Method of manufacture of a composite

Abstract

A method of manufacturing a composite comprises mixing water, an inorganic compound selected from calcium sulphate, calcium carbonate, calcium phosphate, and combinations of these, and a solubility ag

Description

Field of the Invention The present invention relates to a method of manufacture of a composite, in particular a composite material with ceramic-like properties. The invention further relates to a composite article manufactured by said method, and in particular a tile. Background Ceramic tiles are extensively used in construction for both residences and commercial buildings. They are suitable for a range of applications, and can typically be found covering walls and floors, as well as other decorative features. Aside from aesthetic appeal, ceramic tiles may be desirable for their strength properties, moisture and water resistance, low flammability, provision of thermal and acoustic insulation, lack of smell, ease ofcleaning and low maintenance requirements. Conventionally, ceramics are a mixture of clays and other materials such as sand, quartz, and water. Manufacture of ceramic tiles typically involves pulverizing the raw materials, weighing and mixing the chosen proportions of each raw material, pressing the mixture, drying, and optionally glazing. This is then followed by sintering at extremely high temperatures, for example, by firing in a kiln at 1100-1300 °C. This means that tile production has a high environmental impact, primarily due to the high CO2 emissions and intensive energy requirements of the sintering / firing stage. Sintering is the process whereby a loosely consolidated material is converted into a solid though the application of heat to a temperature significantly below its melting point, and is employed to produce ceramic materials such as bricks and tiles. Sintering occurs when sufficient temperatures are reached to mobilise active elements in the surface of a material; surface elements are less thermodynamically stable than elements in bulk material and hence mobilise at temperatures lower than the melting point. The mobile surface elements rearrange in order to minimise their surface free energy, which results in drive to reduce surface area which is achieved through the fusion of adjacent particles. This results in a solid mass (e.g., a ceramic piece). A macroscopic analogy can be observed when a bag of ice cubes is allowed to defrost slightly before being frozen again - resulting in a solid mass. There is a need for alternative materials with ceramic-like properties which can be manufactured more efficiently by using less energy intensive methods, such as alternative sintering processes, and with sustainable raw materials. Known cold sintering methods for manufacturing materials with ceramic-like properties often require pressing at very high pressures and / or require heating during the pressing step (hot-pressing). This makes it difficult to adapt conventional ceramic manufacturing processes to incorporate cold sintering. The present invention has been devised in light of the above considerations. Summary of the Invention The present inventors have developed a method of manufacturing a composite with ceramic-like properties which does not require a high-temperature sintering or firing step. In a first aspect, the present invention provides a method of manufacturing a composite comprising the steps of: (i) mixing (A) an inorganic compound selected from calcium sulphate, calcium carbonate, calcium phosphate, and combinations thereof, with (B) a solubility agent selected from an organic acid, an organic acid salt, and a combination thereof, and (C) water to form a mixture, (ii) compressing the mixture at 10-300 MPa, (ii) drying the mixture at 35-200 °C. Advantageously, this method produces a composite with good ceramic-like properties. Without being bound by any theory, it is believed that the combination of the inorganic compound with a solubility agent and water leads to partial dissolution of the calcium salt. This is thought to be due to stabilisation of the calcium ions by the solubility agent. The subsequent compression densifies the material and removes voids and cavities. In combination with drying, this leads to precipitation of the dissolved calcium salt - as the water is driven off the saturation solubility is exceeded. It is thought that the existing solid calcium salt particles serve as nucleation sites for the precipitation of the dissolved calcium causing the particles grow and mesh together to form a unitary composite. Thus, the method of the present invention induces a particle-fusion mechanism which binds the calcium salt particles together into a solid composite with ceramic-like properties (a “cold sintering” process). The method does not require particularly high pressures during the compression step or high temperatures during the drying stage. Particularly advantageously, no firing step is required. Thus, a composite with ceramic-like properties can be manufactured with much greater energy efficiency and lower carbon emissions than conventional ceramics. Further, the method is compatible with conventional tile manufacturing processes and machinery, as the mixing, pressing and drying steps correspond to the conventional process and no firing step is required. The pressures involved can be achieved in a conventional ceramic manufacturing process and heating is not required during the pressing step. Thus, current tile manufacturers are able to easily adapt to manufacturing composites by the method of the present invention. The method also allows use of alternative and more sustainable raw materials to produce ceramic-like materials. The method is compatible with waste products such as waste plaster-derived gypsum and byproducts from other industries such as cream of tartar, which are also inexpensive. In a second aspect, a composite article is provided, for example one manufactured according to the method of the first aspect is provided. In some examples the composite article is a tile. The composite prepared according to the present invention has good ceramic-like properties making it useful for a wide range of applications similar to conventional ceramic tiles. Thus the composite according to the present invention may provide a more sustainable alternative to conventional ceramic tiles. Advantageously, the composite prepared according to the present invention displays remarkably little shrinkage during fabrication (meeting the requirements for Bill classification tiles), whereas some ceramic tiles undergo significant shrinkage during firing. Minimal shrinkage improves tile installation and quality of tiling finish, and standards are in place to ensure homogeneity. Significant shrinkage can cause breakages or deformities in tiles that result in their rejection. For Bill classification tiles, the standards are as follows: tile length and width (±0.75%), thickness (±10%), straightness of sides (±0.3%), rectangularity (±0.5%), flatness (+0.5, -0.3%) and warpage (±0.5%). The composites prepared according to the present invention may have no significant deviation in dimensions observed during fabrication, thus satisfying these criteria. This enables a low tile rejection / line-breakage rate, improving economics of production. The tiles of the present invention meet the standard minimum breaking strength to assure good performance, which is particularly important in flooring applications. For Bill classification tiles, the minimum breaking strength is >200 N for tiles <7.5 mm in thickness. The composite material prepared according to the present invention is able to satisfy these criteria. For example, tiles according to the present invention have attained a breaking strength of 393 N at <7.5 mm in thickness. In addition to breaking strength, the present invention may provide tiles with a satisfactory ultimate flexural strength (also known as modulus of rupture). For Bill classification tiles, a modulus of rupture of 12 MPa (or 12 N / mm2) or greater is required. For example, the composites prepared according to the present invention have achieved values of 30.7 MPa, comfortably exceeding this threshold. Water absorption is another important characteristic, which may impact the type of adhesion and hence selection of materials for tile installation technique. Water absorption is also related to frost resistance for tiles applied in environments vulnerable to frost. Bill classification tiles may have >10% water absorption, with more stringent classifications requiring <10%. The composite material prepared according to the present invention may have water absorption values of at most 30%, at most 25%, at most 20%, at most 15%, at most 10%, or at most 9%. For example, the water absorption value may be 8.7%. Bill classification tiles have no formal requirement for hardness or abrasion resistance. The composites prepared according to the present invention may have a hardness of at least 2 on the Mohs scale. Application of a coating can improve the hardness and abrasion resistance of the composites prepared according to the present invention. Chemical resistance is important in determining the suitability of the application of the tile and is linked to prevention of impairment of the composite surface (discolouration, surface corrosion etc.). The chemical resistance of composites prepared according to the present invention are affected by the chemical resistance of the inorganic component. For instance, tiles prepared with relatively inert calcium sulphate may have a higher chemical resistance than those prepared with calcium carbonate which may be reactive with acids. Application of a protective coating can enhance chemical resistance to Class A, the highest class. Similarly to chemical resistance, stain resistance is linked to cleanability and resistance to impairment of appearance as a result of absorption of substances that impair the appearance of the tile. With a protective coating or sealant the materials may have Class 5 stain resistance, the highest class. Particularly advantageously relative to ordinary ceramic tiles, since the composites prepared according to the present invention avoid the need for high-temperature firing, a wide range of pigments and dyes can be incorporated within the material (either embedded on the surface or incorporated within the body) to provide colouration and improve the aesthetic appeal. Pigments can be employed that would otherwise not survive the-high temperature conditions of conventional ceramic tile firing, making them unsuitable for ordinary ceramic tiles. 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 a plot of flexural strength experimental results against strain for a composite manufactured according to the method of the present invention. Figure 2 shows scanning electron microscopy (SEM) images of the surface (top row) and cross-section (bottom row) of a composite manufactured according to the method of the present invention. Figure 3 shows a plot of flexural strength experimental results against strain for a composite manufactured according to the method of the present invention. Figure 4 shows a plot of flexural strength experimental results against strain for a composite manufactured according to the method of the present invention. Figure 5 shows a plot of ultimate flexural strength experimental results against concentration of additive for composites manufactured according to the method of the present invention. Figure 6 shows a plot of ultimate flexural strength experimental results against forming pressure for composites manufactured according to the method of the present invention. Figure 7 shows a plot of ultimate flexural strength experimental results for composites manufactured according to the method of the present invention with different solubility agents and a comparative example without any solubility agent. Figure 8 shows a plot of flexural strength experimental results against strain for a composite manufactured according to the method of the present invention. Figure 9 shows a plot of flexural strength experimental results against strain for a composite manufactured according to the method of the present invention. Detailed Description of the Invention Aspects and embodiments of the present invention will now be discussed. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference. In a first aspect the present invention provides a method of manufacturing a composite comprising the steps of: (i) mixing (A) an inorganic compound selected from calcium sulphate, calcium carbonate, calcium phosphate, and combinations thereof, with (B) a solubility agent selected from an organic acid, an organic acid salt, and a combination thereof, and (C) water to form a mixture, (ii) compressing the mixture at 10-300 MPa, (iii) drying the mixture at 35-200 °C. Inorganic compound (A) In some examples, the inorganic compound (A) is selected from calcium sulphate, calcium carbonate, calcium phosphate, and combinations thereof. As used herein “calcium sulphate” refers to the inorganic compound with the formula CaSO4, and hydrates thereof. As used herein “calcium carbonate” refers to the inorganic compound with the formula CaCOj, and hydrates thereof. As used herein “calcium phosphate” refers to the family of inorganic compounds containing calcium ions together with inorganic phosphate anions, and hydrates thereof, for example Ca3(PO4)2. In some examples, the inorganic compound (A) is a combination of calcium sulphate, calcium carbonate, and calcium phosphate. In some examples, the inorganic compound (A) is a combination of calcium sulphate and calcium carbonate. In some examples, the inorganic compound (A) is a combination of calcium sulphate and calcium phosphate. In some examples, the inorganic compound (A) is a combination of calcium phosphate and calcium carbonate. In some examples, the inorganic compound (A) is selected from calcium sulphate, calcium carbonate, and calcium phosphate. Without being bound by any theory, the solubility behaviour of these compounds enables partial dissolution in water when the inorganic compound is mixed with a solubility agent, but the saturation solubility of the solution can easily be exceeded by driving off water in the drying step. In particular, it is thought that the calcium ions are stabilised by the chosen solubility agent to achieve dissolution and reprecipitation of the inorganic compound at desirable concentrations. Calcium carbonate, calcium phosphate, and calcium sulphate are also readily and cheaply available. In some examples the inorganic compound is calcium sulphate (or a hydrate thereof). Advantageously calcium sulphate is less reactive with acids than, for example, calcium carbonate, so any reaction with the solubility agent is minimised. In some examples the inorganic compound is a calcium sulphate hydrate. Preferably, the inorganic compound is calcium sulphate dihydrate (CaSO4-2H2O). This is naturally occurring as gypsum. Gypsum is also available as a significant waste product from the construction industry. Waste gypsum may include excess or used gypsum articles, for example, gypsum boards or blocks. Waste gypsum can otherwise be difficult to recycle or dispose of. Thus, waste gypsum may be preferred for the inorganic compound (A) as it is cheap, and its use enables ceramic-like materials to be produced from a recycled waste product. In particular, gypsum is used in slip-casting, a common process for producing shaped ceramic articles from clay (e.g., tableware, sanitaryware). Waste plaster-derived gypsum (waste gypsum from the ceramics industry) may be particularly preferred. Waste plaster-derived gypsum may be gypsum discarded after use in slip-casting, gypsum obtained from used slip-casting moulds, or gypsum obtained from discarded plaster casts. Without being bound by any theory, it is thought that the slip-casting process draws out species present in the clay, for example, metal salts such as magnesium chloride. These additional species may interact with the calcium sulphate to influence factors such as the solubility. Additionally, various additives such as aluminosilicates may be added to the plaster moulds used in slip casting for beneficial effects such as to reduce shrinkage during setting of the mould. Such additives may also confer beneficial effects such as minimising shrinkage if they are incorporated into the composite of the present invention. Thus, when waste-plaster derived gypsum is used as the inorganic compound (A) the properties of the composite produced by the method of the present invention may be enhanced. In some examples, the amount of inorganic compound (A) used is 70-98 wt.% relative to the total weight of the components (A), (B), and (C) added to the mixture. In some examples, the amount of inorganic compound (A) is 85-97 wt.%. The amount may be at least 70 wt.%, 80 wt.%, 85 wt.%, 90 wt.%, or 93 wt.%. The amount may be at most 99 wt.%, 98 wt.%, 97 wt.%, 96 wt.%, 95 wt.%. In some examples, the inorganic compound (A) is 85-99.5 wt.% of the total weight of the components (A) and (B), that is, the “dry” components (this includes water of crystallisation or hydration present in the components (A) and (B) but does not include the additional “free” water of component (C)). The inorganic compound (A) may be 95-99.5 wt.% of the total weight of the components (A) and (B), that is, the “dry” components (including hydrates but not including additional water). Solubility agent (B) In some examples, the solubility agent (B) is an organic acid, an organic acid salt, or a combination thereof. Without being bound by any theory, it is thought that the solubility agent enhances the solubility of the inorganic compound by stabilising the calcium ion, to enable partial dissolution of the inorganic compound in the water. In some examples, the solubility agent comprises one or more selected from tartaric acid, glutamic acid, aspartic acid, succinic acid, malic acid, oxalic acid, ethylenediaminetetraacetic acid (EDTA), and salts thereof. In some examples, the solubility agent is selected from tartaric acid, glutamic acid, aspartic acid, succinic acid, malic acid, oxalic acid, ethylenediaminetetraacetic acid (EDTA), and salts thereof. In some examples, the solubility agent is selected from tartaric acid, glutamic acid, aspartic acid, succinic acid, malic acid, oxalic acid, ethylenediaminetetraacetic acid (EDTA), sodium salts thereof, and potassium salts thereof. In some examples, the solubility agent comprises an organic diacid or salt thereof. In some examples, the solubility agent consists of an organic diacid or salt thereof. In some examples, the solubility agent comprises an organic diacid, a sodium salt thereof, or a potassium salt thereof. In some examples, the solubility agent consists of an organic diacid, a sodium salt thereof, or a potassium salt thereof. In some examples the solubility agent comprises tartaric acid, or a tartrate salt. In some examples the solubility agent consists of tartaric acid, or a tartrate salt. In some examples the solubility agent comprises tartaric acid, a sodium tartrate salt, or a potassium tartrate salt. In some examples the solubility agent consists of tartaric acid, a sodium tartrate salt, or a potassium tartrate salt. In some examples the solubility agent comprises a potassium tartrate salt. In some examples the solubility agent consists of a potassium tartrate salt. In some examples the solubility agent comprises potassium bitartrate (cream of tartar). In some examples the solubility agent consists of potassium bitartrate (cream of tartar). Cream of tartar may be preferred as a solubility agent because, without being bound by any theory, it is thought that it stabilises the calcium ions of the inorganic compound (A) to provide a desirable level of solubility when present in relatively small amounts. Cream of tartar is also a significant byproduct of the wine industry. Thus, using cream of tartar as the solubility agent keeps manufacturing costs low. In some examples, the amount of solubility agent (B) mixed in step (i) is 0.5-20 wt.% relative to the total weight of the components (A), (B), and (C) prior to mixing. In some examples, the amount of solubility agent (B) is 0.5-4 wt.%. The amount may be at least 0.5%, 1 wt.%, 1.5 wt.%, 2 wt.%, 2.5 wt.%, or at least 3 wt.%. The amount may be at most 30 wt.%, 20 wt.%, 10 wt.%, 7 wt.%, 5 wt.%, 4 wt.%, or at most 3.5 wt.%. The solubility agent (B) may be 0.5-4 wt.% of the total dry ingredients mixed in step (i) (that is, the total weight of all the components excluding water). Using lower concentrations of solubility agent (B) may reduce the overall costs of the manufacturing process. Water (C) In some examples, enough water (C) is mixed with the inorganic compound (A) and the solubility agent (B) in step (i) to partially dissolve the inorganic compound (A). Without being bound by any theory, it is thought that during the drying step (iii) the amount of liquid water in the mixture decreases, and the concentration of inorganic compound (A) dissolved in the remaining liquid water exceeds the saturation concentration so precipitation of the inorganic compound (A) occurs, binding the mixture into one solid composite article. The water (C) may be mixed simultaneously with the inorganic compound (A) and solubility agent (B). The water (C) may be added to a pre-mixture of the inorganic compound (A) and solubility agent (B). The water (C) may be mixed with the solubility agent (B) before the inorganic compound (A) is added. The water (C) may be pure water, e.g., distilled water or deionised water. The water (C) may be tap water. Tap water may be “soft” or “hard”. “Soft” water may contain less than 100 ppm of mineral deposits e.g. calcium and magnesium salts. “Hard” water may contain more than 100 ppm of mineral deposits e.g. calcium and magnesium salts. In some instances, the use of “hard” water is beneficial due to the presence of calcium and magnesium salts. The amount of water (C) added to the mixture in step (i) may be at least 1 wt.%, 2 wt.%, 3 wt.%, or 4 wt.% of the total weight of the components (A), (B), and (C) prior to mixing. The amount may be at most 20 wt.%, 15 wt.%, 10 wt.%, 7 wt.%, or 5 wt.%. The amount of water (C) may be chosen to ensure that enough inorganic compound (A) dissolves to bind the remaining particles together on reprecipitation. The amount of water (C) may be limited to provide a mixture with a good consistency for the compression step (ii) and reduce the energy required to remove the water during the drying step (iii) to improve efficiency of the method. In some examples, the amount of water (C) added may be chosen to provide a hydration of 2-15%, 2-10%, 2-6%, wherein hydration is defined according to formula (1). The amount of water (C) added may be chosen to provide a hydration of 2.5-4.5%. Hydration (%) = Mass of water (g) Total Mass (g) x 100 (1) The “total mass” of formula (1) is the total mass of all the components of the mixture including water (C). Note that this does not take into account any water of crystallisation present in either (A) or (B). Additives In some examples, additional components are included in the mixture. For example, in addition to the solubility agent (B), the mixture may comprise a metal chloride salt. In some examples, the metal chloride salt is selected from magnesium chloride, potassium chloride, sodium chloride, and combinations thereof. Without being bound by any theory, it is thought that when mixed with the inorganic compound (A) and water, the chloride ions stabilise the calcium ion of the inorganic compound (A) to increase the solubility. The chloride ions may help stabilise the hydroxo-aqua calcium ion. [CaOW(W2O)5]+ + cr [CaCl(H20)5]+ + OH~ Thus, the inclusion of a metal chloride salt may improve the properties of the composite produced, e.g. the ultimate flexural strength. In some examples, chloride ions or metal chlorides present from tap water or from impurities present in the waste plaster may have a beneficial effect. In some examples, the mixture does not comprise a metal chloride salt. In some examples, the mixture includes 0.01-5 wt.% based on the weight of the inorganic compound (A) of coloured pigment(s) for colouring of the final composite. In some examples, 0.01-5 wt.% of the inorganic compound (A) is replaced with coloured pigment(s). The coloured pigment(s) are not particularly limited but may include organic pigments, inorganic pigments, or mixtures thereof. For example, iron oxide or other coloured inorganic salts may be used as pigments. Advantageously, the pigments do not need to be stable at high firing temperatures as is the case for conventional ceramic tiles, meaning a much wider range of pigments can be employed. In some examples, other additives are included in the mixture during step (i) to adjust the appearance, texture or other properties of the composite for specific applications. Step (i) - Mixing Without being bound by any theory, it is thought that mixing the inorganic compound (A), the solubility agent (B), and the water (C) causes partial dissolution of the inorganic compound (A) in the water, with the calcium ions being stabilised by the solubility agent (B). In some examples, the inorganic compound (A), the solubility agent (B), and the water (C) are mixed simultaneously. In some examples, the inorganic compound (A) and the solubility agent (B) are mixed in a first stage, then water (C) is added and a second stage of mixing is performed. In some examples, the inorganic compound (A) is mixed with water (C) in a first stage, and the solubility agent (B) is mixed in a second stage. In some examples, the solubility agent (B) is mixed with water (C) in a first stage, and the inorganic compound (A) is mixed in a second stage. In some examples, the inorganic compound (A), the solubility agent (B), and the water (C) are mixed for at least 5 seconds, at least 10 seconds, or at least 15 seconds. In some examples, the inorganic compound (A), the solubility agent (B), and the water (C) are mixed with a high shear mixer to achieve a uniform distribution. In some examples, the inorganic compound (A) and the solubility agent (B) are blended or mixed to form a mixed powder in a first stage. They may be blended for at least 10 seconds. In some examples, after mixing of the (A), (B) and (C) components, the mixture is allowed to rest in a sealed vessel ensure equilibration of the hydration throughout the mixture. The mixture may be allowed to rest for at least 6 hours, or at least 12 hours. In some examples, the inorganic compound (A) is a coarsely ground aggregate. In some examples, the inorganic compound (A) is ground into a particulate or powder form before being mixed with the solubility agent (B). In some examples, the solubility agent (B) is ground into a particulate or powder form before being mixed with the inorganic compound (A). In some examples, the inorganic compound (A) and solubility agent (B) are ground together. In some examples, the inorganic compound (A) and / or the solubility agent (B) are pulverised during the mixing step. In some examples the inorganic compound (A) is provided for step (i) as a powder. In some examples, the solubility agent (B) is provided for step (i) as a powder. In some examples the solubility agent (B) is provided in solution with the water (C). In some examples, the mixture formed comprises 70-98 wt.% of inorganic compound (A), 0.5-20 wt.% of solubility agent (B), and 1.5-20 wt.% of water (C). In some examples, the mixture formed comprises 85-97.5 wt.% of inorganic compound (A), 0.5-10 wt.% of solubility agent (B), and 2-15 wt.% of water (C). In some examples, the mixture formed comprises 90-97.5 wt.% of inorganic compound (A), 0.5-4 wt.% of solubility agent (B), and 2-7 wt.% of water (C). In some examples the mixture of step (i) comprises a combination of solid and liquid phases. Step (ii) - Compression The mixture is compressed to compact and densify the material and remove any voids or cavities. Without being bound by any theory this is thought to encourage binding of the inorganic compound particles on precipitation as the existing solid inorganic compound particles grow and mesh together. Compaction of the mixture is important to achieve desirable strengths in the product composite. Compaction is also desirable to minimise voids and cavities, increasing density and reducing porosity (hence influencing water absorption). The compression step may be used to achieve the desired size and geometry of the final composite article. For example, the mixture may be placed in a mould during compression. Alternatively or additionally, the final material may be shaped after the compression step, for example, by cutting the material to shape. In some examples, pressure is applied to the mixture uniaxially. The mixture may be pressed in a rigid die or flexible mould. The pressure may be applied by a hydraulic press or friction press. In some examples, the compression step is analogous to pressing in the production of a conventional ceramic tile. In this way, new machinery is not required to make the composite of the present invention, so it is easy for manufacturers to convert from conventional ceramic tile manufacturing. Advantageously, application of heat is not needed during pressing (cold pressing). Pressure is only required to be applied for a short time. Pressure may be applied for at least 2 seconds, at least 5 seconds, or at least 8 seconds. Pressure may be applied for at most 30 seconds, at most 20 seconds, at most 15 seconds, or at most 10 seconds. Low pressing time is advantageous as it results in a high throughput. In some examples, the surfaces of the dies may be smooth and flat to produce a smooth, polished-effect finish. In some examples, the surfaces of the dies are deliberately roughened or textured to impart the roughness or texture into the surface of the article. In some examples, the surface of the die may have 3D features to impart the 3D effect into the surface of the tile. In some examples, powders or fine flakes of coloured material may be first placed into the die prior to compression to impart a coloured and textured effect. In some examples, large flakes of coloured material (e.g., dried waste paint flakes) may be first placed into the die prior to compression to impart a terrazzo-like pattern effect. In some examples a pressure of 10-300 MPa is applied to the mixture. In some examples a pressure of 40-200 MPa is applied to the mixture. In some examples a pressure of 80-150 MPa is applied to the mixture. In some examples a pressure of 80-95 MPa is applied to the mixture. In some examples a pressure of at least 10 MPa, at least 20 MPa, at least 30 MPa, at least 40 MPa, at least 50 MPa, at least 60 MPa, at least 70 MPa, or at least 80 MPa is applied. A minimum pressure is required to ensure good compaction of the mixture and thus good strength properties in the product composite. Higher pressures result in a stronger, more compact composite. In some examples a pressure of at most 300 MPa is applied. In some examples at most 250 MPa, at most 200 MPa, at most 150 MPa, at most 120 MPa, at most 110 MPa, at most 100 MPa, at most 95 MPa, or at most 90 MPa is applied. In some examples a pressure of less than 100 MPa is applied. It is desirable to minimise the pressure required to achieve a composite with good properties to improve efficiency of the manufacturing process and reduce the energy requirements - throughput is increased at lower pressures as more articles can be produced per press of a given force capacity. A minimum pressure ensures that the pressed article has sufficient strength to be handled and transferred easily. In some examples, the mixture is compressed at 88 MPa. This pressure is commonly used in the pressing step of manufacturing conventional ceramic tiles. Step (iii) - Drying In some examples, the mixture is dried to harden the material and provide a composite article. During the drying step, the compressed mixture is heated to drive off water. Without being bound by any theory, it is thought that as water is eliminated from the mixture the saturation solubility of the inorganic compound (A) in the remaining water is exceeded causing reprecipitation of the inorganic compound to bind the solid particles together. The temperature at which the mixture is dried may affect the formation of the precipitated particles and thus impact the physical characteristics of the final composite material, for example, the strength and hardness. In some examples, the mixture is dried at 35-200 °C. In some examples, the mixture is dried at 35-150 °C. In some examples, the mixture is dried at 50-130 °C. In some examples, the mixture is dried at 70-95 °C. In some examples, the drying temperature is at least 35 °C, at least 50 °C, at least 60 °C, at least 70 °C, at least 80 °C, at least 85 °C, at least 90 °C, at least 95 °C, or at least 100 °C. In some examples, the drying temperature is at most 200 °C, at most 170 °C, at most 150 °C, at most 130 °C, at most 120 °C, at most 110 °C, at most 100 °C, at most 95 °C, or at most 90 °C. As the temperature is increased the drying time required decreases, but lower temperatures require a lower energy input. In some examples, the mixture is dried for at least 10 minutes. In some examples, the mixture is dried for at most 2 hours, or at most 1 hour. In some examples, the mixture is dried for at most 30 minutes. The drying time may not be particularly limited and depends on the dimensions of the portion of the mixture being dried. For example, a tile of 5 mm thickness may dry in less than 30 minutes at 90 °C, but the drying time increases with the thickness. In some examples, the mixture is dried “naturally” by placing it in the sun to heat. The mixture may be dried by heat from the sun in a greenhouse. Alternatively or additionally, waste heat from industrial processes may be utilised to dry the materials. Advantageously, this reduces energy costs and improves sustainability of the manufacturing process. The final composite material may be a single phase, monolithic material. Composite article and tile In a second aspect the present invention provides a composite article, which may suitably be manufactured according to the method of the first aspect. In some examples, the composite article comprises 70-99.5 wt.% inorganic component (A), 0.5-30 wt.% solubility agent (B), and 0-3 wt.% water (C). Note that this (C) does not include water of crystallisation or water of hydration bound within the inorganic matrix, that is water which formed part of the inorganic compound (A). In some examples, the composite article comprises 90-99.5 wt.% inorganic component (A), 0.5-10 wt.% solubility agent (B), and 0-2 wt.% water (C). Note that this (C) does not include water of crystallisation or water of hydration bound within the inorganic matrix, that is water which formed part of the inorganic compound (A). In some examples, the composition of the final tile is 98.5 wt.% inorganic component (A) and 1.5 wt.% solubility agent. Essentially all free water (the water from component (C)) is eliminated, although some water remains as water of hydration in the crystal structure. In some examples, the composite article has an ultimate flexural strength (modulus of rupture) of at least 12 MPa (N / mm2), at least 15 MPa, at least 20 MPa, at least 25 MPa, or at least 30 MPa, In some examples, the composite article has a minimum breaking strength of at least 300 N, at least 350 N, or at least 390 N, for example over 393 N, for 120x120 mm tiles with a thickness under 7.5mm. In some examples, the composite article has a water absorption from 0 to 30%. In some examples, the water absorption is from 5 % and / or up to 25%. In some examples, the composite article has a Mohs hardness value of at least 2, or at least 3. In some examples, the composite article has negligible deviation in terms of length, thickness, straightness of sides, rectangularity, flatness and warpage upon drying. In some examples, with the additional application of a coating, the tiles may have excellent stain resistance (Class 5), and / or chemical resistance (Class A), and / or even lower water absorption values of <3%. There may be no significant moisture expansion. Since tiles are not glazed, issues arising from differential thermal expansion of the body and glaze may be avoided (e.g., surface defects, crazing, delamination etc.). Unlike some ceramic tiles, there are no lead or cadmium release issues due to nature of the feedstock. The tiles are non-flammable due to the high inorganic component. A tile may be suitably manufactured according to the method of the first aspect. In some examples, the composite article of the second aspect is a tile. In some examples, the tile has a thickness of 4 - 25 mm. In some examples, the tile has a length of 50-300 mm and a width of 50-300 mm, for example dimensions of 50x50mm - 300x300mm. In some examples, the tile has a thickness of 7.5 mm. The thickness of the tiles is not particularly limited. In some examples, the tile has a length and width of 100x200 mm. The size of the tiles is not particularly limited. In some examples, 3D surface features can be imprinted into the tiles for a textured surface or 3D effect. In some examples, pigments such as iron oxide or other coloured inorganic salts may be incorporated into component (A) which may result in coloured tiles. In some examples, the tile is coated. A step (iv) wherein a coating is applied to the surface of the dried mixture (that is, the tile) may be included in the method of manufacture. *** 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 METHODS OF MEASUREMENT Breaking strength and Ultimate Flexural Strength (aka Modulus of Rupture) were determined by means of three-point loading, following ISO 10545-4. Samples were suspended on two supports with rounded edge, with a central point equidistant from the support rods applying an increasing force at a constant rate until failure of the specimen. Test specimens were rectangular or square tiles of a length equal or greater than 4 cm, width at least 1 cm, and a thickness of between 5 mm and 9 mm. Samples were either tested immediately after drying or kept in a sealed container after drying to prevent interference from atmospheric moisture. Test procedure: A test specimen was placed on the support rods with the proper surface facing upwards. The central rod was then positioned equidistant between the support rods. The load was applied evenly at a rate not exceeding 1 MPa per second. The force to break was noted, and Ultimate Flexural Strength determined through the equation: 3FI Of = Modulus of Rupture (or Ultimate Flexural Strength), the stress required to fracture the sample (MPa) F = Load at a given point on the load deflection curve, (N) L = Support span, (mm) b = Width of test beam, (mm) d = Depth or thickness of tested beam, (mm) Water absorption was determined using the procedure outlined in ISO 10545-3. Dry material specimens (tiles) were weighed before being placed in deionised water and left to soak for 24 hours. The tiles were then removed, and excess surface water was dried off with a paper towel before the tiles were weighed again. Water absorption was calculated using the following expression: (M2-M1) / M1*100 Where: M1 = Mass of the dry tile M2 = Mass of the wet tile Mohs hardness was determined by scratching the specimens with another material of known hardness on the Mohs scale and observing if a scratch had been left. This was achieved with a Mohs hardness testing kit. Stain and chemical resistance were determined using the procedures outlines in ISO 10545-13 and ISO 10545-14, respectively. The materials were subject to the action of test solutions with visual determination of attack after a defined period. Various test solutions were prepared, including Ammonium chloride (100 g / L), Hydrochloric acid (3% v / v), red wine vinegar (5% v / v acetic acid), sodium hydroxide (30 g / L), green oil paint etc. A small drop (50 pL) of each was placed on the test specimen and left for a set amount of time (1-24h), before removal of the test solutions and visual inspection of the materials. Classification was based on the extent of deterioration. MANUFACTURE OF A COMPOSITE - EXAMPLE 1 Calcium Sulphate dihydrate (Gypsum) derived from waste plaster casts from the ceramic slip-casting industry was ground into a coarse powder. The residual moisture content of this powder was 1.45 wt.%. This non-dry powder was blended in a high-shear mixer for about 5 seconds with food-grade Cream of Tartar (CoT, i.e. the monopotassium salt of tartaric acid) at a ratio of 98.5:1.5 by mass (1000g in total). Water was subsequently added to the combined powder at a ratio of 97.75:2.25 by mass. The mixture was further blended for about 5 seconds. The final ratios of ingredients, accounting for initial moisture in the plaster, were: 94.9:1.47:3.62 Calcium Sulphate dihydrate, CoT and water, respectively. A portion of the mixture was then transferred to a metal die, before a uniaxial force was applied corresponding to a pressure of 176 MPa for about 10 seconds. The compressed mixture was then placed in a dehydrating oven at 90 degrees Celsius for 30 minutes. This formed a ceramic-like composite which, when tested, had an ultimate flexural strength (Figure 1) of 30.1 MPa, a Moh’s hardness of 3, and a water absorption value of 6.1%. Scanning electron microscopy (SEM) analysis (Figure 2) revealed a morphology consisting of micronscale crystallites meshed and fused into a solid article. The surface of the materials (in contact with the metal die during pressing) was notably more homogenous and less porous than the interior - suggesting pulverisation of the surface inorganics under pressure into finer particles that fill voids. MANUFACTURE OF A COMPOSITE - EXAMPLE 2 Calcium Sulphate dihydrate (Gypsum) derived from waste plaster casts from the ceramic slip-casting industry was ground into a coarse powder. This powder was blended in a high-shear mixer for about 5 seconds with food-grade Cream of Tartar (CoT, i.e. the monopotassium salt of tartaric acid) at a ratio of 98.5:1.5 by mass (1000g in total). Water was subsequently added to the combined powder at a ratio of 96.5:3.5 by mass. The mixture was further blended for about 5 seconds. A portion of the mixture was then transferred to a metal die, before a uniaxial force was applied corresponding to a pressure of 160 MPa for about 10 seconds. The compressed mixture was then placed in a dehydrating oven at 35 degrees Celsius for 48 hours. This formed a ceramic-like composite which, when tested, had an ultimate flexural strength of 18.6 MPa The procedure was repeated, except after compression the material was placed in a greenhouse on a hot day and left to dry for 48 hours. This formed a ceramic-like composite which, when tested, had an ultimate flexural strength of 16.0 MPa. MANUFACTURE OF A COMPOSITE - EXAMPLE 3 Calcium Sulphate dihydrate (Gypsum) derived from waste plaster casts from the ceramic slip-casting industry was ground into a coarse powder. This powder was blended in a high-shear mixer for about 10 seconds with food-grade CoT at a ratio of 86:14 by mass. Water was subsequently added to the combined powder at a ratio of 93:7 by mass. The mixture was further blended for about 5 seconds. A portion of the mixture was then transferred to a metal die, before a uniaxial force was applied corresponding to a pressure of 40 MPa for about 30 seconds. The compressed mixture was then placed in a dehydrating oven at 70 degrees Celsius for 120 minutes. This formed a ceramic-like composite which, when tested, had an ultimate flexural strength (Figure 3) of 14.2 MPa. MANUFACTURE OF A COMPOSITE - EXAMPLE 4 Calcium Sulphate dihydrate (Gypsum) powder obtained commercially (horticultural grade) was blended in a high-shear mixer for about 5 seconds with laboratory grade monopotassium bitartrate (aka, cream of tartar, CoT) at a ratio of 96:4 by mass. Water was subsequently added to the combined powder at a ratio of 93.5:6.5 by mass. The mixture was further blended for about 2 seconds. A portion of the mixture was then transferred to a metal die, before a uniaxial force was applied corresponding to a pressure of 40 MPa for about 60 seconds. The compressed mixture was then placed in a dehydrating oven at 90 degrees Celsius for 60 minutes. This formed a ceramic-like composite which, when tested, had an ultimate flexural strength (Figure 4) of 12.2 MPa. MANUFACTURE OF A COMPOSITE - EXAMPLE 5 In a similar procedure to above, waste plaster-derived gypsum was ground into a powder with Cream of Tartar (CoT, i.e. the monopotassium salt of tartaric acid) at a ratio of 96.5:3.5 by mass, with a small amount of water (hydration of 6.5%). The mixture was compressed at a pressure of 88 MPa. The compressed mixture was dried at 90 °C for 2 hours. On drying the material hardened and displayed ceramic-like properties with an ultimate flexural strength of over 12 MPa. MANUFACTURE OF A COMPOSITE WITH ADDITIVES - EXAMPLE 6 Calcium Sulphate dihydrate (Gypsum) derived from waste plaster casts from the ceramic slip-casting industry was ground into a coarse powder. This powder was blended in a high-shear mixer for about 10 seconds with food-grade CoT at a ratio of 96:4 by mass. Water was subsequently added to the combined powder at a ratio of 93.5:6.5 by mass. The mixture was further blended for about 5 seconds. A portion of the mixture was then transferred to a metal die, before a uniaxial force was applied corresponding to a pressure of 40 MPa for about 30 seconds. The compressed mixture was then placed in a dehydrating oven at 70 degrees Celsius for 120 minutes. This formed a ceramic-like composite which, when tested, had an ultimate flexural strength of 10.6 MPa. The experiment was then repeated with the following changes (Figure 5). MgCh: the water was replaced with a solution of Magnesium Chloride of concentration ranging between 0.5 - 5 M. The ultimate flexural strength increased to 15.2 MPa at a concentration of 1 M, before declining at higher concentrations. EDTA: the water was replaced with a solution of Ethylenediaminetetraacetic acid (EDTA) of concentration ranging between 0.5-10 wt. %. The ultimate flexural strength increased to 15.4 MPa at 5 wt. % EDTA before declining at higher concentrations. Urea: the water was replaced with a solution of urea of concentration ranging between 0.5 and 5 M. The ultimate flexural strength increased 13.4 MPa at 1 M Urea before declining at higher concentrations. MANUFACTURE OF A COMPOSITE WITH VARIED FORMING PRESSURE - EXAMPLE 7 Calcium Sulphate dihydrate (Gypsum) derived from waste plaster casts from the ceramic slip-casting industry was ground into a coarse powder. This powder was blended in a high-shear mixer for about 10 seconds with food-grade CoT at a ratio of 96:4 by mass. Water was subsequently added to the combined powder at a ratio of 93.5:6.5 by mass. The mixture was further blended for about 5 seconds. A portion of the mixture was then transferred to a metal die, before a uniaxial force was applied corresponding to a pressure of 31.4 MPa for about 30 seconds. The compressed mixture was then placed in a dehydrating oven at 70 degrees Celsius for 120 minutes. This formed a ceramic-like composite which, when tested, had an ultimate flexural strength of 7.04 MPa. The experiment was then repeated but with increasing uniaxial forces corresponding to pressures up to 97.0 MPa (Figure 6). The ultimate flexural strength increased with increasing forming pressures, upto a strength of 15.7 MPa at a forming pressure of 97.0 MPa. MANUFACTURE OF A COMPOSITE WITH TARTRATES - EXAMPLE 8 In this experiment, Calcium Sulphate dihydrate (Gypsum) derived from waste plaster casts from the ceramic slip-casting industry was ground into a coarse powder. This powder was blended in a high-shear mixer for about 10 seconds with food-grade CoT at a ratio of 97.5:3.5 by mass. Water was subsequently added to the combined powder at a ratio of 94.5:5.5 by mass. The mixture was further blended for about 5 seconds. A portion of the mixture was then transferred to a metal die, before a uniaxial force was applied corresponding to a pressure of 54 MPa for about 30 seconds. The compressed mixture was then placed in a dehydrating oven at 70 degrees Celsius for 120 minutes. This formed a ceramic-like composite which, when tested, had an average ultimate flexural strength of 12.4 MPa. The experiment was then repeated using high-purity reagent grade monopotassium tartrate (CoT), achieving an ultimate flexural strength of 11.3 MPa. The experiment was then repeated without any CoT (comparative example), achieving an average ultimate flexural strength of 6.45 MPa. This experiment clearly demonstrated the enhancing effect that COT has on the resulting composite material (Figure 7). MANUFACTURE OF A COMPOSITE WITH ORGANIC DIACIDS - EXAMPLE 9 In this experiment, Calcium Sulphate dihydrate (Gypsum) derived from waste plaster casts from the ceramic slip-casting industry was ground into a coarse powder. This powder was blended in a high-shear mixer for about 10 seconds with food-grade CoT at a ratio of 98.5:1.5 by mass. Water was subsequently added to the combined powder at a ratio of 97.5:2.5 by mass. The mixture was further blended for about 5 seconds. A portion of the mixture was then transferred to a metal die, before a uniaxial force was applied corresponding to a pressure of 88 MPa for about 30 seconds. The compressed mixture was then placed in a dehydrating oven at 90 degrees Celsius for 60 minutes. This formed a ceramic-like composite which, when tested, had an average ultimate flexural strength of 13.3 MPa. The experiment was repeated, except CoT was replaced by tartaric acid, giving an ultimate flexural strength of 16.2 MPa. The experiment was repeated, except CoT was replaced by succinic acid, giving an ultimate flexural strength of 9.65 MPa. The experiment was repeated, except CoT was replaced by oxalic acid, giving an ultimate flexural strength of 11.8 MPa. MANUFACTURE OF A COMPOSITE WITH ORGANIC DIACIDS - EXAMPLE 10 Calcium Sulphate dihydrate (Gypsum) derived from waste plaster casts from the ceramic slip-casting industry was ground into a coarse powder. This powder was blended in a high-shear mixer for about 10 seconds. A 25 wt. % solution of tartaric acid in water was added at a ratio of 94.75:5.25 by mass. The mixture was further blended for about 5 seconds. A portion of the mixture was then transferred to a metal die, before a uniaxial force was applied corresponding to a pressure of 54 MPa for about 30 seconds. The compressed mixture was then placed in a dehydrating oven at 70 degrees Celsius for 120 minutes. This formed a ceramic-like composite which, when tested, had an ultimate flexural strength of 20.0 MPa. MANUFACTURE OF A COMPOSITE WITH A COMBINATION OF INORGANIC COMPOUNDS -EXAMPLE 11 Calcium Sulphate dihydrate (Gypsum) derived from waste plaster casts from the ceramic slip-casting industry was ground into a coarse powder. This powder was blended in a high-shear mixer for about 10 seconds with limestone-derived calcium carbonate and food-grade CoT at a ratio of 96:2:2 by mass. Water was subsequently added to the combined powder at a ratio of 94.25:5.75 by mass. The mixture was further blended for about 5 seconds. A portion of the mixture was then transferred to a metal die, before a uniaxial force was applied corresponding to a pressure of 54 MPa for about 30 seconds. The compressed mixture was then placed in a dehydrating oven at 70 degrees Celsius for 120 minutes. This formed a ceramic-like composite which, when tested, had an ultimate flexural strength of 17.3 MPa. MANUFACTURE OF A COMPOSITE WITH CALCIUM PHOSPHATE - EXAMPLE 12 Calcium phosphate powder was blended in a high-shear mixer for about 5 seconds with food-grade Cream of Tartar (CoT, i.e. the monopotassium salt of tartaric acid) at a ratio of 98.5:1.5 by mass (100g in total). Water was subsequently added to the combined powder at a ratio of 96.5:3.5 by mass. The mixture was further blended for about 5 seconds. A portion of the mixture was then transferred to a metal die, before a uniaxial force was applied corresponding to a pressure of 80 MPa for about 10 seconds. The compressed mixture was then placed in a dehydrating oven at 90 degrees Celsius for 2 hours. This formed a ceramic-like composite which, when tested, had an ultimate flexural strength (Figure 8) of 5.7 MPa. MANUFACTURE OF A COMPOSITE WITH ACID ADDITIVES - EXAMPLE 13 Calcium Sulphate dihydrate (Gypsum) derived from waste plaster casts from the ceramic slip-casting industry was ground into a coarse powder. This powder was blended in a high-shear mixer for about 10 seconds with food-grade CoT at a ratio of 98:2 by mass. Water was subsequently added to the combined powder at a ratio of 94.25:5.75 by mass. The mixture was further blended for about 5 seconds. A portion of the mixture was then transferred to a metal die, before a uniaxial force was applied corresponding to a pressure of 54 MPa for about 30 seconds. The compressed mixture was then placed in a dehydrating oven at 70 degrees Celsius for 120 minutes. This formed a ceramic-like composite which, when tested, had an ultimate flexural strength of 18.06 MPa. The experiment was then repeated, except with 5% acetic acid in place of water, and again with 50 wt% tartaric acid in place of water. The ultimate flexural strength was 20.5 MPa for acetic acid and 27.2 MPa for tartaric acid, demonstrating a significant enhancement in ultimate flexural strength with the inclusion of an acid. MANUFACTURE OF A COMPOSITE - EXAMPLE 14 Calcium Sulphate dihydrate (Gypsum) derived from waste plaster casts from the ceramic slip-casting industry was ground into a coarse powder. This powder was blended in a high-shear mixer for about 10 seconds with food-grade tartaric acid at a ratio of 98.5:1.5 by mass. Water was subsequently added to the combined powder at a ratio of 97.5:2.5 by mass. The mixture was further blended for about 5 seconds. A portion of the mixture was then transferred to a metal die, before a uniaxial force was applied corresponding to a pressure of 88 MPa for about 30 seconds. The compressed mixture was then placed in a dehydrating oven at 90 degrees Celsius for 60 minutes. This formed a ceramic-like composite which, when tested, had an ultimate flexural strength (Figure 9) of 21.1 MPa. PROPOSED MECHANISM - EXAMPLE 15 Without being bound by any theory, the inventors propose that the CoT and residual metal salts present in the plaster serve to enhance the solubility of calcium sulphate through the stabilisation of Ca2+ cations (in the cases of calcium sulphate, calcium phosphate, and calcium carbonate the Ca2+ cations are the limiting factor in terms of solubility). After compression to remove any voids, cavities and to generally densify the material, and subsequent dehydration - the dissolved CaSO4 re-precipitates as the saturation solubility is exceeded through the loss of water. This precipitation of CaSO4 is believed to occur on the existing gypsum needles that serve as nucleation sites - preferentially on the faces of the needles (due to thermodynamic preference) causing them to grow, mesh together and merge in a fashion analogous to the partial dissolution theory (PDT) of how Plaster of Paris (calcium sulphate hemihydrate) forms solid monolithic plaster (see Comparative Example 1). The mechanism by which organic acids such as tartaric acid (or CoT), salts such as MgCh and NaCI, and the chelating agent EDTA enhance the solubility of calcium sulphate may be as described below. When gypsum (Calcium Sulphate dihydrate) dissolves in water, both Ca2+ and SO4- ions are formed. These ions do not exist as such, but from solvated ion complexes as shown below: CaS04.2H20 + 10H20 [Ca(H20)6]2+ + [S04(H20)6]2~ The pK1 value of the aqua-calcium ion is 12.6 and it is likely that the reaction of this ion in solution is the loss of a proton to give hydroxo-aqua calcium complex ion [Baruah, M. K., Gogoi, P. C. &Kotoky, P. Sulphate behaviour from dissolution of gypsum in organic acids. Fuel 79, 211-216 (2000)]: [Ca(H2O)6]2+ [CaOH(H2O\] + + H+ A similar mechanism could be reasoned for the sulphate anion: [S04(W20)6]2- HS04(H20\]~ + ho~ However, sulphate anions (being derived from strong acids) likely only protonate at much lower pH values - thus the rate-determining step lies with the calcium cation. This may explain why other anions such as carbonate and phosphate can also be employed with a similar effect being observed. Organic acids such as tartaric, oxalic and succinic acid are known to enhance the solubility of calcium sulphate and are employed as descaling agents for this purpose. Here, the chelating effect of the organic acids may help stabilise the hydrxo-aqua calcium ion, shifting the equilibrium towards dissolution. [CaOW(W2O)5]+ + TA~ + H+ [Ca(TX)(H20)4] + + 2H2O & R Ca2+ O It is also noted that calcium sulphate and calcium carbonate show the unusual property of decreasing solubility in water with increasing temperature, this is understood to be related to the lower solubility of carbonic acid in water at higher temperatures, and the stabilising effect of carbonate ions on the hydrxo-aqua calcium ion. A similar chelation effect by the carbonic anion may stabilise the calcium complex. CO2 + H2O HC03~ + H+ [CaOH(H2O\]+ + HC03~ + H+ [Ca(HC03)(H20)4]++ 2H2O / Q' HO- O EDTA is also known to enhance the solubility of calcium sulphate. EDTA is known to strongly chelate metal ions such as Mg2+, Fe2 / 3+ and Ca2+ and is employed as a descaling agent for this purpose. [CaOW(W2O)5] + + H2Na2EDTA [CaEDTA]2~ + 2Na+ + H+ + 6H2O COMPARATIVE EXAMPLE 1 - PLASTER OF PARIS For reference (and without being bound by any theory), the mechanism of Plaster of Paris is described. When naturally occurring Gypsum (Calcium Sulphate Dihydrate, CaSO4.2H2O) is heated to about 130 °C, it undergoes thermal decomposition liberating water from the crystalline structure to form Calcium Sulphate Hemihydrate (CaSO4.0.5H2O), commonly known as Plaster of Paris or Bassanite. Higher temperatures (200-1000 °C) drive off even more water to produce, for example, flooring plaster (anhydrous Calcium Sulphate, CaSO4). CaS£>4 ■                           --> (130°C) CaS&s * Plaster a / Paris (lierni / galrate) —* yCaS#* Salu&le an / tydrifk (130 - 20O°C) —> (200 — 100CPC) / ?£a5$4 Znsolu&le an / mina's? aCaS©4 Ploorm^r plaster. When water is added to Plaster of Paris, a hydration reaction occurs whereby gypsum (CaSO4.2H2O) reforms, producing a solid monolithic material. The mechanism through which this occurs is not fully understood, however the most widely accepted theory is partial dissolution theory (PDT). [Van Driessche, A. E. S., Stawski, T. M. &Kellermeier, M. Calcium sulfate precipitation pathways in natural and engineered environments. Chemical Geology 530, 119274 (2019)] In PDT, the hemihydrate is understood to dissolve in solution until saturation, at which point precipitation of the dihydrate (CaSO4.2H2O) as high aspect ratio “needles” occurs. The hemihydrate has a significantly higher solubility than the dihydrate. As crystals of the dihydrate nucleate and grow, they interlock and fuse with surrounding crystals and build up strength until complete monolith is formed (the set plaster). After plaster hardens, crystal structure changes over the course of 72h further increasing in strength. Converting old plaster back into Plaster of Paris isn’t feasible as the product results in different crystal morphologies and the cost is prohibitive. References A number of publications are cited above in order to more fully describe and disclose the invention and the state of the art to which the invention pertains. Full citations for these references are provided below. The entirety of each of these references is incorporated herein. 1. Van Driessche, A. E. S., Stawski, T. M. &Kellermeier, M. Calcium sulfate precipitation pathways in natural and engineered environments. Chemical Geology 530, 119274 (2019). 2. Baruah, M. K., Gogoi, P. C. &Kotoky, P. Sulphate behaviour from dissolution of gypsum in organic acids. Fuel 79, 211-216 (2000).

Claims

:

1. A method of manufacturing a composite comprising the steps of:(i) mixing 70-98 wt.% of (A) calcium sulphate , with 0.5-20 wt.% of (B) a solubility agentselected from an organic acid, an organic acid salt, and a combination thereof, and 1.5-20 wt.% of (C) water to form a mixture,(ii) compressing the mixture at 10-300 MPa,(iii) drying the mixture.

2. The method of claim 1, wherein the inorganic compound (A) is gypsum (CaSO4.2H2O).

3. The method of claim 2, wherein the inorganic compound (A) is waste plaster-derived gypsum.

4. The method of any preceding claim wherein the solubility agent (B) comprises one or moreselected from tartaric acid, glutamic acid, aspartic acid, succinic acid, malic acid, oxalic acid, ethylenediaminetetraacetic acid (EDTA), and salts thereof.

5. The method of claim 4, wherein the solubility agent (B) comprises tartaric acid, or a salt thereof.

6. The method of claim 5, wherein the solubility agent (B) comprises potassium bitartrate.

7. The method of claim 1, wherein step (i) comprises mixing 85-97.5 wt.% of the inorganiccompound (A), with 0.5-10 wt.% of the solubility agent (B), and 2-15 wt.% of water.

8. The method of any preceding claim, wherein step (ii) is performed at 10-200 MPa.

9. The method of claim 8, wherein step (ii) is performed at 60-95 MPa.

10. The method of any preceding claim, wherein step (iii) is performed at 35-200 °C.

11. The method of claim 10, wherein step (iii) is performed at 65-95 °C.

12. A composite article manufactured by the method of any preceding claim.

13. The composite article of claim 12, wherein the article is a tile.s

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