process

Abstract

A process for manufacturing cement clinker, comprising providing a first material providing a metal M or an oxide thereof, heating the material in a furnace to form a first heated material, and contac

Description

FIELD OF THE INVENTION The present invention relates to a process for the manufacture of clinker. Such clinker can subsequently be ground to produce cement. One such cement of interest is Portland cement, although other cements can be produced using the present invention. BACKGROUND Concrete is well-known as a construction material. In simple terms, concrete typically consists of a mix of paste and aggregates. Suitable aggregates are coarse and fine particles (sand, gravel, crushed stone, for example). Portland cement is commonly used as the paste. When water is added to Portland cement, hydration reactions occur, leading to the formation of interlocking crystals that provide hydrated Portland cement with strength and hardness. Typically, Portland cement is manufactured using a cement kiln. The starting materials may be limestone, shale, clay and iron ore. The temperature in the cement kiln (typical temperature 1450-1500 °C) leads to the emission of gases such as CO2, calcination and clinkerisation. Cement clinker from the kiln takes the form of lumps or nodules of varying sizes. The cement clinker is typically cooled relatively rapidly from the kiln temperature in order to retain the preferred phase composition formed during clinkerisation. The cement clinker is ground to a fine powder and is usually mixed with gypsum (which acts as an early set retarder). The combination forms Portland cement. Many countries in the world are implementing plans to reduce emissions of greenhouse gases. As an example, the UK is committed to zero emissions by 2050, which with today's technology requires the closure of all cement production plants. Yet building the infrastructure of a zero emissions economy will be impossible without cement. For 30 years, Carbon Capture and Storage (CCS) has been seen as the solution for the remainder of emissions of “hard-to-abate" sectors including cement, yet its deployment has been so tentative that we can now say with confidence that it is very unlikely to be operating at sufficient scale by 2050 to allow anything like today's levels of construction. Therefore there is an urgent need to find alternative means to allow a sufficient level of composite construction to enable a zero emissions economy while itself having zero emissions. Currently, the production of cement leads to both process and production emissions from the decarbonation of limestone and the burning of fuel. A zero-emissions society will still require a large amount of cement and concrete for the building and maintenance of infrastructure and for the construction of low-carbon power sources [1]. It is therefore important to reduce, and eventually eliminate these emissions, and many options have been put forward to that effect. Most of the proposed strategies to abate emissions focus on the process emissions which result from the decarbonation using substitution materials but fall short of eliminating emissions. Further, many proposed solutions to cement production emissions cannot be deployed at the scale required because they depend on relatively rare materials, or materials with no future supply, such as magnesium cement and alkali-activated binders). Many of the proposed substitution materials also have the advantage of lower requirements in terms of temperature, but their production processes remain high temperature processes and thus likely difficult to electrify economically. Electrification, biofuels and hydrogen burning are at present the only realistic options to provide both the heat and temperature required, and they all have significant drawbacks: poor scalability, procurement difficulty or very poor efficiency. A possible solution is the deployment of CCS on adapted current technologies, but this is largely experimental and also unlikely to be available at scale to meet the net zero emissions 2050 target. Therefore, none of the proposed solutions can provide for zero-carbon concrete in 2050, and their combined deployment potential will fall far short of the objective of no emissions. A short review of the proposed routes to abate cement production emissions follows. Replacement of cement by alkali-activated binders. Alkali-activated binders [2, 3, 4], also called “geopolymers" are made from the by-products of primary steel-making and the burning of coal. Accordingly, and to the extent that such activities are required to reduce in order to meet the objective of reducing CO2 emissions, the available supply of raw materials for geopolymers is insufficient by an order of magnitude or two and the supply is likely to decline to zero as the carbon emitting industries which produce them decline in a zero-carbon society. Magnesium sulfo-aluminate cements (MSA) [5, 6, 7]. These new binders are produced from the calcination of mainly magnesite MgCOs at comparatively low temperature. The process emissions are the same as for the production of Portland cement, but the energy and temperature requirements are lower. They have comparable properties to Portland cement, but there are relatively few available deposits on the planet, the main ones being in China and the United States. They do not represent a reasonable alternative in the United Kingdom (and most of the world outside regions of production). Wollastonite / Rankinite cement (Solidia). These binders are made from calcining limestone at 1200 °C [8]. The key difference with Portland cement is that the cure is done under a CO2 atmosphere at 60 °C recapturing most of the CO2 emitted during the calcination. These binders cannot be used as replacement for cement in general, but have potential for producing low-carbon, unreinforced precast elements. Calcium sulfo-aluminate cements (CSA) [5, 6, 7]. These new binders are produced from the calcination of mainly bauxite at comparatively low temperature. They have the valuable property of hardening faster than Portland cement. However, still they require calcination in a kiln (e.g. by burning fuel), cannot be blended with common substituting cementitious materials, and bauxite is not available everywhere, such as in the UK. This approach therefore only helps alleviate partly the emissions from calcination, and suffers from scaling problems. It likely will be confined to precast applications, representing perhaps 20% of the current cement market. Direct calcination (CCS). The LEILAC project [9] separates the limestone from the burning gas in the calciner, allowing a flow of nearly pure CO2 to be captured effectively. This technology addresses the process emissions but would still require the burning of fuel (and hence an additional CCS process to capture the CO2 from the fuel combustion) to reach calcination temperatures. As with all carbon capture-based technologies it heavily depends on the future development of the storage or use of the CO2. Alternative construction techniques and materials. There are many other existing options for construction, mostly for smaller structures: rammed earth, bricks and lime mortar, etc. These all have a place in a future zero-carbon world, but cannot replace cement at scale for all its many uses. A possible exception is timber, which has the potential to be used even in tall buildings. The emissions for the manufacturing of suitable timber elements are not negligible, however, as a kiln is required for the drying and considerable amounts of glue are used. Further, the emissions are captured by timber only if forests are maintained in perpetuity, and other concerns for ecology and biodiversity mitigate against increasing mankind’s appropriation of biomass. Calcined clays as cement replacement (LC3). Calcined clay can replace up to 50 % of Portland cement, with an additional 15 % substituted by ground limestone. The metakaolin contained in the clay not only works pozzolanically, but the product of this reaction can in turn form hydrates with the dissolution products of the limestone, allowing very high replacement rates

[10] . Calcined at lower temperature than Portland, these widely available material are an economic way to abate a large fraction of the emissions associated with the production of cement. A limit to the amount of replacement possible is the reactivity of the Portland cement used. A higher reactivity cement could potentially allow up to 80 % replacement. Using construction and demolition wastes as raw meal for the production of new cement (CDW). Demonstration plants exist which use streams where aggregate and paste have been partly separated, and ongoing projects look at improving separation technologies [11 , 12], In current approaches, after treatment and separation of the aggregates and finely intermixed powders of sand and hydrated cement paste, calcination still occurs in kilns, resulting in emissions from the burning of fuel. Improved structural design. Currently, buildings contain 30-40 % too much embodied CO2, largely due to poorly optimised designs and errors in the choice of frame types [13, 14], The overall need for construction materials can therefore be reduced considerably, using known design techniques which are still under-deployed. This can never eliminate emissions but can magnify the impact of successful substitution and abatement strategies. As summarised in Table 1, most options for the replacement of cement (discussed above) have similar, if lower calcination emissions, except for geopolymers, which has no significant future due to the unavailability of the raw materials. The “+", and “=” mark improvement / degradation / equal (respectively) compared to Portland for emissions, the economic case for large-scale deployment, the potential to be a full-scale replacement, and the expected supply chain disruption. The present invention seeks to overcome the deficiencies of the known cement replacement processes. Table 1 : Summary of the currently proposed options for the replacement of cement. Option Process Calcination Economics Scalability Supply chain Notes Geopolymers + +: +' + 4. 4 4. no future availability Timber 4-4- + ++ + +• + only some uses CCS + -+ + ? ? + + + long-term option IC3 4- + + ++ ++ 44 needs available day Solidia + + + ? + +- ^44 precast, unreinforced Other + + + ? ,,, small scale options CSA ++ 44 4 4.4 mostly precast MSA = 4. -- “. — rare materials Design +4. ++ + -4- with other solutions CDW + +■ + +=:. ++ 4 4 4 needs reroutfog CDW CDW + EAF + + + 4 + 4 ? + -+-’+ — ,,, needs m a tch ing steel demand Of course, in order to achieve zero greenhouse gas emissions a number of techniques would be developed and combined into an industrial strategy. Many of the basic technologies considered here therefore already exist at the state of large industrial demonstrators. This is true for concrete separation into paste and aggregate streams, or the production of calcined-clay limestone mixes. Integration of these can significantly abate emissions in cement production, but do not solve the problem of emissions from calcination. The present inventors consider that the process described herein provides an important step. It is considered that the clinkering process in an environment which has considerable excess iron has not been studied in detail, and many aspects of the strength development of Portland cement are not fully understood

[15] , let alone mixes with high levels of substitution. All the proposed routes to reduce emissions in cement production and use may have a role to play to achieve the UK target of no emissions by 2050. Nonetheless, there would remain a significant gap depending on the speed of deployment of carbon capture and storage solutions, even if these are economically viable, which is currently unproven. The present invention has been devised in light of the above considerations. SUMMARY OF THE INVENTION The present inventors have developed an alternative route to reduce emissions linked to cement production. The present invention is based on the insight of the present inventors that a metal can be used as both a heat source and the source of the requisite balance of elements for the formation of a slag from a calcium-containing material, forming a slag. The investigations of the present inventors reveal that the slag from such a process can have useful properties as a cement clinker. Accordingly, in a first aspect, the present invention provides a process for the manufacture of cement clinker, the process including the steps: providing a first material, the first material providing iron (Fe) or an oxide thereof; heating the first material in a furnace to form a first heated material, wherein the furnace is an electric arc furnace or an induction furnace; providing a second material, said second material providing at least CaO, Ca(OH)2, or a mixture of CaO and Ca(OH)2; contacting a surface of the first heated material with the second material to form a slag; wherein the heat from the first heated material promotes clinkering of the slag to form the cement clinker; wherein the second material consists essentially of, or consists of, lime; and wherein the first material further provides: i) at least one source of silicon; ii) at least one source of aluminium; wherein greater than 50 wt% of the silicon required to form the cement clinker is provided by the first material; and wherein greater than 50 wt% of the aluminium required to form the cement clinker is provided by the first material. The invention includes the combination of the aspects and optional features described except where such a combination is clearly impermissible or expressly avoided. SUMMARY OF THE DRAWINGS Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying drawings in which: Fig. 1 shows a schematic flow diagram of the present industrial production of concrete and the present industrial recycling of steel. Fig. 2 shows a schematic flow diagram of how a process may be integrated with the industrial recycling of steel. Fig. 3 shows a schematic ternary phase diagram for the CaO-SiO2-Al2O3 system, overlaid with typical compositional areas for cement paste and EAF basic flux. Fig. 4 shows the same schematic ternary phase diagram for the CaO-SiO2-Al2O3 system as in Fig. 3, but overlaid with the compositions of cement phases C3S, C2S and C3A and with typical compositional areas for Portland cement and for EAF slag. Fig. 5 shows the results of powder XRD analysis and peak identification for the w / c 0.4 sample produced in Comparative Experiment 1. Fig. 6 shows the results of powder XRD analysis and peak identification for the w / c 0.6 sample produced in Comparative Experiment 1. Fig. 7 shows the results of powder XRD analysis and peak identification for the w / c 0.4 sample produced in Comparative Experiment 2. DETAILED DESCRIPTION OF THE INVENTION Aspects and embodiments of the present invention will now be discussed with reference to the accompanying drawings. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference. A first aspect of the invention is a process for the manufacture of cement clinker, the process including the steps: providing a first material, the first material providing iron (Fe) or an oxide thereof; heating the first material in a furnace to form a first heated material, wherein the furnace is an electric arc furnace or an induction furnace; providing a second material, said second material providing at least CaO, Ca(OH)2, or a mixture of CaO and Ca(OH)2; contacting a surface of the first heated material with the second material to form a slag; wherein the heat from the first heated material promotes clinkering of the slag to form the cement clinker; and wherein the second material consists essentially of, or consists of, lime; and wherein the first material further provides: i) at least one source of silicon; ii) at least one source of aluminium; wherein greater than 50 wt% of the silicon required to form the cement clinker is provided by the first material; and wherein greater than 50 wt% of the aluminium required to form the cement clinker is provided by the first material. First material In the present invention, it is thought that the first material providing iron (Fe), or an oxide thereof, functions as both a heat source and the source of the requisite balance of elements for the formation of a slag when combined with a second material that provides calcium. The first material providing iron (Fe), or an oxide thereof, may therefore be any material that is suitable of providing this dual functionality in the process of a first aspect of the invention. The first material provides iron (Fe) or an oxide thereof. The first material may be, but is not limited to, a material comprising iron oxides, iron oxyhydroxides, iron sulfides, calcium ferrites, iron-containing nesosilicates, iron-containing sorolicates, iron-containing ionosilicates and iron-containing phyllosilicates. The first material may be, or may comprise, iron oxides such as hematite (Fe2Os) or magnetite (FesCU) as well as ferrites containing these oxides and iron-containing neosilicates. The first material may preferably provide Fe2Os and / or the other compounds comprising the elements of this oxide selected from Fe2O3, FesCU, Fayalite, Andradite Staurolite, Datolite, Titanite, Humite, Chloritoid, (Mg,Fe)?(SiO4)3(OH)2and ((Mg,Fe)7(SiO4)3(F,OH)2). The first material may comprise, consist essentially of, or consist of iron flakes, such as electrolytic iron flakes. The first material may comprise, consist essentially of, or consist of pure iron flakes. By “pure” in this context it is meant that the iron flakes, when analysed by optical emission spectroscopy, do not contain any element other than iron up to the detection limit of the optical emission spectroscopy analysis. The first material may comprise, consist essentially of, or consist of an iron-containing alloy. Preferably the first material is an iron-containing alloy. The iron-containing alloy may comprise one or more additional elements, or compounds (such as oxides, hydroxides, carbonates, nitrates, sulphates, phosphates, and the like) of one or more additional elements. These one or more additional elements are preferably metals, selected from the list comprising manganese, silicon, phosphorous, aluminium, nickel, chromium, vanadium, and cobalt. The iron-containing alloy may further comprise carbon. The iron-containing alloy may comprise iron in an amount of between 0.001 and 10 wt%, preferably between 0.001 and 5 wt%, such as between about 0.01 and 5.0 wt%, or between about 0.01 and 4.5 wt%, or between about 0.01 and 4.0 wt%, or between about 0.01 and 3.5 wt%, or between about 0.01 and 3.0 wt%, or between about 0.01 and 2.5 wt%, or between about 0.01 and 2.0 wt%, or between about 0.01 and 1.5 wt%, or between about 0.01 and 1.0 wt%, or between about 0.01 and 0.5 wt%. The iron-containing alloy further comprising carbon may be cast iron or pig iron. The iron-containing alloy further comprising carbon may be steel. Examples of steels suitable for use in the present invention include, but are not limited to, stainless steel and scrap steel. The steel may be a high purity steel. In one embodiment, the iron-containing alloy does not comprise, consist essentially of, or consist of scrap steel. The iron-containing alloy further comprising carbon may further comprise one or more additional elements, or compounds (such as oxides, hydroxides, carbonates, nitrates, sulphates, phosphates, and the like) of one or more additional elements. These one or more additional elements are preferably metals, and are preferably selected from the list comprising manganese, silicon, phosphorous, aluminium, nickel, chromium, vanadium, and cobalt, more preferably selected from the list comprising manganese, silicon, phosphorous, aluminium, nickel, and chromium. The iron-containing alloy further comprising carbon (e.g. a steel) preferably has a sulphur content of less than about 0.1 wt% and more preferably less than about 0.05 wt%. The iron-containing alloy further comprising carbon (e.g. a steel) may, when analysed by optical emission spectroscopy, have the following elemental composition in mass%: C: 0.651; Si: 0.233; Mn: 0.691; P: 0.0132; S: 0.0106; Cr: 0.139; Mo: 0.00099; Ni: 0.0251; Al: 0.0023; N2: 0.0051; Fe: balance The iron-containing alloy further comprising carbon (e.g. a steel) may, when analysed by optical emission spectroscopy, have the following elemental composition in mass%: C: 0.102; Si: 0.200; Mn: 1.180; P: 0.0150; S: 0.002; Cr: 0.280; Mo: 0.000; Ni: 0.0500; Al: 0.0390; N2: 0.0070; Fe: balance In some embodiments of the invention, the first material may be, consist of, consist essentially of, or comprise scrap steel. The terms “scrap steel” and “steel scraps” are used interchangeably herein to refer to materials that are typically used as an input for steel recycling methods known in the art. Electric arc furnaces (EAF) are currently used to recycle steel by melting a wide variety of scraps at high temperature. Scraps are sourced from a wide variety of locations and waste types; thus many types of non-ferrous impurities are mixed with the steel scrap. These non-ferrous impurities need to be removed to produce a steel with a controlled chemical composition. The most common impurity that comes with the scrap is silicon (Si), present as dissolved metallic Si in the “alloy”, or as silicon dioxide (SiO2) coming from dirt or non-metallic impurities. Aluminium (Al) is another major impurity, which can be present as metallic Al not properly separated from the scrap or as alumina (AI2O3), which comes as dirt or other non-metallic impurities. Other common elements are typically manganese (Mn), phosphorus (P), sulphur (S), titanium (Ti), chromium (Cr), and copper (Cu). Some of these elements, such as Mn and Cr, must be present in the steel to at least a lower limit, depending on the target grade of steel. P and S are generally removed to a very low limit, and Cu is challenging to remove. In a steel recycling process, unwanted elements are removed from the scrap and captured by the slag, producing the target composition of steel. The main impurities captured by the slag are SiO?, AI2O3 (either oxidized in the furnace or already oxidized in the scrap), and to a lower extent MnO, Cr20s and even lower extent P2O5, SO3 (all oxidized in the furnace). In some embodiments of the present invention, wherein the first material is (or comprises) scrap steel, the slag may be initially formed with lime (CaO), which has the capability of absorbing oxides released by the scraps. Beneficially, the slag is basic and therefore protects the refractory lining of the furnace. A basic slag is defined by the ratio of CaO to SiO2, also known as V-ratio. A high V ratio, obtained by having a high ratio of CaO to SiO2, makes the slag basic. A low V ratio makes the slag acidic. Additionally, slags with a high basicity remove P and S, processes called dephosphorisation or desulfurisation, which are key processes in steel recycling. The fluidity of the slag also depends on the V ratio. A low V ratio slag is fully melted, making it very fluid - which is bad for the refractory lining and for energy efficiency. The inventors have surprisingly found that careful management of the ratio of lime to the silicon dioxide and alumina present in the scraps used for steel recycling can produce a cementitious slag and a cement clinker, which comprises major Portland cement phases such as Alite (C3S), belite (C2S), tricalcium aluminate (C3A), and tetracalcium alumino-ferrite (C4AF). Heating the first material The first aspect of the present invention includes the step of heating the first material (i.e. the first material as hereinbefore described) in a furnace to form a first heated material. The function of the furnace is to heat the first material to a sufficient temperature to allow the formation of a slag from the combination of the first material and the second material. Preferably the furnace functions to heat the first material to a sufficient temperature that at least a surface of the first material is at least partially molten. Accordingly the furnace may be any furnace that is suitable for enabling the process of the first aspect of the invention. Preferably the furnace is suitable for heating the first material, or at least a surface of the first material, to at least the liquidus temperature of the first material. Preferably, the furnace is capable of heating the first material, or at least a surface of the first material, to a temperature at least 1350 °C, at least 1500 °C, more typically at least 1550 °C, at least 1600 °C, at least 1650 °C or at least 1700 °C. The furnace is not a conventional cement kiln. The furnace will typically comprise a lining. Alternatively, or additionally, the furnace may contain a crucible, the crucible comprising a lining. In the case of a crucible, the lining may simply be the surface of the crucible that is in contact with at least one of the materials used in, or produced by (including intermediate products), the process described herein. The lining may be any lining that is suitable for carrying out the process described herein. Nonlimiting examples of such a lining include those comprising aluminium, carbon (e.g. graphite), magnesium carbide, magnesiochromite, or magnesium oxide. Preferably the lining comprises, or consists of, a refractory material. The lining is preferably not aluminium, or an oxide thereof. More preferably the lining does not consist of aluminium, or an oxide thereof. Still more preferably, the lining does not consist essentially of aluminium, or an oxide thereof. Without wishing to be bound by theory, it is thought that the presence of aluminium, or an oxide thereof, in the lining can result in leaching of aluminium into one or more of the materials present in the furnace during the process of the invention, including the first material described herein, the second material described herein, the first heated material described herein, the slag described herein, or the cement clinker described herein. It may be disadvantageous for the process of the invention for this leaching of aluminium from the lining to occur, particularly where aluminium is leached into the slag (and, hence, the cement clinker). In such cases, it is thought that the increase of aluminium content in the slag (and the cement clinker) that is thought to result from the leaching described herein may have a negative effect on the presence of desirable, reactive, cementitious phases in the slag (and the cement clinker). The inventors have surprisingly found that the leaching as described herein, when the lining is or comprises magnesium or an oxide thereof, or is or comprises aluminium or an oxide thereof, can be mitigated by the use of lime as the second material. Without wishing to be bound by theory, it is thought that by controlling the process described herein such that a high V ratio, obtained by having a high ratio of CaO to SiO?, is present, the slag produced in the process is basic. As a result, the lining comprising aluminium or an oxide thereof is protected, and the extent of leaching of aluminium into the slag in particular (and, hence, the cement clinker) is reduced, preferably substantially reduced, and even more preferably substantially prevented. The furnace is an induction furnace, or an electric arc furnace (EAF). Preferably, the furnace is an electric arc furnace (EAF). The skilled person well understands what an electric arc furnace (EAF) is. That is, a furnace whereby material is heated by means of an electrical arc. The electrical arc may be produced by electrical breakdown of a suitable gas. In use, material in the EAF is heated by contact with the electrical arc, and the passing of current through the material. Typical temperatures in conventional cement kilns can reach up to 1450 °C. On the other hand, the typical maximum operating temperature in an EAF is significantly higher, for example at least 1350 °C, at least 1500 °C, more typically at least 1550 °C, at least 1600 °C, at least 1650 °C or at least 1700 °C. Industrial EAFs may have a typical maximum operating temperature of up to 1800 °C. EAFs for research purposes may of course reach significantly higher temperatures. Accordingly, the EAF may have a maximum operating temperature of up to 1900 °C, up to 2000 °C, up to 2100 °C, up to 2200 °C, or up to 2300 °C for example. A suitable maximum temperature for carrying out the process of the first aspect may therefore be in a range formed by selection of any one of these lower limits with any one of these upper limits, e.g. 1350 °C to 2300 °C, preferably 1500 °C to 2300 °C. Without wishing to be bound by theory, it is thought that the higher temperature of the EAF (compared to cement kilns) both releases as gas the sulphates and chlorides that have previously been a problem in earlier re-clinkering trials and, importantly, favours the production of Alite over Belite in the resulting cement clinker. Different metallurgical operations may take place in the furnace (e.g. the electric arc furnace). Depending on the metallurgical operation(s) being carried out (e.g. melting only, fine alloying etc.) there may be formed EAF slag and / or ladle slag. It is considered that the present invention has utility for EAF slag and ladle slag. In some embodiments, the present invention has particular utility for EAF slag. Note that in the art EAF slag may be referred to as black slag and ladle slag may be referred to as white slag. Second material In the first aspect of the invention as described herein, the process includes the step of providing a second material providing at least CaO, Ca(OH)2, or a mixture of CaO and Ca(OH)2, wherein the second material consists essentially of, or consists of, lime. The following descriptions of preferred second materials that are suitable for use in the present invention are to be understood as describing materials that can be used as the only second material in the present invention, or in combination with the other second materials described herein, unless explicitly stated otherwise. In some preferred processes of the invention, the second material is pelletised before being added to the furnace. This improves the processability and ease of handling of some preferred second materials. In other preferred processes of the invention, however, pelletisation of the second material before it is added to the surface is not required, particularly if the physical properties of the second material are such that pelletisation is not necessary to provide acceptable material handling properties. In order to produce a cement clinker via the process of the first aspect of the invention, the first material further provides: i) at least one source of silicon; ii) at least one source of aluminium; wherein greater than 50 wt% of the silicon required to form the cement clinker is provided by the first material; and wherein greater than 50 wt% of the aluminium required to form the cement clinker is provided by the first material. In preferred processes of the invention, greater than 60 wt% of the silicon required to form the cement clinker is provided by the first material, more preferably greater than 70 wt%, greater than 80 wt%, greater than 90 wt%, greater than 95 wt%, or greater than 99 wt%. Preferably greater than 60 wt% of the aluminium required to form the cement clinker is provided by the first material, more preferably greater than 70 wt%, greater than 80 wt%, greater than 90 wt%, greater than 95 wt%, or greater than 99 wt%. Preferably greater than 60 wt% of the silicon and greater than 60 wt% of the aluminium required to form the cement clinker is provided by the first material, more preferably greater than 70 wt% of the silicon and greater than 60 wt% of the aluminium, greater than 80 wt% of the silicon and greater than 60 wt% of the aluminium, greater than 90 wt% of the silicon and greater than 60 wt% of the aluminium, greater than 95 wt% of the silicon and greater than 60 wt% of the aluminium, or greater than 99 wt% of the silicon and greater than 60 wt% of the aluminium. Preferably greater than 60 wt% of the silicon and greater than 70 wt% of the aluminium required to form the cement clinker is provided by the first material, more preferably greater than 60 wt% of the silicon and greater than 80 wt% of the aluminium, greater than 60 wt% of the silicon and greater than 90 wt% of the aluminium, greater than 60 wt% of the silicon and greater than 95 wt% of the aluminium, or greater than 60 wt% of the silicon and greater than 99 wt% of the aluminium. Preferably, the at least one source of silicon comprises SiO?. Preferably the at least one source of aluminium comprises AI2O3. In some preferred processes according to the first aspect of the invention, no material or additive other than the first material and the second material is added to the furnace. In such processes, the balance of elements (such as Al and Si) required to form the cement clinker are provided entirely by the combination of the first material and the second material. In the first aspect of the invention, the second material (i.e. a second material providing at least CaO, Ca(OH)2, or a mixture of CaO and Ca(OH)2) consists essentially of, or consists of, lime. The inventors of the present application have surprisingly found that the production of cement clinker is possible from a first material as previously defined herein and a second material that is, or contains, lime. The lime functions as the primary, preferably substantially the only source of calcium (Ca) for the cement clinker produced by the process of the invention. Advantageously, the direct use of lime as the provider of calcium for cement clinker rather than the conventional use of calcium carbonates means that the process of the first aspect of the invention can avoid the production of CO2 that results from the decomposition of calcium carbonate to oxides (and / or hydroxides) of calcium during conventional cement clinker production. The direct use of lime in this way for the production of cement clinker has not previously been described in the art, and represents a substantial improvement over cement clinker production methods of the art. In particular, the use of lime as the provider of calcium for cement clinker has surprisingly been found to result in a higher amount of desirable highly reactive Alite in the cement clinker. Thus the cement clinker produced by the process of the first aspect of the invention, or cement clinker according to the second aspect of the invention, is a highly useful material. A further benefit of the use of lime as the provider of calcium for cement clinker is protection of a furnace lining, or crucible lining, from leaching of elements or compounds contained in the lining into one or more of the materials present in the furnace during the process of the invention, including the first material described herein, the second material described herein, the first heated material described herein, the slag described herein, or the cement clinker described herein. It may be disadvantageous for the process of the invention for this leaching of aluminium from the lining to occur, particularly where aluminium is leached into the slag (and, hence, the cement clinker). The use of lime as the provider of calcium for cement clinker can reduce, preferably eliminate, this problem. Therefore, an embodiment of the first aspect of the invention is a process for the manufacture of cement clinker, the process including the steps: providing a first material, the first material providing iron (Fe) or an oxide thereof; heating the first material in a furnace to form a first heated material, wherein the furnace is an electric arc furnace or an induction furnace; providing a second material, said second material providing at least CaO, Ca(OH)2, or a mixture of CaO and Ca(OH)2; contacting a surface of the first heated material with the second material to form a slag; wherein the heat from the first heated material promotes clinkering of the slag to form the cement clinker; and wherein the second material consists essentially of, or consists of lime; wherein the first material further provides: i) at least one source of silicon; ii) at least one source of aluminium; wherein greater than 50 wt% of the silicon required to form the cement clinker is provided by the first material; and wherein greater than 50 wt% of the aluminium required to form the cement clinker is provided by the first material. In this embodiment, the second material preferably comprises argillaceous, silicious, conglomerate, magnesian, or dolomitic lime. In this embodiment, the first material is as previously described herein. Preferably the first material provides iron (Fe). More preferably the first material is an iron-containing alloy, still more preferably an iron-containing alloy further comprising carbon. Examples of suitable iron-containing alloys further comprising iron include, but are not limited to, pig iron, scrap iron, and steel. In this embodiment, when the first material is steel, the first material may be scrap steel. In this embodiment of the first aspect of the invention, the furnace is a furnace as hereinbefore described. Thus in this embodiment the furnace is an induction furnace, or an electric arc furnace (EAF).More preferably, in this embodiment, the furnace is an electric arc furnace (EAF). The furnace will typically comprise a lining. Alternatively, or additionally, the furnace may contain a crucible, the crucible comprising a lining. In the case of a crucible, the lining may simply be the surface of the crucible that is in contact with at least one of the materials used in, or produced by (including intermediate products), the process described herein. The lining may be any lining that is suitable for carrying out the process described herein. Nonlimiting examples of such a lining include those comprising aluminium, carbon (e.g. graphite), or magnesium oxide. Preferably the lining comprises, or consists of, a refractory material. In this embodiment of the first aspect of the invention, the second material providing at least CaO, Ca(OH)2, or a mixture of CaO and Ca(OH)2 (e.g. lime) may be combined with one or more additional materials to assist with fluxing of the first material (e.g. steel) and / or to promote and preferably optimise the production of Alite (tricalcium silicate). Preferably the ratio of lime to additional material is in the range defined by 75wt% lime : 25wt% additional material at one limit to 99wt% lime : 1wt% additional material at the other limit. The upper end of the range may instead be: 80wt% lime : 20wt% additional material 85wt% lime : 15wt% additional material 90wt% lime : 10wt% additional material 95wt% lime : 5wt% additional material Examples of suitable additional materials for use in such embodiments includes, but is not limited to, clays (such as AhOs-rich kaolin clay), reducing additives (such as aluminium metal and / or ferrosilicon), bauxite, silica sand, silica-rich clays (such as shales) and corundum. The function of such additives is to provide the necessary balance of elements for producing a cement clinker as further described herein, and / or to promote the formation of particularly desirable phases in the cement clinker as further described herein, and / or as processing aids for the process of the first aspect of the invention. Clinkerinq process The process of the first aspect of the invention includes the steps of heating the first material as described herein in a furnace as described herein to form a first heated material, and contacting a surface of the first heated material with the second material as described herein to form a slag, wherein the heat from the first heated material promotes clinkering of the slag to form a cement clinker. The inventors have realised that it is possible to make use of the high temperatures (compared with temperatures available in conventional cement kilns) in furnaces as hereinbefore described (e.g. electric arc furnaces) to produce ultra-reactive cements. This can allow higher levels of substitution, lowering the required volumes of Portland cement for a specific application. As previously described, in the process of the invention, the furnace functions to heat the first material to a sufficient temperature that at least a surface of the first material is at least partially molten. Preferably the first material, or at least a surface of the first material, is heated to at least the liquidus temperature of the first material. In this way, the formation of the cement clinker on a surface of the heated first material can enable the production of zero carbon cement. The temperatures used in processes of the invention (e.g. wherein the furnace is an EAF) can go above 1800 °C, well above the 1250 °C lower stability limit for C3S (Alite), the main Portland phase or the 1450 °C used in commercial kilns. Such higher temperatures open the possibility of producing highly reactive cements. This exploits the high temperatures reached by the EAF without burning fuel for the purpose of making cement. By adding a source of calcium to the melt in the form of the second material as defined herein, the slag produced is compositionally close to a commercial Portland cement as shown by Figs. 3 and 4. Preferably, the cement clinker, on removal from the electric arc furnace, is cooled from the operating temperature of the furnace (e.g. the EAF). The cooling rate of the cement clinker may be such that the temperature of the cement clinker cools from the operating temperature of the furnace to 950 °C or below in a time of 20 minutes or less and more preferably to 500 °C or below in a time of 20 minutes. More preferably this time is 15 minutes or less, still more preferably 10 minutes or less, 5 minutes or less, 4 minutes or less, 3 minutes or less, 2 minutes or less, or 1 minute or less. Cooling from the furnace temperature to room temperature may suitably take place in 40 minutes or less, more preferably in 20 minutes or less. Cement clinker The cement clinker that results from the process of the first aspect of the invention may be characterised by the cementing active hydraulic phases that are formed by the clinkering process. These active phases include C3S (Alite), C2S (Belite), C3A (Celite), and C4AF (felite). Alite based clinkers have a generic stoichiometric formula (CaO)i(SiO2)a(Al2O3)b(Fe2O3)c(SO3)d, wherein: “a” is in the range from 0.05 to 1, “b” is in the range from 0.002 to 0.6, “c” is in the range from 0.001 to 0.25 and “d” is in the range from 0 to 0.3. Alite based clinkers require the formation of C3S, which is the most important hydraulic phase. Portland Cement (also called Ordinary Portland Cement, “OPC”) clinkers have a generic stoichiometric formula (CaO)i(SiO2)a(Al2O3)b(Fe2O3)c, wherein: “a” is in the range from 0.2 to 0.5, “b” is in the range from 0.01 to 0.05, “c” is in the range from 0.001 to 0.05. Central to the OPC clinkers is the formation of C3S (Alite) OPCs typically contain the following active hydraulic phases: 25-75% C3S, 5-35% C2S, 0-15% C3A and 0-20% C4AF. The inventors have surprisingly found that the process of the first aspect of the invention is especially advantageous for promoting the formation of the C3S (Alite) phase in the cement clinker produced from the inventive process. Also described is a cement clinker obtained by or obtainable by a process of the first aspect of the invention. The cement clinker may be Portland cement clinker. For example, it may be Portland cement clinker according to EN 197-1. The cement produced from the Portland cement clinker may be CEM I cement. This is sometimes referred to as ordinary Portland cement (OPC). The cement clinker from the process may therefore be a hydraulic material. The cement clinker from the process of the invention may preferably have a CaO content, by X-ray fluorescence (XRF) analysis, of greater than about 20%, greater than about 30% or greater than about 40%; preferably greater than about 45%, more preferably greater than about 50%, still more preferably greater than about 55%, such as greater than about 56%, greater than about 57%, greater than about 58%, greater than about 59%, or greater than about 60%. The cement clinker from the process of the invention may preferably have a CaO content, by X-ray fluorescence (XRF) analysis, in the range of about 40-60%, such as in the range of about 45-60%, preferably in the range of about 50-60%, more preferably in the range of about 55-60%. The cement clinker from the process of the invention may preferably have a SiO2 content, by X-ray fluorescence (XRF) analysis, of less than about 40%, preferably less than about 35%, more preferably less than about 30%, still more preferably less than about 25%, such as less than about 20%, less than about 19%, less than about 18%, less than about 18%, less than about 17%, less than about 16%, or less than about 15%. The cement clinker from the process of the invention may preferably have a SiO2 content, by X-ray fluorescence (XRF) analysis, in the range of about 10-40%, such as in the range of about 10-30%, preferably in the range of about 10-20%, more preferably in the range of about 10-15%. The cement clinker from the process of the invention may preferably have a AI2O3 content, by X-ray fluorescence (XRF) analysis, of less than about 40%, preferably less than about 35%, more preferably less than about 30%, still more preferably less than about 25%, such as less than about 20%, less than about 19%, less than about 18%, less than about 18%, less than about 17%, less than about 16%, less than about 15%, less than about 14%, or less than about 13%. The cement clinker from the process of the invention may preferably have a AI2O3 content, by X-ray fluorescence (XRF) analysis, in the range of about 10-40%, such as in the range of about 10-30%, preferably in the range of about 10-20%, more preferably in the range of about 10-15%. The cement clinker from the process of the invention may have a composition, by X-ray fluorescence (XRF) analysis, comprising: CaO: about 40-60%, SiO2: about 10-40% AI2O3: about 1-40% other: about 0-50% The cement clinker from the process of the invention may preferably have a composition, by X-ray fluorescence (XRF) analysis, comprising: CaO: about 40-60%, SiO2: about 10-20% AI2O3: about 1-20% other: about 0-50% The cement clinker from the process of the invention may more preferably have a composition, by X-ray fluorescence (XRF) analysis, comprising: CaO: about 50-60%, SiO?: about 10-15% AI2O3: about 1-10% other: about 0-50% Considering the calcium silicates content of the cement clinker, the proportion of Alite may be at least 1 wt%, at least 2wt%, at least 3wt%, at least 4wt%, at least 5wt%, at least 10wt%, at least 15wt%, at least 20wt%, at least 25wt%, at least 30wt%, at least 35wt%, at least 40wt%, at least 45wt%, at least 50wt%, at least 55wt%, at least 60wt%, at least 65wt%, at least 70wt%, at least 75wt%, at least 80wt%, at least 85wt%, at least 90wt%, at least 95wt%, or about 100wt%. The balance calcium silicates may be Belite. Also described is a cement obtained by or obtainable by grinding the cement clinker as hereinbefore described. This may include adding one or more supplementary cementitious materials or binders, such as a source of calcium sulfate (e.g. gypsum). DETAILED DESCRIPTION OF FIGURES Fig. 1 shows a schematic flow diagram of the present industrial production of concrete and the present industrial recycling of steel. Taking the production of cement first, this is formed as already described, using a cement kiln. One of the main raw materials for conventional cement production is limestone. Limestone calcination is responsible for the process emissions of cement. Limestone is used because of its very wide availability, low price, and easy quarrying. Lime from calcination of limestone may also be used as flux for EAF steel recycling. Slag from EAFs is not usually considered to be commercially valuable, and may for example be disposed of in landfill. Concrete structures, after the end of their useful life and demolition, are also usually disposed of in landfill. Fig. 1 can be contrasted with Fig. 2, which shows a schematic flow diagram of how an alternative process may be integrated with the industrial recycling of steel. In Fig. 2, crushed concrete derived from CDW is subjected to a separation process in order to separate used cement paste and aggregate (sand and stone). Microwave separation is considered to be useful, but other separation process may be used in order to provide used cement paste. The used cement paste can be loaded into an EAF (optionally with additional components) in order to be used as a flux in the EAF to assist in the operation of the EAF to melt and treat the steel scrap in the EAF. The resultant slag forms cement clinker that can be pulverised and treated to form cement which subsequently can be used to form new concrete. Optionally, the aggregate from the separation process can be re-used in the new concrete. Also optionally, the steel produced from the EAF can be used in construction, for example as rebar in reinforced concrete. In comparison with the process shown in Fig. 1, the cement kiln CO2 emissions from calcination of limestone are avoided. Additionally, lime kiln CO2 emissions from calcination of limestone (for producing CaO flux for the EAF) are also avoided. Accordingly, in general terms embodiments of the present invention provide a route to economical, industrial scale zero-carbon cement production. Fig. 3 shows a schematic ternary phase diagram for the CaO-SiO2-Al2O3 system, overlaid with typical compositional areas for cement paste and EAF basic flux. Fig. 4 shows the same schematic ternary phase diagram for the CaO-SiO2-Al2CO3 system as in Fig. 3, but overlaid with the compositions of cement phases C3S, C2S and C3A and with typical compositional areas for Portland cement and for EAF slag. These diagrams are created from references 22, 23, 24 and 25. The purpose of including Figs 3 and 4 is to show the compositional similarity between an EAF basic flux and a cement paste. As shown in Figs. 3, these compositions overlap in the Ca0-Si02-AI2C>3 compositional space. By “basic” flux, we refer to the tendency effluxes used in EAFs to be basic in order to avoid the flux / slag corroding the refractory bricks lining the EAF. The black dots on Figs. 4 show the main cement phases C3S, C2S and C3A. In a typical Portland cement, these phases combine to give an average composition illustrated by space A on Fig. 4. On the other hand, a typical EAF flux forms an EAF slag with an average composition illustrated by space B on Fig. 4. As can be seen, in this exemplary illustration, A and B overlap. Together, Figs. 3 and 4 indicate that it may be possible to use cement paste as an EAF flux, form an EAF slag with the flux and that, compositionally, it may be possible to form the required phases in the slag for Portland cement clinker. Accordingly, using a steel recycling EAF as a kiln where the clinkering process occurs on top of the molten steel, can enable the production of zero carbon cement. The temperatures reached in an EAF can go above 1800 °C, well above the 1250 °C lower stability limit for C3S (Alite), the main Portland phase or the 1450 °C used in commercial kilns. Such higher temperatures open the possibility of producing highly reactive cements. It is therefore possible in an industrial context to carry out combined cement and steel production in an EAF plant. This exploits the high temperatures reached by the EAF without burning fuel for the purpose of making cement. By adding a source of calcium to the melt in the form of recovered CDW paste, the slag produced is compositionally close to a commercial Portland cement as shown by Figs. 3 and 4. As an additional benefit, this de-sulphurs the steel. It should be noted that it is known to use various slags (e.g. blast furnace slag, basic oxygen furnace slag, EAF slag and ladle furnace slag) for different purposes in the context of construction materials. They have been investigated, at least at the laboratory scale, for the substitution of binder, fine aggregates or coarse aggregates in concrete. However, the present inventors consider that previous work has not sufficiently considered the importance of Alite and Belite formation and phase quenching that would be required in order to provide Portland clinker, for example. As discussed herein, in some embodiments of the present invention, it is considered that the EAF slag should be cooled relatively quickly from the EAF in order to quench in the required Portland cement phases. Gas (e.g. air) quenching can be used, or heat sink quenching, or a combination of these. Alternatively, or additionally, the cooling may be carried out using water. EXPERIMENTAL SECTION Initial trials produced a slag with very high iron content, approaching 80 % Wustite (FeO) measured by XRD and approaching 70 % FeO measured by SEM-EDS. It is assumed that this was oxidised iron from steel entering the cement slag. The method of melting was adapted in order to reduce the FeO content in slag. This included adding carbon to the scrap melt to oxidise in preference to Fe, removing top layer of scrap melt using a chill plate immediately before adding cement paste, removing the slag soon after fluxing. By using this method, the FeO content of subsequent slag samples was found to be 38-51 % (SEM-EDS analysis). It is considered that this is closer to, but still higher than, the FeO content of a typical EAF slag (typically up to 35%). In all of the trials carried out, melting and complete fluxing was successful. Samples of cement slag were analysed using SEM-EDS to illustrate composition. Table 2 - Clinker material mineral chemical composition Common Name Cement Chemist Notation, where appropriate Chemical Formula Alite C3S CasSiOs Belite C2S Ca2SiO4 Aluminate C3A 3CaO«AI2O3 Ferrite C4AF 4CaO’AI2O3’ Fe2Os Periclase MgO MgO Free Lime CaO CaO Table 2 shows the typical phases found in Portland cement clinker. The approximate bulk mineral composition of typical clinker is: 50-70% alite, 15-30% belite, 5-10% aluminate, 5-15% ferrite, 2% free lime, 2% periclase. Bullard

[29] investigated the effect of cooling of cement clinker on exit from a cement kiln. According to the work reported by Bullard

[29] , the cooling rate from the kiln temperature to 950 °C is considered to be determinative of the mineralisation in the clinker - below 950 °C the minerals in clinker are considered to be fixed. Bullard investigated cooling rates between 1500 °C to 950 °C in the range 550 °C / min to 6.9 °C / min. On the basis of that work, it is considered that cooling of clinker from the furnace temperature to 950 °C should take place over a time of 20 minutes or less, more preferably 15 minutes or less, still more preferably 10 minutes or less, 5 minutes or less, 4 minutes or less, 3 minutes or less, 2 minutes or less, or 1 minute or less. Cooling from the furnace temperature to room temperature may suitably take place in 40 minutes or less, more preferably in 20 minutes or less. Bogue calculation The Bogue calculation is used empirically to calculate the approximate proportions of the four main minerals in Portland cement clinker. We refer to ASTM C150 which gives full details. The calculation assumes that the four main clinker minerals are pure minerals with compositions set out in Table 2 above: Alite, Belite, Aluminate, Ferrite. Clinker is typically made by combining lime and silica and also lime with alumina and iron. If some of the lime remains uncombined, then it is necessary to subtract this from the total lime content before carrying out the calculation in order to get the best estimate of the proportions of the four main clinker minerals present. For this reason, a clinker analysis normally gives a figure for uncombined free lime. The calculation takes the following steps: Firstly, according to the assumed mineral compositions, ferrite is the only mineral to contain iron. The iron content of the clinker therefore fixes the ferrite content. Secondly, the aluminate content is fixed by the total alumina content of the clinker, minus the alumina in the ferrite phase. This can now be calculated, since the amount offerrite phase has been calculated. Thirdly, it is assumed that all the silica is present as belite and the next calculation determines how much lime is needed to form belite from the total silica content of the clinker. There will be a surplus of lime. Fourthly, the lime surplus is allocated to the belite, converting some of it to alite. In practice, the above process of allocating the oxides can be reduced to the following equations, in which the oxides represent the weight percentages of the oxides in the clinker: C3S = 4.0710CaO-7.6024SiO2-1.4297Fe2O3-6.7187AI2O3 [i.e. C3S = 4.0710C-7.6024S-1.4297F-6.7187A] C2S = 8.6024Si02+1.0785Fe203+5.0683AI203-3.0710CaO [i.e. C2S = 8.6024S+1.0785F+5.0683A-3.0710C] C3A = 2.6504AI2O3-1.6920Fe2O3 [i.e. C3A = 2.6504A-1.6920F] C4AF = 3.0432Fe2O3 [i.e. C4AF = 3.0432F] If the compositions of the starting materials are known, then these can be used to carry out the Bogue calculation. Alternatively, it is possible to carry out the calculation based on an elemental compositional analysis of the clinker (e.g. using EDS carried out in an SEM). In that case, however, it is typically not known how much free lime should be taken into account, and accordingly the Bogue calculation as applied to the embodiments discussed herein is adapted as necessary. Comparative Experiment 1 Sample preparation To prepare samples resembling the paste found in Construction and Demolition Waste (CDW) paste samples were prepared with a mass water to cement ratio (w / c) of 0.4. These were left to hydrate for 28 days sealed at room temperature before they were crushed into a powder. One type of cement was investigated and reported here, a CEM I (a pure Portland cement). Sample treatment The samples were pre-heated at 500 °C for 30 minutes, then added to the EAF containing molten scrap for 5-15 minutes. In the EAF there was 8 times as much steel as cement by mass. The resulting slag was removed from the furnace, cooled on a copper plate and ground to <125 pm. It is considered that the cooling experienced by the slag ensured that the slag temperature was less than 950 °C within less than 1 minute. For the w / c 0.4 sample, the melt temperature before the cement paste addition was 1600 °C, the melt temperature at slag removal was 1635 °C, and the time from the end of cement paste addition to the slag removal was 5 minutes. For the w / c 0.6 sample, the melt temperature before the cement paste addition was 1556 °C, the melt temperature at slag removal was 1636 °C, and the time from the end of cement paste addition to the slag removal was 15 minutes. Slag characterisation The ground slag can be analysed using SEM-EDS in order to provide information on the elemental composition of the slag. The ground slag was analysed using X-Ray Diffraction (XRD) combined with Rietveld analysis, which can quantitatively identify all the crystalline phases present. Fig. 5 shows the XRD analysis for the CEM I 0.4 sample. Fig. 6 shows the XRD analysis for the CEM I 0.6 sample. Results A first attempt had an uncharacteristically high amount of F (FeO). This was due to a combination of high oxidation in the melt and furnace geometry. These issues were remediated in a second batch. The amount of FeO in the second batch was still higher than would typically be expected for an EAF. Fluxing was normal. This indicates that recycled CDW paste can work as a flux for EAF operation. The composition of the EAF slag, obtained by XRD was as follows. Table 3 - Composition of EAF slag in Comparative Experiment 1 Compound name Chemical Formula (cement notation) Sample: CEM I w / c=0.4 Sample: CEM I w / c=0.6 Wustite (Iron oxide) F 27.0 47.8 Brownmillerite (Aluminoferrite) C4AF 37.8 41.3 Dicalcium silicate (Belite) C2S 35.2 10.9 This indicates that the process can yield a Belite (C2S) cement. This is not the cement type which we want to produce ultimately, but it is a viable cement and a precursor to Portland cement. 5 Analysis To produce Alite (C3S), there must be an excess of calcium. The lime saturation factor (LSF), as calculated by Bogue is: LSF = C / (2.8S+1.2A+0.65F) Based on the above results, we have 10 F = 27 + 72 / (56*4+102+72)*37.8 = 33.8 A = 102 / (56*4+102+72)*37.8 = 9.7 S = 60 / (56*2+60)*35.2 = 12.3 C = 56*2 / (56*2+60)*35.2 = 22.9 LSF = 0.33 15 This suggests that to reach typical cement plant LSF, we would need a mix of 29% CaO-71 % recycled paste. This would produce C3S instead of C2S. Bogue calculations are empirical and based on the normal operating temperature of kilns. Higher temperatures, as experienced in an EAF, thermodynamically favour the formation of C3S. Therefore, this calculated additional CaO 20 requirement is expected to be a ceiling. Based on similar calculations 25% CaO added to the mix is expected to yield a majority C3S result. In a full-sized EAF, the conditions are much more favourable as the F in contact with the slag is much less (perhaps half). This suggests that in a full scale industrial process, the amount of lime 25 required would be at most 10-15%. This represents a 90-85% abatement in the process emissions associated to both cement and recycled steel production. It is noted that the analysis shows that no C3S was present in the ground EAF slag, whereas the nature of the starting materials is that there should have been some input into the furnace. Without wishing to be bound by theory, the inventors consider that it is possible that the 500 °C pretreatment for 30 minutes may affect the results. It is also noted that different results were seen between the 0.4 w / c and the 0.6 w / c cement pastes. There are no particular reasons these should have yielded different results, aside from the different amounts of C3S left in the sample when prepared to be used as a flux. Comparative Experiment 1 is therefore considered to teach that recycled cement paste can work as a flux for EAF operation. It is considered that a small addition of lime will be conducive to the production of high Alite. Comparative Experiment 2 Comparative Experiment 2 corresponded to Comparative Experiment 1 except that only a mass water to cement ratio (w / c) of 0.4 was used and a lime additive was included. The cement investigated is a CEM I (a pure Portland cement). The lime used as additive was industrial grade (98% pure). Sample treatment The samples were pre-heated at 500 °C for 30 minutes, then added to the EAF containing molten scrap for 5-15 minutes as a flux, with a ratio of 75% cement paste and 25% lime. The resulting slag was removed from the furnace, cooled on a copper plate and ground to <125 pm. Slag characterisation The ground slag was analysed using SEM-EDS. The ground slag was analysed using X-Ray Diffraction (XRD) combined with Rietveld analysis, which can quantitatively identify all the crystalline phases present. Results Based on SEM-EDS analysis, the oxide composition is: • F: 6.32 • A: 6.91 • S: 16.85 • C: 66.43 In order to assess the phases present, we use the Bogue calculation method: Thus, C4AF = 3.043 F = 19.2 C3A = 2.65 A- 1.692 F = 7.6 The amount of free lime is not exactly known. However, it is possible to say with certainty that there was an excess, and we cannot have less than 0% C2S. From this we deduce that we have 64% C3S if no C2S was produced: C3S = 4.071 (C - free lime) - 7.600 S - 6.718 A - 1.430 F = 64 free lime = 6.43 C2S = 2.867 S - 0.7544 C3S = 0 We now explain the rationale behind these calculations in more detail. In both Comparative Experiment 1 and Comparative Experiment 2, the amount of free lime will be in the range between zero and the amount of lime added. In Comparative Experiment 1 , there was not enough lime in order to form Alite, and the LSF was very low. Indeed, the XRD analysis found no free lime. Accordingly, in Comparative Experiment 1 , there was not enough lime in order to reach a LSF of 1, based on the XRD of Comparative Experiment 1. Therefore the lowest amount of free lime giving positive quantities for all phases was selected. That amount is 6%. In principle the maximum possible amount of free lime is 17%. This is derived based on the proportion of lime added (25%) and the proportion of measured calcium in the elemental analysis (about 66%). At 17% free lime the Alite and Belite predicted are 19.3% and 33.9%, respectively. In the view of the inventors, 17% free lime represents the worst case scenario, with 53% Alite plus Belite. However, in the view of the inventors this scenario is not technically realistic, because it implies that there is a lot of free lime despite not being lime saturated and assumes that the lime added did not react at all. This is highly unlikely, in particular from a thermodynamic assessment based on the temperature and the residence time in the EAF. Accordingly, considering the mass of the sample, we can provide the full range of possible outcomes as: Alite proportion in the range 64% to 19.3% Belite proportion in the range 0% to 33.9% In the view of the inventors, for the reasons explained, it is considered most likely that the phase composition of the sample is towards the high Alite proportion end of the range expressed above. It is considered that the other end of the range, in which there is 19.3% Alite and 33.9% Belite is highly unlikely. Further, the inventors consider that for a sufficient residence time, 64% Alite is the outcome of 5 this mixture. The inventors also comment that the Bogue calculation is known to underestimate the amount of Alite (by 3-4% absolute, typically). Accordingly, the inventors have confidence based on the calculations and explanations above that the sample from Comparative Experiment 2 was a high Alite cement. 10 Fig. 7 shows the results of powder XRD analysis and peak identification for the w / c 0.4 sample produced in Comparative Experiment 2. The composition of the EAF slag, obtained by XRD was as follows. 15 Table 4 - Composition of EAF slag from Comparative Experiment 2 Compound name Chemical Formula (cement notation) Sample: CEM 1 w / c=0.4 plus 25% CaO Wustite (Iron oxide) F 2.5 Brownmillerite (Aluminoferrite) C4AF 0 Dicalcium silicate (Belite) C2S 46.7 Tricalcium silicate (Alite) CaS 24 Lime C 2.4 Ferrobustamite CFS 24.4 The sample had a large background problem so quantification was uncertain. More than 70% Alite+Belite was formed, although more Belite than Alite according to this analysis. A phase tentatively identified as ferrobustamite was also identified. Analysis Formally speaking, this test indicates that it is possible to produce a Portland clinker using the process as hereinbefore described. According to EN 197-1: Portland cement clinker is a hydraulic material which shall consist of at least two-thirds by mass of calcium silicates (3CaO.SiO2 and 2CaO.SiO2), the remainder consisting of aluminium and iron containing clinker phases and other compounds. The ratio of CaO to SiO2 shall not be less than 2.0. The magnesium oxide content (MgO) shall not exceed 5.0% by mass. It is clear that the use of 25% addition of CaO is an excess. It is considered that reducing the amount of CaO added further improves the quality of the cement clinker produced using the invention. In view of the formation of a significant proportion of Alite in the EAF slag produced in Comparative Experiment 2, it is therefore demonstrated that the requirements for the formation of Portland cement clinker are satisfied. Experiment 3 Experimental protocol The ratio of CaO to SiO? and AI2O3 required to produce a cementitious material is known in cement science as the lime saturation factor (LSF). LSF between 90 and 100 produce cements which have Alite. To produce mainly Belite, LSF needs to be between 60 to 70 at least. Lower LSF would produce slags with non cementitious phases, while LSF higher than 100 will have more free lime in it. Free lime is a waste of resources and at high quantities can be bad for the concrete which is made with the cement. In a process for recycling steel scraps, the initial availability of silicon dioxide and alumina depends on the purity of the scraps. Dirty scraps provide sufficient SiO2 and AI2O3 for cementitious slags. Clean scraps would not provide sufficient impurities, thus additions of Fe, Si, and metallic aluminium would be required. This experiment used scrap steel plates with very low residual Si dissolved in the steel (-0.15%) and no Al. To replicate the dissolved Si and Al coming from scrap, ferrosilicon (-25% Fe and 75% Si) and metallic aluminium was added. After melting, Si and Al remain as metals not reacting with CaO to form cement. Hence, it was necessary to oxidize the Si and Al. For that purpose, O2 was injected with a lance. It first oxidized all the Al to AI2O3 and oxidized Si dissolved in the metal to SiO2. All oxides rise to the slag. Only Lime (CaO) was added as the flux former, and the lime reacted with the SiO2 and AI2O3 oxidized to form Alite (C3S), belite (C2S) and aluminate (C3A), which are reactive cementitious phases. The hot slag was cooled rapidly to stabilise the cementitious phases at room temperature. Flux additions were determined based on the slag sample live results. To produce cementitious slag material, an electric arc furnace was used. The input materials were 5.5 tonnes of steel scrap, 750 kg of lime, 55 kg of FeSi, 35 kg of metallic aluminium and 100 kg of carbon. An electricity consumption of 5.5 MWh was required to heat up the furnace, melt steel and co-produce cement. Several rounds of O2 injection were used to oxidise desired metals into slag. Results The X-ray fluorescence (XRF) analysis of the cement clinker produced via the slag formation described above was as follows: CaO SiO2 AI2O3 Fe2O 3 MgO MnO C^Os NaO K2O LOI 58.99 14.13 12.06 6.69 5.07 2.7 0.55 0.02 0.02 -1.11 The X-ray diffraction (XRD) analysis of the cement clinker produced via the slag formation described above was as follows: Alite C3S Monoclinic Belite (Beta) Belite (Gamma) C3A cubic C4AF FeO Free lime Gelhenite 45.79 9.41 3.56 17.8 5.93 3.07 7.58 1.35 These results show that a cement clinker containing suitable amounts of reactive cementitious phases, and notably Alite C3S, can be obtained using the process described above. The only source of Ca (and oxides and hydroxides thereof) in this experiment was lime. In other words, lime was used as the flux in this process. The optical emission spectroscopy (OES) analysis of the results of the recycled steel co-produced in the cement clinker formation process described above was as follows: c Si Al P S Cu Mn Cr V Ni 0.88 0.036 0.0045 0.027 0.0049 0.315 0.397 0.401 0.005 0.11 9 These results show that the process of the invention may be a co-production process for cement clinker and recycled steel. The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof. While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention. For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations. Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described. Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example + / -10%. References A number of publications are cited above in order to more fully describe and disclose the invention and the state of the art to which the invention pertains. Full citations for these references are provided below. The entirety of each of these references is incorporated herein. [1] Julian Allwood, Jose Azevedo, Adam Clare, Christopher Cleaver, Jonathan Cullen, Cyrille Dunant, Teppo Fellin, William Hawkins, Ian Horrocks, Philippa Horton, et al. Absolute zero, 2019. [2] John L Provis and Susan A Bernal. Geopolymers and related alkali-activated materials. Annual Review of Materials Research, 44:299{327, 2014. [3] MB Mohd Salahuddin, M Norkhairunnisa, and F Mustapha. A review on thermophysical evaluation of alkali-activated geopolymers. Ceramics International, 41(3):4273{4281,2015. [4] Nicoletta Toniolo and Aldo R Boccaccini. Fly ash-based geopolymers containing added silicate waste. A review. Ceramics International, 43(17): 14545(14551,2017. [5] Frank Bullerjahn, Maciej Zajac, and Mohsen Ben Haha. Csa raw mix design: effect on clinker formation and reactivity. Materials and Structures, 48(12):3895{3911, 2015. [6] Frank Bullerjahn, Dirk Schmitt, and Mohsen Ben Haha. Method for producing ternesite-belite calcium sulfoaluminate clinker, June 302015. US Patent 9,067,825. [7] Natechanok Chitvoranund, Frank Winnefeld, Craig W Hargis, Sakprayut Sinthupinyo, and Barbara Lothenbach. Synthesis and hydration of alite-calcium sulfoaluminate cement. Advances in Cement Research, 29(3): 101 {111 ,2017. [8] Sean Quinn and Sadananda Sahu. Compositions and methods for controling setting of carbonatable calcium silicate cements containing hydrating materials, 2019. US Patent 10,196,311. [9] Mark Sceats and Adam Vincent. Direct separation calcination technology for carbon capture: Demonstrating a low cost solution for the lime and cement industries in the leilac project. In 14th Greenhouse Gas Control Technologies Conference Melbourne, pages 21 {26, 2018.

[10] Karen Scrivener, Fernando Martirena, Shashank Bishnoi, and Soumen Maity. Calcined clay limestone cements (Ic3). Cement and Concrete Research, 114:49{56, 2018.

[11] Nicholas Lippiatt and Florent Bourgeois. Investigation of microwave-assisted concrete recycling using single-particle testing. Minerals Engineering, 31:71{81,2012.

[12] Heesup Choi, Myungkwan Lim, Hyeonggil Choi, Ryoma Kitagaki, and Takafumi Noguchi. Using microwave heating to completely recycle concrete. Journal of Environmental Protection, 2014, 2014.

[13] Cyrille F Dunant, Micha I P Drewniok, Stathis Eleftheriadis, Jonathan M Cullen, and Julian M Allwood. Regularity and optimisation practice in steel structural frames in real design cases. Resources, Conservation and Recycling, 134:294(302, 2018.

[14] Cyrille F. Dunant, Micha I P. Drewniok, John J. Orr, and Julian M. Allwood. Good early stage design decisions can halve embodied C02 and lower structural frames' cost. Structures, submitted, 2020.

[15] Cyrille F Dunant, Jose Granja, Arnaud Muller, Miguel Azenha, and Karen L Scrivener. Microstructural simulation and measurement of elastic modulus evolution of hydrating cement pastes. Cement and Concrete Research, 130:106007, 2020.

[16] M De O Carvalho. Theoretical energy requirement for burning clinker. Cement and concrete research, 29(5):695{698, 1999.

[17] Cyrille F Dunant and Karen L Scrivener. Micro-mechanical modelling of alkali-silica-reaction-induced degradation using the amie framework. Cement and Concrete research, 40(4):517(525, 2010.

[18] Alain B Gloria, Yann Le Pape, and Cyrille F Dunant. Computing creep-damage interactions in irradiated concrete. Journal of Nanomechanics and Micromechanics, 7(2):04017001,2017.

[19] Alain B Gloria and Cyrille F Dunant. Microstructural effects in the simulation of creep of concrete. Cement and Concrete Research, 105:44{53, 2018.

[20] Cyrille F Dunant and Karen L Scrivener. Physically based models to study the alkali-silica reaction. Proceedings of the Institution of Civil Engineers-Construction Materials, 169(3): 136{144, 2016.

[21] Cyrille Dunant. Experimental and modelling study of the alkali-silica-reaction in concrete. PhD thesis, 2009.

[22] HAROLD R Kokal and MADHU G Ranade. Metallurgical uses - fluxes for metallurgy. Ind. Miner. Rocks, 15:661 {675, 1994.

[23] P Kumar Mehta and PJM Monteiro. Concrete: structures, properties and materials. Sao Paulo: IBRACON, 2008.

[24] John Espen Rossen. Composition and morphology of cash in pastes of alite and cement blended with supplementary cementitious materials. Technical report, EPFL, 2014.

[25] Alessandra Primavera, Laura Pontoni, Davide Mombelli, Silvia Barella, and Carlo Mapelli. Eaf slag treatment for inert materials' production. Journal of Sustainable Metallurgy, 2(1):3-12, 2016.

[26] D Kulik, U Berner, and E Curti. Modelling chemical equilibrium partitioning with the gems-psi code. Technical report, 2004.

[27] Pawe I T Durdzinski, Cyrille F Dunant, Mohsen Ben Haha, and Karen L Scrivener. A new quantification method based on sem-eds to assess fly ash composition and study the reaction of its individual components in hydrating cement paste. Cement and Concrete Research, 73:111{122, 2015.

[28] Jose Granja and Miguel Azenha. Towards a robust and versatile method for monitoring e-modulus of concrete since casting: Enhancements and extensions of emm-arm. Strain, 53(4):e12232, 2017.

[29] Robert A. Bullard, EFFECT OF COOLING RATES ON MINERALIZATION IN PORTLAND 5 CEMENT CLINKER, University of Missouri-Kansas City, 2015, https: / / core.ac.uk / download / pdf / 62782661.pdf

Claims

1. A process for the manufacture of cement clinker, the process including the steps: providing a first material, the first material providing iron (Fe) or an oxide thereof;5 heating the first material in a furnace to form a first heated material, wherein the furnace isan electric arc furnace or an induction furnace;providing a second material, said second material providing at least CaO, Ca(OH)2, or a mixture of CaO and Ca(OH)2;contacting a surface of the first heated material with the second material to form a slag;10 wherein the heat from the first heated material promotes clinkering of the slag to form thecement clinker;wherein the second material consists essentially of, or consists of, lime; andwherein the first material further provides:i) at least one source of silicon;15 ii) at least one source of aluminium;wherein greater than 50 wt% of the silicon required to form the cement clinker is provided by the first material; andwherein greater than 50 wt% of the aluminium required to form the cement clinker is provided by the first material.>02. The process according to claim 1, wherein the at least one source of silicon comprises SiO2.

3. The process according to claim 1 or claim 2, wherein the at least one source of aluminium 25 comprises AI2O3.

4. The process according to any preceding claim, wherein the first material is an iron-containing alloy.30 5. The process according to any preceding claim, wherein the iron-containing alloy furthercomprises carbon.

6. The process according to any preceding claim, wherein the first material is a steel.

7. The process according to any preceding claim, wherein the second material comprises argillaceous, silicious, conglomerate, magnesian, or dolomitic lime.

8. The process according to any preceding claim, wherein the second material is pelletised before being added to the furnace.

9. The process according to any preceding claim, wherein no material or additive other than the first material and the second material is added to the furnace.

10. The process according to any of claims 1 to 8, further comprising the step of providing one or more further materials and contacting a surface of the first heated material or the slag with the one or more further materials, wherein the one or more further materials comprise a clay, a sand, corundum, or mixtures thereof.

11. The process according to any preceding claim, wherein the furnace is an electric arc furnace.

12. The process according to any preceding claim, wherein the maximum operating temperature experienced by the cement clinker in the furnace is at least 1350 °C.

13. The process according to any preceding claim, further including the step of removing the cement clinker from the furnace.

14. The process according to any preceding claim wherein the cement clinker is a Portland cement clinker.

15. The process according to any preceding claim, wherein the cement clinker is a hydraulic material comprising one or more calcium silicates, wherein the calcium silicates are Alite, Belite, or a combination of Alite and Belite.

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