Effects of Foaming and Drainage Behavior on Structure and Properties of Polyurethane/Water Glass (PU/WG) Grouting Materials for Coal Mines

During the grouting operation in the underground coal mine, abnormal curing behaviors such as foaming and drainage often lead to the loss of reinforcement effect of the polyurethane/water glass (PU/WG) materials on coal walls and even cause safety accidents. Herein, three kinds of PU/WG grouting materials were successfully prepared by changing the type of catalysts, which were the normal sample (C7), the foaming sample (C14), and the sample with drainage (C17) during curing. *e structure, thermal stability, and compressive strength of the three samples were characterized by scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetrydifferential thermal analysis (TG-DTA), and pressure testing machine. *e results showed that the abnormal curing behavior had little effect on the thermal stability of the material, but it had a significant effect on the microstructure and compressive strength of the consolidated body. C7 exhibited a typical three-phase distribution, in which the polysilicate microspheres encapsulated by acicular carbonate were embedded in the polymer continuum. *e structure of C7 had high rigidity and hardness, and the compressive strength was up to 43MPa. *e three-phase structure of C14 disappeared gradually with the increase of catalyst content, the hard block material and matrix are porous, and the compressive strength was only 2.7MPa. *e organic polymer of C17 existed in the form of microsphere and distributed irregularly in the continuum composed of inorganic components, and the compressive strength was 4.9MPa. *e abnormal solidification behavior such as foaming and drainage made the water glass/ polyurethane material lose its basic mechanical properties, which cannot meet the needs of grouting reinforcement in coal mines. *erefore, the type of catalyst had a significant impact on the stability of the system, and it is necessary to avoid selecting catalysts that are likely to cause abnormal solidification during formulation research.

Inorganic materials have some disadvantages, such as slow curing time, low bond strength, difficulty to penetrate the tiny cracks in coal and rock mass due to large slurry size, which make it difficult to be applied in the coal mining face that requires timeliness. e high reaction exotherm, flammability, and high cost of the organic materials violate the original intention of the disaster management. In addition, organic materials are easy to foam in wet cracks [34,35], and the strength decreases greatly. Silicate modified polyurethane material (PU/WG) served as an organic-inorganic hybrid composite system, which overcomes the inherent shortcomings of the polyurethane system, and not only has the advantages of water glass and polyurethane, such as low cost, good flame retardancy, large diffusion radius, and high bonding strength but also has the outstanding advantages of less heat release and high early strength. Furthermore, PU/WG material is brittle, which is not only conducive to the shearer cutting coal but also beneficial for the roadway expanding and shed changing. PU/WG composite grouting material has been widely used in coal mines due to its many advantages.
PU/WG grouting materials have obtained many research achievements, which mainly focused on material development, performance research, and engineering application. He et al. investigated the effect of silane on the compressive strength of the PU/WG composites [32]. Zhang et al. studied the influence of different catalysts on the structure and properties of the PU/WG materials [33]. However, there is almost no research on the influence of abnormal curing behaviors such as foaming and drainage on the structure and mechanical properties of the PU/WG materials, which occur in underground coal mine construction sites. e materials leaking out of the coal wall tend to form foaming and drainage phenomena, resulting in poor reinforcement effects and even collapse of the coal wall. e current research on the abnormal curing behaviors of foaming and drainage analysis involved the microstructure of the material, but almost no one pays attention to the influence on the structure and mechanical properties of the material. Polyurethane/water glass is an organicinorganic composite two-component grouting material, one of which is almost completely inorganic sodium silicate solution, and the other is mainly isocyanate. e main reactions involved include the reaction of isocyanate with water to form CO 2 (foaming reaction), sodium silicate solution to absorb CO 2 to generate carbonate, and isocyanate to react with silicic acid polymer (gel reaction).
ese reactions proceed at the same time and interact with each other to form a consolidated substance. e formation process of consolidated materials is very complex, including interpenetration, coupling, and phase transition of organic and inorganic structures. erefore, the reaction of foaming and gelation reaches equilibrium, and the distribution of the phase of the consolidated body is the prerequisite for the excellent mechanical strength of the material. e behaviors such as foaming and drainage during the reaction inevitably lead to changes in structure and performance.
Herein, three kinds of PU/WG grouting materials were prepared by adjusting and controlling the catalyst, which were the normal samples, the foaming samples, and the samples with drainage during curing. In order to investigate the influence of the abnormal curing behaviors deeply and thoroughly on the structure and properties of the PU/WG grouting materials, the structures of three samples were analyzed, and the thermal stability and compressive strength were compared. e study is helpful to understand the characteristics of the water glass/polyurethane system and provides a reference for the subsequent catalyst screening and formulation research. At the same time, it is of guiding significance for on-site construction; strictly controlling the grouting process; avoiding foaming, drainage, and other phenomena; achieving the reinforcement effect; and ensuring the safety of coal mining.

Preparation of PU/WG Grouting Materials.
Grouting materials are generally composed of organic and inorganic components. e composition of PU/WG grouting materials is listed in Table 1.
Component A (inorganic component): first, a certain amount of catalyst and additives was fully mixed. en, it was added slowly to the WG stirred vigorously. e resulting mixture was uniformly stirred at a speed of 800 rpm for 15 min.
Component B (organic component): PM200 and DL-2000D were added to the reactor, respectively, and the prepolymer was prepared at 80°C for 2 hours. After that, DOP and the prepolymer were stirred in the same manner. e obtained mixture was cooled to room temperature and then sealed in a dry container. Figure 1, components A and B were mixed and stirred with the volume ratio of 1 : 1 at 800 rpm for 15 s. en, slurry obtained was quickly poured into the mold (cylinder: diameter is 50 mm, and height is 100 mm) coated with a release agent. After the samples became obviously hard, they were taken out from the mold, and the compressive strength was tested after aging at room temperature for 3 days.

Materials Characterization.
e curing time was measured at a temperature of 20°C. 50 mL of component A and 50 mL of component B were poured into a disposable plastic cup and then stirred at a constant speed of 15 s. e time from the beginning of stirring to the curing of the materials is recorded. e endpoint was determined according to the reported literature [33]. Compressive strength curves were conducted using the automatic servo pressure testing machine (YAW-300B provided by Testing Technology Co., Ltd). e loading speed was 5 mm/min, and the condition of crushing was set to 5% of the peak value. Compressive strength testing was performed according to the GB/T2567-2008 standard [36]. Scanning electron microscopy (SEM) pictures were obtained using a Hitachi S4800. X-ray diffraction (XRD) patterns were measured on a Bruker D8 Advanced diffractometer (40 kV, 40 mA) using Cu-Ka radiation (wavelength λ � 1.5406Å) from 15 to 60°at a scanning rate of 8°/min. ermogravimetric analysis (TGA) was collected from a NETZSCH STA449F5 with a heating rate of 10°C/min under a flow of 100 mL/min Nitrogen. X-ray photoelectron spectroscopy (XPS) curves were collected from Perkin Elmer Spectrum 100.

e Performance and Selection of Catalysts.
e catalytic properties of 26 kinds of organic amine catalysts for PU/WG grouting materials were investigated. According to the state of the consolidated body after the reaction, the samples can be divided into three categories: the samples with normal curing process, the samples with drainage on the surface of the material during curing, and the samples with foaming during curing. No. 7, No. 14, and No. 17 (the molecular structure is inlaid in Figure 2) catalysts were selected for the PU/WG grouting process, corresponding to the curing process of normal, foaming, and drainage, respectively. erefore, the three catalysts were selected for subsequent experiments. As shown in Figure 2, the curing time was obviously shortened with the increase of the additional amount. When the addition amount was 0.25%, the catalytic activity of C7 was much higher than that of C14 and C17. However, when the addition amount was 1%, the catalytic efficiency of C17 was the highest, and the curing time was  44.7 s. ere are the same functional groups and the different lengths of carbon chain in the C7 and C17 structure, and the activity of the two catalysts was obviously different.

Morphology and Composition
Analysis. e morphologies of the PU/WG samples catalyzed by three catalysts are shown in Figure 3. e samples were cured normally under different dosages of the C7 catalyst, and there was no obvious difference in appearance. When the content of C14 was 0.25%, the appearance and texture of the samples were similar to those obtained by C7 catalysis. When the content of C14 was 1%, the sample foamed seriously, and the volume was about twice that of the normal sample. e isocyanate first reacted with the water in the water glass to release CO 2 under the action of catalyst, and the rate of CO 2 absorption by silicate was less than the release rate; that is, the gel rate was less than the foaming rate, which eventually led to a larger volume of the consolidation and a loose and porous structure. e sample showed drainage phenomenon during the curing process, which was catalyzed by different amounts of C17.
e small droplets gradually precipitated on the surface of the material after the slurry was mixed and solidified, and the number of small droplets increased and finally converged on the surface of the material. e precipitated liquid turned into white powder in a few minutes at room temperature. As can be seen from Figure 2, the larger the amount of catalyst was added, the whiter the powder adhered on the surface of the sample, indicating that the more liquid separated during curing. e surface of the C7-1% sample was smooth, glossy, and white. e color of the C14-1% sample was yellow, and the texture was loose and porous. e surface of the C17-1% sample was covered with a lot of white powder, and the color was yellow. erefore, the type and amount of catalyst not only changed curing time but also had a significant effect on the morphology of samples. is may be ascribed to the fact that different agents had different catalytic tendencies towards foaming reaction and gel reaction of the PU/WG hybrid system, or the foaming reaction and gel reaction were not balanced.
XRD characterization was carried out to determine the main components of white powder with C17-1% precipitation as shown in Figure 4. According to jade software analysis, the main component of white powder with C17-1% precipitation is Na 3 H (CO 3 ) 2 ·2H 2 O.

Structural Analysis
e cross section morphology of the C7-1% sample is shown in Figure 5 and the C7-1% sample is composed of the continuous phase, microsphere phase, and connecting phase. e microspheres of different sizes are embedded in a continuous phase irregularly, and the surface of the crater is covered with another substance. e C7-1% sample was analyzed by energy dispersive spectrometer (EDS) to determine the distribution of elements in the threephase structure, and the enrichment rules of C, N, O, Na, and Si were mainly characterized. Figure 6 shows the mapping distribution of elements. Almost all C elements were distributed in the continuous phase, while O elements were distributed in the continuous phase and microsphere. At the same time, Si elements were concentrated in the microsphere area, while Na did not show apparent regularity. It is preliminarily inferred that the continuous phase and microsphere may be polyurethane polymer phase and siloxane compounds, respectively. e atomic ratio of elements in each phase was scanned by EDS for quantitative analysis. As shown in Figure 7, the elements with high atomic content in the microsphere region (region 1) are O and Si, and the atomic percentage is 3 : 2. It is speculated that the microsphere phase is silicate polymer, not silica. ere are a lot of C, N, and O elements in zone 2, which are mainly polyurethane phases. e main elements in region 3 are Na, C, and O, and the corresponding atomic ratio is 10 : 43 : 41. It is speculated that region 3 may contain both Na 2 CO 3 and polyurethane, which is in a transition state structure. e element state of the C7-1% sample was characterized by XPS, and there were Na, Si, C, O, and N elements in Figure 8. ere was no Si-C bond in the Si 2p spectrum, and the peak at 103.6 eV was attributed to the characteristic peak of silicate polymer, corresponding to Si 4+ . e peaks at 531.9 and 533.4 eV belong to the characteristic peak of Si-O and C � O, respectively, and the peak at 535.9 eV was derived from the interference peak of Na. e peaks at 284.8, 286.0, and 289.8 eV in C 1 s spectra were assigned to the characteristic peaks of C-C, C-O-C, and Na 2 CO 3 . ere was no Si-C bond between the microspheres and the polymer carbon chain, and the microspheres were only filled in the polymer continuum in the form of physical filling. In addition, XPS spectra further confirmed the EDS results. Figure 9 shows the SEM images of C14 samples with different additions of C14 catalyst. C14-1% sample produced foaming, and there were small block-like and rod-shaped crystals stacked on the surface of the porous matrix, but no microspheres. ere is a porous structure on the surface of the small block-like crystal, while the rodshaped crystal is composed of lamellar stacking and dense texture. However, there are disorderly distributed rodshaped crystals in the C14-0.75% sample, and small blocklike crystals disappear. With the decrease of catalyst content, the foaming degree of the material decreased, and microspheres gradually appeared in the C14 sample (as shown in Figure 9 (C2)). Besides, the composition of the C14-0.25% sample is similar to that of the C7 sample.

C14 Sample.
e EDS results of the C14-1% sample are shown in Figure 10. Si and O elements were enriched on the surface of the matrix, and a large amount of Na and O elements was contained in the stacking materials on the surface, while there is no obvious concentration area of the C element. e molecular structure of No. 14 catalyst contains primary amine, which acted as a nucleophilic reagent to attack the NCO group in isocyanate to form an intermediate complex, and the complex reacted with active hydrogen in water to release CO 2 [33]. A large number of CO 2 cannot be absorbed by silicate solution in time, and no rigid structure was formed in the system simultaneously. e final sample existed in foam form with the release of CO 2 .

C17-1%
Sample. SEM images of the C17-1% sample are shown in Figure 11, which are similar to those of C7-1%, with microsphere and continuous phase.
ere was no connecting phase on the surface of the ball pit. e surface of the microspheres is smooth and wrinkled, and the diameter is large, even up to 200 um. Besides, the continuous phase contains a large number of micropores.
e EDS results of the C17-1% sample are quite different from those of C7-1% ( Figure 12). C was almost completely concentrated in the microsphere region, while Si and O atoms were concentrated in the continuous phase. e low content of Na is attributed to the large amount of Na precipitated out of the solution. EDS-dot scanning results showed that the microsphere region was an organic phase, the total number of C, N, and O atoms accounted for 99.54%, and the total number of Si and O atoms in region 2 accounted for about 77%. erefore, the microstructure of C17-1% is opposite to that of C7-1%, and polyurethane exists in the form of microspheres.

ermal Stability Analysis.
e samples were characterized by TG-DTA to analyze the thermal stability and composition differences of the three samples, as shown in Figure 13. e TG curves of the C7-1% and C17-1% samples are similar in Figure 13(a), indicating that the two samples had a similar material composition. Comparing  Figures 13(b) and 13(d), the two samples had an obvious exothermic peak from room temperature to 120°C, which is mainly ascribed to the unreacted water in the water glass raw material and the water adsorbed on the material surface. e weight loss of the C7-1% and C17-1% samples is 22% and 20%, respectively. e water content of the C17-1% sample is relatively low, which may be caused by the loss of part of water during drainage. In addition, there are obvious exothermic peaks between 300°C and 350°C, which was due to 0.5% 0.25% 0.75% 1% C7 5.5cm         Advances in Civil Engineering the decomposition of polyurethane into the secondary polymer. e small difference in decomposition temperature may be caused by the different particle sizes of the samples. As shown in Figure 13(c), there were two exothermic peaks of the C14-1% sample before 120°C, which was attributed to two different forms of water evaporation on the surface of the material and in the pore, and the weight loss of the two parts is 12% and 4%, respectively. e total weight loss of water is 16%, which is the least among the three samples, probably because more water was consumed during the reaction of isocyanate with water to release CO 2 . e decomposition temperature of the C14-1% sample is 336°C, which was much higher than that of the C7-1% and C17-1% samples, which is due to the formation of the polymer with higher thermal stability by strong foaming catalysis.  Figure 12: Element distribution of the C17-1% sample. 1% samples is only 3%-5% when the compressive strength of the material reached a peak value, and the average compressive strengths of the two samples are 2.7 MPa and 4.9 Mpa, respectively. Compared with C7-1%, the compressive strength of C17-1% was reduced by 88.6%, mainly because the polymer phase in the C17-1% sample was stored in the continuous phase composed of inorganic components in the form of microspheres, while the polyurethane continuum was formed in the C7-1% sample, which had strong pressure bearing capacity as a rigid skeleton.

Conclusions
In conclusion, three kinds of PU/WG grouting materials with different curing behaviors were successfully prepared by changing the catalyst, and the structure, thermal stability, and compressive strength of the three samples were compared. e main conclusions are as follows: (1) Carbonate is the main precipitate in the process of drainage. (2) e foaming and drainage behaviors changed the material composition. e normal sample was composed of silicate polymer microspheres, sodium carbonate needle crystal, and polyurethane continuum; there were only Si-O and no Si-C bond in the structure; and the valence state of Si was +4. ere were small pieces and rod-shaped crystals on the porous surface of the foamed sample. In addition, the organic polymer in the drainage sample was embedded in the inorganic component in the form of the microsphere.
(3) e foaming and drainage behaviors had little effect on the thermal stability of the system, while the water content in the structure was different. (4) e normal sample possessed excellent compressive strength, up to 43 MPa. However, the foaming and drainage behaviors led to a significant reduction of the compressive strength, with a reduction of more than 85%.
Data Availability e data are available on request due to privacy or ethical restrictions.

Additional Points
(1) Polyurethane/water glass samples with different curing behaviors were successfully prepared by adjusting catalysts. (2) e structural characteristics of three kinds of polyurethane/water glass composite grouting materials were analyzed by multiple characterization methods. (3) e foaming and drainage behaviors had little influence on the thermal stability of the grouting material, but they greatly reduced the compressive strength of the material.

Conflicts of Interest
e authors declare no conflicts of interest.