Synthesis of nanoSiO2@graphene-oxide core-shell nanoparticles and its influence on mechanical properties of cementitious materials

Construction and Building Materials 236 (2020) 117619

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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Synthesis of nanoSiO2@graphene-oxide core-shell nanoparticles and its influence on mechanical properties of cementitious materials Yue Gu a,⇑, Kailun Xia a, Zhenhua Wei b, Linhua Jiang a, Wei She c, Kai Lyu c a

College of Mechanics and Materials, Hohai University, Nanjing 210098, PR China Department of Civil Engineering and Engineering Mechanics, Columbia University, New York, NY 10027, USA c School of Materials and Science and Engineering, Southeast University, Nanjing 211189, PR China b

h i g h l i g h t s  A kind of NS@GO core-shell nanoparticles was synthesized and firstly applied to reinforce cementitious materials.  The incorporation of NS@GO could more effectively enhance the mechanical property of cement composites.  A hypothesis named sandwich effect was proposed to explain the reinforcing results of NS@GO.

a r t i c l e

i n f o

Article history: Received 27 July 2019 Received in revised form 8 November 2019 Accepted 13 November 2019

Keywords: NanoSiO2 Graphene oxide Cement Mechanical properties

a b s t r a c t Graphene oxide (GO) has enormous potential to improve properties of cementitious materials. Well dispersed GO in hardened cement matrix could be more effective in strengthening concrete compared to aggregated GO. While GO agglomeration is somewhat inevitable due to its fine nature, the degree of agglomeration could be tuned through particle separation. In this study, nanoSiO2@graphene-oxide (NS@GO) nanoparticles were synthesized using GO and colloid nanoSiO2 (NS) by a electrostatic selfassembly method. NS, GO and the prepared NS@GO were characterized by x-ray diffraction, scanning electron microscopy, atomic force microscopy, and Fourier transmission infrared, and, the influence of NS@GO addition on the mechanical properties of cementitious composites was investigated. The results show that NS@GO can increase the flexural strength of cement composites by 49.2% at later age. A ‘‘sandwich effect” was proposed as an important reinforcing mechanism of NS@GO based on the microstructure and hydration products of NS@GO added composites. Compared with the simple NS-GO blend, NS@GO can better maintain the spatial structure of GO when mixed with cement. In addition, there would be more interactions between calcium silicate hydrates and GO during the cement hydration process, which contributes to the strength gain in NS@GO added mix. The research sheds new light on using GO and NS to optimize cementitious material design, as well as spatial coordination of different types of nanoparticles. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Despite the fact that many new types of construction materials have been invented, Portland cement-based concrete is still the most widely used in the global construction and engineering field. Unfortunately, the production of Portland cement is contributing 5–7% of the worldwide carbon emissions [1–3]. To reduce the carbon footprint, one of the judicious and economic approaches is to extend the service life of concrete, which could avoid repair or rebuilding [4,5]. A major drawback of concrete is its brittleness ⇑ Corresponding author. E-mail address: [email protected] (Y. Gu). https://doi.org/10.1016/j.conbuildmat.2019.117619 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

that could easily lead to the formation and propagation of microcracks [6,7]. The microcracks would seriously reduce the service life of concrete under the influence of the environment. As a novel class of nanoparticles with 2D structure, graphene oxide (GO) is considered to have great potential to improve the toughness of cementitious materials. Lu et al. [8] reported that the addition of 0.08 wt% GO could result in 80.6% increase in flexural strength and 105% increase in flexural toughness. Gholampour et al. [9] mentioned that GO with a mild level of oxygen could lead to a maximum reinforcement of 45% in the 28 days tensile strength of cement composites compared with the plain sample. Pan [10] and his co-author indicated that the incorporation of small quantities of GO could improve the compressive strength of cementitious

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materials by 15–33% and flexural strength by 41–59%, however, the exact mechanisms describing the interactions between GO and cement paste remain unclear. Moreover, contrary to most findings, Ghazizadeh et al. [11] suggested that the reinforcing effect of GO in cement paste was not substantial. This disagreement is related to many factors, the dominating one, however, could be the unsatisfactory dispersion of GO in the cement matrix. It is well-known that the dispersion state of nanoparticles in the matrix correlates intensively with their effectiveness in enhancing the properties of the matrix [12–15]. Typically, adopting smallmolecule chemical dispersants that can increase the absolute surface potential of nanoparticles is a reasonable method to prevent agglomeration. However, this method seems unreliable for the dispersion of GO in cement matrix because calcium ions derived from cement hydration would reduce the surface potential of GO, causing the double layer structures that repels GO moieties to fail [16]. In recent studies, two new strategies were proposed based on the steric hindrance mechanism which makes a small progress for the dispersion of GO. One is pre-adding polycarboxylate superplasticizer (PCE) of high molecular weight to stabilize GO in the solution with complex ions [17,18], the other is using silica fume (SF) as filler to form GO-SF composites prior to mixing with cement [19]. Despite the advantages of these strategies, their drawback is similar in nature, i.e., the weak binding force between PCE (or SF) and GO could not hold the composite structures for too long when they are mixed with cement. For instance, most PCEs would be preferentially adsorbed on the surface of cement clinker (or hydration products) instead of GO in the cement environment [11], and the SF would be separated from GO by the shearing effect in the mixing process. To further develop a novel method to promote the dispersion of GO, enhancing interaction forces between GO and the steric hindering additives seems a reasonable direction of exploration. In this study, nanoSiO2@graphene-oxide (NS@GO) nanoparticles were synthesized and characterized using various techniques. Then mechanical tests were carried out to investigate the interesting strengthening effect of NS@GO on cementitious materials, compared with that of references including NS, GO, and simple NS-GO blend. This research presents a new strategy to synergistically using GO and NS to enhance the property of cementitious materials. 2. Materials and methods 2.1. Materials 2.1.1. Chemicals The chemicals used for the synthesis of GO were graphite (size  20 lm); concentrated sulfuric acid (H2SO4, 98%), potassium permanganate (KMnO4), sodium nitrate (NaNO3) and hydrogen peroxide (H2O2), which were all analytically pure. That for the synthesis of NS were ammonium hydroxide (NH3, 25%), ethanol (99.9%), and tetraethoxysilane (TEOS, 98%). The extra chemical for the synthesis of NS@GO was 3aminopropyl triethoxysilane (APTES, 99%). 2.1.2. Cement Ordinary Portland cement compliant with the Chinese national standard GB8076-2008 was used to fabricate the cement paste. The oxide analysis of the cement was performed using X-ray fluo-

rescence spectrometry (XRF, Thermo Fisher ARL). The composition of the cement is listed in Table 1. 2.2. Synthesis of NS@GO The NS solution with a mass concentration of 30% was produced based on the well-known Stober method [20,21]. GO solution with a concentration of 6 mg/ml was prepared according to the modified Hummer’s method [22], more details about the synthesis of GO were reported elsewhere [23]. NS@GO core–shell nanoparticles were prepared by a two-step method. First, amino-functionalized NS was synthesized through a simple process. 5 g of colloid NS and 1 mL APTES were added to 150 mL ethanol under ultrasonication and stirring. The obtained suspension was placed in a flask, then constantly stirred at 50 °C for 12 h after evacuating air with N2 purge for 20 min. The product was washed with alcohol and water several times, precipitated by centrifugation and dried under vacuum at room temperature for 1 day. Second, the self-assembly between NS and GO by electrostatic force was conducted in an aqueous solution. The surface of GO has many oxygen functional groups including –OH and –COOH. As a result, it shows electronegativity in solution and could interact with amino-functionalized NS with a positively charged surface. The amino-functionalized NS was dispersed in deionized water under stirring and ultrasonication to ensure dispersion. Then, the GO solution was added dropwise to the system. The reaction was processed under vigorous stirring at 50 °C for 3 h. The obtained suspension was collected at room temperature. The mass ratio of GO/NS in this study is designed to be 1/30. 2.3. Preparation of cement composites To explore the effect of different nanoparticles on the mechanical properties and microstructure of cement paste, mixtures with a constant water to binder ratio of 0.4 were prepared. The plain cement paste without nanoparticles served as reference sample. For comparison, cement pastes with different types of nanoparticles were fabricated. These samples were labeled as NS-cement, GO-cement, Mix-cement, and NS@GO-cement, respectively. Details of the cementitious mixture proportions are presented in Table 2. The mixing procedure is as follows: The nanoparticle solution was mixed with water and ultrasonically treated for 15mins and then the suspension was added to a stainless steel mixer. Cement was gradually added over a time span of 30 s at a rotating rate of 60 rpm. After a 20 s interval, the mixing process was continued for another 60 s at 500 rpm and 60 s at1500rpm. After mixing, the mixtures were cast into rectangular molds, then vibrated and compacted to ensure compaction. After 24 h sealed curing in ambient condition, the samples were demolded and placed in a moist storage chamber (20 ± 1 °C, 90%RH) until testing. 2.4. Characterization 2.4.1. Characterization of nanoparticles The morphology of nanoparticles was observed by a Quanta 250 Field Emission Scanning Electron Microscope (FEI, Hillsboro, OA, USA). The vacuum oven-dried sample was coated with 15 nm of gold to make it conductive before observation. The mineralogical

Table 1 Chemical composition of the standard Portland cement. SiO2

CaO

MgO

Al2O3

Fe2O3

SO3

Na2Oeq

Loss

19.98

61.86

2.02

4.61

3.07

4.43

0.68

0.9

3

Y. Gu et al. / Construction and Building Materials 236 (2020) 117619 Table 2 Mix proportions of nano-reinforced cement composites with different types of nanoparticles. Mix

W/B

Nano-SiO2

GO

SiO2@GO

BK NS-cement GO-cement Mix-cement NS@GO-cement

0.4 0.4 0.4 0.4 0.4

– 1.5% – 1.5% –

– – 0.05% 0.05% –

– – – – 1.55%

Note: the amounts of NS, GO, and NS@GO are calculated by the weight of cement.

feature of NS was measured by XRD using a Bruker D8 Advance diffractometer in a h-h configuration using Cu- Ka radiation. The scanning range was 5–50 °C (2h) with a scanning rate of 0.02 °C/s. The thickness of GO was measured by an Atomic Force Microscope (AFM, Bruker CO., Dimension ICON) with the tapping mode. The sample for the AFM test was prepared by depositing a drop of a dilute solution (0.5 mg/mL suspension was dilute 1000times) on a piece of monocrystalline silicon and dried at room temperature. Fourier transform infrared spectroscopy (FTIR, BRUCKER CO., EQUINOX55) was employed to characterize the binding in NS@GO. During the FTIR test, the sample was palletized with KBr powers, and the FTIR spectra were collected in the range of 300– 3800 cm 1 at a resolution of 2 cm 1. 2.4.2. Mechanical properties of cement composites Measurement of compressive strength of cement composites was carried out according to ASTM C109. For flexural strength measurement, the specimen with dimensions of 100 mm *20 mm *20 mm was tested using a 3-point bending method. A span of 70 mm and a constant stain rate of 0.1 mm/min were adopted. Measurements were taken at the ages of 3 days and 28 days. The strength data reported herein are the average of three replicates, together with the standard deviation. 2.4.3. Microstructure of cement composites The microstructure of cement composites was investigated by SEM. The vacuum oven-dried sample was coated with a gold layer of 15 nm to make it conductive before observation. 3. Results and discussion 3.1. Characterization of nanoparticles NS was synthesized by the classic Stober method, which produces relatively uniform spherical nanosilica. The morphology of

NS observed by SEM at a magnification of 120000x is shown in Fig. 1(a). The as-prepared NS possesses a monodispersed state while its shape is close to the ideal sphere as expected. The average diameter of the synthetic NS is around 300 nm. Fig. 1(b) shows the XRD pattern of NS. A strong broad peak is seen around 23°, which agrees well with the characteristics of amorphous silica. A relatively large width at half maximum means NS is in a low crystalline state. SEM and AFM images were used to characterize the morphology of GO nanosheets. As shown in Fig. 2(a), the width of irregular GO sheets is at micron level similar to their length. It can clearly be seen that the GO has good dispersion in the aqueous solution, which is likely due to the presence of functional groups on the surface of GO, e.g., hydroxyl and carboxyl groups. The AFM image displayed in Fig. 2(b) confirms the height profile d representing the thickness fluctuation range of GO is below 25 nm indicating the surface is not very smooth, but full of undulating folds. This surface texture also implies a multilayer structure of flake-like nanosheets. Although wrinkles exist, their magnitude is not significant compared to the length–width plane. There is a vague understanding that GO as a kind of additive would play a better enhancement effect in hardened cement matrix if it has fewer layers in the aqueous solution. In this research, we seek a better and cost-effective alternative to avoid excessive agglomeration of GO after mixed with cement paste. SEM images of NS@GO shown in Fig. 3(a) demonstrates that NS has been densely coated on the surface of GO nanosheets. After this surface modification, the point-to-point interaction between GO and NS replaces the face-to-face large area interaction between nanosheets caused by the p-p stacking force [24]. There are some changes in the morphology of GO — the flake-like nanosheet structures become fluffy and porous, and the appearance of some micropores indicates the continuity of hexagonal carbon rings may be disturbed to some extent by NS. Fig. 3(a) also shows that the NS retained the original spherical shape. FTIR spectra collected for

Fig. 1. Characterization of NS (a) SEM image of NS; (b) XRD pattern of NS.

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Fig. 2. Characterization of GO (a) SEM image of GO; (b) topography of GO.

Fig. 3. Characterization of NS@GO (a) SEM image of NS@GO; (b) FTIR spectra of NS@GO and GO.

GO and NS@GO are shown in Fig. 3(b). For the GO, a broad band centered at 3376 cm 1 is attributed to the hydroxyls (OAH) stretching, and the peaks at 1739 cm 1 and 1227 cm 1, 1050 cm 1 could be assigned to carbonyl (C@O) stretching, alkoxy (CAOAC) stretching, and CAO stretching, respectively. Additionally, the absorption peak at 1620 cm 1 could be assigned to the bending mode of H-O–H which is incorporated in graphemic sub-lattice [25]. These peaks indicate the oxygen functional group is immobilized on the surface of GO. In comparison to GO, NS@GO displays some new characteristic peaks. The intensive bands at 1102 cm 1 can be ascribed to the overlapping of Si-O-Si, and SiO-C asymmetric stretching, while the one at 793 cm 1 can be assigned to Si-OH stretching. In addition, the peak at 465 cm 1 is the feature of Si-O-Si bending vibration. These peaks provide evidence that NS and GO are bonded with each other tightly. 3.2. Mechanical property of cement composites In order to evaluate the influence of nanoparticles on the mechanical properties of cement composites, five mixtures containing different kinds of nanoparticles were measured. The compressive strength results are presented in Fig. 4. In general, incorporation of nanoparticles improves the compressive strength at both early and later curing ages. This could be attributed to the seeding and microstructurally refining effects. At 3 days, it

can be seen from Fig. 4(a) the NS@GO is the most effective type of nanoparticle in improving cement paste’s compressive strength. After NS@GO addition, the compressive strength of cementitious composites can reach as high as 50 MPa, which is equivalent to a 65% increase compared to the plain reference sample. The strength enhancing effect of these nanoparticles follows an order: NS@GO > mixed > GO > NS. At 28 days, as shown in Fig. 4(b), all nanoparticles modified samples present a reinforcing effect with NS@GO addition having the strongest strength enhancement. The compressive strength of NS@GO-cement composites is 28.2% higher than that of the plain sample, while only a slight difference in strengthening is noted between the NS-, the GO- and the mixedcement composites. Similar to the compressive strength results, the flexural strength results in Fig. 5 indicate that all types of nanoparticles have a positive effect at both early and later ages. At 3 days, compared to the plain sample, the flexural strength of the NS-, the GO-, the mixed, and the NS@GO-cement composites increases by 16.8%, 28.8%, 32.6%, and 48.5%, respectively. This indicates that the reinforcing effect is more pronounced in the NS@GO-cement composites. At 28 days, as shown in Fig. 5(b), NS@GO addition still produces the most significant strength increase (49.2% increase compared with the plain sample). The fact that this value is close to the strength increase rate resulted from NS@GO addition at 3 days suggests NS@GO could keep the enhancement effect at later

Y. Gu et al. / Construction and Building Materials 236 (2020) 117619

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Fig. 4. Compressive strength of cement composites (a) 3d; (b) 28d.

Fig. 5. Flexural strength of cement composites (a) 3d; (b) 28d.

ages. For NS, however, the inclusion of NS only has a slight effect on the growth of the strength at later ages. Another interesting observation is that the GO modified sample has a flexural strength similar to the mixed sample. This implies that GO and NS are unlikely to have a remarkable synergistic effect in a simple mixed way. Although the improvement in the strength by the addition of nanoparticles is usually ascribed to the filler effect [12], seeding effect [26], and the morphology alteration of hydration products [27], the spatial distribution of nanoparticles may also play a very important role, especially for GO. A more detailed explanation will be further elucidated in Section 3.4. 3.3. Microstructure of cement composites The microstructure of a material is strongly correlated with its mechanical properties such as compressive or flexural strength. To get more details about the microstructure influenced by the local distribution of nanoparticles, morphology images of cement composites at a very early age (12 h) were observed by SEM. As shown in Fig. 6(a), common hydration products such as portlandite, ettringite, and C-S-H are present in the image of the blank cement sample. The flake-like crystals are mainly portlandite while the small rod-like crystals are ettringite. Some unhydrated areas can also be seen in the image. C-S-H gels tend to grow on the surface of cement particles and in the micro-pores. The morphologies of C-S-H gels were further observed at a higher magnification as shown in Fig. 6(b). The needle-like C-S-H gels are interwoven into

each other which may be an important origin of cohesive force in cement composites. It can also be observed that the local growth of new C-S-H gels in confined spaces may cause the micro-cracks in the solid particles. The morphological information of cement composites with NS@GO is shown in Fig. 7(a)–(b). As shown in Fig. 7(a)–(b), many plate-like products have appeared which cannot be detected in Fig. 6. The planes of plate-like products seem to be perpendicular to a certain direction. The layers are not close together, and the space between them is relatively large. These stacked products with smooth surfaces should be a result of GO clustering following the ion polishing process. Scrutinized observations on the interlayer space at a higher magnification are marked in Fig. 8(a)–(b). Irregular needle-like C-S-H gels are filled in the interlayer space derived from the hydration of cement and the pozzolanic reaction of NS. C-S-H is the main binding phase of cementitious composites, and the existence of C-S-H gels would strengthen the interface between GO clumps and the matrix. A two-layer GO clustering incorporating a layer of C-S-H gels in between resembles a sandwich structure, which indicates that NS may function as wedges before it was involved in the pozzolanic reaction. The transformation of NS to C-S-H would further enhance the binding between GO and the matrix. Among fibrous C-S-H clusters, few irregular small particles could be seen. This implies that most NS particles have been involved in the pozzolanic reaction, and their surfaces are covered or etched by the reaction products. Based on the above observation, it is conceivable that the GO clusters could be separated by NS mechanically in the cement matrix.

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Fig. 6. SEM micrographs of cement paste without NS@GO (a) 12,000x; (b) 40,000x.

Fig. 7. SEM micrographs of cement composites at 10,000X.

Fig. 8. SEM micrographs of cement composites at 20,000X.

3.4. Discussion GO has been reported as an excellent additive to reinforce cementitious materials due to facts that GO (i) itself has extraordinary mechanical properties [11]; (ii) could provide more nucleation sites to boost cement hydration; (iii) exhibits a bridging effect to suppress the propagation of micro-cracks at nanoscale [28]; and (iv) could refine the microstructure such as the micropore structure and morphology of composites.

In general, the dispersibility of GO as a nanoparticle in cement matrix strongly determines its reinforcement efficiency. Unlike graphene, GO has a relatively better dispersion in aqueous solutions. However, this does not guarantee that GO could also be well dispersed in a cementitious matrix when mixed with cement particles. The ions derived from the dissolution of cement minerals could break the stability of GO due to neutralization of the surface potential. For instance, a low concentration of Ca2+ released in hydration reactions could cause GO re-agglomeration [29,30]. In

Y. Gu et al. / Construction and Building Materials 236 (2020) 117619

order to minimize the degree of agglomeration of GO in the cement matrix, various strategies including high shear mill, ultrasonication, and assistance of surfactants, have been proposed in previous studies. Among various surfactants, water-reducing agents seem a promising candidate due to their good compatibility with cement systems. As the latest generation of water-reducing agents, polycarboxylate ether (PCE) copolymer has been recently studied to improve the dispersion of GO in cement matrix. Unlike some types of surfactants, the driving force of the repulsive effect of PCE on GO structures is based on its steric hindrance, rather than DLVO interaction. This dispersion mechanism is relatively less affected by ions compared with methods that regulate surface potential. It should be noted that GO-PCE dispersion experiments in many studies were observed in very dilute suspensions which have huge differences from actual dispersion scenarios in cement matrix.

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While some progress has been made, it is still a big challenge to better stabilize the dispersion of GO using PCE admixtures. Inspired by the idea of steric hindrance, some new strategies have been proposed. For instance, the method of utilizing silica fume (SF) as wedges to separate GO mechanically has been reported by Li et al. [31]. However, as pointed out by Lu et al. [32], this method is too ideal due to the priority self-lock of GO, which is due to the fact that GO is more likely to adsorb calcium ions than SF in the cementitious environment. As such, the key to take advantage of the steric hindrance effect lies in maximizing the spatial interlocking between GO and the embedded particles after mixing. In this study, NS and GO are bonded by the strong electrostatic attraction and H-bond in the synthetic NS@GO. The self-lock phenomenon of GO could be controlled to a lower degree in comparison to simple GO-NS mixing. The possible dispersion mechanism

Fig. 9. Schematic of the possible dispersion state (a) GO; (b) NS@GO.

Fig. 10. Schematic of the hypothetical sandwich effect.

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of NS@GO in cement matrix and that of GO are illustrated in Fig. 9 (a)–(b). As shown in Fig. 9(a), GO with homogeneous dispersion state would re-agglomerate in cement matrix after the mixing process, due to its affinity to calcium ions as mentioned before. This re-agglomerate would reduce the contact area between GO and cementitious matrix that weakened the effectiveness of GO. For NS@GO, the pristine spatial structure in the cementitious environment may be well maintained after mixing in cement paste, as shown in Fig. 9(b). The aggregation of nanoparticles in cement matrix could not be fully eliminated, and NS@GO is no exception. Nevertheless, when local aggregation occurs, the interlock mechanism would be activated such that GO could still exhibit a low aggregated state in the matrix. The dispersion enhancement of NS@GO in the local area of cement composites could be referred to as a ‘‘sandwich effect”. During the cement hydration process, since the GO itself is relatively inert, there would be no phase transition. While for NS, it would react with calcium hydroxide derived from the hydration of C2S and C3S to generate the C-S-H gels. For NS in NS@GO, there would be two possible reaction paths as shown in Fig. 10(b) and (c), respectively. As illustrated in Fig. 10(b), the NS in the inner layer has a different exposed surface area, and only part of NS with enough reactive surface would be initially involved in pozzolanic reaction before the surface gets covered. This suggests the newly produced C-S-H gels would not connect NS to GO, but link NS particles or small NS agglomerates to one another. Another possible case scenario shown in Fig. 10(c) depicts that most of NS particles embedded in the inner space would have a similar reaction rate as ions of the high concentrations could diffuse freely in the system. As a result, the surface of most of NS particles would be covered by C-S-H gels. The interface between NS and GO could obtain a stronger adhesion compared with the case scenario shown in Fig. 10(b). The inner space has a high porosity that is convenient for ion diffusion. This may indicate that NS@GO could be most likely all converted to C-S-H@GO for both possible situations shown in Fig. 10(d). It should be noted that, the reaction path of NS particles in the interlayer space and their intermediate state need further research as they may intensively influence the enhancement effect. In addition, for effective application of GO in cement and concrete composites, other promising routes that could expect similar sandwich effect include (i) design of microparticles composed of GO and supplementary cementitious materials (SCMs) (e.g., silica-fume@GO, fly-ash@GO) (ii) reasonable adjustment of raw materials mixing sequence. Specifically, GO could be premixed mechanically with sand or SCMs before mixed with cement.

4. Conclusion In order to explore a new method to enhance the dispersibility of GO in cement-based materials and figure out a better strategy to reinforce cementitious composites, this study has designed a new type of core–shell nanoparticles, i.e., nanoSiO2@graphene-oxide (NS@GO), and investigated its influence on the mechanical properties of cementitious materials. NS@GO core–shell nanoparticles were successfully synthesized using GO and colloid nanoSiO2 by the electrostatic self-assembly method, which has been confirmed by a serious of characterizations including XRD, SEM, AFM, and FTIR. Mechanical tests have revealed that the incorporation of NS@GO can significantly improve the compressive strength of cement paste at both early and later ages by 65% and 28%, respectively. Similarly, the 28-day flexural strength of cement composites can be increased by 49% with the addition of NS@GO. The strengthening effect of NS@GO is much more effective than those that of

NS, GO, and simple NS-GO blend. At later ages, NS@GO exhibits better reinforcing capacity in flexural strength than in compressive strength. Through SEM observation, numerous plate-like products were found in the cement composites containing NS@GO, with some irregular needle-like C-S-H gels filled in the interlayer space. Furthermore, A hypothesis of a sandwich effect was proposed to explain the local reinforcing mechanism due to the introduction of NS@GO in cementitious composites. The outcomes of this research provide new insight into modifying cementitious materials by GO and NS.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments The authors would like to thank the financial support by the National Natural Science Foundation of China (Grant No. 51808188), China Postdoctoral Science Foundation (2018M642151), State Key Laboratory of High-Performance Civil Engineering Materials (Grant No. 2018CEM008), Jiangsu Key Laboratory for Construction Materials (CM2018-13), and Jiangsu Planned Projects for Postdoctoral Research Funds (2018K134C). The authors thank Jiangsu Research Institute of Building Science Co., Ltd and the state key laboratory of high performance civil engineering materials for funding this research project.

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