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Article

Mechanical Properties and Mineral Characteristics of Multi-Source Coal-Based Solid Waste Filling Materials under Different Proportioning

by
Guodong Huang
1,2,3,4,5,*,
Xiaojun Zheng
2,
Miao Gao
3,
Qi Chen
3,
Zheng Qiao
3,
Tianbao Xie
3,
Mengyao Deng
3 and
Qing Wei
3
1
School of Civil Engineering and Construction, Anhui University of Science and Technology, Huainan 232001, China
2
China Construction Second Engineering Bureau Co., Ltd., Beijing 100160, China
3
Institute of Energy, Hefei Comprehensive National Science Center, Hefei 230031, China
4
Engineering Research Center for Geological Environment and Underground Space of Jiangxi Province, East China University of Technology, Nanchang 330013, China
5
Institute of Environment-Friendly Materials and Occupational Health, Anhui University of Science and Technology, Wuhu 241003, China
*
Author to whom correspondence should be addressed.
Crystals 2023, 13(7), 997; https://doi.org/10.3390/cryst13070997
Submission received: 5 June 2023 / Revised: 17 June 2023 / Accepted: 20 June 2023 / Published: 22 June 2023
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Abstract

:
Traditional grouting materials have certain limitations, such as greater cement consumption, high cost, slow setting rate, and insufficient early strength, hindering their wide applicability. In this paper, desulfurization gypsum (DG) and fly ash (FA) are used as the main raw materials, supplemented by a small amount of Portland cement (PC), to develop a low-cost, fast-setting, and high-early-strength filling material. The mechanical properties and setting characteristics were assessed for varying PC, DG, and FA ratios. The effects of different mineral crystal formations on mechanical properties and hydration characteristics were analyzed. The results show that adding DG leads to a sudden decrease in mechanical properties while accelerating the setting. The compressive strength and setting rate increase with increasing DG content. FA can assist in PC hydration and delay the setting time, and the dosage should be limited to 20%. A synergistic enhancement effect between DG and FA can be achieved, forming grossular-type aluminosilicate and promoting compressive strength development. The optimal performance is achieved when PC, DG, and FA are added at 20%, 60%, and 20% dosages, respectively.

1. Introduction

With rapid economic development and industrialization, there have been significant advancements in coal mining, processing, and use. However, this generates huge amounts of solid waste, mainly coal gangue, fly ash, and desulfurization gypsum [1]. The rapid development of coal industries has generated a large amount of solid wastes with a low utilization rate, occupying a large amount of land, severely polluting the environment, and restricting the rapid development of the regional ecological economy and society [2]. Under the ecological development concept of “green water and green mountains are golden mountains and silver mountains”, achieving the safe disposal and comprehensive resource utilization of multi-source coal-based solid waste has become a major issue for national energy and ecological security [3].
Coal mining inevitably causes surface subsidence and ecological environmental damage [4]. Filling mining is an effective method to prevent surface subsidence from the source [5]. It also helps in acquiring coal resources with minimal ecological disturbance and limits the impact on the ecological environment (water and soil resources) and infrastructure within the controllable tolerance limits [6]. In addition, it promotes the coordinated and integrated development of coal mining and the ecological environment [7]. However, traditional underground paste filling requires mixing coal gangue with Portland cement (PC) in the well and transporting it through a long pipeline for subsequent filling [8]. Due to the large volume of filling material, a large amount of PC is consumed, significantly increasing the cost and involving high energy consumption, pollution, and carbon emission issues [9]. Moreover, PC concrete has a slow setting rate and early strength development, affecting underground coal mining operations and the overall filling effect [10].
Many researchers have studied the mechanical properties and setting characteristics of filling materials. Luan et al. [11] used modified tailings powder for paste filling. For performance improvement in filling materials, an addition of 50% tailings powder was found to be optimal. Feng et al. [12] proposed incorporating plant fibers to improve the interfacial transition zone (ITZ) between the filling material aggregate and slurry. The results showed that adding a small amount of fibers could improve the compressive strength of the filling material in the early stage (28-day age); however, the drying shrinkage significantly increased. Yang et al. [13] used various types of solid waste to prepare filling materials, and the flowability and mechanical properties of the filling materials were analyzed, resulting in the most suitable ratio. Zhang et al. [14] used coal gangue as the main raw material to prepare filling materials. In addition, microorganisms were added into coal gangue to form carbonate minerals via microbial fermentation, improving the microstructure and mechanical properties of filling materials. Cheng et al. [15] explored the mechanical properties and microstructure characteristics of coal gangue powder as an underground filling material by combining macroscopic mechanical performance tests with microscopic analysis methods.
Although previous research studies have shown promising results, the dependence on PC has not been surpassed [16,17]. Moreover, utilizing only a single material, i.e., coal gangue or fly ash, as the filling material cannot fully utilize various solid wastes [18,19]. In this paper, desulfurized gypsum and fly ash are taken as the main cementitious materials, replacing PC in a large proportion, significantly reducing energy consumption and carbon emissions, lowering the resulting costs, and leveraging the synergistic enhancement effect between various solid wastes to alleviate the decreased mechanical performance caused by reduced PC content. The compressive strength development and setting behavior of the developed filling material were studied for varying ratios, and the optimal ratio was obtained. The microstructural mineral crystal characteristics of the solid waste filling materials and the synergistic effect on mineral crystals were studied using X-ray diffraction (XRD). The effect of mineral crystal formation on hydration and compressive strength development was analyzed. Finally, a low-cost, rapid-setting, improved-early-strength, and environmentally friendly multi-source solid waste underground filling material with excellent mechanical properties and controllable setting time was achieved.

2. Raw Materials and Experimental Methods

2.1. Raw Materials

2.1.1. Coal-Based Solid Waste

  • Desulfurized gypsum (DG)
DG, known as flue gas desulfurization gypsum, is a coal-based solid waste. Since DG has cementing properties, it can be used as a cementitious material to reduce the amount of PC and lower the preparation cost of filling materials [18]. Its specific surface area reaches 300 m2/kg. The types and contents of elements in DG detected via X-ray fluorescence (XRF) are shown in Table 1. DG exhibits high contents of calcium (32.45%) and sulfur (39.32%), while the silicon (4.36%) and aluminum (1.65%) amounts are relatively low. The DG used in this experimental study was provided by Anhui Huainan Pingwei Power Generation Co., Ltd. (Huainan, China)
  • Fly ash (FA)
In this study, two-level Class-F fly ash conforming to the requirements of the national standard for FA used in cement and concrete (GB/T1596-2017) [19] was used. It has a specific surface area of 250 m2/kg, and the residue on a 45 um square hole sieve is less than 45%. FA only has low hydration reactivity, and it can be used as a cementing material to replace PC, further reducing the amount of cement used and the preparation cost. The types and contents of elemental oxides in the raw materials detected via XRF are shown in Table 1. FA has high silica (55.24%) and alumina (31.35%) content but low calcium (2.07%) content. The FA used in the experiment was also provided by Anhui Huainan Pingwei Power Generation Co., Ltd.
  • Coal gangue (CG)
CG was used as a fine and coarse aggregate in the filling materials after being crushed using a crusher (EP-150 × 250 jaw crusher, provided by Henan Hebi Tianguan Instrument Co., Ltd., Hebi, China). CG particles with a size of 0.1 mm–5 mm were taken after crushing for use as fine aggregate, whereas those with a size of 5 mm–40 mm were used as coarse aggregate. CG has high silica (59.26%) and alumina (20.25%) content, but low lime (1.94%) content, similar to FA (Table 1). The CG used in the experiment was provided by Anhui Huainan Mining Group Co., Ltd. (Huainan, China).

2.1.2. Portland Cement (PC)

Ordinary 32.5-grade Portland cement (PC) provided by Huainan Bagongshan Cement Plant was used in this experimental study. PC has high silica (22.97%) and calcium (56.34%) content (Table 1), and it conforms with the national standard of general Portland cement (GB 175-2020) [20].

2.2. Experimental Methods

2.2.1. Mix Proportions

The mix proportions of the specimens are given in Table 2. Specimen S-1 completely used Portland cement as the cementitious material and was considered to be the control specimen. Specimens S-2 to S-5 had varying amounts of DG, significantly reducing the PC amount. Likewise, Specimens S-6 to S-9 had FA for evaluating the synergistic effect of PC and FA. Lastly, in Specimens S-10 to S-13, the amounts of DG and FA were increased in the same proportion, which gave full play to the synergistic enhancement effect of multisource coal-based solid wastes on PC.

2.2.2. Preparation and Curing

Taking Specimen S-13 as an example, the preparation procedure is explained as follows. First, PC, DG, and FA were evenly mixed (SYH-10 3D Motion Mixer, provided by Changzhou Qibao Drying Equipment Co., Ltd., Changzhou, China). Then, potable water was added to the mix, and mixing was performed for 30 s (HJW-60 concrete single horizontal shaft mixer, provided by Hebei Cangzhou Yixuan Test Instrument Co., Ltd., Hebei, China). Then, CG fine and coarse aggregates were added and mixed for 120 s. After mixing, the fresh mix was cast into 150 mm × 150 mm × 150 mm cubes. The specimens were placed in the laboratory for setting. After 1 d, the specimens were demolded and still placed in the laboratory until the age of experimental design. The temperature difference between day and night was 15 to 26 degrees, and the humidity range was 50% to 70%.

2.2.3. Mechanical Properties

  • Compressive strength
The compressive strength tests were conducted per the standard for the evaluation of concrete compressive strength (GB/T 50107-2010) [21]. A DYE-2000 concrete press (Honghao Jieke Construction Instrument Co., Ltd., Cangzhou, Hebei, China) was used to test the compressive strength of specimens cured for 1 d, 3 d, 28 d, and 60 d. The average of six data points was taken as the representative value.
  • Consistency and setting time
The setting time test was conducted following the test methods for the water requirement of the normal consistency setting time and soundness of the Portland cement (GB/T 1346-2011) [22]. Initial (IS) and final (FS) setting times were determined to study the influence of different solid waste admixtures on cement hydration. To obtain the setting time of the specimen more accurately, the IS and FS were measured every 5 s within the first 10 min, every 10 s within 10 to 30 min, and every 10 min after 30 min. The water/binder ratio of the paste specimen was taken as 0.5.

2.2.4. Micro-Morphological Testing

  • XRD analysis
A smart Lab SE intelligent X-ray diffractometer (RIGAKU, Tokyo, Japan) was used to analyze the types and characteristics of crystal formation of the developed specimens. Paste specimens were used for XRD analysis, and through which the influence of mixing multi-source coal-based solid waste on mineral crystal formation was analyzed. To further determine the effect of FA on hydration in the later stage, the crystal characteristics after 60 d of curing were also examined.

3. Experimental Results and Discussion

3.1. Macro Experimental Analyses

3.1.1. Compressive Strength Analysis

  • Effect of Portland cement on compressive strength
The compressive strength of Specimen S-1 (100% PC) completely prepared with Portland cement after curing for 1 d and 3 d reached 5.8 MPa and 12.4 MPa, respectively. After 28 d and 60 d, it increased to 21.6 MPa and 25.4 MPa, respectively. Due to the high water/binder ratio (0.6) and the use of coal gangue sand and stone as fine and coarse aggregates, the concrete performance was significantly affected [23], reflected by reduced and inadequate compressive strength. However, underground filling materials do not require high compressive strength (usually 2–10 MPa) [24]. Nevertheless, the preparation cost must be controlled, and reducing the Portland cement dosage is the most effective way to reduce the associated costs.
  • Effect of DG dosage on compressive strength
The compressive strength of Specimen S-2 (80% PC, 20% DG) significantly decreased, by up to 2.2 MPa and 4.4 MPa, when curing for 1 d and 3 d, respectively (Figure 1a). However, it only increased to 8.6 MPa (28 d) and 10.8 MPa (60 d), which are 60.2% and 57.5% of the strength compared to Specimen S-1 (100% PC). PC usually has approximately 3% gypsum (CaSO4·2H2O), whose primary function is to delay the setting. However, after desulfurization, the chemical composition of DG mainly consists of bassanite (CaSO4·0.5H2O) [25]. Bassanite first reacts with water to produce gypsum and then continuously precipitates and crystallizes as the colloidal particles solidify and harden. Although DG has certain cementing properties, its setting and hardening mechanisms are entirely different from those of PC [26]. The optimal pH environment for gypsum crystallization and hardening is a neutral reaction solution (between 7 and 9) [27]. However, the pH of the PC hydration reaction environment is approximately 13, which is not conducive to the crystallization and precipitation of gypsum [28]. Therefore, adding DG into PC not only affects the PC hydration reaction and the formation of calcium silicate hydrate (C–S–H) gel, but also affects the crystallization and precipitation reaction of DG, resulting in reduced gypsum crystal formation, which is why there is compressive strength reduction for Specimen S-2.
With the increased DG content, the compressive strength of Specimens S-3 to S-4 significantly improved. At 40% DG dosage, the compressive strength of Specimen S-3 (60% PC, 40% DG) reached 3.1 MPa (1 d) and 5.2 MPa (3 d), respectively, which further increased to 10.5 MPa (28 d) and 12.9 MPa (60 d), i.e., improvement of 22.1% and 19.4%, respectively, compared to Specimen S-2 (80% PC, 20% DG). Furthermore, the compressive strength of Specimen S-4 (40% PC, 60% DG) increased to 5.8 (1 d) MPa and 7.5 MPa (3 d) for 60% DG content, while it further increased to 15.5 MPa and 18.6 MPa when the curing time reached 28 and 60 d, respectively, which are 47.6% and 44.2% higher than that of Specimen S-3 (60% PC, 40% DG). The DG dosage increase also led to a proportionate decrease in PC dosage. A reduced PC hydration reaction leads to decreased Ca(OH)2 formation, thereby lowering the pH of the reaction environment, which gradually forms a lower pH environment conducive to a gypsum crystallization reaction [29]. Therefore, gypsum particles can better continuously crystallize, precipitate, grow, and intersect with each other, ultimately developing strength, which is why the compressive strength increases with DG content [30]. However, when the content of DG increased to 80%, the compressive strength of Specimen S-5 (20% PC, 80% DG) at any given curing age had no further growth but rather a slight decrease compared to that of Specimen S-4 (40% PC, 60% DG), indicating that once DG content exceeds 60%, C–S–H formation cannot be further promoted. Thus, the precipitation, crystallization, and growth of gypsum are severely hindered [31]. Therefore, the addition of DG cannot only reduce the PC amount and preparation cost, but also improve the compressive strength. The optimal dosage of DG has been found to be 60%.
  • Effect of FA dosage on compressive strength
When the content of FA was only 20%, there was no significant reduction in the compressive strength of Specimen S-6 (80% PC, 20% FA) at each curing age compared to Specimen S-1 (100% PC) (Figure 1b). This indicates that adding a small amount of FA does not seriously affect compressive strength development. However, as the FA dosage increased to 40%, the compressive strength of Specimen S-7 (60% PC, 40% FA) after curing for 1 d and 3 d only reached 2.6 MPa and 6.4 MPa, which increased to 14.6 MPa and 18.1 MPa at 28 d and 60 d, i.e., reduced by 38.1% (1 d), 43.5% (3 d), 29.5% (28 d), and 26.4% (60 d), respectively, compared to Specimen S-6. With the increase in FA content (>20%), the impact on compressive strength becomes more pronounced. Additionally, adding FA has a more severe impact on the compressive strength at an early age (1 d and 3 d) but a slightly lower impact in the later stage (28 d and 60 d) [32]. As FA is partially reactive, it only has the filling effect in the early stage (1 d and 3 d). However, after the inert period (>28d), the silicon and aluminum present in FA can react with the Ca(OH)2 formed by PC hydration to form calcium silicate and calcium aluminate minerals [33], which not only eliminates the negative impact of Ca(OH)2 by reducing the pH but also alleviates the reduced compressive strength problem due to a lower PC amount.
However, as the dosage of FA further increased to 60% and 80%, Specimens S-8 (40% PC, 60% FA) and S-9 (20% PC, 80% FA) could not be removed from the molds after 1 d and 3 d of curing until the curing period increased to 60 d. Additionally, the compressive strength was less than 5 MPa. Therefore, although adding FA could reduce the PC amount and resulting cost, exceeding 40% of the content leads to a drastic decline in compressive strength. The strengthening effect of FA at later ages due to secondary hydrate formation cannot offset the loss in compressive strength [34]. Thus, it is advisable to have the content of FA as being less than 40%.
  • Effect of both DG and FA on compressive strength
When 10% DG and 10% FA were added, Specimen S-10 (80% PC, 10% DG, 10% FA) could not be demolded after 1 d of curing (Figure 1c). The compressive strength was only 2.9 MPa after 3 d of curing, whereas the strength increased to 6.8 MPa and 9.1 MPa at 28 d and 60 d. Incorporating DG and FA did not alleviate the incompatibility between DG and PC [28] and resulted in lower strength than Specimen S-2 (80% PC, 20% DG). However, the strength was enhanced as the dosages of DG and FA increased. When the dosage of DG increased to 40%, and that of FA increased to 20%, the compressive strength of Specimen S-12 (40% PC, 40% DG, 20% FA) increased to 10.7 MPa (28 d) and 14.4 MPa (60 d), slightly better than the compressive strength of Specimen S-3 (60% PC, 40% DG). It can also be seen by comparing Specimens S-12 and S-3 (same DG dosage) that Specimen S-12 can achieve better compressive strength for lower PC content, indicating better compatibility and coupling between DG and FA [23]. This is because DG is a mineral with high calcium and low silicon contents, whereas FA is a mineral with low calcium and high silicon content (Table 1), which can help to promote the polymerization of calcium, silicon, and aluminum, thus forming silicate and aluminate gels to compensate for the reduced strength due to lower PC amount [25].
The compressive strength of Specimen S-13 (20% PC, 60% DG, 20% FA) reached 4.2 MPa (1 d) and 8.7 MPa (3 d). It further increased to 15.8 MPa (28 d) and 19.5 MPa (60 d), i.e., an improvement of 47.7% and 35.4%, respectively, compared to Specimen S-12 (40% PC, 40% DG, 20% FA), and was slightly higher than those of Specimen S-4 (40% PC, 60% DG). Increasing the DG content can further increase the content of active calcium in the cementitious matrix, form a better C/S ratio with FA (research shows that 3:1 is the best), and further improve the reactivity [35]. It also promotes the formation of spherical and reticular silicate gel, thus improving the compressive strength while further reducing the PC content [36]. Therefore, the synergistic enhancement effect between DG and FA can be achieved through complementary calcium and silicon, and the compressive strength is not significantly reduced under the condition of significantly reducing PC content.

3.1.2. Hydration and Setting

  • Setting time of PC specimen
The setting time of Specimen S-1 (100% PC) is shown in Figure 2a. Its IS is 210 min, while the FS increases to 370 min, which meets the requirement of PC (i.e., the IS should not be less than 45 min, and the FS should not be longer than 10 h).
  • Effect of DG dosage on setting time
The hydration reaction in Specimen S-2 (80% PC, 20% DG) significantly changed with the addition of DG. The IS of Specimen S-2 was reduced to 9 min and 40 s, while the FS was reduced to 15 min and 10 s. Adding DG was found to accelerate the hydration reaction. This is because the hydration reaction process is slow and long-lasting, and the specimen can only become set when the formation of silicate (C-S-H gel) reaches a certain level [37]. However, the reaction between DG and water generates gypsum, and the precipitation, crystallization, and growth of gypsum are significantly faster than the hydration reaction due to the low solubility of gypsum in water [25]. In addition, the continuous crystallization and precipitation of gypsum can further promote the dissolution of DG. When the precipitation and crystallization of gypsum reached a certain amount, Specimen S-2 became set.
The rates of the IS and FS of the specimen accelerate with the DG content. When the DG content increased to 60%, the IS and FS of Specimen S-4 (40% PC, 60% DG) were reduced to 6 min 15 s and 9 min 30 s, respectively. Due to the constant water/binder ratio and the continuous increase in DG content, the proportion of DG in the mixed slurry continues to increase, promoting faster conversion from DG to gypsum and accelerating the precipitation and crystallization processes [28]. This is why Specimen S-4 accelerates the setting process. However, although rapid solidification improves the early filling effect and compressive strength, higher requirements are suggested for preparing and casting the filling material. In order to adapt to underground casting, it is not easy to significantly reduce the IS. Therefore, using a mixture of PC and DG to prepare the filling materials is preferred, significantly reducing the PC amount but not substantially lowering the compressive strength.
  • Effect of FA dosage on setting time
After adding FA, the IS and FS of Specimen S-6 (80% PC, 20% FA) increased to 280 min and 420 min, respectively, which were 33% and 13.5% longer than those of Specimen S-1 (100% PC). Adding FA can delay the setting and hardening of the PC. Although FA contains many active silicon and aluminum components, its early hydration reaction ability is insufficient, and it only plays an inert filling role in the early stage. The increase in FA dosage leads to reduced PC content, resulting in a decreased hydration reaction rate and delaying the formation of early hydration products, which is the reason for the delayed setting of Specimen S-6.
The IS and FS of Specimens S-7 to S-9 further increased with FA content. Until the FA content increased to 80%, the IS of Specimen S-9 (20% PC, 80% FA) extended to 520 min, while the FS increased to 780 min. Thus, it is inferred from the strength analysis and setting time that adding 20% FA can simultaneously delay the setting and promote the hydration reaction of PC [34]. However, when the FA content is too high (i.e., >20%), it can seriously affect compressive strength development and lead to slow hardening and strength gain. Therefore, an appropriate amount of FA should be introduced into the DG system to utilize its retarding effect and delay the rapid setting of DG.
  • Effect of both DG and FA on hydration reaction
With the combined addition of DG and FA, the IS of Specimen S-10 (80% PC, 10% DG, 10% FA) was prolonged to 18 min 40 s, and the FS reached 37 min 10 s, i.e., it was delayed by 93.1% and 99.1% compared to that of Specimen S-2 (80% PC, 20% DG). The addition of 10% FA significantly delayed the setting of DG. This is because the DG content was low (10%) and FA played a retarding role, promoting a significant increase in the setting time of Specimen S-10. Moreover, with the increased DG and FA amounts, the IS and FS of Specimen S-11 (60% PC, 20% DG, 20% FA) reached 18 min 30 s and 36 min 50 s, respectively, with no significant difference compared to Specimen S-10. This is because although the further increase in DG content accelerates the hydration reaction, the increase in FA content also plays a retarding role [25]. Thus, the resulting hydration rate of Specimen S-11 does not significantly change.
Due to the severe adverse impact on compressive strength due to excessive FA content, Specimens S-12 and S-13 kept the FA content unchanged and continued to increase the DG content. The IS and FS of Specimen S-13 (20% PC, 60% DG, 20% FA) were reduced to 13 min 10 s and 26 min 20 s, respectively, i.e., 14.1% and 16.4% lower than those of Specimen S-12 (40% PC, 40% DG, 20% FA). However, compared with Specimen S-4 (40% PC, 60% DG) with the same DG content, the IS and FS of Specimen S-13 were extended by 95.7% and 177.2%, respectively. Although increased DG content significantly accelerated the hydration of Specimen S-13, the presence of 20% FA could effectively delay the setting, ensuring that the continuous increase in DG content increased compressive strength while avoiding rapid hardening [38].

3.2. Micro Experiment Analysis

XRD Analysis

  • XRD analysis of raw materials
The XRD analysis of DG and FA is shown in Figure 3. The main mineral peak identified in DG was bassanite (PDF #41-0224, CaSO4·0.5H2O). Calcite (PDF #47-1743, CaCO3) with low characteristic peak intensity was also detected. The mineral crystals in DG are compounds containing calcium, which is consistent with the detection results of XRF. This also allows DG to exert its calcium-enhancing effect.
Quartz (PDF #46-1045, SiO2) was the most obvious characteristic peak detected in FA. Additionally, the characteristic peak of mullite (PDF #15-0776, SiO2) with very obvious intensity was detected in FA. The mineral crystals in FA were silicon and aluminum compounds, consistent with the XRF results.
  • XRD analysis of DG-incorporated pastes
The effect of adding DG on the characteristic crystals in the paste of Specimens S-2 and S-4 is shown in Figure 4a. Weak characteristic peaks of tobermorite (1 Na2O·Al2O3·6SiO2; PDF #45-1480), hillebrandite (2 (Ca5(Si6O16)(OH2); PDF #42-0538)), and calcium silicate hydrate (3 Ca3SiO5·3H2O; PDF #42-0551) could be observed in Specimen S-2 (80% PC, 20% DG). The intensity of the characteristic peaks is relatively low, which is attributed to the main reaction product of PC hydration (C-S-H gel) and the main source of Specimen S-2’s compressive strength. The hydration reaction of PC was hindered by DG, resulting in a significant reduction in the formation of silicate crystals and causing a sudden decline in compressive strength. Furthermore, weak characteristic peaks of portlandite (4 Ca(OH)2; PDF #44-1481) and ettringite ((5 3CaO·Al2O3·3CaSO4·32H2O); PDF #41-1451) were observed in Specimen S-2, which are all by-products of the PC hydration reaction [39]. The low intensity of portlandite and ettringite crystals also illustrates that the PC hydration reaction is severely hindered. The characteristic peak of gypsum (6 CaSO4·2H2O; PDF #21-0816) was also found in Specimen S-2, which was formed by the reaction between DG and water [40]. However, the intensity of gypsum crystal was not obvious, indicating that the setting, crystallization, and growth of gypsum were also affected by PC hydration. Therefore, adding 20% DG into PC affects the hydration reaction, and hinders the formation of silicate crystals and the precipitation and crystallization of gypsum, hindering compressive strength development.
The characteristic peaks of tobermorite, hillebrandite, and calcium silicate still appear in Specimen S-4 (40% PC, 60% DG). However, the strength of the characteristic peaks was still significantly lower than that of Specimen S-2 (80% PC, 20% DG), indicating that the degree of PC hydration is further limited with an increased DG dosage [26]. The strength of portlandite and ettringite crystals in Specimen S-4 also did not increase, corroborating the above conclusion. However, the strength of gypsum crystal in Specimen S-4 shows a significant increase compared with Specimen S-2, indicating that the precipitation, crystallization, and growth reactions of gypsum are significantly improved, fully utilizing the setting performance of gypsum and improving the compressive strength and accelerated setting [28].
  • XRD analysis of FA-incorporated pastes
As seen in Figure 4a, obvious tobermorite, hillebrandite, and calcium silicate characteristic peaks appeared in Specimen S-6 (80% PC, 20% FA), and the intensity was significantly higher than that of Specimen S-2 (80% PC, 20% GD) for the same PC dosage conditions. FA can play a better role (compared with DG) with PC in facilitating the hydration reaction and increasing the formation of the C-S-H gel, so that the compressive strength of Specimen S-6 is not considerably reduced with reduced PC content [32]. The significant increase in the characteristic peak strength of portlandite and ettringite in Specimen S-6 (compared with Specimen S-2) also indicates that the hydration reaction of PC is sufficient, and the formation of by-products of hydration products is substantially increased.
However, the characteristic peaks of the calcium silicate crystals in Specimen S-7 (60% PC, 40% FA) significantly decrease compared with Specimen S-6 (80% PC, 20% FA) when the FA content increases to 40%. The characteristic peaks of portlandite and ettringite also demonstrate varying degrees of reduction. The reduction in the PC dosage adversely affects the hydration reaction of PC and the formation of calcium silicate crystals, which causes a decrease in compressive strength [33]. Since PC has high calcium and silica contents, whereas FA only contains a large amount of silicon and aluminum, the calcium content is significantly insufficient (Table 1). The increase in FA dosage thus leads to decreases in the PC amount, resulting in increased silicon and aluminum contents but reduced calcium content in the reaction environment. The formation of calcium silicate crystals requires an appropriate amount of calcium amount. However, the continuous reduction in calcium content seriously affects the formation of calcium silicate crystals [40], which is why the compressive strength of Specimen S-7 was reduced. Therefore, although adding FA can assist the PC hydration reaction, increase calcium silicate crystals, and reduce PC consumption, considering the compressive strength, it is not easy for the FA content to exceed 20%.
  • XRD analysis of pastes containing both DG and FA
As seen in Figure 4b, the characteristic peak of calcium silicate crystals can still be detected in Specimen S-10 (80% PC, 10% DG, 10% FA). However, it is still low, and it is obviously lower than that of Specimen S-6 (80% PC, 20% FA) and even lower than that of Specimen S-2 (80% PC, 20% DG). As long as 10% DG is added, it will have adverse effects on the hydration reaction of PC, and the degree of influence does not significantly change with increased DG dosage. Moreover, incorporating a small amount of FA cannot eliminate the incompatibility between DG and PC [26]. The obvious lack of portlandite and ettringite crystals in Specimen S-10 confirms that the impact of DG on the PC hydration reaction is still relatively serious. Moreover, the gypsum crystal characteristic peak in Specimen S-10 is also significantly weaker, indicating that the reaction of bassanite with water and the crystallization of gypsum are affected to varying degrees. However, the characteristic peak of grossular (8 Ca3Al2(SiO4)3; PDF #39-0368) appeared in Specimen S-10, which was absent in the other specimens. This indicates a synergistic effect between DG and FA, where the active calcium in DG reacts with the active silicon and aluminum in FA to form grossular crystals [28]. However, the grossular crystal characteristic peak is not obvious, indicating the inadequate formation of grossular crystals caused by the low dosages of DG and FA. Therefore, the simultaneous addition of 10% DG and 10% FA does not significantly reduce the amount of PC used, nor assist in PC hydration, resulting in a significant decrease in compressive strength.
However, with the gradual increase in the dosages of DG and FA, their improvement effects considerably change. The strength of calcium silicate crystals in Specimen S-13 (20% PC, 60% DG, 20% FA) did not increase significantly, indicating that the crystalline state of calcium silicate was not improved. This is mainly because the PC content was reduced to 20%. The PC hydration reaction did not dominate the reaction system. However, gypsum crystal in Specimen S-13 increased significantly, and the strength was much higher than that of Specimen S-10 (80% PC, 10% DG, 10% FA) and even higher than that of Specimen S-4 (40% PC, 60% DG). The crystallization, growth, and development of gypsum in Specimen S-13 were very smooth, which is why there was increased compressive strength. Moreover, the Ca(OH)2 formed by the PC hydration reaction can further stimulate the potential bonding performance of DG and FA, making them more actively participate in the reaction [41]. Under the condition of significantly reduced PC content, the increase in the characteristic peak strength of grossular in Specimen S-13 proves the increase in the formation of C–A–H [42,43]. Additionally, it indicates an increase in the reactivity between DG and FA. Therefore, the preparation of Specimen S-13 not only fully utilizes the cementitious properties of DG under low PC content conditions but also reflects the synergistic promoting effect of FA on DG, achieving resource utilization of DG and FA and significantly reducing the associated preparation costs.

4. Conclusions

In this study, the influence of varying PC, DG, and FA proportions in filling material on the compressive strength, hydration, and setting time was analyzed. The effects of PC, DG, and FA amounts and their nature on the formation and transformation of mineral crystals were further analyzed using XRD.
The following conclusions are drawn from the obtained results.
(1) The addition of DG affects the PC hydration reaction, leading to decreased compressive strength and an accelerated setting. The compressive strength gradually increases with increased DG content, whereas the corresponding setting time is reduced.
(2) FA can assist PC hydration, promote silicate mineral formation, and delay the setting. However, the compressive strength is greatly reduced and the setting time is significantly prolonged when an excessive FA dosage (>40%) is used.
(3) The synergistic use of DG and FA can significantly reduce the PC content and alleviate the drastic decline in mechanical properties, as well as control the setting time. The optimal dosages of PC, DG, and FA were found to be 20%, 60%, and 20%, respectively.
(4) XRD analysis showed that DG forms gypsum after reacting with water and undergoes crystallization and precipitation to help to develop strength. Moreover, DG and FA can have a synergistic enhancement effect to form grossular crystals, thereby further promoting mechanical property development.

Author Contributions

Validation, G.H. and M.G.; formal analysis, Q.C. and T.X.; investigation, M.D.; resources, X.Z.; data curation, Z.Q. and Q.W.; writing—original draft preparation, G.H.; writing—review and editing, G.H. and X.Z. All authors have read and agreed to the published version of the manuscript.

Funding

The research described in this paper was financially supported by the Provincial Quality Engineering Project of Colleges and Universities in Anhui Province in 2022 (No. 2022xqhz015) and the Research Foundation of the Institute of Environment-friendly Materials and Occupational Health (Wuhu), Anhui University of Science and Technology (ALW2021YF01). Open Fund from Engineering Research Center for Geological Environment and Underground Space of Jiangxi Province (JXDHJJ2022-012).

Data Availability Statement

The authors confirm that the data supporting the findings of this study are available within the article.

Conflicts of Interest

The authors declare no conflict of interest.

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Crystals 13 00997 g001 550
Figure 1. Compressive strength analysis. (a) Specimens S-1 to S-5; (b) Specimens S-6 to S-9; (c) Specimens S-10 to S-13.
Figure 1. Compressive strength analysis. (a) Specimens S-1 to S-5; (b) Specimens S-6 to S-9; (c) Specimens S-10 to S-13.
Crystals 13 00997 g001
Crystals 13 00997 g002 550
Figure 2. Setting behavior analysis. (a) Specimens S-1 to S-5; (b) Specimens S-6 to S-9; (c) Specimens S-10 to S-13.
Figure 2. Setting behavior analysis. (a) Specimens S-1 to S-5; (b) Specimens S-6 to S-9; (c) Specimens S-10 to S-13.
Crystals 13 00997 g002
Crystals 13 00997 g003 550
Figure 3. XRD analysis of DG and FA.
Figure 3. XRD analysis of DG and FA.
Crystals 13 00997 g003
Crystals 13 00997 g004 550
Figure 4. XRD analysis of raw materials. (a) Specimens S-2, S-4 and S-6; (b) Specimens S-7, S-10 and S-13.
Figure 4. XRD analysis of raw materials. (a) Specimens S-2, S-4 and S-6; (b) Specimens S-7, S-10 and S-13.
Crystals 13 00997 g004
Table
Table 1. Chemical (oxide) composition of raw materials (%).
Table 1. Chemical (oxide) composition of raw materials (%).
Raw MaterialSiO2Al2O3Fe2O3CaOMgONa2OK2OSO3OthersLoss
DG4.361.650.8932.451.240.230.2539.320.8218.52
FA55.2431.354.852.071.130.840.25-1.821.73
CG59.2620.255.371.941.050.252.37-0.368.33
PC22.978.893.7856.342.430.341.48-1.181.85
Table
Table 2. Mix proportions of specimens (kg/m3).
Table 2. Mix proportions of specimens (kg/m3).
PCDGFAWaterLiquid–Solid RatioCG Fine AggregateCG Coarse Aggregate
S-1300001800.66001200
S-22406001800.66001200
S-318012001800.66001200
S-412018001800.66001200
S-56024001800.66001200
S-62400601800.66001200
S-718001201800.66001200
S-812001801800.66001200
S-96002401800.66001200
S-1024030301800.66001200
S-1118060601800.66001200
S-12120120601800.66001200
S-1360180601800.66001200
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MDPI and ACS Style

Huang, G.; Zheng, X.; Gao, M.; Chen, Q.; Qiao, Z.; Xie, T.; Deng, M.; Wei, Q. Mechanical Properties and Mineral Characteristics of Multi-Source Coal-Based Solid Waste Filling Materials under Different Proportioning. Crystals 2023, 13, 997. https://doi.org/10.3390/cryst13070997

AMA Style

Huang G, Zheng X, Gao M, Chen Q, Qiao Z, Xie T, Deng M, Wei Q. Mechanical Properties and Mineral Characteristics of Multi-Source Coal-Based Solid Waste Filling Materials under Different Proportioning. Crystals. 2023; 13(7):997. https://doi.org/10.3390/cryst13070997

Chicago/Turabian Style

Huang, Guodong, Xiaojun Zheng, Miao Gao, Qi Chen, Zheng Qiao, Tianbao Xie, Mengyao Deng, and Qing Wei. 2023. "Mechanical Properties and Mineral Characteristics of Multi-Source Coal-Based Solid Waste Filling Materials under Different Proportioning" Crystals 13, no. 7: 997. https://doi.org/10.3390/cryst13070997

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