energies Article Study on Rheological and Mechanical Properties of Aeolian Sand-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 viscosity, and yield stress 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 porosity 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, soil 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 abovethe 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 leavingstrip and an 8leaving m wide an strip 8 m pillar wide [1 ].strip It was pillar found [1]. that It was after found the filling that slurryafter 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 notof 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 ,11power]. Therefore, plants around using aeolianthe mining sand area and requiresfly ash as large the- fillingscale accumulation materials can notsites, only and reduce has caused the amount serious of wasteenvironmental disposal andpollution 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 thebring 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 offor solids the selection [21,22]. Some of filling scholars pump [23 stat,24]ions studied and thepipelines. 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 solidificationinfluence of strengththe type, is acontent direct, factorand fineness to measure of thefly fillingash on e fftheect, rheologicaland sufficient behavior bearing of strength cement will and directly concrete aff ecthas 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 bearing of aeolian strength sand concrete, will directly and studied affect thethe influencestability ofof thethe content overburden of aeolian and sandthe safetyand fly of ash the on working the