Study of Road Performance and Curing Mechanism of Coal Gangue by Curing Agent

Abstract

Coal gangue is a type of rock waste product with dark gray color during coal mining and washing. The curing agent stabilizes soils by improving their physico-mechanical properties, allowing the soils to be reused in engineering as the subgrade fill. This study investigates the impact of SAHP curing agent on road performance parameters of coal gangue. The results showed that the road performance parameters of coal gangue increase with the curing agent content. The compressive resilience modulus of 7 days and California bearing ratio of coal gangue with 0.2% curing agent meet the specifications. The scanning electron microscope reveals the presence of agglomerated gels and hexagonal prism crystals between coal gangue particles. The observed crystals are ettringite, and the gels are silicate gel (nSiO2·mH2O) formed by the reaction of Na2O·nSiO2 with CO2 and H2O, as determined by combined X-ray diffraction and energy spectrum analysis. The improved coal gangue by the curing agent can be utilized as subgrade fill, supporting the reuse of coal gangue in highway engineering.Mine wastes are generated nearly in all mining operations. As the unwanted by-products of mining, they are often placed in large heaps on the mining sites. Inappropriate disposal of the mine wastes (coal gangue, tailings, and other wastes) would release hazardous substances, which exert great impact on the local ecological environment and human health [1, 2]. The oxidation of sulfide minerals is the main source of acid mine drainage (AMD), which results in the surface and groundwater contamination. As the typical sedimentary rock, coal contains a large amount of carbon, sulfur, and hydrogen elements. The oxidation of pyrite is the major source of AMD or coal mine drainage (CMD) in the coal industry [3]. During the complex oxidation process among water, air, and exposed coal rock, the heavy metals leach into the water gradually. A comprehensive evaluation of the quality of the soils, stream, and water bodies near the coal-washing waste dump from the geochemical perspective is necessary for water remediation plan [4]. It is worth noting that, not all CMD are hated, advanced technology makes it possible to reuse the mining wastes, such as recovering rare earth elements [3].Large size of the mining industry determines the deposition of coal gangue, which occupies a large area of land resources [2, 5]. Oxidation and spontaneous combustion exist during the long-term coal gangue accumulation, and harmful gases such as SO2, NOx, and CO can also be released [1]. Coal gangue has been utilized in the preparation of cement [6], powder asphalt mortar [7], and autoclaved aerated concrete [8]. With the rapid development of transportation infrastructure construction, coal gangue was also proposed to be reused in highway engineering as the filling material.Coal gangue can satisfy the basic requirements for road engineering materials after being compacted or stabilized, which offers certain potential in road construction and rehabilitation [9, 10]. The cinder gravel in Ethiopia can meet the requirements of small-volume road specifications after testing the geochemistry and strength characteristics (California Bearing Ratio or CBR) [11]. The shear strength and compressive strength of the coal gangue can be improved after soaking in acidic solution (pH = 4.5) and alkaline solution (pH = 8.5) by forming gel substances [12]. After being compacted for seven cycles, the coal gangue could meet the requirements of the railway subgrade in Zezhou, Shanxi [13]. Furthermore, freeze–thaw cycle tests of compacted coal gangue revealed that it can also be reused as road-filling materials in cold regions [14]. Unmodified coal gangue can be employed as subgrade filling on low-volume roads. For high-grade roads, subgrade engineering, multiple rolling, and compaction of coal gangue are necessary, which brings out the problem of high energy costs.Adding a curing agent to coal gangue can improve its mechanical performance. Curing agents can be classified as traditional and nontraditional categories [15]. Traditional curing agents, such as fly ash, lime, asphalt, cement, and other inorganic binders, can increase the strength of coal gangue in road construction [16-18]. Inorganic binders, such as lime, cement, and lime-fly ash can be added into the coal gangue to satisfy the requirements of the third-class highway in China [16]. After adding 5.5% cement and 30% gravel to coal gangue, the unconfined compressive strength (UCS) of 7 days exceeded 5.0 MPa, facing the strength requirements for cement stabilized base and subbase in expressway and first-class highway [17]. The mixture of red mud, fly ash, and desulfurized gypsum can also be applied in the stabilization of coal gangue as the solid waste pavement base by providing ettringite and aluminum-modified calcium silicate hydrate gel [9]. Traditional curing agents, especially cement, can enhance the mechanical properties of coal gangue to some extent. However, gaseous emissions like carbon dioxide, sulfur dioxide, and nitrogen oxides have been a serious issue during the clinker manufacturing process [18, 19].Nontraditional stabilizers can be used for soil stabilization, including rice husk ash (RHA), plastic waste, synthetic fibers, and other organic composite soil stabilizer [20]. Organic composite soil stabilizers are mostly liquids, which are mainly composed of one or more combinations of water glass, epoxy resin, polymer materials, and ionic materials. They have been applied to the erosion resistance of the slopes [21] and the improvement of the mechanical properties of clay and gravel [22, 23]. In addition, curing agent can also be utilized to reinforce soils contaminated by heavy metals [24, 25]. However, research on the improvement of coal gangue by new curing agents is rare. Therefore, it is essential to explore new and sustainable nontraditional curing agents for coal gangue [26]. If feasible, the use of nontraditional stabilizers in coal gangue reinforcement can enhance the engineering performance of roadbeds while conserving cultivated land and reducing construction costs.This study examines the physical, chemical, and mechanical properties of coal gangue modified using SAHP nontraditional curing agent. The UCS, compression resilience modulus, shear strength, and CBR of coal gangue at 7 and 28 days under various curing agent contents are determined through laboratory tests and analysis. The curing mechanism of the curing agent on coal gangue is analyzed using scanning electron microscope (SEM), X-ray diffraction (XRD), and energy dispersive spectroscopy (EDS) analysis. The research findings can provide a basis for applying coal gangue in subgrade engineering.In this study, the coal gangue was sourced from an abandoned gangue dump (116° 47′ 24″ E, 35° 23′ 26″ N) near the Zao-He Expressway in Shandong Province, China (Figure 1).Laboratory tests were conducted to determine the liquid plastic limit, particle size distribution, free expansion rate, disintegration resistance index, and crushing value of coal gangue in accordance with standards (in Chinese): JTG 3430 (2020) [27], JTG E41 (2005) [28], and JTG E42 (2005) [29], as shown in Figure 2(a). The SAHP soil stabilizer produced by Zhuonengda Construction Technology Co.Ltd was applied to improve natural coal gangue in this study (Figure 2(b)). Sodium silicate, sodium sulfate, anhydrous ethanol, hydroxypropyl methylcellulose (HPMC), polyacrylamide (PAM), and inorganic compounds of CaCl2 and MgCl2 are the primary components of the SAHP curing agent.This study conducted the UCS test, resilient modulus (RM) test, direct shear test, and CBR test of coal gangue mixed with 0.2%, 0.4%, 0.6%, and 0.8% curing agent based on the JTG 3430 (2020) [27] to investigate the physical and mechanical properties of coal gangue under various curing agent content.The coal gangue samples were divided into two groups based on their curing ages: 7 and 28 days. Water must be added to the natural coal gangue to ensure uniform water distribution. The diluted curing agent cannot be added to the coal gangue during the enclosing process to avoid preconsolidation. Therefore, the water used in the sampling preparation was split into two parts, the first part was added in advance for enclosing material, and the second was utilized to dilute the curing agent. After the enclosing period, the diluted curing agent should be added to the coal gangue within 30 min before the standard specimen preparation.Under the direction of JTG E51(2009) [30], the UCS and RM of the specimens (100 × 100 mm) were performed using the TC-200 F pavement material comprehensive tester manufactured by Hebei Longteng Test Instrument Co., Ltd. Six specimens were prepared for each group test. The maximum compressive load was 0.6 times the average compressive strength. The RM of the specimen was measured by six levels of loading and unloading load (Figure 2(c)).The ZJ strain-controlled direct shear apparatus produced by the Nanjing Soil Instrument Factory was used for the quick shear test. During the shear process, the vertical loads were 100, 200, 300, and 400 kPa, and the shearing rate was kept at 0.1 mm/min. The compaction test was conducted utilizing the JDS-1 numerical control electric compactor manufactured by Nanjing Soil Instrument Factory.In the CBR test, specimens were prepared using the heavy compaction instrument, and the amount of soaking swell after 4 days and nights of immersion was measured. The submerged samples were applied to the penetration test based on the TC-200F pavement material comprehensive tester. The bearing ratio of the 50 mm diameter penetration bar was calculated when the penetration was 2.5 and 5 mm, with the larger value serving as the CBR.The mineral composition of the improved coal gangue was characterized using XRD by a Smartlab SE diffractometer equipped with X-radiation. The scan setting was 5–85° 2θ and 10°/min.The micro-morphology of coal gangue specimens with various curing agent contents was observed at different magnification levels with a Gemini SEM 360 field emission SEM and an energy dispersion spectrometer (EDS) produced by Carl Zeiss AG.Figure 3 depicts the grain size distribution of four parallel groups of coal gangue based on JTG 3430 (2020) [27]. The average value of d10, d30, and d60 of the natural coal gangue are 0.975, 3.650, and 8.185 mm, respectively. Based on formulas 1 and 2, the nonuniformity coefficient Cu and curvature coefficient Cc of the natural coal gangue can be calculated. It reveals that the nonuniformity coefficient of natural coal gangue Cu is 8.39, and the curvature coefficient Cc is 1.66, indicating that the coal gangue is poorly sorted or well graded. The fundamental physical indices of coal gangue are listed in Table 1. As a unique geological material, coal gangue has the property of disintegration. Using the JTG 3430 (2020) [27], the compaction test of coal gangue is conducted by a heavy compactor. Figure 4 displays the relationship curves between water content and dry density of four parallel groups of coal gangue. It illustrates that the optimal water content of natural coal gangue is 7.04%, and the maximum dry density of coal gangue is 2.12 g/cm3.The chemical composition analysis of coal gangue (Table 2) was analyzed by the burette (CaO, MgO, Al2O3), ultraviolet spectrophotometer (TU-1901; SiO2, TiO2), and the atomic absorption spectrophotometer (A3F-12; K2O, Na2O) under the guidance of Method for Chemical Analysis of Silicate Rocks (GB/T 14506-2010) [31]. The primary chemical components of coal gangue are SiO2, CaO, and Al2O3, which account for 41.59%, 14.59%, and 13.09%, respectively. In addition, Fe2O3, Na2O, K2O, MgO, Na2O, TiO2, and other oxides are also presented in the coal gangue with relatively low contents. Coal gangue can be split into calcium magnesium (ω > 10%) and silicon aluminum (ω ≤ 10%) based on the total content of CaO and MgO [32]. In this paper, the content of CaO and MgO in coal gangue is 15.15%, demonstrating that the studied gangue is calcium magnesium.Figure 5 depicts the UCS and RM of coal gangue with various curing agent contents at 7 and 28 days of curing ages. It demonstrates that the UCS and RM of coal gangue rise as the curing agent content increases. Under 7 days of curing age, the UCS of coal gangue containing 0.2% curing agent is 0.77 MPa (Figure 5(a)), and the RM is 90.79 MPa (Figure 5(b)). When the curing agent content is 0.8%, the UCS is 1.12 MPa, and the RM is 154.24 MPa. In other words, at 7 days, the UCS of coal gangue with 0.8% curing agent increases by 45.45% compared to that of 0.2%, and the RM is 69.88% greater. Under 28 days of curing age, the UCS of coal gangue with 0.2% curing agent is 0.87 MPa, and the RM is 111.26 MPa (Figure 5(c)). When the curing agent proportion is 0.8%, the UCS is 1.38 MPa, and the RM is 174.24 MPa (Figure 5(d)). The UCS of coal gangue at the age of 7 days with 0.8% curing agent increases by 58.62% and the RM grows by 56.61% compared to 0.2% curing agent. The UCS and RM of coal gangue after 28 days of curing are higher than the parameters after 7 days.When the curing agent content is 0.8%, the 28-day UCS and RM of gangue are 23.21% and 12.97% higher than those measured after 7 days of curing age, respectively (Figure 5). In JTG D30 (2015) [33]and JTG D50 (2017) [34], it is specified that the RM of the top surface of the roadbed should be the critical design index of the subgrade. The RM of improved coal gangue with 0.2% curing agent at 7 days of curing age is 90.79 MPa (Figure 5(b)), which already meets the RM requirement (70 MPa) of the roadbed top surface under extremely heavy traffic load grade. In addition, at the same curing age, the growth of compressive strength and RM of coal gangue gradually reduces with the curing agent content.Figure 6 illustrates the cohesion (c) and internal friction angle (φ) of coal gangue in the direct shearing test at 7 and 28 days of curing age under various curing agent content. It is revealed that the growing trend of cohesion and internal friction angle with curing agent content is similar to the development trend of UCS and RM in Figure 5. When the mixing ratio of the curing agent is 0.8%, the cohesion and the internal friction angle under the curing age at 7 days are 79.24 kPa and 33.22°, respectively (Figure 6(a) and (b)). Compared to the shear strength of coal gangue with 0.2% curing agent at 7 days, the cohesion of coal gangue with 0.8% increases by 28.47%, and the internal friction angle rises by 32.97% (Figure 6(a) and (b)). At 28 days, the cohesion of coal gangue with 0.8% curing agent is 112.31 kPa (Figure 6(c)), which is 41.73% greater than that with 0.2% curing agent, while the internal friction angle increases by 11.86% (Figure 6(d)). Thus, the improved effect of curing age on cohesion is not as obvious as expected. In addition, the growth rates of coal gangue cohesion and internal friction angle decrease with the dosage of curing agent.Figure 7 depicts the CBR and expansion rate measured by the CBR test for different curing agents. It indicates that the CBR gradually increases, and the expansion rate decreases with the curing agent content. The expansion rate of coal gangue ranges between 0.29% and 0.43%. When the content of the curing agent is 0.2%, the CBR of the stabilized coal gangue is 38.07%, which is higher than the requirement of ≥ 8% bearing ratio of subgrade fill in JTG D30 (2015) [33]. In addition, the CBR of the stabilized coal gangue reaches 62.31% when the curing agent content is 0.8%, which is 63.67% higher than that of 0.2% curing agent content.The physical and mechanical parameters of coal gangue mixed with a curing agent are significantly enhanced compared to natural coal gangue. As the curing agent content rises, the compressive strength, RM, shear strength, and CBR of coal gangue at 7 and 28 days increase (Figures 5-7). When the proportion of curing agent increases from 0.2% to 0.8%, the growth rates of 7 days UCS and RM of coal gangue are 45.27% and 69.89%, respectively (Figure 5). Under the same condition, the growth rates of 28 days UCS and RM of coal gangue are 58.62% and 56.61%, respectively, when the curing agent content climbs from 0.2% to 0.8% (Figure 5). Although the increase of RM of coal gangue at 28 days age under the influence of curing agent is slightly less than that at 7 days, the curing agent still has a significant promotion effect on the coal gangue strength.Currently, the new curing agent is generally applied to the soil rather than coal gangue, and cement can also be utilized as the stabilizer. Soils stabilized by different new curing agents are listed in Table 3. Shen et al. [22] employed a DGSC curing agent with a dosage of 15% to clay, and the results showed that the UCS at 7 and 28 days is 0.177 and 1.944 MPa, respectively. Adabi et al. [23] added PG curing agent to sand to stabilize the loose gravel. Other curing agents can also be utilized to reinforce soils contaminated by heavy metals [24], mucky soils [35], and high plastic clay [36] with various cement dosages.The addition of cement also contributes to the strength improvement of the soils stabilized by CFG, CACO, and PPF (Table 3). Without adding cement, the UCSs of clay mixed with DGSC [22] and sand mixed with PG [23] curing agent at 7 and 28 days are close to the UCS of coal gangue mixed with SAHP curing agent investigated in this paper. Hence, although the coal gangue is loose, the impact of the curing agent on the gangue strength improvement can be compared favorably with other stabilized soils.Figures 8 and 9 depict the microstructure morphology of the natural coal gangue and the improved coal gangue at 7 and 28 days of age, respectively. Based on SEM images, the natural structure of coal gangue is quite loose (Figure 8(a), (b) and (c),). After mixing with a curing agent, the coal gangue debris contains hexagonal prism crystalline with nondirectional distribution (Figure 8(e), (h) and (i)). Furthermore, the corresponding energy spectrum image (Figure 8(p)) demonstrates the distribution of O, Si, Al, S, and Ca in the hexagonal prism crystalline, which indicates the generation of ettringite. At the same time, it is discovered that gel adheres to the stabilized coal gangue, which fills the particle interface (Figure 8 and Figure 9). Also, the energy spectrum image (Figure 8(q)) shows the distribution of O, Si, and Na in the gel, which proves the existence of the silicic acid gel. Under the 28-day curing age, the crystalline in the coal gangue is more intensive (Figure 9), which indicates that the amount of hexagonal prism crystal has a positive correlation with the curing agent proportion for samples in 7 and 28 days. When the curing agent concentration is 0.8%, the crystals are densely packed (Figure 9(k) and (l)).XRD analysis was performed on natural coal gangue and improved coal gangue with the 0.8% curing agent at 28 days of age (Figure 10). Figure 10(a) and 10(b) display the diffraction peaks at 2θ = 26.61° and 24.85°, which correspond to quartz and kaolinite of coal gangue, respectively. Ettringite with a diffraction peak at 2θ = 11.61° can be observed in stabilized coal gangue containing 0.8% curing agent (Figure 10(b)), which is consistent with the hexagonal prism crystals detected in the SEM images (Figures 8 and 9).As a composite curing agent, the main components of SAHP are sodium silicate, sodium sulfate, anhydrous ethanol, HPMC, and PAM. Sodium silicate reacts with CO2 and H2O to form agglomerated silicic acid gels, creating an aggregate structure. In the alkaline solution prepared by the curing agent, the Al2O3 in the coal gangue reacts with Ca2+ and produces hydrated calcium aluminate (C-A-H), which then reacts with SO42− in the curing agent to form ettringite. The formation of silicate gel and ettringite crystal skeleton contributes to the coal gangue’s strength increase. The reaction becomes more thorough as time passes. In addition, HPMC can be dissolved in anhydrous ethanol to increase the viscosity of coal gangue particles. PAM can improve the physical bonding strength between debris.There are few studies on coal gangue stabilization with composite curing agents. Hence, the applications of traditional curing agents, such as cement and fly ash, for coal gangue improvement are discussed here (Table 4). The improved coal gangue strength by 5% cement in 7 days curing time are 5.003 MPa [37] and 3.801 MPa [38]. After mixing with 15% fly ash and 5% cement, the coal gangue’s 7 and 28 days compressive strengths are 5.102 and 8.403 MPa [39]. Cai et al. [40] added carbide slag and fly ash with Ca(OH)2 to coal gangue at 15% content and discovered that the latter period strength exceeds the early period strength. It can be seen that cement can enhance the mechanical strength of the coal gangue. With the increase in curing time, the compressive strength of the stabilized coal gangue increases.Inappropriate disposal of the coal gangue would release hazardous substances such as AMD, which exerts a great impact on the local ecological environment. Applying coal gangue in highway construction would be a great improvement in reusing the mine waste. This study examines the influence of SAHP curing agent on the road performance of coal gangue in the subgrade fill of Zao-He Expressway in Shandong using laboratory tests and analysis. The content of CaO and MgO in coal gangue in this paper is 15.15%, demonstrating it is calcium magnesium. The nonuniformity coefficient of the coal gangue is 8.39, and the curvature coefficient is 1.66, indicating that the coal gangue is well graded. The UCS, RM, cohesion, and internal friction angle of improved coal gangue are positively related to the increase of SAHP curing agent content. Under 7 days of curing age, the UCS of coal gangue containing 0.2% curing agent is 0.77 MPa, the RM is 90.79 MPa and the CBR of the stabilized coal gangue is 38.07%, which meets the requirement for RM (70 MPa) and bearing ratio (8%) of the roadbed top surface subjected to extremely heavy traffic load grade. The UCS, RM, cohesion, and internal friction angle values of 28 days coal gangue stabilized with 0.8% curing agent are 1.38 MPa, 174.24 MPa, 112.31 kPa, and 37.16°, which are 23.21%, 12.97%, 41.73%, and 11.86% higher than those of 7 days (1.12 MPa, 154.24 MPa, 79.24 kPa, and 33.22°), respectively. Coal gangue can be stabilized by curing agent, which leads to silicic acid gel and ettringite crystal formation. The silicic acid gel fills the particle interface, growing the density of packs. Hexagonal prism crystals intersperse coal gangue debris to form a strong skeleton and squeeze the particle spacing. With the increase in curing time, the strength of coal gangue has been greatly improved, which can be applied to highway subgrade engineering.The data supporting the results of the study can be found in the following link: website: https://pan.baidu.com/s/1XfGd7Zx0DxkFGGJdwRc1pA. key:grj5The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.This study was supported by the National Natural Science Foundation of China (NO. 42107191). We acknowledge the editors and the anonymous reviewers for their insightful suggestions on this work.