Cement-based materials

Abstract

A composition comprising a cementitious material, water, graphene nanoplatelets and graphene oxide nanoplatelets, wherein the total graphene material (graphene nanoplatelets and graphene oxide nanoplatelets) content of the composition is from 0.005 to 0.5 wt% relative to the content of the cementitious material, and wherein the concentration ratio of graphene oxide nanoplatelets to graphene nanoplatelets is from 0.025:1 to 1:1. The composition may further comprise aggregate, optionally in an amount of 40-90 wt%. A method of forming a composition is also defined comprising mixing a cementitious material, water, and an additive, wherein the additive comprises graphene nanoplatelets, graphene oxide nanoplatelets and water, and the concentration of graphene nanoplatelets and graphene oxide nanoplates in the additive is from 29-150 mgmL-1 and 1-50 mgmL-1 respectively. A material obtainable by curing the composition is further defined, as is a composition obtainable by the method, and a material obtainable by curing the composition made by the method. A further method is defined for forming a hardened material comprising forming a composition by the method and curing the composition.

Description

Field of the Invention The present invention relates to enhanced compositions for cement-based materials (that is, compositions which comprise a cementitious material as one of their ingredients); in particular, compositions comprising graphene and graphene oxide and methods of making them. Background Much has been written about the need for sustainability in technological fields new and old, whether that is related to sustainability of materials, recycling, CO2 emissions or some other aspect. One of the biggest contributors to CO2 emission globally is the construction industry, and in particular the use of cement-based materials such as concrete. It has been estimated that, through the production chain, concrete is responsible for 4-8% of the entire world’s CO2 emissions. Huge quantities of concrete are used in most large-scale construction projects, often with steel reinforcement alongside the concrete pour. Concrete is deployed in a wide range of projects, and is highly prized for its strength (especially when reinforced). However this comes with challenges. Steel reinforcement within concrete is vulnerable to corrosion. It requires the use of more complex and involved pouring techniques. Accordingly there has been a focus in the construction sector on identifying alternative ways of reinforcing concrete and other cement-based materials. A stronger material might mean less is used (or needed) for a given project, potentially significantly reducing the environmental impact. It might mean that steel reinforcement to achieve a similar effect can be excluded, again leading to an environmentally improved process. Graphene has been much researched since its isolation in 2004. It has been suggested as a possible reinforcement nanomaterial for building materials, for example in WO 2019 / 175564 and WO 2017 / 092778. The related compound graphene oxide has also been suggested as a useful additive to building materials: see, again, WO 2017 / 092778 and WO 2013 / 096990. However there has been difficulty in the art with such options, in particular in properly dispersing graphene or graphene oxide. At low levels, both graphene and graphene oxide have successfully been suspended in water, but scaling this up to allow incorporation of the necessary much higher levels in industrial scale compositions has not been achieved. A low level inclusion cannot provide the reinforcement benefits needed to significantly alter the amount of material needed. Accordingly the ability of graphene and graphene oxide to enhance cement-based materials to the levels needed to make a significant environmental impact is limited. There remains a need for enhanced mixtures. The present invention has been devised in light of the above considerations. Summary of the Invention At its broadest, the present invention relates to compositions comprising a cementitious material, water, graphene and graphene oxide. In a first aspect, the present invention provides a composition comprising a cementitious material, water, graphene nanoplatelets and graphene oxide nanoplatelets, wherein the total graphene material content of the composition (graphene nanoplatelets + graphene oxide nanoplatelets) is from 0.005 to 0.5 wt% relative to the content of cementitious material, and wherein the concentration ratio of graphene oxide nanoplatelets to graphene nanoplatelets is from 0.025:1 to 1:1. The compositions described herein may be suitable for curing / hardening to form cement-based materials, in particular materials comprising cementitious material, graphene nanoplatelets and graphene oxide nanoplatelets. The inventors have found that such compositions harden to form materials with many improved properties. For example, improvements in compressive, tensile and flexural strength characteristics have been observed. These lead to enhanced durability of the hardened materials, not only through plain strength but also crack resistance. This can mean that, for example, less material is needed for a given purpose, or that additional products are not need to provide, for example, anti-cracking measures. In preferred embodiments the graphene nanoplatelets and graphene oxide nanoplatelets are substantially uniformly distributed through the composition. This forms a uniform composition and hence a uniform cured product, with predictable and advantageous performance across its bulk. The present compositions, when cured, and the present cured materials may have enhanced properties, for example, a compressive strength (measured according to BS EN 12390, parts 3 (published 16 July 2019) and 4 (published 10 October 2019)) after 3 days curing of at least 30 MPa; or a compressive strength (measured according to BS EN 12390, parts 3 (published 16 July 2019) and 4 (published 10 October 2019)) after 14 days curing of at least 40 MPa; or a compressive strength (measured according to BS EN 12390, parts 3 (published 16 July 2019) and 4 (published 10 October 2019)) after 28 days curing of at least 44 MPa. They may have, for example, a tensile strength (measured according to BS EN 12390, part 6 (published 30 June 2010)) after 3 days curing of at least 3.2 MPa; or a tensile strength (measured according to BS EN 12390, part 6 (published 30 June 2010)) after 14 days curing of at least 3.6 MPa; or a tensile strength (measured according to BS EN 12390, part 6 (published 30 June 2010)) after 28 days curing of at least 4.0 MPa. They may have, for example, a flexural strength (measured according to BS EN 12390, part 5 (published 19 July 2019)) after 3 days curing of at least 5.0 MPa; or a flexural strength (measured according to BS EN 12390, part 5 (published 19 July 2019)) after 14 days curing of at least 5.6 MPa; or a flexural strength (measured according to BS EN 12390, part 5 (published 19 July 2019)) after 28 days curing of at least 6.0 MPa. Suitably, the composition comprises the cementitious material in an amount 3-80 wt%, preferably 5-30 wt%. Suitably, the composition comprises water in a weight ratio relative to weight of cementitious material of 0.3-0.8, preferably 0.45-0.65. In some embodiments, the composition further comprises an aggregate, optionally in an amount 40-95 wt%, preferably 50-90 wt%. In such embodiments, the aggregate may suitably be made up of coarse aggregate and fine aggregate in a weight ratio fine : coarse of 1:0.5 to 1:4. The present compositions may be converted into hardened materials (cementitious matrix materials) by curing. Accordingly a second aspect of the present invention provides a material obtainable by curing a composition as described herein. A third aspect of the present invention provides a method of forming a composition, the method comprising mixing a cementitious material, water and an additive comprising: (a) graphene nanoplatelets; (b) graphene oxide nanoplatelets; and (c) water, wherein the concentration of graphene nanoplatelets in the additive is from 29 mg.ml-1 to 150 mg.ml-1 and the concentration of graphene oxide nanoplatelets in the additive is from 1 mg.mH to 50 mg.mH. Also described herein is, thus, use of such an additive to increase compressive, tensile or flexural strength of a cementitious material such as concrete; and use of such an additive to reduce cracking of a cementitious material such as concrete. In each case the use comprises mixing the additive with at least a cementitious material and water and then curing the resultant composition, as described herein. In some embodiments, the additive is first mixed with the water component of the composition (that is, is diluted); then after that the cementitious material is added. Where the composition includes aggregate, it may suitably be added after the water but before the cementitious material. A fourth aspect of the present invention provides a composition obtainable by a method according to the third aspect. A fifth aspect of the present invention provides a material obtainable by curing a composition according to the fourth aspect. A sixth aspect of the present invention provides a method of forming a hardened material comprising forming a composition by a method according the third aspect and then curing the composition; alternatively, the method comprising curing a composition according to the first aspect or according to the fourth aspect. These aspects form a cured product comprising a cementitious material in which graphene and graphene oxide are contained; in some embodiments, wherein the total graphene material content of the composition (graphene nanoplatelets + graphene oxide nanoplatelets) is from 0.005 to 0.5 wt% relative to the original content of cementitious material, and wherein the concentration ratio of graphene oxide nanoplatelets to graphene nanoplatelets is from 0.025:1 to 1:1. Of course it will be recognised that the cementitious material is cured in such a product. The additive described herein comprises water with graphene nanoplatelets (GNP) and graphene oxide (GO), both ideally being substantially uniformly distributed therein. The methods to make such a dispersion, permit significantly higher levels of GNP and GO to be suspended in the water than have previously been achieved. Furthermore, the methods lead to suspensions which are remarkably stable over time. The additive may be a graphene dispersion, comprising: (a) graphene nanoplatelets; (b) graphene oxide nanoplatelets; and (c) water, wherein the concentration of graphene nanoplatelets in the additive is from 29 mg.ml-1 to 150 mg.ml-1 and the concentration of graphene oxide nanoplatelets in the additive is from 1 mg.ml1 to 50 mg.ml1. Such an additive may be, as set out above, mixed with (further) water and a cementitious material to make a composition according to the present invention. The dispersion of GNP and GO is preferably substantially uniform within the water. Preferably, the total graphene material content in the dispersion is greater than or equal to 30 mg.ml1. A stable dispersion with these levels of GO and GNP loadings has not been known in the prior art. The present inventors have made dispersions of this type which are stable for more than 6 months, permitting them to be added in a concrete batching plant. This is a significant advantage over previous, less long lived dispersions, even at lower loading levels, which would need to be made in situ to permit dispersion in a further composition. The inventors have found that a larger flake size of the GNP may be preferable. Accordingly, in some embodiments, the graphene nanoplatelets have an average lateral flake dimension of greater than 1 pm, preferably greater than 10 pm. The inventors have also found that a smaller flake size of the GO may be preferable, to increase the surface area (and at% oxygen) and hence the availability of oxygen containing groups. Accordingly, in some embodiments, the graphene oxide nanoplatelets have an average lateral flake dimension of less than 0.9 pm. As explained above, the present additives contain a significant level of GO and GNP. It may be preferred that 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. This can give an ideal balance of stability and potential property improvement. Mentioned here is a method of making the additive, i.e. a graphene dispersion, the method comprising the steps of: (i) mixing graphene oxide nanoplatelets, or a graphene oxide precursor material, with water; (ii) mixing at high shear at at least 4000 rpm for at least 15 minutes; (iii) adding graphene nanoplatelets, or a graphene precursor material; and (iv) mixing at high shear at at least 4000 rpm for at least 15 minutes. It is noted that step pairs (i) and (ii), and (iii) and (iv), may be switched in order. That is, the steps may be performed in the order (i) then (ii) then (iii) then (iv) or may be performed in the order (iii) then (iv) then (i) then (ii). The high shear mixing may preferably (in one or both of steps (ii) and (iv)) be performed under conditions such that the shear rate is at least 1.5x104 s1. The graphene oxide precursor material may be, for example, graphite oxide. The graphene precursor material may be, for example, graphite. In step (i) it may be that a graphene oxide or graphite oxide wetcake is mixed with the water. In order to improve the distribution of the GO towards uniformity, it may be preferable that in step (ii) mixing is performed for at least 45 minutes, preferably about 1 hour. In step (iii), it may be that a graphene nanoplatelets powder is added. In order to improve the distribution of the GNP towards uniformity, it may be preferable that in step (iv) mixing is performed for at least 45 minutes, preferably about 1 hour. For the reasons mentioned above, in step (iv) it may be preferable that graphene nanoplatelets with an average lateral flake dimension of greater than 1 pm, preferably greater than 10 pm, are used. For the reasons mentioned above, in step (i) graphene oxide nanoplatelets or graphite oxide with an average lateral flake dimension of less than 0.9 pm are used. Preferably, in step (i) the graphene oxide nanoplatelets or graphene oxide precursor material is added in an amount to give a graphene oxide nanoplatelet concentration of from 1 mg.ml-1 to 50 mg.ml-1 in the dispersion. Preferably, in step (iii) the graphene nanoplatelets or graphene precursor material is added in an amount to give a graphene nanoplatelet concentration of from 29 mg.ml-1 to 150 mg.ml-1 in the dispersion. 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 to give a concentration ratio of graphene oxide nanoplatelets to graphene nanoplatelets, calculated as graphene oxide nanoplatelet concentration / graphene nanoplatelet concentration, from 0.025 to 1 in the dispersion. The present additive is or may be a graphene dispersion obtainable or obtained by the methods described above. That is, a graphene dispersion obtainable or obtained by a method comprising the steps of: (i) mixing graphene oxide nanoplatelets, or a graphene oxide precursor material, with water; (ii) mixing at high shear at at least 4000 rpm for at least 15 minutes; (iii) adding graphene nanoplatelets, or a graphene precursor material; and (iv) mixing at high shear at at least 4000 rpm for at least 15 minutes, the steps being performed in the order (i) then (ii) then (iii) then (iv) or being performed in the order (iii) then (iv) then (i) then (ii). Graphene dispersions obtained in this way may be stable for at least 24 hours, at least 48 hours, at least 1 week, at least 1 month or most preferably at least 6 months. The invention also provides a reinforced concrete material obtainable by mixing an additive as described herein with a coarse aggregate material, a fine aggregate material (such as sand), water and a cementitious material, followed by curing; as well as a method of making such a concrete comprising mixing an additive for concrete as described herein with a coarse aggregate material, a fine aggregate material (such as sand), water and a cementitious material, followed by curing. The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided. Summary of the Figures Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which: Figure 1 shows compressive strength testing data from the below Example 1. Figure 2 shows tensile strength testing data from the below Example 1. Figure 3 shows flexural strength testing data from the below Example 1. Figure 4 shows compressive strength testing data from the below Example 2. Detailed Description of the Invention Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference. Definitions Initially it is worth discussing the meaning of various terms of art that are used herein. Given the relatively young age of graphene based techniques, there has in the past been some variation in how particular terms are used with reference to it. In particular, “graphene” is used to refer not only to monolayer graphene but also few layer graphene and graphene nanoplatelets according to ISO / TS 80004-13:2017(en). The graphene, sometimes herein interchangeably also called graphene nanoplatelets (GNP), used herein preferably comprise 1-10 layers of monolayer graphene. Suitably, graphene has a layer thickness of 1 nm to 3 nm and lateral dimensions ranging from approximately 100 nm to 100 pm. Such and suitable graphene products are readily commercially available, and various methods of making graphene are well known. It will be recognised that this definition includes reduced graphene oxide (rGO) as graphene (= graphene nanoplatelets). “Graphene oxide” (and, similarly, “graphene oxide nanoplatelets”) refers to graphene (as defined above) that has a significant amount of surface bound oxygen containing groups. For example, hydroxyl (C-OH), ketone (C=O), carboxyl (COOH), epoxide (C-O-C) and other species. Graphene oxide is readily commercially available, being obtained from various methods such as the Hummers process. It may be obtained by, for example, delamination of graphite oxide. The term “nanoplatelet” is used to refer to plate-like particles with at least one dimension (length, width, thickness) in the nanometre order of magnitude. Herein, the concentrations of GNP and GO in the cement-based composition may be discussed in terms of weight content per millilitre of the final formulation of the composition, mg.mH. The content can also be considered in terms of wt% of the final composition. Alternatively, the contents can be stated in terms of the content relative to the content of cementitious material in the cement-based composition. The total graphene material content of a given composition or material can be calculated by simple addition of the contents of all the graphene-based components present. For example, GNP content + GO content. Cement-based Materials and Compositions Herein, the term cement-based material or composition is used to refer to a composition which utilises a cementitious material as an ingredient which will harden on curing, and can act as a binder for other ingredients of the composition on curing. It is noted that such cement-based materials are typically not >50 vol% cementitious material. For example, in atypical concrete mix (a common cement-based material) the cementitious material might only make up 10-15 vol% of the composition, the balance being for example water (15-20 vol%) and aggregates of various types (65-75 vol%), plus any minor additives. Typical approximate mixing ratios (by weight) of cementitious material, fine aggregate and coarse aggregate include 1:1:2; 1:2:2; 1:2:4; 1:3:6; 1:4:8; 1:5:10; 1:2:3; 1:2.5:3; and 1:3:3. These can be suitably used in embodiments of the present invention. It will be recognised that these ratios persists after curing. Typical water: cement ratio (water content / cement content, by weight) is 0.3-0.8, for example 0.4-0.7, and typically 0.45-0.65 for example about 0.5-0.6. Ratios of about 0.5 and about 0.55 are common. The present compositions comprise at least a cementitious material, water, plus graphene and graphene oxide. The graphene and graphene oxide-containing additive used in making the present compositions may suitably have a total graphene material content of 30 mg.mH or greater (that is, about 3.0 wt% or greater); it may suitably have a total graphene material content of 200 mg.mH or lower (that is, about 20.0 wt% or lower). The additive may be included in the mix at a content amount suitable to give a total graphene material content of at least 0.005 wt%, preferably at least 0.01 wt%, more preferably at least 0.08 wt%, relative to the content of cementitious material. The additive may be included in the mix at a content amount of at most 0.5 wt%, preferably at most 0.3 wt%, more preferably at most 0.15 wt%, relative to the content of cementitious material. For example, a total graphene material content of 0.01 wt% to 0.15 wt% may be suitable. Accordingly, it can be calculated that the additive itself may suitably be added in an amount from 0.025 wt% (where an additive with 20.0 wt% total graphene material content is added to give a composition with total graphene material content of 0.005 wt% relative to the cementitious material content) to 16.67 wt% (where an additive with 3.0 wt% total graphene material content is added to give a composition with total graphene material content of 0.5 wt% relative to the cementitious material content) relative to the content of the cementitious material. The content ratio of graphene oxide (GO) and graphene (GNP) may suitably be 0.025:1 to 1:1 [A ratio of 0.025:1 representing 1 part GO to 40 parts GNP; a ratio of 1:1 representing 1 part GO to 1 part GNP.] Therefore if the additive is included in an amount suitable to give a total graphene material content of, say, 4 wt% relative to the content of cementitious material, then, the contents in the composition may be about 0.1 wt% GO and 3.9 wt% GNP to 2 wt% GO and 2 wt% GNP relative to the content of cementitious material. Accordingly, the content of graphene (GNP) in the composition is suitably at least 0.0025 wt%, preferably at least 0.004 wt%, more preferably at least 0.005 wt%, and yet more preferably at least 0.075 wt%, relative to the content of cementitious material. The content of graphene (GNP) in the composition is suitably at most 0.49 wt%, preferably at most 0.4 wt%, more preferably at most 0.25 wt%,, and yet more preferably at most 0.15 wt% relative to the content of cementitious material. Also accordingly, the content of graphene oxide (GO) in the composition is suitably at least 0.00012 wt%, preferably at least 0.0025 wt%, more preferably at least 0.05 wt%, relative to the content of cementitious material. The content of graphene oxide (GO) in the composition is suitably at most 0.25 wt%, preferably at most 0.2 wt%, more preferably at most 0.1 wt%, relative to the content of cementitious material. It will be recognised that these contents persist after curing. The graphene and graphene oxide are each independently suitably substantially homogeneously (uniformly) distributed through the composition. That is, the graphene is suitably substantially homogeneously distributed though the composition; and the graphene oxide is suitably substantially homogeneously distributed through the composition. This can mean that, after curing, the resultant cement-based material has graphene and graphene oxide dispersed through it, given uniformity of strength and advantageous properties through the material. The compositions can be made by mixing (that is, by forming a substantially homogeneous mixture of) water, the cementitious material, and an additive as described herein which contains water, graphene nanoplatelets and graphene oxide nanoplatelets, wherein the concentration of graphene nanoplatelets in the additive is from 29 mg.ml1 to 150 mg.ml1 and the concentration of graphene oxide nanoplatelets in the additive is from 1 mg.mH to 50 mg.mH. In this mix, the cementitious material is suitably used in an amount 3-80 wt%. It will be recognised that many ratios of ingredients are known in the art for producing different cement-bound (cement-based) products, for example concretes, mortars, screeds and so on. It will also be recognised that, where other ingredients such as aggregates are present, the weight content of cement may be relatively low. For example, where the compositions comprises an aggregate the composition might be only 3-40 wt% cementitious material, for example 5-30 wt%, for example 10-25 wt%; with the aggregate(s) making up around 40-95 wt%, for example 50-90 wt%, for example 60-80 wt%, of the composition. The water, including that water included in (that is, introduced into the mixture as a part of) the additive, is suitably used in a weight ratio (relative to weight of cementitious material; that is, water weight / cementitious material weight) of 0.3-0.8, for example 0.4-0.7, and typically 0.45-0.65 for example 0.5-0.6 or about 0.5 or about 0.55. The additive is suitably used in an amount appropriate to give a total graphene materials content of 0.005 to 0.5 wt%, relative to the content of cementitious material. It will be apparent that, while water is present in the additive as well as as a separate ingredient of the composition, the quantity of the additive included is small and so the overall content of water is barely affected by the content of water in the additive. It is therefore reasonable to include the water from the additive in the overall consideration of how much water is in the composition as a whole. Suitable cementitious materials include, for example, Portland Cement or Ordinary Portland Cement, fly ash, ground granulated blast furnace slag (GGBFS), silica fume and ground limestone. It will be recognised that further ingredients can be included in the present compositions as cement-based materials. A common such material is, for example, concrete. Concrete has been used for many hundreds of years for construction purposes. It is made by water, an aggregate of some type, and a cementitious material (usually cement). It is noted that, depending on the exact ones of these used, the mix may be given a different name from “concrete” in the art (for example, “mortar” is a word generally used where the aggregate is sand alone; “screed” is a word used where the aggregate is sharp sand alone). To the concrete mix, various additives can also be added. As explained herein, in the present invention one of those includes graphene and graphene oxide in certain amounts. It is well known in the art that the exact ingredients of a given concrete can be adjusted depending of the properties of the product wanted. For example, various types of aggregate may be useful in various circumstances; different combinations of aggregate and cements may give differing results; and different additives can impart different properties on the concrete itself. The relative amount of each ingredient also impacts the final properties of the concrete. The present additives are useful to improve the mechanical properties of substantially all concrete compositions, it is thought. Suitable aggregates include, for example, sand, gravel, glass, crushed stone (e.g. limestone or granite), and even recycled i.e. crushed concrete. Suitable weight ratios (cementitious material: aggregate) are well known in the art. For example, ratios of 1:0.5 to 1:20 are known; suitably 1:1 to 1:18, and most commonly 1:3 to 1:15. Typical approximate mixing ratios (by weight) of cementitious material and aggregate include 1:3; 1:4; 1:6; 1:9; 1:12; 1:15; 1:5; 1:5.5; and 1:6. These can be suitably used in embodiments of the present invention. It will be recognised that these ratios persists after curing. Sometimes a ‘coarse aggregate’ (particles of diameter about 9.5 mm - 37.5 mm) and a ‘fine aggregate’ (particles with diameter less than about 9.5 mm) are both included in the mix. Each can be independently selected. They may be the same material, with different levels of crushing / grinding. They may be different materials; for example, sand as the fine aggregate and something else such as gravel as the coarse aggregate. The ‘fine aggregate’ many made up of be several types of aggregate each of which is individually a fine aggregate; the ‘coarse aggregate’ content may be made up of several types of aggregate each of which is individually a coarse aggregate. By using a fine and a coarse aggregate, the packing density of aggregate particles is higher (that is, more aggregate can be present in a given volume), leading to a stronger concrete. Furthermore a smaller amount of cementitious material and water is needed to fill the voids between aggregate particles. Where a fine and a coarse aggregate are used, they may be used in a total content (weight) ratio of, for example, 1:0.5 to 1:4, for example 1:0.75 to 1:3, for example 1:1 to 1:2 (fine : coarse). Suitable ratios with respect to the weight of cementitious material used as described hereinabove. lOt can be recognised that these ratios persist after curing. Suitable optional further additives include, for example, water reducing agents, corrosion inhibiting agents, plasticisers, superplasticisers, air entrainment additives, surfactants, pigments and so on. Suitable superplasticisers include polycarboxylate ethers. Suitable air entrainment additives include MC Bauchemie Centrament Air 201. Suitable surfactants include non-ionic surfactants, cationic surfactants, anionic surfactants or non-ionic surfactants, such as Pluronic P-123, Tween 80, Brij 700, Gum Arabic from acacia tree, Triton X-100, Brij 30, Polyvinylpyrrolidone, n-Dodecyl p- D-maltoside I, Poly(sodium 4-styrenesulfonate), 3-[(3-Cholamidopropyl)dimethylammonio]-1-Propanesulfonate, Sodium deoxycholate, Sodium dodecylbenzene-sulfonate, 1-Pyrenebutyric acid, Sodium dodecyl sulphate, Sodium taurodeoxycholate hydrate, and Hexadecyltrimethylammonium bromide. The present additive can be added to the cement-based material mix during the batching process, to provide a dispersion of graphene and graphene oxide through the cement-based material composition batch. The resultant cured cement-based material formed has a substantially uniform dispersion of GNP and GO through it. Suitably that is at a significantly higher concentration than could have been made possibly using previously known techniques. Manufacture of Cement-based Material The general process for making cement-based materials is very well known. At its most basic it involves mixing the desired ingredients at the desired content ratios. The composition can then be formed into a desired shape. That might be, for example, by pouring into a mould or receptacle; or pouring onto a site. After this, the cement-based material composition cures (goes off). This hardens the composition. When mixing the ingredients, in general, the order of mixing is not so important. There is generally stirring, for example to encourage coating of aggregates with the cementitious material and to uniformly mix the other components. The present inventors have found that, where the present additive is used in concrete manufacture, the order of addition may have new importance. It may be suitable to add the additive to the initial water for the concrete batching - effectively a dilution. Then, the non-cementitious ingredients (e.g. the aggregate(s) and any other additives that are wanted) are added and mixed in. Finally, the cementitious material is added. It has been found that this order of addition leads to superior distribution of graphene and graphene oxide through the concrete; in particular, the addition before the cement is added means that the graphene and graphene oxide can be distributed by mixing before any reaction of / with the cement begins. Additive The present additive is a dispersion of graphene, and graphene oxide, in water. It may be characterised by the high levels of GNP that are contained within a stable dispersion, or by its stability overtime (that is, the time over which the graphene remains dispersed without significant agglomeration). Previous such dispersions have included significantly less GNP; and / or have been stable for significantly less time. One way of judging stability of the dispersion is to measure its viscosity at certain time intervals (as viscosity is affected by agglomeration and hence dispersion instability, a significant rise in viscosity indicates a loss of stability). For example, using a standard equipment such as a lab viscometer (for example the DV2T viscometer from AMETEK Brookfield, LV model) the viscosity of the dispersion can be measured directly after production (this may be referred to a Vo). Then, the viscosity can be measured by the same technique (same conditions; hence, the test for stability is independent of exactly what viscosity measurement technique or conditions are used) as a time ‘x’ hours later (this may be referred to as Vx). If the viscosity measured at time x is within about 35% of the viscosity originally measured (that is, if {([Vx / Vo]*1OO) -100} is less than or equal to 35) the dispersion can be said to be stable for at least ‘x’ hours. The viscosity measured at time x may preferably be within about 25% of the viscosity originally measured to demonstrate stability, more preferably within about 20% and most preferably within about 15%. The viscosity measurement may be performed at, for example, 30 rpm, 40 rpm, 50 rpm, 70 rpm, 90 rpm or 110 rpm, preferably at 40 rpm, at 21 °C. Stability (the above mentioned Vx being within about 35% of Vo, preferably about 25% and so on) need only be demonstrated at at least one of 30 rpm, 40 rpm, 50 rpm, 70 rpm and 90 rpm to be considered satisfied in the present invention. However more preferably it is demonstrated at at least two of those rpm counts, more preferably at least four of those rpm counts, and most preferably at all of those rpm counts. The dispersions described herein are preferably stable for at least 24 hours, more preferably at least 72 hours, yet more preferably at least 168 hours and most preferably at least 504 hours. An alternative, or additional, indicator of stability can be by conducting a visual inspection of the dispersion after heating for a certain time. For example, a dispersion may be said to be stable if it can be subjected to heating for at least 5 days at at least 40°C with no change in its visual appearance; for example subjected to heating for at least 7 days at at least 50°C with no change in its visual appearance. [A change of visual appearance here is indicative of the dispersion starting to separate.] The dispersion itself may have a variety of viscosities depending on the contents. It may be in the form of a fluid, or a gel. The present dispersions include (a) graphene nanoplatelets; (b) graphene oxide nanoplatelets; and (c) water, wherein the concentration of graphene nanoplatelets is from 29 mg.ml-1 to 150 mg.ml-1 and the concentration of graphene oxide nanoplatelets is from 1 mg.ml-1 to 50 mg.mH. Such a dispersion can then be added, as described herein, to other ingredients (in particular water and a cementitious material) to form a cement-based composition. These higher concentrations in the additive are, it is believed, enabled by the manufacturing methods described hereinbelow. The concentration of GNP may preferably be from 30 mg.ml-1 to 130 mg.mH, or 40 mg.ml-1 to 125 mg.mH, or 50 mg.mH to 120 mg.mH, and more preferably from 60 mg.mH to 100 mg.mH. The concentration of GO may preferably be from 3 mg.ml-1 to 47 mg.mH, or 5 mg.ml-1 to 45 mg.mH, or 10 to 42 mg.mH, and more preferably from 15 mg.ml-1 to 40 mg.mH. The higher loadings of GNP / GO make the use of graphene dispersion easier at the point of use as less needs to be added to reach a required loading and achieve a required response. For example at a concrete batching plant much less of the present dispersion has to be added within the process meaning existing hardware and software can be used, meaning no disruption to existing batching processes. No additional surfactant is needed in order to achieve a stable dispersion with these ingredients when the present methods are used. Therefore in some embodiments no further surfactant is included in the dispersion. In some embodiments the dispersion may be substantially free of sodium cholate (in such cases, the present cement-based composition may also be substantially free of sodium cholate). In some embodiments, the additive consists of (a) graphene nanoplatelets; (b) graphene oxide nanoplatelets; and (c) water, wherein the concentration of graphene nanoplatelets in the additive is from 29 mg.ml1 to 150 mg.ml1 and the concentration of graphene oxide nanoplatelets in the additive is from 1 mg.ml1 to 50 mg.mH, the balance being water. The total graphene material content of the present dispersions can be calculated by simple addition of the contents of all the graphene-based components present. For example, GNP content + GO content. It may be preferred that the total graphene material content is 30 mg.ml1 or greater. As it is understood by the inventors that having a greater amount of graphene material present helps deliver that material efficiently (smaller amount of dispersion needed to give the same addition of graphene materials), it is more preferable for the total graphene material content to be 40 mg.mH or greater, for example 50 mg.mH or greater, and most preferable for it to be 70 mg.mH or greater, for example 90 mg.mH or greater. As the density of the additive is, approximately, 1 kgm-3, this equates to a preferable total graphene material content of about 3.0 wt% or greater, 4.0 wt% or greater, 5.0 wt% or greater, 7.0 wt% or greater, or 9.0 wt% or greater respectively. It may be preferred that the total graphene material content is 200 mg.mH or lower (20.0 wt% or lower). The present inventors have found that dispersibility in certain systems can be affected by total graphene material content. So, it may be more preferable for the total graphene material content to be 150 mg.mH or lower (15.0 wt% or lower), for example 120 mg.mH or lower (12.0 wt% or lower), and most preferable for it to be 100 mg.ml-1 or lower (10.0 wt% or lower), for example 80 mg.ml-1 or lower (8 wt% or lower), 70 mg.ml-1 or lower (7.0 wt% or lower), 60 mg.ml-1 or lower (6.0 wt% or lower), 50 mg.ml-1 or lower (5.0 wt% or lower) or 40 mg.ml-1 or lower (4.0 wt% or lower). In some a particularly suitable embodiments total graphene material content may be 30-70 mg.ml1, that is 3.0 to 7.0 wt%, for example 30-40 mg.ml1, that is, 3.0 to 4.0 wt%. Stable dispersions including such concentrations of graphene material have not been known in the prior art. Nor have methods which could make such dispersions. The ratio of content of GNP and GO in the dispersion is also of interest. In some circumstances, it may be preferable to include, for example, less GO as a proportion of GNP; that is, to use a lower concentration ratio of graphene oxide nanoplatelets to graphene nanoplatelets, graphene oxide nanoplatelet concentration / graphene nanoplatelet concentration. That ratio is preferably from 0.025 to 1. [A ratio of 0.025 representing 1 part GO to 40 parts GNP; a ratio of 1 representing 1 part GO to 1 part GNP.] The ratio may in some embodiments be towards the lower end of this range. For example, it may be 0.025 to 0.5, more preferably 0.03 to 0.3, yet more preferably 0.04 to 0.2, and most preferably 0.05 to 0.1. In some other embodiments, a higher ratio may be used, for example 0.05 to 0.7, more preferably 0.1 to 0.25. In yet further embodiments, a yet higher ratio may be used, for example 0.5 to 1, more preferably 0.6 to 0.8. These ratios may also apply to the present cement-based compositions and materials made from them; while the absolute content of GNP and GO is much lower, the ratio of those is still as it was in the additive before mixing with the water, cementitious material and any other ingredients of the composition. It may be recognised that GO can act as a thickener, effectively increasing the viscosity of the dispersion. Accordingly, the amount of GO included may be moderated to achieve a desirable viscosity as well as desirable dispersion characteristics. Of course, where a higher viscosity is desired a higher content of GO can be used. While GNP and GO exist in various shapes and sizes, it has been found that the size of flakes used has some effect on the benefits provided by the additive. In particular a higher lateral size of GNP and a lower lateral size of GO have each individually been found to be beneficial. For example, it may be preferred for the GNP to have an average lateral flake dimension of greater than 1 pm, more preferably greater than 10 pm. Equally and separately, it may be preferred from the GO to have an average lateral flake dimension of less than 0.9 pm. The lateral flake dimension of a given flake can be measured by, for example, SEM. Both GNP and GO, as well as suitable precursors, are routinely supplied by manufacturers with information about the flake sizes contained. For example, where a GNP product says that the lateral flake dimension is less than 30 pm, it can be assumed that substantially all flakes within that product have a size smaller than 30 pm. [Generally the lateral flake “dimension” is the longest dimension of the flake. The “average” lateral flake dimension may suitably be the dso.] Manufacture of Additive It has previously been impossible to provide dispersions of the type described above, with GNP substantially uniformly dispersed within, in a stable form. The present inventors have found methods which allow just such dispersions to be formed, with stability radically improved as compared to previously described dispersions. The present inventors use high shear mixing to combine graphene oxide (or a precursor thereof), graphene nanoplatelets (or a precursor thereof) and water. More particularly, the methods involve (i) mixing graphene oxide nanoplatelets, or a graphene oxide precursor material, with water; (ii) mixing at high shear at at least 4000 rpm for at least 15 minutes; (iii) adding graphene nanoplatelets, or a graphene precursor material; and (iv) mixing at high shear at at least 4000 rpm for at least 15 minutes, wherein the steps may be performed in the order (i), (ii), (iii), (iv) or in the order (iii), (iv), (i), (ii). This use of high shear mixing is in contrast to the, for example, ultrasonication mixing techniques that have previously been deployed. Surprisingly it permits stable dispersions of much higher GNP (and GO) concentration to be formed. As the graphene oxide nanoplatelets used in step (i), there is no significant limitation to their form. The preferences and features discussed above with respect to the additive itself of course still apply. In some embodiments, it may be practical to provide the graphene oxide in the form of a wet cake; that may have a GO solids content of 30 wt% - 60 wt%. However it will be appreciated that the important value is how much GO is mixed with how much water, to give a final concentration of GO, rather than the form in which it is before addition. As an alternative, a graphene oxide precursor can be used in the present methods. Suitable precursors include, for example, graphite oxide or other materials which can form graphene oxide by exfoliation. The inventors believe that such precursors can be exfoliated by the high shear mixing in step (ii), leading to the effective formation of graphene oxide nanoplatelets in situ during mixing. As the graphene nanoplatelets used in step (iii), there is again no significant limitation to their form. The preferences and features discussed 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. Again, the amount of graphene added in step (iii) is suitably chosen to provide the desired final concentration in the additive, and in concert with the amount of graphene oxide added in step (i) to provide the desired concentration ratio of graphene oxide and graphene. As an alternative, a graphene precursor can be used in the present methods. Suitable precursors include, for example, graphite or other materials which can form graphene by exfoliation. The inventors believe that such precursors can be exfoliated by the high shear mixing in step (ii), leading to the effective formation of graphene nanoplatelets in situ during mixing. Mixing at at least 4000 rpm, under high shear, has been found to provide a stable dispersion; performing the mixing for at least 15 minutes in each mixing step reinforces that. It will of course be apparent that higher mixing rates can be used, usually in the range 4000-20000 rpm, for example 5000-8000 rpm. The high shear mixing may preferably (in one or both of steps (ii) and (iv)) be performed under conditions such that the shear rate is at least 1.5x104 s'1, preferably at least 2x104 s'1, for example at at least 4x104 S'1. In preferred embodiments, the mixing time in each of steps (ii) and (iv) may independently be at least about 30 minutes, for example at least about 45 minutes, or more preferably at least about 1 hour. Suitably, mixing in each of steps (ii) and (iv) is performed for about 1 hour. The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof. While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above 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 invention. For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations. Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described. Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. It must be noted that, as used in the 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 the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example + / -10%. Examples REFERENCE EXAMPLES - additive REFERENCE EXAMPLE 1 - dispersion Graphite oxide was obtained from The 6th Element (https: / / www.c6th.com / ), product SE2430W-N, as a wet cake with approximately 45% solids content. GNP were sourced from Versarien, product GNP-HP. 69g of the graphite oxide wet cake was placed into a glass beaker, and approximately 700ml of tap water added to it. The mixture was then mixed in a high shear mixer for approximately 1 hour (Silverson high shear mixer with a Square Hole High Shear Screen™ at 5000 rpm). After this mixing, 69g of the GNP powder was added in four aliquots, with approximately 30 seconds of manual mixing between additions. Once all the GNP had been added, the final mixture was then mixed in a high shear mixer for approximately 1 hour (Silverson high shear mixer with a Square Hole High Shear Screen™ at 5000 rpm). The resulting mixture (apx 1 litre) was found to be stable under standard conditions for more than 6 months. REFERENCE EXAMPLE 2 - stability testing Reference Example 2a Following the general experimental method set out in Reference Example 1, a dispersion was made including 7% total graphene concentration (70 mg.mH), with a GO:GNP concentration ratio of 0.25 (that is, 4:1 in terms of GNP:GO). Viscosity (cP) of the resultant dispersion was measured using a DV2T viscometer from AMETEK Brookfield, LV model, at a variety of different rpm counts. Viscosity profiles at day 7 and day 21 were measured. Despite the long time between the readings, the viscosity profile of the dispersion did not significantly change, indicating a long term stability of the dispersion. Viscosity was broadly stable (within about 15%) for each of 30 rpm, 40 rpm, 50 rpm, 70 rpm and 90 rpm. Reference Example 2b Following the general experimental method set out in Reference Example 1, a dispersion was made including 10% total graphene concentration (100 mg.ml-1), with a GO:GNP concentration ratio of 0.25 (that is, 4:1 in terms of GNP:GO). Viscosity (cP) of the resultant dispersion was measured using a DV2T viscometer from AMETEK Brookfield, LV model, at a variety of different rpm counts. Viscosity profiles at day 3 and day 7 were measured. Despite the long time between the readings, the viscosity profile of the dispersion did not significantly change, indicating a long term stability of the dispersion. Viscosity was broadly stable (within about 15%) for each of 30 rpm, 40 rpm, 50 rpm, 70 rpm and 110 rpm. EXAMPLE 1 - concrete Samples of a concrete made according to the following procedure, based on BS 1881-125:2013. In a cement mixer: 1. Fine and coarse aggregates are added to the mixer as thus; a. Approximately half of the coarse aggregate b. The fine aggregate c. The remaining coarse aggregate 2. Mix for 30-45 seconds to blend the aggregates. 3. Continue mixing while gradually adding about 2 / 3 of the water, mixed with the graphene-based admixture (GBA), over the next 15 seconds. 4. Mix the contents for 2 to 3 minutes, then stop the mixer and allow the mixture to rest for 5-15 minutes to allow thorough wetting of the aggregates. 5. Add the cementitious material to the mixture and mix for an additional 30 seconds. 6. Add the remaining 1 / 3 of the mixing water over the next 30 seconds. 7. Continue mixing for another 2-3 minutes until the concrete is thoroughly combined. 8. Mixture was poured into relevant mould for compression, flexural and tensile testing. The fine aggregate used was sharp sand purchased from Jewsons. The coarse aggregate used was a mixture of 10 mm crushed limestone and 20 mm crushed limestone purchased from Jewsons. The cementitious material used was CEM I 52.5 N (Portland Cement) purchased from Hanson. The graphene-based admixture was an additive as described herein, comprising water, graphene and graphene oxide, the total content of graphene + graphene oxide (total graphene material content) being 4 wt% of the additive made up of graphene (80% of the total, i.e. 3.2 wt% of the additive)) and graphene oxide (20% of the total, i.e 0.8 wt% of the additive) at a ratio of 0.25 (1:4, GO: GNP), the balance (i.e. 96 wt% of the additive) being water. The mix ratio of the ingredients was as follows: CEM I 52.5 N: 335 Fine aggregate (sharp sand): 790 Coarse aggregate (10 mm): 566 Coarse aggregate (20 mm): 560 Water to cement ratio: 0.55 with the graphene-based admixture being added in an amount to give 0.1 wt% total graphene based materials (graphene and graphene oxide) to 100 wt% of the cementitious material; its water content was included in the calculation for the amount of water to add to the mixture. Compressive, tensile and flexural strength testing was carried out according to BS EN 12390, parts 3 (published 16 July 2019), 4 (published 10 October 2019), 5 (published 19 July 2019) and 6 (published 30 June 2010). Fig. 1 shows compressive strength of three samples of concrete made according to the above method (GBA-R1, GBA-R2, GBA-R3, uppermost three lines) versus a control concrete (lowermost line) made using the same method and ingredients except excluding the graphene / graphene oxide-containing additive. It can be seen that after about two days’ curing, the compressive strength of each GBA sample is higher than the control (30-33 MPa, as compared to about 27 MPa). Over the next few days compressive strength increases at a rate higher than that for the control, such that after 28 days the compressive strength is yet further enhanced (47-52 MPa, as compared to about 37 MPa). Importantly, it can be seen that the samples of the present Example, GBA, reach the maximum compressive strength achieved by the control sample (about 37 MPa) after only 4-5 days, as opposed to about 14 days for the control sample. This represents a huge increase in the speed to useable strength. Fig. 2 shows tensile strength of three samples of concrete made according to the above method (GBA-R1, GBA-R2, GBA-R3, uppermost three lines) versus a control concrete (lowermost line) made using the same method and ingredients except excluding the graphene / graphene oxide-containing additive. It can be seen that after about two days’ curing, the tensile strength of each GBA sample is higher than the control (3.4-3.8 MPa, as compared to about 3.1 MPa). Over the next few days tensile strength increases at a rate higher than that for the control, such that after 28 days the tensile strength is yet further enhanced (4.4-5.2 MPa, as compared to about 3.6 MPa). Importantly, it can be seen that the samples of the present Example, GBA, reach the maximum tensile strength achieved by the control sample (about 3.6 MPa) after only 2-4 days, as opposed to about 10 days for the control sample. This represents a huge increase in the speed to useable strength. Fig. 3 shows flexural strength of three samples of concrete made according to the above method (GBA-R1, GBA-R2, GBA-R3, uppermost three lines) versus a control concrete (lowermost line) made using the same method and ingredients except excluding the graphene / graphene oxide-containing additive. It can be seen that after about two days’ curing, the flexural strength of each GBA sample is higher than the control (5.1-5.3 MPa, as compared to about 4.7 MPa). Over the next few days flexural strength increases at a rate higher than that for the control, such that after 28 days the flexural strength is yet further enhanced (6.4-7.0 MPa, as compared to about 5.7 MPa). Importantly, it can be seen that the samples of the present Example, GBA, reach the maximum flexural strength achieved by the control sample (about 5.7 MPa) after only 10-14 days, as opposed to about 28 days for the control sample. This represents a huge increase in the speed to useable strength. EXAMPLE 2 - air entrained concrete A sample of air entrained concrete was made, following the method, ingredients and mix ratios set out in Example 1 except that the water added in steps 3 and 6 was mixed not only with the graphene-based additive but also with an air entrainment admixture. The air entrainment admixture was Centrament Air 201 purchased from MC-Bauchemie. It was added in an amount 0.26 wt% with respect to the content of cementitious material. Compressive strength testing was carried out according to BS EN 12390, parts 3 (published 16 July 2019) and 4 (published 10 October 2019). Fig. 4 shows compressive strength of air entrained concrete made according to the above method (GBA1, uppermost line) versus a control air entrained concrete (lowermost line) made using the same method and ingredients except excluding the graphene / graphene oxide-containing additive. It can be seen that, after about two days’ curing, compressive strength is significantly higher (over 30%), and that 5 over the next about six days of curing that enhancement remains. The absolute improvement in compressive strength increases over the time of curing, from 6.2 MPa to 7.5 MPa. The compressive strength of the sample according to the present invention, GBA-1, at about day 2 is higher than the compressive strength achieved by the control sample after about 10 days. 10

Claims

1. A composition comprising a cementitious material, water, graphene nanoplatelets and graphene oxide nanoplatelets,wherein the total graphene material content of the composition (graphene nanoplatelets + graphene oxide nanoplatelets) is from 0.005 to 0.5 wt% relative to the content of cementitious material, and wherein the concentration ratio of graphene oxide nanoplatelets to graphene nanoplatelets is from 0.025:1 to 1:1.

2. A composition according to claim 1, wherein the graphene nanoplatelets and graphene oxide nanoplatelets are substantially uniformly distributed through the composition.

3. A composition according to claim 1 or claim 2, wherein the composition comprises the cementitious material in an amount 3-80 wt%, preferably 5-30 wt%.

4. A composition according to any one of claims 1-3, wherein the composition comprises water in a weight ratio relative to weight of cementitious material of 0.3-0.8, preferably 0.45-0.65.

5. A composition according to any one of the preceding claims, further comprising aggregate, optionally in an amount 40-95 wt%, preferably 50-90 wt%.

6. A composition according to claim 5, wherein the aggregate is made up of coarse aggregate and fine aggregate in a weight ratio fine : coarse of 1:0.5 to 1:4.

7. A material obtainable by curing a composition according to any of the preceding claims.

8. A method of forming a composition, the method comprising mixing a cementitious material, water and an additive comprising:(a) graphene nanoplatelets;(b) graphene oxide nanoplatelets; and(c) water,wherein the concentration of graphene nanoplatelets in the additive is from 29 mg.mH to 150mg.mH and the concentration of graphene oxide nanoplatelets in the additive is from 1 mg.mH to 50 mg.mH.

9. A composition obtainable by a method as set out in claim 8.

10. A material obtainable by curing a composition according to claim 9.

11. A method of forming a hardened material comprising forming a composition by a methodaccording to claim 8 and then curing the composition.

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