Synthesis, microstructure, multifunctional properties of mayenite Ca12Al14O33 (C12A7) cement and graphene oxide (GO) composites

The Pristine Mayenite Ca12Al14O33 (C12A7) Cement was simply synthesized by using solid-state reaction. The C12A7 and Graphene Oxide (GO) composites (C12A7_GO-x) with various contents of the GO suspension loading (x = 0 wt%, 1 wt%, 2 wt%, 3 wt%, and 4 wt%) were directly prepared by mixing the C12A7 and GO. X-ray diffraction results of pristine C12A7 and all C12A7_GO composites indicated a pure phase corresponding to the standard of C12A7 cement. Raman spectroscopy confirmed the existence of GO in all C12A7_GO samples. Scanning Electron Microscopy (SEM) showed the micrometer grain sizes and the occurrence of grain boundary interfaces for GO incorporation in all C12A7_GO samples. UV–Vis spectroscopy revealed the absorption value of all C12A7_GO samples and red shift near longer wavelengths when increasing the GO concentrations. The dielectric constant of C12A7_GO composites can be explained by the high density of free electron charges for the interfacial polarization on the GO surface. The maximum specific capacitance of C12A7_GO-4 electrode of 21.514 at a current density of 0.2 A g−1 can be attributed to the increase in the electrochemically active surface area for the formation of the electrical double layer capacitors behavior and the effects of high surface area GO connections. Also, the mechanical properties exhibited an increase in Vickers indenter hardness (HV) values with increasing GO contents. The highest HV value was 117.8 HV/2 kg at the C12A7_GO-4 sample. These results showed that the composite materials of the pristine C12A7 cement with GO were highly efficient. All in all, the GO material contained a high potential for enhancing low-cost cement materials in multifunctional properties such as optical, dielectric, electrochemical, and mechanical properties.


Results and discussion
XRD analysis. The XRD patterns of the GO, pristine C12A7, and all C12A7_GO samples are shown in Fig. 1a. The Rietveld profile fitting of the pristine C12A7, and C12A7_GO-1, C12A7_GO-2, and C12A7_GO-3 samples are shown in Fig. 1b-e. As seen in Fig. 1a, the characteristic diffraction peak of GO is appeared at 2θ = 10.890°, which was well indexed to the (002) plane and presented the layer distance (d-spacing) of 0.811 nm, corresponding to the standard GO pattern 26 . In line with Dikin et al. 's study 35 , the results indicated a layer distance of approximately 0.8 nm showing one molecule-thick layer of water and hydrogen-bonded between the layer GO sheets. Furthermore, the edges of GO sheets consisted of hydrophilic oxygenated graphene sheets supporting the existing oxygen functional groups, which is vital for the pristine C12A7 cement composite. Moreover, the C12A7_GO samples under all different conditions showed that all of the main diffraction peaks from 2θ = 18.0°-70.0° corresponded to (211), (310), (320), (321), (420), (332), (422), (551), (611), (444), (640), (642), and (831) planes as indexing the pristine Ca 12 Al 14 O 33 cement phase of cubic structure with space group of I-43d in the standard data of JCPDS:00-009-0413 36 . The intensity peak of the C12A7_GO samples was decreased due to the strong continuous network of GO sheets with the agglomeration and over stacking on C12A7 surface, which is consistent with the reported values in the literature 31 . In addition, lattice parameters and fitting parameters of the samples were calculated by using the Rietveld profile fits of the pristine C12A7, C12A7_GO-1, Scientific RepoRtS | (2020) 10:11077 | https://doi.org/10.1038/s41598-020-68073-4 www.nature.com/scientificreports/ C12A7_GO-2, and C12A7_GO-3 samples as summarized in Table 1 and displayed in Fig. 1b-e. Figure 1b-e displays the fitting refinement and calculation lattice parameters of the samples. The results were in a good agreement with the standard data. The calculated lattice parameters along a-axis were increased with increasing GO content as 11.975(4), 12.009 (8), 11.993(3), 12.020(5) and 12 10  www.nature.com/scientificreports/ (D) of the pristine C12A7 and all C12A7_GO samples was calculated using the diffraction peaks and planes by Scherer's Eq. (1): [37] where D is the crystallite size, λ is the X-ray wavelength, k is the shape factor of value 0.9, θ is the diffraction angle, and β is the full width at half maximum. All calculated D values of the pristine C12A7 and all C12A7_GO samples are listed in Raman spectra analysis. Figure 2 shows the Raman spectra of the GO, pristine C12A7 and all C12A7_GO samples. The main peaks of pristine C12A7 phase were observed at the Raman shift region of 100-1,000 cm -1 , which is attributed to the lattice framework of the Ca 12 25 .When the content loading of GO in C12A7_GO is increased, the Raman shift peak of the samples can be observed in two main peaks at around 1,338 and 1586 cm -1 . The first peak (1,338 cm -1 ) corresponds to the hexagonal graphitic layers of D-band, which is related to the defects and disorder. The second peak (1586 cm -1 ) corresponds to the G-band, which related to sp 2 -bonded carbon atom vibration in 2D hexagonal lattice owing to the strong continuous network of GO sheets 26 . Furthermore, the occurrence peaks of 2D and D + G band at around 2,688.83 and 2,942.56 cm −1 , respectively, can be confirmed by the second order Raman spectra. It is forming an orientation, stacking defects, and a defect-induced double resonance of inter-valley scattering process during the composites. Interestingly, the obtained composite phases between pristine C12A7 phases and GO sheets without other phases were detected, which can strongly confirm the C12A7_GO composites by XRD technique. Therefore, the above results confirm that the complete composite system of the C12A7 cement and GO composite structure was due to the existence of free extra framework of O 2in C12A7 structure presented by O 2vibrations mode.
(1) D = k /βcosθ Table 1. Lattice parameters, the obtained parameter of fitting were calculated using the Rietveld profile fits and the crystallite size (D) of the pristine C12A7 and C12A7_GO-1, C12A7_GO-2, and C12A7_GO-3 composite samples.

Sample
Lattice parameters (Å) R brag R profile Weight profile Goodness of fitting D (nm) TGA and DTA diagrams analysis. Figure 4a,b shows the TGA and DTG diagrams analysis which was done to study the behavior and weight loss of the pristine C12A7 and all C12A7_GO samples. As seen in Fig. 4a, the first phase of the weight loss of all samples at the temperature of 200 °C slightly decreased due to the removal of physically absorbed water on the surface of Ca 12 Al 14 O 33 cement. The second phase of weight loss at the temperature ranging from 200 to 320 °C was assigned to the mass loss of the decomposition and/or oxidation process of water, Al 2 O 3 and CaCO 3 structures which lead to the formation of Ca 3 Al 2 (OH) 12 cement 40,41 . In the next phase of TGA curve, the mass loss temperature ranging from 320 to 640 °C was mainly attributed to the decomposition of Ca 3 Al 2 (OH) 12 cement co-occurring with a transformation phase to Ca 3 Al 2 (OH) 6 cement. Finally, in the fourth phase of the weight loss, the decomposition of Ca 3 Al 2 (OH) 6 cement, which is the transformation phase to Ca 12 Al 14 O 33 cement, was observed at higher temperature of 640 o C. In addition, the weight loss of all samples with GO loading C12A7 composites was higher than that of the pristine C12A7 cement. In Fig. 4b, the DTG curve was shifted to lower temperatures with increasing the GO content. This result showed that an improvement of the thermal and phase transformation was due to C12A7 cement content and GO sheets composites.
Bet analysis. Figure 5a-f show Nitrogen adsorption isotherms with inset pore volume curves of the pristine C12A7 and all C12A7_GO samples, the plots of average pore sizes, and specific BET surface area as a function of GO concentration, respectively. As seen in Fig. 5a-e, the adsorption/desorption Nitrogen isotherms exhibited a typical type-IV isotherm, suggesting the characteristic of mesoporous structure in particles. Moreover, the specific BET surface area of the pristine C12A7 and all C12A7_GO samples was estimated from the area of a closed curve as shown in Fig. 5f, which found to be 35.26 for the pristine C12A7 and C12A7_GO-1, C12A7_GO-2, C12A7_GO-3 and C12A7_GO-4 samples of 23.92, 12.08, 27.74 and 41.07 m 2 /g, respectively. Additionally, the average pore sizes of the pristine C12A7 and all C12A7_GO were calculated by using BJG technique as shown the inset Fig. 5a-e. The results were found to be 6.56, 7.63, 10.41, 7.16 and 5.55 nm for the pristine C12A7, C12A7_GO-1, C12A7_GO-2, C12A7_GO-3 and C12A7_GO-4 samples, respectively. The maximum specific BET surface area was 41.07 m 2 /g of C12A7_GO-4 samples. The effect was due to the surface C12A7 cement being uniformly decorated by the network structure of GO sheet, which is promoting the surface area 42 . www.nature.com/scientificreports/ studies conducted by Matsuishi et al. 8 Rudradawong et al. 21 and Rudradawong et al. 24 the result was due to the transition of free electrons from the extra O 2states inside a cage to the CCB. The optical band gaps (E g ) of the pristine C12A7 and all C12A7_GO samples were determined by fitting the absorbance data in Fig. 8a to Eq. (2):

SEM and EDS analysis.
where E g is the direct band gap, hν is the photon energy, α is the optical absorption coefficient, and E D is a constant. The E g values of the pristine C12A7 and all C12A7_GO samples were obtained by extrapolating the linear regions of these plots to zero absorption, as shown in Fig. 8b. As seen in this   Dielectric properties. The dielectric behavior of the pristine C12A7 and all C12A7_GO samples at room temperature is shown in Fig. 9a,b. Figure 9a indicates that the εʹ value increases with increasing GO concentration. The εʹ value of the pristine C12A7, C12A7_GO-1, C12A7_GO-2, C12A7_GO-3 and C12A7_GO-4 samples was determined at 1 kHz and found to be 6.61, 19.44, 16.80, 17.14 and 21.11, whereas tanδ value was found to be 0.107, 0.715, 0.658, 0.740 and 0.974, respectively, as shown in Fig. 9b. These results can be explained by the internal of filler having many free charges on the surface of C12A7_GO samples, which mainly affects the interfacial polarization [45][46][47] . Theoretically, interfacial polarization can be effective at low frequency 46 . In addition, the GO sheets were agglomerating and over stacking on C12A7 surface, which increase the accompanying micro capacitor. Moreover, the results were highly effective in increasing the dipolar polarization at the interfacial region of C12A7_GO samples 45 . www.nature.com/scientificreports/ electrochemical properties. Figure 10a shows CV curves of the pristine C12A7 and all C12A7_GO electrodes in 1.0 to a − 0.0 V voltage window at a scan rate of 100 mV s −1 . Figure 10b-f show CV curves of the pristine C12A7 and all C12A7_GO electrodes in various scan rates with inset cycling stability using CV test at a scan rate of 200 mV s −1 . It is obviously seen in Fig. 10a that the CV curve of the pristine C12A7 had poor capacitive properties, whereas all C12A7_GO electrodes were similar in showing an ideal electrochemical double layer capacitor (EDLC) behavior, which was rectangle shaped with increasing GO content 48,49 . Additionally, Fig. 10b-f show the CV curve remaining quasi-rectangular with increased scan rate indicating good capacitive properties for all C12A7_GO electrodes. Also, the response current was increased with an increase in scan rate suggesting an excellent rate capability of the pristine C12A7 electrode with adding GO concentrations. It can be seen in the inset of Fig. 10b-f that the characterization of CV curve was increased with more cycling test due to the typical pattern of electrochemically GO behavior [48][49][50]53 . Figure 11a-f show approximately triangle-shaped curves suggesting capacitors with high reversibility and ideal capacitor behaviors [48][49][50]53 . Figure 11a indicates GCD curves of the pristine C12A7 and all C12A7_GO where C sc is the specific capacitance (F g −1 ), i is the measured current (A), v is scan rate, m is the mass of active materials in each electrode (g), and ΔV is the total potential deviation (V), respectively. Figure 12a shows the C cs value as found to be 6.764, 11.686, 9.604, 17.006, and 18.512 F g −1 at 10 mV s −1 for the pristine C12A7, C12A7_GO-1, C12A7_GO-2, C12A7_GO-3 and C12A7_GO-4 electrodes, respectively. The C cs values were estimated from the obtained GCD curves using the following Eq. (4): where i is the discharge current density (A g −1 ), ΔV/Δt is the slope of discharge curves after the R i drop, respectively. In Fig. 12b, it can be seen that the C sc values decreased with increasing current density. The C sc values were found to be 5.291, 13.259, 11.646, 21.100, and 21.514 F g −1 at a current density of 0.2 A g −1 for the pristine C12A7, C12A7_GO-1, C12A7_GO-2, C12A7_GO-3 and C12A7_GO-4 electrodes, respectively. These C sc values of the pristine C12A7 and all C12A7_GO electrodes are summarized in Table 2. The excellent electrochemical properties were obtained from C12A7_GO-4 sample as a promising electrode active material for EDLC. Figure 12c shows the capacitance retention rate of the pristine C12A7 and all C12A7_GO electrodes at a current density of 2 A g −1 . The results indicated that the enhanced activity of the composite samples may be due to an enrichment effect via diffusion within the modified electrode and highly useful GO surface area. Also, the free electron charge was transferred to the free extra framework O 2anions and GO surfaces in C12A7_GO lattices 26 . The C12A7_GO-4 electrode retained 98.82% in 1 M KOH aqueous electrolyte after 1,000 cycles, which is an excellent cycle stability indicating stable energy-storage processes during long cycle charging and discharging 51 .
(3) C sc = idv/2m v �V  www.nature.com/scientificreports/ The Nyquist plots of the pristine C12A7 and all C12A7_GO electrodes were studied by EIS measurements as shown in Fig. 12d. Normally, the Nyquist plots were obtained in two different frequency regions. At high frequency region, the intercept on Zʹ axis showed the series resistance (R s ) of these electrodes and found to be 1.019, 0.875, 0.952, 0.869 and 0.869 Ω cm 2 for the pristine C12A7, C12A7_GO-1, C12A7_GO-2, C12A7_GO-3 and C12A7_GO-4 electrodes, respectively. The semicircle loops of the Nyquist plots correspond to the charge transfer resistance (R ct ) of the pristine C12A7 and all C12A7_GO electrodes, which can be determined from the diameter of a semicircle loop. Also, another semicircle at high frequency region was the result of diffusion process of 1 M KOH aqueous electrolyte. All determined R s and R ct values are summarized in Table 2. As seen in Table 2, the R ct of the pristine C12A7 electrode was about 0.793 Ωcm 2 indicating the imperfection diffusion process, whereas the R ct values of C12A7_GO-1, C12A7_GO-2, C12A7_GO-3 and C12A7_GO-4 electrodes www.nature.com/scientificreports/ were lower than those of C12A7 electrode. The result was found to be 0.474, 0.419, 0.274 and 0.217 Ωcm 2 , for C12A7_GO-1, C12A7_GO-2, C12A7_GO-3 and C12A7_GO-4 electrodes, respectively. These outcomes suggested that the effective charge was transferred at the interface between electrode and electrolyte. Obviously, GO loading C12A7 cement electrode can be significantly enabled to more rapidly charge transfer at the C12A7_GO electrode and 1 M KOH aqueous electrolyte interface. At low frequency region, the slope of 45° portion of curve was due to Warburg resistance which represents the ion diffusion and/or transport in the electrolyte [49][50][51][52][53] . The capacitive behavior of the pristine C12A7 and all C12A7_GO electrodes was found to be good.  www.nature.com/scientificreports/

Mechanical properties
The mechanical properties of the pristine C12A7 and all C12A7_GO composites were investigated using a microhardness method with a pyramid on a square base according to the Vickers indenter hardness (HV) technique. The determined micro hardness defined the loading to the regions on the surface area by the following equation 26 : where HV is the Vickers micro hardness value, P is the loading ratio in kg, and d is the average diagonal length of the impression in mm. The mechanical test applied 2 kg of a load weight for 5 s. As seen in Fig. 13, the HV values of the pristine C12A7, C12A7_GO-1, C12A7_GO-2, C12A7_GO-3 and C12A7_GO-4 composites were 64.6, 74.5, 85.2, 112.0, and 117.8 HV/2 kg, respectively. The results showed an increase in the HV values with increasing GO content. These results showed that the improvement of GO loading in pristine C12A7 cement was affected by the movement of free extra framework of O 2directly between the cages through the cages opening 6,28,33 . Furthermore, these effects were due to the reinforcement of GO incorporation with an elastic force at grain boundary surface of C12A7 cement. This indicated that the C12A7_GO composite materials were affected by the stronger bonds between free extra framework of O 2cement and free electrons on GO surfaces. It helped with inhibitive mechanism and the propagation and growth of initial microcracks 54 as observed in SEM results (Fig. 6). Considering the high magnification view inset previously mentioned in SEM results, Fig. 6e suggests that the decrease of grain sizes of the C12A7_GO-4 composite was affected by loading weight percent of GO which has a high HV value. Thus, the enhancement of the mechanical properties of composited C12A7_GO cement specimens was affected by the amount of GO loading in pristine C12A7 cement.

conclusion
In summary, the C12A7_GO composites (C12A7_GO-1, C12A7_GO-2, C12A7_GO-3 and C12A7_GO-4) were successfully prepared by a direct mixing method of the pristine C12A7 and GO at room temperature. XRD result and Rietveld refinement method of pristine C12A7 and all C12A7_GO composites showed a pure cubic phase with a space group Ia-3d and the excellent fitting showed a good agreement with the 3CaO·Al 2  www.nature.com/scientificreports/ that the enhanced activity of the composite samples was due to an enrichment effect via diffusion within the modified electrode and highly useful GO surface area. Also, the free electron charge was transferred between the free extra framework O 2anions and GO surfaces in C12A7_GO lattices. The C12A7_GO cement composites exhibited multifunctional properties appropriate for high-performance multifunctional applications such as smart cement materials, including optical, dielectric, electrochemical and mechanical properties. Future research may investigate the advantages of enhancing cement properties using modified surface cement from the 2-D nanostructure materials of the GO or Reduced Graphene oxide (rGO) suspension loading. Working electrodes assembly for electrochemical properties analysis. The analysis of electrochemical properties of working electrodes with the pristine C12A7 and all C12A7_GO composites was done in 1 M KOH aqueous electrolyte in three-system electrodes consisting of an active material (working electrode), silver/silver collide (Ag/AgCl reference electrode), and platinum wire (Pt counter electrode). According to Duangchuen et al. ' study 34 , the working electrodes were measured by Cyclic Voltammetry (CV test), Galvanostatic Charge-Discharge (GCD) test, and Electrochemical Impedance Spectroscopy (EIS) tests. The fabrication working cells for electrochemical characterization were normally prepared by the composition slurries mixed with a mass ratio of 80:10:10 of each obtaining product of polyvinylidene difluoride (PVDF) binder and acetylene black, respectively dissolved in N-methyl-2 pyrrolidone (NMP) of 0.4 ml. The composition slurries were mixed by using the ball milling method for 24 h. Next, the mixed composition was coated by a square area of 1 × 1 cm 2 on a cleaned nickel foam substrate and subsequently dried at 80 °C for 6 h. Finally, all electrodes were pressed using uniaxial compression at 1.5 ton for 1 min and soaked in 1 M KOH aqueous electrolyte for 24 h before the test. The CV and GCD testing were carried out in the potential range of − 1.0 to 0.0 V at different scan rates of 10, 20, 50, 100 and 200 mV s −1 and different current density of 0.2, 0.5, 1, 2, 5 and 10 A g −1 . The cycling stability testing of all working electrodes was determined using GCD test at a current density of 2 A g −1 for 1,000 cycles. Lastly, EIS testing of all working electrodes was carried out in a frequency range of 100 MHz-0.01 Hz using an open-circuit voltage by applying AC voltage of 10 mV.

Methods
Characterizations. The structure of the pristine C12A7 and all C12A7_GO composites were investigated at 2θ scanning range of 5-85° with a step interval of 0.02°/s by X-Ray Diffractometer (XRD) in Rigaku (Miniflex Cu K-alpha radiation (CuKα) = 1.5406 Å). Moreover, the confirmed characteristic structures of the pristine C12A7 and the interaction interface between GO and pristine C12A7 of all C12A7_GO composites were examined using dispersive Raman microscopy DXR Smart (Thermo Scientific) at a 532 nm excitation wavelength. Also, FTIR spectra (Bruker, Senterra) was used to evaluate the vibration mode of the atomic bonding of all samples. Thermogravimetric Analysis (TGA) and Netzsch STA 449F3 Jupiter were performed over 35-900 °C with a heating rate of 10 °C/min in nitrogen gas to measure the weight loss of all samples. Pore-size distribution and specific BET surface area of all samples were studied from the Nitrogen gas adsorption/desorption isotherm using the BET and Barrett-Joyner-Halenda (BJH) technique in Autosorb-1, Quantachrome. Scanning Electron Microscope (JEOL SEM JSM-5800 LV) was used to observe the morphologies and grain sizes of the pristine C12A7 and all C12A7_GO composites and to perform the homogeneous distribution of atoms on the powder surfaces by energy dispersive X-ray spectroscopy (EDX-mapping). Shimadzu UV-3101PC UV-Vis-NIR www.nature.com/scientificreports/ spectrophotometer was utilized to measure the absorption edge spectra of all samples at room temperature based on Rudradawong et al. 's study 21 . Furthermore, the dielectric properties of all samples at room temperature were investigated within an AC oscillation voltage of 500 mV over the frequency of 40-10 7 Hz in a KEYSIGHT E4990A Impedance Analyzer. Finally, the electrochemical workstation (CS350 Potentiostat/Galvanostat, Wuhan Corrtest Instruments Corp Ltd) conducted the investigation on electrochemical properties of working pristine C12A7 and all C12A7_GO composite electrodes.