Characterization of Slag Reprocessing Tailings-Based Geopolymers in Marine Environment

Article Characterization of Slag Reprocessing Tailings-Based Geopolymers in Marine Environment

Jie Wu, Jing Li, Feng Rao * and Wanzhong Yin *

School of Zijin Mining, Fuzhou University, Fuzhou 350108, China; [email protected] (J.W.); [email protected] (J.L.) * Correspondence: [email protected] (F.R.); [email protected] (W.Y.)

Received: 30 July 2020; Accepted: 10 September 2020; Published: 22 September 2020

Abstract: In this study, copper slag reprocessing tailings (CSRT) were synthesized into geopolymers with 40%, 50% and 60% metakaolin. The evolution of compressive strength and microstructures of CSRT-based geopolymers in a marine environment was investigated. Except for compressive strength measurement, the characterizations of X-ray diﬀraction (XRD), Fourier-transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM) were included. It was found that marine conditions changed the Si/Al ratio in the sodium-aluminosilicate-hydrate (N-A-S-H) gel backbone, promoted the geopolymerization process, led to more Q4(3Al), Q4(2Al) and Q4(1Al) gel formation and a higher compressive strength of the geopolymers. This provided a basis for the preparation of CSRT-based geopolymers into marine concrete.

Keywords: copper slag reprocessing tailings; geopolymer; microstructure; NMR

1. Introduction Massive quantities of copper mine tailings are discharged from the beneficiation of the copper mine, which causes some environmental problems such as contamination of surrounding air, water bodies and soils, when disposed of in tailings storage facilities without rehabilitation [1,2]. Therefore, it is necessary to develop methods to reprocess the mine tailings to relieve their adverse impacts, whilst, at the same time, recycle resources and increase economic benefit. In the past decade, researchers used alkaline activation or the geopolymerization process for the consolidation of tailings [3,4]. Accordingly, the tailings are employed as raw materials in the alkaline activation process, in which the aluminosilicates in tailings are attacked by alkalis to decompose monomers and then to regroup and harden. For example, Ahmari et al. used sodium hydroxide as an alkaline activator to prepare copper mine tailings-based geopolymers, which were suitable for application of road base material at a room temperature [5]. Cristelo et al. applied fly ash as additive, and sodium hydroxide and sodium silicate as alkaline activators to prepare high-sulfur copper mine tailings-based geopolymer, which had a maximum compressive strength of 23.5 MPa [6]. Generally, mine tailings contain a substantial amount of crystal minerals and exhibit inert in geopolymeric reaction, so that the calcination of the tailings or addition of calcined materials (e.g., metakaolin and slag) is usually necessary; the reasons are: (1) the calcined materials (e.g., metakaolin and slag) have a large amount of silicon, aluminum and calcium, which are vital elements required for geopolymerization, (2) the calcined raw materials usually have a fast dissolution and gelation rate that makes geopolymers show high early compressive strength. [7]. The calcination induces dehydroxylation of the precursor materials and enhances their amorphous phase content, so that the dissolution of silicate and aluminate monomers is favorable in the formation of sodium-aluminosilicate-hydrate (N-A-S-H) gel [8].

Minerals 2020, 10, 832; doi:10.3390/min10090832 www.mdpi.com/journal/minerals Minerals 2020, 10, 832 2 of 12 Minerals 2020, 10, x FOR PEER REVIEW 2 of 12

Recently,Recently, geopolymer geopolymer has has become become an an emerging emerging marine marine material, material, because because of of its its particular particular microstructure.microstructure. Some Some researchers researchers considered considered the co thempact compact tetrahedral tetrahedral aluminosilicate aluminosilicate structure structure might protectmight protectthe geopolymer the geopolymer concrete concrete from attack from attackby corro bysive corrosive ions in ions harsh in harshmarine marine conditions conditions [9,10]. [9 For,10]. instance,For instance, a metakaolin-based a metakaolin-based geopolymer geopolymer was was synthesized synthesized with with sodium sodium hydroxide hydroxide and and sodium sodium silicatesilicate mixture asas alkaline alkaline activator activator and and exposed expose tod seawater to seawater for 28 for days, 28 whosedays, compressivewhose compressive strength strengthexceeded exceeded 20 MPa 20 [11 MPa]. Therefore, [11]. Therefore, in this in work, this work, copper copper slag reprocessingslag reprocessing tailings tailings (CSRT)-based (CSRT)- basedgeopolymers geopolymers were preparedwere prepared and characterized and characterized in marine in environment,marine environment, in order in to order provide to guidanceprovide guidancefor the preparation for the preparation of tailings of into tailings marine into concrete. marine Copperconcrete. tailings Copper have tailings a significant have a significant amount of amountsilicon, of aluminum silicon, aluminum and calcium, and calcium, which are which crucial are elements crucial elements needed needed for geopolymerization. for geopolymerization. It was Ithypothesized was hypothesized that (1)that the (1) slagthe slag flotation flotation tailings tailings would would have have higher higher reactivity reactivity thanthan other copper copper minemine tailings tailings in in the the geopolymerization geopolymerization process; process; (2) (2) the the formed formed geopolymers geopolymers would would have have higher higher resistanceresistance againstagainst corrosioncorrosion in in the the marine marine environment environment than than ordinary ordinary Portland Portland cement cement (OPC) concrete.(OPC) concrete.After preparation, After preparation, the CSRT-based the CSRT-based geopolymers geopolymers cured in air, cured artificial in air, seawater artificial and seawater heat–cool and cycles heat– in coolseawater cycles were in seawater analyzed. were analyzed.

2.2. Experimental Experimental

2.1.2.1. Materials Materials CopperCopper mine mine tailings tailings were were obtained obtained from from the the copper copper slag slag reprocessing reprocessing tailings tailings of of Zijin Zijin Shan, Shan, Longyan,Longyan, Fujian Fujian province, province, China. China. Kaolinite Kaolinite was waspurchased purchased in Wuhan, in Wuhan, Hubei Hubeiprovince, province, China, which China, waswhich used was to used prepare to prepare metakaolin metakaolin through through calcination calcination at 800 at 800°C for◦C for6 h. 6 h.Figure Figure 1 1gives gives the the size distributiondistribution of of the the CSRT CSRT and and metakaolin metakaolin measured measured by by a a laser laser diffraction diffraction analyzer analyzer (LS-CWM, (LS-CWM, Omec, Omec, Zhuhai,Zhuhai, China), China), in in which which the the dd5050 andand dd8585 areare 16.81 16.81 and and 36.71 36.71 µmµm for for CSRT, CSRT, and and 2.38 2.38 and and 3.52 3.52 µmµm for for metakaolin,metakaolin, respectively. TableTable1 shows1 shows the chemicalthe chem compositionical composition of the CSRTof the and CSRT metakaolin and metakaolin analyzed analyzedby an X-ray by fluorescencean X-ray fluorescence instrument instrument (XRF, Axios (XRF, advanced, Axios PANalyticaladvanced, PANalytical B.V., Almelo, B.V., The Netherlands).Almelo, The Netherlands).The CSRT mainly The containedCSRT mainly SiO 2contained(28.19%) andSiO2 Fe (28.19%)2O3 (57.83%), and Fe while2O3 (57.83%), SiO2 (52.67%) while andSiO2 Al (52.67%)2O3 (41.37%) and Alwere2O3 the(41.37%) main componentswere the main of components the metakaolin. of the Figure metakaolin.2 shows theFigure main 2 componentsshows the main of the components CSRT and ofmetakaolin the CSRT characterized and metakaolin by X-ray characterized diffraction by (XRD, X-ray D8, diffraction Brucker, Karlsruhe,(XRD, D8, Germany).Brucker, Karlsruhe, The CSRT Germany).mainly contained The CSRT magnetite mainly and contained fayalite, while magnetite metakaolin and showedfayalite, an while amorphous metakaolin peak at showed 2θ of 15–30 an◦ amorphousand crystal peak phase at in 2θ quartz. of 15–30° Sodium and crystal silicate phase (ACS in reagent quartz. grade), Sodium employed silicate (ACS as an reagent alkali activator, grade), employedwas bought as froman alkali Aladdin activator, Chemical was bought Reagent from (Shanghai, Aladdin China) Chemical in the Reagent geopolymerization (Shanghai, China) process. in theDeionized geopolymerization water was used process. throughout Deionized the wate wholer was process. used throughout the whole process.

100 Metakaolin CSRT

Accumulative distribution (%) distribution Accumulative 20

0 1 10 100 Particle size (μm)

FigureFigure 1. 1. SizeSize distribution distribution of of the the tailings tailings and and metakaolin metakaolin samples. samples.

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Table 1. Chemical composition of the, copper slag reprocessing tailings (CSRT) and metakaolin.

Components (%) SiO2 Al2O3 Fe2O3 MgO CaO TiO2 Na2OK2O CSRT 28.19 4.24 57.83 1.101 1.906 0.303 0.525 0.81 MineralsMetakaolin 2020, 10, x FOR PEER 52.67REVIEW 41.37 1.299 0.32 0.02 0.163 0.092 2.083 of 12

1800 ♦ •: Fayalite PDF: 76-0512 • ♦: Magnetite PDF: 88-0866 ∗ 1500 • : Quartz PDF: 99-0088 ♣: Muscovite PDF: 72-1503 • • 1200 ♦ • ♦ ♦ • • • ♦ • ♦ 900 • • • CSRT ♣ ∗ Intensity (cps) ♣ 600 ∗ ♣♣ ♣ ♣ 300 ∗ ♣ ∗ ∗ ∗ Metakaolin ∗ ∗ ∗ 0 15 30 45 60 75 90 2θ (degree)

FigureFigure 2.2. XRD patternspatterns ofof thethe CSRTCSRT andand metakaolin.metakaolin. 2.2. Methods Table 1. Chemical composition of the, copper slag reprocessing tailings (CSRT) and metakaolin. For the synthesis of the CSRT-based geopolymer, the activating solution was prepared by sodium Components (%) SiO2 Al2O3 Fe2O3 MgO CaO TiO2 Na2O K2O silicate solution (6.17 M), and then mixed with the raw materials of CSRT and metakaolin for 10 min, CSRT 28.19 4.24 57.83 1.101 1.906 0.303 0.525 0.81 which were of 40%, 50% and 60% metakaolin and a total solid of 192 g. Subsequently, the mixture was Metakaolin 52.67 41.37 1.299 0.32 0.02 0.163 0.092 2.08 put into cubic steel molds (30 mm 30 mm 30 mm), which were vibrated to liberate air bubbles on a × × vibration table for 5 min. Next, the molds were sealed with a plastic sealing bag and cured at 60 C for 2.2. Methods ◦ 12 h and left at room temperature for 7 days to complete the hydrate process of geopolymers to get the initialFor the CSRT-based synthesis geopolymer. of the CSRT-based After that, geopolymer the initial, geopolymers the activating were solution exposed was or prepared cured under by threesodium types silicate of environment solution (6.17 for M), 30 days:and then (1) inmixed air; (2)with in the artificial raw materials seawater; of (3) CSRT in a heat–cooland metakaolin seawater for cycle.10 min, Geopolymers which were wereof 40%, exposed 50% and to artificial 60% metakaolin seawater atand room a total temperature solid of 192 for g. 12 Subsequently, h and 12 h under the themixture cycle was in a freezerput into ( cubic18 C). steel Artificial molds seawater(30 mm × with 30 mm 10-fold × 30 concentration mm), which waswere made vibrated with to 292.5 liberate g/L − ◦ NaCl,air bubbles 7.45 g on/L KCl,a vibration 36 g/L MgSOtable fo4.r The5 min. artificial Next, seawaterthe molds was were renewed sealed with every a sevenplastic days sealing throughout bag and thecured whole at 60 curing °C for process 12 h and [12 ].left at room temperature for 7 days to complete the hydrate process of geopolymersThe compressive to get the strength initial of theCSRT-based CSRT-based geopol geopolymersymer. After was analyzedthat, the byinitial a YAW-300 geopolymers compression were andexposed flexure or cured machine under from three Jinan types Tianchen of environment manufacture for 30 (Jinan, days: China).(1) in air; Three (2) in geopolymerartificial seawater; samples (3) werein a heat–cool tested and seawater the mean cycle. value Geopolymers was given in were each measurement.exposed to artificial The morphology seawater at and room microstructure temperature offor the 12 geopolymersh and 12 h under were the characterized cycle in a freezer using a(− scanning18 °C). Artificial electron seawater microscope with (SEM, 10-fold Quanta concentration 250, FEI, Hillsboro,was made OR, with USA) 292.5 and g/L XRD NaCl, diffractometer. 7.45 g/L KCl, The 36 geopolymer g/L MgSO was4. The ground artificial to less seawater than 75 µwasm to renewed prepare specimensevery seven for days XRD throughout measurements, the whole in which curing Cu-K processα1 radiation [12]. and a scanning rate of 0.1◦/s from 5◦ to 90◦Theof 2θcompressivewere used. Thestrength geopolymers of the CSRT-based were also analyzed geopolymers for the microstructurewas analyzed byby employinga YAW-300 a Fourier-transformcompression and infrared flexure spectroscopy machine from (FTIR, Jinan Nicolet, Tianchen Thermo Fishermanufacture Scientific, (Jinan, Waltham, China). MA, USA),Three 1 ofgeopolymer which transmittance samples were spectra tested were and obtained the mean over avalue wavenumber was given of 400–4000 in each cmmeasurement.− ; the resolution The 1 29 wasmorphology 2 cm− . The and interactions microstructure of silicate of the in geopolymers the geopolymers were were characterized measured byusing a Sia scanning nuclear magnetic electron resonancemicroscope (NMR, (SEM, AVANCE Quanta 250, III, Bruker, FEI, Hillsboro, Zurich, Switzerland) OR, USA) and instrument. XRD diffractometer. Solid-state 29 TheSi NMR geopolymer spectra was ground to less than 75 µm to prepare specimens for XRD measurements, in which Cu-Kα1 radiation and a scanning rate of 0.1°/s from 5° to 90° of 2θ were used. The geopolymers were also analyzed for the microstructure by employing a Fourier-transform infrared spectroscopy (FTIR, Nicolet, Thermo Fisher Scientific, Waltham, MA, USA), of which transmittance spectra were obtained over a wavenumber of 400–4000 cm−1; the resolution was 2 cm−1. The interactions of silicate in the geopolymers were measured by a 29Si nuclear magnetic resonance (NMR, AVANCE III, Bruker, Zurich, Switzerland) instrument. Solid-state 29Si NMR spectra were collected at 99.35 MHz with a

Minerals 2020, 10, 832 4 of 12 Minerals 2020, 10, x FOR PEER REVIEW 4 of 12 pulsewere collectedwidth of at4 µs 99.35 (i.e., MHz 90 degrees with a for pulse quantitative width of 4 analysis)µs (i.e., 90 and degrees a relaxation for quantitative delay of 3 analysis) s. The spectra and a wererelaxation referenced delay to of tetramethylsilane 3 s. The spectra were (TMS). referenced to tetramethylsilane (TMS).

3.3. Results Results and and Discussion Discussion FigureFigure 33 showsshows thethe compressivecompressive strengthstrength ofof CSRT-basedCSRT-based geopolymersgeopolymers preparedprepared withwith didifferentfferent proportions of metakaolin and diffe differentrent exposures. The The initial initial geopolymers, geopolymers, cured cured for for 7 7 days, days, had had a a compressivecompressive strength of 2.3 MPa, MPa, 7.8 7.8 MPa MPa and and 15.3 15.3 MPa MPa when when the the proportion proportion of of metakaolin metakaolin was was 40%, 40%, 50%50% and and 60%, 60%, respectively. respectively. The The low low value value in in compre compressivessive strength strength suggested suggested that that the the CSRT CSRT had had low low activityactivity in in the the geopolymeric geopolymeric reaction. reaction. Moreover, Moreover, me metakaolintakaolin was was obviously obviously advantageous advantageous to to increase increase thethe compressive compressive strength strength of of the the geopolymer. Compar Compareded to to the initial geopolymers, the compressive strengthstrength of of the the geopolymers that that were were exposed exposed to to air air for for 30 30 days had no obvious improvement or or lesseninglessening of of strength. strength. While While for for the the geopolymers geopolymers ex exposedposed to to seawater seawater for for 30 30 days, the compressive strengthstrength increased increased significantly significantly to to 10.7 10.7 MPa, MPa, 15.7 15.7 MPa MPa and and 32.6 32.6 MPa MPa with with 40%, 40%, 50% 50% and and 60% 60% of of metakaolinmetakaolin addition, addition, respectively. respectively. For For exposure exposure to to heat–cool heat–cool cycles cycles of of seawater, seawater, the the increase increase in compressivecompressive strength of geopolymers was less than than those those in in seawater, seawater, which were 3.4 MPa, 10.2 10.2 MPa MPa andand 23 23 MPa MPa with with 40%, 40%, 50% 50% and and 60% 60% of ofmetakaolin metakaolin addition, addition, respectively. respectively. These These results results show show that thatthe compressivethe compressive strength strength of the of thegeopolymers geopolymers was was impr improvedoved when when exposed exposed to to seawater, seawater, which which is is in agreementagreement with previous studies. Sotya Sotya et et al. al. repo reportedrted that that the the compressive compressive strength strength of of geopolymers geopolymers was not affected affected by seawater immersionimmersion [[11].11]. Jin etet al.al. found that cations inin seawaterseawater (e.g.,(e.g., MgMg22++ andand KK++) )could could diffuse diffuse into into the the aluminosilicate aluminosilicate network network st structureructure of of geopolymers geopolymers to to balance balance the the negative charge,charge, making the gel structure stable [[13].13].

35 in heat-cool cycle in seawater in air 28 initial strength

Compressive strength (MPa) 7

0 40% 50% 60% The proportion of metakaolin (%)

FigureFigure 3.3. CompressiveCompressive strength strength of theof geopolymersthe geopolymers prepared prepared with di ffwitherent different proportions proportions of metakaolin of metakaolinand different and exposures. different exposures. Figure4 gives the XRD spectra of the geopolymers prepared with 60% metakaolin and which Figure 4 gives the XRD spectra of the geopolymers prepared with 60% metakaolin and which underwent different exposures for 30 days. Compared to the XRD patterns of raw materials (Figure2), underwent different exposures for 30 days. Compared to the XRD patterns of raw materials (Figure the crystal phases of quartz, magnetite, fayalite and muscovite remained after the geopolymeric 2), the crystal phases of quartz, magnetite, fayalite and muscovite remained after the geopolymeric reaction. While for exposure in seawater, the solution penetrated pores and precipitated halite (NaCl) reaction. While for exposure in seawater, the solution penetrated pores and precipitated halite (NaCl) was observed. The range of broad peak at 2θ of 15–30° in metakaolin transformed into 2θ of 22–35° in the geopolymers with different exposures, which indicates the formation of N-A-S-H gel [14,15].

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was observed. The range of broad peak at 2θ of 15–30◦ in metakaolin transformed into 2θ of 22–35◦ in theMinerals geopolymers 2020, 10, x FOR with PEER diﬀ REVIEWerent exposures, which indicates the formation of N-A-S-H gel [14,15].5 of 12

•: Muscovite PDF:99-0082 2000 ∗ ♦ : Halite PDF:02-0818 ♦: Fayalite PDF:09-0484 •(α) ♣ ♦ ♣ ♣ ♦♦ : Magnetite PDF:99-0073 1600 • • α: Quartz PDF:99-0088 •♦ ♦ ♦ ♦♦ ♦ ♦ ♦ ♣ α In air •(α) ♦(∗) ∗ 1200 •(α) ♦ ♦ ♣ • ∗ •♣ ♦ ♦ ∗ In heat-cool cycle Intensity (cps) ♦ •♦ ∗ ♦♦ ♦ ♦♣ ∗ α ∗ 800 •(α) ♦(∗) ♦ •(α) ♣ ∗ ♦ ♣ ♦ • ∗ • • ♦ ♦ 400 ♦ ♦ In seawater ♦ ♦ ∗ ♦♣ ∗α ∗ •(α) ∗

15 30 45 60 75 90 θ 2 (degree) FigureFigure 4. 4.XRD XRD spectraspectra ofof thethe geopolymersgeopolymers prepared prepared with with 60% 60% metakaolin. metakaolin.

FigureFigure5 5shows shows the the FTIR FTIR spectra spectra of of geopolymers geopolymers prepared prepared with with 60% 60% metakaolin metakaolin and and di differentﬀerent 1 1 exposures.exposures. The The absorption absorption peaks peaks at at 3440 3440 cm cm− −1and and 1644 1644 cm cm−−1 areare duedue toto stretchingstretching vibrations vibrations of of OH OH−−,, andand H-OHH-OH bondsbonds ofof freefree water,water, respectively, respectively, corresponding corresponding to to adsorbed adsorbed HH22OO inin geopolymersgeopolymers [[16].16]. 1 2 TheThe absorption absorption peak peak around around 1453 1453 cm cm− −1is is assigned assigned to to asymmetric asymmetric stretching stretching of of O-C-O O-C-O bonds bonds in in CO CO3 −32− groups,groups, duedue toto carbonationcarbonation in the the curing curing and and exposi exposingng process process of of geopolymers geopolymers [17]. [17 The]. The peak peak at 699 at 1 699cm− cm1 is− theis theSi-O Si-O bonds bonds in inquartz, quartz, suggesting suggesting the the quartz quartz particles particles are are insoluble insoluble in thethe geopolymericgeopolymeric 1 1 reactionreaction [[18].18]. The absorption peaks at at 560 560 cm cm−−1 andand 460 460 cm cm−1 −representrepresent the the zeolite zeolite framework framework in the in thestructure structure of ofthe the geopolymer geopolymer [19,20], [19,20 ],which which indi indicatescates zeolite zeolite frameworks frameworks are formedformed duringduring thethe 1 process.process. The The band band at at about about 1000 1000 cm cm−−1 isis thethe Si-O-TSi-O-T bondsbonds (T(T representsrepresents thethetetrahedral tetrahedral Al Al or or Si) Si) in in the the geopolymergeopolymer gelgel [21[21].]. ItIt isis sensitivesensitive toto thethe SiSi/Al/Al ratio ratio in in the the geopolymer geopolymer backbone, backbone, which which could could shift shift towardtoward the the lower lower wavenumber wavenumber at at a a low low Si Si/Al/Al ratio ratio [ 22[22].]. The The wavenumber wavenumber transformation transformation indicates indicates thethe exposure exposure in in the the heat–cool heat–cool cyclecycle andand seawater,seawater, which which changedchanged thethe evolutionevolution ofof thethe N-A-S-HN-A-S-H gel.gel. InIn addition, addition, the the higher higher intensity intensity for for the the geopolymers geopolymers exposed exposed to to seawater seawater than than in in the the heat–cool heat–cool cycle cycle andand in in air, air, suggests suggests the the formation formation of of higher higher proportions proportions of of N-A-S-H N-A-S-H gel. gel. NMRNMR spectroscopy spectroscopy is is an an excellent excellent analytical analytical method method for for characterizing characterizing the the short-range short-range ordering ordering andand molecular molecular structure structure of of silicates silicates [ 23[23,24].,24]. ItIt employsemploys GaussianGaussian peak peak deconvolution deconvolution to to overcome overcome the the lacklack of of spectraspectra resolutionresolution andand separate and and quantify quantify QQnn(mAl)(mAl) species species (0 (0≤ m m≤ n ≤n4, m4,, nm =, ninteger).= integer). The ≤ ≤ ≤ Theresonances resonances at − at74 and74 and −79 ppm79 ppm are areassigned assigned to (Q to 0(Q) and0) and (Q1(Q), 1respectively,), respectively, due due to tothe the presence presence of − − ofsilicate silicate monomer monomer and and dimer dimer [25]. [25]. The The resonance resonance at at −104104 ppm ppm represents Q3(R)(R) when H inin OHOH isis − substitutedsubstituted by by alkali alkali metal metal ion ion (Na (Na+ +or or K K++)) inin QQ33 [[2],2], andand thethe resonanceresonance at at −114114 ppmppm correspondscorresponds toto − thethe cristobalite cristobalite in in the the geopolymers geopolymers [ 26[2].6]. In In geopolymer geopolymer N-A-S-H N-A-S-H gel, gel, the theQ Q4(44(4Al),Al), Q Q4(43(3Al),Al), Q Q44((22Al),Al), QQ4(4(11Al),Al), QQ44((00AlAl)) resonateresonate atat aroundaround −84, −89, −93,93, −9999 and and −108108 ppm, ppm, respectively respectively [19]. [19]. − − − − −

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In air 1644 100 1452 694 567 3439 In heat-cool cycle 1654 1466 460 80 699 In seawater 1000 557 3436 1643 1453 60 705 459 3444 1004 560 40

Transmittance (A.U.) Transmittance 460 1009 20

4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm-1)

Figure 5. FTIR spectra of the geopolymers prepared with 60% metakaolin and different exposures. Figure 5. FTIR spectra of the geopolymers prepared with 60% metakaolin and different exposures. Figure6 gives the 29Si NMR spectra and deconvolution of the raw metakaolin and CSRT-based Figure 6 gives the 29Si NMR spectra and deconvolution of the raw metakaolin and CSRT-based geopolymers prepared with 60% metakaolin and exposed differently for 30 days. Metakaolin geopolymers prepared with 60% metakaolin and exposed differently for 30 days. Metakaolin predominantly peaked at around 105 ppm. After geopolymerization, the 29Si NMR lines shifted predominantly peaked at around −−105 ppm. After geopolymerization, the 29Si NMR lines shifted to to less shielded values due to the geopolymeric reaction taking place to form N-A-S-H gel. For the less shielded values due to the geopolymeric reaction taking place to form N-A-S-H gel. For the geopolymers exposed to air, the heat–cool cycle and seawater, the Si sites in silicate monomers (Q0) geopolymers exposed to air, the heat–cool cycle and seawater, the Si sites in silicate monomers (Q0) are 1.39%, 1.26% and 1.23%, those in silicate dimer (Q1) are 8.63%, 6.65% and 6.10%, and in Q3(R) are are 1.39%, 1.26% and 1.23%, those in silicate dimer (Q1) are 8.63%, 6.65% and 6.10%, and in Q3(R) are 7.69%, 1.32% and 3.52%, but those in N-A-S-H gel (Q4(mAl), 0 m 4) are 79.21%, 90.77%, and 87.78%, 7.69%, 1.32% and 3.52%, but those in N-A-S-H gel (Q4(mAl), 0 ≤ m ≤≤ 4) are 79.21%, 90.77%, and 87.78%, respectively. The Si sites in Q0 and Q1 slightly decrease and those in N-A-S-H gel increase when respectively. The Si sites in Q0 and Q1 slightly decrease and those in N-A-S-H gel increase when the the geopolymers are exposed to seawater, indicating that the Q0 and Q1 transformed into N-A-S-H geopolymers are exposed to seawater, indicating that the Q0 and Q1 transformed into N-A-S-H gel. It gel. It has been reported that the compressive strength relies on the amount of Q4(3Al), Q4(2Al) and has been reported that the compressive strength relies on the amount of Q4(3Al), Q4(2Al) and Q4(1Al) Q4(1Al) [27]. The percentages of Q4(3Al), Q4(2Al) and Q4(1Al) are 51.08%, 63.78% and 64.78% for [27]. The percentages of Q4(3Al), Q4(2Al) and Q4(1Al) are 51.08%, 63.78% and 64.78% for the exposures the exposures in air, heat–cool cycle and seawater, respectively, which explained the variation in in air, heat–cool cycle and seawater, respectively, which explained the variation in compressive compressive strength of geopolymers. These results suggest that exposure in seawater promotes the strength of geopolymers. These results suggest that exposure in seawater promotes the geopolymerization process. geopolymerization process. Figure7 shows the SEM morphology of the geopolymers that underwent di fferent exposures for Figure 7 shows the SEM morphology of the geopolymers that underwent different exposures for 30 days. The geopolymers exposed to both the heat–cool cycle and seawater showed homogenous and 30 days. The geopolymers exposed to both the heat–cool cycle and seawater showed homogenous compact structures, indicating geopolymers with 60% metakaolin can be consolidated well. However, and compact structures, indicating geopolymers with 60% metakaolin can be consolidated well. cracks were exhibited in geopolymers exposed to air. This result further confirmed that more N-A-S-H However, cracks were exhibited in geopolymers exposed to air. This result further confirmed that gel was formed in the geopolymers that were exposed to both the heat–cool cycle and seawater, thereby more N-A-S-H gel was formed in the geopolymers that were exposed to both the heat–cool cycle and bridging the cracks, and thus increasing their compressive strength. seawater, thereby bridging the cracks, and thus increasing their compressive strength. In this study, the geopolymers exposed to seawater showed a high formation of N-A-S-H gel, thereby exhibited a higher compressive strength than the geopolymers exposed to air. When geopolymers were exposed to the marine environment, alkaline ions (e.g., Mg2+, Na+) in seawater functioned as balancing cations to the negative charge of the tetra-coordinated aluminum, thereby promoting the geopolymerization process [28]. Therefore, for the geopolymers exposed to seawater, more Si-O-T gel was formed (Figure5) and the Q0 and Q1 species were transformed into N-A-S-H gel (Figure6).

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180,000 Q4(4Al): 1.02% Metakaolin Q4(3Al): 8.10% 150,000 Q4(2Al): 22.85% Q4(1Al): 30.35%

) 120,000 4 Q (0Al): 33.37% 4 Q 0 cps Cristobalite: 4.30% 4 ( Q 1 y

it 90,000 Q4 ens 2 t n I 60,000

4 30,000 Q 3

4 Cristobalite Q 4 0 -70 -80 -90 -100 -110 -120 Chemical shift (ppm)

70,000 0 In air Q : 1.39% Q1: 8.63% 3 56,000 Q (R): 7.69% 4 4 Q 3 Q (4Al): 24.24% 4 4 Q 4 Q (3Al): 21.35% Q4 42,000 2 Q4(2Al): 17.72% Q4(1Al): 12.01% Q4(0Al): 3.89% 4 28,000 Q 1

Intensity(cps) Cristobalite: 3.07% Q1 Q3(R) 14,000 4 Cristobalite Q0 Q 0

0 -70 -80 -90 -100 -110 -120 Chemical shift (ppm)

Figure 6. Cont.

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In heat-cool cycle Q0: 1.26% 75,000 Q1: 6.65% Q3(R): 1.32% Q4(4Al): 25.84% 60,000 Q4(3Al): 26.26% Q4 4 4 4 3 Q 2 Q (2Al): 27.07% Q 4 45,000 Q4(1Al): 10.45% Q4(0Al): 1.15%

Intensity (cps) 30,000

4 Q 1 15,000 Q1 Q0 3 4 Q (R) Q 0 0 -70 -80 -90 -100 -110 -120 Chemical shift (ppm)

In seawater Q0: 1.23% 75,000 Q1: 6.10% Q3(R): 3.52% Q4(4Al): 21.52% 60,000 Q4 Q4(3Al): 24.88% Q4 2 3 Q4(2Al): 28.11% 4 4 45,000 Q 4 Q (1Al): 11.79% Q4(0Al): 1.48%

Intensity (cps) Intensity Cristobalite: 1.36% 30,000 4 Q 1 1 15,000 Q 0 Q 3 Q (R) Cristobalite Q4 0 0 -70 -80 -90 -100 -110 -120 Chemical shift (ppm)

Figure 6. 29Si NMR spectra and deconvolution of the geopolymers prepared with 60% metakaolin. Figure 6. 29Si NMR spectra and deconvolution of the geopolymers prepared with 60% metakaolin.

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In air

Crack

In heat-cool cycle

Figure 7. Cont.

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In seawater

Figure 7. SEM images of the geopolymers prepared with 60% metakaolin. Figure 7. SEM images of the geopolymers prepared with 60% metakaolin. 4. Conclusions InMine this tailings study, afterthe geopolymers the flotationof exposed copper slagto seaw areater synthesized showed intoa high geopolymers formation withof N-A-S-H the addition gel, therebyof metakaolin exhibited (40–60%), a higher which compressive enhances their strength compressive than the strength. geopolymers For the geopolymersexposed to exposedair. When to geopolymersseawater, the Siwere/Al changedexposed into thethe N-A-S-H marine environment, gel backbone, alkaline more Q4 (ions3Al) ,(e.g.,Q4(2 Al)Mgand2+, NaQ+4)( 1inAl) seawatergel and functionedfewer cracks as formed, balancing resulting cations in to a higherthe negative compressive charge strength of the tetra-coordinated being observed for aluminum, these geopolymers thereby promotingthan for those the exposedgeopolymerization to the heat–cool process cycle [28]. of Therefore, seawater and for inthe air. geopolymers This is attributed exposed to to the seawater, fact that morealkaline Si-O-T ions gel in seawaterwas formed balance (Figure the 5) negative and the chargeQ0 and ofQ1 the species aluminum were transformed tetrahedrons into in geopolymers, N-A-S-H gel which(Figure promote 6). the formation of N-A-S-H gel. As understanding the evolution of geopolymer in marine environments is important, this study is of significance and would benefit the development 4.of Conclusions marine concrete incorporating tailings. The optimization of compressive strength and long-term durabilityMine oftailings the CRST-based after the flotation geopolymer of copper and tailoring slag are of CRST-basedsynthesized geopolymerinto geopolymers marine with concrete the additionshould be of studied metakaolin in the (40%–60% future. ), which enhances their compressive strength. For the geopolymers exposed to seawater, the Si/Al changed in the N-A-S-H gel backbone, more Q4(3Al), Q4(2Al) and Author Contributions: J.W.: figures, study design, data collection, data analysis, and writing. J.L.: literature Qsearch,4(1Al) figures. gel and F.R.: fewer study cracks design, formed, data analysis,resulting data in a interpretation, higher compressive and writing. strength W.Y.: being study observed design, data for theseanalysis, geopolymers and data interpretation. than for those All authors exposed have to read the and heat–cool agreed to cycle the published of seawater version and of thein manuscript.air. This is attributedFunding: This to the study fact was that financially alkaline supported ions in seaw by theater National balance Natural the negative Science Foundationcharge of the of Chinaaluminum under tetrahedronsthe project No. in 51974093 geopolymers, and the which Fuzhou promote University the Qishan formation Scholar of N-A-S-H Talent Foundation gel. As understanding under the Grant the No. GXRC-18042, for which the authors are grateful. evolution of geopolymer in marine environments is important, this study is of significance and would benefitConflicts the of development Interest: We declare of marine that weconcrete do not incorporating have any commercial tailings. or associativeThe optimization interest thatof compressive represents a conflict of interest in connection with the work submitted. strength and long-term durability of the CRST-based geopolymer and tailoring of CRST-based geopolymer marine concrete should be studied in the future. References

Author1. Park, Contributions: I.; Tabelin, C.B.; J.W.: Jeon, figures, S.; Li, study X.; Seno, design, K.; Ito,data M.; collection, Hiroyoshi, data N. Aanalysis, review and of recent writing. strategies J.L.: literature for acid search,mine figures. drainage F.R.: prevention study design, and minedata tailingsanalysis, recycling. data interpretation,Chemosphere 2018and ,writing.219, 588–606. W.Y.: [ CrossRefstudy design,][PubMed data] analysis,2. Tian, and X.; data Xu, interpretation. W.; Song, S.; Rao, All authors F.; Xia, L.have Eﬀ ectsread of and curing agreed temperature to the published on the version compressive of the strengthmanuscript. and Funding:microstructure This study ofwas copper financially tailing-based supported geopolymers. by the NationChemosphereal Natural2020 Science, 253 Foundation, 126754. [CrossRef of China][ PubMedunder the] project3. Yang, No. 51974093 F. Geopolymerization and the Fuzhou of Copper Univ Mineersity Tailings Qishan; The Scholar University Talent of Foun Arizona:dation Tucson, under the AZ, Grant USA, No. 2012. GXRC- 18042, for which the authors are grateful.

Conflicts of Interest: We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

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