Geopolymer-based modification of blasting sealing materials and optimization of blasting block size in coal seams of open pit mines

Xiaohua Din ,Zhonchen Ao,c, *,Wei Zhou,c ,Hao Qin ,Zhonao Yan ,Wen An,e ,Xiaoshuan Li,Honlin Liu

a School of Mines,China University of Mining and Technology,Xuzhou 221116,China

b High Tech Research Center for Open Pit Mines,China University of Mining and Technology,Xuzhou 221116,China

c State Key Laboratory of Coal Resources and Safe Mining,China University of Mining and Technology,Xuzhou 221116,China

d College of Environmental Science and Engineering,Liaoning Technical University,Fuxin 123000,China

e Wuhan Marine Machinery Plant Co.Ltd,Wuhan 430010,China

f School of Civil Engineering,Shaoxing University of Arts and Sciences,Shaoxing 312010,China

g School of Geology and Mining Engineering,Xinjiang University,Urumqi 830046,China

Keywords:

ABSTRACT This research proposes the utilization of a geopolymer-based blasting sealing material to improve the profitability of coal sales and reduce the rate of coal fragmentation during blasting in open pit mines.The study first focused on optimizing the strength of the sealant material and reducing curing time.This was achieved by regulating the slag doping and sodium silicate solution modulus.The findings demonstrated that increasing slag content and improving the material resulted in an early rise in strength while increasing the modulus of the sodium silicate solution extended the curing time.The slag doping level was fixed at 80 g,and the sodium silicate solution modulus was set at 1.5.To achieve a strength of 3.12 MPa,the water/gel ratio was set at 0.5.The initial setting time was determined to be 33 min,meeting the required field test duration.Secondly,the strength requirements for field implementation were assessed by simulating the action time and force destruction process of the sealing material during blasting using ANSYS/LS-DYNA software.The results indicated that the modified material meets these requirements.Finally,the Shengli Open Pit Coal Mine served as the site for the field test.It was observed that the hole-sealing material’s hydration reaction created a laminated and flocculated gel inside it.This enhanced the density of the modified material.Additionally,the pregelatinized starch,functioning as an organic binder,filled the gaps between the gels,enhancing the cohesion and bonding coefficient of the material.Upon analyzing the post-blasting shooting effect diagram using the Split-Desktop software,it was determined that the utilization of the modified blast hole plugging material resulted in a decrease in the rate of coal fragmentation from 33.2% to 21.1%.This reduction exhibited a minimal error of 1.63% when compared to the field measurement,thereby providing further confirmation of the exceptional plugging capabilities of the modified material.This study significantly contributes to establishing a solid theoretical basis for enhancing the blasting efficiency of open pit mines and,in turn,enhancing their economic advantages.

1.Introduction

Open-pit coal mining commonly employs loose blasting as a predominant method,with perforation blasting constituting a significant share of the overall mining costs.The efficacy of blasting operations is influenced by numerous variables,among which the nature of sealing materials stands out as a crucial element in optimizing blasting outcomes.Currently,the prevalent practice in open-pit coal mines involves using drilling chips generated during perforation operations as sealing materials in the coal seam blasting process.The filling length of the large-diameter blast holes typically ranges from 5 to 8 m.The actual filling length on-site is usually determined by the site personnel based on their experience,and the filling operation is semi-mechanical and semimanual.Once the hole is sealed,the sealing materials must withstand the dynamic load from the explosion.In instances where the hole remains unsealed or the sealing effect is suboptimal,the shockwave produced by the explosion propagates along the vulnerable surfaces.This not only damages the hole but also causes the punching phenomenon,thus failing to achieve the desired blasting effect.Furthermore,incomplete reactions during blasting can result in the release of hazardous gases and particulate matter.As documented by Wang et al.[1] and Luan et al.[2],these emissions not only pollute the mining environment but also pose risks to operators and adjacent equipment.

The use of traditional pulverized coal as a filler material is not without its limitations.In an attempt to reduce the occurrence of punch holes,traditional coal seam blasting methods often choose to increase the filling length and reduce the amount of field charge.However,this approach leads to the downward movement of the charge center,resulting in the unsatisfactory upper step blasting effect.Recognizing the challenges associated with coal seam blasting,numerous scholars have undertaken extensive investigations.For instance,Mabroum et al.[3] and Nikolić et al.[4] explored the possibility of using marl,an aluminosilicate,in the manufacturing of alkali-activated geopolymers.Their findings suggest that marl holds promise as a feasible material for producing geopolymers.Ridtirud et al.[5]and Guo et al.[6]focused on the reduction in size of fly ash geopolymers.The findings indicated a strong correlation between the shrinkage of geopolymers and the ratio of liquid ash,as well as the temperature during the curing process.The impact of cured geopolymers under sealed and unsealed conditions on their mechanical strength was examined in the literature[7-9].It was observed that retaining water during the curing stage could enhance the strength of geopolymers.Wang et al.[10]examined the fracture mechanics of sandstone formations during rock breaking.They employed a combination of field experiments and numerical simulations using ANSYS/LS-DYNA software.The results demonstrated that employing the mud as a sealing agent for boreholes can significantly enhance blasting effectiveness,minimize explosive gas wastage,and extend the duration of its impact.Yang et al.[11]and Jang et al.[12]introduced a novel and environmentally friendly hole plug to enhance the effectiveness of mine blasting through the implementation of multi-stage buffer compression blasting within the blast hole.Experimental results demonstrated a significant extension,ranging from 14% to 63%,in the duration of radial displacement within the hole when utilizing the newly developed sealing materials.Ding et al.[13] studied the effects of various plugging materials,revealing that rubber plugs and angular stone chips of appropriate size (3-7 mm) placed within the blast hole can effectively reduce flying stones and minimize energy loss.

In a comprehensive exploration of bench blasting scenarios,Zhang[14]employed the LS-DYNA finite element program to simulate the effects of dual-millisecond hole blasting.It was found that peak particle velocity and energy dissipation were observed in close proximity to the blasting source,providing crucial insights for enhancing the fundamental understanding necessary to safeguard structures located near blasting sources.Similarly,Gan et al.[15] determined the propagation of blasting waves and the multi-cavity response,validated the accuracy of the simulation,and confirmed that the stability of the cavity remained unaffected by the bench blasting test.Li et al.[16] and Shen et al.[17] performed numerical simulations and experiments on coke rock bench blasting using LS-DYNA software,optimizing the initial blasting parameters to enhance explosive efficiency and minimize blasting dust production.Dehghani et al.[18] discovered that emulsion explosions in wet holes exhibited a more consistent distribution of fragment sizes compared to ANFO explosions.Conversely,ANFO explosions in dry holes produced fewer large fragments.These results underscore the importance of optimizing pre-cracking blast hole network parameters to achieve efficient blasting operations.Liu and Li [19] investigated the transmission of strain waves within composite coal blocks and explored the impact of fragmentation on coal.Their study found a close correspondence between the size of the crushing zone in composite coal blocks and numerical calculations,with the observed distribution of cracks in laboratory tests closely resembling the calculated outcomes.Zaid and Shah[20]explored rock tunnel performance under blast loading using finite element analysis.Employing the coupled Euler-Lagrange method for advanced modeling,they simulated blast internal loads by modeling TNT and air in tunnels,revealing that shallow-buried tunnels incurred more severe damage compared to highly buried tunnels [21].The existing body of research lacks a comprehensive examination of simulating the response of sealing materials to blasting forces;instead,it primarily concentrates on enhancing the mechanical properties of these materials.In the domain of simulation research,the predominant focus has been on exploring the relationship between explosive charge participation and the explosion effect,with limited attention given to understanding the influence of the sealing materials on the blasting effect.The predominant focus has been either on studying material mechanical properties or simulating blasting force effects,often leaning towards theoretical investigations.There is a scarcity of research that bridges the gap between theoretical considerations and practical field applications,specifically in terms of utilizing original drilling materials to fabricate sealing materials with specific strength characteristics aligned with on-site construction requirements.These gaps underscore the need for a more comprehensive exploration of the dynamic interplay between sealing materials and blasting forces,moving beyond theoretical frameworks to incorporate practical field applications and address the inherent complexities of real-world construction scenarios.

In this research,the mechanical properties of geopolymer coal seam blasting sealing materials were studied through laboratory tests,and the influence of hole-sealing materials on the blasting effect was verified using numerical simulation.Finally,holesealing materials were applied in the field to realize the optimization of the blasting block,reduce the coal crushing rate,minimize the dust generated by blasting,and significantly improve production efficiency.Numerical simulation and field tests verify that the sealing material has a positive effect on the blasting effect.

2.Experiments and methods

This investigation revolves around the analysis of sealing materials utilized in coal seam blasting,with a particular focus on the development,design,and field application of modified materials to optimize the efficiency of coal seam blasting.A comprehensive approach was employed,encompassing laboratory experiments,computer simulations,deductive theory,and on-site testing to thoroughly investigate the setting time and compressive strength of the sealing material.Material properties were examined across various ingredient ratios,ensuring originality,and minimizing similarities with existing literature.The mechanical failure process of sealing materials during blasting was simulated using ANSYS/LS-DYNA software.Drawing on rock blasting theory and static theory,three distinct failure mechanisms related to coal seam blasting in open pit mines were analyzed,and the characteristics of regional blasting were identified.Subsequently,the damage radius of the crushing area,fracture area,and elastic vibration area was calculated employing impact force theory.Finally,the geopolymer with the best proportions was applied on-site at Shengli Open Pit Coal Mine to assess the blasting performance of the sealing material in a real-world setting.

2.1.Testing raw materials

The raw materials used in the research for preparing blasting sealing materials include slag,pulverized coal,alkali activator,water,and pregelatinized starch.In the experiment,S95 grade granulated blast furnace slag,a commonly used industrial waste,was chosen.Pulverized coal was selected from the coal sample of 5# coal in the south of the 855 level of the Shengli Open Pit Coal Mine.The alkali activator employed comprised a combination of sodium silicate solution and solid sodium hydroxide,serving as a catalyst to enhance the hydration reaction of slag and improve the efficiency of interaction between slag and water.Compared with ordinary starch,pregelatinized starch serves as an organic binder that enhances bonding properties,and water absorption,and reduces the difficulty of its dissolution in water.The specific combination of these materials is detailed in Table 1,providing a comprehensive overview of the raw materials used in the experiment.

Table 1 Testing raw material.

2.2.Preparation and maintenance of modified material

(1) Preparation stage: A tray was used to evenly distribute dry pulverized coal and S95 grade slag powder.The particle size of the slag powder was measured using a laser particle size analyzer(Fig.1)to ensure it remained below 0.3 mm,while the particle size of aggregate (pulverized coal) varied from 0.2 mm to 1 mm.The prepared sodium hydroxide solution was allowed to stand for 30 min before being used.

Fig.1.Specimen preparation process and performance test.

Firstly,pulverized coal,slag,and pregelatinized starch were weighed according to the pre-designed ratio using a balance(Fig.1)and then poured into the Cement and Sand Mixer)and stirred for about 1 min.Once the dry pulverized coal and S95 grade slag powder were thoroughly blended,specific quantities of sodium silicate solution,aqueous sodium hydroxide solution,and water were added to the mixer.The mixture was then stirred at both low and high speeds for a duration of 60 s each.The specimen size of ϕ50 mm×100 mm type cylindrical mold was selected,and lubricating oil was applied to the inner wall and base of the cleaned abrasive tool to make the demolding process smoother.The mold was placed on the shaker(Fig.1),and the stirred material was added twice,keeping the height of the slurry higher than that of the mold by about 2 mm,while carrying out vibration operation to exclude the gas inside the slurry.

(2) Maintenance stage: The specimens were grouped according to the concentration ratio and put into the constant temperature maintenance box (Fig.1) for maintenance operation and labelled with label paper.The tests on mechanical properties,setting time,and microscopic morphology of the materials were conducted after the maintenance was completed.

2.3.Design of indoor experimental program for modified materials

2.3.1.Design of uniaxial compressive scheme for specimens

(1) Uniaxial compressive strength test protocol for modified materials

The instrumentation used for the test was a WDW-300 Electronic Universal Testing Machine(Table 2).The tester had a maximum load of 300 kN,with a controlled relative error size of approximately 0.5.The initial testing load was set at 300 N,and the specimens used for uniaxial compression had dimensions of ϕ50 mm × 100 mm.The compression process was regulated through displacement loading,with a speed set at 0.005 mm/s.The loading was completed when the overall percentage of specimen fracture reached about 70%.The loading procedure was halted once the universal testing machine reached its maximum load of 300 kN.Subsequently,the stress-strain curve was obtained,and the material’s uniaxial compressive strength was determined through calculation.

Table 2 Experimental instrument and equipment.

(2) Slag dosing on compressive strength scheme design

Based on the principle of the controlled variable method,the sodium silicate solution was set at a modulus value of 3.2.With 200 g of pulverized coal,60 g of activator,and 40 g of water,the amount of slag was set as variables.Specifically,six sets of tests were conducted with slag amounts set at 40 g (A1),50 g (A2),60 g (A3),70 g (A4),80 g (A5),and 90 g (A6),respectively.

(3) Design of sodium silicate solution modulus on compressive strength scheme

The group with the highest matching ratio of compressive strength was selected for subsequent testing.This was done to examine the impact of different sodium silicate solution moduli,specifically at a modulus of 1.0 (M1),1.2 (M2),1.5 (M3),and 2(M4),on the compressive strength of the modified materials.

2.3.2.Determination of setting time

The blasting operation in the open pit poses a high requirement for safety and efficiency.Accordingly,the initial setting time of the geopolymer materials needed to be controlled within 0.5 h.The instrument used to test the sealing materials specimens was the JXT/T729 Vicator (Table 2).

The initial setting test needle was first installed and tightened.Subsequently,the prepared specimen was poured into the mold,and the sliding rod switch was released to allow the test needle to make complete contact with the glass plate.Following this,the pointer was adjusted to align with 70 mm.With the sliding rod raised and the locking screw tightened,the test needle was wiped with a clean cotton cloth.The position of the test mold was then adjusted to ensure that the test needle was maintained at a 10 mm distance from the inner wall of the test while simultaneously contacting the surface of the specimen.When the test needle sunk about 4 mm from the plate,the specimen reached the initial setting state.

2.3.3.Microscopic morphology observation

The test was conducted using the Scanning Electron Microscope Hitachi Regulus 8100(Table 2)to observe the microstructure of the modified material and indirectly reflect material porosity.The required length,thickness,and width of the testing specimen block sample were limited to more than 1 cm,and the sample mass was not to exceed 200 g.Different groups were established using pulverized coal-infused specimens.To examine the microstructure and porosity size of geopolymer-modified materials,a control group was formed using pulverized coal-infused specimens,while the test group consisted of the modified materials.This experimental setup was designed to ensure controlled variables.The organic binder used in this test was pregelatinized starch with mass fractions of 1%,2%,and 5%,respectively,and its effect on the porosity of the geopolymer-modified materials was also investigated.

2.4.Numerical simulation of coal seam blasting

To establish a theoretical framework for coal seam blasting,the stress wave propagation process and the resulting damage state of coal seam blasting were simulated using LS-DYNA software.The sealing materials were modified to enhance the accuracy of the simulation.A comparison was made between the numerical simulation results and the findings from field tests to validate the model’s credibility.

The model parameters were established based on the constraints of bench blasting at the open-pit coal seam mine.This process involved setting the parameters for both the blast hole and sealing material.Specifically,the length of the blast hole explosives was set to 10 m,and the plugging length was set to 6 m.The model was built using the ANSYS/LS-DYNA software,with the manager overseeing the model calculation and LS-Prepost software handling the pre-and post-processing.The overall model was constructed from bottom to top for the explosives,hole-sealing materials,and ideal air model.The explosives parameters were based on the actual parameters of the mine,utilizing the HIGH EXPLOSIVE BURE material model to simulate the explosion process.The bursting pressure was set at 0.18 MPa,and the bursting speed was established at 4500 m/s,ensuring that the explosives released energy to conform to the JWL equation.A,B,R1,R2,ω are constants,and the detailed parameters are shown in Table 3.

Table 3 Explosive parameters.

The RHT model was utilized to represent the characteristics of the coal rock medium.A comprehensive dataset dealing with the parameters of the coal rock is provided in Table 4.

Table 4 Coal rock parameters.

As for the hole network parameters are shown in Table 5,the actual data from the site were used as the reference standard.The triangular hole arrangement in the local stress propagation can be extended to the steps of a large range of stress changes.Due to the rapid diffusion of stress waves,micro-differential blasting was employed for coal seam blasting.

Table 5 Hole network parameters.

2.5.Design of field test

2.5.1.Site construction plan

The modified sealing materials were applied to the 870 coal seam of Shengli Open Pit Coal Mine.Considering the characteristics of the coal seam within the open pit,a triangular arrangement was employed to position the blast holes,and ammonium-based explosives were selected as the preferred option.A continuous charge structure was applied in the field test.The drilling inclination was set to 90°,that is vertical drilling.The safety distance of vibra-tion is determined as 55 m,with a maximum charge of 105 g in a single section.

2.5.2.Analysis of fragmented rock blocks

Split-Desktop is a specialized software designed for determining the particle size of fragmented materials.This powerful tool utilizes advanced image processing algorithms to analyze the size and shape of each fragment.It computes a range of block distribution data by processing multiple digital images.The block picture,based on image recognition,necessitates the presence of one or more references within the image.Simultaneously,the image must be pre-cropped in areas devoid of blasted and fragmented rock or coal blocks,with pixels reduced to a reasonable value according to the specified resolution limit section before processing.

3.Results and discussion

3.1.Indoor test results of modified materials

3.1.1.Analysis of uniaxial compressive strength

(1) Impact of slag additive on compressive strength

With an increase in slag dosing,there was a notable escalation in the difficulty of material mixing.However,upon adjusting the water-glue ratio to approximately 0.5,the mixing process for geopolymer material became more manageable,resulting in shorter specimen forming time and obvious compressive strength.After the test,in conjunction with the principle of improving the economy of the mine blasting process and reducing the punching rate,three groups of slag dosing (A3,A4,and A5) were chosen for further investigation.Uniaxial compression tests were conducted three times for each ratio to obtain the mean value.Fig.2 illustrates the results of the uniaxial compressive strength tests.

Fig.2.Compressive strength of different slag dosing groups.

It was observed that the average strength of the polymer with A3,A4,and A5 slag dosage exhibited a gradual increase from 2.40 MPa to 3.12 MPa over time,with a growth rate of 30%.When compared with that of A3 and A4,the compressive strength of group A5 was the highest.As indicated,with the variables of sodium silicate solution modulus,alkali activator dosing,and water-glue ratio remaining the same,the increase in slag dosing improved the overall strength of the polymer material.Group A5 was found to be more suitable for on-site operation,with a ratio of 200 g pulverized coal,80 g slag,60 g sodium silicate solution,and 40 g water.

(2) Impact of the sodium silicate solution modulus on the compressive strength

In line with the previous investigation,where the A5 served as the designated control experimental group,subsequent experiments were carried out by modifying the modulus of the sodium silicate solution.The modulus of the sodium silicate solution was varied to 1.0(M1),1.2(M2),1.5(M3),and 2(M4)to conduct a comparative analysis.Fig.3 illustrates the results obtained from these experiments.

Fig.3.Compressive strength of specimens with different moduli.

Fig.3 illustrates that as the modulus of the sodium silicate solution decreased from 2.0 to 1.5,the average uniaxial compressive strength of the modified material increased from 3.62 MPa initially to 4.69 MPa.This occurs because,at higher values of sodium silicate solution modulus,the alkalinity of the mud environment is reduced.The increased occupancy of[SiO4]4-inhibits the slag particles from reacting with water,affecting the content of the final hydration product,and making the early strength of the material lower.When the modulus continues to decrease to 1.2 and 1.0,the strength fluctuation range is not obvious,measuring 3.96 and 3.55 MPa,respectively.When the modulus is lower than a certain critical value,although the overall alkaline environment of the slurry is strong,the amount of free [SiO4]4-decreases,and the double promotion effect of the hydration reaction of the alkali activator is not obvious.Consequently,when the sodium silicate solution’s modulus falls slightly below the critical threshold,it hampers the development of compressive strength in the geopolymer material.As sodium silicate solution modulus decreases from 2.0 down to 1.5,the modified material slurry flow capacity gradually reduces.This is because,with the increase of sodium hydroxide,there are free Na+and OH-in the slurry.This promotes the rate of hydration reaction and improves the alkalinity of the slurry.Accordingly,the viscosity of the slurry gradually increases and the fluidity decreases.However,as the modulus continues to decrease,the more viscous the slurry is,the more difficult the specimen preparation and mixing process will be,which will reduce the actual construction efficiency on site.Therefore,appropriately raising the alkalinity of the slurry can effectively improve the early compressive strength of the modified materials,but too high alkali content is not conducive to specimen preparation.

3.1.2.Study of the setting time

Taking into account the early strength of the modified materials,the A5 group with better properties was selected to continue the setting time measurement.In order to explore the influence of slag dosing and sodium silicate solution modulus on the coagulation time of geopolymer sealing materials,a total of 8 groups of tests were designed,and the variable slag dosing was 60,70,80,and 90 g,respectively.The modulus of sodium silicate solution was set to 1.2,1.5,2.0,and 3.2,respectively,in the conducted tests.The effect of slag dosing on the setting time of the sealing material was assessed by measuring the setting time of the geopolymer sealing material,as shown in Fig.4.Additionally,the effect of modulus of sodium silicate solution on the setting time of the sealing material is shown in Fig.5.

Fig.4.Effect of slag dosing on the setting time of sealing materials.

Fig.5.Effect of modulus of sodium silicate solution on the setting time of sealing materials.

With the increasing amount of slag dosing,the coagulation of the geopolymer modified material is significantly accelerated because OH-inhibits the [Si—O] and [Al—O] produced after the hydration reaction of slag in a strongly alkaline environment from further coagulating with Ca2+and finally forms a gel structure with certain strength structure (CaO—SiO2—nH2O).Consequently,the higher the slag dosing,the faster the coagulation of the slurry.As the modulus decreases,the alkalinity of the solution increases and the gelation effect of the cementitious material becomes better.Accordingly,the setting time is reduced more significantly compared to the original time.However,the increase in modulus makes the specimen preparation process more complicated and does not contribute significantly to the improvement of compressive strength.

3.1.3.Microscopic morphology analysis of modified materials

(1) Microscopic analysis of raw pulverized coal after remodeling and modified geopolymer

Fig.6 shows the microstructure of the raw pulverized coal after remodeling with the microscopic morphology of the geopolymer modified materials.

In Fig.6,lumpy coal chips are visible in the microscopic morphology image of the original pulverized coal after remodeling.In contrast,there are more flocculent C—S—H(CaO—SiO2—nH2O)gels and lamellar gel products and less lumpy coal chips in the image of the modified materials,and a large amount of coal chips have been wrapped by the gel.At the same time,a noticeable difference is observed when comparing the modified materials with the original pulverized coal remodeled specimen.Following the occurrence of the hydration reaction,the modified materials exhibit a denser arrangement between particles.This densification is attributed to a substantial amount of gel filling the voids between the particles within the original material.From a macroscopic perspective,this densification reduces the porosity of the sealing materials,leading to a decrease in the decay rate of stress waves within the material.Furthermore,the gel formed through the process of hydration plays a crucial role in enhancing the strength of the modified material.As the slag admixture content increases,there is a progressive rise in compressive strength,consistent with the observed variations in uniaxial compressive strength outlined in the preceding section.

(2) Microscopic analysis of the ground polymer after adding different mass fractions of pregelatinized starch

The organic binder used in this experiment was pregelatinized starch,and its effect on the porosity of the geopolymer modified materials was investigated by adding pregelatinized starch at different mass fractions (0.5%,1%,2%,and 5%,respectively).The results are shown in Fig.7.

Fig.7.Microstructure under different pregelatinized starch dosing.

It is evident from Fig.7a that pregelatinized starch adheres to the inner surface of the material,forming a doughnut or flocculent binder.Pregelatinized starch contains functional groups like—COOH and —OH,which interact with corresponding functional groups on the geopolymer modified materials’ surface.This bonding process effectively consolidates the particles and enhances the physical properties of the modified materials.Observing Fig.7b,c,and d,it becomes apparent that an increase in the dosage of pregelatinized starch results in a wider distribution of the doughnut or flocculent bonding products formed by starch.These products fill the gel pores of the slag,densifying the material and optimizing its compressive properties.

3.2.Results of numerical simulation

3.2.1.Damage mechanism of coal seam blasting

The simulation results obtained using ANSYS/LS-DYNA software reveal that during explosive detonation,the high-temperature and high-pressure shock waves consistently impact the wall surrounding the blast hole.This action generates an intense stress wave within the coal seam.Due to the significant disparity between the pressure of the stress wave and the dynamic compressive strength of the coal seam,excessive fracturing occurs in the surrounding area of the pillar.As the stress wave advances towards the exposed surface,it generates a tensile stress wave.This leads to the occurrence ‘‘slice off” phenomenon in the coal seam,as its tensile strength is comparatively lower than its compressive strength.The propagation and diffusion process of the stress wave within the coal rock layer is illustrated in Fig.8.

Fig.8.Stress wave propagation process.

The observation in Fig.8 reveals that the coal seam adjacent to the hole wall experiences the initiation of impact compression damage.The impact compression from the blasting wave on the surroundings generates a shock wave that rapidly transforms into a radial compression stress wave propagating outward.This compression effect of the shock wave can lead to the complete crushing of the coal seam around the explosive,forming a compression crushing area.

3.2.2.Influence of hole-sealing materials on hole wall pressure

The simulation of the pressure on the blast hole shows variations in the pressure(P)on the hole wall over time,comparing scenarios with and without sealing materials,as illustrated in Fig.9.

Fig.9.Variation of pore wall pressure with time.

As in Fig.9,under the two operation conditions,the energy released by the explosives exerts similar pressure on the blast hole wall.The sealing extends the pressure and stress action time of explosive gas on the wall of the blast hole,which can significantly improve the expansion of the coal seam and throwing effect.

Based on the specifications outlined in Section 2.4.1,the construction of the LS-DYNA 3D model was carried out,incorporating a grid division.To enhance the software calculation efficiency and simplify the model construction process,a design choice was made to incorporate a total of 5 holes.These holes were positioned with a spacing of 10 m between them and a row spacing of 8 m.LSPP software was used to observe the process of stress cloud diagram,and the force cloud diagram of coal seam blasting,as shown in Fig.10.

Fig.10.Force cloud diagram of coal seam blasting.

Upon analyzing the force cloud diagram of coal seam blasting,it is observed that the explosives generate a stress wave circle with its center at the pillar’s center.This stress wave propagates outward along the diameter of the blast hole,impacting the surrounding coal seam.Due to the relatively lower dynamic compressive strength and tensile strength of the coal seam in comparison to the initial stress wave,damage occurs.Moreover,the blasting gas further contributes to the development and expansion of existing fissures.

3.3.Field application of modified materials

3.3.1.On-site implementation

The field test utilized the optimal laboratory ratio from the previous group M3.The process involved dissolving the weighed sodium hydroxide solution,ensuring complete dissolution through stirring equipment.Subsequently,the sodium hydroxide solution was mixed with the sodium silicate solution by gently pouring it into a container and stirring.The sealed container was then stored in a cool location.

As the field mixing of modified materials is similar to that of ordinary silicate cement,combined with the actual requirements for the operational safety and efficiency of the mine,the manual mixing method was used.After the initial mixing of geopolymer and pulverized coal,the cooled 1.5 modulus sodium silicate solution and water were added,and the slag was mixed twice while the hydration reaction occurred with water to make it fully react.After the material was presented as a slurry,it was filled into the blast hole,and finally,the material was compacted.

To evaluate the effectiveness of modified sealing materials in achieving a plugging effect,the blasting site was divided into two sections:a designated test zone and a control zone.In the control zone,the original material (pulverized coal) was used to seal the plugging operation,whereas in the test zone,modified materials were used for sealing.To avoid interference with the actual blasting operations in the mine,the modified materials were filled at a distance of 2 m from the hole location,reducing the sealing operation time.The schematic diagram of the charging structure is illustrated in Fig.11.

Fig.11.Schematic diagram of the charging structure in the test area.

After the blasting operation was prepared,the 975 flat pan was selected as the filming location,and the filming equipment was used to record the blasting process in the blasting area.Subsequently,the blasting blockage in the test and control areas was analyzed separately,and the punching rate was calculated.As depicted in Fig.12,the control area displays a larger punch hole with greater height,accompanied by a substantial number of coal blocks.In contrast,the test area features a smaller-scale punch hole,demonstrating an overall effective sealing effect.Large-scale punch hole phenomena are notably absent in the test area.Initial observations indicate that the enhanced materials exhibit superior sealing capabilities,thereby extending the duration of their effectiveness within explosive shell holes.Consequently,this contributes to a notable reduction in the rate at which explosive gases escape.

Fig.12.Blasting effect of the burst area.

3.3.2.Analysis of blasting block size

To analyze the size of blasting blocks,the pictures were initially imported into the Split-Desktop software.The helmet was chosen as the reference in the system.The boundary was demarcated following the configuration shown in Fig.13,involving marking the known length of the reference and setting the helmet to the standard diameter of 22 cm.

Fig.13.Split-Desktop blasting block degree image recognition.

After inputting the reference size,post-processing was conducted to obtain the percentage of particle size analysis for the burst pile size in both areas.The results are illustrated in Fig.14.

Fig.14.Cumulative particle size distribution percentages.

In conjunction with the actual production operations in the mine,it is evident that the size of crushed coal is below 25 cm,while lump coal typically falls within the range of 25-120 cm.With the comprehensive crushing station,electric shovel,front loader,and other equipment,as well as the operational needs of the various production processes,and as discerned from Fig.14,the crushed coal rate in the region employing modified material sealing is 21.1%,contrasting with a 33.2% crushed coal rate in the blasting area devoid of modifications.This reflects a notable reduction in the crushed coal rate by 12.1%.In addition,according to the image recognition results,the bulk rate of the test area did not significantly increase compared to the original material,and the lump coal in the test area surpassed that in the control area at the size of 25 cm,which can prove that the excellent sealing material increases the lump coal rate and reduces the crushed coal rate of the blasting of coal seams in mines.

To mitigate statistical errors in the field test block size,a comprehensive analysis was conducted on the blasting block size of the open pit coal seam.This analysis involved integrating scheduling reports from the production system of the open pit mine.The accompanying Fig.15 illustrates the data obtained from the crushing station.

Fig.15.Crushing station lump coal and crushed coal data.

In Fig.15,the test area and the control area of the actual crushing station statistics show that the average rate of coal fragmentation of 25.4% and 37.0%,respectively.The actual rate of coal fragmentation output rate reduced by about 11.6%.

Image recognition results from Split-Desktop reveal a 12.1%reduction in the rate of coal fragmentation.The minimal 0.5% discrepancy between the actual statistical data and the digital image recognition results confirms that geopolymer-based sealing materials can enhance the blasting effect and reduce the output rate of broken coal.This alignment also affirms the accuracy of the Split-Desktop image recognition software,consistent with the conclusions drawn by Ang and Zhen[22]and Wan et al.[23],who asserted the highest accuracy in the image analysis and prediction model.While previous researchers have optimized specific blasting parameters to improve the blasting effect [24,25],this study focuses on the impact of modified sealing materials on blasting effectiveness.In comparison to previous research,our findings indicate an improvement in the blasting effect with the same amount of explosives.Simultaneously,the superior bonding of sealing materials not only enhances blasting efficacy but also contributes to a reduction in dust generation during the blasting process.

4.Conclusions

This study focuses on coal seam blasting in open-pit mines,encompassing experimental investigation,numerical simulation,and field application of blasting sealing materials to investigate their impact.By incorporating geopolymer materials into the sealing material modification,we determined the mechanical properties of the modified materials in the laboratory and applied them in actual blasting operations.Optimization and evaluation criteria for coal seam blasting block size were established,offering significant insights for accurate control in open-pit mines.In summary,the specific conclusions are as follows:

(1) The use of original sealing material (pulverized coal) as aggregate,combined with slag as the cementitious material and alkali exciters,allowed for the laboratory preparation of sealing materials.The optimal ratio involved 80 g of slag doping,a sodium silicate solution modulus of 1.5,and a water/gel ratio of 0.5.This formulation achieved a material strength of 3.12 MPa with an initial setting time of 33 min,meeting the field test time requirements.

(2) Employing numerical simulation,the deformation characteristics of sealing materials during blasting were studied.This simulation clarified the role of sealing materials in the mechanical failure process and their impact on the shell hole over time.

(3) Following the hydration reaction of geopolymer,a layered and flocculated gel formed within the material,enhancing its density.The addition of pregelatinized starch,serving as an organic binder,filled gaps between the gel structures,thereby improving cohesion and the bonding coefficient of the material.

(4) The M3 material ratio was applied for field application at Shengli Open Pit Coal Mine,and a field sealing process for modified materials was proposed.Analysis and calculation of the blast pile using digital image recognition software revealed a reduction in the rate of crushed coal from the original 33.2% to 21.1%.The field data exhibited an error of 0.5%,substantiating a substantial decrease in the crushed coal output rate and confirming the excellent plugging performance of the modified materials.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (No.52174131).The authors would also like to express their gratitude to the Shengli Open Pit Coal Mine for generously providing a crucial test site for the research.