DESC:FIELD OF THE INVENTION

This invention relates to a chemical admixture developed for mixing with a cement based material and thereby enhancing the performance of the cement based material, especially the flexural strength and compressive strength, of cement based materials. More particularly, the present invention relates to a chemical admixture which is an aqueous dispersion of carbon nanotubes along with a dispersing agent. Further, the present invention relates to a process of preparation of the chemical admixture and cement based composite material comprising the chemical admixture and a cement based material.

BACKGROUND OF THE INVENTION

Cementitious materials are the most commonly and widely employed construction materials. Cementitious materials, though quite strong in compression, are relatively weak in flexure and have low bending strengths and fracture toughness. Fibre reinforcement of cementitious matrices improves their flexural strength as well as toughness by impeding crack formation and growth. Different types of fibres including polymer, glass and steel fibres have been used in reinforcement of cementitious materials. Reinforcement of cementitious material with high strength nanodiameter fibres such as CNT can be helpful in achieving high performance cement based composites.

CNT is a tubular form of carbon, configurationally equivalent to a two dimensional sheet rolled into a tube. These are just a few nanometers in diameter and several microns long. CNT exhibits extraordinary mechanical properties including high Young’s modulus (higher than some of the metals), as stiff as diamond, high tensile strength and high strength to weight ratio. CNTs have very high elastic modulus as well as tensile strength. CNTs can be excellent reinforcing materials because of their extremely high strength, toughness and aspect ratios. Since the first report on CNTs by Iijima in 1991, numerous attempts have been made to strengthen materials (especially polymer based materials) with nanotubes. Use of CNT in concrete mixes can enhance the strength and toughness of concrete structures resulting in rationalization in the use of materials and design of structures leading to better utilization of space and enhanced durability of buildings. In addition to their high strength and elastic constant, CNTs have high aspect ratios also. The size and high aspect ratios of CNT mean that they can be distributed on a much finer scale than commonly used reinforcing fibers. Cracks can be interrupted much more quickly during propagation in a CNT reinforced matrix. CNTs normally exist in agglomerated state. The major challenge for use of CNT as a reinforcing fibre is to achieve its proper dispersion in cementitious matrix. A number of approaches have been reported to prepare OPC (Ordinary Portland cement)-CNT composites. These are dry mixing, stirring and sonication with and without dispersing agents. Further, it is also required to establish the extent of improvement achievable in the properties of cement based materials on incorporation of CNT as reinforcement.

CN104030634B provides high-strength and high-toughness reactive powder concrete of a carbon doped nano-tube. The high-strength and high-toughness reactive powder concrete was prepared by taking cement, a carbon nano-tube, silica fume, a water reducing agent, silica sand, coal ash, quartz powder, steel fiber and water as raw materials, wherein all the components in the mixture are calculated in parts by mass: 1000-1200 parts of cement, 250-350 parts of silica fume, 250-350 parts of slag powder, 40-50 parts of water reducing agent, 1200-1400 parts of silica sand, 180-230 parts of water, 190-230 parts of steel fiber, 180-250 parts of coal ash, 80-120 parts of quartz powder, 0.1-5 parts of carbon-nano-tube dispersing agent and 1-10 parts of carbon nano-tube powder.

WO 2018/103814 Al relates to a composition, in particular a construction material composition, comprising at least one binder, at least one filler, carbon nanotubes, and optional further ingredients, wherein the proportion of the carbon nano tubes is, according to the broadest aspect less than 200 ppm and is preferably 0. l ppm to 150 ppm by mass in terms of the dry mass of the
construction material composition. The said patent application also provides a method for the preparation of a stabilized aqueous carbon nanotube dispersion comprising adding carbon nanotubes to an, optionally alkaline, aqueous medium and dispersing to obtain an aqueous carbon nanotube dispersion followed by exposure to a pulsed electrostatic field and addition of a superplasticizer to the aqueous carbon nanotube dispersion.

Liebscher et al. (Liebscher et al., Journal of Materials Science volume 52, pages 2296–2307(2017)) provides a report on impact of the molecular architecture of polycarboxylate superplasticizers on the dispersion of multi-walled carbon nanotubes in aqueous phase elucidating the working mechanism of carbon nanotube (CNT) dispersion by distinct methacrylate ester-based polycarboxylates (PCEs), all of which are highly efficient cement dispersants. After mixing accompanied by ultrasonication, pronounced differences regarding the quality of CNT dispersion at equal dosages of PCEs under investigation were observed by means of optical microscopy. Using equivalent numbers of PCE molecules (calculated from their Mw) with respect to CNT mass enabled the disclosing of a clear structure/performance relationship with the best dispersion being achieved by PCEs with a long backbone and a high grafting density of PEG laterals.

Mendoza et al. (Mendoza et al., Construction and Building Materials 47 (2013) 771–778) reports the influence of super plasticizer and Ca(OH)2 on the stability of functionalized multi-walled carbon nanotubes dispersions for cement composites applications. This work studied the effect of superplasticizer and Ca(OH)2 on the stability of OH- functionalized multi walled carbon nanotube (MWCNT)/water dispersions produced via sonication and the MWCNT/water/superplasticizer dispersions were quantified using UV–Vis spectroscopy. According to the report, a polycarboxylate super plasticizer (SP) or an anionic dispersant is not the most adequate dispersant to generate stable MWCNT that can be applied to Portland cement matrixes, because the alkaline Ca(OH)2 rich environment generated during cement hydration prevents the SP adsorption onto the MWCNT surface.

The data available in literature is still inconclusive as to the effects of CNT on cement matrix with both negative and positive effects being reported. Incorporation of CNTs into cement composite materials such as concrete has the potential to overcome their mechanical limitations, i.e., low tensile strength, low strain capacity, and brittleness. But at present there many gaps that need to be overcome to gain an understanding of how the CNT dispersion in cementitious matrixes affects the microstructure and ultimate mechanical properties of the composite. The prior arts though have reported use of sonication as well as dispersing agents for dispersion of CNT, it has not reported the stability of dispersions to obtain a stable chemical admixture. CNT possess very high tensile strength as compared to metallic materials and is seen as good reinforcing materials particularly with the materials that have good compressive strength but lack in tensile and flexural strength. Therefore, one of the promising applications of stable CNT dispersions can be in bulk materials such as cement and construction industry. Use of CNT in concrete mixes can enhance the strength and toughness of concrete structures resulting in rationalization in the use of materials and design of structures leading to better utilization of space and enhanced durability of buildings. This in turn necessitates the exploration of the application of CNT in cement materials.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a chemical admixture wherein a dispersion of CNT is prepared in a suitable medium that is water in presence of a surface modifying dispersing agent and subsequently mixing the same with a cement based material to achieve improvement in compression and flexural strength of the cement based material. Further, the present invention provides a method of preparation of stable aqueous dispersions of CNT using sonication along with use of dispersing agents, especially PCE (Polycarboxylic ether) based super-plasticizer to obtain a chemical admixture for enhancing the strength characteristics of cement based materials (cement mortar and cement paste).

The present invention provides a chemical admixture for mixing with a cement based material, wherein the chemical admixture comprises:
carbon nanotubes (CNT) in an amount ranging between 0.01-1.0 % by weight of the cement based material; and
a dispersing agent in an amount ranging between 0.1 to 1.0 % by weight of the cement based material;
wherein the chemical admixture is an aqueous dispersion of carbon nanotubes.

Further, the present invention provides a process for preparing a chemical admixture for mixing with a cement based material for enhancing the performance of the cement based material, wherein the process comprises:
(a) adding carbon nanotubes (CNT) and a dispersing agent in water to obtain a mixture, wherein the CNT is present in an amount ranging between 0.01-1.0 % by weight of the cement based material and the dispersing agent is present in an amount ranging between 0.1 to 1.0 % by weight of the cement based material;
(b) shaking vigorously and ultrasonicating the mixture of step (a) to obtain the chemical admixture, wherein the chemical admixture is an aqueous dispersion of carbon nanotubes;
wherein the aqueous dispersion of CNT is stable upto 120 days.

OBJECTIVES OF THE INVENTION

The primary objective of the present invention is to provide a chemical admixture for mixing with a cement based material, comprising CNT and a dispersing agent.

Another objective of the present invention is to provide a chemical admixture based on stable aqueous dispersion of carbon nanotubes (CNT) for performance enhancement of cement based materials, especially the flexural strength and compressive strength of cement based materials.

Another objective of the present invention is to provide a method of preparation of a chemical admixture developed for enhancing the performance of cement based materials.

Yet another objective of the present invention is to prepare and characterize cement based composite material, particularly OPC-CNT composite paste and mortar with improved performance characteristics using an aqueous dispersion of CNT as compared to the OPC without any CNT.

DESCRIPTION OF THE INVENTION

For the purpose of promoting an understanding of the principles of the present disclosure, reference will now be made to the specific embodiments of the present invention further illustrated in specific language to describe the same. The foregoing general description and the following detailed description are explanatory of the present disclosure and are not intended to be restrictive thereof. It will nevertheless be understood that no limitation of the scope of the present disclosure is thereby intended, such alterations and further modifications in the illustrated composition, and such further applications of the principles of the present disclosure as illustrated herein being contemplated as would normally occur to one skilled in the art to which the present disclosure relates. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one ordinarily skilled in the art to which this present disclosure belongs. The products, methods, and examples provided herein are illustrative only and not intended to be limiting. As used herein, the term “chemical admixture” is synonymous with the term “composition”, intended to encompass the constituents for mixing with a cement based material for enhancing the mechanical properties of the cement based material.

The present invention provides a chemical admixture developed for enhancing the performance, especially the flexural strength and compressive strength, of cement based materials. The chemical admixture is based on an aqueous dispersion of carbon nanotubes that is mixed with a cement based material for improving the performance of the cement based material. CNT’s are nanomaterials hydrophobic in nature. Its dispersion in water is a critical issue to mix up with micron size cementitious matrix materials. Its aqueous dispersion is one of the most challenging task and mix-up with cementitious grain matrix is another challenging task. Accordingly, aqueous dispersions are produced by application of the process of sonication in which ultrasound energy is employed for agitating the particles in a mixture/ suspension. The developed admixture also contains chemical having surfactant properties which assist in dispersion of carbon nanotubes, enhance the stability of the CNT dispersion and also affect a reduction in the water requirement for producing cement based pastes and mortars of required workability. The developed product of the present invention is a CNT based aqueous dispersion which when added to cementitious matrix provides reinforcement of the matrix at nanoscale leading to significant improvement in the flexural strength of cement based materials like cement mortar.
The present invention provides a chemical admixture for mixing with a cement based material, wherein the chemical admixture comprises:
carbon nanotubes (CNT) in an amount ranging between 0.01-1.0 % by weight of the cement; and
a dispersing agent in an amount ranging between 0.1 to 1.0 % by weight of the cement;
wherein the chemical admixture is an aqueous dispersion of carbon nanotubes.
In an embodiment, the present invention provides that the amount of CNT in the chemical admixture ranges from 0.05 to 0.1 % by weight of the cement.
In another embodiment, the present invention provides that the carbon nanotubes (CNT) are multi-walled carbon nanotubes (MWCNT) having average length ranging from 2 to 4 micrometers and diameter ranging from 30-80 nanometers; and wherein the CNT has an aspect ratio in the range of 25-133.
In an embodiment, the present invention provides that the dispersing agent is selected from Polycarboxylic ether (PCE) based superplasticizer, Cetyltrimethylammonium bromide (CTAB), Sodium dodecyl sulfate (SDS) or combinations thereof. Preferably, the dispersing agent is Polycarboxylic ether (PCE) based superplasticizer.
In a preferred embodiment, the present invention provides that the amount of PCE in the chemical admixture is 0.5 % by weight of the cement,
In an embodiment, the chemical admixture is an aqueous dispersion of the carbon nanotube that is stable upto 120 days.
In another embodiment, the chemical admixture is mixed with a cement based material that is selected from cement paste, cement mortar, cement concrete, ordinary Portland cement (OPC) and other similar materials.
In yet another embodiment, present invention provides that the aqueous dispersion of carbon nanotubes is mixed with an ordinary Portland cement (OPC) to obtain an OPC-CNT composite;
wherein flexural strength of the OPC-CNT composite is 24.2 to 40.8% higher than that of the cement based material in the absence of the carbon nanotubes at the age of 28 days, and compressive strength of the OPC-CNT composite is 5.3 to 19.6% higher than that of the cement based material in the absence of the carbon nanotubes at the age of 28 days. Though, at the age of 3 days, the improvement in flexural strength of OPC-CNT composite has been observed as 73.9%.

Further, the present invention provides a process for preparing a chemical admixture for mixing with a cement based material for enhancing performance of the cement based material, wherein the process comprises:
(a) adding carbon nanotubes (CNT) and a dispersing agent in water to obtain a mixture, wherein the CNT is present in an amount ranging between 0.01-1.0 % by weight of the cement and the dispersing agent is present in an amount ranging between 0.1-1.0 % by weight of the cement;
(b) shaking vigorously and ultrasonicating the mixture of step (a) to obtain the chemical admixture, wherein the chemical admixture is an aqueous dispersion of carbon nanotubes;
wherein the aqueous dispersion of CNT is stable upto 120 days.

In a preferred embodiment, the present invention provides that in step (b) ultrasonication is carried out for 3-5 hours using an ultrasound energy ranging 40-60 kJ per litre to obtain the aqueous dispersion of CNT.

In another embodiment, the present invention provides that in the process of preparation of the chemical admixture disclosed herein, the dispersing agent is selected from Polycarboxylic ether (PCE) based superplasticizer, Cetyltrimethylammonium bromide (CTAB), Sodium dodecyl sulfate (SDS) or combinations thereof. Preferably, the dispersing agent is Polycarboxylic ether (PCE) based superplasticizer.

In yet another embodiment, the present invention provides that in the process of preparation of the chemical admixture disclosed herein, the carbon nanotubes (CNT) are multi-walled carbon nanotubes (MWCNT) having average length ranging from 2 to 4 micrometers and diameter ranging from 30-80 nanometers, and wherein the CNT has an aspect ratio in the range of 25 to 133.

Additionally, the present invention also provides a process of making a composite cement based material, comprising mixing an aqueous dispersion of CNT with a cement based material,
wherein the aqueous dispersion of CNT comprises the CNT in an amount ranging between 0.01-1.0 % by weight of the cement and a dispersing agent in an amount ranging between 0.1-1.0 % by weight of the cement;
wherein the aqueous dispersion of CNT and the cement based material are mixed at a water to cement (w/c) ratio ranging from 0.35 to 0.50; and
wherein the cement based material is selected from cement mortar, cement concrete, cement paste, ordinary Portland cement (OPC) other similar materials.

In an embodiment, the present invention provides that in the presently disclosed process of preparation of the composite cement based material, the cement based material is OPC and the composite cement based material is an OPC-CNT composite; wherein flexural strength of the OPC-CNT composite is 24.2 to 40.8% higher than that of the cement based material in absence of the carbon nanotubes at the age of 28 days, and
compressive strength of the OPC-CNT composite is 5.3 to 19.6 % higher than that of the cement based material in absence of the carbon nanotubes at the age of 28 days.

The present invention thus provides that 0.05 and 0.1 wt% of cement CNT composite shows significant improvement in compressive strength and flexural strength 5.3 to 19.6% and 24.2 to 40.8% after curing of 28 days respectively.

Further, use of the aqueous dispersions of CNT in preparation of OPC-CNT paste and mortar led to well dispersed CNTs in the cement matrix exhibiting the following advantages:
1. The water requirement for preparing cement paste of normal consistency is lowered on use of aqueous CNT dispersions for preparation of cement paste and mortar. This lowering in water requirement is considered to be caused by the dispersing agent present in the CNT dispersion.

2. The setting of cement pastes is somewhat retarded on account of presence of PCE in aqueous dispersion of CNT. Further, the extent of the retardation of setting time is different for different cements at the same content of PCE. The retardation in setting time is within acceptable limits.

3. The OPC comprising CNT shows significantly high improvements in the flexural strength of OPC-CNT composite mortars. The CNT contents of 0.05 and 0.1 percent by weight of cement have consistently yielded more than 20% improvement in the flexural strength of mortar at the age of 28 days as compared to OPC without containing the aqueous CNT dispersion.

4. The improvements in compressive strength varies with both strength reduction and strength enhancement. The improvements in compressive strength at CNT contents of 0.05 and 0.1% are upto 30%, 26% and 19.6 % at the ages of 3, 7 and 28 days respectively.

5. The CNTs result in a very significant improvement in the flexural strength of OPC-CNT composite mortars. Compressive strengths are also generally improved at the ages of 3, 7 and 28 days. Therefore, it is indicated that the CNTs are acting as the physical reinforcement at nanoscale leading to significant improvement in flexural strength as well as some improvement in compressive strength of the OPC-CNT composites.

6. Further, CNT contents in the range of 0.05 – 0.1 % by weight of cement, generally yield higher improvements in compressive as well as flexural strengths than the improvements observed at other CNT contents.

7. For CNT contents in the range of 0.05 – 0.1 % by weight of cement, PCE is indicated to be a suitable for aqueous dispersing agent.

8. For CNT contents in the range of 0.05 – 0.1 %, PCE dose of 0.5 % by weight of cement is found to be optimum.

9. The CNT reinforcement imparts tensile strength to cementitious materials.

Advantages of the present invention:

CNT possesses very high tensile strength as compared to metallic fibre materials and is seen as a good reinforcing material particularly with the cementitious materials that have good compressive strength but lack tensile and flexural strength. Therefore, one of the promising applications of CNT is in bulk materials such as cement and construction industry. The cementitious materials, though quite strong in compression, are relatively weak in flexure and have low bending strengths and fracture toughness. Fibre reinforcement of cementitious matrices improves their flexural strength as well as toughness by impeding crack formation and growth. Reinforcement of cementitious binders with high strength nano diameter fibres such as CNT helps to achieve high performance cement based mortar/composites.

The present invention is further illustrated based on some non-limiting examples as described below. The examples provide a representative elaboration for the detailed description of the present invention.

Examples:
The aqueous dispersion of CNT was prepared by adding 0.20 gram to 0.571 gram of CNT and 2 gram to 2.86 gram PCE in 200 ml of water, vigorously shaking and sonicating up to 5 hours with ultrasound energy. The CNT content is in the range of 0.05 – 0.1 % by weight of cement, with a PCE dose of 0.5 % by weight of the cement. The dispersion was found to be stable up to 120 days. The dispersed CNT was mixed with cement mortar. The required quantities of water, CNT and PCE at various w/c ratio for preparation of mortar with 100 gm cement is given in Table 1. The test samples prepared by such mortar were evaluated for its compressive and flexural strength. The improvement in case of compressive strength has been obtained 5.3 to 19.6 % and the same in case of flexural strength has been obtained 24.2 to 40.8%.

Table 1: The required quantities of water, CNT and PCE at various w/c ratio for preparation of mortar with 100 gm cement

Cement (gram) Water/Cement (w/c) ratio Water Quantity (gram) CNT
(wt % of Cement) CNT
(gram) PCE
(gram)
100 gram 0.35 35gram 0.05% 0.05 gram 0.5 gram
35 gram 0.10% 0.10 gram 0.5 gram
100 gram 0.40 40 gram 0.05% 0.05 gram 0.5 gram
40 gram 0.10% 0.10 gram 0.5 gram
100 gram 0.50 50 gram 0.05% 0.05 gram 0.5 gram
50 gram 0.10% 0.10 gram 0.5 gram

The effect of varying the concentrations of CNT on the compressive strength and flexural strength of the cement based material was studied based on different OPC-CNT composites as shown in Table 2. The optimum quantity of CNT has been found in the range of 0.05 to 0.10 wt% of cement.

Table 2: Effect of varying the concentrations of CNT on the compressive strength and flexural strength of the cement based composite material

Sl. No. CNT content (wt%) PCE content (wt%) Enhancement % of Compressive strength Enhancement % of Flexural strength
1. 0.01 0.5 3.6 33.8
2. 0.05 0.5 19.6 27.8
3. 0.1 0.5 9.7 40.8
4. 0.25 0.5 3.9 -
5. 0.5 0.5 3.9 30.1

The CNT content in OPC-CNT composite mortars was varied in the range of 0.01 to 1.0 % by weight of cement. W/c was varied from 0.35 to 0.50 and the results indicate that CNT contents in the range of 0.05 – 0.1 % by weight of cement, yield consistent improvements in compressive as well as flexural strengths as shown in Table 3.

Table 3: Effect of varying W/C value on the compressive strength and flexural strength of the cement based composite material

Sl. No. water/ cement ratio (w/c) PCE
(wt %) CNT (wt %) Enhancement % of Compressive strength Enhancement % of Flexural strength
1. 0.35 0.5 0.05 6.8 35.4
0.10 9.7 37.6
2. 0.40 0.5 0.05 9.7 24.2
0.10 10.7 25.7
3. 0.50 0.5 0.05 5.3 27.8
0.10 8.0 34.0

The strength improvements have been observed at all the three water/ cement ratios studied. Hence, selection of water/ cement ratio is made based on the workability requirement of the composite mortar for a given application of cementitious materials which are widely employed as construction materials and are quite strong in compression but are relatively weak in flexure and have low bending strengths and fracture toughness.

Table 4 shows the technical advancement of the present invention with respect to the efficacy of the chemical admixture in enhancing the mechanical properties of the cement based material, as compared to that achieved based on conventionally employed admixtures and methods. It was observed from Table 4 that for a similar improvement in compressive strength, the conventional methods require higher CNT concentration. Moreover, some researchers have obtained downward trend also. Therefore, the present invention exhibits an improvement over exiting state of art.

Table 4: Comparative data showing technical advancement of the present invention in terms of increase in compressive strength

Sl. No. CNT
(wt %) Change (%) of Compressive strength
Comparative Example-1
A. Chaipanich, T. Nochaiya, W. Wongkeo, and P. Torkittikul,“Compressive strength and microstructure of carbon nanotubes-fly ash cement composites,” Materials Science and Engineering A, vol. 527, no. 4-5, pp. 1063–1067, 2010. 1.0 10.0% increase
Comparative Example-2
S.Musso, J.-M. Tulliani, G. Ferro, and A. Tagliaferro, “Influence of carbon nanotubes structure on the mechanical behavior of cement composites,” Composites Science and Technology, vol. 69, no. 11-12, pp. 1985–1990, 2009 0.5 11.0% increase
Comparative Example-3
Parveen, S.; Rana, S.; Fangueiro, R.; Paiva, M.C. Microstructure and mechanical properties of carbon nanotube reinforced cementitious composites developed using a novel dispersion technique. Cem. Concr. Res. 2015, 73, 215–227. 0.1 8.98% decrease
Example 1 (present invention) 0.10
(w/c: 0.35) 9.7% increase
Example 2 (present invention) 0.10
(w/c: 0.40) 10.7% increase
Example 3 (present invention) 0.10
(w/c: 0.50) 8.0% increase

Similarly, Table 5 signifies that for a similar improvement in flexural strength, the conventional methods require higher CNT concentration. Moreover, some researchers have obtained downward trend also. Therefore, the present invention shows an improvement over existing state of art.

Table 5: Comparative data showing technical advancement of the present invention in terms of increase in flexural strength

Sl. No. CNT (wt %) Change (%) of Flexural strength
Comparative Example-1
Carbon nanotubes cement composites, G. Ferro et alii, Cassino (FR), Italia, 13-15 Giugno 2011, ISBN 978-88-95940-36-6 0.5 30.4% increase
Comparative Example-2
G. Y. Li, P. M. Wang, and X. Zhao, “Mechanical behavior and microstructure of cement composites incorporating surface treated multi-walled carbon nanotubes,” Carbon, vol. 43, no. 6, pp. 1239–1245, 2005. 0.5 25% increase
Comparative Example-3
Parveen, S.; Rana, S.; Fangueiro, R.; Paiva, M.C. Microstructure and mechanical properties of carbon nanotube reinforced cementitious composites developed using a novel dispersion technique. Cem. Concr. Res. 2015, 73, 215–227. 0.1

16.09% decrease
Example 1 (present invention) 0.10
(w/c: 0.35) 37.6% increase
Example 2 (present invention) 0.10
(w/c: 0.40) 25.7% increase
Example 3 (present invention) 0.10
(w/c: 0.50) 34.0% increase
,CLAIMS:A chemical admixture for mixing with a cement based material, wherein the chemical admixture comprises:
carbon nanotubes (CNT) in an amount ranging between 0.01-1.0 % by weight of the cement; and
a dispersing agent in an amount ranging between 0.1-1.0 % by weight of the cement;
wherein the chemical admixture is an aqueous dispersion of carbon nanotubes.

2. The chemical admixture as claimed in claim 1, wherein the amount of CNT ranges from 0.05 to 0.1 % by weight of the cement based material.

3. The chemical admixture as claimed in claim 1, wherein the carbon nanotubes (CNT) are multi-walled carbon nanotubes (MWCNT) having average length ranging from 2 to 4 micrometers and diameter ranging from 30-80 nanometers; and wherein the CNT has an aspect ratio in the range of 25 to 133.

4. The chemical admixture as claimed in claim 1, wherein the dispersing agent is selected from Polycarboxylic ether (PCE) based superplasticizer, Cetyltrimethylammonium bromide (CTAB), Sodium dodecyl sulfate (SDS) or combinations thereof.

5. The chemical admixture as claimed in claim 1, wherein the dispersing agent is Polycarboxylic ether (PCE) based superplasticizer.

6. The chemical admixture as claimed in claim 5, wherein the amount of PCE is 0.5 % by weight of the cement based material.

7. The chemical admixture as claimed in claim 1, wherein aqueous dispersion of the carbon nanotube is stable upto 120 days.

8. The chemical admixture as claimed in claim 1, wherein the cement based material is selected from cement mortar, cement concrete, cement paste, ordinary Portland cement (OPC).

9. The chemical admixture as claimed in claim 8, wherein the aqueous dispersion of carbon nanotubes is mixed with an ordinary Portland cement (OPC) to obtain an OPC-CNT composite;
wherein flexural strength of the OPC-CNT composite is 24.2 to 40.8% higher than that of the cement based material in absence of the carbon nanotubes at an age of 28 days, and compressive strength of the OPC-CNT composite is 5.3 to 19.6% higher than that of the cement based material in absence of the carbon nanotubes at an age of 28 days.

10. A process for preparing a chemical admixture for mixing with a cement based material for enhancing performance of the cement based material, wherein the process comprises:
(a) adding carbon nanotubes (CNT) and a dispersing agent in water to obtain a mixture, wherein the CNT is present in an amount ranging between 0.01-1.0 % by weight of the cement based material and the dispersing agent is present in an amount ranging between 0.1-1.0 % by weight of cement based material;
(b) shaking vigorously and ultrasonicating the mixture of step (a) to obtain the chemical admixture, wherein the chemical admixture is an aqueous dispersion of carbon nanotubes;
wherein the aqueous dispersion of CNT is stable upto 120 days.

11. The process as claimed in claim 10, wherein in step (b) ultrasonication is carried out for 3-5 hours using an ultrasound energy ranging from 40kJ to 60kJ to obtain the aqueous dispersion of CNT.

12. The process as claimed in claim 10, wherein the dispersing agent is selected from Polycarboxylic ether (PCE) based superplasticizer, Cetyltrimethylammonium bromide (CTAB), Sodium dodecyl sulfate (SDS) or combinations thereof.

13. The process as claimed in claim 10, wherein the carbon nanotubes (CNT) are multi-walled carbon nanotubes (MWCNT) having average length ranging from 2 to 4 micrometers and diameter ranging from 30-80 nanometers, wherein the CNT has an aspect ratio in the range of 25 to 133.

14. A process of making a composite cement based material, comprising mixing an aqueous dispersion of CNT with a cement,
wherein the aqueous dispersion of CNT comprises the CNT in an amount ranging between 0.01-1.0 % by weight of the cement based material and a dispersing agent in an amount ranging between 0.1-1.0 % by weight of the cement;
wherein the aqueous dispersion of CNT and the cement based material are mixed at a water to cement (w/c) ratio ranging from 0.35 to 0.50; and
wherein the cement based material is selected from cement mortar, cement concrete, cement paste, ordinary Portland cement (OPC).

15. The process as claimed in claim 14, wherein the cement based material is an ordinary Portland cement (OPC) and the composite cement based material is an OPC-CNT composite;
wherein flexural strength of the OPC-CNT composite is 24.2 to 40.80% higher than that of the cement based material in absence of the carbon nanotubes at the age of 28 days, and
compressive strength of the OPC-CNT composite is 5.3 to 19.60 % higher than that of the cement based material in absence of the carbon nanotubes at the age of 28 days.