Graphene Dispersion

Abstract

A stable aqueous dispersion of graphene nanoplatelets and a method for making such a dispersion are provided.

Description

[Technical field]

[0001] The present invention relates to a graphene dispersion, particularly in the form of an additive for introducing graphene into compositions such as concrete, and a method for making same. [Background technology]

[0002] Graphene has been much studied since its isolation in 2004. For example, WO2019 / 175564 and WO2017 / 092778 have suggested its potential as a reinforcing nanomaterial for building materials. Indeed, there has been much discussion of it being a potentially useful additive for a wide variety of purposes, from inks to polymer composites.

[0003] A commonly faced problem is the dispersion of graphene within another composition or material. Generally, graphene has a tendency to aggregate and distribute unevenly in (for example) the liquid phase. Therefore, graphene has only been successfully suspended in (for example) water at very low concentrations. Generally, much higher concentrations are required to provide an economical additive that can be used in a wide range of applications. Therefore, there is still a need for suspensions with higher graphene concentrations.

[0004] Moreover, previous attempts to suspend graphene in water have resulted in very limited stability ("shelf life"). Due to its tendency to aggregate, even low concentrations of graphene remain uniformly dispersed for only a relatively short time, on the order of a few hours. This means that such suspensions need to be used soon after formation, making them impractical for large-scale production. There remains a need for suspensions that are stable for days or even months, making them more readily available for industrial use.

[0005] Both of these issues are important in various industries, such as construction, where it would be desirable to include graphene in materials such as concrete.

[0006] The present invention has been devised in light of the above considerations. Summary of the Invention

[0007] The present invention relates in its broadest sense to graphene nanoplatelet dispersions.

[0008] The dispersion comprises water containing graphene nanoplatelets (GNPs) and graphene oxide (GO), both ideally dispersed substantially uniformly in the water. The method that is the subject of the present invention for producing such a dispersion allows significantly higher concentrations of GNPs and GO to be suspended in water than previously achieved. Moreover, the method results in a suspension that is significantly more stable over time.

[0009] Therefore, in a first aspect, the present invention provides a graphene dispersion comprising: (a) graphene nanoplatelets; (b) graphene oxide nanoplatelets; and (c) water, wherein the concentration of the graphene nanoplatelets in the additive is 29 mg ml -1 ~150 mg ml -1 and the concentration of graphene oxide nanoplatelets in the additive is 1 mg ml -1 ~50 mg ml -1 The present invention provides a graphene dispersion comprising:

[0010] It is preferred that the dispersion of GNPs and GO is substantially uniform in water.

[0011] Preferably, the total content of the graphene material in the dispersion is less than 30 mg ml -1 That's all.

[0012] Stable dispersions with such loading levels of GO and GNPs are not known in the prior art. The inventors have created this type of dispersion that is stable for over six months, allowing loading at a concrete batch plant. This is a significant advantage over previous less long-lived dispersions that, even at lower loading levels, needed to be made on-site to allow for dispersion in further compositions.

[0013] The inventors have discovered that larger flake sizes of GNPs can be preferred. Thus, in some embodiments, the graphene nanoplatelets have average lateral flake dimensions greater than 1 μm, preferably greater than 10 μm.

[0014] The inventors have also discovered that smaller flake sizes of GO may be preferred to increase the surface area (and oxygen atom concentration) and thus the availability of oxygen-containing groups. Thus, in some embodiments, the graphene oxide nanoplatelets have average lateral flake dimensions of less than 0.9 μm.

[0015] As can be seen, the additive of the present invention contains significant levels of GO and GNPs. The concentration ratio of graphene oxide nanoplatelets to graphene nanoplatelets, calculated as graphene oxide nanoplatelet concentration / graphene nanoplatelet concentration, may be preferably 0.025 to 1. This provides an ideal balance between stability and potential property improvements.

[0016] A second aspect of the invention relates to a method of making a graphene dispersion comprising the steps of: (i) mixing graphene oxide nanoplatelets, or graphene oxide precursor material, with water; (ii) mixing at high shear of at least 4000 rpm for at least 15 minutes; (iii) adding graphene nanoplatelets, or graphene precursor material; and (iv) mixing at high shear of at least 4000 rpm for at least 15 minutes.

[0017] It should be noted that the order of step pairs (i) and (ii) and (iii) and (iv) may be interchanged, i.e. steps may be performed in the order (i), then (ii), then (iii), then (iv), or steps may be performed in the order (iii), then (iv), then (i), then (ii).

[0018] The high shear mixing preferably (in one or both of steps (ii) and (iv)) comprises a shear rate of at least 1.5×10 4 s -1 The reaction can be carried out under the conditions described above.

[0019] The graphene oxide precursor material may be, for example, graphite oxide. The graphene precursor material may be, for example, graphite.

[0020] In step (i), the graphene oxide or graphite oxide wet cake may be mixed with water.

[0021] To improve the distribution of GO towards homogeneity, in step (ii), it may be preferred that the mixing is carried out for at least 45 minutes, preferably about 1 hour.

[0022] In step (iii), graphene nanoplatelet powder may be added.

[0023] To improve the distribution of the GNPs towards homogeneity, in step (iv) it may be preferred that the mixing is carried out for at least 45 minutes, preferably about 1 hour.

[0024] For the reasons mentioned above, it may be preferred that in step (iv) graphene nanoplatelets having an average lateral flake dimension of more than 1 μm, preferably more than 10 μm are used.

[0025] For the reasons stated above, in step (i) graphene oxide nanoplatelets or graphite oxide having an average lateral flake dimension of less than 0.9 μm are used.

[0026] Preferably, in step (i), the graphene oxide nanoplatelets or the graphene oxide precursor material is dispersed in a dispersion having a concentration of graphene oxide nanoplatelets of 1 mg ml -1 ~50 mg ml -1 It is added in an amount to

[0027] Preferably, in step (iii), the graphene nanoplatelets or graphene precursor material is dispersed in a dispersion having a concentration of graphene nanoplatelets of 29 mg ml -1 ~150 mg ml -1 It is added in an amount to

[0028] Preferably, in steps (i) and (iii), the graphene oxide nanoplatelets or graphene oxide precursor material and the graphene nanoplatelets or graphene precursor material are added in an amount such that a concentration ratio of graphene oxide nanoplatelets to graphene nanoplatelets in the dispersion, calculated as graphene oxide nanoplatelet concentration / graphene nanoplatelet concentration, is between 0.025 and 1.

[0029] Another aspect of the invention relates to a graphene dispersion obtainable or obtainable by the above-mentioned method, i.e., the present aspect relates to a method of making a graphene dispersion comprising the steps of: (i) mixing graphene oxide nanoplatelets or graphene oxide precursor material with water, (ii) mixing at high shear of at least 4000 rpm for at least 15 minutes, (iii) adding graphene nanoplatelets or graphene precursor material, and (iv) mixing at high shear of at least 4000 rpm for at least 15 minutes, wherein the steps are performed in the order of (i), then (ii), then (iii), then (iv), or in the order of (iii), then (iv), then (i), then (ii).

[0030] The graphene dispersion thus obtained may be stable for at least 24 hours, at least 48 hours, at least one week, at least one month, or most preferably at least six months.

[0031] The present invention includes combinations of the aspects and preferred features described herein except where such combinations are clearly unacceptable or clearly avoided.

[0032] BRIEF DESCRIPTION OF THE DRAWINGS Embodiments and experiments illustrating the principles of the present invention will now be described with reference to the accompanying drawings. [Brief description of the drawings]

[0033] [Figure 1] 1 illustrates the viscosity profiles of an exemplary dispersion of the present invention at 7 days and 21 days. [Diagram 2] 1 illustrates the 3-day and 7-day viscosity profiles of an exemplary dispersion of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0034] Aspects and embodiments of the present invention will now be described with reference to the accompanying drawings. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.

[0035] definition First, it may be useful to clarify the meaning of the various terms used in this document. Because graphene-based technology is still relatively new, there has been some variability in the way certain terms relating to graphene are used up until now.

[0036] In particular, "graphene" is used to refer to not only single-layer graphene but also several-layer graphene. The graphene nanoplatelets used in the present invention preferably contain 1 to 10 single-layer graphene. Such graphene products are readily commercially available, and various methods for making graphene are well known.

[0037] "Graphene oxide" refers to graphene (as defined above) that has a large amount of surface-bound oxygen-containing groups, such as hydroxyl (C-OH), ketone (C=O), carboxyl (COOH), and epoxide (COC) species. Graphene oxide can be obtained by a variety of methods, such as the Hummers process, and is readily available commercially. For example, graphene oxide can be obtained by exfoliation of graphite oxide.

[0038] Herein, the concentrations of GNPs and GO in the additives are expressed as their weight content per milliliter of the final formulation of the additive (mg ml -1 The content may be described in units of weight percent of the final additive.

[0039] dispersion liquid The dispersions are aqueous dispersions of graphene and graphene oxide. The dispersions may be characterized by a high level of GNPs contained within a stable dispersion or by their stability over time (i.e., the time that the graphene remains dispersed without significant aggregation). Previously, such dispersions have contained significantly less GNPs and / or have been stable for a significantly shorter period of time.

[0040] One way to judge the stability of a dispersion is to measure its viscosity at specific time intervals (viscosity is affected by flocculation and therefore instability of the dispersion, so a significant increase in viscosity indicates loss of stability).

[0041] For example, using standard equipment such as a laboratory viscometer (e.g., AMETEK Brookfield's DV2T Viscometer, Model LV), the viscosity of the dispersion can be measured directly after production (this is sometimes called V0). Then, using the same technique (and under the same conditions; therefore, the stability test is completely independent of the viscosity measurement technique or conditions used), the viscosity can be measured after "x" hours (this is called V). x If the viscosity measured at time x is within about 35% of the initial viscosity measured (i.e., {([V x / V0]*100)-100} is less than or equal to 35), the dispersion is said to be stable for at least "x" hours.

[0042] The viscosity measured at time x may be preferably within about 25%, more preferably within about 20%, and most preferably within about 15% of the initially measured viscosity to demonstrate stability.

[0043] Viscosity measurements can be performed, for example, at 30 rpm, 40 rpm, 50 rpm, 70 rpm, 90 rpm or 110 rpm, preferably at 40 rpm, at 21° C. Stability (see above) xWithin about 35%, preferably about 25% etc., of V0 need only be demonstrated at at least one of 30 rpm, 40 rpm, 50 rpm, 70 rpm and 90 rpm to be considered satisfactory in the present invention, but more preferably at at least two of those rpm counts, more preferably at at least four of those rpm counts, and most preferably at all of those rpm counts.

[0044] The dispersions of the present invention are preferably stable for at least 24 hours, more preferably at least 72 hours, even more preferably at least 168 hours, and most preferably at least 504 hours.

[0045] An alternative or additional indicator of stability may be by visual inspection of the dispersion after heating for a period of time. For example, a dispersion may be said to be stable if it is heated at least 40° C. for at least 5 days without any discernible change in appearance. It may also be said to be stable if it is heated at least 50° C. for at least 7 days without any discernible change in appearance.

[0046] (This change in appearance indicates that the dispersion is beginning to separate.)

[0047] The dispersion itself may have a variety of viscosities depending on the contents. The dispersion may take the form of a liquid or a gel.

[0048] The dispersion contains (a) graphene nanoplatelets, (b) graphene oxide nanoplatelets, and (c) water, and the concentration of the graphene nanoplatelets in the additive is 29 mg ml -1 ~150 mg ml -1 and the concentration of graphene oxide nanoplatelets in the additive is 1 mg ml -1 ~50 mg ml -1 It is.

[0049] Such higher concentrations are believed to be possible with the preparation methods described below. The concentration of GNPs is preferably 50 mg ml -1 ~120 mg ml -1 , more preferably 60 mg ml -1 ~100 mg ml -1 The concentration of GO is preferably 5 mg ml -1 ~45 mg ml -1 , more preferably 15 mg ml -1 ~40 mg ml -1 It could be.

[0050] The higher the GNP / GO loading, the easier it is to use the graphene dispersion at the point of use since less needs to be added to reach the required loading and achieve the required reaction. For example, a concrete batching plant would need to add significantly less of this dispersion in the process and would be able to use existing hardware and software, without the need to interrupt the existing batch process.

[0051] When using the methods of the present invention, no additional surfactant is required to achieve a stable dispersion with these components. Thus, in some embodiments, no additional surfactant is included in the dispersion. In some embodiments, the dispersion may be substantially free of sodium cholate. In some embodiments, the dispersion comprises (a) graphene nanoplatelets, (b) graphene oxide nanoplatelets, and (c) water, wherein the concentration of graphene nanoplatelets in the additive is greater than or equal to 29 mg ml -1 ~150 mg ml -1 and the concentration of graphene oxide nanoplatelets in the additive is 1 mg ml -1 ~50 mg ml -1 and the remainder is water.

[0052] The total content of graphene material in the dispersion can be calculated by simply adding up the content of all graphene-based components present, e.g., GNP content + GO content.

[0053] The total graphene content is 30 mg ml -1 It may be preferred that the total graphene material content is greater than or equal to 40 mg ml, as it is understood by the inventors that the presence of a greater amount of graphene material aids in the efficient delivery of the material (requiring less volume of dispersion to load the same amount of graphene material). -1 More than 50 mg / ml, for example -1 More preferably, it should be 70 mg ml -1 More than 90 mg / ml, for example -1 More preferably, it is equal to or greater than this.

[0054] A stable dispersion containing such a concentration of graphene material is not known in the prior art, nor is a method by which such a dispersion can be made.

[0055] The ratio of the GNP and GO content in the dispersion is also of interest: in some circumstances it may be preferable to use, for example, a lower GO as a proportion of GNPs, i.e. a lower concentration ratio of graphene oxide nanoplatelets to graphene nanoplatelets, i.e. concentration of graphene oxide nanoplatelets / concentration of graphene nanoplatelets.

[0056] The ratio is preferably 0.025 to 1.

[0057] (A ratio of 0.025 represents GO1 to GNP40. A ratio of 1 represents GO1 to GNP1.)

[0058] In some embodiments, the ratio can be toward the lower end of the range. For example, the ratio can be 0.025 to 0.5, more preferably 0.03 to 0.3, even more preferably 0.04 to 0.2, and most preferably 0.05 to 0.1.

[0059] In some other embodiments, higher ratios may be used, such as 0.05 to 0.7, and more preferably 0.1 to 0.25.

[0060] In still further embodiments, even higher ratios may be used, such as 0.5 to 1, and more preferably 0.6 to 0.8.

[0061] It can be appreciated that GO can act as a thickener, effectively increasing the viscosity of the dispersion, and therefore the amount of GO included can be reduced to achieve the desired viscosity and dispersion characteristics.

[0062] Of course, if a higher viscosity is desired, a higher content of GO can be used.

[0063] GNPs and GO exist in a variety of shapes and sizes, and it has been found that the size of the flakes used has some impact on the benefits provided by the additive. Specifically, increasing the lateral size of GNPs and decreasing the lateral size of GO have each been found to be independently beneficial.

[0064] For example, it may be preferred that the GNPs have an average lateral flake dimension greater than 1 μm, more preferably greater than 10 μm.

[0065] Similarly and separately, it may be preferred that the GO have an average lateral flake dimension of less than 0.9 μm.

[0066] The lateral flake dimensions of a given flake can be measured, for example, by SEM. Both GNP and GO, as well as suitable precursors, are routinely provided by manufacturers with information regarding the size of the flakes contained therein. For example, if a GNP product is described as having flake lateral dimensions of less than 30 μm, it can be assumed that substantially all of the flakes in that product are less than 30 μm in size.

[0067] (Generally, the lateral flake "dimension" is the longest dimension of the flake. The "average" lateral flake dimension is suitably d 50 It could be.)

[0068] Additive manufacturing It has not been possible so far to provide a dispersion of the above-mentioned type in a stable form, in which the GNPs are substantially uniformly distributed. The inventors have now discovered a method which allows the formation of exactly such a dispersion, the stability of which is fundamentally improved compared to the dispersions described so far.

[0069] In the broadest sense, the method of the present invention uses high shear mixing to combine graphene oxide (or a precursor thereof), graphene nanoplatelets (or a precursor thereof), and water. More specifically, the method of the present invention comprises (i) mixing graphene oxide nanoplatelets, or a graphene oxide precursor material, with water, (ii) mixing at a high shear force of at least 4000 rpm for at least 15 minutes, (iii) adding graphene nanoplatelets, or a graphene precursor material, and (iv) mixing at a high shear force of at least 4000 rpm for at least 15 minutes, where the steps may be performed in the order of (i), (ii), (iii), (iv), or in the order of (iii), (iv), (i), (ii).

[0070] This use of high shear mixing is in contrast to previously implemented ultrasonic mixing techniques, for example, and surprisingly allows for the formation of stable dispersions with much higher GNP (and GO) concentrations.

[0071] There is no significant limitation on the shape of the graphene oxide nanoplatelets used in step (i). The basic settings and characteristics described above with respect to the additive itself still apply, of course. In some embodiments, it may be practical to provide the graphene oxide in the form of a wet cake, which may have a GO solid content of 30% to 60% by weight. However, it will be understood that the important value is not the form before adding GO, but how much GO is mixed with how much water to obtain the final concentration of GO.

[0072] Alternatively, graphene oxide precursors can be used in the process of the invention. Suitable precursors include, for example, graphite oxide or other materials that can form graphene oxide by exfoliation. The inventors believe that such precursors can be exfoliated by the high shear mixing in step (ii) to effectively form graphene oxide nanoplatelets in situ during mixing.

[0073] The graphene nanoplatelets used in step (iii) are again not subject to any significant limitations on their shape. The basic settings and characteristics described above with respect to the additive itself of course still apply. In some embodiments, it may be practical to provide the graphene nanoplatelets in the form of a powder.

[0074] Additionally, the amount of graphene added in step (iii) is appropriately selected to provide a desired final concentration in the additive, which, together with the amount of graphene oxide added in step (i), provides a desired concentration ratio of graphene oxide to graphene.

[0075] Alternatively, graphene precursors can be used in the process of the invention. Suitable precursors include, for example, graphite or other materials that can form graphene by exfoliation. The inventors believe that such precursors can be exfoliated by the high shear mixing in step (ii) to effectively form graphene nanoplatelets in situ during mixing.

[0076] Mixing at at least 4000 rpm under high shear has been found to result in stable dispersions, which is enhanced by carrying out mixing for at least 15 minutes at each mixing step.

[0077] It will of course be apparent that higher mixing speeds can be used, typically in the range 4000-20000 rpm, for example 5000-8000 rpm.

[0078] The high shear mixing preferably (in one or both of steps (ii) and (iv)) comprises a shear rate of at least 1.5×10 4 s -1 , preferably at least 2 × 10 4 s -1 , e.g. at least 4 × 10 4 s -1 The reaction can be carried out under conditions where

[0079] In a preferred embodiment, the mixing time in each of steps (ii) and (iv) may be at least about 30 minutes, such as at least about 45 minutes, or more preferably at least about 1 hour. Suitably, the mixing in each of steps (ii) and (iv) is carried out for about 1 hour.

[0080] Uses of graphene dispersions The graphene dispersions described herein may be useful for inclusion in a variety of other compositions and materials.

[0081] A particular use is as an additive to mixtures containing cementitious materials. Such mixtures may be for forming, for example, concrete, mortar, grout, etc. Such mixtures generally include a cementitious material, water, and at least one aggregate (e.g., a fine aggregate such as sand and / or a coarse aggregate such as gravel). Such mixtures and ingredients are very well known in the art.

[0082] The dispersions of the present invention can also be added to a variety of other mixtures to introduce graphene and improve specific properties, for example they may be added to coating mixtures, mixed with resins for use in fiber reinforced plastics, or used to form aerogels or membranes.

[0083] ***

[0084] The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, and presented in a particular form or in terms of means for performing a disclosed function, or a method or process for achieving a disclosed result, may be used separately or in any combination of such features, as appropriate, to realize the invention in its diverse forms.

[0085] Although the present invention has been described in conjunction with the above exemplary embodiments, many equivalent modifications and variations will be apparent to those skilled in the art given this disclosure. Accordingly, the above exemplary embodiments of the present invention are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the present invention.

[0086] For the avoidance of doubt, the theoretical explanations provided herein are provided for the purpose of enhancing the understanding of the reader, and the inventors do not wish to be bound by any of these theoretical explanations.

[0087] Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

[0088] Throughout this specification, including the claims which follow, unless the context specifically requires, the words "comprise" and "include", and variations such as "comprises", "comprising" and "including", are understood to imply the inclusion of a stated integer value or step, or group of integer values ​​or steps, but not the exclusion of other integer values ​​or steps, or group of integer values ​​or steps.

[0089] It should be noted that, as used in this specification and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from "about" one particular value and / or to "about" another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values ​​are expressed as approximations, by use of the antecedent "about," it will be understood that the particular value forms another embodiment. The term "about" in connection with numerical values ​​is arbitrary and may mean, for example, + / - 10%. EXAMPLES

[0090] Example 1 - Dispersion Graphite oxide is The 6 th It was obtained from Element (https: / / www.c6th.com / ) product SE2430W-N as a wet cake with a solids content of approximately 45%.

[0091] GNP was sourced from Versarien's product GNP-HP.

[0092] 69 g of graphite oxide wet cake was placed in a glass beaker and about 700 ml of tap water was added to it. The mixture was then mixed in a high shear mixer (Silverson High Shear Mixer with Square Hole High Shear Screen™ at 5000 rpm) for about 1 hour.

[0093] Following this mixing, 69 g of GNP powder was added in four portions with approximately 30 seconds of hand mixing between additions.

[0094] Once all the GNPs had been added, the final mixture was then mixed in a high shear mixer (Silverson High Shear Mixer with Square Hole High Shear Screen™ at 5000 rpm) for approximately 1 hour.

[0095] The resulting mixture (approximately 1 liter) was found to be stable for more than 6 months under standard conditions.

[0096] Example 2 - Qualitative Testing Example 2a Following the general experimental method described in Example 1, a 7% total graphene concentration (70 mg ml -1 ) and had a GO:GNP concentration ratio of 0.25 (i.e., 4:1 in terms of GNP:GO).

[0097] The viscosity (cP) of the resulting dispersion was measured at various different rpm counts using an AMETEK Brookfield DV2T viscometer, model LV. Data showing the viscosity profile at 7 and 21 days is shown in Figure 1. It can be seen that despite the long time between readings, the viscosity profile of the dispersion did not change significantly, indicating the long-term stability of the dispersion. The viscosity was broadly stable (within about 15%) at each of 30 rpm, 40 rpm, 50 rpm, 70 rpm, and 90 rpm.

[0098] Example 2b Following the general experimental method described in Example 1, a 10% total graphene concentration (100 mg ml -1) and had a GO:GNP concentration ratio of 0.25 (i.e., 4:1 in terms of GNP:GO).

[0099] The viscosity (cP) of the resulting dispersion was measured at various different rpm counts using an AMETEK Brookfield DV2T viscometer, model LV. Figure 2 shows the data showing the viscosity profile on days 3 and 7. It can be seen that despite the long time between readings, the viscosity profile of the dispersion did not change significantly, indicating the long-term stability of the dispersion. The viscosity was broadly stable (within about 15%) at each of 30 rpm, 40 rpm, 50 rpm, 70 rpm, and 110 rpm. The relatively large change in viscosity at 90 rpm is considered anomalous.

Claims

1. (a) graphene nanoplatelets; (b) graphene oxide nanoplatelets; and (c) water; 1. A graphene dispersion comprising: The concentration of the graphene nanoplatelets in the additive is 29 mg ml －１ ~150 mg / ml －１ and the concentration of the graphene oxide nanoplatelets in the additive is 1 mg ml －１ ~50 mg / ml －１ The graphene dispersion,

2. 10. The dispersion of claim 1, wherein the graphene nanoplatelets have an average lateral flake dimension greater than 1 μm.

3. 10. The dispersion of claim 1, wherein the graphene nanoplatelets have an average lateral flake dimension greater than 10 μm.

4. 4. The dispersion of claim 1, wherein the graphene oxide nanoplatelets have an average lateral flake dimension of less than 0.9 μm.

5. 4. The dispersion of claim 1, wherein the concentration ratio of graphene oxide nanoplatelets to graphene nanoplatelets, calculated as graphene oxide nanoplatelet concentration / graphene nanoplatelet concentration, is from 0.025 to 1.

6. 1. A method of making a graphene dispersion, comprising: (i) mixing graphene oxide nanoplatelets, or graphene oxide precursor materials, with water; (ii) mixing at high shear at least 4000 rpm for at least 15 minutes; (iii) adding graphene nanoplatelets or graphene precursor materials; (iv) mixing at high shear at least 4000 rpm for at least 15 minutes; The method includes the steps of: The method as described above, wherein the steps are performed in the order (i), (ii), (iii), (iv), or (iii), (iv), (i), (ii).

7. 7. The method of claim 6, wherein in step (i), graphene oxide wet cake or graphite oxide wet cake is mixed with the water.

8. 8. The method of claim 6 or claim 7, wherein in step (ii), the mixing is carried out for at least 45 minutes, preferably about 1 hour.

9. 8. The method of claim 6 or claim 7, wherein in step (iii) graphene nanoplatelet powder is added.

10. 8. The method of claim 6 or claim 7, wherein in step (iv), the mixing is carried out for at least 45 minutes, preferably about 1 hour.

11. 8. The method of claim 6 or claim 7, wherein in step (iv) graphene nanoplatelets having an average lateral flake dimension of more than 1 μm, preferably more than 10 μm, are used.

12. 8. The method of claim 6 or claim 7, wherein in step (i) graphene oxide nanoplatelets or graphite oxide having an average lateral flake dimension of less than 0.9 μm are used.

13. In step (i), the graphene oxide nanoplatelets or the graphene oxide precursor material are added to the additive for concrete so that the concentration of the graphene oxide nanoplatelets in the additive is 1 mg ml －１ ~50 mg / ml －１ The method according to claim 6 or claim 7, wherein the amount of the compound added is in the range of 0.1 to 1.

0.

14. In step (iii), the graphene nanoplatelets or the graphene precursor material are added to the additive for concrete to achieve a concentration of graphene nanoplatelets of 29 mg ml －１ ~150 mg / ml －１ The method according to claim 6 or claim 7, wherein the amount of the compound added is in the range of 0.1 to 1.

0.

15. 8. The method of claim 6 or 7, wherein in steps (i) and (iii), the graphene oxide nanoplatelets or the graphene oxide precursor material and the graphene nanoplatelets or the graphene precursor material are added in amounts such that a concentration ratio of graphene oxide nanoplatelets to graphene nanoplatelets in the dispersion, calculated as graphene oxide nanoplatelet concentration / graphene nanoplatelet concentration, is 0.025 to 1.

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