The Effect of Carbon Black, MWCNT and Carbon Fiber on Flexural behavior of cement nano composites

: One of the most popular and commonly utilised building materials is concrete. The recent studies of materials science explains influence of nano materials on cement composite at the macro, micro or structural level and their application in construction industry. In present study flexural behavior of cement nano composite containing nano materials such such as Multi-walled carbon nanotubes (MWCNTs) and Carbon fibers (CFs) and carbon black (CB) are studied. The various sample are prepared and flexural test is carried out. The flexural responses of plain cement beams and CB-CF-CNTs/cement beams were compared using single point bending tests results. The hybridization of MWCNTs, CFs, and Carbon Black coated with cement indicates improvement in mechanical properties compared to other samples. The SEM indicates the strong bonding between CFs, MWCNTs, and the cement matrix.


Introduction
Since a decade ago, the majority of structural components use fiber-reinforced polymeric composites.Traditional structural materials exhibit isotropic behavior.Previous research has demonstrated that when measured in the fibers longitudinal direction, the strength and modulus of fiber-reinforced composites reach their maximum values [1,2].Nanoscience and nanotechnology refer to the control and manipulation of matter at nanometer dimensions techniques.Nano-fibers can be added to concrete to improve performance.Carbon nanofibers have the ability to strengthen cement composites, efficiently stop cracks from spreading, and serve as nucleating agents [3,4].
Nano-scale materials, like nano-silica, nano-fibers, and nanotubes, are of considerable interest in the field of civil engineering [5,7].It is envisaged that the introduction of nanofibers and nanotubes will enable nano-reinforcement, which will enable a reduction in the amount of steel reinforcement needed and difficulties with corrosion impacting concrete structures.With the use of nanoscience, researchers can create new materials with radically different physical and chemical properties by breaking down molecules atom by atom.[8,9].Hydrated cement paste (HCP) is essentially a nanomaterial.Hydrated cement paste has a structure akin to clay, with interlayer and adsorbed water-filled gel holes separating thin layers of particles.Concrete's performance is greatly impacted by HCP's sensitivity to moisture migration, which can result in shrinkage and eventual cracking if changes to element sizes are not performed [10,11].A fundamental understanding of how the system functions has not been extensively explored up until the last several years, when strength was the only criterion of importance.But as the number of elements in concrete systems rises due to the inclusion of additional cementitious materials and numerous chemical admixtures, the systems become more complex.Yining Ding explains increase of compression and flexural strengths of concrete due to inclusion of nanocarbon black and carbon fiber [12].Zakaria Hossain claims that adding 3 and 6 mm fibers improves the bearing capacity to crack-and ultimate-stresses as well as the Young's modulus of the cement composites, resulting in the desired performances following the start of cracks and pre and post-crack load-deflection characteristics of these composites [13].Maker showed that, the hydration of C3S in OPC is accelerated by the presence of single wall carbon nano tube (SWCNT).The shape of the early C3A and the C3S hydration products are both impacted by the presence of SWCNT [14].The hydration products operate as nucleating sites on nanotubes, which quickly coat them in C-S-H.Fiber pull out of SWCNT bundles observed after 24 hours of hydration confirms the classical reinforcing behavior.It is also an evidence of a strong interfacial bonding that exists between C-S-H and SWCNT.Hence in the propose study, the effect of carbon fibre, carbon black and Multiwall carbon nano tube on flexural behaviors of cement nano composite is investigated.

Ordinary Portland cement:
Ultratech 43 grade OPC cement was used in the present investigation.It was evaluated in accordance with the IS-4031-1988 hydraulic cement recommendation.Compressive strength for 28 days is 43 MPa.

Carbon Fibers
The length of carbon fibers used was of 5 mm length as per the recommendations for the minimum specimen size.I.e.The minimum specimen size should be at least three times the length of the fiber.In order to ensure that the fibers are randomly oriented, this minimum size is used. .The properties of Carbon fibers are presented in Table 2.

Multi-walled Carbon Nanotubes (MWCNTs)
In the present, MWCNT is used as reinforcement materials at nano level in cement composite.The properties of MWCNTs are presented in Table 2.The Carbon black used in the present work was obtained from the Phillips Carbon Black Limited, Palej Gujarat, India.The properties are as shown in Table 3.

Dispersion of Multiwalled Carbon Nanotubes
Carbon Nanotubes were pre dispersed before adding them with the cement, Ethanol (Ethyl Alcohol) of quantity 37% by weight of water required for mixing is taken.In order to obtain adequate dispersion, multi-walled carbon nanotubes in the amount of 0.25% by weight of cement were added.A 90-minute ultrasonication process was then performed on the mixture.All MWCNT proportions are distributed using the same methodology.Care should be taken to see that the ethanol is completely evaporated; otherwise it will induce layered cracks in the casted specimens.

Dispersion of Carbon Fibers
By adding 3/5 of the entire amount of water, these fibers were pre-dispersed in the water.The ultrasonic wave was used to vibrate the beaker for 10 minutes.The pre-dispersed slurry was then given a 0.6% by weight of cement addition of the dispersant HYDROXYETHYL CELLULOSE (HEC), and the beaker was once more sonicated for 10 minutes while being continuously stirred.Bundled carbon fibers become scattered when HEC is added because it turns the fibers into a sticky paste.

Dispersion of Carbon Black
Through trial and error, the best dispersion (visually established) and workability was achieved when carbon black was mixed with 1/5th of mix water.The mixture was sonicated in ultrasonicator.Carbon nanotubes, carbon fibers and carbon black were sonicated for 30 minutes.

Methodology
For the preparation of the nanocomposites, Ordinary Portland cement and nanofibers such as Multi-walled carbon nanotubes (MWCNTs) and Carbon fibers (CFs) and carbon black (CB) were used.Moulds were kept on an even surface, cleaned, and oiled.Before being mixed with cement the nanotubes were diffused using ethanol with ultrasonic energy and Carbon fibers were dispersed in water using Hydroxyethyl Cellulose (HEC) with sonication.The CNT, Carbon Fiber, and Carbon Black dispersions were then combined with the cement, and the mixture was thoroughly incorporated.After the paste had been properly compacted into the moulds, the cast specimens were removed after 24 hours and maintained for up to 28 days of cure [15,16].The effectiveness of using uniformly dispersed randomly oriented MWCNTs, carbon fibers, and carbon black as reinforcement for cement composites was tested experimentally.In this study macro, micro and nano level reinforcement were provided in the form of Carbon black, Carbon Fibers and MWCNTs.Carbon black of 2.5% by weight of cement, Carbon fibers 0.25% by weight of cement and MWCNTs of 0.75% by weight of cement were used.The flexural responses of plain cement beams and CB-CF-CNTs/cement beams were compared using single point bending tests on 20mmx20mmx100mm cement nano composite beams.

Testing
Flexure tests were used to assess the nanocomposites' mechanical performance examined at 28 days curing.Samples are cased as per Table 4 of in the size of 20 mm x 20 mm x 100.The three point bending test is performed using load frame having capacity of 10kN with strain rate of 0.05 mm/min [17][18][19].As the concentrated load in center produces distribution of moments and due to this the shear forces also exist in the beam section.From the bending test results such as load and corresponding deflections, the flexural modulus of the specimens was calculated from deflection criteria.

Results and Discussions
From the experimental study mechanical properties such as flexural strength, toughness and ductility index are calculated as below.

Flexural strength
From the experimental test Load v/s deformation graph is plotted to determine the ideal nano materials weight percentage and their impact of micro and nano level fibres on cement composite.Table 6 represents the ultimate load and flexural strength of all samples.

Ductility Index:
Ductility index is used measured the load carrying capacity of the composite.The deflection "Δmax", "Pmax," and deflection "Δy" which correlate to the ultimate load and yield load, respectively, were noted from experimental.The Ductility Index is calculated as the difference between the deflection corresponding to the highest load and the deflection corresponding to the yield load of the composite was measured for single point bending tests.The ductility index is calculated as below.From Table 7 it is found that, plain cement beams with 2.5% carbon black exhibited a 37% improvement in ductility, whereas beams reinforced with 0.25% of carbon fiber exhibited a ductility enhancement by 46%, plain cement beams with 0.75% MWCNT showed around 106% increase in ductility and hybridized beam showed 79% increase in ductility.Thus MWCNTs proves to be a promising reinforcing material.It was established that the incremental improvement in flexural strength of the CNT-containing samples over plain concrete may be primarily attributable to the influence of increased CNT content on the cohesiveness of the samples.

Toughness Index:
Toughness index is the capacity to withstand more severe deformations before failing; it is frequently assessed using a toughness index.Measurement of toughness index of the different proportions of the composite was carried out using the numerical method.Trapezoidal rule is used to find out the approximate area under the load deflection curves.The area up to maximum load only is considered, since the test set up used was not a closed loop test set up and the load deflection behavior after ultimate load point was not available.Area under load deflection curve is a measure of the toughness of the tested specimen.Areas of load deflection curves are compared to measure the relative toughness of the different proportions From Table 7 it is found that, A3 composite showed enhanced toughness with minimum dosage of fiber ie., 0.25% of CFs.Although case A4 with 0.75% of MWCNT showed greater toughness but deformations are greater in that case.Which means the load may increase at large deflections for high fiber volume fractions, but deformations are typically higher for these fibers.MWCNTs' pulling out and crack bridging decreased the development of micro cracks and boosted the matrix's tensile strength.The ability of the composite to withstand significant deformations before failing is indicated by higher toughness

Micro structural Analysis
The Micro structural analysis Scanning electron microscope is performed to the effect of Carbon fibers, Carbon black and MWCNT on morphology of cement composites.Figure 4 presents the SEM images of the internal structures of the A2, A3, A4, A5 samples.(a) shows that the SEM image of A2 sample, which shows the hydrated cement paste with carbon black addition.Carbon black enhanced the post-peak performance, increasing the ductility.Carbon black acts at macro level which reduces the brittle failure of the composites.(b) shows the SEM image of A3 sample, which shows the hydrated cement paste having crystalline structure with pores.Carbon fibers acts like bridge between microcracks.CF's resisted crack propagation.(c) shows the SEM image of A4 sample surface showed that MWCNT were not visible as there was proper dispersion.The image shows that the MWCNT was tightly wrapped by C-S-H gel resulting in good bond between tube and the hydrated paste which causes increase in its mechanical strength.The findings suggest that MWCNT addition can control matrix fracture at the nanoscale.Also the microstructure shows dense hydrated paste which may be due to incorporation of MWCNTs which were capable in reducing the pores.

Conclusions
The above experimental study, the findings demonstrated that the load-bearing capacity of the composite beams has significantly improved when compared to the reference beams.Plain cement beams with 0.25% carbon fiber reinforcement increased their ultimate load by 105%, plain cement beams with 0.75% MWCNT increased their load carrying capacity by roughly 121.65%, and hybridized beams increased their load bearing capacity by 115%.The load carrying capability benefits from having reinforcement at both the micro and nano levels.With 2.5% CB content, the composite's flexural strength improved by 23.33%; with 0.25% CF content, it increased by 104.33%; with 0.75% MWCNT content, it increased by 121.67%; and with a hybridized composite beam, it increased by 115%.Maximum flexural modulus was displayed in the composite with the 0.25% CFs.Higher deflections were visible in composites containing 0.75% MWCNT.With greater load carrying capability, they endured significant deformation.This may be because MWCNTs are preventing fissures from forming.Comparing hybridized composite beam to other nanocomposites, the composite containing 2.5% carbon black, 0.25% carbon fiber, and demonstrated less deflection.Due to sufficient bonding between the CNTs and the host matrix was difficult to develop, the results did not indicate any improvement in the ductility of the composite beam.Effective the absorption of energy during their exceptionally elastic and flexible activity, nanotube reinforcements will improve the durability of composite materials.This demonstrates their potential to develop into the ultimate reinforcing material.It has been proven that carbon black increases the durability of cement-based materials.This could result in the cement paste performing better and lasting longer.It serves as a filler substance and aids in minimizing the mixture's porosity to produce a denser and stronger substance.The hydration products operate as nucleating sites on nanotubes, which quickly coat them in C-S-H.Greater deflections were seen in composites when MWCNT weight percentages were larger.This could be as a result of the MWCNTs' crack inhibition.The rapid interaction between the nanotubes and the C-S-H gel increased the cement's ductility and toughness index.Contrarily, the amount of carbon black-infused cement has decreased as a result of the composite material's increased stiffness.

Table 2 .
Properties of MWCNTs as per suppliers Specifications Volume Resistivity 0.1-0.15ohm.cm (measured at pr. in powder) a. Ethanol it contains Ethyl Alcohol AR.99.9%,Alkaline -0.01%Max, Methanol-0.05%Max and Water-0.1% Max b.Hydroxyethyl Cellulose A non-ionic, water-soluble polymer called hydroxy ethyl cellulose (HEC) can thicken, suspend, bind, emulsify, create films, stabilize, scatter, hold water, and provide protective colloid activity.It can be used to make solutions with a variety of viscosities and is easily soluble in hot or cold water.c.Carbon Black Powder:

Table 3 .
Properties of Carbon Black

Table 4 .
Sample Details

Table 6 .
Flexural strength of samples respectively.At the nano level, 0.75wt% of MWCNT showed the greatest improvement.The composite beam with 2.5% CB & 0.25% CF's showed lesser load carrying capacity when compared to MWCNT's.This is because of the improper dispersion and entanglement of CF's.Less deflections result from higher flexural modulus, which means that the element behaves stiffer.In the present case the flexural modulus is greater in the PC+0.25% of CFs.

Table 7 :
Ductility Index of the hybrid Composites under Single Point Loading Case.

Table 8 .
Toughness Index of the Composites under Single Point Loading Case.