Removal of Hexavalent Chromium in Portland Cement Using Ground Granulatedblast-Furnace Slag Powder

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Article Removal of Hexavalent Chromium in Portland Cement Using Ground Granulated Blast-Furnace Slag Powder

Sungchul Bae 1,* ID , Fumino Hikaru 2, Manabu Kanematsu 2, Chiaki Yoshizawa 3, Takafumi Noguchi 4, Youngsang Yu 5 ID and Juyoung Ha 6 1 Department of Architectural Engineering, Hanyang University, Seoul 04763, Korea 2 Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan; [email protected] (F.H.); [email protected] (M.K.) 3 Research Institute of Science and Technology, Nihon University College of Science and Technology, Chiyoda, Tokyo 101-8308, Japan; [email protected] 4 Department of Architecture, Graduate school of Engineering, The University of Tokyo, Bunkyo, Tokyo 113-8654, Japan; [email protected] 5 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA; [email protected] 6 School of Environmental and Sustainability Sciences, Kean University, Union County, NJ 07083, USA; [email protected] * Correspondence: [email protected]; Tel.: +82-2-2220-0302

Received: 23 October 2017; Accepted: 20 December 2017; Published: 22 December 2017

Abstract: Using ground granulated blast-furnace slag (GGBS) under different alkaline conditions, we studied the mechanisms and extents of Cr(VI) reduction and sorption and compared them to reactions with Portland cement (PC). We also investigated the effects of mixing PC/GGBS ratios on Cr(VI) dissolution after carbonating the substrates. We observed a complete sorption and reduction of Cr(VI) to Cr(III) in a GGBS-in-Ca(OH)2 solution (pH > ~12.5) after 10 h, whereas in distilled water (pH = ~11.5) GGBS exhibited only marginal sorption and reduction (20%). Cr reactions with dissolved ions in supernatants derived from GGBS indicated that the anions dissolved from GGBS act as a reducing agent for Cr(VI) in a Ca(OH)2 solution. Soft X-ray absorption microscopy identified a partial reduction of Cr(VI) to Cr(III) on the GGBS surface. The carbonation of pure PC paste substantially increased the amount of dissolved Cr(VI) in a solution phase whereas a 5 wt % replacement of PC with GGBS significantly reduced the amount of dissolved Cr(VI). We concluded that in the mixed paste during the early curing stage GGBS reduced a significant fraction of Cr(VI) to Cr(III) and that the Cr(III) adsorbed in the GGBS-PC mixture’s hydration products does not readily dissolve, even under carbonation conditions.

Keywords: hexavalent chromium; reduction; immobilization; slag; Portland cement

1. Introduction Hexavalent chromium [Cr(VI)] is found in concrete as chromate, which results from the oxidation process of trivalent chromium [Cr(III)] during Portland cement (PC) clinker production [1,2]. Chromium (Cr) can exist in several different oxidative states and its physical and chemical properties, such as solubility, toxicity, and sorption afﬁnity, vary with environmental conditions. For example, Cr(III) is likely to precipitate in alkaline solutions, whereas Cr(VI) is amphoteric with a concentration-dependent solubility [3]. Cr(VI) is highly mobile but very toxic under aerobic conditions [1,4,5]. Cr(III) has a strong afﬁnity to soils and other substrates; while it is less soluble and toxic than Cr(VI), its potential for oxidation to Cr(VI) should be carefully examined in order to fully understand Cr transport and distribution in the environment. Therefore, examining Cr(VI) dissolution,

Materials 2018, 11, 11; doi:10.3390/ma11010011 www.mdpi.com/journal/materials Materials 2018, 11, 11 2 of 17 sorption, and reduction in PC clinker is of the utmost important for environmental concerns and for the long-term sustainability of PC materials in construction materials [3,6]. In the past, stabilization and solidification (s/s) have been practiced on immobilized PC-associated Cr(VI) [7–9]. Stabilization involves a reductive transformation of Cr(VI) to Cr(III) and a subsequent chemical transformation into a highly insoluble compound, such as calcium chromate [10,11]. Solidification involves the co-precipitation of Cr(VI) with dissolved PC hydration products during hydration reactions [10]. Calcium silicate hydrate (C-S-H), which is the primary hydration and binding product in PC-based materials, contains most of the microporosity, resulting in a large specific surface area that provides physical sorption and co-precipitation sites for heavy metal ions like Cr(VI). It should also be noted that Zamorani et al. [12] observed co-precipitated phase of CaCrO4 when Cr(VI) reacted with C-S-H, suggesting that molecular-level surface reactions are important in determining the reaction rates and pathways when hydrated products of PC-based materials react with Cr(VI) [3]. Ettringite (3CaO·Al2O3·3CaSO4·32H2O) has also been identified as one of PC’s hydration products and, with its highly reactive and needle-like column structure, has been proposed as a surface 2− 2− site for Cr(VI) immobilization via anion substitution, where CrO4 is exchanged for SO4 [13,14]. Another study identified that reductive reactions occur during the curing phases and that the resulting Cr(III) also replaces Al(III) in the ettringite crystalline structure [15]. Ionic substitutions in ettringite were also observed to be Cr(VI)–ettringite and Cr(III)–ettringite phases that further limit the mobility 2− of Cr [8,16]. Cr(VI) can be substituted for SO4 in the interlayer of calcium aluminate monosulfate (hereafter referred to as monosulfate, 3CaO·Al2O3·CaSO4·12H2O) [16]. However, the pH stability range of reactions for Cr(VI)–ettringite and Cr(VI)-hydration products of PC is very narrow and varies significantly [16,17]. The alkalinity of the PC-based matrix is important for decreasing the dissolution of Cr ions [3,11]. However, the carbonation process neutralizes the alkaline conditions and increases the Cr(VI) solubility for PC pastes [18]. A decrease in the pH conditions due to carbonation causes the decomposition of Cr(VI)-bearing hydration products in cementitious materials, which leads to more dissolved aqueous Cr(VI) [19]. Ground granulated blast-furnace slag (GGBS) is a well-known by-product of the pig-iron manufacturing process, and up to 60–70% of GGBS is often incorporated into concrete in order to recycle industrial waste materials [20]. GGBS is capable of immobilizing Cr(VI) via a reductive process: dissolved S2− ions from GGBS transform Cr(VI) into Cr(III) [3]. On the contrary, Macias et al. [18] observed an increased dissolution of Cr(VI) in GGBS-PC pastes due to the Cr(VI) that was released into the cement materials’ pore solution. The effect of GGBS on Cr(VI) reduction and immobilization has not yet been quantitatively studied, and the exact mechanism of GGBS remains unclear. The aim of this study is to elucidate the mechanism of GGBS on Cr(VI) speciation under different alkaline levels. Ca(OH)2 was utilized in order to maintain a high alkaline condition for the activation of GGBS. We also studied GGBS’s capacity to reduce and immobilize Cr(VI), comparing it to that of pure PC. We performed quantitative determinations of the Cr(VI) using a UV-visible spectrophotometer. The dissolved cations and anions in Cr(VI)’s reductive transformation in the solution phase were quantified under different alkaline conditions using ion chromatography. The phases for each sample that was made to react with different Cr(VI) concentrations were identified using X-ray diffraction (XRD). Synchrotron-based X-ray microscopy and spectroscopy were used to identify the detailed chemical speciation of Cr that had reacted with the Ca(OH)2-activated GGBS particles. Finally, an accelerated carbonation test was performed to examine the effects of carbonation on Cr(VI) leaching from a GGBS-PC paste. Materials 2018, 11, 11 3 of 17

2. Experimental Methods

2.1. Material and Sample Descriptions The chemical compositions of PC and GGBS were determined using X-ray fluorescence (XRF, Bruker, Karlsruhe, Germany) (Table1). Research-grade PC was used instead of commercial PC to minimize any possible impurities and their subsequent interference with our experimental results.

Table 1. The chemical compositions of the raw materials.

Chemical Compositions (%) Materials SiO2 Al2O3 Fe2O3 CaO MgO SO3 Na2OK2O TiO2 P2O5 MnO Total PC 20.94 5.45 2.83 64.96 1.5 2.05 0.32 0.48 0.27 0.31 0.08 99.22 GGBS 33.03 14.82 0.42 41.44 6.51 2.07 0.27 0.27 0.53 0.04 0.18 99.9

Table2 shows the mix proportions and experimental measurements for the different samples. Four different series of experimental samples were prepared, and they are denoted as S1, S2, S3, and S4. In the S1 samples, we examined Cr(VI) reduction and immobilization using PC or GGBS. A Cr(VI) solution (0.5 mg/L) was prepared by dissolving potassium dichromate (K2Cr2O7) in distilled water. The GGBS or PC powder (40 g) was added into the Cr(VI) solutions, and the suspensions were agitated at 20 ◦C in 60% relative humidity (RH) conditions. The supernatant of the S1 samples was filtered, and the concentration of the Cr(VI) remaining in the filtered solution was determined. The changes in pH of the S1 suspensions were measured via a hand-held pH meter (D-52, Horiba, Tokyo, Japan).

Table 2. The mix proportions of the specimens.

Mix Proportions Sample Distilled GGBS CH PC Cr(VI) Concentration Measurements Water (L) (g) (g) (g) (mg/L) PC-S1 - - 40 UV-vis, XRD, pH GGBS-S12 40 - -0.5 UV-vis, XRD, pH GGBS-CH-S1 40 20 - UV-vis, XRD, pH, Soft X-ray absorption microscopy PC-S2 - - 48 GGBS-S22.4 48 - - 0.5 UV-vis GGBS-CH-S2 48 24 - GGBS-CH-S3 2 40 20 - 0.5~100 UV-vis, XRD GGBS-S4 2 40 - - - Ion chromatography GGBS-CH-S4 2 40 20 - -

GGBS = Granulated ground blast-furnace slag; CH = Ca(OH)2; PC = Portland cement; XRD = X-ray diffraction; UV-vis = Ultraviolet-visible spectrophotometry.

S2 samples were prepared in order to verify what effects the dissolved ions from PC or GGBS had on Cr(VI) reduction in the solution phase. The PC or GGBS powder (48 g) was mixed with distilled water (2.4 L), and the substrates were equilibrated in distilled water for 24 h to ensure the dissolution of the substrates. After 24 h, the supernatant was filtered, and 1.2 mg of K2Cr2O7 was added to the filtered solutions containing the dissolved ions from the paste samples. Changes to the initial Cr(VI) concentration (0.5 mg/L) was measured at 0, 1, 3, 6, 12, and 24 h in the reacted solutions to identify the extent of the reductive reaction that was due to the dissolved ions from the substrates in the solutions. The S3 experiments series aimed to determine the Cr(VI) sorption capacity of GGBS. Different amounts of Cr(VI) ranging from 0.5 to 100 mg/L were added to a GGBS suspension. After agitating the suspension, the remaining Cr(VI) concentration was determined at 120 and 150 h. Lastly, the S4 series was used to determine the different chemical species of the dissolved free ions from the GGBS samples under different alkaline condition. Ion chromatography (Dionex ICS-2000, Sunnyvale, CA, USA), using suppressed conductivity detection, was performed to quantify the ions that were dissolved in S4 samples. The Cr(VI) concentration in S4 samples was measured using a UV-visible spectrophotometer (DR 5000™, Hach, Materials 2018, 11, 11 4 of 17

London, ON, Canada) and the EPA 7196A method with 1,5-diphenylcarbazide (Cr(VI) Detection limit, Max: 1.0 mg/L, Min: 0.03 mg/L). All the solutions were filtered through a 0.45-µm membrane filter, and the alkaline conditions of the solutions were maintained using pure Ca(OH)2.

2.2. Accelerated Carbonation and Tank Leaching Test GGBS-PC pastes (ø50 × 100 mm) were made at GGBS replacement ratios of 0, 5, 10, and 50 wt %. The water to total solid (GGBS + PC) ratio (w/s) of the pastes was maintained at 0.6 for all the samples. The total initial Cr(VI) concentration was at 0.5 mg/L per 100 g of paste. During the first 24 h of curing, plastic sheets covered the exposed surfaces of the specimen in the mold. The specimens were demolded after 24 h and were cured for an additional 28 days at 20 ◦C in 60% RH conditions. For accelerated carbonation, the final sample was finely grounded and placed in a carbonation chamber at 20 ◦C with 5% CO2 and 60% RH for 28 days. After the carbonation period was completed, the specimens were kept in an ethanol suspension and stored in a vacuum drying chamber until further analysis. The hydration products were identified both before and after 28 days of carbonation. After carbonation, the remaining Cr(VI) concentration in the samples was measured according to Regulation No. 49 (The Ministry of Land, Infrastructure and Transport, Japan).

2.3. Phase Identiﬁcation The raw material phases, the hydration products in the filtered solids derived from the S1 and S3 suspensions, and the accelerated carbonation test specimens before and after carbonation were all identified via powder XRD using monochromatized CuKα radiation (λ = 1.54 Å, at a voltage of 30 kV, a current of 10 mA, in the range of 2θ = 5◦–75◦, D2 Phaser, Bruker, Karlsruhe, Germany). The residues collected after filtering were dried and used for the XRD analysis. Prior to the sample preparation, both the carbonation of the raw materials and the purity of Ca(OH)2, which was used as a reference compound, were checked using XRD. Although great care was taken to avoid carbonation of the pure Ca(OH)2, very small peaks of CaCO3 were seen in the XRD pattern, and the amount of CaCO3 was quantified as ~2 wt % via Rietveld refinement using the Topas software (version 4.3, Bruker, Karlsruhe, Germany). This result may be due to the unavoidable carbonation of pure Ca(OH)2 during the XRD measurements.

2.4. Scanning Transmission X-ray Microscopy (STXM) Spatial correlations between the chemical compositions and their oxidation states were mapped by a combination of scanning transmission X-ray microscopy (STXM) and X-ray absorption spectroscopy (XAS) across Cr L2,3-edges (560–600 eV) and the O K-edge (525–560 eV), which allows us to image distributions of the chemical species with spectral sensitivity from very small spots (~35 nm). STXM measurements were performed at the bending magnet beamline (5.3.2.1) at the Advanced Light Source (ALS) in the Lawrence Berkeley National Laboratory, Berkeley, CA, USA [21]. To obtain transparency in the soft X-ray region, the diluted mixtures (dried leachate of GGBS-CH-S3 and 10 mg/L of added Cr(VI)) were drop-casted with isopropanol onto a 50-nm-thick Si3N4 membrane (Silson Inc., Warwickshire, England). STXM measurements utilized a 25-nm outer-width Fresnel zone plate for focusing the X-ray. A raster scanned onto 8 × 8 µm2 field-of-views with a square scan grid of 35 nm steps. Image spectra (i.e., repeated images at different energies), whose finest energy step was 0.1 eV near the absorption resonance, were recorded by a phosphor photomultiplier tube (PMT). The intensity transmitted through the samples (I) was converted into the optical density (OD) by taking the log ratio of the incident (I0) and transmitted (I) intensities (Equation (1)):

I OD = − ln = σ·t = µ·ρ·t, (1) I0 where I0: incident X-ray photon intensity, t: specimen thickness, µ: mass absorption coefficient, and ρ: density of the specimen. Materials 2018, 11, 11 5 of 17 Materials 2018, 11, 11 5 of 17

To overcome spectral distortions resulting from the acquisition time gap between the I0 and I To overcome spectral distortions resulting from the acquisition time gap between the I0 and I measurements, thethe incidentincident beam beam intensity intensity was was simultaneously simultaneously measured measured through through an openan open area area of the of sample.the sample. After After image image registration registration by a by phase-correlation a phase-correlation algorithm, algorithm, the XASthe XAS spectra spectra of each of each pixel pixel as a functionas a function of energy of energy were corrected.were corrected. All data All processing data processing was carried was outcarried using out the using Axis2000 the Axis2000 software (versionsoftware 8.3,(version McMaster 8.3, Univ.,McMaster Hamilton, Univ., ON, Hamilt Canada)on, ON, [22]. Canada) Further details[22]. Further regarding details applications regarding of STXMapplications to the of study STXM of cementitiousto the study of systems cementit canious be foundsystems elsewhere can be found [23–26 elsewhere]. [23–26].

3. Results and Discussion

3.1. Cr(VI) Removal Using PC and GGBS-PC Samples Figure 11aa showsshows thethe timetime profilesprofiles ofof thethe pHpH valuesvalues forfor thethe suspensionssuspensions retrievedretrieved fromfrom thethe S1S1 samples over over 120 120 h. h. The The pH pH value value for for PC-S1 PC-S1 increased increased from from 12.7 12.7 to 13.2 to 13.2 in 12 in h 12and h decreased and decreased to 12.5. to 12.5.This is This in good is in goodagreement agreement with withPC paste, PC paste, which which has a has water a water to binder to binder ratio ratio (W/B) (W/B) of 0.3 of [27]. 0.3 [27No]. Nosignificant significant changes changes in pH in pH were were observed observed afterwar afterwards.ds. On On the the other other hand, hand, the the pH pH value value for for GGBS-S1 GGBS-S1 increased from from 11.1 11.1 to to 11.7 11.7 in in24 24h and h and remained remained in the in 11.6–11.8 the 11.6–11.8 range range until 120 until h 120had hpassed. had passed. When Whencompared compared to PC-S1 to PC-S1 and andGGBS-S1, GGBS-S1, GGBS-CH-S1, GGBS-CH-S1, to towhich which Ca(OH) Ca(OH)2 2waswas added, added, exhibited comparatively small changes in pH within the first first 20 h. It had a constant value of 12.7 until 120 h had passed. Past studies indicate that GGBS’s activa activationtion is is strongly strongly dependent dependent on on pH pH values values [28,29]. [28,29]. Higher pHs lead to improved GGBS activation, with a value higher than 11.5 leading to an effective activation [[30].30]. Therefore, Therefore, alkaline alkaline conditions conditions for for GGBS GGBS activation can be maintained by adding Ca(OH)22 (GGBS-CH-S1).(GGBS-CH-S1). TheThe effect effect of of alkalinity alkalinity on on GGBS GGBS activation activation was was further further confirmed confirmed in the in XRD the resultsXRD results (see Section (see Section 3.4). 3.4).

(a) 14 (b) 0.9 0.8 PC-S1 GGBS-S1 13 0.7 GGBS-CH-S1 0.5 mg/L 0.6 12 0.5

pH 0.4 11 0.3

PC-S1 0.2 10 GGBS-S1 0.1 GGBS-CH-S1

Concentration of Cr(VI) in Solution (mg/L) 0.0 9 0 20 40 60 80 100 120 0 20406080100120 Time (Hours) Time (Hours)

Figure 1.1.Time Time proﬁles profiles of (aof) the (a) pH the values pH ofvalues S1 samples’ of S1 suspensionssamples’ suspensions and (b) the Cr(VI)and ( concentrationsb) the Cr(VI) inconcentrations the S1 samples’ in the ﬁltered S1 samples’ solutions filter measureded solutions from measured 0 to 120 h. from 0 to 120 h.

Figure1 1bb depicts depicts the the Cr(VI) Cr(VI) concentration concentration of of the the supernatants supernatants ([Cr(VI)] ([Cr(VI)]supernatantssupernatants)) retrieved retrieved from the S1S1 samplessamples asas aa functionfunction of of time. time. Within Within 1 1 h, h, [Cr(VI)] [Cr(VI)]supernatantssupernatants increasedincreased from from 0.50 toto 0.700.70 mg/Lmg/L for PC samples. FollowingFollowing this,this, [Cr(VI)][Cr(VI)]supernatantssupernatants graduallygradually decreased decreased over over 120 120 h h to to below below the detection limit (0.03(0.03 mg/L). mg/L). The The initial initial increase increase in the in Cr(VI) the Cr(VI) concentration concentration is likely is due likely to the due initial to dissolutionthe initial ofdissolution the cement of clinkersthe cement in the clinkers PC samples. in the In PC the samples. GGBS-S1 withoutIn the GGBS-S1 Ca(OH)2 ,without the [Cr(VI)] Ca(OH)supernatants2, the decreased[Cr(VI)]supernatants from 0.5decreased to 0.4 mg/L fromwithin 0.5 to 0.4 5 h mg/L and remained within 5 relativelyh and remained constant relatively at 0.40 mg/L constant over at 120 0.40 h. Inmg/L contrast, over 120 a rapid h. In decrease contrast, was a rapid observed decrea forse the was GGBS-CH-S1 observed for in Ca(OH)the GGBS-CH-S12 solution, within Ca(OH) its the2 [Cr(VI)]solution,supernatants with its thereaching [Cr(VI)] 0supernatants mg/L within reaching 24 h. 0 Thismg/L is within an interesting 24 h. Th observationis is an interesting because observation increasing thebecause pH condition increasing is the expected pH condition to increase is expected [Cr(VI)]supernatants to increase. Instead, [Cr(VI)] wesupernatants observed. Instead, a [Cr(VI)] we observedsupernatants a decrease[Cr(VI)]supernatants in thepresence decrease ofinthe the GGBS presence paste. of the Our GGBS results pa suggestste. Our that results GGBS suggest has an that impact GGBS on has Cr(VI) an reactionimpact on pathways Cr(VI) inreaction Ca(OH) pathways2 conditions in andCa(OH) that2 alkali-activated conditions and GGBS that canalkali-activated remove and immobilizeGGBS can Cr(VI)remove more and immobilize than the untreated Cr(VI) more GGBS than in distilled the untreated water can.GGBS in distilled water can.

Materials 2018, 11, 11 6 of 17 Materials 2018, 11, 11 6 of 17

3.2.3.2. Cr(VI) Cr(VI) Reduction Reduction via via Dissolved Dissolved Ions Ions from from GGBS GGBS

FigureFigure 22 showsshows thethe evolutionevolution ofof thethe Cr(VI)Cr(VI) concentration concentration in in the the ﬁltered filtered solutions solutions ([Cr(VI)] ([Cr(VI)]aqueousaqueous)) ofof the the S2 S2 samples samples as as a afu functionnction of of time. time. [Cr(VI)] [Cr(VI)]aqueousaqueous decreasesdecreases when when it reacts it reacts with withdissolved dissolved ions fromions fromPC and PC GGBS. and GGBS. Water-S2, Water-S2, the control the control sample, sample, is a mixture is a mixture composed composed of distilled of distilled water water and Kand2Cr2 KO27Cr mixed2O7 mixed with a with pre-determined a pre-determined Cr(VI) Cr(VI) concentr concentrationation of 0.5 of mg/L, 0.5 mg/L, brought brought to reaction. to reaction. The initialThe initial [Cr(VI)] [Cr(VI)]aqueousaqueous for allfor the all samples the samples was was 0.5 0.5mg/L. mg/L. During During the the first ﬁrst hour, hour, a aslight slight increase increase in in [Cr(VI)][Cr(VI)]aqueousaqueous forfor the the PC-S2 PC-S2 sample sample (~0.52 (~0.52 mg/L), mg/L), when when compared compared to to Water-S2 Water-S2 (~0.47(~0.47 mg/L),mg/L), was was likelylikely due due to to Cr(VI) Cr(VI) dissolving dissolving from from the the cement cement clinker, clinker, which which we we also also observed observed in in Figure Figure 11bb because PCPC inherently inherently contains a small amount of trace Cr(VI).

1.1 0.6

1.0 PC-S2 0.5 GGBS-S2 0.4 0.9 GGBS-CH-S2 0.3 0.8 Water-S2 0.2

0.5 mg/L (mg/L) Cr(VI) 0.7 0.1 0.0 0.6 02468 Time (Hours) 0.5

0.4

0.3

0.2

0.1

Concentration of Cr(VI) in Solution (mg/L) Concentration of Cr(VI) 0.0 0 2 4 6 8 10 12 14 16 18 20 22 24 Time (Hours)

FigureFigure 2. 2. TimeTime profile proﬁle of of the the Cr(VI) Cr(VI) concentration concentration in in the the S2 S2 specimens’ solutions from 0 to 24 h.

TheThe [Cr(VI)] aqueousaqueous inin GGBS-S2 GGBS-S2 decreased decreased to to 0.38 0.38 within within 1 1 h h before before reaching reaching a a constant constant concentration.concentration. On On the the contrary, contrary, due to the presence of Ca(OH)2,, GGBS-CH-S2 GGBS-CH-S2 showed showed a a significant significant andand continuous continuous Cr(VI) Cr(VI) reduction reduction ov overer 6 h before finallyfinally reaching 0.05 mg/L at at 24 24 h, h, which suggests anan almost a complete removal of Cr(VI) from the solution. Based Based on on the the resu results,lts, we attributed the the reducedreduced amount amount of of Cr(VI) Cr(VI) in in the the so solutionlution to to the the solution solution chemistry chemistry between between the the dissolved dissolved ions ions in in the the GGBS-CH-S2GGBS-CH-S2 sample withwith Cr(VI).Cr(VI). In In contrast, contrast, our our experimental experimental data data indicated indicated that that the dissolvedthe dissolved ions ionsfrom from the PC the did PC not did reduce not thereduce Cr(VI). the Our Cr(VI). results Ou suggestedr results thatsuggested the significant that the Cr(VI) significant concentration Cr(VI) concentrationdecrease for the decrease GGBS for in Ca(OH) the GGBS2 solution in Ca(OH) was2 solution primarily was due primarily to the reducing due to the reaction reducing between reaction the betweendissolved the ions dissolved from the ions GGBS from and the Cr(VI). GGBS and Cr(VI). FigureFigure 3 shows the the evolution evolution of of the the concentrations concentrations of the of thedissolved dissolved ions ions from from the GGBS the GGBS in water in (GGBS-S4)water (GGBS-S4) and GGBS and GGBS in Ca(OH) in Ca(OH)2 solution2 solution (GGBS-CH-S4) (GGBS-CH-S4) within within 25 25h. h.In In both both GGBS-S4 GGBS-S4 and and GGBS-CH-S4GGBS-CH-S4 samples, the concentrationsconcentrations ofof AlAl[III][III] werewere relatively relatively lower lower (lower (lower than than 1 mg/L)mg/L) than thosethose recorded for thethe otherother cations.cations. ForFor GGBS-S4,GGBS-S4, the the three three cations cations (Al (Al[III][III],, SiSi[IV][IV],, and CaCa2+2+)) showed showed similarsimilar dissolution trends.trends. WeWe presumed presumed that that the the concentration concentration changes changes in those in those cations cations resulted resulted from fromthe slow the dissolutionslow dissolution of GGBS of GGBS in water. in water. The Si [IV]Theconcentration Si[IV] concentration in GGBS-CH-S4 in GGBS-CH-S4 was very was low very due low to duethe rapidto the consumptionrapid consumption of Si in of GGBS’s Si in GGBS’s alkali-activation alkali-activation process. process.

Materials 2018, 11, 11 7 of 17 Materials 2018, 11, 11 7 of 17

1.4 1.0 16 1.0 22 700 1.0 GGBS-S4 (a) (b) (c) 1.2 GGBS-CH-S4 14 600 GGBS-CH-S2 (Cr(VI)) 0.8 0.8 20 0.8 1.0 12 10 18 500 0.8 0.6 0.6 0.6

8 16 400 (mg/L) 0.6 (mg/L)

[III] 0.4 0.4 [IV] 6 0.4 Al Cr(VI) (mg/L) Si Cr(VI) (mg/L) Cr(VI) (mg/L) (GGBS-S4) (mg/L) 0.4 (mg/L) Cr(VI) 14 300 2+ 4 (GGBS-CH-S4) (mg/L) 2+ 0.2

0.2 0.2 Ca 0.2 2 12 200 Ca

0.0 0.0 0 0.0 0.0 10 100 0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 Time (Hours) Time (Hours) Time (Hours) 24 1.0 24 1.0 24 1.0 (d) (e) (f) 20 20 0.8 0.8 20 0.8

16 16 16 0.6 0.6 0.6

12 (mg/L) 12 12 (mg/L) 2- 2- 3 4 0.4 0.4 0.4 O 2 SO Cr(VI) (mg/L) S Cr(VI) (mg/L)Cr(VI)

8 Cr(VI) (mg/L) 8 Total S (mg/L) 8 0.2 0.2 0.2 4 4 4

0.0 0.0 0 0 0 0.0 0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 Time (Hours) Time (Hours) Time (Hours) Figure 3. Time profiles for the ion dissolution from GGBS in distilled water (GGBS-S4) and GGBS in Figure 3. Time proﬁles for the ion dissolution from GGBS in distilled water (GGBS-S4) and GGBS Ca(OH)2 solutions (GGBS-CH-S4) for 0 to 24 h. The Cr(VI) concentration in the GGBS-CH-S2 solution in Ca(OH) solutions (GGBS-CH-S4) for 0 to 24 h. The Cr(VI) concentration in the GGBS-CH-S2 (Figure 2) 2is presented in the same plot. The maximum value of the y-axis was adjusted to the solution (Figure2) is presented in the same plot. The maximum value of the y-axis was adjusted to the [III] [IV] 2+ 2− 2− concentration value of each ion. (a) Al[III]; (b) Si [IV]; (c) Ca ;2+ (d) S2O3 ; (2e−) SO4 ; and2− (f) total S. concentration value of each ion. (a) Al ;(b) Si ;(c) Ca ;(d)S2O3 ;(e) SO4 ; and (f) total S.

2+ The initial CaCa2+concentrationconcentration in in GGBS-CH-S4 (~200 mg/L) was was much much higher higher than for GGBS-S4 (~12 mg/L) because of the added Ca(OH)2. After 1 h, 2+the Ca2+ concentration in GGBS-CH-S4 (~12 mg/L) because of the added Ca(OH)2. After 1 h, the Ca concentration in GGBS-CH-S4 increased significantlyincreased significantly to ~650 mg/L to ~650 but thenmg/L decreased but then todecrea 400 mg/Lsed to at400 3 h.mg/L After at this,3 h. weAfter recorded this, we no recorded changes 2+ inno thechanges Ca2+ concentrationin the Ca concentration in GGBS-CH-S4. in GGBS-CH-S4. The initial The rapid initial increase rapid and increase the subsequent and the subsequent decrease 2+ 2+ ofdecrease the Ca of2+ theconcentration Ca concentration represent, represent, respectively, respectively, the dissolution the dissolution of Ca2+ of fromCa from the GGBSthe GGBS and and the 2+ ion’sthe ion’s consumption consumption via the via formation the formation of alkali-activation of alkali-activation products. Theproducts. constant The Ca2+ constantconcentration Ca thatconcentration we recorded that after we 3recorded h resulted after from 3 theh result balanceed from between the thebalance GGBS between dissolution the processGGBS dissolution on the one 2+ handprocess and on the the Ca one2+ consumptionhand and the viaCa the consumption precipitation via of the alkali-activation precipitation of products alkali-activation on the other. products on the other. 2− 2− The S2O3 and SO4 concentrations were measured to represent the anions’ dissolution form 2− 2− the GGBS-S4The S2O3 and and GGBS-CH-S4 SO4 concentrations samples. were In GGBS-S4, measured a to relatively represent small the anions’ amount dissolution of anions wereform dissolvedthe GGBS-S4 from and GGBS, GGBS-CH-S4 and this amount samples. was In constant GGBS-S4, for 25 a h.relatively Conversely, small in theamount GGBS-CH-S4 of anions sample, were dissolved2− from GGBS,2− and this amount was constant for 25 h. Conversely, in the GGBS-CH-S4 the S2O3 and SO4 dissolution rapidly increased after 7 h. Most importantly, in contrast to the sample,2− the S2O32− and SO42− dissolution2− rapidly increased after 7 h. Most importantly, in contrast to S2O3 (0~0.5 mg/L) and SO4 (1~2 mg/L) concentrations, we observed that a significantly greater 2− 2− amountthe S2O3 of (0~0.5 total sulfur mg/L) (S) and (7~13 SO4 mg/L) (1~2 mg/L) was dissolved concentrations, within 5 we h (the observed rapid Cr(VI) that a removalsignificantly duration greater for GGBS-CH-S2,amount of total in sulfur Figure (S)2) in(7~13 GGBS-CH-S4 mg/L) was samples dissolved than within in GGBS-S4. 5 h (the Previousrapid Cr(VI) XAS removal study found duration that for GGBS-CH-S2, in Figure 2) in GGBS-CH-S42− samples than in GGBS-S4.2 −Previous XAS study found the S in GGBS exists mostly in the form of S with a minor amount of SO4 [31]. Our results therefore that the S in GGBS exists mostly in the form of S2− with a minor amount of SO42− [31]. Our results suggested that GGBS samples in Ca(OH)2 solution the significant Cr(VI)-concentration decrease in thetherefore solution suggested phase was that primarily GGBS samples caused by in the Ca(OH) fact that2 solution the reducing the significant reaction between Cr(VI)-concentration the dissolved ionsdecrease (S2− )in acted the solution as a reducing phase agentwas primarily for the Cr(VI) caused in by the the solution, fact that as the has reducing previously reaction beenfound between by the dissolved ions (S2−) acted as a reducing agent for the Cr(VI) in the solution, as has previously others [3,11]. We concluded that the dissolved ions from the GGBS activated in Ca(OH)2 solution play abeen critical found role by in theothers reductive [3,11]. transformationWe concluded of that Cr(VI) the intodissolved Cr(III) inions solution. from the GGBS activated in Ca(OH)2 solution play a critical role in the reductive transformation of Cr(VI) into Cr(III) in solution. 3.3. Cr(VI) Adsorption onto GGBS-PC Samples 3.3. Cr(VI) Adsorption onto GGBS-PC Samples We measured the changes in the Cr(VI) concentration for the S3 samples at 0, 120, and 150 h in order to furtherWe measured evaluate the the capacity changes of in GGBS the Cr(VI) to immobilize concentration Cr(VI) for in athe Ca(OH) S3 samples2 solution at 0, (Figure 120, and4). For150 each h in specimen,order to further the initial evaluate Cr(VI) the concentrations capacity of varied GGBS from to immobilize 0.5 to 100 mg/L. Cr(VI) After in a 120Ca(OH) h of agitation,2 solution we (Figure observed 4). noFor trace each of specimen, Cr(VI) in GGBS-CH-S3the initial Cr(VI) that hadconcentrations a Cr(VI) concentration varied from of 0.5 up to to 100 10 mg/L, mg/L. whichAfter indicated120 h of agitation, we observed no trace of Cr(VI) in GGBS-CH-S3 that had a Cr(VI) concentration of up to 10 mg/L, which indicated the complete removal of Cr(VI) from that solution. The GGBS specimen containing 50 mg/L of Cr(VI) had approximately 0.7 mg/L of Cr(VI) after 120 h. The Cr(VI)

Materials 2018, 11, 11 8 of 17

Materials 2018, 11, 11 8 of 17 the complete removal of Cr(VI) from that solution. The GGBS specimen containing 50 mg/L of Cr(VI) hadconcentration approximately in the 0.7 specimen mg/L of containing Cr(VI) after 100 120 mg/L h. The of Cr(VI)Cr(VI) concentration was reduced into the8.6 specimenmg/L. Furthermore, containing 100for mg/Lall samples, of Cr(VI) we was observed reduced no to 8.6further mg/L. change Furthermore, in the Cr(VI) for all samples,concentrations we observed even noafter further 150 changeh. It is inpossible the Cr(VI) that concentrationsall GGBS’s sorption even after sites 150 for h. Cr(VI) It is possible are saturated that all at GGBS’s this concentration, sorption sites which for Cr(VI) would are saturatedexplain why at this no concentration,further removal which of Cr would(VI) from explain the whysolution no further was observ removaled. ofIt Cr(VI)is also frompossible the solutionthat the wassaturation observed. of GGBS’s It is also surface possible sites that hinders the saturation any further of GGBS’s reduction surface of sites Cr(VI) hinders to Cr(III). any further Based reduction on there ofbeing Cr(VI) no tofurther Cr(III). changes Based onin the there Cr being concentrations no further after changes over in 150 the h, Cr a concentrationspseudo-equilibrium after over condition 150 h, awas pseudo-equilibrium attained under our condition experimental was attained condition. under We our do experimentalnot have the condition.experimental We data do not to havemeasure the experimentalactivated complexes data to measure at the transition activated complexesstate for th ate theoverall transition Cr transformation state for the overall reaction Cr transformation mechanisms, reactionand as a mechanisms, consequence and of asthis a consequencewe cannot calculate of this we th cannote pseudo-equilibrium calculate the pseudo-equilibrium constant for the constant reactions. for theFurther reactions. detailed Further work detailed is needed work isto needed confirm to confirmthe exact the exactmechanism mechanism at play at play during during surface surface site saturation, along along with with its its impact impact on on Cr(VI) Cr(VI) uptake upta andke reductionand reduction by GGBS. by GGBS. Additionally, Additionally, GGBS in GGBS distilled in waterdistilled (GGBS-S1) water (GGBS-S1) exhibited aexhibited limited capacity a limited for capa Cr(VI)city removal for Cr(VI) within removal 120 h: within 0.025 mg/L120 h: per0.025 gram mg/L of GGBS.per gram Related of GGBS. to this Related result is to the this fact result that GGBS’sis the fa capacityct that GGBS’s to immobilize capacity and to stabilize immobilize Cr(VI) and in Ca(OH)stabilize2 solutionCr(VI) in was Ca(OH) particularly2 solution large: was approximately particularly 2.3large: mg/L approximately per gram of GGBS.2.3 mg/L This per result gram indicates of GGBS. that This the alkalineresult indicates conditions that improve the alkaline the dissolution conditions of GGBS. improve This resultsthe dissolution in the enhancement of GGBS. of This the GGBS’s results capacity in the toenhancement remove Cr(VI), of inthe addition GGBS’s to itscapacity potential to toremove do so in Cr(V distilledI), in water addition conditions. to its All potential the reaction to do products so in ofdistilled the experimental water conditions. samples wereAll the further reaction analyzed prod usingucts XRDof the in experimental order to examine samples the possible were surfacefurther precipitateanalyzed using phase. XRD in order to examine the possible surface precipitate phase.

0.5mg/L 100 1mg/L 3mg/L 5mg/L 50 10mg/L 50mg/L 100mg/L

0 Concentration of Cr(VI) in Solution (mg/L)

0 20 40 60 80 100 120 140 160 Time (Hours) Figure 4. A comparison of the Cr(VI) concentrations in the S3 specimens’ solutions at 0, 120, and Figure 4. A comparison of the Cr(VI) concentrations in the S3 specimens’ solutions at 0, 120, and 150 h of150 reaction. h of reaction.

3.4. XRD Characterization of the Reacted Phases 3.4. XRD Characterization of the Reacted Phases Figure 5 shows the diffraction patterns of raw materials and of the S1 samples, measured using Figure5 shows the diffraction patterns of raw materials and of the S1 samples, measured XRD. We recorded no diffraction peaks for K2Cr2O7 in S1 specimens, indicating the complete using XRD. We recorded no diffraction peaks for K2Cr2O7 in S1 specimens, indicating the dissolution of K2Cr2O7. PC-S1 exhibited very strong crystalline peaks for the Ca(OH)2 formed from complete dissolution of K Cr O . PC-S1 exhibited very strong crystalline peaks for the Ca(OH) the PC hydration, but such2 crystalline2 7 peaks were absent in GGBS-S1’s XRD pattern, indicating that2 formed from the PC hydration, but such crystalline peaks were absent in GGBS-S1’s XRD pattern, here GGBS was mostly amorphous and had not been eﬀectively activated under a pH of circa −11.5 indicating that here GGBS was mostly amorphous and had not been effectively activated under (Figure 1a). In the XRD patterns of PC-S1 and GGBS-CH-S1 samples, we observed a formation of a pH of circa −11.5 (Figure1a). In the XRD patterns of PC-S1 and GGBS-CH-S1 samples, calcium aluminate hemicarbonate (hereafter referred to as hemicarbonate, we observed a formation of calcium aluminate hemicarbonate (hereafter referred to as hemicarbonate, 2Ca4[Al(OH)6]2(CO3)0.5OH·5.5H2O) and calcium aluminate monocarbonate (hereafter referred to as 2Ca4[Al(OH)6]2(CO3)0.5OH·5.5H2O) and calcium aluminate monocarbonate (hereafter referred to as monocarbonate, 3CaO·Al2O3·CaCO3·10.68H2O) while only the PC-S1 sample formed ettringite. monocarbonate, 3CaO·Al2O3·CaCO3·10.68H2O) while only the PC-S1 sample formed ettringite.

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■ : Ca(OH)2 ◆ : PC Clinkers ● ◇■ : Ca(OH)Ettringite2 ◆ :: PCC-S-H Clinkers ▽◇ : EttringiteHemicarbonate● : C-S-H▲ : Monocarbonate ▽ : Hemicarbonate ▲ : Monocarbonate ◆ ◆ ◆ ◆ ■ ◆ ◆ ◆ ◆ ■ ◆ ◆ ◆ ◆ ◆ ◆◆ ◆ ◆ ◆ ◆◆ ▲ ◆ ◇ ● PC-Raw ▽▲ ◇ ◇ ◇ ◇ ◆ PC-Raw ◇ ● ▽ ■◇ ◇ ■ ■ ■ ■ PC-S1 Intensity (A.U) ▽▲ ■ ■ ■● ■ ■PC-S1

Intensity (A.U) ▽ ■ ▲ ● ■ GGBS-CH-S1 GGBS-CH-S1 GGBS-S1 GGBS-S1 GGBS-Raw GGBS-Raw

10 15 20 25 30 35 40 45 50 55 60 10 15 20 25 30 35 40 45 50 55 60 2Theta 2Theta Figure 5. XRD patterns for raw materials (PC and GGBS), PC-S1, GGBS-CH-S1, and GGBS-S1. FigureFigure 5. 5.XRD XRD patterns patternsfor for rawraw materialsmaterials (PC and and GGBS), GGBS), PC-S1, PC-S1, GGBS-CH-S1, GGBS-CH-S1, and and GGBS-S1. GGBS-S1. Figure 6a shows the XRD patterns for GGBS-CH-S3 with Cr(VI) concentrations ranging FigureFigure6a shows6a shows the XRDthe XRD patterns patterns for GGBS-CH-S3 for GGBS-CH with-S3 Cr(VI)with Cr(VI) concentrations concentrations ranging ranging between between 0.5 and 100 mg/L. Regardless of the initial Cr(VI) concentration, all the specimens 0.5between and 100 mg/L.0.5 and Regardless 100 mg/L. of theRegardless initial Cr(VI) of the concentration, initial Cr(VI) all concentration, the specimens exhibitedall the specimens very strong exhibited very strong crystalline peaks due to the Ca(OH)2. In specimens with a Cr(VI) exhibited very strong crystalline peaks due to the Ca(OH)2. In specimens with a Cr(VI) crystallineconcentration peaks of due 0.5 tomg/L the Ca(OH)(Figure 26b),. In specimenshemicarbonate with aand Cr(VI) monocarbonate concentration formed, of 0.5 mg/L as observed (Figure 6inb), hemicarbonateconcentrationand of 0.5 monocarbonate mg/L (Figure formed,6b), hemicarbonate as observed and in GGBS-CH-S1. monocarbonate No formed, ettringite as orobserved monosulfate in GGBS-CH-S1.GGBS-CH-S1. NoNo ettringiteettringite oror monosulfatemonosulfate phases were foundfound inin anyany ofof thethe S3S3 specimens.specimens. InIn phases were found in any of the S3 specimens. In contrast, in the GGBS-CH with 50 and 100 mg/L of contrast,contrast, in in the the GGBS-CH GGBS-CH withwith 5050 andand 100100 mg/L of Cr(VI), thethe hemi-hemi- oror monocarbonatemonocarbonate XRD XRD peak peak Cr(VI), the hemi- or monocarbonate XRD peak intensity was very low. The addition of 100 mg/L of intensityintensity was was veryvery low.low. TheThe additionaddition ofof 100100 mg/L of Cr(VI) encouragedencouraged thethe formationformation ofof calcium calcium Cr(VI)aluminum encouraged Cr oxide the hydrates formation ((Ca of4Al calcium2O3(CrO aluminum4)·9H2O), CAC-9), Cr oxide which hydrates suggests ((Ca4 Althat2O the3(CrO amount4)·9H 2ofO), aluminum Cr oxide hydrates ((Ca4Al2O3(CrO4)·9H2O), CAC-9), which suggests that the amount of CAC-9), which suggests that the amount of Cr(VI) inﬂuences the2 formation of GGBS products under Cr(VI)Cr(VI) influences influences the the formationformation ofof GGBSGGBS products under Ca(OH)2 activation.activation. PreviousPrevious studies studies have have Ca(OH)shownshown2 thatactivation.that Cr(VI)Cr(VI) Previous cancan bebe studiesimmobilizedimmobilized have shownby pure that PC Cr(VI) hydrates, can be suchsuch immobilized asas ettringite,ettringite, by pure mono-,mono-, PC hydrates, andand suchhemicarbonatehemicarbonate as ettringite, [11,18].[11,18]. mono-, Additionally,Additionally, and hemicarbonate high-surfacehigh-surface [11 area,18]. hydration Additionally, by-products,by-products, high-surface suchsuch asas area C-S-H C-S-H hydration from from by-products,PCPC hydration hydration such reactions, reactions, as C-S-H cancan removeremove from PC Cr(VI)Cr(VI) hydration from so reactions,lutions viavia can physicalphysical remove adsorptionadsorption Cr(VI) from[3].[3]. Based Based solutions on on our our via physicalfindings,findings, adsorption i.e.,i.e., thethe rapid [rapid3]. Based Cr(VI)Cr(VI) on ourreductionreduction ﬁndings, and i.e., immobilization the rapid Cr(VI) throughthrough reduction GGBSGGBS and underunder immobilization alkalinealkaline throughconditionsconditions GGBS maintainedmaintained under alkaline byby Ca(OH) conditionsCa(OH)22,, wewe maintained conclude by thatthat Ca(OH) thethe 2 ,by-productsby-products we conclude produced thatproduced the by-products byby thethe producedalkali-activatedalkali-activated by the GGBS alkali-activatedGGBS can can contributecontribute GGBS toto canboth contribute the immobilization to both the ofof Cr(VI) immobilizationCr(VI) ionsions via via the the of complexation complexation Cr(VI) ions via thewithwith complexation dissolveddissolved anionsanions with dissolved resultingresulting anionsfromfrom the resulting GGBS fromhydration the GGBS reaction, hydration andand thethe reaction, Cr(VI)Cr(VI) reduction andreduction the Cr(VI) to to reductionCr(III).Cr(III). to Cr(III).

■■ ■ : Ca(OH) ❖❖: CAC-9 (a)(a) : Ca(OH)22 (b)(b) : CAC-9 ■■ ● : C-S-H ▽▽:: HemiHemi ▲▲:: MonoMono

■ Cr(Cr(ⅥⅥ)) concentration concentration ❖❖ ❖❖ ■■ ■ ●● ■ [mg/L][mg/L] ❖❖ ■ ▲▲ 100100 ❖❖ ❖❖ ❖❖▽▽▲▲ 5050 ▲▲ ▽▽ 1010 ▲▲ ▽▽ 5

Intensity (A.U) ▲ 5

Intensity (A.U) ▲ ▽▽ ▲▲ 33 ▽▽

▽ 1 ▽ 1 ▲ ▲ 0.50.5

1010 15 15 20 20 25 25 30 30 35 35 40 45 50 55 60 1010 15 15 2Theta FigureFigure 6. 6. ( a(a) )XRD XRD patterns patterns ofof GGBS-CH-S3GGBS-CH-S3 in solutions containing 0.50.5 toto 100100 mg/Lmg/L of of Cr(VI) Cr(VI) and and ( b(b) ) Figure 6. (a) XRD patterns of GGBS-CH-S3 in solutions containing 0.5 to 100 mg/L of Cr(VI) and thethe magnified magnified XRD XRD patterns patterns inin thethe rangerange ofof 9 to 15 degrees. (b) the magniﬁed XRD patterns in the range of 9 to 15 degrees.

Materials 2018, 11, 11 10 of 17 Materials 2018, 11, 11 10 of 17

3.5.3.5. Chromium Chromium Speciation Speciation via via STXM STXM InIn exhibiting exhibiting differentdifferent Cr Cr oxidation oxidation states, states, Cr(III) Cr and(III) Cr(VI)and Cr(VI) can be readilycan be resolvedreadily byresolved comparing by L comparingthe sample the spectra sample with spectra suitable with reference suitable spectrareference that spectra are taken that atare the taken Cr at2,3 -edge.the Cr ThisL2,3-edge. is because This isnear because edge X-raynear edge absorption X-ray fineabsorption structure fine (NEXAFS) structure is (NEXAFS) highly sensitive is highly to both sensitive the oxidation to both states the oxidationand the geometry states and of the Cr’s geometry first coordination of Cr’s first shell coordination [32]. shell [32]. FigureFigure 77 presentspresents the the X-ray X-ray absorption absorption image imag ofe the of GGBS-CH-S3the GGBS-CH-S3 sample sample taken belowtaken CrbelowL3-edge Cr Lat3-edge a photon at a photon energy energy of 560 eV, of 560 and eV, the and image the contrast image contrast maps of maps Cr and of O.Cr Cr and appeared O. Cr appeared to be localized to be localizedat the edge at the of theedge GGBS of the particles, GGBS particles, whereas wherea O was equallys O wasdistributed equally distributed in the GGBS. in the In GGBS. the Cr In image the L Crcontrast image map,contrast Area map, 1, as Area illustrated 1, as illustrated in Figure in8a, Figure was selected 8a, was forselected the Cr for 2,3the-edge Cr L NEXAFS2,3-edge NEXAFS analysis. analysis.We chose We pure chose K2 Crpure2O7 Kas2Cr a2O reference7 as a reference for Cr(VI) for spectra Cr(VI) andspectra used and a seriesused ofa series Gaussian of Gaussian peaks to peaksfit the to spectra. fit the spectra. In Figure In Figure8b, K 28b,Cr2 KO27Crexhibits2O7 exhibits four four distinct distinct X-ray X-ray absorption absorption features features in in the the Cr CrL2,3 L-edge2,3-edge spectra, spectra, indicated indicated as as a 2a2(~578.5 (~578.5 eV),eV), aa33 (~580.9(~580.9 eV), b2 (~587.7(~587.7 eV), eV), and and b b33 (~589.5(~589.5 eV), eV), wherewhere the the peaks peaks defined defined as as a3 aand3 and b2 bare2 are induced, induced, respectively, respectively, by the by theCr L Cr3, andL3, L and2 absorptions.L2 absorptions. The additionalThe additional peaks peaks a2 and a2 andb2 result b2 result from from the thecrystal crystal field field prod produceduced by by the the ligands ligands with with different different electronegativities,electronegativities, which which are are no non-linearlyn-linearly related related to to the the magnitu magnitudede of of the the crystal-field crystal-field parameter parameter (10Dq)(10Dq) [33,34]. [33,34]. Since Since the the absorption absorption shoulder shoulder features features of of Cr(VI) Cr(VI) on on the the low low energy energy side side are are very very close close toto Cr(III)’s Cr(III)’s main main absorption absorption [32], [32], the the absorption absorption peaks peaks of of a a22 andand b b22 areare overlapping overlapping contributions contributions fromfrom both both Cr(VI) Cr(VI) and and Cr(III). Cr(III). The The low low energy energy side side features features a1 and a1 band1 (the b 1arrows(the arrows in Figure in 8b) Figure solely8b) correspondsolely correspond to the tooctahedrally the octahedrally coordinated coordinated Cr(III Cr(III),), which which is in is inexcellent excellent agreement agreement with with the the chromitechromite (FeCr (FeCr2O2O4)4 spectrum) spectrum and and Cr Cr2O23O [32,35,36].3 [32,35,36 The]. The coexistence coexistence of Cr of Cras both as both Cr(VI) Cr(VI) and and Cr(III) Cr(III) is presumablyis presumably due due to tothe the superposition superposition of ofthe the partially-reduced partially-reduced Cr(VI) Cr(VI) and and the the physically physically adsorbed adsorbed Cr(VI)Cr(VI) onto onto the Ca(OH)Ca(OH)22-activated GGBS.GGBS. The The remaining remaining areas areas illustrated illustrated in Figurein Figure8a show 8a show the same the sametrends trends as Area as 1,Area i.e., the1, i.e., coexistence the coexistence of Cr(III) of and Cr(III) Cr(VI). and However, Cr(VI). dueHoweve to ther, lowerdue to Cr the concentration, lower Cr concentration,the areas have the lower areas X-ray have absorption lower X-ray intensities absorpti andon intensities a higher background and a higher noise. background noise.

Figure 7. (a) Scanning transmission X-ray microscopy (STXM) image of GGBS-CH-S3 taken at 560 Figure 7. (a) Scanning transmission X-ray microscopy (STXM) image of GGBS-CH-S3 taken at 560 eV; eV; (b) image contrast map of Cr; and (c) O for GGBS-CH-S3. (b) image contrast map of Cr; and (c) O for GGBS-CH-S3.

TheThe O O KK-edge-edge NEXAFS NEXAFS analysis analysis provided provided further further evi evidencedence for for the the coexistence coexistence of of Cr(VI) Cr(VI) and and Cr(III) on the Ca(OH)2-activated GGBS (GGBS-CH-S3), as shown in Figure 9. The O K-edge NEXAFS Cr(III) on the Ca(OH)2-activated GGBS (GGBS-CH-S3), as shown in Figure9. The O K-edge NEXAFS spectrum of the K2Cr2O7 exhibited intense pre-edge X-ray absorption peaks at 529.2 and 531.2 eV, spectrum of the K2Cr2O7 exhibited intense pre-edge X-ray absorption peaks at 529.2 and 531.2 eV, and these can be attributed to the electronic excitation from the O 1s core level into the unoccupied and these can be attributed to the electronic excitation from the O 1s core level into the unoccupied t2g t2g and eg molecular orbitals, respectively [37]. The O K-edge NEXAFS spectrum of pure Ca(OH)2 and eg molecular orbitals, respectively [37]. The O K-edge NEXAFS spectrum of pure Ca(OH)2 differs differs from the absorption features of 2CrO− 42− [24]; in this study, the presence of crystalline Ca(OH)2 from the absorption features of CrO4 [24]; in this study, the presence of crystalline Ca(OH)2 was wasnot observednot observed in the in entirethe entire area ofarea the of specimen. the specimen. Area1 Area and Area1 and 2 appearedArea 2 appeared to contain to twocontain pre-edge, two pre-edge, low-intensity absorption peaks arising from2− CrO42−. In addition to the peaks, the features low-intensity absorption peaks arising from CrO4 . In addition to the peaks, the features indicated indicatedby arrows by at ~533arrows eV at in ~533 the Area eV 1in and the Area Area 2 spectra1 and Area correspond 2 spectra to the correspond O K-edge absorptionto the O K peak-edge of absorption peak of Cr(III)-oxyhydroxide, which has previously been observed in the spectrum of Cr(III)-oxyhydroxide, which has previously been observed in the spectrum of pyrite (FeS2) reacting pyrite (FeS2) reacting with Cr(VI) [35]. Area 3 did not exhibit any CrO42− or Cr(III)-oxyhydroxide features because of the absence of any Cr in an oxidation state, as observed in Figure 7b.

Materials 2018, 11, 11 11 of 17

2− with Cr(VI) [35]. Area 3 did not exhibit any CrO4 or Cr(III)-oxyhydroxide features because of the Materialsabsence 2018 of, any 11, 11 Cr in an oxidation state, as observed in Figure7b. 11 of 17 Materials 2018, 11, 11 11 of 17

Figure 8. (a) Illustration of the analyzed areas in GGBS-CH-S3 for the Cr L2,3-edge NEXAFS in the Cr FigureFigure 8. 8. ((aa)) Illustration Illustration of of the the analyzed analyzed areas areas in in GGBS-CH-S3 GGBS-CH-S3 for for the the Cr Cr L2,32,3-edge-edge NEXAFS NEXAFS in in the the Cr Cr image contrast map (Figure 7b); (b) Cr L2,3-edge NEXAFS spectra for Area 1 in (a) and spectra from imageimage contrast contrast map map (Figure (Figure 77b);b); (bb)) Cr Cr LL2,32,3-edge-edge NEXAFS NEXAFS spectra spectra for for Area Area 1 1 in in ( (aa)) and and spectra spectra from from K2Cr2O7. KK22CrCr2O2O7.7 .

Figure 9. (a) Illustration of the analyzed areas in GGBS-CH-S3 for the O K-edge NEXAFS; (b) O Figure 9. (a) Illustration of the analyzed areas in GGBS-CH-S3 for the O K-edge NEXAFS; (b) O KFigure-edge 9.NEXAFS(a) Illustration spectra offor the each analyzed area in areas (a) and in GGBS-CH-S3for K2Cr2O7. for the O K-edge NEXAFS; (b)O K-edge K-edge NEXAFS spectra for each area in (a) and for K2Cr2O7. NEXAFS spectra for each area in (a) and for K2Cr2O7. 3.6. Cr(VI) Dissolution and the Effects of Accelerated Carbonation 3.6. Cr(VI) Dissolution and the Effects of Accelerated Carbonation A comparison of the dissolved Cr(VI) concentration from the specimens before and after A comparison of the dissolved Cr(VI) concentration from the specimens before and after accelerated carbonation is shown in Figure 10. At 56 days, all the specimens that had not been accelerated carbonation is shown in Figure 10. At 56 days, all the specimens that had not been exposed to atmospheric CO2 showed that relatively minimal amounts of Cr(VI) (i.e., less than the exposed to atmospheric CO2 showed that relatively minimal amounts of Cr(VI) (i.e., less than the detection limit of 0.03 mg/L) were released from the GGBS-PC paste, regardless of the replacement detection limit of 0.03 mg/L) were released from the GGBS-PC paste, regardless of the replacement

Materials 2018, 11, 11 12 of 17

3.6. Cr(VI) Dissolution and the Effects of Accelerated Carbonation A comparison of the dissolved Cr(VI) concentration from the specimens before and after accelerated carbonation is shown in Figure 10. At 56 days, all the specimens that had not been exposedMaterials 2018 to,atmospheric 11, 11 CO2 showed that relatively minimal amounts of Cr(VI) (i.e., less than12 of the 17 detection limit of 0.03 mg/L) were released from the GGBS-PC paste, regardless of the replacement ratio.ratio. Conversely,Conversely, in in the the accelerated accelerated carbonation carbonation samples samples the dissolution the dissolution of Cr(VI) of dependedCr(VI) depended strongly onstrongly the GGBS on the replacement GGBS replacement ratio for ratio PC. Infor pure PC. In PC, pu 0.7re PC, mg/L 0.7 ofmg/L Cr(VI) of Cr(VI) was leached, was leached, whereas whereas only 0.1only mg/L 0.1 mg/L of Cr(VI) of Cr(VI) was leached was leached from the from GGBS-PC the GGBS-PC paste with paste a 5 wtwith % GGBSa 5 wt replacement. % GGBS replacement. The Cr(VI) leachedThe Cr(VI) from leached the PC from paste the was PC paste the sum was of the the sum pre-added of the pre-added Cr(VI) (0.5 Cr(VI) mg/L) (0.5 and mg/L) additional and additional Cr(VI) (~0.2Cr(VI) mg/L) (~0.2 containedmg/L) contained in the PC in clinker, the PC as clinker, already as observed already inobserved the dissolution in the dissolution experiment experiment (Figure1b). For(Figure a sample 1b). For with a sample 50% of GGBS-replacedwith 50% of GGBS-replaced PC paste, we PC measure paste, a we very measure small amount a very small of Cr(VI) amount below of theCr(VI) detection below limit the ofdetection 0.03 mg/L. limit Pore of solutions0.03 mg/L. extracted Pore solutions from PC extracted paste generally from PC possess paste a pHgenerally in the rangepossess of a 12.4 pH toin 13.5the range [38]. Ourof 12.4 work to 13.5 revealed [38]. Our that work the alkalinity revealed resultingthat the alkalinity from the resulting naturally from formed the naturally formed Ca(OH)2 in PC hydration was sufficient for GGBS activation and enhanced GGBS’s Ca(OH)2 in PC hydration was sufﬁcient for GGBS activation and enhanced GGBS’s capacity to reduce andcapacity immobilize to reduce Cr(VI), and immobilize as observed Cr(VI), in the solutionas observed samples in the (Figures solution1b samples and2). Consequently, (Figures 1b and in a2). realisticConsequently, PC paste, in a replacing realistic PC PC paste, with areplacing minimal PC amount with a of minimal GGBS had amount a signiﬁcant of GGBS impact had a significant on Cr(VI) reductionimpact on and Cr(VI) immobilization, reduction and even immobiliza with a 5tion, wt % even replacement. with a 5 wt % replacement.

0.8 56d Curing 0.7 28d Carbonated 0.6

0.5

0.4

0.3

0.2 Concentration of Cr(VI)(mg/L) 0.1

0.0 0 5 10 50 Substitution Ratio of GGBS (wt.%)

FigureFigure 10.10.Tank Tank leaching leaching test resultstest results for the GGBS-PCfor the GGBS-PC paste before paste and afterbefore an acceleratedand after carbonationan accelerated test. carbonation test. Figure 11 shows the XRD patterns for the GGBS-PC paste, with different GGBS replacement ratios and eitherFigure with 11 shows or without the XRD carbonation. patterns Peaksfor the arising GGBS- fromPC paste, monosulfate, with different Ca(OH) GGBS2, and replacement C-S-H were detectedratios and for either all GGBS-PC with or without pastes without carbonat carbonation.ion. Peaks arising For the from 50 wt monosulfate, % GGBS-PC paste,Ca(OH) we2, notedand C-S-H very weakwere peaksdetected due for to theall GGBS-PC presence of pastes hydrotalcite without (Mg carbonation.6Al2(CO3(OH) For 16the·4H 502O)) wtand % GGBS-PC vaterite. This paste, result we indicatednoted very that weak replacing peaks due GGBS to inthe PC presence increases of thehydrotalcite hydration (Mg products’6Al2(CO susceptibility3(OH)16·4H2O)) to and carbonation vaterite. dueThis toresult the reactionindicated with that atmospheric replacing GGBS CO2 .in Previous PC increases studies the have hydration also conﬁrmed products’ that susceptibility carbonation to cancarbonation change thedue PCto the paste’s reaction porosity with andatmospheric that this CO can2. alsoPrevious affect studies the hardened have also specimen’s confirmed COthat2 permeabilitycarbonation can [39, 40change]. the PC paste’s porosity and that this can also affect the hardened specimen’s CO2 Monosulfatepermeability [39,40]. diffraction patterns were not detected in all samples with carbonation, whichMonosulfate suggests that diffraction complete patterns monosulfate were not decomposition detected in occurredall samples in thewith carbonated carbonation, samples. which Insuggests addition, that we complete detected monosulfate the presence decomposition of CaCO3 polymorphs occurred suchin the as carbonated vaterite, aragonite, samples. In and addition, calcite; thesewe detected were the the main, presence crystalline, of CaCO accelerated3 polymorphs carbonation such as products vaterite, for aragonite, all the specimens. and calcite; In these particular, were asthe the main, carbonation crystalline, progressed, accelerated the peakcarbonation intensities produc of thets CaCOfor all3 polymorphsthe specimens. became In particular, stronger, asas hasthe carbonation progressed, the peak intensities of the CaCO3 polymorphs became stronger, as has been previously found [41]. Presumably, the formation of aragonite is linked to the carbonated C-S-H formed during PC clinker hydration, as has been observed in the young post-carbonation tricalcium silicate paste [42]. In general, Ca(OH)2 and C-S-H are the main hydration products involved in the carbonation of PC-based systems [43]; the carbonation occurs preferentially on Ca(OH)2, and precipitating CaCO3 polymorphs in the system [44,45]. However, carbonation in PC systems that contain 75% amounts of GGBS mainly occurs on the C-S-H phases, which results in vaterite

Materials 2018, 11, 11 13 of 17 been previously found [41]. Presumably, the formation of aragonite is linked to the carbonated C-S-H formed during PC clinker hydration, as has been observed in the young post-carbonation tricalcium silicate paste [42]. In general, Ca(OH)2 and C-S-H are the main hydration products involved in the carbonation of PC-based systems [43]; the carbonation occurs preferentially on Ca(OH)2, Materialsand precipitating 2018, 11, 11 CaCO3 polymorphs in the system [44,45]. However, carbonation in PC systems13 of 17 that contain 75% amounts of GGBS mainly occurs on the C-S-H phases, which results in vaterite formation [46]. [46]. C-S-H C-S-H formed formed in GGBS-PC systems has a generallygenerally lowlow Ca/SiCa/Si ratio ratio because because of the Ca(OH)2 consumptionconsumption compared compared to to that of pure PC systemssystems [[47–50].47–50]. WhenWhen C-S-HC-S-H withwith aa lowlow Ca/SiCa/Si ratio is exposed to COCO2,, both vaterite and aragonite,aragonite, whichwhich areare CaCOCaCO33 polymorphs, preferentially precipitate [51[51].]. InIn the the present present study, study, after after carbonation, carbonation, C-S-H C-S-H exhibited exhibited a peak intensitya peak inintensity GGBS50C in GGBS50Cthat was lower that was (2θ lower= ~29◦ (2) thanθ = ~29°) forthose than for contained those contained less GGBS. less Furthermore, GGBS. Furthermore, we observed we observed that the thatmore the GGBS more there GGBS was, there the higherwas, the was higher the vaterite-related was the vaterite-related peak, which peak, formed which due formed to the carbonation due to the carbonationof C-S-H with of aC-S-H low Ca/Si with a ratio. low Ca/Si ratio.

◆ ◆ ◎ ◆ ◆ ◆ ■ ◆ ◇ ● ■ ◆ ▣ ◎ ◇ ● ◇ GGBS50C

GGBS10C After GGBS5C Carbonation

GGBS0C

■ ✳ ■ ◆ ● ◆ ■ ▣ ✳ ◆ ✳ ● ◆■ GGBS50 Intensity (A.U) GGBS10 Before GGBS5 Carbonation

GGBS0

10 15 20 25 30 35 40 45 50 55 60 2Theta Figure 11. XRD patterns of the GGBS-PC paste with GGBS replacement ratios of 0, 5, 10, and 50 wt % Figure 11. XRD patterns of the GGBS-PC paste with GGBS replacement ratios of 0, 5, 10, and 50 wt % before and after 28 days of accelerated carbonation. ◆ : Vaterite, ♢: Aragonite, ■: Ca(OH)2, before and after 28 days of accelerated carbonation. : Vaterite, ♦: Aragonite, : Ca(OH)2, }: Calcite, ◎:: C-S-H,Calcite, ●:: Monosulfate, C-S-H, ✳: Monosulfate, and : Hydrotalcite. and ⎕: Hydrotalcite. 2 Figure 1212 shows shows the the XRD XRD peak peak intensity intensity of the of 001 the basal 001 reflectionbasal reflection from Ca(OH) from2 Ca(OH)for the GGBS-PC2 for the GGBS-PCpastes before pastes and before after carbonation.and after carbonation. We observed We a observed very intense a very peak intense for crystalline peak for Ca(OH) crystalline2 in Ca(OH)the pre-carbonation2 in the pre-carbonation pure PC paste pure (GGBS0). PC paste Like (GGBS0 anticipated,). Like anticipated, as GGBS’s replacementas GGBS’s replacement ratio in PC ratioincreased, in PC Ca(OH) increased,2’s peak Ca(OH) intensity2’s peak decreased intensity due decreased to the dilution due to ofthe PC dilution and to of the PC consumption and to the consumptionof Ca(OH)2 by of GGBS.Ca(OH) After2 by GGBS. carbonation, After carbonation, more Ca(OH) more2 decreased Ca(OH)2 decreased in the pure in PCthe pastepure PC (GGBS0) paste (GGBS0)than Ca(OH) than2 inCa(OH) the GGBS-PC2 in the paste.GGBS-PC As a paste. result, As all thea result specimens, all the exhibited specimens similar exhibited Ca(OH) similar2 peak Ca(OH)intensities,2 peak which intensities, supported which the supported findings that the findings C-S-H is that more C-S-H readily is more carbonated readily thancarbonated Ca(OH) than2 in Ca(OH)GGBS-PC2 in paste GGBS-PC because paste the carbonationbecause the of carbonatio C-S-H dependsn of C-S-H on the depends initial Ca(OH) on the2 amountinitial Ca(OH) present2 amountprior to carbonationpresent prior [46 to]. Moreover,carbonation it can[46]. be Moreover demonstrated, it can that be thedemonstrated sufficient Ca(OH) that the2 in sufficient pure PC Ca(OH)paste can2 in buffer pure PC C-S-H paste carbonation. can buffer C-S-H Presumably, carbonation. the lowerPresumably, Cr(VI) the dissolution lower Cr(VI) in the dissolution carbonated in 2− theGGBS-PC carbonated resulted GGBS-PC from a pre-carbonationresulted from a reductivepre-carbonation process reductive caused by process dissolved caused anions by such dissolved as S 2− anions(which aresuch continuously as S2− (which released are continuously from GGBS released during hydration) from GGBS because during S hydration)can inhibit because the oxidization S2− can inhibitof Cr(III) the to oxidization Cr(VI) [18 ].of ItCr(III) should to beCr(VI) noted [18]. that It weshould do notbe noted have thethat direct we do evidence not have we the need direct to evidenceidentify thewe exactneed surface-promotedto identify the exact reductive surface- transformationpromoted reductive of Cr(VI) transf to Cr(III).ormation It isof likelyCr(VI) that to Cr(III). It is likely that S2− released during the hydration reaction is responsible for the reducing reactions in solution and that the reduced Cr species are physisorbed to hydrated materials. Equally, we cannot rule out the possibility that physisorbed S2− located on hydrated-materials surfaces act as reducing agents for Cr reductive transformations. Further detailed studies are needed to distinguish the two different reaction pathways. Moreover, C-S-H formed in the GGBS-substituted PC system, which has a longer mean chain length [52], resulting in a larger surface area that can offer better surface adsorption, inclusion, and physical entrapment of Cr(VI). Based on the XRD and Cr(VI) dissolution results before and after carbonation, we concluded that partially replacing PC with

Materials 2018, 11, 11 14 of 17

S2− released during the hydration reaction is responsible for the reducing reactions in solution and that the reduced Cr species are physisorbed to hydrated materials. Equally, we cannot rule out the possibility that physisorbed S2− located on hydrated-materials surfaces act as reducing agents for Cr reductive transformations. Further detailed studies are needed to distinguish the two different reaction pathways. Moreover, C-S-H formed in the GGBS-substituted PC system, which has a longer Materialsmean chain 2018, 11 length, 11 [52], resulting in a larger surface area that can offer better surface adsorption,14 of 17 inclusion, and physical entrapment of Cr(VI). Based on the XRD and Cr(VI) dissolution results before GGBSand after can carbonation, greatly reduce we and concluded immobilize that Cr(VI) partially via replacing dissolved PC anions with into GGBS the canpore greatly solutions reduce and andvia physicalimmobilize adsorption Cr(VI) via onto dissolved C-S-H. anionsEven though into the C-S- poreH solutions and hydration and via products physical are adsorption more susceptible onto C-S-H. to carbonationEven though in C-S-H the GGBS-PC and hydration system products than in are the more pure susceptible PC system, to Cr(VI) carbonation was not in readily the GGBS-PC leached system from thethan GGBS-PC in the pure paste PC system,after carbonation. Cr(VI) was not readily leached from the GGBS-PC paste after carbonation.

Figure 12. Comparison of the 001 basal reflection from Ca(OH)2 in GGBS-PC pastes before and after Figure 12. Comparison of the 001 basal reﬂection from Ca(OH)2 in GGBS-PC pastes before and after an accelerated carbonation. The numbers followed by GGBS indicate the replacement ratios of GGBS for PC.

4. Conclusions We found that without alkali activation GGBS is is limited in in its its capacity capacity to to reduce reduce Cr(VI). Cr(VI). In contrast, the Ca(OH)2-activated GGBS demonstrated a considerably greater capacity for reducing In contrast, the Ca(OH)2-activated GGBS demonstrated a considerably greater capacity for reducing and immobilizing Cr(VI). Therefore, we conclude that alkalinity is one of the dominating factors in determining thethe extentextent and and mechanisms mechanisms of reductiveof reductive Cr(VI) Cr(VI) dissolution dissolution and uptakeand uptake reactions reactions for GGBS. for GGBS.Furthermore, Furthermore, we found we experimental found experimental evidence eviden that Cr(VI)ce that removal Cr(VI) fromremoval the from solution the wassolution due towas its 2− duereactions to its withreactions the dissolved with the dissolved anions, such anions, as S 2such−, resulting as S , resulting from GGBS from being GGBS under being high under alkaline high alkalineconditions. conditions. X-ray absorption X-ray absorption spectroscopy spectroscopy data identiﬁed data theidentified coexistence the ofcoexistence Cr(III) and of Cr(VI) Cr(III) on and the Cr(VI) on the Ca(OH)2-activated GGBS. This partial reduction of Cr(VI) to Cr(III) on the surface of Ca(OH)2-activated GGBS. This partial reduction of Cr(VI) to Cr(III) on the surface of GGBS suggests GGBSthe presence suggests of a the surface-site-activated presence of a surface-site-a reductive mechanism.ctivated reductive The Cr(VI) mechanism. we observed The on Cr(VI) GGBS likewe observedresults from on physicallyGGBS like adsorbedresults from Cr(VI) physical on hydrationly adsorbed products. Cr(VI) on hydration products. Our accelerated-carbonation experimental data data in indicatedicate that the hydration products formed in the GGBS-PCGGBS-PC paste,paste, such such as as C-S-H, C-S-H, possessed possessed a greater a greater resistance resistance to Cr(VI) to Cr(VI) leaching leaching than those than formed those formedin pure PC.in pure Our studyPC. Our suggests study thatsuggests even thethat used even of the 5% used weight-based of 5% weight-based partial GGBS partial replacement GGBS replacementfor PC-based for materials PC-based would materials be a suitable would methodbe a suitable for reducing method and for immobilizing reducing and Cr(VI) immobilizing without Cr(VI) without violating the mechanical and physical integrity of PC [53]. We hypothesize that violating the mechanical and physical integrity of PC [53]. We hypothesize that Ca(OH)2 generated Ca(OH)during the2 generated early stage during of PC the hydration early stage reactions of PC hydration promotes thereactions dissolution promotes and activationthe dissolution of GGBS, and whichactivation results of GGBS, in an enhanced which results Cr(VI) in reduction.an enhanced It should Cr(VI) bereduction. noted that It observingshould be Snoted2− concentrations that observing in 2− Sthe concentrations dissolved solution in the will dissolved provide useful solution insights willfor provide determining useful andinsights identifying for determining further detailed and identifying further detailed reaction pathways. Future studies that use additional information to research similar experimental conditions are necessary to identify Cr(VI)’s exact reductive transformation pathways in hydrated cementitious materials. In addition, in order to understand the Cr(VI) reductive mechanism in GGBS, further detailed mechanistic and molecular research is needed to identify the exact state of physisorbed Cr(VI) on GGBS’s hydration products, how different types and amounts of GGBS alkaline activators influence Cr(VI) reduction and immobilization, and the factors that control Cr(VI) reduction pathways in carbonated GGBS-PC materials.

Materials 2018, 11, 11 15 of 17 reaction pathways. Future studies that use additional information to research similar experimental conditions are necessary to identify Cr(VI)’s exact reductive transformation pathways in hydrated cementitious materials. In addition, in order to understand the Cr(VI) reductive mechanism in GGBS, further detailed mechanistic and molecular research is needed to identify the exact state of physisorbed Cr(VI) on GGBS’s hydration products, how different types and amounts of GGBS alkaline activators inﬂuence Cr(VI) reduction and immobilization, and the factors that control Cr(VI) reduction pathways in carbonated GGBS-PC materials.

Acknowledgments: This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2016R1C1B1014179). Author Contributions: Sungchul Bae designed the experiments and wrote most of the paper; Juyoung Ha contributed to the analysis of the entire experiments and writing of the paper; Fumino Hikaru contributed to the preparation the experiments and the analysis of the data of Cr reduction and XRD; Manabu Kanematsu, Chiaki Yoshizawa, and Takafumi Noguchi contributed to the analysis of the data and the writing of the paper; Youngsang Yu contributed the analysis of the STXM data and the writing of the paper. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

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