Performance Evaluation of MWCNTs Reinforced Cement Mortar Composites using Natural and Commercial Surfactants

Qareeb Ullah, Rao Arsalan Khushnood,2, Sajjad Ahmad, Muhammad Usman,Shad Muhammad, Jean-Marc Tulliani

(1.School of Civil and Environmental Engineering, National University of Sciences and Technology, Islamabad-44000, Pakistan; 2.Department of Structural, Geotechnical and Building Engineering, Politecnico di Torino, Turin-10129, Italy; 3.Department of Civil Engineering,FE&T, Bahauddin Zakariya University, Multan-56000, Pakistan; 4.Department of Applied Science and Technology, Politecnico di Torino,Turin-10129, Italy)

Abstract: An efficient and promising approach for effectively dispersing multi-walled carbon nanotubes(MWCNTs) in cementitious composites has been investigated. The naturally occurring organic extracts from species of indigenously known ‘Keekar’ trees scattered along tropical and sub-tropical regions; is found as an exceptional replacement to the non-natural commercial surfactants. In the initial phase of investigation, ideal surfactant’s content required for efficient dispersion of MWCNTs in solution was determined using ultraviolet spectroscopy. The experimental investigations were then extended to five different cement composite formulations containing 0.0, 0.025, 0.05, 0.08 and 0.10% MWCNTs by weight of cement. It was observed that the natural surfactant produced efficient dispersion at much reduced cost (approx. 14%) compared with the commercial alternate. The estimated weight efficiency factor φ was found 6.5 times higher for the proposed sustainable replacement to the conventional along with remarkable increase of 23% in modulus of rupture on 0.08 wt% addition of MWCNTs. Besides strength enhancement, the dispersed MWCNTs also improved the first crack and ultimate fracture toughness by 51.5% and 35.9%, respectively. The field emission scanning electron microscopy of the cryofractured samples revealed efficient dispersion of MWCNTs in the matrix leading to the phenomenon of effective crack bridging and crack branching in the composite matrix. Furthermore, the proposed scheme significantly reduced the early age volumetric shrinkage by 39%.

Key words: MWCNTs; acacia nilotica gum; keekar; triton X-100; fracture; crack bridging; cement mortar; early age micro-cracking; sustainability

1 Introduction

Numerous materials and techniques have been explored by several researchers that include the incorporation of secondary raw materials[1,2], wellengineered steel fibers[3], carbon nano/microfibers[4],carbon nanotubes[5], graphene-oxide[6]and carbonaceous nano/micro inerts[7,8]to modify the traditional properties of cement paste and mortars composites. Due to the exceptional characteristics in terms of strength,fracture toughness and elastic modulus, multi walled carbon nanotubes (MWCNTs) are being considered as an idealized additive for reinforcing approximately all types of composites. There are two major hurdles that hinder their use on large scale in composites and especially cementitious composites,i e, high synthesis cost and attainment of efficient dispersion[9]. MWCNTs being strong in tensile strength have potential to modify the mechanical properties of cementitious composites such as modulus of rupture and fracture toughness by offering crack hindrance at nano and micro levels[5]. The tensile strength and elastic modulus of polyacrylonitrile (PAN) based carbon fibers were found to be 3-7 GPa and 400 GPa, respectively[10]while on the other hand, the tensile strength and elastic modulus of MWCNTs reported by Yuet al[11]were around 63 GPa and 950 GPa, respectively. MWCNTs exhibiting such extraordinary properties cannot produce superior matrices until and unless they are dispersed homogeneously in the composite matrix. The problem which is normally associated with improper dispersion of MWCNTs is their agglomeration and bundling in the matrix at certain locations, leading to inferior results by producing spots for stress concentrations[12,13]and defective sites due to strong van der Waals interactions[14]. The strong agglomeration(Fig.1) may be reduced with the help of high-energy sonication and employing surfactants but usually the dispersion process produces a huge amount of foam during mixing, thus requiring additional chemicals to deal with it; but in the case of cementitious composites,these chemicals/ surfactants may sometimes disturb the chemistry of materials. Anionic, nonionic, cationic and amphoteric[15]surfactants having a hydrophilic head and hydrophobic tail plays a significant role in the formation of nano-emulsion that disperse MWCNTs efficiently and improves the compressive strength of cementitious matrix by 40%[16]. The selection of surfactants is very crucial in cementitious matrices incorporating MWCNTs because in the absence of suitable surfactant it is impossible to attain sufficient dispersion of MWCNTs in the matrix thus affecting their mechanical and microstructural properties[17].Researchers have reported that the utilization of unsuitable surfactants may produce other problems.Fox example, sodium dodecyl sulfate (SDS) helps in dispersion of MWCNTs but produces large amount of foam during mixing of cement thus drastically affecting the composite strength[18,19]. Initially, most of the research was focused on the dispersion and production techniques of MWCNTs[20,21]demonstrating the fabrication of MWCNTs by three main techniques,i e, electric arc discharge, laser ablation and chemical vapor deposition (CVD). Among the above-mentioned techniques, CVD is the most economical and controllable in terms of MWCNTs growth direction on the substrate and large-scale production. Dispersed with gum arabic, MWCNTs were found to modify the microstructure and fracture energy significantly by reinforcing and bridging the newly generated cracks[22].Such improved dispersion enhances the early-age strain properties and fracture toughness with very small concentrations 0.025 wt%- 0.08 wt%[23]and leads to 19% enhancement in compressive strength and 25% in flexural strength[24]. The insertion of MWCNTs in micro and nano pores and its combination with silica fume were also found helpful in improving the porosity,interfacial interaction, mechanical and electrical properties[25].

Furthermore, the autogenous shrinkage measurements as the ASTM C 341 and ASTM C 490 presented demonstrates[23]around 40% decrease in shrinkage due to lowering of capillary stresses. The current research work investigates the dispersion efficiency of selected natural and commercial surfactants with their possible impact on the overall behavior of mortar matrices containing CNTs in smaller proportions. The explored characteristics include shrinkage parameters, fracture toughness, compressive and flexural strength, hydration kinetics and microstructure refinement of the modified matrix.

The present research is focused on the investigation of an economical naturally occurring surfactanti e,acacia gum (AG) and its performance comparison with commercially available Triton X-100 (TX) for achieving efficient dispersion of MWCNTs in a cementitious environment. The comparison is extended to include hydration kinetics, fracture properties, microstructure and shrinkage parameters of cementitious composites prepared with AG and TX. The AG is chosen in the present study because it has been barely studied in all above mentioned aspects. Therefore, this study will enable the researchers to investigate an overall picture about the suitability and effectiveness of this naturally occurring surfactant.

2 Experimental

2.1 Materials

The investigated MWCNTs in the present workwere purchased from US Research Nanomaterials, Inc.The MWCNTs have outside diameters of 20-30 nm and lengths ranging from 20 to 30 µm[26]. The MWCNTs were synthesized by CVD technique with their properties mentioned in Table 1.

Table 1 Physical and chemical properties of MWCNTs

The Raman spectra reveals the defect band (ID)and band for degree of graphitization (IG) at Raman shifts of 1 350 cm-1and 1 590 cm-1, respectively. TheIDtoIGratio, which is considered as the indication of material’s quality, came out to be 0.849, thus confirming the presence of moderate number of defects in MWCNTs (Fig.2(a) refers). Literature suggests that the presence of defective sites in MWCNTs help in the development of strong bondage between the host cement matrix and MWCNTs[27]. X-Ray diffractogram of pristine MWCNTs is displayed in Fig.2(c) with a sharp peak of carbon depicted at 2-theta position of 26.37 degrees having crystallographicd-spacing of 0.337 9 nm. The scanning rate was fixed at 0.02o/sec for the scatter range of 10o-80ousing Cu as the anode material28. Fig.2(b) represents the temperature-programmed oxidation of MWCNTs.

The selection of suitable surfactant is an extremely important phase to attain optimum dispersion level in the host medium. This research utilizes gum obtained from the acacia nilotica gum (AG), a species of trees scattered along tropical and sub-tropical countries and indigenously known as “Keekar”, as the natural surfactant to be explored in conjunction with cement[29].

AG is a branched-chain complex polysaccharide,and its chemical composition can vary with its source,the age of the trees from which was obtained, under climatic condition and soil environment[30]. Based on extensive literature and market survey Triton X-100 was chosen as the commercial surfactant for the present work[31-33]. The level of dispersion was gauged via UVVis spectrophotometer (T60 UV) against the selected concentrations of MWCNTs,i e, 0.025 wt%, 0.05 wt%,0.08 wt%, and 0.10 wt% additions[23].

FTIR analysis of AG and TX was performed in the range of 450 - 4 000 cm-1to assess the presence of attached functional groups and their bond type. In FTIR spectra (Fig.3(a)), the peak at 3 324 cm-1appears as the absorbed moisture from atmosphere, wavenumbers 1 602 and 1 415 cm-1might be the vibrations of carbon-carbon single and double covalent bond and 1 024 cm-1evidences the presence of C-O bond. In the case of TX, as shown in Fig.3(b), the benzene ring that is covalently attached to the tail of structure whose vibrations are evident from the spectrum at 1 511 and 1 244 cm-1may improve the π-π interactions and the dispersing ability of MWCNTs. A similar analogy may explain the spectrum at 1 100 cm-1as elaborated in AG at the corresponding wavenumber of 1 024 cm-1[29,34].

Table 2(a) Chemical composition and physical properties of cement

Table 2(b) Physical properties of acacia gum

Table 2(c) Physical properties of Triton-X

Ordinary Portland cement grade 53 according to ASTM C150[35]was used for the preparation of cementitious composites. The physical properties and the chemical composition of cement along with the investigated surfactants are reported in Table 2.

2.2 Dispersion scheme

Due to strong intermolecular interactions, it is difficult to disperse nano particles in water without using a surface activating agent and supplying energy to break the bonds[36]. The authors tested several mixing regimes by varying vibrational amplitudes and times and found that the optimum dispersion is achieved by using 150 W,JAC-1505 high energy sonicator operated at 40 kHz for 30 min in the presence of surfactants. MWCNTs were dispersed using each of the selected surfactants in five different formulations as summarized in Table 3. Influence of chemical and mechanical means on the dispersion efficiency was evaluated based on absorbance values attained through UV-spectrum.

Table 3 Detailed dispersion scheme of MWCNTs

2.3 MWCNTs reinforced mortar samples preparation

The prepared homogenized suspensions of MWCNTs were used to prepare the cement mortar specimens. Water to cement ratio was kept to 0.45 for all the formulations. During mixing, the TX generated foam due to its inherent deterging nature. Therefore,Tri-n-butyl phosphate (TBP) was added by 0.15 wt%of cement as an anti-foaming agent[22]. For preparing the cement mortar composites, initially the measured amounts of cement and sand were mixed in dry condition in Hobart mixer and then the suspension containing well dispersed MWCNTs was transferred into the mixer and slow mixing at 145 rpm was carried out for 3 min followed by the fast mixing at 285 rpm for 2 min, thus making the total mixing time of 5 min which is considered suitable for the proper activation of surfactant molecules in cementitious environment[37].Then the mortar was transferred into the standard prism molds of 40 mm×40 mm×160 mm prepared as the protocol explained in Table 4 and stored in 95% humidity at 25±1oC for 24 h. Afterwards immersed water curing was carried out until the day of testing. The specimens were tested for their fracture response in flexure and compression as well as for their hydration kinetics and shrinkage aspects at specified ages.

Table 4 Composition of mortar samples＊

Before carrying out the flexural tests the specimens were smoothened by rotary polishing device with#180 WX FLEX paper. Then 12 mm deep notches were carefully machined in the center of each specimen using Remet type TR100S, s/n 3714 abrasive cutter with 4 mm thick diamond cut-off wheel under a constant flow of water. Finally notched specimens were tested using ZwickiLine z010 single column flexural testing machine shown as inset in Fig.13, under CMOD control mode with the opening rate fixed at 0.003 mm/min. Highly accurate and sensitive extensometer (clip on gauge) was employed for the measurement of crack opening and the data were digitally recorded. The flexural performance of mortar composites was evaluated at 3, 7 and 28 days of curing and the broken halves of each prism were then tested for compressive strength as per ASTM C349. The microstructural investigations were done on a squared centimeter small fractured chips using FESEM.

3 Results and discussion

3.1 Dispersion of MWCNTs

Dispersion with AG and TX was investigated with varying ratios of surfactant/CNTs within the range of 0.0 to 5.0. The dispersed formulations were characterized by UV-Vis spectroscopy after 30 and 60 minutes of sonication. The UV-Vis spectrophotometer results for TX are presented in Fig.4.

The UV absorption spectrum reveals a gradual increase in the values of absorbance with proportionate increase in surfactant/CNTs from 1 to 4 and then reduction afterwards. This might be due to the fact that at lower concentrations the surfactant quantity is not sufficient to encapsulate the whole surface area of MWCNTs but this comes to equilibrium at surfactant/CNTs ratio of 4. Further increase in the surfactant content exhibits slight reduction in dispersion due to the presence of excess quantity in the suspension.Based on the results, surfactant/CNTs ratio of 4 was considered as the optimum proportion for further usage with cementitious matrices. The stability of the prepared suspensions was also evaluated via UV spectrophotometry after 60 min of sonication.The results given in Fig.6 clearly illustrates that the formulation in the earlier phase remained most efficient in the stability prospects as well. Prolonged sonication was found to have no significant effect on further refinement of dispersed formulations as shown in Fig.6.

Similar procedure was followed to check the dispersing capability of naturally extracted AG that concluded an optimum AG to CNTs ratio of (1:1). The values of absorbance at 500 nm wavelength usually stated as the unaffected wavelength at ambient conditions[38].The comparison against the investigated proportions of the selected surfactants was shown in Fig.7. The comparison clearly highlights that natural surfactant has the potential to disperse MWCNTs in much effective and efficient way compared with the commercial alternate.The calculated weight efficiency factor (φ) using Eq.(1)revealed AG to be 649% more efficient in dispersion than TX as plotted in Fig. 7.

3.2 Flexural and compressive behavior of cement mortar composites

The flexural and compressive resistance plots of the cement mortar samples modified with MWCNTs additions using AG and TX as surfactants are shown in Fig.8 and 9, respectively.

Overall, the addition of MWCNTs reinforces the cementitious mortar matrices with respect to the controlled formulation. Maximum enhancement in the mechanical response was observed with 0.08%MWCNTs addition.

Fig.8 (a) Flexural and (b) compressive strength of modified mortar composites containing AG as surfactant

Table 5 Comparison of flexural strengths of MWCNTs reinforced composites

All cement formulations containing AG as a dispersing agent showed an incremental trend with a maximum increase of 23% in flexure and 29.5%in compression attained with 0.08 wt% addition of MWCNTs at the age of 28 days. Similar enhancement has been noticed by other researchers while working with MWCNTs at relatively higher concentrations[39].

Previous literature as summarized in Table 5 reports maximum increase by 25% in flexural resistance of modified cementitious mortars with 0.5 wt% addition of MWCNTs. In paste specimens, a similar level of enhancement was reported by Konsta-Gdoutoset al[39]with 0.08 wt% addition of MWCNTs. In current work,the authors remained successful to attain that level of refinement in mortar matrix on the similar additive fraction as specified by Konstaet alwhile dealing with cement paste systems.

In the case of TX even after the addition of an antifoaming agent, the samples were found to be sufficiently porous as shown in Fig.10. The induced porosity has adversely affected the strength parameters by hindering the reinforcing action of the intruded CNTs.

Among the four formulations, an increase by 15.8% in flexural strength was observed with least addition of CNTs at the age of 28 days. The use of TX badly affected the compressive resistance of specimens by producing foam that ultimately reduced the density of prepared samples as illustrated in Fig.11.

3.3 Load-displacement response

The fracture toughness of the cement composites strongly depends upon the effective dispersion of MWCNTs in the matrix. A typical load-CMOD response of cement composite specimen incorporating 0.08 wt% MWCNTs is presented in Fig.13. The incorporation of MWCNTs not only enhanced the flexural strength but also substantially improved the energy absorption capacity during the phases of crack generation and propagation.

The modulus of rupture (MOR) and fracture energy is determined using Eqs.(2) - (3), whereFmaxis the maximum force during three-point bending test,Lis the span length,wis the width andhis the height of specimen.Uis the area under Load-CMOD curve and A is the ligament area equal toB(W-ao)[42,43]:

An appreciable increase by 20.75% in MOR and 35.9% in total fracture energy has been observed by an optimum addition of 0.08 wt% in cementitious mortar matrix. Moreover, significant enhancement in the first crack and ultimate fracture toughness by 51.5 and 35.9% was attained using load-CMOD response.Therefore, a paradigm shift in the fracture pattern of resultant composites from fragility towards ductility may be concluded. The increase in the value of fracture energy evidences the offered hindrance by MWCNTs to limit the crack from any further propagation in the same orientation or tilting them to change their genuine trajectory.

3.4 Hydration kinetics

To investigate the influence of MWCNTs additions on the overall hydration kinetics of the cementitious matrix, Field calorimetry (Fcal-8 000) was performed. The prepared formulations with AG were kept in the drain of calorimeter for 48 hrs with the collected data summarized in Fig.13. The curve reveals that there is not any significant variation in the hydration kinetics of the modified matrices that affirms the absence of any chemical interaction of inducted carbon with the host mortar matrix. Since the presence of individual CNTs in cementitious matrix acts as a nucleation site for the deposition of hydration product[44], the CNTs addition slightly accelerates the hydration process with a proportionate increase from 0.025 wt% to 0.1 wt%. On the sole usage of AG in conjunction to cement, delays in the hydration process have been reported[29]due to the possible coating of reacting grains on direct injection with cement. While as dispersant, it actually gets deposited on the surface of nanotubes to overcome their intermolecular interactions instead of any direct interaction with the cement grains in the mixing phase.

3.5 Shrinkage

Due to reinforcing action, the addition of CNTs in the matrix tends to resist the formation of cracks and thereby improves the shrinkage response of the resultant mixed formulation. Shrinkage apparatus (Schwindrine Germany) which follows linear protocol was used to measure the shrinkage of cement mortar composites.The samples were prepared with sand/cement of 1.3 against the selected five formulations and kept in drains of shrinkage apparatus for 48 h. The shrinkage plots shown in Fig.14 depicted significant reduction in the values of shrinkage with the proportionate increase in CNTs content. Maximum reduction by 39% was observed on the optimum addition of CNTs,i e0.08 wt%.Thus it can be concluded that small concentrations of effectively dispersed MWCNTs possess the potential of significantly reducing shrinkage of the cement matrix.Increasing content of CNTs affects the present moisture of reaction; hence, the setting time needs to be separately addressed as the pattern of modified spectrums slightly deviates from controlled as seen in the region of 5-12 hrs.

3.6 Microstructure

To have the micrographic evidence, small chunks of the efficient formulation were explored in detail using field emission scanning electron microscope(FESEM). The micrographs shown in Fig.15 confirmed that the added CNTs are effectively dispersed with a little or no signs of agglomeration. Furthermore, crack bridging and crack branching phenomenon of CNTs evidenced their strong interaction with the host matrix.At some occasional sites, CNTs were pulled out that might be associated with reduced development length along such critical sections.

Table 6 Cost comparison for 1 m3 mortar with MWCNTs and selected surfactants (＊ $ stands for US dollars [26])

These factors are contributing towards the attainment of 35.2% increase in the overall fracture toughness and leading to the failure towards ductile nature.In order to confirm the existence of CNTs in host matrix, spot EDX was performed with the spectrum shown in Fig.15(d), evidencing the content of carbon beyond 80%.

3.7 Cost comparison

To have an idea related to percent reduction in cost on utilizing natural surfactant AG instead of commercial TX to disperse CNTs in cubic meter cement mortar, cost analysis was performed as per the purchased price from the market (as of September 2016). Formulations containing 0.08 wt% MWCNTs were selected as they attained much enhanced fracture properties in comparison with pristine cement mortar.Based on Eq.(4), detailed calculations of quantities and associated expenses are summarized in Table 6. After analysis, it appears that AG reduces the cost by approx.14.2% in comparison with corresponding CNTs/mortar matrices formulated using TX as a dispersant.

4 Conclusions

a) Natural surfactant AG is ranked on priority as it exhibits 649% enhanced dispersing power in comparison with TX, and ultimately reducing the overall project expenses by approximately 14%.

b) TX has the ability to disperse MWCNTs in the cementitious environment but it degrades the mechanical strength up to 50% of the control mix due to the foam formation. The remarkable improvement in the fracture properties (i e, 20.75% increase in MOR and 35.9% in fracture toughness) of cement mortar matrix was attained on 0.08 wt% addition of MWCNTs dispersed using AG

c) An appreciable increase in fracture energy by more than 35% and the enhanced fracture area evidences the crack bridging / crack branching phenomena due to the micrographs limiting the crack from any further propagation in the same orientation or tilting them to change their genuine trajectory

d) The addition of MWCNTs has weak or no contribution in the chemical hydration of cementitious materials

e) Significant reduction in the values of total shrinkage by 39% was attained on the inclusion of 0.08 wt% of MWCNTs in cement mortar matrix