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Article

Comparison of Different Additives and Ages on Mechanical and Acoustic Behavior of Coal Gangue Cemented Composite

by
Meng Xiao
State Key Laboratory of Intelligent Construction and Healthy Operation and Maintenance of the Underground Engineering, China University of Mining and Technology, Xuzhou 221116, China
Appl. Sci. 2024, 14(22), 10418; https://doi.org/10.3390/app142210418
Submission received: 11 October 2024 / Revised: 26 October 2024 / Accepted: 7 November 2024 / Published: 12 November 2024
(This article belongs to the Special Issue New Insights into Digital Rock Physics)
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Abstract

:
Cemented backfill represents a significant trend in mine filling methods; however, it often exhibits high brittleness and limited resistance to failure, which can restrict its practical application. This study investigates the mechanical properties and damage evolution of fiber-reinforced coal gangue cemented materials (CGCMs) at various curing times using uniaxial compressive tests, acoustic emission (AE) analysis, and scanning electron microscopy (SEM). Specimens were created with different fillers, including carbon fibers (CFs), steel fibers (SFs), and carbon black (CB), and subjected to uniaxial compression until failure. Control specimens without fillers were also tested for comparison. The microstructure of the specimens was examined using scanning electron microscopy (SEM). The findings indicate that (1) the compressive strength of filler-reinforced CGCMs increases between 7 and 14 days of curing but decreases thereafter, with CB significantly improving early-age strength; (2) specimens reinforced with CFs and SFs exhibit significantly enhanced toughness in their post-cracking response; (3) AE events during specific stages can effectively identify the reinforcing effects of CFs and SFs; (4) the presence of fillers improves resistance to shear cracks, with CFs and SFs being more effective than CB; and (5) adding CB results in a denser and more stable hydration product structure, while CFs and SFs lead to a more porous structure with increased cracking.

1. Introduction

Coal gangue (CG) is solid waste generated during coal mining and washing, comprising about 10–25% of total coal output. In China, CG production ranges from 600 to 800 million tons annually, with a utilization rate below 70%. Currently, CG stockpiles exceed 7 billion tons, increasing by 300–350 million tons each year. This accumulation leads to land wastage, severe air and water pollution, and potential environmental disasters like landslides and spontaneous combustion [1]. Therefore, it is crucial to explore effective utilization strategies. Over the past two decades, technologies have emerged for CG disposal in various sectors, such as power generation, concrete production, and road construction. However, due to the complex mineral composition and transport costs, only limited disposal options are viable. Recently, cemented backfills—composed of cementitious materials and mining waste—have gained popularity for stabilizing mine goafs. This method does not require specific mineral content and allows for on-site use of CG, making it a convenient and cost-effective solution [2].
CG cemented materials (CGCMs) are vital for supporting surrounding rock, helping to maintain stability in underground spaces. However, their high brittleness makes concrete susceptible to cracking over time due to challenging environmental conditions. If issues like micro-cracks are not quickly resolved, the lifespan of CGCM structures can be greatly diminished, leading to severe consequences.
Incorporating additives like fibers and nanoparticles is considered an effective method to improve the mechanical properties of CGCMs. There are many studies focusing on additive materials for multifunctional concrete. Among them, carbon fibers (CFs), steel fibers (SFs), and carbon black (CB) are widely used because of their high performance and lower price. Perez et al. [3] evaluated the mechanical properties of concrete with SFs and found that the addition of fibers to self-compacting concrete improved its mechanical behavior after cracking load. Wang et al. [4] studied the flexural behavior of CF-reinforced concrete beams under impact loading, and their test results showed the addition of CFs improved concrete’s ability to absorb impact energy. Yildirim et al. [5] analyzed the self-sensing capability of concrete incorporated with CFs or CB, and they found that carbon-based materials improved the mechanical properties of concrete, and CFs were the best for improving the self-sensing capacity of concrete. Nguyen et al. [6] studied the tensile self-sensing and mechanics of SF-reinforced concrete. Their research showed that SFs produced a significant enhancement of both self-sensing capacity and tensile strength. Nalon et al. [7] compared the electrical and mechanical properties of concrete with different CB, and they found that a CB particle with high structure and surface area significantly enhanced electrical properties of concrete but had a harmful impact on its compressive strength. Cao et al. [8] studied the mechanical properties and damage evolution of waste tire SF-reinforced CGCM. Their results demonstrated a significant enhancement in both the compressive strength and peak strain of the CGCM. Ou et al. [9] found that incorporating CB significantly improved the mechanical strength and workability of cemented materials. A similar finding was reported in the study by Kang et al. [10], which showed that CB achieved comparable compressive strength due to its internal curing, interlocking, and bridging effects. The literature mentioned above focuses mostly on macro-mechanical properties of concrete with different admixtures. The macro-mechanics of concrete is the result of deformation and damage of micro- or meso-structure, so it is important to study the meso-mechanical characteristics of multifunctional concrete. However, literature about this field is rare.
Acoustic emission (AE) can be used to detect damage and deformation of the micro- and mesostructure of concrete, and many studies have shown that acoustic emission is an efficient way of analyzing the effect of admixture in concrete. Mu et al. [11] studied the mechanism of the reinforcement of aligned SFs on the mechanical properties of cement-based composite by AE signal analysis, and their results showed that the concrete with aligned SFs had more AE signals produced by fiber pull-out. Jiao et al. [12] studied the reinforced mechanism of concrete with basalt and SFs by comparing their AE counts and duration. The test results showed that there were satisfactory correlations between the curve characteristics of AE parameters and the failure loads of specimens. The literature authorized by Aggelis et al. [13] focused on the effect of chemical coating and shapes of SFs on the AE characteristics monitored during the fracture process of multifunctional concrete. Their results showed that AE analysis could be used for interpretation of the fracturing stage and characterization of the fracture mode. An experimental study conducted by Park et al. [14] demonstrated that micro-damage of CF-cemented composite can be detected by AE. Topolář et al. [15] monitored AE activity during three-point bending tests on concrete with CB, and the effect of CB on micro-damage of concrete was analyzed.
The above studies have demonstrated that incorporating additives like fibers changes the mechanical properties of cementitious materials, including CGCMs, significantly, and the AE technique is an effective measure for characterizing damage and fracture in cemented materials. However, most research has focused on single additive materials, and comparative studies between different fillers are scarce. Every piece of research has focused on the effect of fibers on the mechanical characteristics of CGCMs. To address this gap, this paper conducts an experimental study on the AE characteristics of CGCMs with various additive materials, including CFs, SFs, and CB, with plain CGCM specimens prepared for comparison. The effects of different additives and curing ages on compressive strength, mechanical properties, and failure mechanisms are analyzed, and the damage mechanisms are further examined using SEM. The test results can provide valuable references for enhancing the supporting ability of backfill materials and the comprehensive utilization of coal-based solid waste.

2. Materials and Methods

2.1. Materials and Preparation of Specimens

The CG used in this experiment was sourced from Xuzhuang Coal Mine in Xuzhou City, Jiangsu Province, China (Figure 1). X-ray diffractograms of coal gangue aggregate (CGA) were obtained with a Bruker AXS D8 Advance instrument with Cu anode (40 kV, 40 mA) (Billerica, MA, USA). Samples were step-scanned at a rate of 2°/min, with 2θ in the range 5–70°, with a step size and 2s/step count time. The results, shown in Figure 2, revealing that the primary mineral components are kaolinite, quartz, and feldspar, consistent with previous studies. The chemical composition of CGA was determined using X-ray fluorescence spectroscopy (XRF, AXIOSmAX, Panalytical B.V., Alemelo, The Netherlands). The results, presented in Table 1, reveal that the primary constituents of CGA are SiO2 and Al2O3, accounting for 62.113% and 28.169%, respectively.
The preparation of the CGCMs also involved the use of 42.5 ordinary Portland cement (PC), fine silica fume, polycarboxylate superplasticizer (SP), and potable water. Table 2 provides the chemical composition and physical characteristics of both the PC and silica fume.
Fillers used to improve conductivity in cement-based composites include carbon fibers (CFs), steel fibers (SFs), and carbon black (CB). Among these, CB is a powder filler, whereas CFs and SFs serve as flexible and rigid fiber fillers, respectively. CFs have a length of 12 mm, a diameter of 7 μm, 95% carbon content, 1.53% elongation, a tensile strength of 3500 MPa, an elastic modulus of 230 GPa, and a density of 1.75 g/cm3. SFs, on the other hand, have a diameter of 200 μm, a length of 6 mm, a tensile strength of 2060 MPa, and an elastic modulus of 200 GPa. For CB, the surface area is 254 m2/g, the average particle size is 30 nm, and the density is 0.0961 g/cm3.
The CF content in this study matched that used by Chen and Liu [16], while the SF and CB content was based on the findings of Lee et al. [17] and Li et al. [18], respectively, both of which reported satisfactory mechanical performance. The names and proportions of the CGCM mixtures are listed in Table 3, with each mixture named according to the specific functional additive it contained. The water-to-binder ratio was maintained at 0.4, and workability was enhanced using polycarboxylic acid SP (Feike Ltd., Yuncheng, China) at a dosage of 0.25%. The mass ratio of CFs to PC was set at 0.08, while the volume percentages of SFs and CB were 2% and 9%, respectively, and these proportions were converted to mass ratios in Table 3. Additionally, a control CGCM mixture without any functional fillers was also prepared for comparison.
A 60 L mixer was used to prepare the CGCM specimens. First, 30% of the total water and the SP were added and allowed to sit for 20 min to ensure complete dissolution. The functional filler was then manually incorporated. Next, the remaining water and silica fume were added, and the mixer operated at a slow speed for about 1 min. Finally, PC and CGA were added, and mixing continued for an additional 2 min.
Cubic specimens measuring 100 mm × 100 mm × 100 mm were then cast, with nine specimens produced for each mix. After 24 h, the specimens were demolded and placed in a moist-curing room for curing durations of 7, 14, and 28 days, respectively.

2.2. Mechanical and AE Test

AE analysis is employed to monitor micro-crack initiation and propagation within the CGCMs during mechanical testing. It captures transient elastic waves generated by the release of energy from internal microstructural changes, providing real-time data on damage onset and progression. This approach enables the assessment of the material’s mechanical behavior, identification of critical stress points, and determination of failure modes. Consequently, AE analysis offers valuable insights into the material’s integrity and durability, facilitating a comprehensive evaluation of performance under various additive conditions. In this study, a PCI-2 AE system (manufactured by Physical Acoustic Corporation, Princeton Junction, Princeton, NJ, USA) was used to capture AE signals during mechanical testing. Two probes, attached to the CGCM specimen surface, collected acoustic emission signals simultaneously during loading. The sampling frequency was set to 140 kHz, with a signal amplification of 45 dB.
Compression tests were then carried out using a YNS 2000 Servo hydraulic testing system (Changchun Machinery Factory, Changchun, China) at a strain rate of 0.05 mm/min. The layout of the test equipment and CGA-based specimen is shown in Figure 3.

2.3. SEM Test

CGCM is a heterogeneous multiphase system comprising a solid phase with various hydration products, along with water and air occupying the void spaces [19]. The mechanical properties of CGCM are closely linked to the specimen’s microscopic porosity structure, which is influenced by the type and quantity of hydration products formed. To investigate the characteristics of these hydration products, their morphology and microstructural features were analyzed using a Hitachi SU8000/8220 scanning electron microscope (Hitachi High-tech Corporation, Minako-ku, Tokyo, Japan).

3. Results and Discussion

3.1. Strength of Concrete with Different Fillers and Ages

3.1.1. Effects of Fillers

Figure 4 and Table 4 present the compressive strength of concrete after different curing times. The results indicate that, compared to plain paste, CB significantly enhances the compressive strength of samples at 7- and 14-day curing periods but reduces it at 28 days, consistent with findings by Dai et al. [20] and Dong et al. [21]. The high specific surface area of CB leads to increased water adsorption, which negatively impacts the rate of cement hydration [7]. However, as the curing age increases, strength decreases due to insufficient hydration reactions. As observed in similar studies [22,23,24], the effect of SFs on enhancing compressive strength is minimal. CFs have a negative impact on compressive strength due to the poor bonding between the cement and the fibers. The compressive strength of SF-reinforced CGCM is higher than that of CF-reinforced CGCM, which can be attributed to the rigidity of SFs compared to CFs. SFs are more effective at bridging macro-cracks than CFs, thereby resulting in higher compressive strength compared to CF-reinforced specimens.

3.1.2. Effects of Ages

Figure 5 presents the compressive strength of specimens reinforced with different fillers at various ages. The results show that the strength of all specimens increases from 7 to 14 days, primarily due to the hydration of cement. It should be noted that SFs and CB enhance the strength of specimens at an early age, demonstrating that compactness and bridging action significantly contribute to improved mechanical properties [25,26]. However, after 14 days, only the plain specimens continue to gain strength, while the strength of specimens reinforced with CFs remains nearly unchanged, and the strength of those reinforced with CB and SFs decreases. This reduction can be attributed to poor cohesion between the cementitious matrix and fillers, leading to higher porosity and reduced mechanical properties [27]. Additionally, the adsorption of CB on the surface of the cement further hinders the hydration process [28].

3.2. Mechanical Characteristics of Concrete with Different Fillers and Ages

The deformation modulus changes continuously during the loading process, and its variation reflects the micro-damage development in the samples. Figure 6 presents the stress–time curve (blue lines) and the stress growth rate–time curve (red dotted lines). To facilitate a better comparison with the AE results, time data were used on the horizontal axis. Typically, strain data are used on the horizontal axis. In this experiment, a strain-controlled loading method was employed, resulting in a linear correlation between the deformation modulus and the stress growth rate. As a result, the stress–strain curves and stress–time curves exhibit identical shapes.
The loading process can be divided into three or four stages based on the stress growth rate–time curve. In stage I, the growth rate fluctuates slightly around 0, which is due to the compaction of voids and cracks inside the samples. In stage II, the stress growth rate rises steadily, indicating an improvement in the load-bearing capacity of the samples, attributed to further compaction of micro-voids and cracks. Upon entering stage III, the stress growth rate decreases continuously, with a sharp drop near the peak stress, caused by the development of micro-cracks, which negatively affects the load-bearing capacity. In stage IV, the stress growth rate stabilizes around a certain value, reflecting the residual load-bearing capacity of the samples.
Interestingly, the distribution of the four stages during the loading process varies with sample age and fillers. The reference and CB-reinforced samples show a higher rate of decline around point C compared to CF- and SF-reinforced samples, while stage IV in fiber-reinforced samples lasts longer and is more stable than in reference and CB-reinforced samples. Notably, reference and CB-reinforced samples do not exhibit stage IV at 28 days of age. Based on the experimental results, the post-cracking response in the compressive tests of CF- or SF-reinforced specimens is characterized by significant toughness, which can be attributed to the bridging action of fibers and is not observed in plain or CB-reinforced concrete [24,29]. It is also observed that the duration of stage IV in all samples decreases significantly with age, due to the reduction in residual load-bearing capacity as the hydration reaction of cement progresses.

3.3. AE Hits Analysis of Concrete with Different Fillers

AE hit counts are often regarded as an important parameter associated with crack events, and the number of AE counts has a positive correlation with both micro- and macro-crack occurrences [30,31]. Specimens at 7-day curing ages were selected to analyze the effect of fillers on AE characteristics, as shown in Figure 7, where the compressive stress is also provided. To highlight different cracking states during the cracking process of specimens, four points were marked on the stress–time curve. Points a, b, and c were determined by the method described in Section 3.2.
Based on the growth rate, the cumulative AE count curves can be classified into two categories. The first category includes plain and CB-reinforced samples, where the growth rate of cumulative counts is lower at first and then increases. The second category comprises CF- and SF-reinforced samples, both reinforced with fiber materials, whose growth rate remains almost linear throughout the loading process. The lower growth rate in the first category corresponds to the compaction and elastic-like stage of the stress–time curves, indicating that the closing process of micro pores in these samples is easier than in fiber-reinforced samples, which is consistent with results in the literature [32]. The cumulative count growth rate of the CB-reinforced sample is significantly lower at the beginning of loading compared to plain samples, likely due to the denser microstructure of the CB-reinforced sample [33].
Figure 7 also shows the distribution of AE counts during the loading process (red line). It can be seen that the plain sample exhibits more AE events with a large number of counts compared to the filler-reinforced samples. Notably, the CB-reinforced concrete shows a more uniform distribution of AE events, with only a few large-count events occurring around point a and after point c, attributed to the filling effect of CB particles on the matrix’s micro pores [33]. The AE strength of CF-reinforced samples is lower during the compaction stage, but some high-strength AE events appear in stages III and IV, which is quite different from the plain and CB-reinforced samples. This is due to the bridging effect of CFs, where the pull-out events can be detected by AE signals. A similar phenomenon is observed in SF-reinforced samples, with high-strength AE events appearing in stages II and III, indicating that AE signals can reflect the reinforcing effect of fillers.
Moreover, there are notable differences between CF- and SF-reinforced samples. AE events in SF-reinforced samples are higher in stage I but significantly lower in stage IV. Generally, rigid fibers (SFs) and flexible or soft fibers (CFs) have different effects because rigid fibers do not deflect to fill voids within the concrete [31,32]. During the compaction stage, CF-reinforced samples are more easily deformed due to the flexibility of CFs. After peak loading, SFs exhibit a better reinforcing effect on the matrix, leading to fewer AE events. This is further demonstrated by comparing the stress–time curves of CF- and SF-reinforced samples: the stress decline rate of SF-reinforced samples is slower, indicating that the reinforcing effect of rigid fibers is better than that of flexible fibers [34].

3.4. Failure Mode from AE Events

The instability failure of CGCM results from the initiation, propagation, and coalescence of cracks during loading, with the distribution pattern of these cracks significantly influencing the material’s fracture behavior [35]. Therefore, studying the crack patterns of CGCMs from a microscopic perspective is crucial for understanding their instability failure mechanism. Previous research has shown that different crack modes, such as tensile and shear cracks, produce distinct waveform characteristics during loading, allowing failure modes to be identified through AE signal analysis [32]. Crack modes can be differentiated using two key AE parameters: the average frequency (AF), defined as the number of counts over the signal duration (measured in kHz, see Figure 8), and the RA value, which is the ratio of rise time to the maximum amplitude of the waveform (measured in μs/V) [36]. These parameters help characterize crack types, with tensile cracks exhibiting lower RA and higher AF values, while shear cracks display lower AF and higher RA values. Each AE event was classified as either a tensile or shear crack by calculating the AF/RA ratio, enabling a quantitative analysis of crack evolution across different loading stages.
To analyze the failure modes of concrete reinforced with different fillers, the classification of cracks was performed using an oblique line based on the RA–AF characteristics (see Figure 9). When selecting the relative ratio of RA and AF, there is currently no definitive value [37]. According to the literature [38], a ratio of AF:RA = 50 kHz × V/μs was suggested as a boundary to classify crack types based on moment tensor analysis. In Figure 9, cracks are classified as tensile when the AF signal is lower than 50RA, while shear cracks occur when the AF signal is higher than 50RA. Additionally, cracks were counted separately for each stage of loading.
Table 5 shows the proportion of shear and tensile cracks at different loading stages. It can be observed that shear cracks form during the initial stages, specifically stages I, II, and III. As the loading progresses, the proportion of shear cracks gradually decreases, while the proportion of tensile cracks increases. By stage IV, tensile cracks become the predominant mode of failure. Aggelis et al. [13] reported similar findings, indicating that AE activity in the earlier stages is associated with micro-cracking in the matrix, which is the primary active mechanism at that point, whereas extensive matrix cracking and fiber pull-out occur during the main fracture. The fiber pull-out can increase tensile failure [32].
Compared to the reference group, during stages I to III, the average proportion of shear cracks in samples reinforced with CB and CFs decreases significantly from 19.53% to 17.41% and 14.82%, respectively, indicating that CB and CFs effectively enhance resistance to tensile cracking. Conversely, the SF-reinforced samples show a slight reduction in shear crack proportion, from 80.47% to 79.87%, suggesting that the rigid nature of SFs helps to prevent shear crack formation. Additionally, the results indicate that curing time has no significant effect on the failure mode of the samples.

3.5. SEM Analysis

Figure 10 shows the microstructure of CGCM mixes cured for 28 days. The reference sample generated a substantial amount of flocculent hydration products, primarily C-S-H gels, along with a small amount of Ca(OH)2. The C-S-H gels contribute to a denser structure, thereby enhancing the strength of the cement. In Figure 10b, the distribution of CB is difficult to observe. Due to the dilution effect caused by the carbon materials, the microstructure of the CB-reinforced sample exhibited cracks and pores. Compared to the reference sample, the CB-reinforced sample produced less C-S-H gel and more needle-shaped AFt crystals, which interlaced with each other to form a denser skeletal structure, thereby enhancing the strength of the CGCM.
The CF-reinforced sample produced a significant amount of flaky Ca(OH)2, which has weak cementitious properties and contributed to the lowest mechanical strength of the CGCM. This can be explained by the hydrophobic nature of CFs, which can increase the water–cement ratio due to its bleeding effect, making it easier for Ca(OH)2 crystals to form [37]. The SF-reinforced sample displayed a loose and porous structure, primarily consisting of needle-like AFt crystals. This is attributed to the stiff SFs particles introducing more air bubbles into the mix [38].

4. Conclusions

In this study, the effects of curing time and fillers, including SFs and CFs, and CB, on the mechanical properties of CGCMs under uniaxial loading were demonstrated using AE. The following conclusions can be reached.
(1)
CB enhances the compressive strength of the samples by 10.84% and 13.85% at 7 and 14 days of curing, respectively. However, it leads to a slight reduction in compressive strength, decreasing by 3.27%, at 28 days of curing. SFs increase the compressive strength of the samples by 4.16% at 14 days of curing but reduce it by 8.45% and 13.69% at 7 and 28 days, respectively. CFs negatively impact compressive strength, decreasing it by 21.11%, 17.94%, and 24.08% at 7, 14, and 28 days, respectively. The compressive strength of samples reinforced with CB, SFs, and CFs increases by 14.2%, 17.14%, and 26.5%, respectively, from 7 to 14 days, but shows only minor changes beyond 14 days of curing. In contrast, the compressive strength of plain cemented paste continues to increase steadily, by 11.2% and 10.1%, over time.
(2)
The damage process of cemented paste can be classified into three or four stages according to the variations in stress growth rate. The post-cracking behavior in compressive tests of specimens reinforced with CFs or SFs exhibits notable toughness, while the reference samples and those reinforced with CB show a sharp decline at the end of stage III. Additionally, the duration of stage IV for CF- and SF-reinforced samples decreases significantly by 91% and 45%, respectively, as the curing age increases from 7 to 28 days.
(3)
The reinforcing effect of additives can be detected by the AE method, and the AE characteristics vary according to the types of fillers. The cumulative count growth rate of plain and CB-reinforced samples is lower at first and then becomes higher, while the cumulative count growth rate of CF- and SF-reinforced samples is almost linear during the loading process. AE events at stage III can effectively realize the identification of the reinforcing effects of CFs and SFs.
(4)
From stages I to III, shear cracks are the predominant crack type, with their average proportion across all samples decreasing from 86.61% to 76.61%. In stage IV, the primary failure mode shifts to tensile cracks, which account for 77.62% of the total. Additionally, the presence of CB and CFs positively influences resistance to tensile cracking, while SFs effectively mitigate shear cracking.
(5)
Adding CB results in a denser and more stable hydration product structure, whereas CFs and SFs contribute to a more porous structure with increased cracks.

Funding

This research was financially supported by the Nature Science Foundation of China (52304162), the Chinese Central Government-Guided Special Fund for Regional Scientific and Technological Development (ZYYD2024JD16), and the Major Science and Technology Projects of Xinjiang Uygur Autonomous Region (2012A01002).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data is contained within the article.

Conflicts of Interest

The author declares no conflicts of interest. All authors have read and agreed to the published version of the manuscript.

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Applsci 14 10418 g001
Figure 1. The crushed CGA.
Figure 1. The crushed CGA.
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Figure 2. XRD result sheet for CGA.
Figure 2. XRD result sheet for CGA.
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Figure 3. Layout of the test equipment.
Figure 3. Layout of the test equipment.
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Figure 4. Compressive strength of samples with different fillers.
Figure 4. Compressive strength of samples with different fillers.
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Figure 5. Compressive strength of concrete in different ages.
Figure 5. Compressive strength of concrete in different ages.
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Applsci 14 10418 g006aApplsci 14 10418 g006b
Figure 6. Mechanical characteristics of samples during loading: (a) D7Re; (b) D7CB; (c) D7CF; (d) D7SF; (e) D14Re; (f) D14CB; (g) D14CF; (h) D14SF; (i) D28Re; (j) D28CB; (k) D28CF; (l) D28SF.
Figure 6. Mechanical characteristics of samples during loading: (a) D7Re; (b) D7CB; (c) D7CF; (d) D7SF; (e) D14Re; (f) D14CB; (g) D14CF; (h) D14SF; (i) D28Re; (j) D28CB; (k) D28CF; (l) D28SF.
Applsci 14 10418 g006aApplsci 14 10418 g006b
Applsci 14 10418 g007aApplsci 14 10418 g007b
Figure 7. AE characteristics of concrete with different fillers: (a) D7Re; (b) D7CB; (c) D7CF; (d) D7SF.
Figure 7. AE characteristics of concrete with different fillers: (a) D7Re; (b) D7CB; (c) D7CF; (d) D7SF.
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Figure 8. AF and RA definitions of the simplified waveform of an AE signal.
Figure 8. AF and RA definitions of the simplified waveform of an AE signal.
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Applsci 14 10418 g009aApplsci 14 10418 g009bApplsci 14 10418 g009c
Figure 9. Relationship between RA value and AF under uniaxial loading: (a) D7Re; (b) D7CB; (c) D7CF; (d) D7SF; (e) D14Re; (f) D14CB; (g) D14CF; (h) D14SF; (i) D28Re; (j) D28CB; (k) D28CF; (l) D28SF.
Figure 9. Relationship between RA value and AF under uniaxial loading: (a) D7Re; (b) D7CB; (c) D7CF; (d) D7SF; (e) D14Re; (f) D14CB; (g) D14CF; (h) D14SF; (i) D28Re; (j) D28CB; (k) D28CF; (l) D28SF.
Applsci 14 10418 g009aApplsci 14 10418 g009bApplsci 14 10418 g009c
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Figure 10. SEM investigation of CGCM mixes after 28 days normal curing: (a) reference sample; (b) CB-reinforced sample; (c) CF-reinforced sample; (d) SF-reinforced sample.
Figure 10. SEM investigation of CGCM mixes after 28 days normal curing: (a) reference sample; (b) CB-reinforced sample; (c) CF-reinforced sample; (d) SF-reinforced sample.
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Table 1. The chemical composition of CGA.
Table 1. The chemical composition of CGA.
Material TypeSiO2Al2O3Fe2O3K2OCaOTiO2MgONa2O
CGA62.11328.1693.5082.2311.2240.9810.6670.400
Material TypeSO3Others
CGA0.3790.328
Table 2. Chemical composition and physical properties of PC and SF.
Table 2. Chemical composition and physical properties of PC and SF.
ConstituentsChemical Composition, %Physical Properties
SiO2Al2O3Fe2O3MgOCaONa2OK2OLoss on
Ignition		Specific Gravity
(Unitless)		Blaine Fineness
(m2/kg)
PC20.55.053.502.4965.10.150.742.403.05350
Silica fume92.01.00.90.70.31.30.451.60.6519.5
Table 3. Material proportions of different self-sensing mixtures.
Table 3. Material proportions of different self-sensing mixtures.
Mixture IDMaterial Proportion (kg/m3)Curing Time
(Days)
CGAPCSFSPWaterFiller
D7Re13751375412.541.2511000.07
D14Re13751375412.541.2511000.014
D28Re13751375412.541.2511000.028
D7CB13751375412.541.25110077.847
D14CB13751375412.541.25110077.8414
D28CB13751375412.541.25110077.8428
D7CF13751375412.541.251100227
D14CF13751375412.541.2511002214
D28CF13751375412.541.2511002228
D7SF13751375412.541.251100155.437
D14SF13751375412.541.251100155.4314
D28SF13751375412.541.251100155.4328
Table 4. Average compressive strength of samples after different curing times.
Table 4. Average compressive strength of samples after different curing times.
FillersCompressive Strength (MPa)
7 Days14 Days28 Days
Reference28.77 ± 0.62
(--)		31.99 ± 0.39
(--)		35.22 ± 2.67
(--)
CB31.89 ± 0.56
(10.84%)		36.42 ± 1.16
(13.85%)		34.07 ± 1.07
(−3.27%)
CF22.41 ± 1.44
(−21.11%)		26.25 ± 0.41
(−17.94%)		26.74 ± 1.04
(−24.08)
SF26.34 ± 1.00
(−8.45%)		33.32 ± 0.45
(4.16%)		30.40 ± 1.72
(−13.69%)
Table 5. Proportion of crack types at different loading stages.
Table 5. Proportion of crack types at different loading stages.
Mix IDTensile CracksShear Cracks
S1S2S3S4S1S2S3S4
D7Re13.6314.3420.4281.3686.3785.6679.5818.64
D7CB12.8921.9827.6767.9387.1178.0272.3332.07
D7CFs10.5714.5715.2387.6489.4385.1384.7712.36
D7SFs21.5126.9432.8877.5378.4973.0667.1222.47
D14Re14.1615.0433.9578.3785.8484.9666.0521.63
D14CB10.4714.9117.2984.9689.5385.0982.7115.04
D14CFs11.4214.1921.4474.3988.5885.8178.5625.61
D14SFs8.8611.8122.3280.1591.1488.1977.6819.85
D28Re15.4621.2027.53-84.5478.8072.47-
D28CB0.1517.2318.94-84.6582.7781.06-
D28CFs11.5515.6718.4567.2288.4584.3381.5532.78
D28SFs14.8317.5124.5476.6685.1782.4975.4623.34
Note: S1, S2, S3, and S4 represent stage I, stage II, stage III, and stage IV, respectively.
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Xiao, M. Comparison of Different Additives and Ages on Mechanical and Acoustic Behavior of Coal Gangue Cemented Composite. Appl. Sci. 2024, 14, 10418. https://doi.org/10.3390/app142210418

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Xiao M. Comparison of Different Additives and Ages on Mechanical and Acoustic Behavior of Coal Gangue Cemented Composite. Applied Sciences. 2024; 14(22):10418. https://doi.org/10.3390/app142210418

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Xiao, Meng. 2024. "Comparison of Different Additives and Ages on Mechanical and Acoustic Behavior of Coal Gangue Cemented Composite" Applied Sciences 14, no. 22: 10418. https://doi.org/10.3390/app142210418

APA Style

Xiao, M. (2024). Comparison of Different Additives and Ages on Mechanical and Acoustic Behavior of Coal Gangue Cemented Composite. Applied Sciences, 14(22), 10418. https://doi.org/10.3390/app142210418

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