The typical unburned coal gangue in the Heilongjiang region is mainly composed of SiO₂, Al₂O₃, and Fe₂O₃, which accounts for approximately 91% of the total mass. The unburned coal gangue meets the minimum CBR requirement of 8% after 7 freezing–thawing cycles. This study helps fully and reasonably utilize typical unburned coal gangue in the local area, providing technical support for achieving the overall goal of “green development, conservation and intensification, and low-carbon environmental protection”.

1. Introduction

Coal, as a resource with abundant reserves, is a major component of China’s energy structure. In the past 10 years, China’s average annual coal production has been approximately 3.809 billion tons. As the solid waste generated in the process of coal mining and coal washing, the amount of coal gangue accounts for about 10~20% of the total amount of coal output [1]. Coal gangue is a mixture of coal and rocks around the coal seam. Besides the major gangue components of coal, Al₂O_3, and Fe₂O₃, gangue components also include some metal sulfides, heavy metals, and nitrogen-containing organic matter (mainly pyrite). At present, the cumulative storage of coal gangue exceeds 7 billion tons in China, covering an area of about 70 km² and facing problems of land occupation and environmental pollution. In China, the engineering application of unburned coal gangue is mainly used for concrete aggregate or cement production after the process of overfire [2]. This method has a high recycling cost, limited consumption, and is prone to environmental pollution. At present, the utilization rate of unburned coal gangue in China is only about 30%, which is relatively low compared with other developed countries [3].

Due to the relatively good physical and mechanical properties of unburned coal gangue, some scholars have begun to explore the application of unburned coal gangue in road engineering construction. Jiang and Zhang et al. [4,5] performed research on unburned coal gangue samples from Xuzhou, Huaibei, and other regions. The particle size range of coal gangue in these areas has characteristics of a wide size with poor grading, high content of coarse particles, low content of fine particles, and extremely uneven particle size distribution. At the same time, there is also the problem of discontinuous particle size distribution. The research results of the French Ministry of Highway Technology Research and the Road and Bridge Test Center have found that unburned coal gangue is a high-performance building filling material, which can be beneficial for roadbed paving [6]. Experimental roadbed construction was conducted using unburned coal gangue. The results show that unburned coal gangue can be compacted to a higher degree under vibration compaction conditions, with a dry density of up to 1.91 g/cm³ [6]. Researchers such as Buter, Michalski, and Solesbury [7,8,9] conducted on-site compaction simulation experiments using different types of coal gangue and found that the degree of compaction of coal gangue is closely related to its particle size distribution. Unburned coal gangue with a high content of fine particle size usually has good compatibility and consolidation performance. Skarzynska et al. [10,11] found a strong correlation between the compressibility of unburned coal gangue and the particle size non-uniformity coefficient. The higher the non-uniformity coefficient, the higher the compressibility of unburned coal gangue. Meng [12] analyzed the physical properties and chemical composition of coal gangue raw materials through methods such as mass loss in ignition test and hydration test, and obtained the unique physical and chemical properties of coal gangue as a building foundation material. Wu Jun and others [13,14,15] conducted an evaluation on the strength and deformation characteristics of unburned coal gangue through a large-scale triaxial shear test to determine the influences of factors such as gradation, confining pressure, and compactness on the strength and deformation characteristics of unburned coal gangue. Li and He [16,17] analyzed the impact of different soil content on the physical and mechanical properties of unburned coal gangue in the Loudi area and summarized the variation pattern between the soil content and the physical and mechanical parameters of unburned coal gangue.

Moreover, domestic and foreign scholars have also conducted relevant research on the road performance of unburned coal gangue. Yin [18,19] systematically studied the road performance of unburned coal gangue in the Handan mining area. Test results showed that the bearing capacity of the unburned coal gangue met the requirements of highway construction as a filling material. Di Shengguan [20] studied the physical and chemical properties of unburned coal gangue in the Handan mining area of Hebei Province and analyzed the effects of chemical composition, mineral composition, and activity of unburned coal gangue on roadbed filling. The distribution, bearing capacity, and other indicators of unburned coal gangue were studied, and the feasibility of using unburned coal gangue as a roadbed filling material was determined. Liu et al. [21] conducted research on the basic physical and mechanical properties of unburned coal gangue, and the results showed that unburned coal gangue is a high-performance road-building material. Based on its physical and mechanical properties, feasible plans and technical approaches for roadbed filling were proposed. He [16] studied the influence of soil content on the bearing capacity and compaction characteristics of unburned coal gangue and proposed the technical index requirements for unburned coal gangue as a filling material for highways. Renpeng [22] conducted a series of consolidation tests on unburned coal gangue samples under the coupling effect of load and dry–wet cycles. He studied the dehumidification and moisture absorption characteristics of unburned coal gangue under the combined action of load cycles and dry wet cycles. Zhang et al. [23,24,25] carried out a static triaxial shear test based on the results of a field screening test of coal gangue with different weathering conditions and obtained the decay law of mechanical parameters of coal gangue undergoing different weathering and degrees of saturation. Wang [26] conducted a systematic study on the water stability of coal gangue railway embankment slopes by combining physical and mechanical tests, the FLAC3D numerical simulation method, and the image processing method.

The physical and chemical properties of unburned coal gangue are the basis for studying its road performance as there are significant differences in unburned coal gangue in different regions. Therefore, before studying the road performance of typical unburned coal gangue in Heilongjiang Province, it is necessary to study its basic physicochemical properties. In addition, the road performance of unburned coal gangue should be considered in conjunction with local geographical and climatic conditions. The application of unburned coal gangue not only alleviates the difficulty of land acquisition and soil acquisition in mining area road engineering, but also consumes a large amount of accumulated unburned coal gangue, which is conducive to building a low-carbon, environmentally friendly society, achieving sustainable development, and responding to China’s “dual carbon” strategy. Whether from the perspective of economic construction, environmental protection, or social development, the application of unburned coal gangue in road engineering has broad prospects. This article investigates and samples typical unburned coal gangue in Heilongjiang Province, China, and studies the road performance of unburned coal gangue roadbeds in cold regions.

2. Materials and Methods

2.1. Materials

2.1.1. Basic Properties of Unburned Coal Gangue

This study investigates and samples 12 typical unburned coal gangue sites in Heilongjiang Province. The sample numbers and sampling locations of unburned coal gangue are shown in Figure 1. Heilongjiang Province has a good reputation for its abundance in coal storage, and 90% of the coal was stored in the eastern part of the province. In addition, it is estimated that there are approximately 292 unburned coal gangue hills, accounting for a total mass of 576 million tons. The moisture contents of the samples are between 1.2% and 4.9%, and the average specific gravity is 2.71.

2.1.2. Particle Size Distributions

According to the Chinese standard “Test Methods of Soils for Highway Engineering” (JTG 3430-2020) [27] (hereinafter referred to as the standard), sieve analysis was performed for the collected unburned coal gangue samples, and the test results are shown in Figure 2. Most of the unburned coal gangue samples were coarse-grained soils, with an effective particle size of less than 0.3 mm with particle sizes ranging from 8 mm to 12 mm. The C_u values of unburned coal gangue samples in each mining area are greater than 5, indicating that the distribution of unburned coal gangue particles is uniform, and the particle size difference is significant. Most samples have a C_c between 1 and 3, indicating that the particle size of unburned coal gangue is complete, continuously distributed, well-graded, and conducive to compaction.

2.1.3. Mass Loss on Ignition, Crush Value, and Compaction Characteristics

According to the specification, a loss-on-ignition test was conducted on typical unburned coal gangue samples. The results are shown in Figure 3a, and the loss-on-ignition of unburned coal gangue is relatively large, ranging from 9.74% to 35.18%. However, most samples have a loss-on-ignition of less than 20%, which meets the requirements of the Chinese standard “Specifications for Design of Highway Subgrades” (JTG D30-2015) [28] for roadbed construction.

The crushing value test was also conducted on some samples of unburned coal gangue, and the test results are shown in Figure 3b. The crushing value of unburned coal gangue varies greatly among different mining areas, but it is generally high. Among them, the crushing value of samples from sites 2, 3, and 5 is less than 30%, while the crushing value of samples from sites 1, 4, and 7 is between 31% and 35%. The unburned coal gangue contains soft rocks, residual coal, organic matter, and other easily broken substances, resulting in a generally high crushing value of unburned coal gangue. Before using unburned coal gangue as roadbed filling material, it is necessary to check whether the crushing value meets the requirements of roadbed filling specifications [28].

Through the compaction test, we can explore the characteristics of unburned coal gangue. It can be seen from Figure 3c that the optimal moisture content distribution of unburned coal gangue is 6.9~8.5%, and the maximum dry density distribution is 1.98–2.12 g/cm³.

2.1.4. Chemical Components

Unburned coal gangue is a mixture of carbonaceous rocks and other rocks, with a relatively complex composition and structure. The chemical composition and sulfur content were analyzed in accordance with the specification, and the results are shown in Figure 4. The main chemical composition of each coal gangue sample is shown in Figure 4a, with SiO₂, Al₂O₃, and Fe₂O₃ accounting for about 91% of the total mass. The content of SiO₂ is the highest, at approximately 64.4%. The content of Al₂O₃ takes the second place, at about 22.2%, with a ratio between 2.8 and 3.2. Soluble SiO₂ and Al₂O₃ in unburned coal gangue can react with lime, cement, fly ash, and other materials at room temperature to produce gel hydration products. Al₂O₃ can form ettringite with calcium ions to increase the bonding strength between particles. The sulfur content of the unburned coal gangue is shown in Figure 4b, and the sulfur content of each unburned coal gangue sample is relatively close and far below 1.0%.

2.2. Test Methods

2.2.1. CBR Test

According to the specification, the CBR test is carried out at room temperature. The CBR test method is as follows: the samples of unburned coal gangue from different mines are dried and mixed with clay (0%, 5%, 10%, and 15%) and deionized water to achieve the target water content. After soaking in a closed environment for 24 h, the samples are compacted in 3 layers in the mold, and each layer is compacted according to the set number of times (98 times, 50 times, and 30 times). Place the prepared specimen into a water tank and read the dial indicator value. Then, soak it in water for 4 days and read the dial indicator value again. After taking out the test piece and letting it stand for 15 min, a penetration test is performed for each test sample.

The CBR test method under freeze–thaw cycle conditions is as follows: after drying the unburned coal gangue sample, prepare the sample according to the optimal moisture content. Then, immerse the sample in a closed environment for 24 h and compact it in 3 layers with 98 blows per layer (lifts per layer). After that, soak the prepared sample in a water tank for 4 days. Finally, take out the sample, wait for 15 min, then place a temperature sensor in the center of the sample. Seal the test sample and freeze it in a refrigerator at −15 °C. The penetration test is performed after the sample has experienced 7 freezing–thawing cycles.

2.2.2. Dynamic Resilient Modulus Test

A dynamic rebound modulus test is conducted according to JTG D30-2015 [27]. The elastic response in unbounded granular materials and subgrades was termed the “Resilient Modulus” to indicate its nonlinearity, and it is the ratio between an applied deviatoric stress and its induced recoverable axial strain. Firstly, the raw unburned coal gangue from each test site is screened, oven-dried, and cooled to room temperature. Then, the sieved unburned coal gangue is mixed with clay at 0%, 5%, 10%, and 15% content by weight and deionized water is added to achieve the target water contents. After that, the sample is placed in a sealed environment for 12 h to ensure that water is well distributed within the soil sample. Finally, the samples are compacted at the target density using the modified Proctor test, and the dynamic resilient modulus test is carried out according to the specified load loading sequences. As for the samples under freezing–thawing cycles, the sample preparation process is the same, and the specimen is sealed and placed in a refrigerator at −15 °C. After that, the sample is melted at room temperature. The dynamic resilient modulus test is performed after the sample has experienced 5 freezing–thawing cycles.

3. Test Results and Analyses

3.1. CBR Test Results

In this section, the CBR test results are analyzed to evaluate the effects of compaction effort, clay content, and number of freezing–thawing cycles on the CBR values of the unburned coal gangue.

3.1.1. Influence of Compaction Effort

The CBR value of each typical unburned coal gangue is shown in Figure 5a. The CBR value of unburned coal gangue samples is far greater than 8%, which is the minimum requirement for subgrade filler of expressways and first-class highways in the specification. Based on the sample particle size results in Figure 1, under the same compactness, the higher the coarse aggregate content of unburned coal gangue, the greater its CBR value. In addition, taking samples from sites 9 and 11 as examples, the effects of different compaction efforts (98, 50, and 30 blows per layer, respectively) on the CBR of unburned coal gangue samples were investigated, as shown in Figure 5b. With the decrease in compaction effort, the compactness of the samples decreased and the CBR values showed a downward trend. The CBR values of unburned coal gangue under three different compaction times are all greater than 8%, meeting the requirements for the bearing capacity of roadbed filling.

3.1.2. Influence of Clay Admixture Content

The unburned coal gangue from five different sites was selected and different clay admixture contents of 0%, 5%, 10%, and 15% were used to explore the effect of clay contents on the CBR performance of unburned coal gangue. As shown in Figure 6, the CBR values of the unburned coal gangue significantly increased when mixed with clay soil for site 10. In contrast, the CBR value first shows an increasing and then decreasing trend as the clay content increases for site 4. The CBR values for gangue from sites 1, 2, and 3 decreased continuously with an increasing amount of clay content. Based on Figure 2, when the number of coarse-grained particles in the unburned coal gangue is high, the clay admixture serves as a compensating content that fills within the pores of the soil skeleton and reduces the internal porosity of the specimen. Therefore, the CBR values show an increasing trend. However, after adding too much clay, there are too many small particles within the unburned coal gangue sample, which reduces the CBR value of the unburned coal gangue.

3.1.3. Influence of Freezing–Thawing Cycles

The internal temperature changes within the test samples are presented in Figure 7a. The required freezing time for the unburned coal gangue specimen is 24 h, and the required melting time is 8 h. The CBR test results after the freeze–thaw cycles are shown in Figure 7b. After seven freeze-thaw cycles, the CBR values of each unburned coal gangue sample show a decreasing trend but are still greater than 8%. This indicates that unburned coal gangue has a certain ability to resist freeze–thaw cycles and can be used as a filling material for roadbeds in cold regions.

3.2. Dynamic Resilient Modulus Test Results

3.2.1. Influence of Compaction Effort

The influence of different compaction degrees ranging from 94% to 98% on the dynamic resilient modulus of unburned coal gangue is shown in Figure 8a–c. At the 94% compaction degree, the resilient modulus values range from 100.0 MPa to 39.7 MPa. The resilient modulus values vary from 107.0 MPa to 33.6 MPa at 96% degree of compaction, while the modulus values change from 127.5 MPa to 40.5 MPa at 98% degree of compaction.

The representative values of the dynamic resilient modulus of the unburned coal gangue are determined and presented in Figure 8d. Compared with 98% degree of compaction, the representative values of the dynamic rebound modulus of 96% compaction and 94% compaction decrease by 10.6% and 14.2%, respectively. The compaction degree has a significant impact on the dynamic rebound modulus value, and the dynamic rebound modulus increases with the increase of the compaction degree.

3.2.2. Influence of Clay Admixture Content

The effect of clay content within the unburned coal gangue on the dynamic resilient modulus is shown in Figure 9a–d. The dynamic rebound modulus ranges from 160.1 MPa to 67.0 MPa at a 5% clay content. The dynamic resilient modulus range at 10% clay content is 162.4 MPa~57.6 MPa. The dynamic resilient modulus range at 15% clay content is 76.5 MPa~38.1 MPa. The representative values of the dynamic resilient modulus of unburned coal gangue with different clay contents are shown in Figure 9e. Compared to the specimens with 0% clay content, the representative values of dynamic rebound modulus of specimens with 5% clay content, 10% clay content, and 15% clay content increased by 139.4%, 107.8%, and 9.1%, respectively. The influence of clay content on the dynamic resilient modulus value is obvious. Adding an appropriate amount of clay to the unburned coal gangue can effectively increase the dynamic resilient modulus of the unburned coal gangue. The maximum increment is observed at a clay content of 5%, after which the resilient modulus values decrease with increasing clay content.

3.2.3. Influence of Freezing-Thawing Content

The influence of the number of freezing–thawing cycles on the dynamic resilient modulus of unburned coal gangue specimens is shown in Figure 10a–d. For samples with 0% clay content and five freezing–thawing cycles, the dynamic resilient modulus ranges from 136.6 MPa to 34.6 MPa. The dynamic resilient modulus range is 152.6 MPa~68.1 MPa for samples at a 5% clay content. The representative values of the dynamic resilient modulus of unburned coal gangue after freezing–thawing cycles at different clay contents are shown in Figure 10e. When no clay is added, the representative value after five freezing–thawing cycles decreased by 8.1% compared to that without freezing–thawing cycles. When the clay content is 5%, the representative value of dynamic resilient modulus after five freezing–thawing cycles decreases by 4.9% compared to that without freezing–thawing cycles. Under the conditions of 98% compaction and optimal moisture content, the impact of freezing–thawing cycling on the dynamic resilient modulus of unburned coal gangue is relatively small, and unburned coal gangue has good resistance to freezing–thawing-induced reduction in the resilient modulus.

4. Conclusions

The unburned coal gangue samples were collected from 12 typical sites in Heilongjiang Province, China. A series of laboratory tests have been performed to determine the physical and chemical properties of the unburned coal gangue and its performance as a filling material in cold regions. The influence of compaction effort, clay content, and number of freezing–thawing cycles on the mechanical performance of the unburned coal gangue was determined via CBR and dynamic resilient modulus tests. The major findings of this study are summarized as follows:

(1) The typical unburned coal gangue in the Heilongjiang region is mainly composed of SiO₂, Al₂O₃, and Fe₂O₃, which accounts for approximately 91% of the total mass of unburned coal gangue. The low sulfur content of the unburned coal gangue indicates that the risk of spontaneous combustion is very low. The crushing value and ignition loss are relatively large, and there is a significant difference between different mining areas. Therefore, it is recommended to check the crushing value and burning loss of the unburned coal gangue before using it as a filling material.

(2) The typical unburned coal gangue in the Heilongjiang region belongs to coarse-grained soil with high compactabilities. The particle size distribution of unburned coal gangue in different mining areas varies greatly, with most samples of unburned coal gangue effectively being less than 0.3 mm and ranging between 8 mm and 12 mm. The gradation curve is relatively smooth and continuous, making it easy to compact the sample made of unburned coal gangue.

(3) The unburned coal gangue has high strength and certain resistance to freezing–thawing cycles, with a CBR value much greater than the minimum requirement of 8%. After seven freezing–thawing cycles, the bearing capacity of the sample still meets the minimum requirement.

(4) The representative value of the dynamic resilient modulus of unburned coal gangue is 27.74 MPa at a 94% degree of compaction. The representative value of the dynamic resilient modulus increases with an increasing degree of compactness. When the unburned coal gangue is in the optimal moisture content state, the effect of freeze–thaw cycles on the dynamic rebound modulus of unburned coal gangue is relatively small.

The traditional application of unburned gangue has limited consumption and environmental issues. Although unburned coal gangue has good physical and mechanical properties, using unburned coal gangue as roadbed filler not only alleviates the difficulty of land acquisition and soil extraction in mining area road engineering, but also consumes a large amount of accumulated unburned coal gangue, which will not cause environmental pollution problems.