energies

Article Study on Rheological and Mechanical Properties of Aeolian -Fly Ash-Based Filling Slurry

Xiaoping Shao 1,2,*, Long Wang 1, Xin Li 1, Zhiyu Fang 1,*, Bingchao Zhao 1,2, Yeqing Tao 1, Lang Liu 1,2, Wuliang Sun 1 and Jianpeng Sun 1

1 Energy School, Xi’an University of Science and Technology, Xi’an 710054, China; [email protected] (L.W.); [email protected] (X.L.); [email protected] (B.Z.); [email protected] (Y.T.); [email protected] (L.L.); [email protected] (W.S.); [email protected] (J.S.) 2 Key Laboratory of Western Mines and Hazards Prevention, Ministry of Education of China, Xi’an 710054, China * Correspondence: [email protected] (X.S.); [email protected] (Z.F.)

 Received: 9 February 2020; Accepted: 5 March 2020; Published: 9 March 2020 

Abstract: Backfill mining is the most environmentally friendly mining method at present, which can effectively control the surface subsidence, improve the recovery rate, and has good social and economic benefits. The purpose of this study is to solve the environmental problems caused by solid waste, combined with the rich geographical advantages of aeolian sand in the Yushenfu mining area of China. The rheological properties of the aeolian sand-fly ash-based filling slurry with different fly ash content are studied by experiments, and the strength development law of the filling body under different age and fly ash content are studied from the macroscopic and microscopic points of view. The rheological experiments showed that the increase of the amount of fly ash has a significant effect on the thixotropy, plastic , and yield of the filling slurry. Additionally, rheological properties of aeolian sand-fly ash-based filling slurry conform to the Bingham model. With the increase of the amount of fly ash, the performance of the filling slurry has been significantly improved. Uniaxial test and scanning electron microscope observation showed that the influence of fly ash on the strength of the filling body was mainly reflected in the late stage of maintenance, but was not obvious in the middle stage. Fly ash particles mainly bear the role of “water reduction” and a physical filling effect, which makes the filling slurry thicker and the internal structure more closely spaced. The volcanic ash reaction of fly ash is lagging behind the hydration reaction of cement; the secondary product of the delayed reaction is filled in the pores of cement hydrates, which can greatly reduce the of the backfill body and increase the later strength of the backfill body. It provides a guarantee for the safe replacement of coal pillars in the working face.

Keywords: coal mining backfill; fly ash content; uniaxial compression; aeolian sand

1. Introduction For a long time, coal has played a pivotal role in China’s energy economy. In the process of underground mining, traditional mining methods have caused a series of environmental problems such as surface subsidence, erosion, and desertification of the surface soil mass, which need to be resolved. In the northern Shaanxi Province Yushenfu mining area of China, the old room-pillar mining and the “narrow strip” coal mining methods have been used for a long time [1]. Although the purpose of controlling surface subsidence and water retention has been achieved within a period of time, the recovery rate can only reach about 50%, which has caused serious waste of resources, and roof disasters have occurred frequently in recent years [2]. Strip mining areas also have hidden

Energies 2020, 13, 1266; doi:10.3390/en13051266 www.mdpi.com/journal/energies Energies 2020, 13, 1266 2 of 13 Energies 2020, 13, x FOR PEER REVIEW 2 of 13 dangersdangers of of overburden overburden disasters duedue toto long-termlong-termcreep creep of of coal coal pillars pillars [3 [3]]. Backfill. Backfill mining, mining, as as a trenda trend in inmodern modern mining, mining, can can effectively effectively alleviate alleviate the above the ab problemsove problems [4–7]. [4 We–7]. have We proposedhave proposed a strip backfilla strip backfillmining mining method method of “mining of “ 7mining m to retain 7 m to 8 m”retain for 8 a m Shanghe” for a Shanghe coal mine, coal that mine, is mining that is a 7mining m wide a strip7 m wideand leaving strip and an 8 leaving m wide an strip 8 m pillar wide [1 ]. strip It was pillar found [1]. that It was after found the filling that slurry after the was filling pumped slurry from was the pumpedground fillingfrom the station ground to the filling goaf station for solidification, to the goaf thefor fillingsolidification, strength the needed filling to strength be more need thaned 6.0 to MPa be moreto ensure than the6.0 stabilityMPa to ofensure the overburden the stability in of the the process overburden of coal in pillar the process replacement of coal [1]. pillar The fillingreplacement system [1]is. shown The filling in Figure system1. is shown in Figure 1.

Figure 1. Strip filling diagram. Figure 1. Strip filling diagram. The selection and proportion of filling materials are very important to rheological and mechanical properties.The selection However, and conventional proportion filling of filli materialsng materials mostly are use cementvery important as a gelling to agent,rheological and cement and mechanicalaccounts for properties about 70%. However, of the total conventional filling cost filling [8]. Meanwhile materials mostly the filling use materialscement as (gangue, a gelling tailing)agent, andcommonly cement used accounts in Western for about China 70% are not of the abundant total filling [9], resulting cost [8 in]. higherMeanwhile filling the material filling costs. materials Most (gangue,of the surface tailing) of thecommonly Yushenfu used mining in Western area is typicallyChina are aeolian not abundant dunes, where [9], resulting there is abundantin higher aeolianfilling materialsand. Moreover, costs. Most fly ashof the in the surface power of plants the Yushenfu around the mining mining area area is requirestypically large-scale aeolian dunes, accumulation where theresites, is and abundant has caused aeolian serious sand. environmental Moreover, fly pollution ash in the [10 ,11 power]. Therefore, plants around using aeolian the mining sand area and requiresfly ash as large the- fillingscale accumulationmaterials can not sites, only and reduce has caused the amount serious of waste environmental disposal and pollution disposal [10 costs,,11]. Thereforebut also save, using cement aeolian and sand reduce and the fly filling ash as cost the [filling12]. Engineering materials can practice not only has provenreduce the that amount adding of fly wasteash to disposal structural and concrete disposal can costs, improve but also concrete save cement performance, and reduce increase the filling concrete cost strength [12]. Engineering in the later practiceperiod, increasehas proven the compactnessthat adding fly and ash resistance to structural to concrete, concrete reduce can theimprove shrinkage concrete of concrete performance, [13–17], increaseand bring concrete good economic strength andin the social later benefits period, [ 18increase–20]. Therefore, the compactness aeolian sandand andresistance fly ash to are concrete, some of reducethe more the cost-e shrinkageffective of paste concrete filling [1 materials3–17], and in the bring Yushenfu good economic mining area, and China. social benefits [18–20]. Therefore,During aeolian the filling sand andand miningfly ash process,are some the of rheologicalthe more cost performance-effective paste of the filling filling materials slurry is in a the key Yushenfuconsideration mining for area the,selection China. of filling pump stations and pipelines. Previous experiments have shownDuring that the fly ashfilling can and improve mining the process, flow propertiesthe rheological of the performance slurry in pipelines of the filling and theslurry suspension is a key considerationperformance of for solids the selection [21,22]. Some of filling scholars pump [23 stat,24]ions studied and the pipelines. influence Previous of different experiments fly ash content have shownon the that rheological fly ash can properties improve of the waste flow rock properties filling slurry of theand slurry gangue in pipelines filling slurryand the by suspension slump test performanceand rheological of solids test; furthermore,[21,22]. Some thescholars influence [23,2 of4] thestudied type, the content, influence and of fineness different of fly fly ash ash content on the onrheological the rheological behavior properties of cement of andwaste concrete rock filling has been slurry reported and gangue by many filling scholars slurry [25 by]. slump After the test filling and rheologicalslurry is injected test; furthermore, into the goaf, the its solidification influence of strength the type, is acontent direct, factorand fineness to measure of the fly filling ash on e ff theect, rheologicaland sufficient behavior bearing of strength cement will and directly concrete aff ect has the been stability reported of the by overburden many scholars and the [25] safety. After of the the fillingworking slurry face is [26 injected]. Dong into et al. the [27 goaf,] established its solidification the evolution strength equation is a direct between factor the to tensilemeasure strength the filling and effect,compressive and sufficient strength bearingof aeolian strength sand concrete, will directly and studied affect the the influence stability of of the the content overburden of aeolian and sand the safetyand fly of ash the on working the mechanical face [26]. properties Dong et ofal. concrete.[27] established Cao et al. the [ 28 evolution] showed equation that aeolian between sand andthe tensilemass concentration strength and havecompressive the least strength influence of on aeolian the compressive sand concrete, strength. and studied When the the content influence of aeolianof the content of aeolian sand and fly ash on the mechanical properties of concrete. Cao et al. [28] showed that aeolian sand and mass concentration have the least influence on the compressive strength.

Energies 2020, 13, x FOR PEER REVIEW 3 of 13 Energies 2020, 13, 1266 3 of 13 When the content of aeolian sand exceeds 45%, the compressive strength of the filling body gradually decreases. Agrawal et al. [29] found that fly ash concrete has higher mechanical strength sand exceeds 45%, the compressive strength of the filling body gradually decreases. Agrawal et al. [29] and durability through the experiment of replacing river sand concrete with fly ash. Although in the found that fly ash concrete has higher mechanical strength and durability through the experiment of past there was less research study on the performance of aeolian sand-fly ash-based filling materials, replacing river sand concrete with fly ash. Although in the past there was less research study on the there is still significant further research. performance of aeolian sand-fly ash-based filling materials, there is still significant further research. In China, the Yushenfu mining area is different from other mining areas; this area is rich in In China, the Yushenfu mining area is different from other mining areas; this area is rich in aeolian aeolian sand materials. In order to solve the mining subsidence problem of the Yushenfu mining sand materials. In order to solve the mining subsidence problem of the Yushenfu mining area, this area, this article proposes a new type of mixture material, aeolian sand-fly ash-based filling material. article proposes a new type of mixture material, aeolian sand-fly ash-based filling material. Aeolian Aeolian sand is used as the filling aggregate in this paper; the influence of fly ash contents on sand is used as the filling aggregate in this paper; the influence of fly ash contents on rheological rheological properties and mechanical properties of aeolian sand fly ash based filling slurry is properties and mechanical properties of aeolian sand fly ash based filling slurry is emphatically emphatically studied. The study results obtained the ratio of filling materials favorable for the studied. The study results obtained the ratio of filling materials favorable for the Yushenfu mining area. Yushenfu mining area. This study not only improves the recovery rate safely, and effectively This study not only improves the recovery rate safely, and effectively controls the surface subsidence, controls the surface subsidence, but also makes full use of the waste material (fly ash) and local but also makes full use of the waste material (fly ash) and local abundant aeolian sand, greatly reducing abundant aeolian sand, greatly reducing the filling cost and environmental pollution, which has the filling cost and environmental pollution, which has good social and economic benefits. It will good social and economic benefits. It will provide reference to safe filling mining in the Yushenfu provide reference to safe filling mining in the Yushenfu mining area and similar mines in the world. mining area and similar mines in the world. 2. Experimental Materials and Methods 2. Experimental Materials and Methods 2.1. Experimental Materials 2.1. Experimental Materials In this experiment, we choose aeolian sand and fly ash as filling materials. Particle morphology and mineralIn this experiment, composition we of cho aeolianose aeolia sandn and sand fly and ash fly were ash analyzed as filling by materials. laser particle Particle size morphology scanner and andX-ray mineral fluorescence composition (XRF). Theof aeolian following sand is and a detailed fly ash description were analyzed of the by two laser filling particle materials: size scanner and XAeolian-ray fluorescence sand: It is(XRF). taken The from following the vicinity is a detailed of Yulin description area, China. of the Its two main filling light materials minerals: are quartz,Aeolian feldspar sand: and It calcite, is taken accounting from thefor vicinity more than of Yulin 90% ofarea, its minerals. China. Its The main analysis light mineralsof its mineral are quartz, feldspar and calcite, accounting for more than 90% of its minerals. The analysis of its mineral composition is shown in Table1. Its main mineral component is SiO 2; its SiO2 content is 67.8%. composition is shown in Table 1. Its main mineral component is SiO2; its SiO2 content is 67.8%. As As shown in Figure2a, d 50 and d90 are 214.8 and 357.3 µm, respectively, and the average particle showndiameter in is Figure 210.6 µ2m.a, d The50 and sieving d90 are test 214.8showed and that 357.3 the particleμm, respectively, size distribution and the of aeolian average sand particle was diameterrelatively is uniform, 210.6 μm. and The the sieving sortability test showed was good. that The the aeolianparticle sand size hasdistribution very few of sticky aeolian particles sand was and relativelypowder particles. uniform, and the sortability was good. The aeolian sand has very few sticky particles and powder particles.

10 3.5 Incremental volume 100 Incremental volume 100 Cumulative volume 3.0 Cumulative volume 8 d50=214.8;d90=357.3;dm=210.6 d50=11.67;d90=136.2;dm=38.08 80 2.5 80

6 2.0 60 60

1.5 4 40 40 1.0

Cumulative volume (%)

Incremental volume (%)

Cumulative volume (%)

Incremental volume (%)

20 2 20 0.5

0 0 0.0 0 0.1 1 10 100 1000 0.1 1 10 100 1000 Size (μm) Size (μm) (a) Aeolian sand (b) Fly ash

FigureFigure 2. 2. DistributionDistribution curves curves of of the the particle particle size size of of aeolian aeolian sand sand and and fly fly ash ash..

TableTable 1. AAeolianeolian sand sand compo composition.sition.

CompositionComposition AL2O AL3 2O3 SiOSiO2 2 KK22OO CaOCaO Fe2OFe32O3 OthersOthers ContentContent (%) (%)10.3 10.367.8 67.8 7.5 7.5 5.35.3 5.85.8 3.3 3.3

Energies 2020, 13, 1266 4 of 13

Fly ash: It was taken from Hanglaiwan power plant of China and belongs to class II fly ash. The loss on ignition is 4.34%. As shown in Figure2b, d 50 is 11.67 µm, d90 is 136.2 µm, and the average particle size dm is 38.08 µm. The active components of fly ash are mainly SiO2, A12O3 and CaO. Because the content of CaO in fly ash is generally low, the main active components are SiO2 and Al2O3 (as shown in Table2).

Table 2. Fly ash composition.

Composition SiO2 AI2O3 Fe2O3 CaO K2O Others Content (%) 35.6 11.9 25.5 16.4 4.0 6.6

Cement: P.O42.5type cement was used. The 28 d compressive strength is 48.0 MPa. Its composition is shown in Table3.

Table 3. Cement composition.

Composition CaO SiO2 AL2O3 MgO Fe2O3 Others Content (%) 65.08 22.36 5.53 1.27 3.46 2.30

2.2. Experimental Schemes Fix paste mass fraction of 78%, keep the quality of water and cement unchanged, increase the amount of fly ash from 0% to 30% [22] and study its effect on the rheological properties and compressive strength of the paste material. The specific proportions are shown in Table4.

Table 4. Proportion of aeolian sand-fly ash-based filling paste.

Mass Ratio of Coal Mine Filling Paste Group Fly Ash Content (%) Aeolian Sand (g) Fly Ash (g) Cement (g) Water (g) I 281 0 70 99 0 II 255 26 70 99 7.5 III 228 53 70 99 15 IV 202 79 70 99 22.5 V 176 105 70 99 30

Experimental process: In this experiment, we tested the rheological properties and mechanical properties of the paste material. The experiment was performed with a HAAKE rheometer (Figure3a) and a 500 mL beaker. The speed of the rheometer can be controlled within the range of 0.001–157 rad/s, and the torque resolution is 0.01 mNm. In the experiment, the control variable method was used to obtain the backfill body rheology curve by controlling the shear rate (increased from 1 1 0.1 s− to 100 s− in 180 s) and analyzing its rheological properties by means of linear fitting evaluation. The mechanical properties test of the filling body uses a cylindrical steel mold of 50 mm 100 mm. × After the specimens were cured for 1 day, they could be formed and demolded. After the mold was removed, the specimens were numbered and were next placed in the curing box (Figure3c). After the curing time (3 d, 7 d, 14 d, 28 d, 90 d) [28], the MTS electronic universal testing machine (Figure3b) was used to test the uniaxial compressive strength (UCS) of the test piece. The test machine can achieve force control in the range of 1 N to 50 kN and can accurately obtain the filling compressive strength value of the body. A cube sample with a side length of 5mm was from the center of the sample. Remove the dust on the surface of the sample with a rubber suction cup to obtain a fresh and clean section, and then apply a layer of metal conductive film in the vacuum coating machine to the sample used for scanning electron microscope (SEM) observation [30]. The specific experimental process is shown in Figure4. Energies 2020, 13, x FOR PEER REVIEW 5 of 13

Energies 2020, 13, 1266 5 of 13 Energies 2020, 13, x FOR PEER REVIEW 5 of 13

(a) Rheometer (b) UCS test machine (c) Standard conservation box

Figure 3. Experimental equipment.

After the curing time (3 d, 7 d, 14 d, 28 d, 90 d) [28], the MTS electronic universal testing machine (Figure 3b) was used to test the uniaxial compressive strength (UCS) of the test piece. The test machine can achieve force control in the range of 1 N to 50 kN and can accurately obtain the filling compressive strength value of the body. A cube sample with a side length of 5mm was cut from the center(a) Rheometer of the sample. Remove(b) UCS the test dust machine on the surface(c) Standard of the sample conservation with a box rubber suction cup to obtain a fresh and clean section, and then apply a layer of metal conductive film in the Figure 3. Experimental equipment. vacuum coating machine to the Figuresample 3. usedExperimental for scanning equipment. electron microscope (SEM) observation [30After]. The thespecific curing experimental time (3 d, process7 d, 14 isd, shown 28 d, in 90 Figured) [28] 4,. the MTS electronic universal testing machine (Figure 3b) was used to test the uniaxial compressive strength (UCS) of the test piece. The test machine can achieve force control in the rangeStandard of 1 N to 50 kN and can accurately obtain the Forming Conservation 3d/7d/14d filling compressive strength value of the body. A cube sample with a /28sided/90 dlength of 5mmBroken was cut 95%,22 fromAeolian the center of the sample. Remove the dust on the surface of the sample with a rubber suction cup tosand obtain a fresh and clean section, and then apply a layer of metal conductive Sampling film in the Test mode vacuum coating machine to the sample used for scanning electron microscope (SEM) observation [30]. The specific experimental process is shown in Figure 4. SEM test Fly ash Rheological Standard UCS test Formingtest Conservation 3d/7d/14d /28d/90d Broken 500ml 95%,22 Aeolian Slurry SEM picture sand Cement Rheometer Sampling Test mode UCS test machine FigureFigure 4. 4.Experimental Experimental flowchart. flowchart. SEM test Fly ash 3. Results and Discussion Rheological 3. Results and Discussion. test UCS test

DuringDuring the the experiment, experiment,500ml two two rheological rheological experiments experiments were were carried carried out on out the onsame the ratio same of aeolian ratio of sand-fly ash-based fillingSlurry slurry. The uniaxial strength test was carried out on two cylindricalSEM specimens picture Cementaeolian sand-fly ash-based filling slurry.Rheometer The uniaxial strength test was carried out on two withcylindrical the same specimens ratio, and with the averagethe same value ratio,of and the the two average testsUCS test was value taken of asthe the two experimental tests was taken data; as the the machine experimentalexperimental results data; the are experimental shown in Table results5. are shown in Table 5. Figure 4. Experimental flowchart. TableTable 5. 5.Experimental Experimental data. data. 3. Results and Discussion. Compressive Strength/MPa Group Yield Stress /PaPlasticPlastic Viscosity/(Pa s) Compressive Strength/MPa GroupDuring theS tress/Pa experiment, V iscosity/ two rheological experiments· 3 dwere carried 7 d out 14 d on the 28 d same ratio90 d of 3 d 7 d 14 d 28 d 90 d aeolian I sand-fly ash- 14.71based （ fillingPa·s ） slurry. 1.107 The uniaxial 0.309 strength 2.725 test was 4.653 carried 4.482 out 5.989on two cylindricalⅠ II specimens 22.54with the same ratio, 1.417 and the average 0.74 value 3.345of the two 5.289 tests 5.417was taken 8.699 as the III14.71 30.11 1.107 1.6890.309 2.725 1.68 3.1614.653 5.4864.482 5.995 8.8065.989 experimentalⅡIV data22.54; the 52.05 experimental1.417 results 3.052 are0.74 shown in3.345 Table 1.953 5. 3.2395.289 5.0965.417 6.302 11.3998.699 ⅢV 30.11 97.26 1.689 4.7921.68 3.161 2.358 4.0535.486 6.1345.995 9.444 15.0338.806 Notes:Ⅳ 1. When52.05 the content of fly3.052 ash is higherTable than 5.1.953 Experimental 30%, the filling3.239 data slurry . becomes5.096 very thick and6.302 the fluidity of the11.399 paste cannot meet the working requirements; 2. The compressive strength value of the test piece is the average of two test piecesYield with the samePlastic proportion. Compressive Strength/MPa Group Stress/Pa Viscosity/ 3 d 7 d 14 d 28 d 90 d （Pa·s） Ⅰ 14.71 1.107 0.309 2.725 4.653 4.482 5.989 Ⅱ 22.54 1.417 0.74 3.345 5.289 5.417 8.699 Ⅲ 30.11 1.689 1.68 3.161 5.486 5.995 8.806 Ⅳ 52.05 3.052 1.953 3.239 5.096 6.302 11.399

Energies 2020, 13, x FOR PEER REVIEW 6 of 13

Ⅴ 97.26 4.792 2.358 4.053 6.134 9.444 15.033 Notes: 1. When the content of fly ash is higher than 30%, the filling slurry becomes very thick and the fluidity of the paste cannot meet the working requirements; 2. The compressive strength value of the test piece is the average of two test pieces with the same proportion.

3.1. Rheological Test Results

3.1.1. Rheological Model, Yield Stress and Plastic Viscosity In order to determine the rheological model of aeolian sand-fly ash-based filling slurry, we fitted the rheological curve of filling slurry with different fly ash content, and analyzed the rheological curve of filling slurry with 15% fly ash content, as shown in Figure 5. In the process of increasing the shear rate from 0 s–1 to 100 s–1, it can be found that the shear rate meetsEnergies a 2020linear, 13, relationship 1266 with the . The shear stress increases with the increase of6 the of 13 shear rate. It is found that the shear rate and shear stress satisfy a linear function relationship, and the relationship is  = 1.689  + 30.11, where represents the shear stress, represents the shear3.1. Rheologicalrate, and where Test Results the coefficient of determination is 0.97119. It is not difficult to see that the rheological characteristics of aeolian sand-fly ash-based filling slurry conforms to the law of the 3.1.1. Rheological Model, Yield Stress and Plastic Viscosity Bingham model [31,32]:  =  + 0 , where  is the plastic viscosity and 0 is the yield stress. In order to determine the rheological model of aeolian sand-fly ash-based filling slurry, we fitted Therefore, when the fly ash content is 15%, the plastic viscosity of the filling slurry is 1.689 Pa·s and the rheological curve of filling slurry with different fly ash content, and analyzed the rheological curve the yield stress is 30.11 Pa. See Table 5 for the yield stress and plastic viscosity of backfill slurry, of filling slurry with 15% fly ash content, as shown in Figure5. when the fly ash content is 0%, 7.5%, 22.5%, and 30%.

120 τ-γ 180 η-γ Linear fitting 160 Fly ash content:15% 100

140 80 120 =1.689 +30.11 60 100

τ/Pa

η/Pa·s Equation: y = Intercept + Slope*x 80 40 Intercept 30.1142 60 Slope 1.68906 20 R2 0.97119 40

0 20

0 20 40 60 80 100 -1 γ/s

FigureFigure 5. 5. TheThe relation relation curve curve between between shear shear stress stress and and shear shear rate rate and and the the relation relation curve curve between between viscosityviscosity and and shear shear rate rate..

1 1 TheIn thedeformation process of and increasing flow characteristics the shear rate of from the filling 0 s− to slurry 100 sunder− , it can the beexternal found thatforce the are shear the keyrate factors meets a that linear guide relationship the selection with the of shear the field stress. filling The shear pipeline stress system. increases Figure with the 5 shows increase the of relationshipthe shear rate. between It is found the amount that the of shear fly ash rate and and the shear shear stress yield satisfy stress a and linear plastic function viscosity relationship, of the backfilland the body relationship slurry when is τ the= 1.689 shearγ rate+ 30.11, is increased where τfromrepresents 0.1 s–1 to the 100 shear s–1 at stress,a constantγ represents rate within the 180shear s. rate, and where the coefficient of determination is 0.97119. It is not difficult to see that the rheologicalAs shown characteristics in Figure 6 ofa, aeolian when sand-flythe amount ash-based of fly filling ash is slurry increased conforms from to 0% the to law 30%, of the the Bingham shear = + yieldmodel stress [31,32 of]: theτ slurryηγ τ increases0, where η fromis the the plastic original viscosity 14.71 and Pa τ to0 is 97.26 the yield Pa. By stress. fitting Therefore, the data, when we the fly ash content is 15%, the plastic viscosity of the filling slurry is 1.689 Pa s and the yield stress is found that the fly ash content and the shear yield force satisfy a quadratic· function relationship within30.11 Pa.a certain See Table error5 for range, the yieldand the stress relationship and plastic is viscosity y = 17.14 of – backfill0.8x + 0.11x slurry,2. The when shear the flyyield ash stress content of is 0%, 7.5%, 22.5%, and 30%. The deformation and flow characteristics of the filling slurry under the external force are the key factors that guide the selection of the field filling pipeline system. Figure5 shows the relationship between the amount of fly ash and the shear yield stress and plastic viscosity of the backfill body slurry 1 1 when the shear rate is increased from 0.1 s− to 100 s− at a constant rate within 180 s. As shown in Figure6a, when the amount of fly ash is increased from 0% to 30%, the shear yield stress of the slurry increases from the original 14.71 Pa to 97.26 Pa. By fitting the data, we found that the fly ash content and the shear yield force satisfy a quadratic function relationship within a certain error range, and the relationship is y = 17.14 0.8x + 0.11x2. The shear yield stress of the paste slurry − is proportional to the square of the fly ash content. As the fly ash content increases, the ability of the filling slurry to resist deformation also increases, and its plasticity increases. Conversely, the lower the fly ash content, the weaker the paste slurry’s ability to resist deformation. Energies 2020, 13, x FOR PEER REVIEW 7 of 13

the paste slurry is proportional to the square of the fly ash content. As the fly ash content increases, the ability of the filling slurry to resist deformation also increases, and its plasticity increases. Conversely, the lower the fly ash content, the weaker the paste slurry’s ability to resist deformation. Plastic viscosity is the main factor affecting the fluidity of the filling slurry. As shown in Figure 6b, as the amount of fly ash increases, the plastic viscosity also increases. When the fly ash content is 30%, the plasticity viscosity can reach the maximum value of 4.79 Pa·s in this experiment. Increasing the amount of fly ash can improve the cohesiveness of the slurry, which is conducive to the formation of a “plunger flow” that does not segregate, settle, nor bleed [33]. However, the fluidity of the slurry is also weakened, which is not conducive to pumping the slurry. This is mainly because in the aeolian sand-fly ash-based filling slurry, with the increase of fly ash content, the proportion of fine particles increases, and the between particles also increases. Moreover, with the increase of fly ash particles, aeolian sand particles are gradually replaced by finer fly ash particles. The fly ash particles are evenly distributed in the basic phase of the filling slurry; because of its fine particles and large specific surface area, the particle surface easily absorbs more water [34], which plays the role of “water reducing agent”, making the slurry

Energiesthicker,2020 and, 13, helps 1266 to improve the uniformity of the fresh and hardened paste [35]. At the same time,7 of 13 the fine fly ash particles filled in the capillary pores of the slurry, reduce the porosity and makes the structure compact.

110

100 5 4.792 catter plot 97.26 90 Fitting line

80 4 Equation: y = Intercept + B1*x + B2*x^2 70 3.052 price error 3 60 Intercept 17.14486 4.97262

/Pa B1 -0.80063 0.78539

0

τ B2 0.11318 0.0251 50 η/Pa·s R2 0.98734 2 40 1.689 y = 17.14 -0.8x + 0.11x2 30 1.417 1.107 20 1 10 14.71 0 0 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 Fly ash content/% Fly ash content/%

(a) Yield stress (b) Plastic viscosity

FigureFigure 6. 6. EEffectffect ofof flyfly ash content on on shear shear yield yield stress stress and and plastic plastic viscosity viscosity of slurry of slurry..

3.1.2.Plastic Thixotropic viscosity Properties is the main factor affecting the fluidity of the filling slurry. As shown in Figure6b, as the amountThixotropy of fly refers ash increases, to the phenomenon the plastic viscosity that the alsoconsistence increases. of When the filling the fly slurry ash content decreases is 30%, the(increases) plasticity when viscosity sheared, can reachand increases the maximum (decreases) value when of 4.79 the Pa shearings in this stops experiment. [36]. The Increasingthixotropic the · amountcharacteristics of fly ash of can the improve aeolian the sand cohesiveness-fly ash-based of the backfill slurry, whichbody isslurry conducive were to analyzed the formation by the of a “plungerthixotropic flow” ring that method. does not The segregate, area of the settle, thixotropic nor bleed ring [reflected33]. However, the thixotropy the fluidity of the of theslurry. slurry Figure is also weakened,7a shows, whichthe up isand not down conducive flow curves to pumping of different theslurry. fly ash-filled backfill body slurry measured in a 3 minuteThis is experime mainly becausent. In the in entire the aeolian range sand-flyof shear ash-basedrate (0.1 s filling–1–100.1 slurry, s–1), when with the increasefly ash content of fly ash content,exceeds the 15%, proportion the upward of flow fine curve particles of the increases, filling slurry and theis on friction the higher between side of particles the downward also increases. flow Moreover,curve. As with shown the in increase Figure of7b fly, the ash area particles, of the aeolian thixotropic sand ring particles increases are graduallywith the increase replaced in by the finer flyamount ash particles. of fly ash. The fly ash particles are evenly distributed in the basic phase of the filling slurry; because of its fine particles and large specific surface area, the particle surface easily absorbs more water [34], which plays the role of “water reducing agent”, making the slurry thicker, and helps to improve the uniformity of the fresh and hardened paste [35]. At the same time, the fine fly ash particles filled in the capillary pores of the slurry, reduce the porosity and makes the structure compact.

3.1.2. Thixotropic Properties Thixotropy refers to the phenomenon that the consistence of the filling slurry decreases (increases) when sheared, and increases (decreases) when the shearing stops [36]. The thixotropic characteristics of the aeolian sand-fly ash-based backfill body slurry were analyzed by the thixotropic ring method. The area of the thixotropic ring reflected the thixotropy of the slurry. Figure7a shows, the up and down flow curves of different fly ash-filled backfill body slurry measured in a 3 minute experiment. In the 1 1 entire range of shear rate (0.1 s− –100.1 s− ), when the fly ash content exceeds 15%, the upward flow curve of the filling slurry is on the higher side of the downward flow curve. As shown in Figure7b, the area of the thixotropic ring increases with the increase in the amount of fly ash. In the process of increasing the amount of fly ash from 0% to 30%, the area increases from 212.46 Pa/s to 6482 Pa/s. When the amount of fly ash is 22.5%–30%, the area of thixotropic ring increases very obviously, the consistency of the slurry reaches the maximum value of this test, the upper and lower shear stresses show obvious differences, and the rising curve shows obvious stress overshoot. There are obvious fluctuations and bending phenomena in the rising stress curve, and then gradually decrease in steady state; this is due to the greater friction between the ultra-multiple fly ash particles, and the fine fly ash particles will fill the slurry. Among the capillary pores, the slurry has dense pores and can only be destroyed by large shear forces [37,38]. Therefore, in the aeolian sand-fly ash-based filling material, Energies 2020, 13, x FOR PEER REVIEW 8 of 13

0% 7.5% 15% 22.5% 30% 400 30%

350 22.5% 15% 2666.76 300 7.5% 0% 250 Energies 2020, 13, 1266 1298.59 8 of 13

/Pa

τ 200

150 the amount of fly ash is positively related to the consistency of the filling slurry. The larger648.45 the amount 100 212.46

of fly ash, the thicker the slurry and the greater the thixotropy. But6482 as thixotropic properties increase, the fluidity50 of the slurry also decreases. Energies 2020, 13, x FOR PEER REVIEW 8 of 13 0 0 10 20 30 40 50 60 70 80 90 100 110 0% 7.5% γ/s 15%-1 22.5% 30% 400 30%

350 (a) Thixotropic ring 22.5% (b) Thixotropic ring area 15% 2666.76 300 Figure 7. Thixotropic characteristics 7.5% . 0% 250 In the process of increasing the amount of fly ash from 0% to 30%, the area increases1298.59 from 212.46

/Pa

τ 200

Pa/s to 6482 Pa/s. When the amount of fly ash is 22.5%–30%, the area of thixotropic ring increases very obviously,150 the consistency of the slurry reaches the maximum value of this test, the upper and 648.45 lower shear100 stresses show obvious differences, and the rising curve shows obvious stress212.46 overshoot.

There are obvious fluctuations and bending phenomena in 6482 the rising stress curve, and then 50 gradually decrease in steady state; this is due to the greater friction between the ultra-multiple fly 0 ash particles,0 10 and20 the30 fine40 fly50 ash60 particles70 80 90 will100 fill110 the slurry. Among the capillary pores, the slurry -1 has dense pores and can onlyγ/s be destroyed by large shear forces [37,38]. Therefore, in the aeolian sand-fly ash-based(a )filling Thixotropic material, ring the amount of fly ash is positively(b related) Thixotropic to the ringconsistency area of the filling slurry. The larger the amount of fly ash, the thicker the slurry and the greater the Figure 7. Thixotropic characteristics. thixotropy. But as thixotropic propertiesFigure 7.increase,Thixotropic the fluidity characteristics. of the slurry also decreases. 3.1.3.In Rheological the process Propertiesof increasing at Constantthe amount Shear of fly Rate ash from 0% to 30%, the area increases from 212.46 3.1.3. Rheological Properties at Constant Shear Rate Pa/s toIn 6482 the BinghamPa/s. When model, the amount when the of shearfly ash rate is 22.5 is constant,%–30%, the the area shear of forcethixotropic and viscosity ring increases meet a very Inobviously, the Bingham the consistency model, when of thethe slurry shear reachesrate is constant,the maximum the shear value force of this and test, viscosity the upper meet and a1 proportional relationship. For the convenience of research, we controlled the shear rate to 50 s− , -1 proportionalloweras shown shear in stresses Figurerelationship.8 showa,b. The Forobvious higherthe convenience differences, the content andof of research, flythe ashrising is,we thecurve contro higher showslled the theobvious viscosity shear stress rate curve to overshoot. 50 and s , the as shownThigherhere in are the Figure shear obvious 8 stressa and fluctuations curve. b. The When higher and the the bending fly content ash content phenomena of fly isash less is, than inthe the higher 7.5%, rising the the stress stressviscosity curve curve, curve and and viscosityand then the highergraduallycurve change the decreaseshear smoothly stress in steady duringcurve. state Whenthe; entirethis the is shearingdue fly ashto the contentprocess. greater is When friction less the than between content 7.5%, of the fly ultra stress ash- reachesmu curveltiple and30%, fly viscosityashthere particles, is a curve significant and change the di fine ff smoothlyerence fly ash between particles during the the will stress entire fill curvethe shearing slurry. and viscosity procesAmongs. the curve When capillary after the content shear pores, stabilization ofthe fly slurry ash reacheshasand dense its initial30%, pores there height, and is a whichcan significant only shows be differencedestroyed the ability between ofby resistinglarge the shear deformationstress forces curve [3 7 ofand,3 slurry,8]. viscosity Therefore, is improved curve in theafter by aeolian flyshear ash stabilizationsandin the-fly slurry. ash - andbased That its isfilling initial to say, material, height, the yield whichthe stress amount shows of the of the slurryfly abilityash is is increased. positively of resisting Atrelated the deformation beginning to the consistency of of slurry the shear, ofis improvedtheexperiment, filling by slurry. the flyrotor ash The in needs largerthe toslurry. overcome the amountThat ais large to of say fly yield, t ash,he stress yield the to thickerstress reach of a the stablethe slurryslurry state. andis When increased. the the greater content At the of beginningthixotropy.fly ash is low,of But the the as shear yieldthixotropic experiment, stress isproperties small, the sorotor increase, the needs curve the changesto fluidityovercome gently. of athe large slurry yield also stress decreases. to reach a stable state. When the content of fly ash is low, the yield stress is small, so the curve changes gently. 3.1.3. Rheological Properties at Constant Shear Rate In the Bingham model, when the shear rate is constant, the shear force and viscosity meet a proportional relationship. For the convenience of research, we controlled the shear rate to 50 s-1, as shown in Figure 8a and b. The higher the content of fly ash is, the higher the viscosity curve and the higher the shear stress curve. When the fly ash content is less than 7.5%, the stress curve and viscosity curve change smoothly during the entire shearing process. When the content of fly ash reaches 30%, there is a significant difference between the stress curve and viscosity curve after shear stabilization and its initial height, which shows the ability of resisting deformation of slurry, is improved by fly ash in the slurry. That is to say, the yield stress of the slurry is increased. At the beginning of the shear experiment, the rotor needs to overcome a large yield stress to reach a stable state. When the content of fly ash is low, the yield stress is small, so the curve changes gently.

(a) Shear stress (b) Plastic viscosity

Figure 8. Rheological curve at constant shear rate. 3.2. Mechanical Test Results The determination of the compressive performance of the filling paste is the key to the filling and mining of coal mines, which will directly affect the filling effect. Reasonable strength requirements are

(a) Shear stress (b) Plastic viscosity

Energies 2020, 13, x FOR PEER REVIEW 9 of 13

Figure 8. Rheological curve at constant shear rate.

3.2. Mechanical Test Results Energies 2020, 13, 1266 9 of 13 The determination of the compressive performance of the filling paste is the key to the filling and mining of coal mines, which will directly affect the filling effect. Reasonable strength requirementsthe guarantee are of the safe guarantee mining. Forof safe fly ashmining. filling For material, fly ash thefilling strength material, development the strength of thedevelopment filling body ofunder the filling different body curing under time different and fly curing ash content time and is of fly great ash significancecontent is of to great filling significance mining. to filling miningAs. shown in Figure9, as the amount of fly ash increases, the strength of the backfill body generally increases.As shown The in compressive Figure 9, as strength the amount of the test of fly specimen ash increases, No.I (0% the fly strength ash content) of the on backfill the curing body time generallyof 90 days increases. is only aboutThe compressive 6.0 MPa. The strength compressive of the strengthtest specimen of No.V No. (30%Ⅰ (0% fly fly ash ash content) content) can on reachthe curing15.0MPa, time which of 90 is days doubled is only compared about 6.0 to theMPa. compressive The compressive strength strength of No.I ratio.of No.V It can (30% be seenfly ash that contentthe amount) can reach of fly 15.0 ash MPa, has played which ais key doubled role in compared improving to thethe latercompressive strength strength of the paste. of No.I At ratio. 7 days It andcan be 14 daysseen that of curing the amount time, the of strengthfly ash has of played the test a specimens key role in is improving concentrated theat later about strength 3.24 MPa of the and paste.5.1 MPa At 7 respectively. days and 14 Atdays the of time curing of 3 time, days the of curing,strength the of strength the test ofspecimens filling the is bodyconcentrated of No.V at can aboutreach 3.24 about MPa eight and times 5.1 MPa of that respectively. of No.I ratio. At the In summary,time of 3 days the impact of curing, of fly the ash strength on the of filling filling body the is bodymainly of No.V reflected can inreach the enhancementabout eight times of the of strength that of No.I at the ratio. later In stage, summary, and the the impact impact on of the fly strength ash on of thethe filling filling body body is in mainly the middle reflected period in isthe not enhancement obvious. This of is the mainly strength because at the during later thestage, development and the impactof the on strength the strength of the filling of the body, filling it first body experienced in the middle the hydration period is reaction not obvious. of the This cement, is mainly and the becauseC-H generated during thefrom development the hydration of of the the strengthcement corroded of the filling the surface body, of it the first fly ash,experienced which in the turn hydrationstimulated reaction the volcanic of the ashcement, reaction and of the the C fly-H ashgenerated [39]. Volcanic from the ash hydration reaction of of fly the ash cement is lagging corroded behind thethe surfac hydratione of the reaction fly ash, of cement. which in The turn secondary stimulated product the volcanic of the delayed ash reaction reaction of is the filled fly in ash the [3 pores9]. Volcanicof cement ash hydrates, reaction whichof fly ash can is greatly lagging reduce behind the the porosity hydration of the reaction backfill of body cement. and increaseThe secondary the later productstrength of ofthe the delayed backfill reaction body [40 is]. filled in the pores of cement hydrates, which can greatly reduce the porosity of the backfill body and increase the later strength of the backfill body [40].

FigureFigure 9. 9.EffectEffect of offly fly ash ash content content on on strength strength of ofbackfill backfill..

AsAs shown shown in in Figure Figure 10,10, materials such such as as C C2S,2S, G G3S,3S, C C3A,3A, and and C 4 CAF4AF in in cement cement particles particles react react with withwater water to undergo to undergo a series a seriesof hydration of hydration reactions. reactions. The hydration The hydration products products are: calcium are: hydroxide calcium hydroxide(C-H), water (C-H), calcium water aluminate calcium a (C-A-H),luminate calcium (C-A-H), ferric calcium aluminate ferric (C-S-F-H),aluminate etc. (C- [S34-F].-H), Among etc. [3 these4]. Amonghydration these products, hydration C-H products, is an active C-H activator is an active of fly ashactivator in the of cement fly ash environment, in the cement and environment, it is in the form andof flake it is crystals.in the form Compared of flake with crystals. other Compared hydration products with other of cement, hydration its products composition of cement, is the purest. its compositionC-H develops is the crystals purest. on C the-H surface develops of flycrystals ash to on form the an surface alkaline of thinfly ash film to solution. form an Thealkaline active thin SiO 2 filandm solution. Al2O3 in The fly ash active react SiO with2 and the Al flake2O3 C-Hin fly crystals ash react formed with by the hydration flake C- ofH cement crystals to formed form C-S-H by and C-A-H . Although the reaction is slow, it has a great influence on the later strength [23,41]. Fly ash itself has only potential activity. In the absence of admixtures, fly ash generally does not produce self-setting. However, the reaction of active SiO2, Al2O3 in fly ash and Ca(OH)2 released Energies 2020, 13, 1266 10 of 13

Energies 2020, 13, x FOR PEER REVIEW 10 of 13 from the hydration of cement clinker minerals can make its gelling properties appear, and finally the strengthhydration of the of cementfilling body to form is gradually C-S-H and increased. C-A-H gel. Although the reaction is slow, it has a great influence on the later strength [23,41].

FigureFigure 10. 10.SEM SEM picturespictures of filling filling body body with with ratio ratio of of No. No. Ⅲ III in indifferent different curing curing age age..

4. DiscussionFly ash itself has only potential activity. In the absence of admixtures, fly ash generally does not produce self-setting. However, the reaction of active SiO2, Al2O3 in fly ash and Ca(OH)2 released In this paper, the rheological properties of the aeolian sand-fly ash-based filling slurry with 0–30% from the hydration of cement clinker minerals can make its gelling properties appear, and finally fly ash of aeolian sand fly ash based filler are studied, and the strength development rule of the filler the strength of the filling body is gradually increased. under different fly ash content and curing age (3–90 d) is analyzed. It is found that the fly ash content is4. positively Discussion related to viscosity and yield stress, and the strength increases with the increase of fly ash content and age. When the content of fly ash is less than 15%, although the fluidity of filling slurry is In this paper, the rheological properties of the aeolian sand-fly ash-based filling slurry with 0– excellent, the bearing strength cannot meet the requirements (more than 6.0 MPa). When the content 30% fly ash of aeolian sand fly ash based filler are studied, and the strength development rule of the of fly ash is more than 15%, the support strength can be satisfied after 28 days of curing. However, filler under different fly ash content and curing age (3–90 d) is analyzed. It is found that the fly ash with the increase of the content of fly ash, the fluidity of filling slurry will be weakened. When the content is positively related to viscosity and yield stress, and the strength increases with the content of fly ash is 30%, the plastic viscosity of the slurry reaches 4.792 Pa s and the consistency of the increase of fly ash content and age. When the content of fly ash is less· than 15%, although the slurryfluidity is relativelyof filling slurry large, is which excellent, is not the conducive bearing strength to pumping, cannot which meet the is consistent requirements with (more the research than results6.0 MPa). of many When scholars the content [42–44 of]. fly When ash is fly more ash contentthan 15%, is 15%the support and 22.5%, strength both thecan fluiditybe satisfied and after bearing strength28 days are of consideredcuring. However, as the idealwith materialthe increase ratio of for the filling content and of mining fly ash, in the Shanghe fluidity coal of filling mine afterslurry field verificationwill be weakened. [1]. Unfortunately, When the ourcontent experiment of fly ash did is not 30%, design the plastic longer viscosity curing time of the (such slurry as 180 reaches d) [45 ]. Some4.792 scholars Pa·s and studied the consistency found that of withthe slurry the increase is relatively of fly large, ash content, which is the not workability conducive andto pumping, mechanical strengthwhich is of consistent cement paste with backfillthe research (CPB) results increase, of many and scholars when they [42– reach44]. Wh theen maximum, fly ash content they is decrease 15% withand the 22.5%, increase both of the fly fluidity ash content and [ bearing24,46]. However, strength are in this considered experiment, as the we ideal did not material find the ratio strength for inflectionfilling and point mining of the in fillingShanghe specimen; coal mine further after field research verification is needed. [1]. Unfortunately, In a future study, our experiment we will study thedid strength not design development longer curing law time of aeolian (such as sand-fly 180 d) ash-based[45]. Some fillingscholars slurry studied with found higher that content with the of fly ashincrease and at of a longerfly ash content, curing period the workability (180 d),and and themechanical rheological strength characteristics of cement paste of filling backfill slurry (CPB) under time-varyingincrease, and characteristics. when they reach This the promotes maximum, the treatment they decrease of solid with waste the and increase the e offfi cient fly ash development content of[2 coal4,46 resources;]. However, it inhas this a great experiment, significance we did to the not development find the strength of the inflection green mining point of industry. the filling It not onlyspecimen provided; further a basis research for the is selectionneeded. In of a fillingfuture study, ratio and we pipelinewill study transportation the strength development parameters law in the safeof fillingaeolian mining sand-fly of ash the-based Yushenfu filling mining slurry area with in high Northerner content Shaanxi, of fly but ash it and also at has a certain longer curingreference period (180 d), and the rheological characteristics of filling slurry under time-varying characteristics. significance for filling mining under the conditions of similar mines in the world. This promotes the treatment of solid waste and the efficient development of coal resources; it has a 5.grea Conclusionst significance to the development of the green mining industry. It not only provided a basis for the selection of filling ratio and pipeline transportation parameters in the safe filling mining of the YushenfuIn this mining paper, area the rheologicalin Northern propertiesShaanxi, but of it the also aeolian has certain sand-fly reference ash-based significance filling for slurry filling with diminingfferent flyunder ash the content conditions are studied of similar by mines experiments, in the world. and the strength development law of the filling body under different age and fly ash content is studied from the macroscopic and microscopic points of5. view. Conclusions The main conclusions of this study are as follows: (1) TheIn this rheological paper, the properties rheological of properties aeolian of sand-fly the aeolian ash-based sand-fly filling ash-based slurry filling conform slurry with to the differentBingham fly ash model. content are studied by experiments, and the strength development law of the filling (2)bodyThixotropy under different of aeolian age and sand-fly fly ash ash-based content is filling studied slurry from increases the macroscopic with the and increase microscopic of fly ash pointcontent.s of view. When The main the amount conclusions of fly of ashthis isstudy 22.5%–30%, are as follows: the area of the thixotropic ring increases very significantly.

Energies 2020, 13, 1266 11 of 13

(3) The fly ash content and the shear yield force meet a quadratic function relationship of y = 17.14 0.8x + 0.11x2. As the fly ash content increases, the backfill body’s resistance to deformation − increases and its plasticity increases. (4) Increasing the amount of fly ash can improve the cohesiveness of the slurry, which is beneficial to the slurry and the slurry does not segregate. When the fly ash content is 30%, the plastic viscosity can reach the maximum value of 4.79 Pa s in this experiment. However, with the increase of the · amount of fly ash, the fluidity of the slurry also weakens, which is not conducive to pumping the slurry. (5) The strength of aeolian sand based fly ash filling increases with the increase of fly ash content; the influence of fly ash on the strength of the filling body is mainly reflected in the enhancement of the strength at the later stage, but the influence on the medium-term strength is not obvious.

Author Contributions: Conceptualization, L.W.; data curation, X.L. and W.S.; formal analysis, L.W.; funding acquisition, L.L.; investigation, Z.F. and Y.T. and J.S.; methodology, Z.F.; resources, B.Z.; software, X.L.; supervision, X.S.; validation, X.S.; writing—original draft, X.S.; writing—review and editing, Z.F. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science of China (No. 51874230, 51674188, 51874229, 51504182), the Innovation Support Program (Science and Technology Innovation Team) Project Funding of Shaanxi Province of China (No. 2018TD-038), and the Shaanxi Innovative Talents Cultivate Program-New-star Plan of Science and Technology (No. 2018KJXX-083). Conflicts of Interest: The authors declare no conflict of interest concerning the publication of this paper.

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