E ﬀ ects of graphene oxide on the geopolymerization mechanism determined by quenching the reaction at intermediate states

The e ﬀ ects of graphene oxide on the geopolymerization reaction products at di ﬀ erent times were investigated by quenching the reaction. The phase composition and valence bond structure evolution were investigated systematically. The results show that the ethanol/acetone mixture helps isolate reaction products early in the process (0 – 24 h). RGO bonded well with the geopolymer matrix during the geopolymerization. The degree of densi ﬁ cation increased and the amorphous nature of the material decreased with reaction time. The addition of rGO accelerated the conversion of ﬁ ve and six coordinate Al – O sites into four coordinates and Si atoms forming Q 4 (3Al) network structure.

7][28][29][30][31][32][33][34] Thus, Xu et al. 27 used ve-membered alumino-silicate framework ring models in Ab initio calculations.Weng et al. 29,30 compared the hydrolysis and condensation reactions between low and high Si/Al ratio geopolymers.The partial charge model predictions and experimental results suggested possible Al and Si monomeric species that might form during the dissolution and condensation process.Mitchell et al. 34 reported that polar solvents (acetone or isopropyl alcohol) could help to stop hydration reactions.But aldol reactions can occur by the presence of nC-H during the derivative thermo gravimetric analysis of the reaction products.Chen et al. 31 demonstrated a solvent extraction method using a mixture of alcohol and acetone to stop the reaction and acquire samples for nuclear magnetic resonance (NMR) studies.Unfortunately, a detailed understanding of the entire process remains largely elusive.
Since the reaction products of geopolymers depended highly on the raw material, alkali-activated solution and curing conditions, 32 the structure changes in the geopolymer matrix used have not been quantitatively characterized especially in the early stages during the geopolymerization process.The reaction products oen have considerable water, that interferes with characterization tools, such as Nuclear Magnetic Resonance (NMR) and X-ray diffraction (XRD).Thus, it is quite necessary to develop a method of arresting reaction during the geopolymerization process.
In the present work, we describe such a method and there-aer systematically characterize the effects of graphene oxide on the initial products using micrographs, phase composition, functional group and valence bond structure evolution.

Preparation of reaction products
The alkaline mixture was synthesized by mixing silica sol with KOH for 3 days with the help of magnetic stirring.The rGO/ geopolymeric alkaline mixture (GO powders/metakaolin 1 wt%) was prepared by dropping the obtained GO dispersion to the alkaline mixture and stirred for 15 min.Thereaer, the rGO/ geopolymer slurry was prepared by adding the metakaolin powders into the alkaline mixture and mixing for 45 min using a high-shear mixer and ultrasonication in an ice bath.The continuous stirring under ice bath ensured complete distribution and fully dissolution of both metakaolin particles and rGO powders resulting in a low viscosity, homogeneous slurry.Then, the slurry was casted in plastic containers and cured at 60 C for different times (0-24 h) to get the reaction products.
In order to remove water, some preliminary preparations were carried out.Slurry samples (if they had not set, 0-2 h) were directly added into a 50/50 (vol) ethanol/acetone mixture and stirred hard for round 5 min (around 1 g samples and 100 mL ethanol/acetone mixture), then continued adding new solvent.This procedure was repeated for three times, until the reaction products became particles.Samples (if they had set, 3-24 h) were rst divided into micron-sized particles using a mortar and  pestle, and then mixed with 50/50 (vol) ethanol/acetone mixture for round 5 min to remove water.Then, taking out and drying the particle samples at room temperature, as can be seen in Fig. 1.Finally, use a mortar and pestle to grind the samples for characterization.

Characterization
The micrographs of the geopolymer (KGP) and rGO/geopolymer (rGO/KGP) reaction products were observed using scanning electron microscope (SEM, FEI, Helios NanoLab 600i).Fouriertransform infrared (FT-IR) spectra of raw kaolin, metakaolin and reaction products during geopolymerization were obtained on a Nicolet Nexus 6700 Fourier transform infrared spectrometer.The phase compositions of the reaction products were examined by X-ray diffraction (XRD, Rigaku, RINT-2000) with Cu-Ka radiation.The 27 Al and 29 Si solid state NMR were conducted using Bruker Avance III400 spectrometer with a magnetic eld strength of 9.4 T and 4 mm rotor.The relaxation delay times were 2 and 1 s for 27 Al and 29 Si NMR spectra, respectively.The spectrometers were 5 kHz for Al and 12 kHz for Si.The chemical shis of 27 Al and 29 Si nuclei were referenced to AlNO 3 and tetramethylsilane, respectively.

Results and discussions
Micrographs of particles for reaction products of both the KGP and rGO/KGP samples formed at various reaction times are shown in Fig. 2. Aer stirring for 45 min, the reaction products exhibit a porous and spongy morphology, with many irregular surface voids (Fig. 2(a) and (h)).The rGO bonded well with the reaction products of KGP matrix.The voids decreased signicantly as time increased.At 3 h, the reaction products became dense, without obvious voids (Fig. 2(e) and (l)).It could be found that the rGO was wrapped around by the particles of KGP matrix.Aer 24 h, both the KGP and rGO/KGP offers a relatively homogeneous and dense microstructure (Fig. 2(g) and (n)).
Fig. 3 shows XRDs of the KGP and rGO/KGP products formed at various times.There was no obvious difference between the Fig. 7 27 Al NMR spectras of reaction product of rGO/KGP formed at various reaction times in geopolymerization process: (a)-(g) corresponding 0 min, 30 min, 1 h, 2 h, 3 h, 6 h and 24 h, respectively.
two type samples.The patterns samples displayed typical geopolymer broad amorphous humps around 17-32 2q.The amorphous content decreased with the reaction time.The minor a-quartz remained.
Fig. 4 and 5 show the FT-IR spectra of kaolin, metakaolin and reaction products of KGP and rGO/KGP formed at various reaction times.As for the kaolin samples in Fig. 4, the bands at 3694, 3670, 3654 and 3620 cm À1 are associated with nO-H in kaolinite structure. 37,38The bonds at 1115, 1099, 1032 and 1008 cm À1 corresponded to nSi-O from SiO 4 .Sharp bands located at 937 and 913 cm À1 were attributed to nAl-OH vibrations. 38The IR peaks at 795 and 755 cm À1 are assigned to nSi-O-Al.The bands located at 696, 539 and 470 cm À1 was attributed to the Si-O, Si-O-Al and Si-O vibrations from AlO 6 , respectively. 39Aer treating at 800 C for 2 h, the transformation to metakaolin removes most of these bands, leaving three obvious peaks located at 1090, 803 and 463 cm À1 , which attributed to the nSi-O from SiO 4 , nAl-O from AlO 4 and Si-O vibrations, respectively.It can be concluded that the Si-O and Al-O bonds hydrolyze during geopolymerization. 32he FT-IR spectras during geopolymerization over 24 h are also shown in Fig. 4 and 5.There was no obvious difference between the FT-IR spectra of the KGP with and without rGO.The bands positioned at 3500 and 1659 cm À1 were attributed to nO-H, indicating that adsorbed atmospheric water existed in the molded geopolymer sample. 40The intensity of the band located at 463 cm À1 increased with reaction time, suggesting formation of a greater number of Si-O-Si units.At 6 h, the intensity of the bands at 593 and 717 cm À1 increased, related to increases in Si-O-Al and the four-coordinated AlO 4 structure units in the reaction product.These represent the structural changes and rearrangement occurring during geopolymerization of Al units.Different from the FT-IR spectrum of the metakaolin, a weak shoulder located at 880 cm À1 appeared and increased with time, which was corresponding to the carbonate from atmospheric air.
The intense band related to nSi-O shis from 1090 to 1022 cm À1 aer 24 h, caused by the presence of Al-O bonds owing to silicon substitution by aluminum in the second coordination sphere and the generation of the Si(Al)-O units. 41The Al-O band at 803 cm À1 decreases with the reaction time and disappears aer 6 h, conrming the dissolution of metakaolin and the disruption of the Al environment during the geopolymerization process.
It is difficult to distinguish the characteristic rGO peaks from the FT-IR spectra of the rGO/KGP products due to their small content.Further inuence of rGO on the reaction products could be detected by the 27 Al and 29 Si solid state NMR.Fig. 6, 7 and Table 1 show the 27 Al NMR spectra and the Gaussian t of raw kaolin, metakaolin and reaction products of KGP and rGO/KGP formed at various times.As shown in Fig. 6(a), the 27 Al chemical shi of raw kaolin is 0.92 ppm, corresponding to six-coordinated aluminum.The three peaks at 53.3, 29.5 and 3.1 ppm in the 27 Al NMR spectrum of metakaolin Fig. 6(b), can be attributed to 4-, 5-and 6-coordinate Al.The 5-coordination accounts for 48% of total Al atoms, the major species in metakaolin.
The broad peaks arise due to highly distorted geometry at aluminum sites. 32Aer metakaolin reacts with alkaline solution for 0-30 min, the Al coordination states of both the KGP (Fig. 6(c) and (d)) and the rGO/KGP (Fig. 7(a) and (b)) show no obvious change.The three peaks in the 27 Al NMR spectrum were attributed to 4-, 5-and 6-coordinate Al.The 4-coordinate Al in the reaction products of rGO/KGP was relatively more than the KGP matrix (Table 1), which may be attributed to the acceleration of rGO on the initial products early in the geopolymerization process.
The 5-and 6-coordinated Al atoms in the KGP samples appear to transform to 4 coordination gradually during geopolymerization over 1-3 h.The relative content of 4-coordinated Al atoms increased to 71.7% within 2 h (Fig. 6(f)).However, as for the rGO/KGP samples, the relative content of 4-, 5-and 6coordinate Al atoms was 46.8%, 37.5% and 15.7% within 1 h, respectively (Table 1).The species of the Al atoms were not changed, whereas, the 4-coordinated Al atoms increased to 69.8% within 2 h (Fig. 7(d)).When the reaction time was longer than 3 h, the 4-coordinated Al atoms (56.6 ppm) became the mainly species (Fig. 7(e)).However, at the same time, the vecoordinated Al in the KGP and rGO/KGP products was disappeared aer 6 h (Fig. 6(h) and 7(f)).
Aer 24 h, the peaks of both the KGP and rGO/KGP products at 56.Compared with the KGP matrix products, the addition of rGO accelerated the conversion of 4-coordinated Al in the rGO/KGP products at the early stage (0-30 min) during the geopolymerization process.While, the effects of rGO was not obvious at the later stage (1-24 h).This was due to the KGP particles likely priority formed, attached and grown on the Fig. 8 29 Si NMR spectras of kaolin and reaction product of KGP formed at various reaction times in geopolymerization process: (a)-(i) corresponding to kaolin, metakaolin, 0 min, 30 min, 1 h, 2 h, 3 h, 6 h and 24 h, respectively.
surface of the rGO sheets.However, the matrix was solidicated, surface of the rGO was all parceled with the geopolymerization products.Thus, the geopolymerization promotion was not obvious. 29Si NMR spectra of kaolin, metakaolin and reaction products of both the KGP and rGO/KGP formed at various reaction times are given in Fig. 8, 9 and Table 2.The 29 Si MAS NMR spectrum of raw kaolin consists of a single resonance centered at À92.4 ppm assigned to Si-O-Si linkages only (Fig. 8(a)). 42ccording to previous studies of aluminosilicate materials, 29,30,32 the broad 29 Si NMR spectra peak could be divided into ve possible silicon Q 4 (mAl) species.Two peaks at À99.3 and À90.8 ppm correspond to tetra-coordinated Si atom, (Q 4 (1Al) and Q 4 (3Al)).Q 4 (1Al) is the major species in the metakaolin (Fig. 8(b)).
However, aer 24 h, only one broad 29 Si NMR peak of KGP at À90.1 ppm is seen (Fig. 8(i)).The peaks at À87.0 and À90.1 ppm of rGO/KGP are observed (Fig. 9(g)), indicating that Si is present mainly as Q 4 (3Al) structural units.Compared with the pure KGP, the addition of rGO accelerated the conversion of Si structure unit from Q 4 (1Al) to Q 4 (4Al), Q 4 (3Al), Q 4 (2Al) and Q 4 (1Al) species.Thus, the addition of rGO affects the Si structure during the geopolymerization and has an positive inuence on the generation of Q 4 (3Al) species.
To sum up, the effects of GO on the geopolymerization of the rGO/KGP could be illustrated in Fig. 10.For the KGP (Fig. 10(a)), the geopolymerization mechanism can be rationally expressed according to the experimental analysis as follows.First, metakaolin particles dissolve from the surface aer mixing with alkaline silicate solutions; the Si-O bond and Al-O bond hydrolyze; Si and Al monomers diffuse; polycondense and rearrange; ve and six coordinate Al-O sites convert to four coordination, condensing Si species with Al species mainly in the form of Q 4 (3Al) and Al in four coordinate.
GO can be in situ reduced to rGO in alkaline silicate solutions and have positive effects on the geopolymerization during the reaction process. 22As described in Fig. 10(b), the rGO accelerates the conversion of Al-O sites into four coordinates and Si atoms in the form of Q 4 (3Al), but has not changed the nal  network structure of the rGO/KGP.The reaction products bond well with the rGO sheets show denser microstructure and lower amorphous degree with the increased in reaction time.Based on the above analyses, the addition of GO is proper in preparing rGO/KGP composites.Meanwhile, the presence of rGO contributed to the enhancement of mechanical performance of geopolymer.As reported in our previous studies, 23,24 compared with pure geopolymer, improvements in mechanical properties were achieved through rGO reinforcement of 0.05-1 wt% at room temperature.With 1 wt% GO addition, the fracture toughness and exural strength of rGO/geopolymer composites increased by approximately 30% and 7%, respectively, attributing to the proper interface bonding, crack deection and propagation and rGO pull-out.

Conclusions
In the present study, the method to stop geopolymerization reaction was provided and the effects of graphene oxide on the geopolymerization mechanism of geopolymer based on natural metakaolin were investigated systematically.
(1) The reaction products of KGP and rGO/KGP in the geopolymerization process (0-24 h) can be isolated by introducing ethanol/acetone mixtures.The voids in the reaction products decrease signicantly with the geopolymerization time.The products display typical broad amorphous humps around 17-32 2q and the amorphous degree decrease with the reaction time.
(2) During geopolymerization, the metakaolin dissolved in the alkaline silicate solutions, and Si-O bond and Al-O bond hydrolyzed, ve and six coordinates of Al-O sites converted into four coordinates, condensing the network structure in which Si mainly in the form of Q 4 (3Al) and Al in four coordinate.
(3) The addition of GO accelerated the conversion of Al-O sites into four coordinates and Si atoms in the form of Q 4 (3Al).The reaction products of geopolymer matrix bonded well with the rGO sheets and showed denser microstructure and lower amorphous degree with the increase in reaction time.

Fig. 1
Fig. 1 Photographs of (a) KGP and (b) rGO/KGP samples obtained at different reacting time.

Fig. 3
Fig. 3 XRD patterns of the reaction products of (a) KGP and (b) rGO/KGP formed at various reaction times in geopolymerization process.

Fig. 4
Fig.4FT-IR spectras of kaolin, metakaolin and reaction products of KGP formed at various reaction times in geopolymerization process.

Fig. 5
Fig.5FT-IR spectras of metakaolin and reaction products of rGO/ KGP formed at various reaction times in geopolymerization process.
3 ppm became sharper, indicating the coordination of Al-O in nal geopolymer converts to four coordinate (Fig. 6(i) and 7(g)).The shi to 4-coordinated Al (from 53.3 to 56.3 ppm) likely arises because of extensive formation of Si-O-Al links.29,32

Table 1
27ussian fit results of27Al NMR spectra of reaction products formed at various reaction times

Table 2 29
Si NMR spectra results of reaction products formed at various reaction times