Stabilizing effect of methylcellulose on the dispersion of multi-walled carbon nanotubes in cementitious composites

Multi-walled carbon nanotubes (MWCNTs) have been added in the plain cementitious materials to manufacture composites with the higher mechanical properties and smart behavior. The uniform distributions of MWCNTs is critical to obtain the desired enhancing effect, which, however, is challenged by the high ionic strength of the cement pore solution. Here, the effects of methylcellulose (MC) on stabilizing the dispersion of MWCNTs in the simulated cement pore solution and the viscosity of MWCNT suspensionswere studied. Further observations on the distributions of MWCNTs in the ternary cementitious composites were conducted. The results showed that MC forms a membranous envelope surrounding MWCNTs, which inhibits the adsorption of cations and maintains the steric repulsion between MWCNTs; thus, the stability of MWCNT dispersion in cement-based composites is improved. MC can also work as a viscosity adjuster that retards the Brownian mobility of MWCNTs, reducing their re-agglomerate within a period. MC with an addition ratio of 0.018 wt.% is suggested to achieve the optimum dispersion stabilizing effect. The findings here provide a way for stabilizing the other dispersed nano-additives in the cementitious composites.


Introduction
Carbon nanotubes (CNTs), categorized as single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) [1,2], have low density, superior high aspect ratio and excellent mechanical properties [3,4]. The tensile strength and Young's modulus of a single MWC-NTs was found to be about 11-63 GPa and 270-950 GPa [3]. Due to the excellent material properties, CNTs has found its applications in enhancing various types of composites including organic polymers [5], ceramics [6], and cementbased composites [7][8][9]. Studies show that the CNTs embedded in cement-based matrices can work as nano-filler as well as play the crack-bridging role [10], and it can improve the load transferring efficiency of the composites through the pulling-out behavior [11]. The enhancement of CNTs at a low addition ratio (<0.5 wt.%) in the cementitious composites has been observed in the strength and stiffness [9,10,12]. CNTs, by virtue of the excellent electrical conductivity (>10 3 S/m) [13], has been incorporated in cement-based materials for the resistivity-reducing [14]. In addition, the characteristic of the varied resistivity of the CNTs enhanced cementitious materials versus the external load [15], moisture [16], and chloride concentration [17], providing the composites with the monitoring functions.
To incorporate CNTs properly into cement, their uniform dispersion is required because only the individually distributed CNTs are effective in terms of reinforcement or resistivity decline [7,18]. The agglomerated CNTs in cement-based materials, however, work as weak spots or introduce defects in the form of voids or unreacted cement particles [18]. Unfortunately, the strong van der Waal's interaction makes the pristine CNTs to attach to each other in bundles or clumps [19]. Preparing the CNTs aqueous suspensions by ultrasonication [7,18,20] or high shear treatment [20,21] is the most used to separate CNTs from agglomerates, at present. Meanwhile, the non-covalently attached surface-active agents (SSAs) that can enhance the steric repulsion between CNTs are generally incorporated for maintaining the stable dispersion of CNTs in aqueous solutions [18,19]. Some researchers introduced the covalently attached functional groups like -COOH on the outer walls of MWCNTs to provide the hydrophilicity and repulsion [22]. In fresh cement pastes, more than 95% of CNTs exist in the pore solution which is characterized as a strong Ca 2+ based alkaline environment (with pH value of higher than 10) [22,23]. In contrast to the organic solutions [24,25] and the near neutral aqueous liquid [26], such an alkaline environment is not conductive to the stable CNT dispersion. The dispersion degree of CNTs declines over time at a measurable rate before the hardening of slurries and the CNTs attach to each other; thus, sedimentation of CNT agglomerates happens [23]. It has been reported that after 5 hours, the maximum decline of the dispersion degree can be about 80% [23]. Stabilizing the well-dispersed CNTs in fresh cement is necessary for making full use of their material properties. In previous study [27], methylcellulose (MC), a thickening agent for concrete [28], was found to be helpful in stabilizing the dispersed CNTs in cement pore solution according to the UV-vis-NIR test results. In Ref. [27], it was found that at 4 h, the MWCNT suspension with 0.18 wt.% MC had about 51% of dispersed MWCNTs, which was almost double of the amount of MWCNTs in suspension without MC. MC, on the other hand, is also helpful for avoiding the precipitation of cement particles during the hardening process. The bleeding rate of the fresh pastes was lower than 4.0% when 0.10 wt.% MC was incorporated. However, in Ref. [27], researchers have focused on the influences of MC on the fluidity, porosity, compressive strength and elastic modulus of the CNTs enhanced cementitious composites, whereas the interactions between MC and CNTs, and the distribution of CNTs in the composites are neglected. Little information about the exact role of MC in the mixtures has been provided. Understanding of the working mechanism of MC on stabilizing the wellseparated CNTs in cementitious materials requires further research.
Here, the stabilizing effects of different dosages of MC on the dispersion of MWCNTs in the simulated cementitious pore solution were identified by measuring the Zeta potential of MWCNT suspensions and the hydrodynamic diameter of the agglomerates [22]. Influences of MC on the viscosity of MWCNT suspensions were tested. Distributions of MWCNTs in the pore solution and in cement were visually observed using TEM (transmission electron  ) and SEM (scanning electron microscopy), respectively. This study shows MC, with a moderate addition, can forms a membranous envelope surrounding MWCNTs that inhibits the adsorption of cations, thus maintaining the steric repulsion, and works as viscosity modifier that retards the Brownian mobility of MWCNTs, which enhance the resistance of MWCNTs to the re-agglomeration within a period. Findings provide a method for maintaining the stable dispersion of other nano-additives in alkaline cementitious environments.

Raw materials and instrumentation
The structure properties of the non-functionalized MWCNTs used here are presented in Table 1 and fly ash (FA) were adopted to prepare the cementitious blends. FA and SF were added to obtain a more general cementitious composite [29,30]. The addition of FA and SF allows the use of less amount of cement and thus decreases the associated carbon dioxide emission and reduces the potential economic cost [31,32].
A horn ultra-sonicator with a cylindrical tip, SONICS VCX 500W, was used to separate the MWCNTs in distilled water. The end-cap diameter of the sonicator tip is 13 mm. A zeta potential analyzer, NanoBrook 90 Plus was used to measure the Zeta potential of suspensions and the hydrodynamic size (D) of nanoparticles. The morphology of MWCNT bundles in the simulated cement pore solutions was visually characterized using TEM (Tecnai G2 F20). A digital rotary viscometer, type NDJ-9S was used to measure the coefficient of viscosity (µ) of the MWCNT suspensions that contains different content of MC. The embedded MWC-NTs in the hardened matrix was observed using SEM (HI-TACHI SU8220).

Fabrication of MC-MWCNT suspensions
With the pulse ultrasonic method, MWCNTs were separated in the PC aqueous solutions under ultra-sonication for producing the uniform MWCNT suspensions. The ultrasonic process lasted for 12 min and about 750 J/mL ultrasonication energy was input into the mixtures. The waterice bath treatment was used during sonication to prevent the overheating of the mixtures. Then, MC was dissolved into the suspensions by stirring at a rate of 200 rpm for about 10 min. MC was infiltrated by ethanol to accelerate its dissolution. The mixtures were left for 120 min to until the bubbles disappeared before the subsequent experiments. The mass concentrations of MWCNTs, PC, and MC in the suspensions are shown in Table 2. The weight of PC was about eight times that of MWCNTs, as suggested in the literature [10]. Five groups of MC-MWCNT suspensions MC were prepared.

Dispersion and viscosity tests of the MC-MWCNT suspensions
A simulated cement pore solution, with the chemical concentrations suggested in Refs was firstly prepared to obtain the alkaline cementitious environment [23,33]. Then, MC-MWCNT suspensions were diluted to 1/50 of their initial concentrations with the pore solution for simulating introducing MWCNTs into cement. Zeta potential of the MC-MWCNT suspensions and the hydrodynamic size (D) of nanoparticles were measured to characterize the dispersion degree of MWCNTs in this environment. Volumes (3 mL) of the mixtures were stored in phials and transferred to the zeta-potential analyzer. The variations of Zeta potential and D every 30 min for 4 h were measured. All suspensions were equilibrated for 2 min at room temperature before the measurements. To confirm the reproducibility, two samples were prepared for each group of suspensions for the measurement of Zeta potential and D.
The diluted suspensions were stored in phials for visual observation. And at the times of 0, 60 min, 120 min and 240 min after dilution, a drop of liquid was dropped on the copper grid and dried in room temperature for investigating the morphology of the MWCNT agglomerates. Only the MWCNT agglomerates in the suspensions MC-0, MC-1 and MC-4 were observed by TEM. During the dispersion testing process, the coefficient of viscosity (µ) of the diluted MC-MWCNT suspensions was tested simultaneously. The rotation speed was fixed at 6 rpm and the No. 0 rotor was adopted. Three samples for each group of suspensions were prepared for the viscosity measuring.

Distributions of MWCNTs in cement-based matrixes
The distributions of MWCNTs in the hardened cementbased matrixes were observed by SEM. Suspensions MC-1, MC-2, and MC-4 were used to make the MWCNTs enhanced cementitious composites. Suspensions were added into the mixed ternary powders containing OPC, SF, and FA. The mass contents of OPC, SF and FA were 80%, 10% and 10%, respectively, as suggested in literature [29,30]. The water to powder ratio (w/c) of the slurries was 0.6. Such a high or higher w/c is often used to prepare the high-fluidity cementitious materials [31]. More details about the fabrication of the pastes and the curing conditions are included in Ref. [27]. In order to reveal the MWCNTs covered by the C-S-H gels or other hydration products [34], before the SEM investigation, the hardened composites were treated with the acid etching. To be exact, matrixes were immersed in the hydrochloric acid with pH of 4 for half a minute for dissolving the hydration products, and then rinsed in ultrapure water for the acid residue removing. With the dilution acid and the limited etching time, the influences of etching on the distributions of MWCNT could be ignored.

Effects of MC on the dispersion of MWCNTs in pore solution
The variations of Zeta potential of MC-MWCNT suspensions versus time are shown in Figure 2. The Zeta potential of the suspensions decreased gradually with the increasing storage time, indicating the decreased dispersion stability of MWCNTs in the pore solution [22]. After about 1 hour and a half, the Zeta potential of all the suspensions was with values of over 12 mV. The suspension MC-1 showed the highest Zeta potential value, then the suspension MC-2, implying these suspensions have higher dispersion degree of MWCNTs than other suspensions [22].  All the observations on the variations of Zeta potential and hydrodynamic size imply that an appropriate amount of MC is required to stabilize the dispersed MWCNTs, and excess MC accelerates the sedimentation of MWCNTs in cement pore solution. Adding 0.018-0.036 wt% of MC is preferred to give the favorable stabilizing effect. Figure 5 presents the investigated morphology of MWCNTs agglomerates in cement pore solutions at different time.

TEM investigations on the distribution of MWCNTs in pore solutions
More TEM results are covered in Figure S1. The observed regions were randomly chosen. Figure 5 (a) shows that the matter in the suspensions start to adsorb and accumulate on the surface of individual MWCNTs immediately after mixing. The amount of the accumulated microparticles on MWCNTs was more and more, and the MWCNTs tended to connect to each other. About   Figure 5 (b) shows that the molecular MC could form a layer of a membrane-like structure that wraps the individual MWCNTs to protect them effectively from the adsorption of solute. After 120 min, the amount of microparticles surrounding the MWCNTs was much lower. 4 hours later, re-agglomerated MWCNTs could be found as well, but with a smaller volume, corresponding well with the results presented in Figure 4. When an excess amount of MC was added, the netlike MWCNT agglomerates were observed from the beginning time ( Figure 5 (c)). After 60 min, most of the MWCNTs appeared in an interwoven form or in clumps. The clumped MWCNTs were stilled wrapped by MC. As most of the MWCNTs appeared in the agglomeration in the suspension MC-4 after about 1 hour, therefore, no TEM images were taken anymore in the following time.   Figure 6 demonstrates the Energy Dispersive X-Ray Spectroscopy (EDX) result of the agglomerated particles observed in suspension MC-0 after mixing. The resource of copper (Cu) here is the copper grid substrate. The carbon (C) was provided by MWCNTs and the oxygen (O) came from the impurities in MWCNTs. Thereby, both the sodium (Na), potassium (K) and calcium (Ca) should be the constituents of the microparticles surrounding MWC-NTs, and they came from the pore solutions, indicating that the cationic (Ca 2+ , K + , Na + ) in the pore solutions will adsorb on the surface of the individual MWCNTs after mixing. In MWCNT aqueous suspensions, molecular PC can wrap onto the surface of the MWCNTs via the hydrophobic groups, whereas the hydrophilic parts are pointed outwards, making the MWCNTs disperse well through steric interaction [19]. The adsorption of ions will lower the surface potential of the dispersed MWCNTs and decline the steric repulsion, resulting in the re-attachment of MWCNTs ( Figure 5 (a)) [23]. With the existence of the MC membrane, the adsorption of ions can be inhibited effectively ( Figure 5 (b)); thus, the re-agglomeration of MWCNTs in the cement pore solutions can be retarded. However, at higher concentration, the MC chains are more readily to connect into a network that can link the MWCNTs into the large agglomerations ( Figure 5 (c)) [35]. Figure 7 shows the viscosity measurement results of MC-MWCNT suspensions with different mass contents of MC. Figure 7 shows that µ of the MC-MWCNT suspensions increases significantly with increasing dosages of MC. The maximum increase in µ can be higher than 600%. Since the averaged µ value of a MWCNT suspension with no MC is about 1 mPa·s, very close to that of pure water (0.9 mPa·s), the higher value of µ is caused mainly by the addition of MC. MC, a long-chain substituted cellulose, is a typical non-ionic water-soluble polymer [36]. The diluted molecular MC in the suspensions spreads out and the free movement of molecular water will be restrained [28]. Therefore, the viscosity of the suspensions is increased. Brownian motion is a physical process that the suspended microscopic particles in liquids or gases move randomly and it is resulted from the impact of molecules of the surrounding mediums [37]. Owing to the small size char-acteristic of MWCNTs (Table 1), the potentially effect of Brownian motion on the MWCNT aggregation should not be discounted [38]. The aggregation of MWCNTs is an assembly process of the singly distributed fibers that influenced by the inter-particle separation [38]. Assuming the equidistantly separated MWCNTs, over a period of time, the longer the mean travel distance of MWCNTs, the more likely the nanoparticles are to agglomerate. Studies [39] have pointed out that the mean travel distance of particles declines logarithmically versus the increasing viscosity of surrounding medium. That is to say, the intensity of Brownian motion is negatively influenced by the increasing viscosity. From this perspective, in suspensions containing a moderate amount of MC, the non-interacting nanotubes have the deficient Brownian motion to re-agglomerate over a period of time [40,41]. Namely, MWCNTs can be remained separately within the time period. Therefore, MC also helps to avoid the re-aggregation of MWCNTs via viscosity modifying effect.

Distributions of MWCNTs in cement matrixes
The SEM observation results of the distribution of MWC-NTs in cement-based composites are presented in Figure 8. More SEM observation results can be found in Figure S2. All the SEM images were randomly taken on the samples. As shown in Figure 8 (a), when the cement-based composites were manufactured without adding any MC, MWC-NTs in the matrix were in clumps, whereas MWCNTs were more likely to be embedded in the matrix as single fibers or in the separated state when moderate amount of MC (with MC/s of 0.18 wt.% and 0.35 wt.%) was used (Figure 8 (b), (c)). For the composites prepared by adding suspension MC-4, as presented in Figures 8 (d) and (e), the MWCNTs in the hardened matrixes were more likely to be in clumps or agglomerations. More details can be found in Figure 8 (f) to (h). The distributions of MWCNTs in cement-based matrixes is very consistent with the dispersion test results (Section 3.1), showing that moderate additions of MC to MWCNT suspensions can improve the dispersion degree of nanotubes in cement pastes, which benefits the uniform distribution of MWCNTs in cement-based matrixes. The more uniformly embedded MWCNTs in the hardened composites can play their role in strength enhancement much better, which has been experimentally verified. In Ref. [27], authors have reported that adding 0.018 wt% MC to the MWCNT suspensions achieves the maximum enhancing efficiency of MWCNTs on the strength and stiffness of the cementitious composites. The maximum increase in the Moreover, in addition to the CNTs, other nanomaterials, such as carbon nanofibers [42,43], graphene [44,45], graphene oxide [46][47][48], and boron nitride [49], have been incorporated to make high-strength cementitious materials. The dispersion of all these nanomaterials in the cement environment is challenging. The findings here provide a new way to stabilize the dispersion of nanoscale additives in cementitious materials. In addition, MC is nontoxic and it has good compatibility with cementitious materials [28]. Even there is a trade-off between fluidity and stability when MC is used, the thickening effect of MC that helps to prevent the separation of water and cement grains allows the use of less cement when an appropriate mixing rate is adopted [27], which can be cost-saving, eco-friendly and beneficial especially for the cementitious materials with high water content.

Conclusions
Here, the role of MC on stabilizing the dispersion of MWCNT in simulated cementitious pore solution was studied, and distribution of MWCNTs in the cementitious composites was investigated. The following conclusions can be drawn: 1. In MWCNT suspensions, molecular MC can form a membrane that wraps MWCNTs. The MC membrane can protect the well-dispersed MWCNTs from the adsorption of ions in cementitious pore solution, which helps to maintain the steric interaction and retards the re-agglomeration. However, at higher concentration, the connected MC chains can link the adjacent MWCNTs into a large gel, accelerating the formation of MWCNT agglomerates. 2. The addition of MC can increase the viscosity of MC-MWCNT suspensions. This also contributes to the stable dispersion of MWCNTs since the higher viscosity gives the individual carbon nanotubes insufficient Brownian motion ability to re-agglomerate within a period. 3. The SEM images show that MWCNTs are more likely to distribute individually in the cement-based matrixes with a moderate addition of MC, whereas the nanotubes appeared in clumps or agglomerates when excess MC is incorporated. A content of 0.018 wt% MC is preferred to give the optimum dispersion results.