Effects of chloride on the early mechanical properties and microstructure of gangue-cemented paste backfill

Construction and Building Materials 235 (2020) 117504

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Effects of chloride on the early mechanical properties and microstructure of gangue-cemented paste backfill Shaojie Chen a,b, Zhaowen Du a,b,⇑, Zhen Zhang a,b, Huawei Zhang c, Zhiguo Xia a,b, Fan Feng a,b a

State Key Laboratory Breeding Base for Mining Disaster Prevention and Control, Shandong University of Science and Technology, Qingdao 266590, PR China College of Mining and Safety Engineering, Shandong University of Science and Technology, Qingdao 266590, PR China c College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, PR China b

h i g h l i g h t s
Chloride can significantly increase the strength gain rate of paste backfill at early ages.
Chloride content is important for the self-consolidation of paste backfill.
Chloride content of paste backfill is important for the cycle frequency of mining and filling.
An initial chloride content of 10‰ is the most beneficial for gangue-cemented paste backfill.

a r t i c l e

i n f o

Article history: Received 11 April 2019 Received in revised form 16 October 2019 Accepted 5 November 2019

Keywords: Gangue-cemented paste backfill Chlorine salt Cement hydration Early-age strength Coal mine

a b s t r a c t The addition of an early strength agent as an additive in cement-based materials provides several securityrelated, technical, and economic advantages, such as improving the early strength of the paste and accelerating the cycle frequency of mining and filling. In this paper, we present the results of an experimental investigation of the effects of chloride on the early age (3, 7, and 28 days) strength of gangue-cemented paste backfill (GCPB). The GCPB specimens had an initial chloride concentrations of 0‰, 5‰, 10‰, 20‰, 30‰, and 40‰, and the obtained samples were analyzed by performing uniaxial compressive strength tests, X-ray diffractometry, scanning electron microscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The results show that chloride significantly affects the early-age strength of GCPB. At early ages, chloride can have positive and negative effects, i.e., the GCPB strength increased with an initial chlorine content of 10‰, while it decreased for GCPB with an initial chlorine content of 40‰. These positive or negative effects depend primarily on the initial concentration of chloride in the GCPB. The key reason for this characteristic is the promotion or suppression of cement hydration by chloride ions. The essential factor that affects the GCPB strength is the relationship between the volume of voids and the amounts of calcium silicate hydrate gel, ettringite, and Friedel’s salt formed. These results indicate that it is important to consider the influence of chloride on the early strength of GCPB to improve the self-stabilizing strength of GCPB and the economic benefits of coal mines. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Gangue-cemented paste backfill (GCPB) is a new cemented filling material that is mainly composed of cement, fly ash, and gangue [1–3]. GCPB is widely used in ‘‘three-under” mining (under buildings, under water, and under railways) and in the secondary utilization of mine solid waste [4–8]. This emerging technology not only increases the economic and social benefits of mining,but can also ensures the safety of mines for workers [1–8]. ⇑ Corresponding author at: State Key Laboratory Breeding Base for Mining Disaster Prevention and Control, Shandong University of Science and Technology, Qingdao 266590, PR China. E-mail address: [email protected] (Z. Du). https://doi.org/10.1016/j.conbuildmat.2019.117504 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

The early-age mechanical strength (within 28 days) is the most important GCPB property and plays a crucial role in backfill stability and underground workplace safety [1,9]. Generally, during the period from the injection to the self-consolidation of the GCPB, the roof above the filling face is in a hanging state [10]. Sufficient early age mechanical strength is needed to prevent roof collapse and ensure the safety of the filled working face [11–15]. However, the special backfilling hydraulic supports, which acts as baffles, cannot continue to be used with advancing coal mining faces. Therefore, GCPB and coal mining cannot operate in parallel. To obtain more coal resources and increase profits [16], coal enterprises need to accelerate the cycle frequency of mining and filling. In other words, shortening the self-consolidation time and increas-

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ing the early age mechanical strength of GCPB can reduce the mining cycle. In recent years, in order to improve backfill strength and shorten the self-consolidation time, researchers have focused on the factors affecting GCPB or CPB (cemented paste backfill) strength [17–26]. One important factor is the use of an early strength agent (chemical additive) [1]. Previous studies have shown that the backfill strength can be improved by adding an appropriate amount of an early strength agent. For example, Fall and Pokharel [2] studied the effect of the double coupling of sulfate and temperature on CPB strength. They found that an initial sulfate content of 15,000 ppm had a beneficial effect on the early strength development of CPB when the curing temperature was 20 °C. Huang et al. [27,28] studied the influence of the triisopropanolamine (TIPA) contents in sulfate cement on the strength development. The results showed that TIPA is the key factor in improving the strength of the cement slurry over 28 days. Adding sulfate can increase the hydration of C3S, which can promote the early strength of the cement slurry. Abdul-Hussain and Fall [29] studied the effect of sodium silicate as a chemical additive on the thermo-hydro-mechanical properties of cemented paste tailings. The results showed that the mechanical properties of cemented tailings are closely related to the saturation, thermal development, and inhalation development. Ma et al. [30] conducted a comprehensive experimental study of the basic characteristics of recycled powder concrete and its chloride ion permeability. The results showed that the chloride ion diffusion coefficient increases linearly as the relative dynamic elastic modulus decreases. Tan et al. [31] explored a new way of utilizing lithium slag powder in a sulfoaluminate cement system. Studies have shown that the early strength of sulfoaluminate cement paste can be significantly improved when the content of wet ground lithium slag is less than 10%. The main reasons for the improvement are the filling effect of the fine particles formed during wet grinding and the induction of nanoparticle seed nucleation. Aldhafeeri and Fall [32,33] studied the effects of different initial sulphate contents on CPT reactivity. They found that the reactivity of the CPT sample increased with increasing sulfate content, except when the sulfate content was 5000 ppm. Dong et al.[34] prepared four kinds of CPB samples with different sulfate contents (2 wt%, 5 wt%, 10 wt%, and 15 wt%) and found that the sulfide performed as an early strength agent. When the initial sulfide content was 5 wt%, the early strength of the CPB was significantly improved. Extensive work on improving the properties of CPB has been conducted previously [35–37]. In fact, sulfate, TIPA, sodium silicate, chloride, and lithium slag all belong to admixtures or additive substances added based on conventional CPB, GCPB or concrete. Among these substances, sulfate and chloride are classified as early strength agents in the classification of admixtures. However, there have been no studies of the effects of chloride as an early strength agent on the mechanical properties and microstructure of GCPB, although complete understanding of the effects of chloride on these features of GCPB is necessary. Therefore, this paper mainly discusses the effects of different chlorine salt contents on the mechanical strength and microstructure of cement filling at early ages.

2. Experimental section 2.1. Materials 2.1.1. Gangue The gangue selected in this experiment came from the solid waste produced during roadway excavation, coal mining, and separation in Daizhuang Coal Mine, Zibo Mining Group, Shandong Province. The particle size was less than 25 mm. The screening of gangue with different particle sizes is shown in Fig. 1, and Fig. 2 illustrates the specific composition and content of the gangue.

Fig. 1. The grading diagram for the gangue grain size.

Fig. 2. The main chemical composition and contents of the gangue.

Fig. 3. The main chemical composition of the fly ash.

2.1.2. Fly ash The fly ash selected in this experiment was grade II fly ash with a light grey appearance from Huangdao Power Plant in Qingdao City, Shandong Province. Its main chemical components are depicted in Fig. 3, and its physical properties are summarized in Table 1. 2.1.3. Cement The cement used in this experiment was 42.5 slag Portland cement produced by the Second Cement Plant of Yishui, Shandong Province. The chemical composition of the clinker is shown in Fig. 4. 2.1.4. Early strength agent The sodium chloride used in this experiment was of analytical purity, produced by the Chengdu Kelong Chemical Reagent Factory, and its sodium chloride content was more than 96%.

S. Chen et al. / Construction and Building Materials 235 (2020) 117504 Table 1 The main physical and chemical properties of the fly ash. Apparent density/kg/m3

Bulk density/kg/m3

Combustion quantity/%

45 lm square hole screening margin/%

1950

780

6.34

19

3

2.3.4. FTIR experiment The functional groups on the surfaces of the samples were characterized by FTIR analysis on a Nicolet 6700 FTIR spectrometer. The resolution factor was 2 cm1 in the 400–4000 cm1 range, and scans were collected at a scan speed of 5 kHz. 2.3.5. XPS experiment XPS measurements were conducted using a Thermo ESCALAB 250 x, using the Al Kr line with a C 1s binding energy of 284.6 eV.

3. Results and discussion 3.1. UCS results and analysis

Fig. 4. The main chemical composition of the slag Portland cement.

2.2. Material ratio and experimental design The required amounts of samples were composed of cement, fly ash, gangue, sodium chloride and water. Previous studies of Daizhuang Coal Mine of the Zibo Mining Group [38,39], have indicated that when the ratio of cement to fly ash to gangue is 1:4:6 and the solid mass fraction is 74%, then the filling paste has the best performance and good engineering application effects. Therefore, we prepared samples with the aforementioned ratio with different sodium chloride contents (5‰, 10‰, 20‰, 30‰, and 40‰). The samples were mixed using an NJ-160 mixer for approximately 8 min until a homogeneous paste was obtained. Then, the prepared GCPB was poured into a mold with a length, width, and height of 70.7 mm. Manual vibration and tamping were used to prevent bubbles in the samples. Then, the specimens were stored in a constant temperature and humidity box for 3, 7, or 28 days. In addition, we prepared a control sample without sodium chloride for comparison and performed uniaxial compressive strength (UCS) tests, X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) on the specimens. 2.3. Testing method 2.3.1. Testing of mechanical properties UCS testing is the most commonly used method of evaluating the mechanical quality of GCPB. The testing machine employed to test the mechanical properties of the specimens in this study was a Shimazu ag-x250 electronic universal testing machine. This type of test machine is driven by a motor servo, loaded with a double screw structure, and can perform tests of many kinds of mechanical properties. The maximum load is 250 kN, the speed is 0.0005–500 mm/min, the precision is up to ±0.1%, the crosshead stroke measurement resolution is 0.0104 lm, and the effective test width is 595 mm. In these tests, the loading mode of the specimens was controlled by the displacement and the loading rate was 0.01 mm/s. According to Chinese concrete test standards (GB/T 50107-2010), the UCS of GCPB should be tested at 3, 7, and 28 days. 2.3.2. XRD experiment To achieve a full understanding of the mineral composition of the GCPB with different chloride contents, the selected samples were analyzed via XRD. The XRD tests were performed using a D/max 2500 PC X-ray diffraction device (radiation, 2h = 5°–80°) with an operating voltage of 40 kV, an emission current of 40 mA, and a step size of 0.02. 2.3.3. SEM experiment To elucidate the effect of chloride on GCPB more directly, SEM analysis was performed. A JSM-6510LV high- and low-vacuum scanning electron microscope was used to scan the specimens. Its high-vacuum resolution is 3 nm (30 kV)/8 nm (3 kV)/15 nm (1 kV), and its low-vacuum resolution is 4.0 nm (30 kV). Before the SEM observations, samples were cut directly from GCPB with different curing ages and immersed in an alcohol solution to prevent any further hydration. The samples were dried after immersion. Then, a layer of conductive metal was sprayed onto the specimen surface and observed by SEM.

3.1.1. Stress–strain analysis of specimens with different curing times Fig. 5 shows the stress–strain curves of the specimens with different curing times. These curves indicate that all of the specimens underwent the elastic stage, rupture stage, and failure stage. It can be seen from Fig. 5(a) that the peak stress of the specimens with an initial chloride content of 10‰ is 432.01 kPa higher than those of the other four groups. The elastic modulus of the specimens with an initial chloride content of 10‰ (27.68 MPa) is the second largest and is lower than that of the specimens with an initial sodium chloride content of 5‰ (37.25 MPa). The elastic modulus of the specimens with a 5‰ chloride content is the largest, which indicates that the resistance of the specimens to deformation in the elastic deformation stage is greater than that of the other groups. Fig. 5(b) shows the stress–strain curves of specimens cured for 7 days. It is obvious that the curves of the specimens with an initial chloride content of 40‰ are quite different from those of the specimens with initial sodium chloride contents of 0‰, 5‰, 10‰, 20‰, and 30‰. The stress–strain curves of the specimens with 0‰, 5‰, 10‰, 20‰, and 30‰ contents tend to be consistent and similar. However, the specimen with 40‰ chloride content has not only a peak stress less than those of the other five groups, but also a lower elastic modulus than those of the other five groups (658.56 kPa and 2.89 MPa, respectively). Fig. 5(c) shows the stress–strain curves of specimens cured for 28 days. It is obvious that the peak stress decreases in the order of 10‰ > 20‰ > 5‰ > 0‰ > 30‰ > 40‰ chloride content. The maximum peak stress is 1.7 times the minimum peak stress. At the same time, it can be clearly seen that the elastic moduli of the samples with different chloride contents are not different in the elastic stage except for that of the 40‰ chloride sample. The elastic modulus is in the range of 73.32–91.37 MPa. However, the elastic modulus of the specimens with 40‰ chloride content is different from those of the other groups. It is obvious that the elastic modulus is larger than that in Fig. 5(b) and has increased by 88.48 MPa. 3.1.2. Effects of different initial chloride content on the UCS strength of GCPB Fig. 6 shows the UCS changes in GCPB with different initial chloride contents (0‰, 5‰, 10‰, 20‰, 30‰, and 40‰) and curing times (3, 7, and 28 days). From this figure, it can be clearly seen that the chloride content significantly affects the UCS of the GCPB after curing from 3 to 28 days. It can also be clearly observed that the UCS shows significant differences on the same and different curing days. With increasing curing time, the peak stresses of the samples with different initial chloride contents increase. Among these, the UCS of the control specimens used for comparison has a linear relationship with the curing age, and the growth rates are similar. The UCS of the control specimens cured for 7 days was calculated to be 1.38 times that of the samples cured for 3 days, and the UCS of the control specimens cured for 28 days was 2.09 times that of the samples cured for 7 days. At all curing times, only the specimens with initial chloride contents of 5‰, 10‰, and 20‰ had higher strength and strength growth rates than those without chloride

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Fig. 6. UCS of specimens with different concentrations at different curing times.

Fig. 5. Stress-strain curves at different curing times: (a) 3 d, (b) 7 d, and (c) 28 d.

because chloride reacts with tricalcium aluminate (C3A) in cement to form insoluble Friedel’s salt (C3ACaCl210H2O) and reacts with calcium hydroxide (CH) in the hydrated products to form an alkaline compound salt (CaCl23Ca(OH)212H2O), which is insoluble in the chloride solution. On the positive side, these hydration reactions were important in improving the strength of the pastes. During hydration, the capillary pores in the hardening cement paste were gradually filled with hydration products. Further, the solid phases formed a dense microstructure, contributing to the associated increase in strength [40–42]. The chemical equations for the hydration products are as follows:

CaðOHÞ2 þ 2NaCl ¼ CaCl2 þ 2Naþ þ 2OH

ð1Þ

CaCl2 þ C3 A þ 10H2 O ¼ C3 A  CaCl2  10H2 O

ð2Þ

CaCl2 þ 3CaðOHÞ2 þ 12H2 O ¼ CaCl2  3CaðOHÞ2  12H2 O

ð3Þ

Compared with the UCS of the specimens cured for 3 days, the growth rate of the specimens with initial chloride contents of 30‰ cured for 7 days was the highest and was 1.79 times higher than that of the specimens cured for 3 days, because the 30‰ chloride content enhanced the chemical reaction and produced more insoluble substances to fill the pore space in a short time. However, when cured for 28 days, the strength of the GCPB with an initial chloride content of 30‰ was 1008.25 kPa, which is less than that without chloride and less than those of the specimens with 5‰, 10‰, and 20‰ chloride. With increasing curing time, the UCS of the specimens cured for 28 days decreased, mainly for two reasons. Firstly, more ettringite (AFt) was produced by chloride acceleration in the specimens. AFt is an expansive hydration product and may have caused expansion cracking or even damage of the joint skeleton structure, thus reducing the UCS of the specimens [43–45]. In addition, the high chloride content reacted with C3A in the cement clinker to produce a large amount of Friedel’s salt (C3A3CaCl230H2O), which could have reduced the strength of the GCPB [46–50]. In addition, the peak stress of the specimens with 40‰ chloride content cured for 7 and 28 days is lower than those of the other groups. The low peak stress indicates the decrease in cement reaction rate. It is worth noting that there are only UCS results for 7 and 28 days of curing, but not for 3 days, due to the failure of the specimens to be molded. The reason for this phenomenon is that the high chloride content hindered the hydration of the cement in the specimen at the initial stage. Because the high chloride content absorbed more water in GCPB, the liquid phase between particles increased. Therefore, a higher initial chloride content negatively impacted on the early strength of GCPB. 3.2. XRD analysis results The mechanical properties of the samples with different chloride contents were tested as described above, and the test results showed that the initial chloride content and curing time had significant effects on the GCPB. To improve understanding of the effects

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of these factors on the GCPB, XRD was performed to analyze the phases of some samples containing chloride. After analyzing the UCS results of the GCPB, representative samples with initial chloride contents of 10‰, 30‰, and 40‰ were selected for XRD analysis. Figs. 7–9 present the XRD patterns of the hydration products of the above three initial chloride content specimens cured at 7 and 28 days. Fig. 7 shows the XRD pattern of the hydration product of GCPB with an initial chloride content of 10‰. Fig. 7(a) reveals that the strongest peak in the pattern after curing for 7 days is that of silica, which indicates that the GCPB body had not been fully hydrated at 7 days and that only a small amount of calcium silicate hydrate (C-S-H) gel formed. However, it is apparent in the 28 day XRD pattern that the silica peak decreased, indicating that the calcium silicate (C3S) continued to hydrate over time, producing more C-S-H gel. In the initial stage, C-S-H gel functions to connect the particles in the GCPB body into a net preliminarily, which makes the cement slurry solidify. The hydration reaction of C3S can generate C-S-H and CH, and the combination of CaCl2 and CH further accelerates the hydration of C3S to generate numerous C-S-H gels. With continuous gel formation, the network structure of joints is strengthened and the strength increases accordingly. At the same time, it can be seen from the XRD pattern shown that AFt was generated at different curing times. The amount of AFt increases with increasing curing time. It can be seen from the comparative analysis of the pattern that the peak strength of AFt after curing for 28 days is greater than that at 7 days because when CaCl2 exists simultaneously with Ca2SO4 (gypsum) and C3A, CaCl2 can also accelerate the reaction of C3A and Ca2SO4 in the GCPB and initially generate C3A3Ca2SO432H2O (AFt). When SO24 is exhausted, C3A combines with CaCl2 to form Friedel’s salt (C3ACaCl210H2O). The formation of the AFt and Friedel’s salt expands in volume and compacts the paste, thus increasing its early strength. According to the UCS and the reaction mechanism of GCPB with an initial chloride content of 10‰, it can be inferred that the formation of AFt and Friedel’s salt did not reach the maximum space value of GCPB in the early stage (within 28 days). The expansive crystals precipitated

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were always in the process of filling the pore space, which resulted in the UCS of the GCPB being in the enhancement stage. Fig. 8 shows the XRD pattern of the hydration product of GCPB with an initial chloride content of 30‰. It can be clearly seen that the phase composition of Fig. 8(a) is not different from that of Fig. 7 (a), showing that there are similar hydration products. The strongest peaks are all silica. According to the XRD pattern of the hydration products in Fig. 8(a), it can be inferred that the GCPB is not completely hydrated. The maximum amount of C-S-H gels, which are formed in the hydration process, is not reached. However, in the XRD pattern (Fig. 8(b)) after curing for 28 days, a relatively strong AFt peak is clearly observable, which proves that a large amount of AFt was generated. A large amount of AFt enters the confined compact space, which causes expansion, cracking and even destruction of the connecting skeleton structure between the particles in the GCPB. In addition, as a strong electrolyte, chloride disassociates completely in water, providing abundant Ca2+ and Cl- in the solution. Due to the influence of the salt effect, the mineral composition of the cement clinker becomes more soluble and accelerates the dissolution process. Furthermore, due to the ion effect, the nucleation of the cement hydration products and the crystal growth process will be accelerated. In other words, the crystals (AFt, Friedel’s salt, and others) enlarge the pores of the GCPB as they separate out and grow. This behavior also confirms why the UCS of GCPB with an initial chloride content of 30‰ in Fig. 6 is lower than those of the other samples (0‰, 5‰, 10‰, and 20‰) after curing for 28 days. Fig. 9 shows the XRD pattern of the hydration product of GCPB with an initial chloride content of 40‰. Unlike in Figs. 7 and 8, the sodium chloride peak is clearly observable in the XRD pattern in Fig. 9. The presence of this peak indicates that there is a large amount of sodium chloride in the GCPB that does not participate in the hydration reaction. The sodium is an anti-alkali aggregate, which can lead the hydration rate of GCPB to decrease. In addition, the Ca2+ ions in C-S-H can be replaced by Na+ in the pore solution. This characteristic explains why the UCS of GCPB with an initial chlorine content of 40‰ in Fig. 6 is lower than those of the other samples.

Fig. 7. XRD patterns of specimens with an initial chloride content of 10‰.

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Fig. 8. XRD patterns of specimens with an initial chloride content of 30‰.

Fig. 9. XRD patterns of specimens with an initial chloride content of 40‰.

3.3. SEM observations To observe the internal microstructure more intuitively, SEM tests of the GCPB specimens with different initial chloride contents were performed. Specimens of the control mixture and with 10‰, 30‰, and 40‰ chloride addition were tested. Figs. 10–12 present SEM photographs of the GCPB with different initial chloride contents after curing for 7 and 28 days. The results

confirm the presence of an improved microstructure in the GCPB with different initial chloride contents. It can be seen from Fig. 10(a) that there are still certain sizes and numbers of voids in the GCPB after curing for 7 days. However, it is worth noting that there are basically no voids inside the specimen after curing for 28 days (Fig. 10(b)), due to the voids being filled with numerous hydration products. The main reason for this result is that as the hydration reaction continues, C-S-H gels, AFt,

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Fig. 10. SEM images of GCPB with 10‰ chloride: (a) 7 d; (b) 28 d.

Fig. 11. SEM images of GCPB with 30‰ chloride: (a) 7 d; (b) 28 d.

Fig. 12. SEM images of GCPB with 40‰ chloride: (a) 7 d; (b) 28 d.

Fig. 13. FTIR spectrum of samples with different chloride contents cured for 28 days.

and Friedel’s salt and other hydration products appear on the surface of the clinker particles. They grow and spread over time, eventually covering the entire clinker grain, which explains why the UCS of the specimen with an initial chlorine content of 10‰ is high. As shown in Fig. 11(a), numerous voids, pores, and discontinuities were present in the control specimen after curing for 7 days. In contrast, the microstructure of the specimen after curing for 28 days (Fig. 11(b)) is more compact and denser and no separation phases are observable inside the GCPB. The results show that the sample with an initial chloride content of 30‰ had relatively high hydrated mineral precipitation or the formation of a large amount of C-S-H gels. These hydration products fill the inside of the sample, improving the degree of GCPB densification, which was also confirmed as described above. However, in Fig. 11(b), it can also be observed that some positions have enlarged capacities.

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Fig. 14. (a and c): the survey XPS spectra of samples with an initial chloride content of 10‰ and 40‰; (b and d): the Cl XPS spectrum of products with an initial chloride content of 10‰ and 40‰.

Fig. 12 shows the SEM photographs of the GCPB with 40‰ added chloride. It can be clearly seen from Fig. 12(b) that the void and pores became significantly smaller, indicating that hydration products were generated inside the GCPB after curing for 28 days to fill the voids and pores, thus improving the compactness. However, on the surfaces of the GCPB specimens, it is obvious that excessive admixtures are doped or coated with hydration products. This finding also agrees with the XRD analysis results presented above. 3.4. FTIR analysis To investigate the effects of chloride salts on GCPB further, especially appropriate and excessive amounts of chloride salts, the chemical structures of the prepared samples were characterized by FTIR spectroscopy. Fig. 13 shows the infrared spectra of samples with different initial chlorine contents (10‰ and 40‰) after curing for 28 days. As can be seen, most of the functional groups of the two samples have similar energies, indicating that the samples contained similar components. The pronounced peaks in the spectra at 3436 cm1, 1090 cm1, and 1009 cm1 were assigned to the O–H, Si-O, and Si-O-Si stretching vibrations, respectively [51,52]. However, there is a significant difference at 778 cm1. Compared with the specimen with an initial content of 40‰, a chlorine-containing functional group appears for the sample with 10‰ chloride, indicating that the GCPB with 10‰ chloride content produced a detectable amount of Friedel’s salt. 3.5. XPS analysis To verify the effects of appropriate and excessive amounts of chloride salts on GCPB further, XPS was employed to determine the electronic states of the samples. Fig. 14 shows the XPS spectra of the samples with initial chloride contents of 10‰ and 40‰ after curing for 28 days. As shown in Fig. 14(a) and (c), five elements—

Na, O, Ca, Cl, and Al—are observable in the material based on the XPS spectra. The high-resolution XPS spectra in Fig. 14(b) and (d) depict the binding energies of Cl 2p at 198.6 eV. The analysis of the proportion of binding compounds of Cl shows that the proportion of CaCl2 in the binding compound with an initial chlorine content of 10‰ is 31.5%, while the proportion of CaCl2 in the binding compound with an initial chlorine content of 40‰ is 26.3%. The XPS spectra further illustrate that GCPB with an initial chlorine content of 10‰ generated more Friedel’s salt. The increase in the amount of Friedel’s salt further filled the internal space of the specimen, which is also consistent with the results shown above. 4. Conclusions In this study, experiments were performed to evaluate the effects of chloride on the early mechanical properties of GCPB. Several GCPB samples with different initial contents of chloride were prepared and cured under the same conditions and curing times (3, 7, and 28 days). The reuse of solid mine wastes in cementbased materials was shown to be plausible, especially when accompanied by an additive such as chloride. The following results were obtained: (1) An initial chloride content of 10‰ promotes the early strength of GCPB the most effectively, while an initial chloride content of 40‰ promotes the early strength the least. The main reason for the effectiveness in the first case is that chloride salts could promote hydration reactions in the specimens, and the insoluble hydration products in the specimens did not completely fill the pores and voids. Meanwhile, the main reason for the lack of effectiveness in the latter case is that the excessive chloride prevented the formation of hydration products. Thus, the microscopic experimental analysis using SEM provided valuable evidence.

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(2) The results of the XRD, FTIR, and XPS experiments showed that the hydration products included AFt (C3A3Ca2SO432H2O), Friedel’s salt (C3ACaCl210H2O), and alkaline compound salt (CaCl23Ca(0H)212H2O). When the initial sodium chloride content was less than 30‰, the insoluble hydration products increased with increasing initial chloride content. When the initial sodium chloride content was greater than 30‰, the insoluble hydration products decreased increasing initial chloride content. When the initial sodium chloride content was less than or equal to 10‰, the hydration products did not fill the internal space of the GCPB, increasing the UCS of the GCPB. When the initial sodium chloride content was 10‰–30‰, the amount of hydration products was larger than the internal space of the GCPB, decreasing the UCS of the GCPB. When the initial sodium chloride content was greater than 30‰, the antialkali aggregate reduced the hydration rate and decreased the UCS of the GCPB. Conflict of interest statement We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. Acknowledgements This study was supported by the National Key R&D Program of China (2018YFC0604704), the National Natural Science Foundation of China (51774194), the Shandong Provincial Natural Science Fund for Distinguished Young Scholars (JQ201612), the Shandong Provincial Key R&D Plan (2017GSF17112), the Shandong Provincial Natural Science Fund (ZR2018ZC0740), and the Taishan Scholars Project. References [1] W. Li, M. Fall, Sulphate effect on the early age strength and self-desiccation of cemented paste backfill, Constr. Build. Mater. 106 (2016) 296–304. [2] M. Fall, M. Pokharel, Coupled effects of sulphate and temperature on the strength development of cemented tailings backfills: Portland cement-paste backfill, Cem. Concr. Compos. 32 (2010) 819–828. [3] M. Pokharel, M. Fall, Combined influence of sulphate and temperature on the saturated hydraulic conductivity of hardened cemented paste backfill, Cem. Concr. Compos. 38 (2013) 21–28. [4] S. Chen, D. Yin, F. Cao, Y. Liu, K. Ren, An overview of integrated surface subsidence-reducing technology in mining areas of China, Nat. Hazards 81 (2016) 1129–1145. [5] G. Akbar, M. Ali, S. Homayon, R. Mahasa, Effects of groundwater withdrawal on land subsidence in Kashan Plain, Iran, Bull. Eng. Geol. Environ. 75 (2016) 1157– 1168. [6] S. Ouellet, B. Bussiere, M. Mbonimpa, et al., Reactivity and mineralogical evolution of an underground mine sulphidic cemented paste backfill, Miner. Eng. 19 (2006) 407–419. [7] Q. Sun, J. Zhang, N. Zhou, W. Qi, Roadway backfill coal mining to preserve surface water in western China, Mine Water Environ. 37 (2018) 366–375. [8] J.H. Zhao, N. Jiang, L.M. Yin, L.Y. Bai, The effects of mining subsidence and drainage improvements on a waterlogged area, Bull. Eng. Geol. Environ. (2018) 1–17. [9] N. Zhou, H. Yan, S. Jiang, Q. Sun, S. Ouyang, Stability analysis of surrounding rock in paste backfill recovery of residual room pill, Sustainability 11 (2019) 1– 13. [10] O. Robin, D. Craig, M. Rusty, Void fill techniques for stabilizing roof conditions during longwall recovery, Int. J. Ming Sci. Technol. 26 (2016) 119–122. [11] M. Pokharel, M. Fall, Coupled thermochemical effects on the strength development of slag-paste backfill materials, J. Mater. Civil Eng. 23 (2010) 511–525. [12] X. Feng, Q. Zhang, The effect of backfilling materials on the deformation of coal and rock strata containing multiple goaf: a numerical study, Minerals 8 (2018) 1–17. [13] Q. Zhang, J. Zhang, Y. Huang, F. Ju, Backfilling technology and strata behaviors in fully mechanized coal mining working face, Int. J. Mining Sci. Technol. 22 (2012) 151–157.

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