Experimental Study and Application of a Novel Foamed Concrete to Yield Airtight Walls in Coal Mines

Airtight walls are vital to prevent spontaneous combustion of residual coal as caused by air leakage. A new type of foamed concrete (FC) was developed to control air leakage. FC specimens of four densities (250, 450, 650, and 850 kg/m) were prepared for use in a series of physical andmechanical tests. A thickener was used to control the FC shrinkage and collapse.-e permeability of the FC decreased approximately exponentially with an increasing density. On the contrary, the compressive strength (σ) and elastic modulus (E) increased in exponential and linear relationship separately with the increase in density, under uniaxial compression conditions. Under triaxial compression, the compressive strength of the FC increased with an increase in confining pressure and appeared slight plastic.-e impact experiment showed that the dynamic compressive strength of the FC appeared to be strain-rate dependent, and it increased with an increase in the strain rate and pressure under a confining pressure. Without a confining pressure, the variation in compressive strength exhibited a slow decrease. Applied FC resulted in a 5MPa 28-day compressive strength of the airtight wall with no remaining fissures and with air leakage suppression to the gob.


Introduction
e spontaneous combustion of residual coal in gob as triggered by air leakage has posed major threats to the safety of miners and mine facilities [1][2][3].Approximately 56% of mines have been threatened by spontaneous combustion in China's state-owned collieries, which account for 90-94% of Chinese coal mines [4][5][6].In the Shenfu-Dongsheng coalfield, Shaanxi, the "U + L"-type ventilation system has been used extensively in the collieries.us, many cross headings were left in the gob during mining process.Building an airtight wall is thought to be a most effective way to control air leakage [7,8], that is caused by the cross headings that remain in the gob.
Because of limitations of materials and construction techniques, the construction of an airtight wall has been problematic over recent decades.Problems include the following aspects.Earlier airtight walls were built mostly from rigid materials [9], such as bricks and concrete [10].
e airtight walls cracked easily because of the huge surrounding rock stresses.Under such conditions, air would leak and penetrate into the gob by pressure difference and promote oxidation of the residual coal [11].To obtain an improved sealing and to prevent incidences of spontaneous combustion [12,13], new filling materials, such as gel materials, fly ash composite colloids, rigid polyurethane foams, Rocsil foams, Marithan, and Igloneige, have been applied to build airtight walls [1,[14][15][16][17].However, these filling materials have shortcomings.For instance, gel materials have a relatively small compressive strength.Organic filling materials often release toxic gases and significant heat, which promotes coal oxidation [15,17,18].e development of new materials and improved construction techniques is critical for increasing mining depths and the application of new mining technologies.Foamed concrete (FC), as an inexpensive high-quality green material, has raised the attention of many experts and scholars.Since the 1980s, foreign countries with advanced mining safety technologies, such as Poland, Britain, Germany, France, South Africa, Australia, and the United States, have developed and applied various fissure plugging materials [19].Because of their long solidification times, low foam expansion rates, and poor resistance to dynamic pressure, these materials cannot be used extensively in deep coal mines.To overcome these shortcomings, Wen et al. [20] and Tan et al. [21] developed a type of inorganic solidified foamfilling material and analyzed the effects of constituent materials on its basic physical and chemical properties in various experiments.Jambor [22] and Tang [23] probed the effects of pore structure on the FC strength.Kuzielová et al. [24] studied the effect of activated foaming agents on the FC properties.Based on the unique advantages of magnesium phosphate cement, Ma and Chen [25] used sodium bicarbonate as a foaming agent and developed new quicksetting and high-early-strength FC.Tan et al. [26] conducted a series of tests to investigate the influence of high temperature on the FC properties.
Although many experts and scholars have carried out extensive research on FC for use in coal mines, minimal attention has been given to the influence of thickeners, rock pressure, and explosion shock waves on the properties of FC filling materials.Suitable filling equipment for underground coal mine environment has seldom been mentioned in prior research and patents.
In this study, FC samples of 4 densities (250, 450, 650, and 850 kg/m 3 ) were prepared for test.e effects of thickener on the concrete performance, the change in permeability, and compressive strength with density, and the variation rules in failure patterns with confining pressures and impact loading are presented.A set of suitable filling systems was also developed and improved.Its efficiency was tested at working face 31404 of the Halagou coal mine in 2016.

Mixture Proportions.
No standard method exists for proportioning FC. e orthogonal experiment design (OED) method was used to select the optimum mix proportions [27][28][29][30].en, the analysis of variance (ANOVA) was performed to estimate the relative significance of each factor in terms of percentage contribution to overall quality of FC.Consequently, the W/C ratio and foam volume played more significant roles in the FC in terms of dry density, compressive strength, tensile strength, and so on.Moreover, the optimal mix proportions could also be determined.Accordingly, we designed the FC specimen with different target densities according to the W/C ratio.Correspondingly, the constituent materials were mixed according to the above results strictly.Here, we take 1 m 3 FC as an example, and the method can be described as follows: where ρ d is the target density of the FC specimen (kg/m 3 ); δ is the empirical coefficient, 1.2 is suitable for the current constituent material ratio; and m s is the mass of solid constituent materials required per cubic meter of FC (kg/m 3 ).e foam expansion volume can be obtained from the following equation [26]: where V f and V cs are the volumes of foam expansion and cement slurry, respectively (m 3 ), and k is a coefficient related to the foam quality, which varies from 1.1 to 1.3.According to the previous experiments, the foam agent was stable, so 1.18 was used.m c , m w , and m fa are mass of cement, water, and fly ash per cubic meter, respectively (kg); ρ c , ρ w , and ρ fa are the densities of cement, water, and fly ash (kg/m 3 ), with ρ c � 3100 kg/m 3 , ρ w � 1000 kg/m 3 , and ρ fa � 2600 kg/m 3 , respectively.e water-cement ratio (W/C) is 0.5, namely, m w � 0.50 m c .
e mass of foaming agent can be obtained by the following equations [31]: where m foa is the mass of the foaming agent and α is the dilution ratio, α 20. e mass of ber, water-reducing agent, coagulant, and thickener added into the mixtures account for 0.8%, 1.0%, 3.0%, and 1.5% of the cement mass, respectively.e details of the mix constituent proportions with the 4 FC densities of 250, 450, 650, and 850 kg/m 3 are outlined in Table 1.

Specimen Preparation.
As shown in Figure 1, the specimen preparation can be summarized as follows: (i) Mix the foam agent and water in 1 : 40 proportion and store these in a tank for foam generation.Advances in Materials Science and Engineering (vi) Pour the evenly stirred slurry into 50 mm diameter × 100 mm high moulds, cast for 24 h, remove the specimens from the moulds, and seal in a curing box for 28 days before removal for further testing.

Actual Density Calculation of Specimens.
After the specimens were removed out from the moulds, three standard specimens (Ø50 mm × 100 mm) in each density were chosen for actual density calculation.First, the actual dimensions of three specimens are gauged to calculate their volume, and then, these specimens were put into the oven for 24 h at 65 °C.After 24 h, the baking temperature was adjusted to 100 °C until the weight of specimens was constant.en, the mass of these specimens were measured in ambient temperature.Ultimately, densities of the specimens can be obtained by the following expression: where M a , V a , and ρ a are the actual mass, volume, and density of the specimen, respectively.

Experimental Apparatus.
Specimen preparation and experiments on the physical and mechanical properties were carried out by the following test apparatus: (1) NJ-160B cement blender with an autocontrol rotating speed of 0∼120 r/min (Kexi Instrument Equipment Co., Ltd.Hebei, China).(2) Servo-controlled testing system (RMT-150B) is manufactured by the Institute of Rock and Soil Mechanics, the Chinese Academy of Sciences.is system can be used to conduct many types of tests, such as uniaxial compression, triaxial compression, tension, and fatigue tests [26,32,33].Its maximum load is 1000 kN and the maximum confining pressure is 40 MPa.Using a self-designed testing container and the air supply system, the system can also be used to carry out the permeability test, and the maximum of gas injection pressure can reach 12 MPa.(3) e Split Hopkinson pressure bar (SHPB) apparatus consists of an emitting cavity, a cone-shaped striker, an input bar, a transmitter bar, an absorption bar, and a data acquisition system [34,35].e lengths and diameters of the t input and output bars are 2500 m and 50 mm, respectively.e bars and striker are made of high-strength 40 Cr steel with a density of 7850 kg/m 3 and Young's modulus of 210 GPa. e wave propagation velocity c 0 � ��� E/ρ  � 5172 m/s.

Experimental Methodology.
Compressive strength is the basic physical property of FC, which can be defined as the ability of the concrete specimen to sustain the axial load.In this study, the 28-day compressive strength of FC specimens of densities 250, 450, 650, and 850 kg/m 3 was conducted in the RMT-150B machine according to the ASTM standards [36,37].To ensure the accuracy of the test, three specimens were prepared for compression at each density.To clarify the relationship between the uniaxial compressive strength and density quantitatively, the compressive strength was expressed as where σ is the compressive strength (MPa), F is the maximum failure load, and A is the compressive area of the specimen (mm 2 ).Triaxial compressive tests of samples of two FC densities (250 and 450 kg/m 3 ) under different confining pressures (0.00, 0.25, 0.50, and 1.00 MPa) were also conducted in the RMT-150B machine.
e FC permeability was determined in the servopermeability test device, which was designed based on an RMT machine.e maximum gas injection pressure can reach 12 MPa.e FC permeability was measured under the steady-state method.Darcy's equation was used to estimate the permeability of FC according to the experimental results [38]: where Q is the gas flow rate, p 0 is the atmospheric pressure, μ is the gas viscosity, L is the sample length, and p 1 and p 2 are the injection and outlet pressures, respectively.During the experiment, p 1 , p 2 , and Q were recorded when the gas was at steady state.In this study, the N 2 permeability of FC was calculated at given set of confining pressures at 1.0 MPa fixed injection pressure.
e confinement pressures ranged from 0 to 3.5 MPa and varied with the interval of 0.25 MPa.A similar series of tests were conducted on the FC specimens with the other three densities, and finally, the permeability as function of confining pressure and density was estimated.
e dynamic compressive properties of FC were studied experimentally by using SHPB.Twenty specimens with a density of 850 kg/m 3 were prepared and tested under various confining pressures (0, 2.5, 5.0, and 7.5 MPa).In each series of tests with the same confining pressure, the impact loading was increased from 0.3 MPa to 0.7 MPa.Prior to the experiment, specimens were sandwiched between the input and transmitter bars.
e cone-shaped striker was ejected from the emitting cavity under pressurized nitrogen to strike the input bar and generate an incident pulse.Because of the impedance mismatch of the specimen and the input bar, only part of the incident pulse, as a transmitted pulse, was transmitted through the specimens into the transmitted bar, and the other part was reflected into the input bar.Strain gauges on the input bar and transmitted bar were used to record the incident, reflected, and transmitted pulses.Pulses collected by the data acquisition system were used to generate dynamic stress-strain curves.
e response that was obtained by using SHPB was voltage signals, which need to be calculated according to a three-wave formula to obtain the stress-strain relation.

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Results and Discussion
e results of actual density calculation of specimens of four densities (250, 450, 650, and 850 kg/m 3 ) are shown in Table 2.

E ect of
ickener on the Performance of Foamed Concrete.In common FC, only cement is used directly as a cementitious material, and it accounts for 70%-80% of the mass of the FC.During the lling of the airtight wall, the lower cohesive performance of the cement yields a maximum lling height of the airtight wall of 30 cm-50 cm.Once the lling height exceeds 50 cm, the bubble is easy to burst subject to the slurry gravity, which leads to a large shrinkage in the upper part of the airtight wall.us, the homogeneity and construction quality of the airtight wall cannot be guaranteed.
To overcome the shortcomings mentioned above, a new special thickener, which was composed of silicon powder, bentonite, and xanthan gum in a mass ratio of 60 : 70 : 1, was mixed in the FC. e specimen shrinkage was reduced signi cantly and it did not collapse, after thickener addition.e performance of the special thickener is shown in Figure 3.    Advances in Materials Science and Engineering 3.2.Permeability Experiment of Foam Cement.Figure 4 presents the relationship between permeability and confining pressure for FC at 4 different densities.e variation in the red ball symbols in Figure 4 indicates that the FC permeability decreased to varying degrees with an increase in confining pressure and density.e larger buried depth and airtight wall density yield a better tightness.
e relationship between the permeability, density, and confining pressure can be expressed as k � 1.32 × 10 −7 + 1.10 × 10 −5 • e ((−(C/54.06))−(ρ/536.14)), (8) where k decreased gradually with an increase in confining pressure and density when 0 ≤ C ≤ 4 MPa and 200 ≤ ρ ≤ 850 kg/m 3 .Combined with the field situation and experience, the FC permeability with a density of 250 kg/m 3 also reached 6.1 × 10 −6 m/s, which was sufficient to suppress the oxidation of residual coal by preventing air from entering the mined area.

Uniaxial Compressive Test of Foamed Concrete.
Figure 5 shows the typical failure patterns of FC specimens with four densities (250, 450, 650, and 850 kg/m 3 ).e FC at different densities presented different failure patterns, which can be grouped into the following three categories: (1) Specimens were damaged from top to bottom with a continuing downward movement of the indenter, and no penetrating fractures were found in the specimen surface and interior.ese failure characteristics were attributed mainly to a lower compressive strength.With fiber addition, the tensile strength approximated the compressive strength.e large porosity of the low-density FC created a relatively large inward deformation space, which enabled the specimen to maintain a better integrity after large deformation, as shown in Figure 5 (3) Figures 5(c) and 5(d) show that the main inclined crack throughout the specimen appeared on the specimen surface, along with several small shear failures that distribute around the main failure.As the FC density increased, the fractional small shear failures decreased gradually.
Because the three duplicate tests of each density gave consistent results, the average values of the three tests were used and are plotted in Figure 6(a).As mentioned in previous papers, the compressive strength was a function of porosity or density.Its mathematical expressions could be depicted as linear, exponential, power, and logarithmic.After the test data analysis, we found that the compressive strength increased approximately exponentially with an increase in density [26], and R 2 reached 0.994.
e expression is as given below: e relationship between the elastic modulus and density is presented in Figure 6(b).And we fitted the average test values (R 2 � 0.997) and obtained the following expression: where E is the elastic modulus.6 Advances in Materials Science and Engineering

Triaxial Compressive Test of Foamed Concrete.
Figure 7 shows that under uniaxial compression, the stress showed an increasing trend with increasing strain in an approximately linear relationship until the stress equaled the destroyed loading.en, the stress decreased slightly and was maintained at a steady level with a continuous increasing strain.is occurred because along with the collapse of partial framework, the internal stress was transferred to another partial framework of the FC.erefore, the FC was a type of ideal elastoplastic material.However, under triaxial compression, because of the in uence of the conning pressure on radial deformation, the FC density would increase when the volume was compressed to a critical value.Consequently, the compressive and residual strengths increased with an increase in density.us, the stress-strain curves transformed gradually from a softening to a hardening strain with an increase in con ning pressure.

Foam Cement Impact Resistance.
e failure patterns of the specimens under di erent con ning pressures are compared in Figure 8.With no con ning pressure, specimens were crushed into small pieces with an increase in impact pressure, as shown in Figure 8(a).
is occurred because ssures with a dip angle larger than the internal friction angle could not endure any loading [39].However, because of the increase in con ning pressure, the loading capacity of the ssures with the steep angle increased rapidly, which improved the specimen compressive strength of the specimen and deformation performance and prevented shear-slip damage.Under the same impact loading and with an increase in con ning pressure, the damaged section of the specimen was reduced gradually.e specimen remained intact after an impact of 0.3 MPa and 0.7 MPa, whereas the con ning pressure reached 2.5 MPa.
e observation demonstrates that the compressive strength increased with an increase in con ning pressure.e con ning pressure 8 Advances in Materials Science and Engineering that constrains the formation and expansion of cracks in concrete may be a major reason for the larger compressive strength.e variation in compressive strength versus the strain rate (R i ) for FC specimens with a density of 850 kg/m 3 is shown Figure 9. e dynamic compressive strength of the FC is clearly strain-rate dependent.Under no con ning pressure, the compressive strength of the FC decreased with an increase in the strain rate.However, under a con ning pressure, the compressive strength increased with an increase in the strain rate.Under a similar strain rate, the compressive strength of the FC under the high con ning pressure was larger than that of the low con ning pressure.
Figure 10 shows the stress-strain curves of the FC with a density of 850 kg/m 3 under impact loading with di erent con ning pressures.
e compressive strength of the FC increased with an increase in con ning pressures.e stressstrain curves of the FC have similar features at di erent con ning pressures.During the initial loading stage, the load was sustained mainly by the specimen frameworks, and the stress increased almost linearly to an elastic limit with an increasing strain, in accordance with Hook's law.e elastic limit corresponds to the rst in ection point of the stressstrain curves.Beyond the elastic limit, the stress increased continuously, but the increasing rate decreased gradually, and sometimes the stress-strain curves showed oscillatory features, which is related closely to the collapse of the internal FC framework.During this period, the specimen could still bear the load to a certain degree, and the partial energy of the incident stress wave converted to a plastic deformation of the specimen.
e pore structure of the FC was destroyed gradually, and the bearing capacity of the specimen decreased gradually until the FC framework was destroyed completely.In summary, the con ning pressure increased the plastic deformation of FC.

Advances in Materials Science and Engineering
As shown in Figure 11, under di erent con ning pressures, the curves of the FC compressive strength versus pressures showed similar features compared with Figure 9.Under con ning pressure conditions, the compressive strength increased with an increase in pressure.A higher con ning pressure yielded a larger compressive strength.In contrast, the compressive strength showed a downward trend without con ning pressure.e variations in compressive strength indicated that the con ning pressure promoted the compressive strength, which prevented explosions caused by spontaneous combustion.
e results obtained could provide suggestions for the selection of foamed concrete density.

Engineering Application
4.1.Rapid-Filling System.As mentioned above, an airtight wall is a key factor to prevent the spontaneous combustion of residual coal in gob.Normally, airtight walls are built using bricks and loess [13,40].As shown in Figure 12(a), rst, two external walls were built using the bricks, and then, the loess was lled in the interior space between the two brick walls.It would take four days to build an airtight wall with a width of 5.6 m, a height of 6 m, and a thickness of 5 m by using the traditional construction techniques.
To shorten the construction period, a new type of rapid construction technique was used at no. 31404 working face in the Halagou Coal Mine, in the Shengdong mining area.Figure 12(b) shows that a new type of airtight wall was built by the cast-in-place concrete formwork technology.First, we dug a 500 mm groove in the roof, oor, and on both sides of the tunnel.
en, two walls were built using concrete formworks to enclose a con ned zone.Subsequently, an injection pipe was connected with a slurry pump and a grouting machine to complete the lling process.
Figure 13 shows the lling system, which was composed mainly of a screw conveyor, ZMJ-G-type slurrymaking machine, and a pneumatic pump.e ZMJ-G-type slurry-making machine was improved based on the ZMJ-F type.e screw conveyor could deliver and mix the cement and y ash in certain proportion evenly, which alleviates the labor intensity of the workers and improves the eciency.
e eld tests reveal that the average FC productivity reaches 8-10 m 3 /min, achieving long-distance transportation and continuous lling at a large capacity.
e cost of the lling material can be controlled to within ¥350-500 yuan/m 3 .

4.2.
e Filling E ect.Figures 14(a A new type of filling materials, with advantages of a higher strength, an improved tightness, and an excellent antiexplosive performance, is suitable for wide promotion and application.

Conclusions
Tests were carried out to study the properties of FC. e following conclusions were obtained: (1) e thickener could enhance the cohesive performance of the cement, prevent foam from breaking, and resist FC shrinkage during moulding.(2) e FC permeability decreased to varying degrees with an increase in pressure and density.(3) e experimental results on the mechanical properties of the FC showed that the compressive strength increased approximately the exponential trend with an increase in density and the elastic modulus increased linearly with an increase in density.Because of the constraint of confining pressure on the radial deformation, the compressive strength of FC increased with an increase in confining pressure and the stress-strain curves showed plastic features.(4) e impact experiments showed that the compressive strength of the FC was clearly strain-rate dependent.e dynamic compressive strength increased with an increase in the strain rate and pressure under a confining pressure.Without the confining pressure, the variation in compressive strength decreased significantly.
(5) e engineering practice showed that the new filling system could shorten the construction period, alleviate worker labor intensity, and achieve an FC productivity of 8-10 m 3 /min.e 28-day compressive strength of the airtight wall reached 5.0 MPa.

2
Advances in Materials Science and Engineeringwhere m fo and ρ fo are the foam mass and density, respectively.

(
ii) Add measured ordinary Portland cement, water, y ash, ber, and water-reducing agent to a blender and stir for 5 min at the speed of 60 r/min.(iii) During stirring, activate the air compressor to an accumulated pressure of 3.5 MPa.Add the foaming agent solution to the foam tank, open the ball valve, and allow solution to ow through the foam generator, as shown in Figure 2. (iv) Measure the foam volume using the graduated cylinder, and add foam into the cement slurry twice.Add a small quantity of foam (accounting for 1/4-1/5 of the foam quantity) to ensure the slurry is easy to dilute, and then add the remaining foam and change the stirring speed to 120 r/min.(v) Maintain stirring for 2-3 min, add the coagulant and thickener rapidly, and continue stirring for 30-40 s.

Figure 3 :
Figure 3: Contrast of the foam concrete specimen before (a) and after (b) the new special thickener was added.
f i n i n g p r e s s u r e ( M P a )

Figure 4 :
Figure 4: Relationship between the permeability coe cient of foamed concrete and con ning pressures.
(a). (2) As shown in Figure 5(b), under a quasistatic loading, several visible splitting cracks propagated parallel to the compressive loading, on the specimen surface.Only one penetrated the specimen from top to bottom.ese failure characteristics were mainly caused by a lower tensile strength.

Figure 12 :Figure 11 :
Figure 12: Schematic diagram of (a) traditional construction technique and (b) the new type of construction technique.
20 H 40 N 2 O 3 ) and potassium monoalkyl phosphate (C 12 H 25 OPO 3 K 2 ) in a 7 : 3 ratio.e foam stabilizers were composed of triethanolamine (C 6 H 15 NO 3 ) and oleic acid (C 18 H 34 O 2 ) in a 1 : 1 ratio.(5) Fiber was modified polypropylene from the Yonggu Engineering Fiber Equipment Co., Ltd, Jiangsu, (4)e foaming agent was generated by mixing the foam activators and foam stabilizers in a 1 : 1 mass ratio.efoam activators consisted of lauryl amidopropyl betaine (C

Table 1 :
Mix proportions for di erent densities of foamed concrete.

Table 2 :
Comparison of target density and actual density.