Nuclear magnetic resonance (NMR) study of fly ash hydration with graphene oxide Gang Xu, P.E. Xianming Shi, Ph.D., P.E. Department of Civil & Environmental Engineering Washington State University

Air pollution from fly ash Coal ash spill in Tennessee Background Cement industry accounts for around 5% of global CO2 emissions. The U.S. generated approximately 70 million tons of fly ashes in 2014, only 27% were recycled. Cement Production (photo by Shaila Dewan) Here is the background showing why do we need to study the fly ash hydration. Because currently we are facing many environmental problems associated with fly ash and cement. To solve these problems, the best solution is to make concrete out of fly ash, it will not only reduce the usage of cement, and also recycle the fly ash from waste stream. Previous studies show the possibilities of using fly ash as the sole cementitious binder to make concrete that has moderate strength. Air pollution from fly ash (photo by Shaila Dewan) Coal ash spill in Tennessee (photo by Shaila Dewan)

Fly Ash Indtroduction Fly ash Bottom ash Carbon content General transformation of coal during combustion (Kutchko, 2006) Before burning the coal, the coal will be ground into this very small particle. After burning, the carbon content will become CO2, but the mineral content inside the coal will liquefied first in the high temperature. Some of them will vaporize and go up. Some of them will not so they go down. Those vaporized will cool down and form these sphere-shape particles, it’s called fly ash. these two show you what fly ash look like under microscope.

Fly Ash Composition Cement Fly ash Specific gravity 3.2 2.7   Cement Fly ash Specific gravity 3.2 2.7 Bulk Density (lbs/ft3) 76 54 SiO2 (wt. %) 21 23.5 CaO (wt. %) 65 23.2 Al2O3 (wt. %) 4 13.8 Fe2O3 (wt. %) 3.5 4.8 MgO (wt. %) 0.2 4.2 WA ash MT ash OR ash We got three ash samples from 3 states. And the typical fly ash composition is oxides including ……. The major difference between cement and fly ash is the the content of Cao and Al2O3, fly ash has more Al, but Cement has more CaO. The minerology of fly ash is very complex, more than 150 kinds of minerals in fly ash Typical cement only has 5-10 kinds of minerals. Chemical Composition The contents of principal oxides are usually SiO2, Al2O3, Fe2O3, CaO, MgO, K2O, Na2O and SO3. 2. Minerology Composition Fly ash has approximately 316 individual minerals and 188 mineral groups.

Precursors Identification 40 min 5 min fly ash has the same precursors. Use this to help audiences get a picture for fly ash case, It shows Al exists as anion, Si exists as both anion and neutral species in pore solution. Raman spectra were collected from the extracted pore solution of OPC paste at 5 min. and 40 min. Three regions, -Si-O-Si- vibrational modes (475-650 cm-1) in oligomers, Si-O* symmetric stretching mode (750-800 cm-1) in monomer and Si-O¯ stretching modes in all deprotonated species (850-1125 cm-1) [19], were marked in Fig. One study identified v1 mode = 981 cm-1, v2 mode = 453 cm-1, v3 mode = 1126 cm-1 and v4 mode = 614 cm-1 for SO42¯. Correspondingly, v1 mode = 978 cm-1, v2 mode = 445 cm-1and v3 mode = 1136 cm-1 were identified in the experimental results, which was a strong indication of SO42¯ in the pore solution.   Since the major Si species in alkaline solution (pH > 8) are SiO(OH)3¯ and SiO2 (OH)22¯anions, the low-intensity 782 cm-1 band was assigned to the symmetric stretching of the tetrahedral Si(OH)4. This agrees with previous experimental and calculated results. The 1080 cm-1 band was attributed to the SiO(OH)3¯ anion. Gout and Hunt also observed Si-O stretching in SiO(OH)3¯ at 1020 cm-1 and 1141 cm-1, respectively. SiO2(OH)22¯ anions were identified at 970 cm-1. This was close to 928 cm-1 band and 927 cm-1 band observed from Si-O stretching in SiO2(OH)22¯ in previous two studies. The intensity of 1080 cm-1 band was increased and that of 970 cm-1 band was decreased (by subtracting the shoulder of 978 cm-1 from SO42¯) from 5 min. to 40 min. This was mainly due to the pH value decrease by carbonation of pore solution, as SiO(OH)3¯ became dominant with a decreasing pH value [21]. The carbonation was evidenced by a broad band around 1355 cm-1 The 618 cm-1 band was attributed to Al(OH)4¯ molecules, which is the dominant Al species in the alkaline solutions. The shoulder around 618 cm-1 band can be attributed to the v4 mode of SO42¯. Raman spectra of fly ash pore solution taken at ambient condition

Interaction between GO and Ca2+ Showing the function GO on Ca in Cement (attract and bond with Ca). Also applies to fly ash, Use this to help audiences get a picture for fly ash case The obvious Ca-distribution changes in Fig strongly suggested some interactions occurred between calcium ions and GO. According to a previous study, six models describing the chemical reactions between the calcium ions and GO are presented in Fig: (1) Ca2+ is attracted by the π-electrons of the GO basal plane; (2) Ca2+ interacts with "=O" groups, where Ca2+ breaks one of the O−C bonds and binds strongly to the oxygen atom; (3) Ca2+ binds to "−OH" groups on the edge; (4) Ca2+ removes two "−OH" groups from the basal plane to form Ca(OH)2; (5) Ca2+ is attracted by "=O" groups on the edge; (6) Ca2+ is attracted by "−COOH " groups on the edge. Surface plot of average Ca-concentration at 28-d: (a) paste without GO; (b) paste with GO. Interactions between the calcium ions and the GO (Archanjo et al., 2014)

GO Modified Mortar EPMA (Electron probe micro-analyzer) Ca Si Element mapping (Ca and Si) (a) mortar without GO; (b) GO-modified mortar (a) Wavelength dispersive X-ray spectroscopy (WDS) was used to examine the elemental composition of pure fly ash mortar. Ca Si (b)

GO’s effect on elemental distribution This slide summarize the The box plots reflects the GO’s influence on the Al, Fe (intermediates), Ca (network modifier), Si (network former) position of box moved upward for Al- and Fe-concentrations and downward for Ca- and Si-concentrations. This suggested that GO acted primarily as a reduction factor of Ca-concentration and an increase factor of Al- and Fe-concentrations for OPC hydrates inside the box, rather than changing their distribution patterns. Since Ca was mainly present as positively-charged ions, it was attracted and consumed by electronegative GO, resulting a decrease in Ca-concentration in the pore solution. This GO‘s influence on the Ca-concentration was reflected in the final hydrates, as the box of Ca-concentration moved downward with median reduced by 6% (Fig. a) after the addition of GO. The Al and Fe mainly existed as electronegative tetrahedra (e.g. Al(OH)4-), so the negatively-charged GO reduced the Al- and Fe-distribution area in the pore solution through repulsion, resulting in an increase in Al- and Fe-concentrations. This GO-induced effect was also reflected in the final hydrates as the box of Al- and Fe-concentration moved upward with median increased by 12% and 10%, respectively (Fig. c&d). The box of Si-concentration moved downward slightly with median reduced by 3%, because Si existed as both neutral (H4SiO4°) and electronegative (H3SiO4-, H2SiO42-) species (Fig.b). After adding GO, the difference between maximum and minimum values showed a decreasing trend for all elements. This was mainly because GO improved the OPC hydration, resulting in a decrease in the extreme level of Al, Ca, Si and Fe concentrations. Boxplot of elemental concentrations of pastes at 28-d: (a) Ca, (b) Si, (c)Al, (d) Fe.

GO Modified Mortar Fly ash without GO Fly ash with GO In order to get more info from elemental mapping. We did mole ratio analysis. by summarize the mole ratio data pionts in EMPA mapping, you will have these two scatter plots. The x-axis is Si to Ca ratio. The y-axis is Al to Ca ratio. The data cluster after adding GO can be explained by the knowledge gained from OPC paste study. GO promote the formation of Jennite and C-S-H in fly ash mortar. The line labelled with C-(A)-S-H is the average composition line of C-(A)-S-H from previous studies The observations of this slide can be used later to confirm the NMR study Al/Ca against Si/Ca mole ratio plot for an area (50 x 50 μm2, with 0.1 μm increment).

Graphene Oxide (GO) Modified Fly Ash Mortar   0.03% GO- modified fly ash mortar Regular fly ash mortar Compressive strength increase 7-day fc’ (psi) 3353.2 2705.9 24% 14-day fc’ (psi) 4688.0 3721.1 26% 28-day fc’ (psi) 5998.2 4877.9 23% Cement mortar (left); GO-modified fly ash mortar (middle); fly ash mortar (right) Here is the GO modified fly ash mortar with strength improvement and surface condition change. GO can improve the fly ash mortar strength by more than 20% (Left) Fly ash mortar; (Right) GO-modified fly ash mortar

Microscopic Investigation Secondary Electron Imaging (SEI) Analysis A B You can see the fly ash particle, Left is still in perfect shape. But with activators, the dissolved fly ash formed these fibrous structures and linked everthing togther, which is able to provide mechanical strength. SEI micrograph of mortar surface cured for 28 days. A) Fly ash sphere. B) Fibrous structure. Figure 2. 3D contour diagram of 3-day compressive strength model Figure 2. 3D contour diagram of 3-day compressive strength model

NMR Instrument Bruker Avance III 400MHz NMR machine If an external magnetic field is applied, nuclei will do energy transfer and the energy will be emitted at certain frequency. Different nuclei bonding structure will have different frequency. Once probe pick up those radio waves from energy transfter, you can tell the nuclei structures. Didn’t use freqency unit, because it is have more than 10 digits after decimal point and it also depends on strength of magnetic field. Use PPM because it is convenient to use (less than 3 digits) and independent of magnetic field strength. Bruker Avance III 400MHz NMR machine (photo by Bruker Inc.)

29Si NMR Study of Fly Ash Hydrates 29Si NMR spectra at 56-day Increase of Q1(J) and Q2(1Al) Add GO Increase of Low Quartz Shaded area are major differences. NMR also shows fly ash binder is a very complex material (so many different peaks) Decrease of Q1 and amorphous quartz means the dissolution of fly ash was accelerated by GO Increase of Q1(J) and Q2(1Al) means more hydrates formed, J is jennite Decrease of amorphous Quartz in fly ash Decrease of Q1 in fly ash

29Si NMR Coupling with EPMA Increase of Q1(J) and Q2(1Al) XRD confirmed increase of Low Quartz By coupling the NMR data with EPMA, we are able to confirm the finding of NMR analysis. Jennite and C-S-H formation found in EPMA mapping analysis was confirmed by the increase of Jennite and Q2 peaks in NMR BSE show the morphology of similar Jennite found in cement hydrates BSE image of GO-induced Jennite

29Si NMR Coupling with XRD Increase of Low Quartz XRD confirmed increase of Low Quartz Another important change is the increase of low quartz. XRD confirmed the increase of Low quartz in NMR BSE show the morphology of similar low quartz found in cement hydrates BSE image of GO-induced Low Quartz

27Al NMR Study of Fly Ash Hydrates 27Al NMR spectra at 56-day Al(VI) in AFm Al(IV) in Fly Ash Decrease of Al(IV) in Fly Ash Interlayer Al(IV) in C-S-H Al(VI) in AFm Al(VI) in AFt Increase of Interlayer Al(IV) in C-S-H Add GO confirmed the increase of Interlayer Al(IV), indicating more Al was pushed out of C-S-H by GO, becoming interlayer Al tetrahedron between C-S-H layers. Decrease of Al(IV) in fly ash, meaning fly ash hydration was accelerated by GO. Other 2 peaks was relatively unchanged.

29Si NMR Spectra Comparison Fly ash hydrates More than half of peak area is on the network structure side. Fly ash hydrates is essentially Geopolymer Cement hydrates Because of 3D network and cross-link of fly ash hydrates, its structure should be stronger and more stable than cement hydrates theoretically More than half of peak area is on the chain structure side, Cement hydrates is different from Geopolymer

27Al NMR Spectra Comparison Fly ash hydrates Great peak area of Al(IV) from fly ash itself and C-A-S-H hydrates. Al (IV) acts as reservoir to improve the resistance to sulfur attack. Cement hydrates The shaded area represents the great peak area……………. Although Al (IV) acts as reservoir to improve the resistance to sulfur attack, it can decrease the strength of geopolymer. Most of Al present as Al(VI) in AFt and TAH (amorphous Al hydroxide). No enough Al reservoir for later sulfur attack.

Functions of GO Exclude the intermediates. Consume network modifiers.   Cement Fly ash SiO2 (wt. %) 21 23.5 Al2O3 (wt. %) 4 13.8 Fe2O3 (wt. %) 3.5 4.8 CaO (wt. %) 65 23.2 Element Function in structure Ca Network Modifiers Fe Intermediates Al Si Network Formers This is again the illustration of proposed GO function. It still needs to be improved, but it shows basic ideas. Exclude the intermediates. Consume network modifiers. Not affect network formers.

Fly Ash Pervious Concrete Samples Pervious concrete 4X8 cylinders (left to right) cement, cement + GO, fly ash, fly ash + GO (a): cylinders with capping (b): Close-up view of surface (a) We also made medium strength fly ash pervious concrete at room temperature and compare it with cement ones. Showing how GO improved fly ash concrete (b)

Tests – Compressive and Split Tensile Strength Compressive strength test results Split tensile strength test results Showing how GO improved fly ash concrete strength in terms of compressive and split tensile strength. Relationship between split tensile strength and compressive strength at 28 days

Summary 0.03 wt.% GO improved overall performance of fly ash-based binder, e.g. the 28-day fc’ of fly ash pervious concrete was improved by more than 50%. GO shows the ability to directionally select network formers (Si) and intermediates (Al, Fe and Mg) to form hydrates. GO can also consume network modifiers (Ca). NMR is a powerful tool to study the structure of hydrates and reaction mechanism of GO in the fly ash hydration.

Acknowledgements Thanks for funding from CESTiCC. Thanks to BASF, Boral and Lafarge for donated materials. Dr. Owen K. Neil, Dr. Mehdi Honarvarnazari, Jiang Yu, Sen Du, Jialuo He at WSU also provided valuable assistance in experiments.

Thank you and Questions ？