Process and system

Abstract

A process for producing a supplementary cementitious material (SCM) 114, comprising the steps; providing a calciferous feedstock 101, determining, before and / or after size reduction, the free lime content of the calciferous feedstock, subjecting the calciferous feedstock to size reduction 102 under phase transition conditions effective to produce a first crude particulate material, and thereby reducing the free lime content of the calciferous feedstock, and if the free lime content determined prior to size reduction is greater than about 4 wt.%, optionally reacting the first crude particulate material to produce a second crude particulate material with a reduced free lime content of below about 4 wt.%, or if the free lime content determined after size reduction is greater than about 1 wt.%, optionally reacting the first crude particulate material to produce a second crude particulate material with a reduced free lime content of below about 1 wt.%, and processing the first and / or second crude particulate material to give the supplementary cementitious material 114, wherein the calciferous feedstock 101 comprises slag. The size reduction may be achieved by atomisation or granulation. Further processes, as well as systems and products thereof, are also defined.

Description

TECHNICAL FIELD The present invention concerns processes for producing a supplementary cementitious material, and associated systemsand process therefor. BACKGROUND Carbon capture and storage is the process by which carbon dioxide gas (CO2) from a range of sources, whether it be industry or the atmosphere is separated, treated, and transported to a long-term storage location. The aim of carbon capture technology is to reduce greenhouse gas emissions and thus mitigate climate change and its effects. CO? captured can be stored in different ways, for example in deep geological formations or as mineral carbonates. Carbon capture has the potential to achieve 14% of the global greenhouse gas emissions reductions needed by 2050 and is widely viewed as the only practical way to achieve decarbonization in the industrial sector, particularly in heavy industry. Amidst a global drive to reduce carbon emissions and move towards a circular economy, heavy industries, such as steelmaking and cement production, aim to decarbonise as fast as possible. Concurrently, the demand for products of such industries, such as steel and cement, is expected to increase. Thus, effective methods and strategies for decarbonising such heavy industries are urgently needed. According to the World Steel Association, every tonne of steel produced emits on average 1.85 tonnes of CO?, equating overall to about 8% of global CO? emissions. The transition to net-zero production methods in this highly carbon intensive industry requires large capital investment. Additionally, waste materials, such as steel or copper slags, produced from steel production or smelting, in themselves pose significant environmental challenges, especially given that for every tonne of steel produced, 0.13 tonnes of steel slag is produced as a byproduct. Copper slag, from copper smelting processes, poses a significant waste issue as around 4.5 million tons of copper slag is produced each year, but only 15 to 20% of it is being used as of 2015. Since these are both heavily wasted materials, finding ways to use them in different industries would reduce overall waste. To move towards a truly circular economy, methods are required to convert these waste materials to useful products. Supplementary cementitious material (SCM) is a material that can be added to concrete mixtures in addition to, or as a partial replacement of, traditional Portland cement or blended cements to improve both fresh and hardened concrete properties. SCMs may be added to alter properties of the overall cement blend, including durability, permeability, pumpability and finishability, mitigating alkali reactivity and the overall hardened properties of concrete through hydraulic or pozzolanic activity (or both). Cement clinker is a solid material produced in the manufacture of Portland cement as an intermediary product. The production of Portland cement clinker is estimated to account for about 5% of the total man-made CO2-emissions. Production of composite cements containing supplementary cementitious materials (SCMs) is seen as a solution for reducing the carbon emissions impact in cement production. However, the cost and availability of high quality SCMs such as granulated blast furnace slag or fly ash are limited. An affordable and reliable replacement of these materials by other cementitious materials is needed. BS EN 197-1 categorises cements into five types based on their composition, these are: CEM I, II, III, IV and V. Typically in general construction, CEM I, which is pure (100%) Portland cement, and CEM II, which is a mixture of Portland cement and SCMs such as fly ash, ground granulated blast furnace slag (GGBFS) or limestone, up to a maximum additive content of 35% by weight, are mostly used. Ground granulated blast furnace slag (GGBFS) is a cementitious material produced as a by-product from blast-furnaces used to make iron and is already widely used as a 'green' cement replacement in CEM II, III and V cements, but demand for this material far outstrips supply. GGBFS is currently used as a type of SCM to reduce the embedded emissions of a given cement blend. CEM III cements are specifically focused on mixtures of Portland cement and GGBFS and is further divided into three sub-types; A, B and C, depending on the amount of slag contained within. For example, A contains the least slag at 40% by weight, and C contains the most at 90% by weight. However, in the art, the maximum feasible substitution of Portland cement by steel slag is approximately 20% by weight due to stability challenges observed when higher proportions are used. The heat of hydration is the heat generated when water and Portland cement react. The heat of hydration is most influenced by the proportion of C3S and C2S in the cement but is also influenced by the water-cement ratio, fineness and curing temperature. As each one of these factors is increased, heat of hydration increases. In large mass concrete structures such as gravity dams, hydration heat is produced significantly faster than it can be dissipated, which can create high temperatures in the centre of these large concrete masses that, in turn, may cause undesirable stresses as the concrete cools to ambient temperature. The use of GGBFSs is advantageous due to the lower heat of hydration of these materials, which is favourable in large infrastructure projects. In contrast, normal cement, which has a higher heat of hydration, can result in the internal temperature of the concrete reaching high temperatures. In the future, changes in steel making techniques are predicted to decrease the supply of GGBFS, whereas cement demand is predicted to grow. The shift in steel manufacture from blast furnaces and blast oxygen furnaces to direct iron reduction and electric arc furnaces will also contribute to this predicted reduction in GGBFS supply. Under the current EU Emissions Trading System (ETS) allowance, steel producers have a carbon allowance, and their taxes are calculated accordingly. The ETS definition now allows storage of CO2 in existing minerals to count as ETS-compliant. Therefore, storing carbon in ex-situ cement has potential positive tax implications. Whilst it is known that steel or copper slags may be used to produce materials for cement production, these methods are limited to the addition of small quantities of slag. These methods also teach that using a higher slag proportion in a cement mix results in soundness issues owing to the free lime content. Examples of current processes provided in the art are given below: CN1486951 describes a steel slag cement produced with limestone, clay, sandstone, and steel slag. The steel slag is used as an additive alongside GGBFS in the production of a cement mix, using a maximum of 17% steel slag. KR20110050611 describes an admixture composition for cement to reduce the generation of carbon dioxide. The steel slag is similarly used as an additive alongside GGBFS in the production of a cement mix, using a maximum of 10% steel slag. The physical properties of slag may be altered to improve the cementitious nature of the slag and improve the suitability of the material for cement applications. Whilst it is known that the properties of slag may be altered through chemical reaction, such as calcination, challenges concerning the free lime content in the slag are the cause of volume instabilities. Examples of current processes provided in the art are given below: CN104556754 describes a method for preparing a low-cost steel slag cement, comprising 5-20% steel slag, using a calcination process to activate the cementitious properties of the slag. EP4157795 describes a process for synthesizing high purity calcium carbonate precipitate from waste or by-products bearing calcium compounds, including steel slags. The method utilises an extraction solvent to produce a calcium rich solution from the slag calciferous feedstock to which it carries out carbonation to produce the precipitate product. Carbonation is another known type of reaction that is used to improve the cementitious nature of slags and improve the suitability of the material for cement applications. Examples of current processes provided in the art are given below: US2023 / 0110452 describes a method of preparing a carbonated supplementary cementitious materials, where a carbonatable mixture is carbonated to obtain a first carbonated cementitious material, which is milled and carbonated a second time to obtain the carbonated supplementary cementitious material product. EP3778525 describes a method of manufacturing supplementary cementitious materials by carbonation of a waste material rich in carbonatable Ca and Mg phases, wherein a starting material is provided, the starting material is carbonated in the presence of a sulphate, and a carbonated product is obtained and provided as the supplementary cementitious material. EP4265582 describes a method for the preparation of a carbonated mineral component in particular for the use as a substituent of cement in hydraulic binder compositions wherein a raw material comprising at least one of the group of concrete demolition waste, hardened concrete paste, hardened cement paste, or any mineral component containing hydrated calcium silicates, calcium aluminates is carbonated and heated, a product and use thereof. EP4155278 describes a method for manufacturing a supplementary cementitious material with improved reactivity from waste concrete, wherein a starting material comprising waste concrete is provided, subjected to carbonation to provide a carbonated product, and the carbonated product is heat treated at a temperature ranging from 120 to 350 °C until constant mass to provide the supplementary cementitious material with improved reactivity, the product and use thereof. EP3498681 describes a method for manufacturing a supplementary cementitious material from recycled concrete fines comprising providing recycled concrete fines as starting material, carbonation of the starting material to provide carbonated material and deagglomerating the carbonated material to form the supplementary cementitious material. Atomisation is a process for breaking a bulk material in the fluid state down into smaller particles. Methods of atomisation may be used with slag for the purposes of metal recovery or slag granulation. However, it is not known to use the process of atomisation to enhance a process for the production of a SCM, both in terms of throughput and improvements in cementitious properties. Examples of current atomisation processes provided in the art are given below: WO2016197244 describes a process for upgrading titania-rich slags using gas atomisation of a liquid slag to decrease particle size, and processing post atomisation to produce relevant products. AU2015367203 describes a process of atomisation utilising oxygen to atomise liquid slag, to increase the magnetic iron content for the purpose of enhanced metal recovery. DE19632698 describes a process for the production of fine grained slag sand by atomising liquid blast furnace slag (or other liquid ironworks slags) at above 1400°C with a gas at above 1 x 10-5 Pa, using an atomiser of the type used for molten metal and non-metal atomisation, to form particles of less than 90 pm in size, which are quenched for vitreous solidification. None of the above noted prior art provides a process in which waste material from heavy industries, such as steelmaking and cement production, can be efficiently upcycled into a supplementary cementitious material in a versatile and controllable method, nor a process in which CO2 can be captured and removed from the atmosphere in the process. Thus, new carbon capture and storage solutions are needed for the steel and cement industries, preferably those that would allow use of significant quantities of industrial waste from steel and cement production for CO2 capture. SUMMARY OF THE INVENTION According to a first aspect of the present invention, there is provided a process for producing a supplementary cementitious material, comprising the steps; i. providing a calciferous feedstock, ii. subjecting the calciferous feedstock to size reduction under phase transition conditions effective to produce a particulate material, and thereby reducing the free lime content, iii. reacting the particulate material to produce a crude particulate material product, and iv. processing the crude particulate material product to give the supplementary cementitious material, wherein the calciferous feedstock comprises slag. In particular embodiments of the first aspect, the crude particulate material product may have below about 4 wt.%, below about 3 wt.%, below about 2 wt.%, below about 1 wt.%, or below about 0.5 wt.% free lime content. In preferred embodiments, the crude particulate material product will have below about 1 wt.% free lime content. In most preferred embodiments, the crude product will have from about 0 to about 1 wt.% free lime content, for example from about 0 to about 0.75 wt.% free lime content, optionally from about 0 to about 0.5 wt.% free lime content. According to a second aspect of the present invention, there is provided a process for producing a supplementary cementitious material, comprising the steps; i. providing a calciferous feedstock, ii. determining, before and / or after step iii, the free lime content of the calciferous feedstock, iii. subjecting the calciferous feedstock to size reduction under phase transition conditions effective to produce a first crude particulate material, and thereby reducing the free lime content of the calciferous feedstock, iv. if the free lime content determined prior to step iii is greater than about 4 wt.%, optionally reacting the first crude particulate material to produce a second crude particulate material with a reduced free lime content of below about 4 wt.%, v. if the free lime content determined after step iii is greater than about 1 wt.%, optionally reacting the first crude particulate material to produce a second crude particulate material with a reduced free lime content of below about 1 wt.%, and vi. processing the first and / or second crude particulate material to give the supplementary cementitious material, wherein the calciferous feedstock comprises slag. In preferred embodiments, the free lime may comprise quicklime or maglime, or a combination thereof. Determination of the free lime content, and then selectively reacting it to reduce the free lime content to below the threshold of about 4 wt.% prior to phase transition or about 1 wt.% after phase transition, is advantageous because in the calcination of cement raw materials, such as SCM, a material called clinker is produced, and the presence of free lime (CaO) in said clinkers can result in undesirable effects such as volume expansion, increased setting time or reduced cement strength. Advantageously, the process of the second aspect comprising the determination of free lime content provides a flexible process that is adaptable to the changing nature of calciferous feedstocks, enabling them to be selectively altered to improve their cementitious properties. The process is especially flexible as the free lime content may be determined either before, after, or both before and after the size reduction under phase transition step. In particular embodiments of the second aspect, the calciferous feedstock may be optionally reacted if the free lime content is greater than about 4 wt.%, greater than about 5 wt.%, greater than about 6 wt.% greater than about 8 wt.%, or greater than about 10 wt.%. Advantageously, subjecting the calciferous feedstock to size reduction under phase transition conditions effective to produce a first crude particulate material reduces the free lime content of the calciferous feedstock, and can abate the requirement of further reaction to reduce the free lime content. In particular embodiments of the second aspect, the calciferous feedstock is subjected to size reduction under phase transition conditions, or additionally reacted to produce a crude product with below about 4 wt.%, below about 3 wt.%, below about 2 wt.%, below about 1 wt.%, or below about 0.5 wt.% free lime content. In preferred embodiments, the crude product will have below about 1 wt.% free lime content. In most preferred embodiments, the crude product will have from about 0 to about 1 wt.% free lime content, for example from about 0 to about 0.75 wt.% free lime content, optionally from about 0 to about 0.5 wt.% free lime content. In particular embodiments of the second aspect, the determination of the free lime content of the calciferous feedstock and / or the crude particulate product in step (ii) may be carried out by ethylene glycol dissolution of any free lime present followed by titration with hydrochloric acid. According to a third aspect of the present invention, there is provided a process for producing a supplementary cementitious material, comprising the steps; i. selecting a calciferous feedstock with less than about 15 wt.% free lime content, ii. providing the calciferous feedstock, iii. subjecting the calciferous feedstock to size reduction under phase transition conditions effective to produce a crude particulate material, and thereby reducing the free lime content, and iv. processing the crude particulate material to give the supplementary cementitious material, wherein the calciferous feedstock comprises slag. In preferred embodiments, the free lime may comprise quicklime or maglime, or a combination thereof. In the third aspect, selection of a calciferous feedstock less than about 4 wt.% free lime content, which may be referred to as material pre-selection, is advantageous as in the calcination of cement raw materials, a material called clinker is produced, and the presence of free lime (CaO) in said clinkers can results in undesirable effects such as volume expansion, increased setting time or reduced strength. By pre-selecting the material used for the SCM production process, a higher quality and more re-producible SCM may be produced, without need for further reaction of the calciferous feedstock. In particular embodiments of the third aspect, the calciferous feedstock selected may have less than about 10 wt.%, less than about 8 wt.%, less than about 6 wt.%, or less than about 4 wt.% free lime content. In particular embodiments the crude particulate material may have less than about 4 wt.%, less than about 3 wt.%, less than about 2 wt.%, less than about 1 wt.%, or less than about 0.5 wt.% free lime content. In preferred embodiments, the crude particulate material selected will have below about 1 wt.% free lime content. In most preferred embodiments, the crude product will have from about 0 to about 1 wt.% free lime content, for example from about 0 to about 0.75 wt.% free lime content, optionally from about 0 to about 0.5 wt.% free lime content. In particular embodiments of the third aspect, the selection of the calciferous feedstock in step (i) may be carried out by pre-processing a small sample of the feedstock in a lab scale atomiser and characterising the free lime content in the quenched product, or using chemical thermodynamic analysis of the feedstock chemistry to predict free-lime content in a processed product.. The calciferous feedstock used in the processes according to any one of the first, second or third aspects of the invention may be a steel or copper slag. Advantageously, a process using such calciferous feedstocks such as slags aids in the reduction of carbon emissions. This may be achieved by upcycling waste that otherwise is difficult to recycle, which in turn results in lower handling costs. Particularly, steel slags, which previously could only be recycled as aggregates in road construction or sent to landfill, may now be able to generate revenue and contribute to the circular economy. In preferred embodiments the steel or copper slag calciferous feedstock may be molten slag or remelted aircooled slag. The steel or copper slag calciferous feedstock may be obtained from an electric arc furnace, a basic oxygen furnace, a blastfurnace, or any other conventional steelmaking facility. Advantageously, using molten slag direct from a furnace, as opposed to solid slag, also requires less energy. Particularly, the use of a steel slag calciferous feedstock from a steelmaking facility in the process is able to significantly reduce the overall net emissions of the steelmaking process. Similarly, the use of a copper slag calciferous feedstock from a smelting facility in the process is also able to significantly reduce the overall net emissions of the copper smelting process. In embodiments where the calciferous feedstock is a molten steel or copper slag, or remelted air-cooled slag, the slag temperature may be from about 1000 °C to about 2000 °C, from about 1300 °C to about 2000 °C, about 1300 °C to about 1800 °C, about 1300 °C to about 1700 °C, about 1400 °C to about 1600 °C or about 1400 °C to about 1500 °C. In a preferred embodiment, the temperature of the molten slag may be about 1500 °C. In some embodiments of the processes according to any one of the first, second or third aspects of the invention, the calciferous feedstock may be doped with additional material. Preferably, the additional material is selected from silica, elemental metals, metal oxide powders, reductants, or oxygen, however, the skilled person would know and be able to select any suitable dopant to add as additional material to alter or improve the cementitious properties of the calciferous feedstock. In the processes according to any one of the first, second or third aspects of the invention, the calciferous feedstock is subjected to size reduction to produce a particulate material. In these processes, size reduction may be achieved by atomisation or granulation. In embodiments when size reduction is carried out by atomisation, the process may be carried out by direct mechanical means, or by contacting with a high velocity fluid. Direct mechanical means may be selected from centrifugal atomisation, mechanical disintegration, impact atomisation, or any other conventional direct mechanical atomisation process. In embodiments where the atomisation is carried out by contacting with a high velocity fluid, the high velocity fluid may be selected from water, oil, or a gas, or any compatible mixture thereof. In a preferred embodiment, at least part of the high velocity fluid may be recycled from downstream in the process. For example, the fluid may comprise a gas stream, such as recycled off-gas from a downstream carbonation reactor. Consequently, the invention provides a process wherein size reduction is affected by fluid atomisation. In this case the calciferous feedstock is contacted with a fluid, preferably a fluid provided at least in part by a recyclate from the process downstream of the size reduction stage, under conditions effective to comminute the feedstock. Preferably, size reduction is carried out by atomisation, and the atomisation process may comprise rapidly cooling the calciferous feedstock and concurrently removing heat. Heat may be removed by a working fluid, optionally wherein the working fluid is a processed flue gas recycled from downstream in the process. The heat removed from the cooling of the calciferous feedstock may be recycled and used downstream in the process, for example in drying the supplementary cementitious material product. In such an embodiment, where heat is extracted from the atomiser by using the flue gas flow in the system as a heat transfer fluid, overall process energy efficiency is greatly increased. Alternatively heat extraction or recovery may be achieved using gas, water, or thermal photovoltaics (PV). In preferred embodiments, the particulate material produced by the size reduction step may have increased amorphous content, and / or reduced crystal size. In conventional technology slag is tipped and / or tapped off the furnace and left to cool on the ground, and the resulting mineralogy is not particularly cementitious. In contrast, we find that when quenched rapidly the material undergoes a phase transition and the resulting metastable structure has improved reactivity during the subsequent process steps, in particular the reacting steps, when present. Advantageously, using a size reduction step, such as atomisation, can reduce the energy consumption by up to 94% versus traditional comminution techniques and size reduction of calciferous feedstock slag increases surface area and creates highly reactive minerals. Additionally, rapid quenching of the calciferous feedstock in this manner is able to generate a highly amorphous, hydraulic material, which is more suitable for mineralisation processes than known processes in the art. For example, dry granulation is a known atomisation technique using centrifugal / mechanical atomisation to atomise blast furnace slag, and the size of the resulting granules is typically 1-2 mm. In contrast, the process according to the invention produces particles at least an order of magnitude smaller, thus increasing the reactive surface area and therefore leading to more reactive crystal structures. The size reduction process, such as atomisation, is therefore highly advantageous as it may provide significant energy savings and accelerate chemical processes. The particulate material may comprise particles from about 20 pm to about 3000 pm, from about 20 pm to about 1000 pm, from about 20 pm to about 500 pm, from about 20 pm to about 200 pm, from about 20 pm to about 100 pm, from about 50 pm to about 200 pm, from about 50 pm to about 150 pm, from about 50 pm to about 100 pm, or from about 80 pm to about 100 pm in diameter. Alternatively, the particulate material may comprise particles of less than about 700 pm, les than about 600 pm, less than about 500 pm, less than about 400 pm, less than about 300 pm, less than about 200 pm, less than about 150 pm, or less than about 100 pm. The inventors have found that this particulate size advantageously allows the particulate material to rapidly cool down and produce a higher fraction of amorphous phase within the calciferous material, which is better suited for use as a SCM. In a particular embodiment, about 80% of the particulate material may be less than about 700 pm, less than about 600 pm, or less than about 500 pm. In most preferred embodiments, the particulate material may comprise particles of about 500 pm or less in diameter. The person skilled in the art will appreciate that there may be a trade-off between processing a high feedstock throughput and providing a relatively small particle size (which will aid subsequent processing / reaction of the particulate material). The specific preferred particle size will therefore depend to some extent on the end application. The processes according to the first, second and third aspects of may further comprise an additional step of grinding the particulate material to reduce the average particle size as required, and the additional grinding step may be included at any point after the size reduction step, such as, for example, directly after an atomisation process, or alternatively as a final processing step of the supplementary cementitious material. In some embodiments of the present invention at least one additional grinding step may be included. The particles produced by such an additional grinding step may be fine particles. The fine particles produced by an additional grinding step may be from about 2 pm to about 20 pm, from about 5 pm to about 10 pm, from about 5 pm to about 15 pm, from about 5 pm to about 20 pm, from about 10 pm to about 20 pm or from about 15 pm to about 20 pm in diameter. In a preferred embodiment the fine particles produced by an additional grinding step may be about 20 pm in diameter, in the form of a fine powder. In preferred embodiments of processes according to the first, second and third aspects of the present invention, the process may further comprise an additional step of hydrating the particulate material to produce a slurry, and this step may follow the size reduction step and precede the reaction step, when present. The slurry produced from the hydration of the particulate material may comprise from about 10% by weight to about 80% by weight, 20% by weight to about 80% by weight, about 30% by weight to about 80% by weight, about 40% by weight to about 80% by weight, about 50% by weight to about 80% by weight, about 60% by weight to about 80% by weight or about 70% by weight to about 80% by weight particulate material, wherein the particulate material is atomised calciferous feedstock (e.g. slag). In a preferred embodiment of the invention, the slurry may comprise greater than about 10% by weight atomised calciferous feedstock. In preferred embodiments of processes according to the first, second and third aspects of the present invention, in the reaction step, when present, where the particulate material is reacted to produce a crude particulate material product, the particulate material may be subjected to a carbonation and / or sulphonation reaction under carbonation and / or sulphonation conditions to produce the crude particulate material product. In a carbonation reaction, which may also be referred to as a mineralisation reaction, the particulate material may be contacted with a CO2-rich gas stream. In a sulphonation reaction, the particulate material may be contacted with a sulphur-containing gas, such as a flue gas. In some embodiments, the particulate material may be fully carbonated. In other embodiments, the particulate material may only be partially carbonated. Partial carbonation may be favourable in order to achieve optimum SCM performance by leaving some reactive oxides still present in the material. The CO2-rich gas stream may be obtained from exhaust gases from the steelmaking process (such as EAF), combustion gases from mill burners, or a high CO2-concentration gas source obtained for example from a carbon capture system, or from another industrial process. Preferably, the CO2-rich gas stream comprises flue gas from industrial processes, for example flue gas from a steelmaking or milling processes. Additionally, the CO2-rich gas stream may include CO2-rich off-gas recycled from downstream in the process, for example from a product drying stage. The CO2-rich gas stream may comprise from about 5% by weight to about 100% by weight, about 20% by weight to about 80% by weight, about 20% by weight to about 70% by weight, about 30% by weight to about 60% by weight, about 30% by weight to about 50% by weight or about 40% by weight to about 50% by weight CO2. In a preferred embodiment of the invention, the CO2-rich gas stream may comprise greater than about 25% by weight CO2. In an embodiment where the CO2-rich gas stream is from EAF exhaust gas, the stream may comprise about 30% et. CO2. In an embodiment where the CO2-rich gas stream is from the combustion gases of mill burners, the stream may comprise about 8% by weight CO2. In an embodiment where the CO2-rich gas stream is from pure CO2 obtained from a carbon capture system, the stream may comprise about 99% by weight CO2. In some embodiments, the CO2-rich gas stream may be pressurised. In such embodiments the gas stream pressure may be about 2 bar, about 3 bar, about 4 bar, about 5 bar, about 6 bar, about 7 bar, about 10 bar, about 50 bar, or about 100 bar. In a preferred embodiment of the invention, the CO2-rich gas stream may be about 5 bar. Advantageously, carbonation of the size reduced calciferous feedstock, such as a steel or copper slag, enables the storage of CO2, which serves as a means of sequestering this harmful greenhouse gas, removing it from the atmosphere. Not only does this process provide significant environmental benefits, but carbonation also improves the cementitious properties and thus the properties of the feedstock as a construction material. The reaction step, when present, may be carried out under maintained or enhanced pressure. The reaction pressure may be from about 1 to about 100 bar, from about 1 to about 50 bar, from about 1 to about 10 bar, from about 1 to about 5 bar, from about 2 to about 5 bar, from about 2 to about 4 bar, from about 3 to about 4 bar, or from about 3 to about 5 bar. Upon completion of the reaction step, when present, the pressure may be released to initiate product precipitation of the product. The use of pressure change to facilitate precipitation of carbonate-containing products back into the slag slurry is able to reduce the amount of limestone addition required in a cement blend. The reaction step, when present, may be enhanced through the addition of one or more catalysts. Exemplary catalysts may include biocatalysts such as enzymes. In some embodiments of the reaction step, when present, product precipitation may be enhanced further by adding calcium to increase the precipitation of carbonates, particularly CaCO3, or by heating the reaction mixture. The person skilled in the art would be aware of other methods of enhancing this process. Advantageously, using a CO2-rich gas stream to effect conversion of the particulate material to produce a crude product is able to facilitate the carbonation of free lime in the calciferous feedstock. Free lime can be a problem when calciferous feedstock such as slag is added directly to cement blends in aggregate form, as it can result in volume instabilities overtime, especially when present in amounts of greater than 4 wt.%. In turn, by altering the mineralogy of the calciferous feedstock, the process of the invention greatly increases the quantity of calciferous feedstock, such as steel or copper slags, which may be added to a cement mix, including up to levels comparable with GGBFS. Additionally, conventional processes in the art make no use of the high-pressure gas, nor of its potential energy after use, wasting significant amounts of energy. In contrast, in the present invention, the pressurised CO2 from the carbonation reaction, vented from the reactor, may be used as a working fluid in the heat recovery process, providing greater energy efficiency to the overall process. In preferred embodiments, the reaction step, when present, of the particulate matter may take place in at least one reactor vessel, or in at least two reactor vessels, in parallel or in series. In a most preferred embodiment, the reaction of the particulate matter may take place in six reactor vessels in parallel or in series and provided with a gas contraflow system. Use of multiple batch reactors connected together in this manner may allow for continuous processing of calciferous feedstocks, such as slags, and CO2 containing gas. Advantageously, the use of multiple batch reactors achieves high CO2 capture efficiencies from industrial flue gases. In preferred embodiments of processes according to the first, second and third aspects of the present invention, the processing step of processing the crude particulate material product to give the supplementary cementitious material (step iv or v) may comprise the following steps; a) thickening b) dewatering, and / or c) drying. In a thickening process, the water content may be reduced using a settling tank. The water removed in the thickening step may be recycled to the particulate hydration step (when present). In the thickening step, the water content may be reduced from at least about 90% by weight to about 50% by weight or less, for example from at least about 90% by weight to about 40% by weight or less. Advantageously, a thickening step increases the amount of time available for carbonate precipitation to occur, increasing the amount of carbon-containing product obtained, and thus contributing to carbon capture. In a dewatering process, the water removed in the dewatering step may be recycled to the particulate hydration step (when present). In the dewatering step, the water content may be reduced from at least about 90% by weight to about 60% by weight or less, from at least about 90% by weight to about 50% by weight or less, from at least about 90% by weight to about 40% by weight or less, from at least about 90% by weight to about 30% by weight or less, or from at least about 90% by weight to about 20% by weight or less. In a preferred embodiment, the water content may be reduced to at least about 30% wt. or less. In a drying process, the drying may be supplemented by heat recovered from the size reduction step, such as an atomisation process. In such embodiments, the hot air and flue gas stream from the atomiser may provide from about 0% to about 100%, from about 10% to about 100%, from about 20% to about 80%, from about 30% to about 70%, from about 40% to about 70%, from about 40% to about 60% of the energy required for the drying process. In a preferred embodiment, the recycled heat provides at least about 50% of the energy required for the drying process. The heat may be recovered from the atomisation process in the form of a high temperature air stream. The temperature of the high temperature air stream may be from about 200 °C to about 500 °C, from about 250 °C to about 400 °C, from about 275 °C to about 400 °C, from about 250 °C to about 300 °C, from about 300 °C to about 400 °C or from about 350 °C to about 400 °C. In some embodiments the temperature may be at least 250 °C, at least 275 °C, at least 300 °C, at least 350 °C or at least 400 °C. In the drying step, when present, the amount of water in the processed material may be further reduced by at least about 60% by weight, by at least about 50% by weight, by at least about 40% by weight, by at least about 30% by weight, by at least about 20% by weight, by at least about 10% by weight, by at least about 5% by weight, by at least about 2% by weight, by at least about 1% wt. As a result, the amount of water present in the processed material after the drying step may be up to about 2% by weight or less, up to about 1.5% by weight or less, up to about 0.75% by weight or less, up to about 0.5% by weight or less, up to about 0.25% by weight or less, for example up to about 0% by weight. In a preferred embodiment of the present invention the moisture content of the processed material may be reduced to less than about 1% by weight water. Advantageously, the drying step results in additional precipitation of carbonates, increasing the amount of carbon-containing product obtained, and thus contributing to carbon capture. The reaction step, when present, may be monitored and controlled by a reaction monitoring system and said system may collect data, such as spectroscopic, micrographic, thermal or diffractive data. The composition of the calciferous feedstock, and in particular steel or copper slag when such is used as the feedstock or as part of it, can vary significantly (e.g., from steelworks to steelworks, or from smelting facility to smelting facility), so the reaction conditions need to be adaptable. Therefore, in some embodiments of the present invention, the data collected from the reaction monitoring system may be analysed by machine learning techniques to control the reaction conditions. The machine learning techniques may be applied in combination with sample analysis, such as X-ray diffraction and X-ray fluorescence. Advantageously, by monitoring and controlling the calciferous feedstock composition, atomisation process, reaction conditions, and further processing steps, a more consistent supplementary cementitious product can be produced, with properties tailored to the desired end application. Monitoring the calciferous feedstock composition is especially important in the second aspect of the invention, where the free lime content of the calciferous feedstock is monitored to determine whether a reaction step is required to reduce the free lime content to acceptable levels. In some embodiments according to the first, second and third aspects, the process may be powered using renewable energy. According to a fourth aspect of the present invention, there is provided a system for carrying out the process according to the firstand second aspect of the invention, comprising an atomiser, and at least one reactor vessel. The system may further comprise a slag crucible, a grinding mill, a compressor, a calciferous feedstock monitoring system, a reaction monitoring system, a settling tank, dewatering press, and / or a dryer. The system may comprise multiple reactor vessels in parallel or in series with one another. The vessels may be configured to enable contraflow contacting of the CO2-rich gas with the particulate material. The calciferous feedstock monitoring system (when present) may be used to determine the composition of the feedstock, particularly to determine the free lime content of the feedstock. This system may use machine learning to analyse the properties of the feedstock and control the subsequent processing steps. The reaction monitoring system (when present) may use machine learning to control the conditions of the reactor vessel. Particularly, the reaction monitoring system may determine the pH and / or the alkalinity. Advantageously, the system of multiple batch reactors may achieve higher CO2 capture efficiencies from industrial flue gases than conventional systems. This is partly as a result of the ability to use the reactors in contraflow, whereby the CO2-rich gas is flowed from one reactor vessel to the next, ensuring a high reaction rate in each reactor vessel. Additionally, the use of high-pressure carbonation exhaust gas in each reactor vessel improves energy efficiency of the overall process. The system is able to facilitate the capture of a significant amount of CO2 through the carbonation of calciferous feedstocks, such as slag, and the amount of CO2 captured can be monetised which favourably helps offset the overall processing costs, allowing for similar margins to GGBFS production. According to a fifth aspect of the present invention, there is provided a supplementary cementitious material produced according to the processes according to the first, second and third aspects of the invention. The supplementary cementitious material product may have a free lime content of about 4% by weight or less, about 3% by weight or less, about 2% by weight or less, or about 1% by weight or less. In most preferred embodiments, the supplementary cementitious material product will have a free lime content from about 0 to about 1% by weight, for example from about 0 to about 0.75% by weight free lime content, optionally from about 0 to about 0.5% by weight free lime content. Most ideally, the free lime content of the supplementary cementitious material product will be at, or approaching, 0% by weight. In most preferred embodiments according to the first, second and third aspects, the supplementary cementitious material products may have a free lime content of less than about 1 wt.% (i.e. by weight). Advantageously a material in which the free lime content is reduced to these levels does not exhibit latent soundness issues, and thus, has enhanced cementitious properties. Advantageously, the processes of the second aspect of the invention, directed towards the inclusion of a step comprising the determination of free lime content, and of the third aspect of the invention, directed towards the inclusion of a material pre-selection step based on the free lime content, provide processes in which the free lime content is an essential characteristic considered to ensure that the free lime content is minimised to towards 0% by weight. The supplementary cementitious material may be used to produce a blend with cement, which may be used in the production of concrete. Advantageously, the supplementary cementitious material product significantly reduces the embedded emissions of cement production, by removing CO2 from the atmosphere through the storage of carbon in the form of a useful end product, and a reduction in the amount of emissions produced in other industry,, such as the amount of emissions produced during the clinkering of Portland cement. According to a sixth aspect of the present invention, there is provided an atomisation process for converting a calciferous feedstock to a particulate material, wherein the calciferous feedstock is atomised and rapidly quenched to provide the particulate material. By "rapidly quenched", we mean for preference that the calciferous feedstock is simultaneously cooled and comminuted under conditions effective to generate an at least partly amorphous, preferably at least 50% by weight amorphous, particulate material. Advantageously the rapid cooling of the molten calciferous material produces a high fraction of amorphous phases within the atomised material structure. When ground and mixed with other cementitious materials, this amorphous content will react rapidly and contribute to enhancing the resultant strength of the material. Further advantageously, atomisation as a size reduction technique can reduce the energy consumption by up to 94% versus traditional comminution techniques and size reduction of calciferous feedstock slag increases surface area and creates highly reactive minerals. Additionally, rapid quenching of the molten calciferous material in this manner is able to generate a highly amorphous, hydraulic material, which is more suitable for use in mineralisation processes than known processes in the art. The calciferous feedstock used in the atomisation process, may be a molten calciferous feedstock, as previously described. In preferred embodiments according to this aspect, the calciferous feedstock may be a steel or copper slag. The atomisation process may use an atomiser. The atomiser may be a rotary disk or cup atomiser, an electrostatic atomiser, an ultrasonic atomiser, or any other conventional atomiser known in the art. Preferably, the atomiser may be a rotary disk atomiser. The atomised calciferous feedstock may be rapidly quenched with a stream of air. The stream of air may be a high energy air, which preferably may comprise recycled flue gas. An off gas may be produced from the atomisation process. The off-gas may be a high-temperature off-gas, and this may be used in a heat recovery process. Recovering energy in this manner is advantageous as it greatly improves the overall process energy efficiency. Alternatively, heat extraction or recovery may be achieved using other working fluids, or by thermal photovoltaics (PV). The atomisation process may be preferably integrated into a process of carbon capture. Further embodiments of the atomisation process are described in our co-pending GB application No. 2416425.3 and are incorporated herein by reference. According to a seventh aspect of the present invention, there is provided a particulate product produced according to the atomisation process according to the sixth aspect of the invention. The particulate material may comprise particles from about 20 pm to about 3000 pm, from about 20 pm to about 1000 pm, from about 20 pm to about 500 pm, from about 20 pm to about 200 pm, from about 20 pm to about 100 pm, from about 50 pm to about 200 pm, from about 50 pm to about 150 pm, from about 50 pm to about 100 pm, or from about 80 pm to about 100 pm in diameter. In a preferred embodiment, the particulate material may comprise particles of about 100 pm or less in diameter. In preferred embodiments, the atomised particulate material may have increased amorphous content, and / or reduced crystal size. The atomised particulate matter is preferably amorphous. In a preferred embodiment, the particulate matter may be greater than about 50%, about 60%, about 70%, about 80% or about 90% by weight amorphous. According to an eighth aspect of the present invention, there is provided a process for the carbonation of a calciferous feedstock slurry, wherein the slurry is treated using counter current flow of a COz-rich gas stream through multiple parallel or connected-in-series reactor vessels. The reactor vessels may be batch reactor vessels. The counter current flow of a CO2-rich gas stream through multiple reactor vessels is preferable as it allows for continuous processing of the calciferous feedstock slurry using the gas stream, which achieves high CO2-capture efficiencies and also improves the reaction yields. In some embodiments, the carbon capture efficiency can be even more greatly enhanced, where the CO2-rich gas stream may be flue gas from industrial processes. For example, the flue gas may be from the steelmaking or milling processes. The CO2-rich gas stream may also include CO2-rich off-gas recycled from downstream in the process, such as the gas vented from the reactor vessels at completion of the reaction. The CO2 rich gas may be first introduced into the reactor vessel that is most near to completion and may be sequentially flowed through each subsequent reactor vessel, ending in the reactor that is furthest from completion. The calciferous feedstock slurry may be produced from the hydration of particulate matter, for example, the particulate matter may be produced by the atomisation of a steel or copper slag. As referenced above, the calciferous feedstock slurry may be partially carbonated, meaning that the carbonation reaction does not proceed to completion. In such embodiments, the calciferous feedstock slurry may be up to about 20% by weight, up to about 30% by weight, up to about 40% by weight, up to about 50% by weight, up to about 60% by weight, up to about 70% by weight, up to about 80% by weight, or up to about 90% by weight carbonated with respect to the total amount of carbonatable reactants within the material. Upon completion of the reaction, the CO2-rich gas stream may be removed, and the pressure may be released to initiate product precipitation. The use of pressure changes to facilitate precipitation of the carbonatecontaining products back into the slag slurry is able to reduce the amount of limestone addition required in a cement blend. Residual CO2 may be vented from the reactor, in the form of an off-gas, and recycled in other processes, and preferably it may be vented from the final reactor in the contraflow system. The slurry in each reactor vessel may be recirculated by a slurry pump. According to a ninth aspect of the present invention, there is provided a reactor system for carrying out the process according to the eighth aspect of the invention. The reactor system according to this aspect may comprise multiple parallel or connected-in-series reactor vessels, a gas compressor, and a contraflow system. Each reactor vessel may be fitted with a pump to circulate the contents of the vessel. The reactor system may also comprise a reaction monitoring system, which may use machine learning to control the conditions of the multiple reactor vessels. According to a tenth aspect of the present invention, there is provided a process for drying a product, wherein the process comprises using heat recovery from an atomisation process. The process may use a high temperature air stream from an atomisation process to dry the product. The high temperature air stream may comprise a mixture of air and flue gas. The high temperature air stream from an atomisation process may provide from about 0% to about 100%, from about 10% to about 100%, from about 20% to about 80%, from about 30% to about 70%, from about 40% to about 70%, from about 40% to about 60% of energy required for the drying process. In a preferred embodiment, the recycled heat provides at least about 50% of the energy required for the drying process. In a preferred embodiment of this aspect, the product may be a supplementary cementitious material. According to a eleventh aspect of the present invention, there is provided a process for producing a supplementary cementitious material, the process comprising treating a molten slag calciferous feedstock through a carbonation reaction, wherein machine learning is applied in combination with sample analysis to determine the supplementary cementitious material composition and to control the reaction conditions. For the avoidance of doubt, all features relating to the first aspect of the present invention may apply, where appropriate, to any of the other aspects of the present invention and vice versa. It will also be apparent to the skilled addressee that the respective steps of providing the calciferous feedstock, subjecting the calciferous feedstock to size reduction, reacting the particulate material, and processing the crude particulate material product may each be conducted at independently remote locations. It should be apparent that each of the latter aspects of the invention, and each or any of their described variants, may be provided in combination with the first aspect of the invention and each or any of its described variants and / or in combination with any one or more of each other. DETAILED DESCRIPTION Slag is a by-product formed in smelting, welding, and other metallurgical and combustion processes from impurities in the metals or ores being treated. Slag predominantly comprises mixed oxides of elements such as silicon, sulphur, phosphorus, and aluminium; ash; and products formed in their reactions with furnace linings and fluxing substances such as limestone. In pyrometallurgical processes, such as those carried out in the steelmaking process, slag floats on the surface of the molten metal. Variants of slag are often further defined by reference to the type of precursor and / or processing conditions used to produce them, such as blast furnace (BF) slags basic oxygen furnace (BOF) slag, and electric arc furnace (EAF) slag. Cooled slag has two solid phases: a crystalline phase is formed when slag is cooled slowly, and a glassy phase (amorphous phase) is formed when slag is cooled quickly. During a rapid cooling process, the substances in the molten slag do not have time to form stable crystalline compounds, and the thermal energy that cannot be released is converted into chemical energy stored in the state of glass, thus having potential chemical activity, and making the amorphous material highly reactive. Therefore, rapidly cooled slag has favourable cementitious properties and may be used as a supplementary cementitious material. Steel slag is a waste material created as a by-product of steel production, particularly when molten steel is separated from impurities in a basic oxygen furnace (BOF) or electric arc furnace (EAF). Steel slag is typically a complex mixture of silicates and oxides that solidifies into a solid material when it cools, which is rich in metal oxides and other minerals. The physical and chemical properties of the material are affected by its chemical composition and how it's cooled. Steel slag can be used as a cementing component in various cementing systems, for example it can be used in place of conventional cements in many applications, including road construction. As described in embodiments of the invention, steel slag can be carbonated to store CO2 and improve its properties as a construction material. Currently, low carbon steel is produced by methods that alter the steelmaking process itself. Steel can be produced via two main processes: either using an integrated blast furnace (BF) / basic oxygen furnace (BOF) or an electric arc furnace (EAF). Efforts to decarbonise the industry largely seek to improve the efficiency of these processes, or to use alternate fuel sources, such as biomass or hydrogen, rather than the conventional coal. For example, current efforts include using hydrogen in the direct reduction of iron (DRI) and increasing the amount of steel recycling using electric arc furnaces. However, these approaches are also challenging. For example, DRI requires orders of magnitude more green hydrogen than is currently produced worldwide, as well as large capital investment. Furthermore, only 2.5% of current iron ore production is suitable for DRI. Electric arc furnaces may be used for steel recycling, but if the electricity used to power such furnaces is not in itself carbon neutral, no ultimate reduction of the carbon footprint results. The use of EAF for recycling uses less energy than BF-BOF processes because it is not a primary steelmaking process. Despite its advantages, EAF still has a considerable carbon footprint, mainly because they often use some fossil fuels, such as oxyfuels, in their heating processes. Additionally, since EAF steelmaking mainly uses recycled materials such as scrap iron and scrap steel, the resulting EAF slag and dust can be toxic. EAF dust frequently contains toxic elements, persistent organic pollutants, or dioxins, and is categorized as a hazardous industrial waste. Copper slag is a glassy, granular, or lumpy by-product of copper smelting that contains valuable metals and hazardous elements. For example, copper slag is typically made up of iron, silica, alumina, calcium oxide, and other elements. The chemical composition varies depending on the furnace and treatment process. Copper slag can be used as an aggregate in cement and concrete, which can provide environmental and economic benefits. Lime is an inorganic material composed primarily of calcium oxides and hydroxides, usually calcium oxide and / or calcium hydroxide. In steel and copper slags, a portion of the CaO found in the material may reside as free lime, as opposed to in a crystal phase, such as Ca-silicate. Free lime poses problems for upcycling steel and copper slag waste into concretes, as it causes volume instabilities. This is because the expansion of residual CaO when hydrated results in cracking of the concrete, which can sometimes occur years after pouring and setting. Free lime can act as a useful activator of cementitious phases, but in excess causes the soundness issues mentioned above. Typically, GGBFS will have no more than 4% free lime content. Free Lime, otherwise known as quicklime or calcium lime, or more broadly as just lime, is another term for free, or un-combined, calcium oxide or calcium hydroxide. Free lime is a critical process parameter for cement producers, with high levels of fee lime leading to unfavourable properties of resultant cement and concretes, such as longer setting times, and soundness issues. Typically, most cement manufacturers target a level of free lime of between l%-2%. For example, a value of about 1.5% or less free lime is the generally advisable quantity for Portland cement. MagLime, otherwise known as magnesium limestone, is a dolomitic limestone comprising calcium carbonate with magnesium carbonate. Typically, for a product to be classed as MagLime, there must be a minimum 5% MgO or 5-35% MgCO3 present. Free lime is often present in slags, such as steel slags. It has been proven from different approaches that the effect of free CaO on the stability of slag derived SCMs not only depends on the amount, but also on the form and grain size of the free CaO. Analysis has shown that two types of free lime often exist in steel slags. The first is free CaO and the second is a mix of CaO and FeO. Excessive free lime, especially in the presence of solid solution with small amounts of FeO, is the primary cause of volumetric unsoundness of SCM based concretes. Supplementary cementitious material (SCM) is a material that can be added to concrete mixtures in addition to, or as a partial replacement of, traditional Portland cement or blended cements to improve both fresh and hardened concrete properties. SCMs may be added to alter properties of the overall cement blend, including durability, permeability, pumpability and finishability, mitigating alkali reactivity and the overall hardened properties of concrete through hydraulic or pozzolanic activity (or both). Currently, SCMs have a great variability in their physicochemical properties, which presents a distinct challenge for their application in cement production as this introduces uncertainty in end product strength. Therefore, provision of a consistent SCM, whose production is controllable would be greatly advantageous and would enable increased use of SCMs in cement mixes. Cementitious, or hydraulic, materials such as Portland cement set and harden when they come into contact with water through hydration reaction. Cement hydration is an irreversible chemical reaction resulting in hydrated cement paste, a strong, stiff material. The primary hydration products are calcium silicate hydrate, which provides strength and performance properties in concrete, and calcium hydroxide, which does not contribute to performance and is prone to chemical attack. SCMs may be designed to increase long term strength, as well as increase durability, by reducing permeability. Increased SCM content may also delay set time by slowing the rate of hydration. This is helpful during hot weather but may not be in cooler weather. SCM addition also reduces risk of thermal cracking by lowering peak hydration temperatures. Metal cations may be leached from steel or copper slags, whilst also storing CO2 through the following reactions: CaSiOj + CO2 -> CaCO3 + SiO2 Ca(OH)2 + CO2 -> CaCO3 + H2O These two carbonation reactions are both naturally occurring in weathering, in processes such as the carbonatesilicate cycle. Carbon mineralization is a versatile and thermodynamically downhill process that can be harnessed for capturing, storing, and utilizing CO2 to synthesize products with enhanced properties. In a carbonation process, which may also be referred to as a mineralisation process owing to the fact that the metal ions are converted into carbonate minerals, there are usually three distinct reactions necessary. First, the calcium / magnesium-silicates and calcium / magnesium hydroxides found in the slag need to be dissolved into water. Secondly, CO2 is contacted with the water. Thirdly, the products are extracted out of the process to precipitate the calcium and magnesium carbonates. The overall process may be accelerated by increasing the acidity of the water solution. However, a highly acidic environment does not induce precipitation of carbonate minerals which means the equilibrium needs to be adjusted to induce the precipitation. To this end, the CO2, which may be introduced in the form of flue gas, may be introduced at high pressure, preferably in a contraflow manner. Achieving contraflow is more difficult with a slurry, and hence mechanical assistance using a slurry pump may be required to ensure constant movement of the slurry. A number of vessels may be used, for instance four vessels, each at a different state of reaction. The amount of CO2 may vary in each vessel and the CO2 reactant will be moved through the vessels in a continuous system, starting in the reactor vessel that is closest to completion, flowing through each vessel in order, ending in the reactor vessel that is furthest from completion. The compressed CO2-rich gas flows through the reaction vessels sequentially to achieve contraflow contacting. The pressurised gas dissolves CO2 into the slurry, leaching the metal cations from the slag and forming carbonates within solution. The vessel containing the most reacted slurry is contacted with the most concentrated flue gas to improve reaction rates. Once the contents of a vessel have fully leached all of its calcium and magnesium, it is disconnected from the flue gas stream. The fully reacted vessel is then depressurised, or placed under vacuum, to initiate, and enhance, the precipitation of carbonates out of solution. The remaining CO2 may be subsequently taken out, and recycled to other processes, and the reactor vessel may then be depressurised. The pressure change can induce precipitation, which therefore is a batch precipitation process. At the beginning of the reaction the solution pH may for example be at least approximately neutral. The addition of a calciferous feedstock, such as a steel or copper slag, or any other feedstock containing free lime, will increase the pH of the solution. The increase of pH is a result of the hydration of the free lime (CaO), which forms Ca(OH)2 and, subsequently, a saturated 'lime solution' comprising Ca2+ cations and hydroxyl ions (OH ). The pH of the reaction mixture after steel or copper slag addition may be about pH 11, or more. If the reaction mixture is saturated the pH will be approximately 12 (Knapp et al., Environ. Sci. Technol., 2023, 57, 26, 9854-9864). The high concentrations of hydroxyl ions in the reaction mixture aids the dissolution of CO2 into the solution due to the formation of bicarbonate (HCO3) and carbonate (CO32) ions. The availability of hydroxyl ions in pure water is very small as only a small amount of water is dissociated, therefore, the generation of a lime solution from a calciferous feedstock can produce an effective solvent to increase the rate of dissolution and the quantity of CO2 that may be dissolved. As a result of the dissolution of CO2 and Ca2+, in the reaction mixture, the saturation state of CaCO3 (at a given set of reaction conditions) may be increased. CaCO3, which is poorly water soluble, exhibits retrograde solubility and therefore depressurising and heating of the reaction chamber also increases the saturation state of the solution, enabling more metal carbonates to precipitate and significantly increasing the levels of CO2 capture. Additional calcium may then be introduced to increase the solubility of the calcium carbonate products while keeping the pH constant. As used herein, the term "contraflow" is used to refer to a direction which is generally opposite to the direction in which a mixture circulates in a general direction. Thus, a gas introduced in a contraflow manner will flow against the direction of the circulating reaction mixture. Advantageously, a continuous, contraflow capture system for flue gases into a slag slurry as described herein additionally allows for the batch removal of volumes of slag that have fully reacted with CO2 and then accelerates the precipitation of carbonates from solution. The system enables a carbon capture efficiency of greater than 95% from flue gases from steel production. The invention will now be more particularly described with reference to the following examples and figures, in which; Figure 1 illustrates a simplified flow diagram of the process according to the present invention. Figure 2 illustrates a detailed process flow diagram of the process in accordance with an embodiment the present invention. Figure 3 illustrates a contraflow contacting reactor system in accordance with an embodiment of the present invention. Figure 4 shows an X-ray diffraction spectra comparing pre- and post-carbonated slag. Figure 5 shows a graph detailing the cumulative particle size distribution of an atomised steel slag sample by the atomisation process of the invention. Figure 6 shows a graph detailing the relationship between the glass content of the particle and the particle size for an atomisation process of the invention. Referring to Figure 1, there is shown a simplified flow diagram which describes the process according to an embodiment of the present invention. Industrial waste material (101), in the form of steel or copper slags is provided to an atomiser (102). The waste material may also be supplemented by additional materials (103), such as silica, metal oxide powders, reductants (such as aluminium or graphite) or oxygen, added to alter and improve the atomisability of the waste material and cementitious properties. The atomization process is rapidly quenched by a stream of air (104) provided to the atomiser. The stream of air may comprise pressurized off-gas from downstream in the process. The particulate product is passed to a grinder (105), with the residual gas stream (106) being removed. The particles subsequently pass from the grinder (105) to a hydration chamber (107), in which they are mixed with water (108) to provide a slurry. The resultant slurry is then passed to mineralization reactor (109). In the mineralization reactor, the slurry is contacted with carbon dioxide (110), which may be in the form of a flue gas. This reaction may be carried out under pressure and produces carbonate products in a carbonation reaction. The reaction may be a full carbonation reaction or a partial carbonation reaction. The off gas (104) is removed from the reactor and may be recycled to the atomization stage. The resultant slurry is de-watered (111), with the water effluent (112) being removed. The water effluent (112) may be reused within the process, particularly in the hydration stage. The dewatered slurry passes to a drier (113), in which it is dried to produce the supplementary cementitious material product (114). In the drying stage, heat may be recovered from earlier in the process, such as from the residual gas stream (106) from the atomization process. Referring to Figure 2, there is shown a detailed process flow diagram of the process in accordance with an embodiment the present invention. Molten slag (201) is poured into a slag crucible (202), where it is doped with additional material (1), such as silica, metal oxide powders , reductants (such as aluminium or graphite) or oxygen, to improve its cementitious behaviour and atomisability. Doped molten slag (2) is transferred to an atomiser (203), where it is atomized to an atomized powder (4). The atomization process may convert the slag into particles of sizes of approximately 100 pm. The atomised molten liquid is then rapidly quenched by a stream of air (3). The stream of air (3) may comprise recycled flue gas from downstream processes. The hot off-gas (5) from the atomization process is vented and may be used in a downstream heat recovery process. Atomised powder (4) is transferred to a grinding mill (204) in which it may be ground down to a particle size of approximately 20 pm. The powder is then mixed with water (6) to produce a slurry (7). The slurry (7) may have a weight content of at least 10% slag. The slag slurry (7) is pumped into one of a number of reaction vessels (205). For simplicity, only one is shown here (see Figure 3 for an example of the multi-vessel set-up). The slurry (7) contained in the vessel is recirculated by a pump (8) and the reaction conditions are monitored by a reaction monitoring system (206). Flue gas (9), comprising CO2, is provided to a gas mixing chamber (207), where it may be mixed with CO2 rich gases from downstream in the process. The mixed flue gas (10) is compressed by a compressor (208) to produce a compressed flue gas (10), which may be at 5 bar. The compressed flue gas (10) is flowed into the reaction vessel. In some cases, the compressed flue gas (10) is flowed through multiple reaction vessels sequentially to achieve contraflow contacting, starting with the vessel containing the most reacted slurry, and ending in the least. In the reaction vessel (205), the pressurised gas dissolves CO2 into the slurry, leaching the metal cations from the slag and forming carbonates within solution, with the reaction progression monitored by the reaction monitoring system (206). Once the contents of a vessel have fully leached all of its calcium and magnesium, the vessel may be disconnected from the flue gas stream, and the fully reacted vessel is then de-pressurised to initiate the precipitation of carbonates out of solution. The vented flue gas (13) that exits the vessel is still under significant pressure, and to maximise the use of the energy in this pressurised gas stream, it is passed through a venturi (209) that entrains approximately three times as much air (14) into the gas stream while reducing the stream pressure to approximately 1.4 bar. This high mass flow rate of air and flue gas may be used as the stream of air (3) used in the atomiser to quench the molten slag. In doing so this air stream can heating up the process to approximately 350°C. Once fully precipitated, the carbonate slurry (15) is pumped out of the reaction vessel (205) and into a settling tank (210), where the water content is reduced to produce a concentrated carbonate slurry (16), for example from approximately 90% by weight to 50% by weight. The water extracted from the settling tank (17) is transferred to a water tank (211), from where it can be recycled to the grinding mill (204). The concentrated carbonate slurry (16) is passed to a dewatering press (212), such as a filter press, where the water content is reduced to produce a dewatered carbonate slurry (18), for example from approximately 50% by weight to 25% by weight. The water extracted from the press (19) is transferred to a water tank (211), from where it can also be recycled to the grinding mill (204). The dewatered carbonate slurry (18) is then passed into a dryer (213), where the moisture content is reduced to <1% by weight water, to produce the end product of a dry SCM (214). The hot air and flue gas stream (5) from the atomiser can provide, for example, 50% of the energy required for the drying process. Steam (20) produced from the drying process is transferred to a condenser (215) which is cooled by external water-cooling loop (21) and the resultant condensed water (22) is transferred to water tank (211) from where it can be recycled back into the atomiser. Cleaned flue gas (23), which may comprise unreacted CO2, may also be collected and transferred to gas mixing chamber (207) to be further recycled in the process. The end product of a dry SCM (214) is outputted from the system. Referring to Figure 3, there is shown a contraflow contacting reactor system in accordance with an embodiment of the present invention. The contraflow contacting reactor system comprises a number of different reaction vessels (301-306). Each reaction vessel (301-306) is fitted with reaction monitoring equipment (307), which monitor the pressure, pH, temperature and alkalinity conditions of each reactor, and a slurry pump (308), which recirculates the slurry held in each reaction vessel. The slag slurry (31) is pumped, by a slurry pump (308) into one of the reaction vessels (301). The other reaction vessels contain a fixed amount of slurry at different levels of progression through the mineralization reaction. CO2-rich flue gas (32) is passed into a compressor (309), where it is compressed to produce a pressurized flue gas stream (33), which may for example be at 5 bar pressure. The compressed flue gas (33) flows through four of the reaction vessels sequentially to achieve contraflow contacting, starting with the most reacted vessel (305), and ending at the least reacted vessel (302), passing through lines (34, 35 and 36). The vented flue gas (37) that exits the final vessel is still under significant pressure and may be used in other processes such as the quenching of molten slag in an atomiser. The pressurized gas dissolves CO2 into the slurry, leaching the metal cations from the slag and forming carbonates within solution. By contacting the most reacted slurry with the most concentrated flue gas, reaction rates may be improved. Once the contents of a reaction vessel have fully reacted, and has leached all of its calcium and magnesium, for example vessel (306), it is disconnected from the flue gas stream, and the vessel that is full of fresh slag slurry, for example vessel (301), may be connected at the end of the vessel cascade. The pressurized flue gas stream (33) supply may then be reconnected to the vessel that was next furthest through reacting. The fully reacted vessel (306) is then depressurized, initiating the precipitation of carbonates from solution. The product stream (38) is then pumped by a slurry pump (308) to downstream processes, such as a dewatering step (310). Referring to Figure 4, there is shown an X-ray diffraction spectra comparing pre- and post-carbonated slag. Powder X-ray Diffraction (XRD) was conducted prior to the carbonation process and post the carbonation process. The carbonation process alters the slag mineralogy, removing CO2 and forming stable CaCO3 minerals, permanently locking up CO2. This is demonstrated by the spectrum for the material post-carbonation, which has a clear calcite (CaCO3) peak, in contrast to the pre-carbonation sample, where it is not present. This provides clear evidence that the material has captured CO2 in the reaction chamber and has stored it as a stable carbonate mineral. Both materials showed typical peaks for wustite solid solution, derived from the high iron (Fe) content in the slag sample, and also p-C2S, a calcium silicate mineral that has favourable cementitious properties. EXAMPLES Example 1: XRD Assessment of Carbonated Slag A sample of electric arc furnace (EAF) slag was obtained and analysed to quantify the carbon capture potential of the material and potential of the slag to be upcycled into a valuable supplementary cementitious material (SCM). The sample provided was powdered in a ball mill and melted via alkali fusion with a lithium metaborate and lithium tetraborate flux. The cooled sample formed a metal oxide glass, which was dissolved in a 2.5 % HNO3 solution. Bulk chemistry was then measured by inductively-coupled-plasma mass spectrometry with internal standards. Reproducibility of standards and repeat measurements of the sample was <5%. Powder X-ray Diffraction (XRD) was conducted prior to carbonation experiments and post carbonation experiments (Fig. 4) to assess the mineralogy. Bulk chemistry was also assessed by inductively-coupled-plasma mass spectrometry. Critically, the material post carbonation had a clear calcite (CaCO3) peak, in comparison to the pre-carbonation sample. This provides clear evidence that the material captured CO2 in the reaction chamber and stored it as a stable carbonate mineral. Both materials showed typical peaks for wustite solid solution, derived from the high Fe content in the sample, and also 0-C2S, a calcium silicate mineral that has cementitious properties. Post carbonation a clear CaCO3 peak was observed in the XRD spectra, proving the material is capable of being used as a calciferous feedstock in mineral carbonation reactions, which capture CO2. Analysis showed that per kg of slag generated, 100g of CO2 could be captured and stored in a stable mineral form. Additionally, by converting excess free lime (CaO) in the slag to stable CaCO3 via the carbonation process, the material may be sold as SCM. The combined value of the slags use in carbon capture and as an SCM was assessed in a techno-economic assessment and this showed that the slag produced could generate a profit of $56 per tonne processed from tax credits awarded from the inflation reduction act to industrial carbon capture and storage facilities and the sale of an SCM into the cement industry. Example 2: Atomisation of Slag A molten steel slag was atomised into two impinging planar water jets, produced by a total of 4 nozzles. In this setup, the nozzles were running at a water pressure of 70 bar and a flow rate of 4 L / min. The resultant atomised particles were recovered, and their particle size distribution recorded. The PSD was determined by using a series of sieves of decreasing sizes. The particle size distribution is shown in Figure 5, which shows the cumulative particle size distribution of the atomised steel slag sample. The amorphous content was assessed for different particle sizes, and the relationship between them is shown in Figure 6, which clearly shows that the smaller the particle size, the higher the glass content of the particles. Example 3: Standard EAF slag, moderate Fe content, moderate f-CaO content A sample of electric arc furnace (EAF) slag was obtained with the aim of processing into a supplementary cementitious material (SCM). Initially the material was analysed to determine the material's bulk chemistry and the quantity of free CaO (often referred to as free lime) present. Upon determination of bulk chemistry and free lime content (Table 1) the optimum processing conditions were determined using a combination of computational modelling and empirical observations. The material was then remelted and processed to generate a crude product, whose properties were assessed (Tables 2, 3,4). The crude product was then ground to a specific Blaine fineness, using a ball mill, and then used to replace Ordinary Portland Cement (OPC) in mortar cubes at a 20% and 30% by weight replacement level, whose strength was assessed (Table 5). Methods ICP-OES for bulk chemistry determination: The sample provided was powdered using a mortar and pestle. The sample was then melted via alkali fusion with a lithium metaborate and lithium tetraborate flux. The cooled sample formed a metal oxide glass, which was dissolved in a 2.5 % HNO3 solution. Bulk chemistry was then measured by inductively-coupled-plasma mass spectrometry with internal standards. Reproducibility of standards and repeat measurements of the sample was <5%. Free lime content determination: The free lime content of the material was determined using the ethylene glycol method. A 1 g sample was placed along with 50 mL of ethylene glycol in a 100 mL conical flask, which was placed in a water bath maintained at 60 °C for 30 min. Each treated sample was filtered using two layers of No. 5B filter bed through a Buchner funnel and was washed thrice with 30 mL of ethylene glycol. The filtrate was then collected in an induction conical flask and titrated with N / 10-HCI standard solution with 2-3 drops of Brome-cresol green solution added as the indicator. The terminal point was set when an N / 10-HCI standard solution turned from blue to green. Using the amount of N / 10-HCI standard solution consumed, the amount of free CaO was calculated by the following: / ,n / x „„ HC1 quantity (mL) ■ Normality of HC1 FreeCaO (wt%) = 28—-1---——----------. 10 ■ Sample mass XRD Rietveld refinement for material characterisation: A 10g sample of steel slag was ground to a fine powder (<10 microns) using a pestle and mortar. The sample was mixed with ~lg of a known quantity of a crystalline compound that does not appear in the sample, typically flint or corundum. 1.25 g of the sample was packed into a sample container and the surface was smoothed. The sample was then analysed using a Bruker D2 Phaser benchtop XRD in Bragg-Brentano geometry (30 kV, 10 mA, Cu Ka radiation, A = 1.5406 A). Data was collected in a 26 range of typically 10° - 90° to capture the key diffraction peaks for steel slag (CaO, MgO, Fe2O3, CaSiO3 phases, etc.) and the flint (SiO2) doping agent (step size = 0.02° 20). The phases in the material were subsequently quantified using the Rietveld refinement method. Determination of Particle Size distribution: Samples were automatically shaken in a sieve stack to separate different size fractions into their respective sieves. The sample from each individual sieve were then individually collected and mass is determined using a 4-point balance. Cube making: Mortar cubes were made in compliance with ASTM C109. Pre-processing results Bulk chemistry of the material and contents of free lime is displayed in Table 1: Component Amount (wt. %) MgO 11.6 AI2O3 4.5 SiO2 12.6 P2O5 0.55 S 0.1 CaO 28.5 TiO2 0.6 Cr2O3 0.9 MnO 3.2 FeOx 37.2 Free-CaO 0.6 Table 1. Bulk chemistry and free lime data for material concerned in Example 3 Modelling and determination of processing conditions Upon the determination of the material properties shown in Table 1, the physical properties of the melt were determined via computational analysis (i.e., liquidus temperature = 1437 °C, viscosity = 0.11 Pa-S) and the processing conditions were tailored to the desired performance using a droplet cooling model developed using COMSOL. In this instance it was noted that the material had a high content of Fe-oxides, and presence of a small component of free lime. Both of these components can be detrimental to the use of the material as a supplementary cementitious material (SCM). Free lime can latently hydrate to form CaCO3, which results in a significant volume increase. The pressure caused by this volume change can result in premature concrete failure. Free lime is required to be below lwt % of the crude product, but ideally <0.5wt%, so that all available Ca is present for reaction in the glass phase. Presence of Fe(lll) in the glass phase of the material has been shown to have a significant impact on the formation of the major strength-giving phases during cement hydration. For instance, Fe( 111) has been shown to incorporate into the C-S-H gel, creating imperfections and lower degrees of crosslinking (Tian, H., Stephan, D., Lothenbach, B. and Lehmann, C., 2021. Influence of foreign ions on calcium silicate hydrate under hydrothermal conditions: a review, Construction and Building Materials, 301, 124071). Furthermore, 15wt% of Fe(lll)-oxides in material were shown to decrease 7-day strength by ~9%, and 28-day strength by as much as 40% (Olmo, I.F., Chacon, E. and Irabien, A., 2001. Influence of lead, zinc, iron (III) and chromium (III) oxides on the setting time and strength development of Portland cement, Cement and concrete research, 31(8), 1213-1219). Given the initial material scoping, the material was atomised to a particle fineness D50 of ~300 pm. It was deemed via computational modelling that this would remove all free lime and immobilise approximately half of the Fe into inert phases such as magnetite, which would not react in cement and weaken strength-giving phases. The partitioning of Fe into inert phases like magnetite was clearly observed in SEM-EDX scans of material surface. It would be apparent to those skilled in the art that there is a trade off between developing a soluble glass phase and filler minerals that immobilise undesirable components of the initial mixture (e.g., Fe-oxides). Processing The processing conditions were selected as a result of experimental work and proprietary empirical models. Based on understanding of the slag being processed, the configuration of their atomiser, and the fluid dynamics involved in the process; the inventors have developed an experimentally validated proprietary model to optimise the atomisation to slag to achieve a desired particle size. This model was used to select the atomisation operating conditions in each of the below examples. On the basis of this model, the atomiser conditions were tuned to produce a particle size D50 of 300 microns. The atomiser was configured to a pressure and flow rate of 170 bar and 15 Lmin4 respectively. Impingement angle was set to 42°. The slag as received was kibbled in a jaw crusher to <10 mm particles and then remelted in a MgO ceramic crucible at 1475 °C. The material was superheated above liquidus to ensure minimum heat loss between removal from the furnace and introduction to the process. The molten slag in the MgO crucible was manually poured into a tundish that interfaces with a custom refractory nozzle of a given size (10 mm in this case), creating a jet of molten material that falls into the impinging jets and becomes atomised. The atomised material was collected, dewatered, and then ground to a specific fineness (~500 m2 kg1). This product was subsequently processed in post-processing into a cube for strength testing. Post-processing results The particle size distribution of the material post-atomisation is displayed in Table 2, where the D50 = 283 pm. Size Quantity (g) >2mm 38.9759 1 - 2mm 100.9877 0.71 - 1mm 55.6254 500 - 710um 51.5430 280 - 500um 105.3581 180 - 280um 132.4538 125 - 180um 94.0712 63 - 125um 119.1117 40 - 63um 23.1966 <40um 19.011 Table 2. Particle size distribution of material post atomisation The mineralogy of the samples was quantified by XRD Rietveld refinement prior to and post atomisation, the results of which are displayed in Table 3. Free lime data was determined by the glycol method mentioned 5 previously. Sample D50 (mm) Glass % Beta-C2S (wt.%) C3A-Cubic (wt.%) Enstatite (wt.%) Magnetite (wt.%) Wustite (wt.%) Free lime (wt.%) Sample as received 58.80 20.80 3.84 2.74 13.80 0.6 Atomised 0.28 69.99 3.47 7.51 - 14.82 4.63 <0.1 Table 3. Rietveld refinement of industrially processed material, as received from supplier, and material post remelting and atomisation 10 The composition of glass and crystalline phases was assessed and shows that atomisation generates a marked increase in the amount of Ca and Si hosted in the amorphous phases and an increase in Fe in the crystalline phase, as shown in Table 4. Sample Amorphous Si (% of all Si) Amorphous Ca (% of all Ca) Crystalline Fe (% of all Fe) Sample as received 24.3 52.6 48.9 Atomised 82.9 70.3 61.5 Table 4. Partitioning of Si, Ca and Fe in the atomised sample and as received sample 15 Samples were made into cement cubes following the methods described previously and crushed in compliance with ASTM C109. The relative strengths of the product and unprocessed material at varying replacement levels are given in Table 5. Sample Replacement level (%) Day of testing MPa at failure (average of 3 replicates) OPC Activation (% activation) OPC reference std - 7 29.19 100.0 Atomised sample 20 7 28.21 96.7 Sample as received 20 7 24.82 85.0 Atomised sample 30 7 23.42 80.2 Sample as received 30 7 19.01 65.1 Table 5. Compressive strength of material relative to OPC at 20 and 30 wt.% replacement Example 4: EAF slag, high Fe content, high f-CaO content A sample of electric arc furnace (EAF) slag with high Fe and f-CaO content was obtained with the aim of processing into a supplementary cementitious material (SCM) in the manner as described in Example 3. Initially the material was analysed to determine the material's bulk chemistry and the quantity of free CaO (often referred to as free lime) present (Table 6). Upon determination of bulk chemistry and free lime content the optimum processing conditions were determined using a combination of computational modelling and empirical observations. The material was then remelted and processed to generate a crude product whose properties were evaluated (Tables 7, 8, 9). The crude product was then ground to a specific Blaine fineness, using a ball mill, and then used to replace Ordinary Portland Cement (OPC) in mortar cubes at a 20% and 30% by weight replacement level, in compliance with ASTM standards, whose strength was assessed (Table 10). Pre-processing results Bulk chemistry of the material and contents of free lime are displayed in Table 6. Component Amount (wt. %) MgO 11.4 AI2O3 6.34 SiO2 15.13 P2O5 0.16 S - CaO 33.0 TiO2 0.43 C^Oa 1.55 MnO 5.27 FeOx 24.5 Free-CaO 1.57 Table 6. Bulk chemistry and free lime data for material concerned in Example 4 Modelling and determination of processing conditions Upon the determination of the material properties shown in Table 6, the physical properties of the melt were determined via computational analysis and the processing conditions were tailored to the desired performance using a droplet cooling model developed using COMSOL (i.e., liquidus temperature = 1594 °C, viscosity = 0.07 Pa-S). In this instance it was noted that the material had a high content of free lime, and a moderate amount of Fe-oxides present. Given the initial material scoping, the material was atomised to a particle fineness D50 of ~200 pm. It was deemed via computational modelling that this would remove all free lime and immobilise approximately half of the Fe into inert phases such as magnetite, which would not react in cement and weaken strength-giving phases. Processing The atomiser was configured to a pressure and flow rate of 170 bar and 15 L min 1 respectively. Impingement angle was set to 42°. The slag as received was kibbled in a jaw crusher to <10mm particles and then remelted in a MgO ceramic crucible at 1600 °C. The material was superheated above liquidus to ensure minimum heat loss between removal from the furnace and introduction to the process. The molten slag in the MgO crucible was manually poured into a tundish that interfaces with a custom refractory nozzle of a given size (15 mm in this case), creating a jet of molten material that falls into the impinging jets and becomes atomised. The atomised material was collected, dewatered, and then ground to a specific fineness (~500 m2kg -1). This product was then made into a cube. Post-processing results The particle size distribution of the material post-atomisation is displayed in Table 7, where the D50 = 235 pm. Size Quantity (g) >2mm 3.1061 1 - 2mm 25.3700 0.71 - 1mm 21.7524 500 - 710um 19.9871 280 - 500um 22.4701 180 - 280um 29.8493 125 - 180um 31.7568 63 - 125um 49.4076 40 - 63um 23.1183 <40um 14.8624 Table 7. Particle size distribution of material post atomisation The mineralogy of the samples was quantified by XRD Rietveld refinement prior to and post atomisation, the results of which are displayed in Table 8. Free lime data was determined by the glycol method mentioned 5 previously. Sample D50 (mm) Glass % Beta-C2S (wt.%) Gamma- C2S (wt.%) C3A-Cubic (wt.%) Enstatite (wt.%) Magnetite (wt.%) Wustite (wt.%) Free lime (wt.%) Sample as received 60.00 14.1 4.21 13.13 2.89 4.94 1.57 Atomised 0.23 65.30 9.01 - 8.25 - 17.48 - <0.1 Table 8. Rietveld refinement of industrially processed material, as received from supplier, and material post remelting and atomisation 10 The composition of glass and crystalline phases was assessed and shows that atomisation generates a marked increase in the amount of Ca and Si hosted in the amorphous phases and an increase in Fe in the crystalline phase, as shown in Table 9. Sample Amorphous Si (% of all Si) Amorphous Ca (% of all Ca) Crystalline Fe (% of all Fe) Sample as received 34.6 47.2 42.7 Atomised 84.0 69.4 64.6 Table 9. Partitioning of Si, Ca and Fe in the atomised sample and as received sample Samples were made into cement cubes following the methods described previously and crushed in compliance with ASTM C109. The relative strengths of the product and unprocessed material at varying replacement levels are given in Table 10. Sample Replacement level (%) Day of testing MPa at failure (average of 3 replicates) OPC Activation (% activation) OPC reference std - 7 29.19 100.0 Atomised sample 20 7 32.71 112.8 Sample as received 20 7 28.86 99.5 Atomised sample 30 7 25.24 87.0 Sample as received 30 7 25.44 65.1 Table 10. Compressive strength of material relative to OPC at 20 and 30 wt.% replacement Example 5: BOF slag, very high f-CaO content A sample of blast oxygen furnace (BOF) slag was obtained with the aim of processing into a supplementary cementitious material. Typically, BOF slag has a high content of free lime, which inhibits its use as a replacement for OPC in cement blends. Free lime contents can be so great that cooling rates required to process the material to an acceptable level (<lwt% free lime, but preferably 0.5wt%) exceed that which can be achieved by the present invention. In this instance, the remaining free lime must be removed via other means, for example carbonation. Upon determination of bulk chemistry and free lime content of the sample in this example (Table 11), the optimum processing conditions (including a decision on carbonation) were determined using a combination of computational modelling and empirical observations (Tables 12, 13), in the same manner as discussed above in Example 3. Pre-processing results Bulk chemistry of the material and contents of free lime are displayed in Table 11. Component Amount (wt. %) MgO 8.82 AI2O3 2.60 SiO2 10.94 P2O5 0.63 S 0.06 CaO 42.14 TiO2 1.43 Cr2O3 - MnO 2.96 FeOx 19.22 Free-CaO 8.60 Table 11. Bulk chemistry and free lime data for material concerned in Example 5 Modelling and determination of processing conditions Upon the determination of the material properties shown in Table 11, the physical properties of the melt were determined via computational analysis (i.e., liquidus temperature = 1656 °C, viscosity = 0.05 Pa-S) and the processing conditions were tailored to the desired performance using a droplet cooling model developed using COMSOL. In this instance it was noted that the material had a very high content of free lime, and a moderate amount of Fe-oxides present. Given the initial material scoping, modelling revealed that a particle diameter less than 500 pm would be sufficient to remove free lime to an acceptable level, which was deemed to be <lwt%, and generate the desired microstructure for the material to behave as a latent hydraulic material. Modelling suggests that slags with free lime contents >15 wt.% (extreme case, e.g., Setien, J., Hernandez, D., and Gonzalez, J J., 2009. Characterization of ladle furnace basic slag for use as a construction material, Construction and Building Materials, 23(5), 1788-1794) would require carbonation. The atomiser conditions were tuned to produce a particle size of <500 pm. Given knowledge of the technoeconomics of the process, this equates to an economically viable set of operating conditions to remove free lime. It is evident that in some instances the quantity of free lime in a sample may be far too great, making the operating conditions for atomisation unfeasible. In this instance, carbonation becomes the cost-effective alternative for removing excess free lime. Processing The atomiser was configured to a pressure and flow rate of 50 bar and 8 Lmin1 respectively. Impingement angle was set to 52°. The slag as received was kibbled in a jaw crusher to <10 mm particles and then remelted in a Pt crucible at 1690 °C. The material was superheated above liquidus to ensure minimum heat loss between removal from the furnace and introduction to the process. The molten slag in the Pt crucible was manually poured into a tundish that interfaces with a custom refractory nozzle of a given size (12mm in this case), creating a jet of molten material that falls into the impinging jets and becomes atomised. Post-processing results The particle size distribution of the material post-atomisation is displayed in Table 12, where the D50 = 360 pm. Size Quantity (g) >2mm 1.2 1 - 2mm 7.0 0.71 - 1mm 8.2 500 - 710um 11.7 280 - 500um 20.9 180 - 280um 13.8 125 - 180um 7.5 63 - 125um 5.2 40 - 63um 2.7 <40um 1.4 Table 12. Particle size distribution of material post atomisation The mineralogy of the samples was quantified by XRD Rietveld refinement prior to and post atomisation, the results of which are displayed in Table 13. Free lime data was determined by the glycol method mentioned 5 previously. Sample D50 (mm) Free lime (wt.%) Sample as received - 8.60 Atomised 0.36 0.45 Table 13. Rietveld refinement of industrially processed material, as received from supplier, and material post remelting and atomisation 10

Claims

1. A process for producing a supplementary cementitious material, comprising the steps;i. providing a calciferous feedstock,ii. determining, before and / or after step iii, the free lime content of the calciferous feedstock,iii. subjecting the calciferous feedstock to size reduction under phase transition conditions effective to produce a first crude particulate material, and thereby reducing the free lime content of the calciferous feedstock,iv. if the free lime content determined prior to step iii is greater than about 4 wt.%, optionally reacting the first crude particulate material to produce a second crude particulate material with a reduced free lime content of below about 4 wt.%,v. if the free lime content determined after step iii is greater than about 1 wt.%, optionally reacting the first crude particulate material to produce a second crude particulate material with a reduced free lime content of below about 1 wt.%, andvi. processing the first and / or second crude particulate material to give the supplementary cementitious material,wherein the calciferous feedstock comprises slag.

2. A process for producing a supplementary cementitious material, comprising the steps;i. selecting a calciferous feedstock with less than about 15 wt.% free lime content,ii. providing the calciferous feedstock,iii. subjecting the calciferous feedstock to size reduction under phase transition conditions effective to produce a crude particulate material, and thereby reducing the free lime content, andiv. processing the crude particulate material to give the supplementary cementitious material, wherein the calciferous feedstock comprises slag.

3. A process for producing a supplementary cementitious material, comprising the steps;i. providing a calciferous feedstock,ii. subjecting the calciferous feedstock to size reduction under phase transition conditions to produce a particulate material, and thereby reducing the free lime content,iii. reacting the particulate material to produce a crude product, andiv. processing the crude reaction product to give the supplementary cementitious material, wherein the calciferous feedstock comprises slag.

4. The process according to any one of Claims 1 to 3, wherein the calciferous feedstock comprises steel or copper slag, optionally molten slag or remelted air-cooled slag, optionally obtained from an electric arc furnace, a basic oxygen furnace, and / or a blast furnace.

5. The process according to any one of Claims 1 to 4, wherein the calciferous feedstock is doped with additional material, optionally wherein the additional material is selected from silica, elemental metals, metal oxide powders, reductants, or oxygen.

6. The process according to any one of Claims 1 to 5, wherein size reduction of the calciferous feedstock is achieved by atomisation or granulation.

7. The process according to Claim 6, wherein the calciferous feedstock atomisation process is carried out by direct mechanical means, or by contacting with a high velocity fluid, optionally wherein the direct mechanical means is selected from centrifugal atomisation, mechanical disintegration, or impact atomisation, or the high velocity fluid is selected from water, oil, or a gas, optionally wherein the high velocity fluid comprises material recycled from downstream in the process.

8. The process according to Claim 7, wherein the atomisation process comprises rapidly cooling the calciferous feedstock and concurrently removing heat, optionally wherein the heat is removed by a working fluid, optionally wherein the working fluid is a processed flue gas recycled from downstream in the process, and / or wherein the heat removed from the cooling of the calciferous feedstock is recycled and used downstream in the process.

9. The process according to any one of Claims 1 to 9, wherein the particulate material has increased amorphous content, and / or reduced crystal size.

10. The process according to any one of Claims 1 to 10, wherein the particulate material comprises particles from about 20 pm to about 3000 pm, from about 20 pm to about 1000 pm, from about 20 pm to about 500 pm, from about 20 pm to about 200 pm, from about 20 pm to about 100 pm, from about 50 pm to about 200 pm, from about 50 pm to about 150 pm, from about 50 pm to about 100 pm, or from about 80 pm to about 100 pm in diameter.

11. The process according to any one of Claims 1 to 11, wherein the process further comprises an additional step of grinding the particulate material to reduce the average particle size, wherein the additional grinding step is included at any point after atomisation, optionally wherein at least one additional grinding step is included, optionally wherein the particles produced are fine particles, optionally wherein the particles are from about 2 pm to about 20 pm, from about 5 pm to about 10 pm,, from about 5 pm to about 15 pm, from about 5 pm to about 20 pm, from about 10 pm to about 20 pm, or from about 15 pm to about 20 pm in diameter.

12. The process according to any one of Claims 1 to 12, wherein the process further comprises an additional step of hydrating the particulate material to produce a slurry, wherein this step follows the size reduction step and preceding the reaction step (when present).

13. The process according to any one of Claims 1 to 13, wherein in the reaction step (when present), the particulate material is subjected to a carbonation reaction, optionally wherein the particulate material is contacted with a CO2-rich gas stream, optionally wherein the CO2-rich gas is obtained from exhaust gases from the steelmaking process, combustion gases from mill burners, or pure CO2 obtained from industrial processes such as a carbon capture system, optionally wherein the CO2-rich gas stream includes CO2-rich off-gas recycled from downstream in the process.

14. The process according to Claim 14, wherein the particulate material is only partially carbonated during the reaction step, and / or wherein the CO2-rich gas stream is pressurised, optionally wherein the CO2 partial pressure is at least about 0.1 bar, at least about 1 bar, at least about 2 bar, at least about 3 bar, at least about 4 bar, at least about 5 bar, at least about 10 bar, at least about 50 bar, or at least about 100 bar.

15. The process according to any one of Claims 1 to 15, wherein the reaction step (when present) is carried out under pressure, optionally wherein the reaction pressure is from about 1 to about 100 bar, from about 1 to about 50 bar, from about 1 to about 10 bar, from about 1 to about 5 bar, from about 2 to about 5 bar, from about 2 to about 4 bar, from about 3 to about 4 bar, or from about 3 to about 5 bar, optionally wherein upon completion of the reaction, the pressure is released to initiate product precipitation, and / or heated.

16. The process according to any one of Claims 1 to 16, wherein in the processing step, processing the crude particulate material comprises the following steps;a) thickening,b) dewatering, and / orc) drying.

17. The process according to Claim 17 wherein In the thickening step, the water content is reduced from about 90% by weight to about 40% by weight, or from about 90% by weight to about 50% by weight, and / or wherein the water removed in the dewatering step is recycled to the particulate hydration step (when present), optionally wherein the water content is reduced from about 90% by weight to about 20% by weight, from about 90% by weight to about 30% by weight, from about 90% by weight to about 40% by weight, from about 90% by weight to about 50% by weight, or from about 90% by weight to about 60% by weight, and / or wherein in the drying step the amount of water is reduced from about 60% by weight to about 0% by weight, from about 50% by weight to about 0% by weight, from about40% by weight to about 0% by weight, from about 30% by weight to about 0% by weight, from about 20% by weight to about 0% by weight, from about 10% by weight to about 0% by weight, from about 5% by weight to about 0% by weight, from about 2% by weight to about 0% by weight, or from about 1% by weight to about 0% by weight.

18. The process according to any one of Claims 1 to 18, wherein in the reaction step (when present) the reaction of the particulate matter takes place in at least one reactor vessel, optionally at least two reactor vessels in parallel or in series, and / or wherein the reaction of the particulate matter in the reaction step (when present) is monitored and controlled by a reaction monitoring system, wherein the system collects data, optionally wherein the data collected from the reaction monitoring system is analysed by machine learning techniques to control the reaction conditions, optionally wherein the machine learning is applied in combination with sample analysis.

19. A system for carrying out the process according to any one of Claims 1 to 19, comprising;an atomiser, andat least one reactor vessel.

20. The system according to Claim 20, further comprising;a calciferous feedstock monitoring system,a slag crucible,a grinding mill,a compressor,a reaction monitoring system,a settling tank,a dewatering press, and / ora dryer; and / or whereinmultiple reactor vessels are provided in parallel or in series with one another, optionally wherein the vessels are linked to enable contraflow contacting of CO2-rich gas, and / or wherein the reaction monitoring system (when present) uses machine learning to control the conditions of the reactor vessel.

21. A supplementary cementitious material produced according to the process of any one of Claims 1 to 19.

22. An atomisation process for converting a molten calciferous feedstock to a particulate material, wherein the calciferous feedstock is atomised and rapidly quenched to provide the particulate material.

23. A process for the carbonation of a calciferous feedstock slurry, wherein the slurry is treated using counter current flow of a CO2 rich gas stream through multiple parallel reactor vessels.

24. A process for drying a product, wherein the process comprises using heat recovery from an atomisation process.

25. A process for producing a supplementary cementitious material, the process comprising treating a molten slag calciferous feedstock through a carbonation reaction, wherein machine learning is applied in combination with sample analysis to determine the supplementary cementitious material composition and to control the reaction conditions.

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