Mechanical activation of coal gasification fine slag and mechanical and thermal properties of coal gasification fine slag–poly(vinyl chloride) composites

To facilitate the high-value utilization of activate coal gasification fine slag (CGFS), a wet mechanical activation process was used. As a result of this treatment, CGFS samples with different particle size distributions were obtained. The effects of mechanical activation on various physical and chemical properties of CGFS were investigated, including its particle size distribution, mineral composition, specific surface area, pore size, crystallinity, particle morphology, chemical bonding, and binding energy. Poly(vinyl chloride) (PVC)/CGFS composites were prepared via a melt blending process, and their mechanical and thermal properties were evaluated. It was found that with increasing levels of mechanical activation, the CGFS particle size distribution became more concentrated and the particle spacing became more uniform. With the increasing mechanical activation, the crystallinity was found to decrease and the content of amorphous mineral matter (such as SiO2 and Al2O3) increased. The observed increase in specific surface area and decrease in average pore diameter due to the mechanical activation was seen to lead to an increase in the number of active sites. The produced PVC/CGFS composite materials were found to exhibit good mechanical properties and dynamic thermal stability. The thermal stability of the PVC composites was also found to improve relative to the composites produced without the use of mechanical activation.


Introduction
Polymer composites filled with inorganic filler particles have received considerable attention due to their wide range of potential applications and their low cost. Previous studies have shown that the addition of inorganic mineral fillers to polymers can improve the mechanical, thermal, acoustic, and flame-retardant properties of the polymers [1][2][3][4]. Poly(vinyl chloride) (PVC), which is a thermoplastic material, is known for its good cost-toperformance ratio and has been the subject of substantial quantities of research related to further improving its properties. The most common method for modifying PVC is via physical modification; in this technique, inorganic fillers are added to improve the mechanical and thermal performance of the polymer. Currently, inorganic fillers, such as glass microspheres, calcium carbonate, talcum powder, waste rubber powder, and wood fiber, are used [5][6][7][8][9]. Although these fillers can result in materials of superior performance, the improvement typically affects only a single property of the composite, and the production cost of such materials is high. Therefore, finding a low-cost and high-quality filler to modify the properties of PVC has become an important research topic [10].
Coal gasification slag is a solid waste that is generated during the coal gasification process; this material can be divided into two categories, coal gasification coarse slag and coal gasification fine slag (CGFS), depending on the characteristic particle size of the material. CGFS primarily consists of flocculent residual carbon and amorphous glass beads; the chemical composition of the glass beads includes metal oxides, such as silicon dioxide, aluminum oxide, calcium oxide, and iron oxide [11]. The size of the particles that make up CGFS ranges from 1 to 100 μm. With the increasingly strict quality requirements within the coal chemical industry and the 'dual carbon' target, [12] the utilization of CGFS has become one of the most important factors limiting developments within the coal chemical industry. At present, the more common uses of gasification slag are related to the production of adsorbents [13][14][15], construction materials [16][17][18], soil improvement [19][20][21], and the production of PVC composite materials using the CGFS as a filler (applications for tubes, sheets, profiles, etc) [22][23][24]. Considering both its technical feasibility and economic viability, the production of composite materials represents a more efficient and higher-quality utilization of CGFS.
It is well known that the physical and chemical characteristics of CGFS determine its suitability for use in a wide range of applications [25]. For example, the dense glassy surface of CGFS leads to initial activity, and the surface structure and properties are also crucial for activity [26]. Physical activation (mechanical force) is one of the most important methods to improve the activity of CGFS [27]. However, few experimental studies on the use of mechanical activation to enhance the activity of CGFS have been undertaken, and little research exists on the effects of mechanical activation methods on the basic physicochemical properties of CGFS [26,28]. Additionally, previous investigations have not systematically explored the interfacial properties of CGFS fillers and the relationships linking the interfacial properties of CGFS with their chemical activity and interactions with polymer matrices. It is well established that fillers with differing surface properties exhibit varied responses during interfacial interactions with polymers [29,30].
Motivated by the above observations, the focus of this experiment is to investigate the effects of mechanical activation of CGFS on particle size, chemical composition, mineral composition, crystal structure, microscopic morphology, elemental surface composition, specific surface area, and porosity. A comprehensive understanding of the surface properties and reaction activity of CGFS after different periods of mechanical activation treatment was obtained permitting the verification of the feasibility of using CGFS as a PVC filler. In addition, the mechanical properties and thermal stability of PVC composites filled with CGFS with varying mechanical activation characteristics were investigated.

Mechanical activation of CGFS
Here, the basic physicochemical properties of CGFS were studied, and the CGFS was selected to have a wide particle size distribution, so the median diameter D 50 (the particles with a particle size below D 50 account for 50% of all particles) was used. Studies have shown that wet mechanical activation, which consumes less energy than dry mechanical activation, can increase the fineness and improve processability of CGFS particles [31][32][33]. The optimum quality concentration of 60% CGFS slurry was prepared using deionized water as the grinding medium prior to mechanical activation [34,35]. The CGFS were then mechanically ground in a mill for different time durations (0, 5, 10, 20, and 30 min) with a mass ratio of the balls to CGFS material of 10:1. After drying at 105°C for 24 h in an oven, various samples of CGFS were obtained and designated as CGFS1-CGFS5 (where the number in the name ascends with increasing milling time). The chemical composition of the samples is shown in table 1.

Preparation of PVC/CGFS
PVC/CGFS composites were prepared via the melt blending process in a mini conical twin-screw extruder (SZS-20, Wuhan Ruiming Experimental Instruments Co., Ltd (Hubei, China)). Standard sample strips (150×10×4 mm 3 ) were then produced using an injection molding machine (SJZS-10B, Wuhan Ruiming Experimental Instruments Co., Ltd (Hubei, China)). The specific process used was as follows: First, the PVC powder, DOTP, and various additives were mixed in a high-speed mixer (SUS316, Jinhua Mofei Household Appliances Co., Ltd (Zhejiang, China)). In previous work, it was shown that PVC/CGFS composites with a CGFS with content higher than 10 wt% exhibit a higher tensile and impact modulus, but these materials show a low flexural strength; the tensile and impact modulus are widely considered to be the most important properties of the composite [3,36,37]. Therefore, in this experiment, it was decided to investigate the effects of a single parameter on the mechanical properties of the composites for a CGFS filler content of approximately 10 wt% in order to achieve the positive effects of the CGFS filler on the mechanical properties of the composites. From inlet to outlet, the extruder had three temperature-controlled zones, the feed zone, circulation zone, and metering zone; the temperatures of these zones were 160°C, 180°C, and 190°C, respectively. The head temperature was set to 200°C, and the template temperature was set to 80°C. The main shaft speed of the machine was 30 rpm, and the filling speed was set to 20 rpm. A micro injection molding machine was used to finalize the preparation of standard samples by injecting strips into the mold closing time of 1 min, injection time 1 and mould closing time 2 were set to 4 and 20 s, respectively, and the injection pressures 1 and mould closing pressures 2 were 0.6 and 0.2 MPa, respectively. The strips were then annealed in an oven at 80°C for 2 h and then left at room temperature for 24 h. Numerous samples were prepared and designated as PVC/CGFS1-PVC/CGFS5. Figure 1 shows the mechanical activation of CGFS and preparation of the PVC/CGFS composites.

Testing and characterization
The chemical composition of the CGFS samples was determined using an x-ray fluorescence spectrometer (ARL-980, ARL, Switzerland,) and boric acid as the carrier. The particle sizes of the samples were determined using a laser particle size analyzer (BT-2003, Danyang Baxter Instruments Co., Ltd (Jiangsu, China)). The phase composition of the mechanically activated samples was examined via x-ray diffractometry (MSALXD-3, Beijing Puyang General Co., Ltd (Beijing, China)) in the 2θ range from 12°to 80°. The morphology of the samples was examined with a scanning electron microscope (ESCAN VEGA3 SBH, TESCAN, Germany) using an acceleration of 30 kV. Fourier transform infrared (FTIR) spectroscopy (NICOLET 380, USA) was used to detect the chemical bond structure in the wavenumber range 4000-400 cm −1 . The specific surface area of the CGFS samples was measured using a surface analyzer (NOVA 4000e, USA)); the samples were degassed in vacuum at 300°C to remove moisture and gas absorbed on the surface prior to this analysis. The surface area of the samples was calculated by measuring the volume of N 2 adsorbed and using a modified one-point Brunauer-Emmett-Teller (BET) analysis. X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, USA) was used to determine the atomic morphology of the surface of the samples. A universal testing machine (WDW-50, Shenzhen KQL Co., Ltd, (Guangzhou, China)) was used to evaluate the mechanical properties of the material; the properties investigated included tensile strength, flexural strength, and elongation at break. The tests were conducted by the standards GB/T1040-1992 and GB/T1048-2008. The impact strength of the samples was calculated using a simple beam impact tester (TGG-25, Jilin Province Taihe Testing Machine Co., Ltd (Jilin, China)). A dynamic mechanical analysis (DMA, TA Instruments Q800, USA) was performed considering a single cantilever beam with a fixed vibration frequency of 1 Hz; a test temperature of 30°C-120°C and a heating rate of 2°C min −1 were used for this testing. The storage modulus, loss modulus, and mechanical loss factor (tan δ) were recorded and plotted against temperature. A thermogravimetric analysis (TGA, Netzsch STA 449 F3, Germany) was undertaken in a nitrogen atmosphere instrument with a heating rate of 10°C min −1 to reach the set temperature of 800°C.

Properties of mechanically activated CGFS
3.1. Particle-size distribution of activated CGFS The particle size distribution of the CGFS activated via different mechanical activation methods was measured using a laser particle size analyzer; the results of this analysis are shown in figure 2.The particle sizes of the activated CGFS are also shown in table 2. It can be observed that the particle size distribution and the abundance distribution of the CGFS activated using the various grinding methods considered here are very similar. The raw CGFS material exhibits large pores with a characteristic diameter of more than 18 μm. Significant differences in the particle size distribution of the CGFS samples were observed in the samples subject to different mechanical activation times; longer mechanical activation times were found to result in a shift of the particle size distribution and frequency distribution curves toward smaller particle sizes. The median diameter of sample CGFS5 was 3.070 μm, representing a significant change in particle size compared with sample CGFS1. This could be due to the fact that the particles collide with each other and with the mechanical activation spheres when subject to mechanical force, and size decreases rapidly along the defect boundaries, resulting in a decrease in the macroscopic particle size and a narrowing of the particle size distribution [15]. At the same time, the average diameter of the particles in the CGFS1 and CGFS5 samples decreased from 8.644 to 4.025 μm, respectively, and the specific surface area increased from 223.9 to 480.8 m 2 kg −1 , respectively.

Surface area and pore structure of the particles
The Barrett-Joyner-Halenda (BJH) desorption pore size distribution and N 2 adsorption/desorption isotherms of the CGFS samples activated using various mechanical activation methods are shown in figure 3, and the pore properties of the activated CGFS samples are listed in table 3. As can be seen from figure 3, the N 2 adsorption/ desorption isotherms of all the CGFS samples correspond to type-III isotherms in the International Union of Pure and Applied Chemistry isotherm classification [38], indicating the existence of a considerable amount of mesoporous pores in the CGFS samples after the activation treatment [39]. Moreover, with increasing mechanical activation time an increase in the specific surface area, pore volume, and average pore size of the CGFS samples was observed. The increase in specific surface area is primarily due to the decomposition of the CGFS particles during grinding rather than the opening of gas pores [40]; this observation is consistent with previous studies [41,42]. Typically, in systems with fillers that have larger specific surface areas, larger contact areas between the filler with the polymer matrix are observed, resulting in more binding sites and stronger  binding forces. We note that sample CGFS5 has a specific surface area of 290.731 m 2 g −1 , which is more conducive to bonding with the polymer matrix compared with sample CGFS1. With increasing extents of mechanical activation, the distribution curve of the BJH adsorption pore size of the samples shows that the total pore volume first decreases and then increases and that the average pore size decreases, indicating that the pore structures of CGFS are destroyed during mechanical activation and the extent of this destruction becomes more large with increasing activation levels. The cumulative mercury intrusion curve shifts toward smaller pores sizes, indicating the formation of small pores [42].   10 : the particles with a particle size below D 10 account for 10% of all particles. b D 50 : the particles with a particle size below D 50 account for 50% of all particles. c D 75 : the particles with a particle size below D 75 account for 75% of all particles. d D 90 : the particles with a particle size below D 90 account for 90% of all particles.

Mineral composition analysis
The x-ray diffraction (XRD) spectra of the CGFS samples subject to mechanical activation of varying durations are shown in figure 4; these samples consist of a large number of glass phases and a small number of crystalline phases (quartz and mullite). The corresponding XRD peaks are located at 20.75°, 26.52°, and 49.95°( PDF#47-1144) for crystal planes of the quartz phase and 29.40°, 33.13°, and 47.55°(PDF#370702)) for the mullite phase. Mechanical activation of CGFS results in a slightly reduced with increasing peak FWHM (full width at half maximum) corresponding to the quartz and mullite phases. With increasing activation extents, the intensity of the amorphous hump increases, and the diffraction peaks become more diffuse, indicating the formation of amorphous components as a result of mechanical activation [43]; this may be due to the structure of CGFS being altered by the mechanical processes, resulting in particle refinement and the destruction of the dense surface structure of the glass body, causing the release of active SiO 2 and Al 2 O 3 from the particles [44]. It has been found that mechanical activation can reduce the crystallinity of mineral phases and increase the proportion of amorphous fractions with active sites [45]. The transition from crystalline to amorphous states increases the reactivity of CGFS, which is likely to increase its surface activity and promote its interfacial bonding with the polymer.

Chemical bond analysis
To better understand the changes in the functional groups of the CGFS samples induced by mechanical activation, FTIR spectroscopy was used; the results of this analysis are shown in figure 5. The spectra demonstrate the differences between the CGFS samples before and after mechanical activation. The presence of quartz leads to a series of band differences, including those observed at 1150, 1087, 989, and 481 cm −1 . The stretching vibrations of the O-H bonds cause strong absorption peaks in the spectra of the CGFS samples at 3441 cm −1 and 1592 cm −1 ; this finding is consistent with studies present in the existing literature [46]. The band at 560 cm −1 is related to the bending vibrations of Al-O-Si in octahedral alumina, indicating the presence of residual mullite in the reaction matrix. This result is consistent with the XRD analysis of the same CGFS samples. Figure 5(b) provides a more detailed view of the two shaded regions of the obtained spectra. Region A covers the range 900-1300 cm −1 , where the peak is due to the asymmetric stretching vibrations of the Si-O-T (T: Si or Al) bonds. The peak intensity and absorption band intensity increase, which can be explained by the increase in amorphous silica or alumina in CGFS samples [38]. Region B covers the range 3400-3500 cm −1 ; in this range, with increasing mechanical activation, a slightly increased OH-intensity can be seen, which is absent in the original CGFS sample. This feature is a result of the bending and stretching vibrations of Si-OH adsorbed water on the surface of the CGFS sample. According to the study by Li et al [38], the formation and presence of Si-OHis indicative of the breakdown of the silica structure. Other studies have shown that the increase in the surface exposure of amorphous silica leads to increased intensity signals in this region [47].

Morphology analysis
Using scanning electron microscopy (SEM) with a magnification of 350×, the micromorphology of the CGFS particles subject to different degrees of mechanical activation was analyzed; the obtained images are shown in figure 6 The results indicate that the samples CGFS1 and CGFS2 were primarily composed of particles characterized by large sizes, a wide range of particle sizes, and particles of various shapes (including coarse, flake, spherical, slag-like, and irregular particles). As the duration of the mechanical activation process increased, the samples exhibited a progressive decomposition of the larger block particles into numerous irregular small particles (this observation can be inferred from CGFS3), accompanied by a reduction in the size of the spherical particles. Sample CGFS4 can be seen to exhibit a more uniform morphology with more irregular, angular particles. Sample CGFS5 can be seen to have a more dense particle distribution and a more uniform particle spacing per unit area. Considerable research has shown that with decreasing particle size in composites, internal stresses can be reduced tensile strength and toughness can be improved, fatigue resistance can be increased, and hardness and tensile strength can be increased [48,49].

Surface binding energy
To investigate the changes in the chemical states of several typical elements in the particles of CGFS as a result of the mechanical activation treatments considered here, XPS was used; the results of this analysis are presented in figure 7. The results of the full-spectrum scan indicate that the main constituents of the CGFS particles are oxygen, silicon, aluminum, and calcium. Other elements, such as potassium and sodium, were not observed in a significant quantity in the spectra. Fine spectral scans were then performed for O1s, Si2p, and Ca2p to obtain the chemical forms of the elements; these scans were then used for qualitative analysis of the structure and activity of the mineral present within the CGFS. It was found that the O1s spectrum of CGFS1 mainly exhibited a single peak structure, and, with increasing mechanical activation time, the peak position shifted toward lower binding energies. A secondary peak was observed at 531-532 eV, corresponding to the Si-O-T (T: Si or Al) bonds, indicating that the observed changes in the oxygen binding energy as a result of mechanical activation were primarily due to changes in the Si and Al content. It has been shown that Si2p binding energy is related to the activity of cementitious materials with lower Si2p binding energies indicating higher cementitious activity [44,48]. The binding energy of the CGFS1 particles was found to be 103.61 eV, and that of the CGFS5 particles was found to be the lowest (102.87 eV). This indicates that the surface activity of CGFS can be improved by increasing the mechanical activation of the samples. A peak fitting analysis of the Si2p curve indicated that the area of the peak at approximately 103 eV increased in size, corresponding to an increase in the amorphous SiO 2 content of the sample, which is consistent with the results obtained via XRD and FTIR spectroscopy. The reactivity of the material results from its large surface area, and a higher Ca content on the sample surface indicates higher activity of the material. According to the XPS spectrum, the Ca2p content and peak area increased with increasing duration of the mechanical activation treatment, indicating an increased participation of Ca 2+ in the hydration reaction [50]. Based on the results presented here, we conclude that mechanical activation can promote the surface activity of CGFS by shifting the surface electrons of the O1s, Si2p, and Ca2p orbitals toward lower binding energies and also causing changes in the mineral structure [42].

Mechanical properties
The mechanical properties of the PVC composites constructed using CGFS subject to different activation treatments are summarized in figure 8. It can be seen that the tensile strength and elastic modulus of PVC/ CGFS1 composites were 58.97 and 436.85 MPa, respectively. However, the composite made by introducing CGFS5 into a PVC matrix was found to have a tensile strength and elastic modulus of 66.45 and 555.55 MPa, respectively; these values represent an increase of 12.68% and 27.17%, respectively, compared with PVC/ CGFS1 (see figure (8a)). This is due to the fact that the smaller the particle size, the more uniformly the CGFS particles can be distributed within the PVC matrix. As shown in figures 8(e)-(f), this uniform distribution leads to an increase in the interfacial area/volume and better adhesion and load transfer between the filler and the polymer, thus improving the mechanical properties [3]. This study suggests that this is an effective way to improve the mechanical properties of polymer composites. Moreover, it was observed that the elongation at break of the PVC/CGFS5 sample is higher than that of the PVC/CGFS1 sample (see figure 8(b)). This is also attributed to the different characteristic sizes of the particles within the CGFS, which dictate the strength of the interactions between the CGFS and the PVC matrix, resulting differing extents of wetting of the fly ash surface by the polymer, which in turn affects the likelihood of the filler detaching from the polymer matrix [51]. With increasing mechanical activation extents, the interfacial bonding becomes stronger and increased load transfer from the polymer matrix to the CGFS particles can occur. This leads to an increase in the flexural strength and modulus of PVC/CGFS composites with increasing mechanical activation treatment times (see figure 8(c)). This work indicates that the impact strength of the PVC/CGFS composites decreases sharply with increasing CGFS particle size (see figure 8(d)) [52]. XRD measurements on unground CGFS show that it contained a large amount of amorphous glass material; the presence of this material was found to result in weak interactions between the CGFS and the PVC matrix and made the PVC/CGFS composite brittle.

Thermal stability
The properties of the PVC/CGFS composite with a 10 wt% filler were investigated via a dynamic mechanical analysis (DMA). Figure 9 shows the storage modulus and loss modulus (figure 9(a)) and the loss factor, tan δ ( figure 9(b)). It can be seen that increasing mechanical activation leads to PVC/CGFS composites with higher storage moduli, indicating an improvement in the elastic properties of the composite material. This is because when the CGFS and PVC matrix have higher interfacial compatibility, the CGFS powder can effectively restrict the movement of the PVC molecular chains and the material can store more energy. This work also shows that the increase in the storage modulus depends on the extent of the dispersion of the CGFS powder particles in the PVC matrix [53,54]. The loss modulus represents the viscous behavior of the composite material [9]. The loss modulus of the composite material first increases and then decreases with increasing temperature. The measurements related to the loss modulus indicate that the increase in mechanical activation can improve the adhesion interaction between the PVC matrix and the CGFS filler. The glass transition temperature (Tg) represents the degree of polymer chain movement [55]. This work shows that strong interfacial bonding can shift the maximum peak of tan δ of the composite material to higher temperatures. It can be seen that with the increase in mechanical activation, Tg moves towards slightly higher temperatures, indicating that the PVC molecular chain movement can be suppressed by stronger interfacial interaction, limiting the chain migration rate of the PVC matrix and resulting in a higher value of Tg. The TGA and DTG curves of the PVC/CGFS composites are shown in figure 10. The thermal decomposition of the composite can be divided into two stages. The first stage of weight loss occurs between 253.4°C and 385.6°C, which may be due to the dehydrochlorination of the PVC and the formation of double bonds. The presence of conjugated double bonds adds stability to the polymer chain. The second stage of weight loss occurs between 420.4°C and 503.6°C, which is due to the breakage of single and multiple covalent bonds (polyacetylene cracking) [56], which results in the formation of volatile aromatic compounds and stable carbonaceous residues. Table 4 summarizes the main parameters of the thermal behavior of PVC/CGFS composites. As shown in table 4, with increasing mechanical activation of the CGFS, the initial degradation temperature and the maximum decomposition temperature of PVC/CGFS composites decrease. In addition, the maximum thermal decomposition rate (dW/dt max ) of the composite increases, indicating that CGFS has a large surface area, high surface energy, and many surface atoms, which accelerates the diffusion of air. In addition, this work suggests that more amorphous and carbon structures provide more active sites, which improve the thermal conductivity of the material and contribute to homogeneous distribution and heat dissipation, thereby improving thermal stability. At 800°C, the residual weight after thermal degradation consists of inorganic filler and ash, which are difficult to remove via combustion, leading to an increase in the thermal stability of PVC composites to a certain extent [57].

Conclusions
In this study, the effects of a wet mechanical activation process on the physical and chemical properties of CGFS were investigated. With the increase in mechanical activation, the particle size distribution of CGFS was found to become more concentration, the specific surface area increase significantly. The intensity of the non-crystalline peak was found to increase, and the diffraction peak was found to become more diffuse, resulting in the leaching of active SiO 2 and Al 2 O 3 . The increase in the amorphous silica or alumina content of the samples was also confirmed by results related to the stretching vibration of the Si-O-Al bonds and the bending vibration of the OH-in the infrared spectra. XPS results showed that mechanical activation promotes the surface activity of CGFS, resulting in a shift in the binding energies of the surface electronic orbitals of O1s, Si2p, and Ca2p toward lower energies, and a change in the mineral structure of CGFS. CGFS was successfully incorporated into PVC, and PVC/CGFS composites were prepared using a melt blending process. The mechanical properties of the composite were found to improve with decreasing CGFS particle size. This indicates that the interfacial bond strength is higher for CGFS characterized by smaller particle sizes, leading to better adhesion and load transfer between the filler and the polymer. The intermolecular interactions, well-dispersed nature of the CGFS powder in the PVC matrix, and the strong interfacial bonding result in a higher storage modulus, loss modulus, and glass transition temperature of the composites made using CGFS characterized by smaller particle sizes. The residual weight after thermal degradation consists of inorganic filler and ash, which are difficult to remove by incineration and improve heat resistance.