Method for producing a carbonatable binder composition

Abstract

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Description

Method for producing a carbonatable binder compositionTechnical field

[0001] The present invention is related to a method for producing a carbonatable binder composition, i.e. a binder composition that is capable to react with CO2 during a carbonation reaction. The invention is further related to such carbonatable binder compositions.Background art

[0002] Cements enable the manufacture of a wide variety of building materials and structures such as mortar, concrete or stucco. Upon hydraulic setting, i.e. exposure to or being contacted with water, the cement phase acts as an adhesive for other components or articles and imparts strength thereto. However, cement manufacturing, usually carried out at temperatures of around 1450 °C, is an energy- intensive process that releases high quantities of CO2.

[0003] An approach to limit the overall release of CO2 is the use of carbonatable binder materials which develop strength by reaction with CO2. Consequently, the CO2 consumed during the hardening process at least partially counterbalances the CO2 produced during manufacturing of the carbonatable binder material.

[0004] Particularly interesting carbonatable binder materials are calcium silicate (Ca-silicate) based binder materials. An advantage of calcium silicates is that they are abundantly present in natural mineral materials, but also in by-products of mining processes and metal production processes.

[0005] A wide variety of industrial by-products and waste streams have proven to be suitable for use in carbonatable binder materials, including fly ash, mine tailings and metal or furnace slags resulting from the production of steel, stainless steel and other metallic materials.

[0006] Further, such industrial by-products or waste streams often also comprise Mg-silicates. However, Mg-silicates show lower reactivity to CO2, requiring high temperatures (> 100 °C) and high pressures (> 50 bar), often in combination with the use of chemical additives, to reach an acceptable conversion. Consequently, these Mg- silicates typically do not carbonate during the carbonation conditions used for the Ca- silicates, leading to inferior carbonated (i.e. carbonate bonded) articles, e.g. carbonated articles having insufficient mechanical strength (e.g. compressive strength).

[0007] EP 3694 818 discloses a method of producing a binder composition which can be hardened by both hydration and carbonation. The method comprises heating a starting material to 600-1200 °C for 1 minute to 5 hours to convert at most 80 % of the starting material. The starting material comprises CaO, SiC>2, up to 30 wt.% of other elements than CaO and SiO with regard to the total mass calculated as oxides, and the molar ratio of Ca to Si is between 0.5 and 1 .8 The binder composition comprises a hydratable material, which comprises at most 10 wt.% monocalcium silicate, at least 15 wt.% dicalcium silicate, at least 3 wt.% CaO, less than 10 wt.% wollastonite, and from 20 to 80 wt.% phases from the raw materials and / or formed during the transformation step e.g. quartz, Ca3Si2O?, CasSiOs, CasAhOe, Ca4AhFe20io, gehlenite, merwenite, altered hydrated phases, glassy and / or X-ray amorphous components, and other rock- and soil-forming minerals. Although some akermanite seemed to be present, it was found to have a negative impact on the carbonation.

[0008] EP 3 177 578 discloses a carbonatable Ca-silicate composition that can be carbonated with CO2 at a temperature between 30 °C and 90 °C to form CaCCh with a mass gain of at least 10 %. The composition comprises 30 % by weight or more, based on the total weight of the Ca-silicate phases, of one or more discrete, crystalline Ca-silicate phases selected from CaSiCh, Ca3Si2O? and Ca2SiC>4, and an amorphous Ca- silicate phase, as well as at most 30 % by weight of metal oxides of Al, Fe and Mg. The composition comprises Ca and Si in a molar ratio of elemental Ca to elemental Si between 0.8 and 1.2. The composition can further comprise one or more inert phases (i.e., non-carbonatable under typical carbonation conditions) such as melilite type minerals (for example gehlenite or akermanite) with the general formula (Ca,Na,K)2 [(Mg, Fe2+,Fe3+,AI,Si)3O?] or ferrite type phases with the general formula Ca2(AI, Fe3+)20s. The composition may further comprise magnesium silicate in naturally-occurring or synthetic form(s) ranging from trace amount (1 %>) to about 50%> or more by weight. In order to obtain a sufficient degree of carbonation and a sufficient strength of the obtained carbonate bonded article, at least 30 % by weight of reactive phases (crystalline and amorphous Ca-silicate) are required, whereas the amount of metal oxides of Al, Fe and Mg should be limited to a maximum of 30 % by weight.

[0009] US2020 / 0055774 discloses a process for producing a clinker composition that can be carbonated from municipal solid waste incinerator residues (ashes). The ashes are primed to uniform the powder size, and are then converted into nodules. The nodules comprise between 56 and 70 wt.% of CaO, AI2O3 and SiO2, with the following target ranges: 35-45 wt.% CaO, 2.0-7.5 wt.% AI2O3 and 12-20 wt.% SiO2. The nodules are fired at a temperature between 1000 and 1100 °C for durations between30 and 60 minutes. The obtained clinker is meant to comprise one or more of the following CCh-reactive phases: ellestadite, mono-calcium silicates (e.g. wollastonite), dicalcium silicates (e.g. belite) and tri-calcium silicates (e.g. alite), which comprise 40 to 70 wt.% of the clinker. The remaining portion, not necessarily contributing to binding and strength, may comprise one or more of wadalite, mayenite, akermanite, gehlenite, bredigite, anhydrite, ronderfite, quarz, halite / syl vite and an amorphous fraction.

[0010] A disadvantage of the foregoing methods of producing carbonatable binder compositions is the significant amount of calcium present as Ca-silicates that is required as active components in the binder composition, which means the production of significant amounts of CO2 during manufacturing of the binder compositions resulting from the production of these Ca-silicates. As a result, the potential use or valorisation of Mg-silicate resources is restricted, and the maximal strength might not result after carbonation. Further, the Ca-silicate compositions may include quantities of inert (i.e., non-reactive) phases that are non-carbonatable under typical carbonation conditions. These inert phases are expected to have a negative impact on the compressive strength development during carbonation.Summary of the invention

[0011] The present invention aims to overcome one or more of the above drawbacks. It is an aim to provide a method for producing a carbonatable binder composition that can be carbonated at mild temperatures and CO2 pressure, thereby producing carbonate bonded articles having a high compressive strength. Yet another aim of the invention is to provide methods that allow the conversion of a wide range of industrial waste streams and by-products, in particular mine tailings and metal slags, typically comprising Mg-silicates and mixed Ca-Mg-silicates, into a carbonatable binder composition, in particular a carbonatable binder composition that can be carbonated at mild temperatures and CO2 pressure. In other words, it is an aim of the invention to provide valorisation methods for Mg-silicate and mixed Ca-Mg-silicates comprising industrial by-products (in particular slags and tailings) and waste streams, thereby also reducing the environmental footprint of industrial processes producing these by-products (e.g. the production of steel), since the by-products do no longer needs to be disposed of, for example by landfilling.

[0012] It is a further aim to provide a method for producing a carbonatable binder composition which, upon carbonation, in particular carbonation in mild conditions, provides mechanical properties, in particular mechanical strength such as compressive strength, similar or even better than existing carbonatable binder compositionscomprising or based on Ca-silicates. A further aim is to provide a method for producing a carbonatable binder composition which, upon carbonation, provides articles having an excellent resistance against water, and thus to any possible damage imparted by water.

[0013] Yet another aim is to provide a carbonatable binder composition which allows to obtain carbonate bonded articles having mechanical properties, in particular mechanical strength, similar or even better than existing carbonate bonded articles. A further aim is to provide a carbonatable binder composition which allows to obtain carbonate bonded articles having an excellent resistance against water, and thus to any possible damage imparted by water.

[0014] The term “carbonatable binder composition” is used interchangeably with “binder composition” in the present disclosure, and is used to describe a binder composition that is capable to undergo a carbonation reaction, i.e. that is capable to react with CO2. Upon reaction with CO2, i.e. during the carbonatation reaction, the binder composition advantageously hardens, thereby obtaining a carbonate bonded article. When applied between materials or articles that need to be connected or bonded together, the carbonatable binder composition will provide a bond upon exposure to CO2, thereby imparting mechanical strength.

[0015] Advantageously, the term “carbonatable binder composition” is used in the present disclosure for a non-hydraulic binder composition, i.e. a binder composition that does not require exposure to, or being contacted with, water to react and form a binder that binds the materials, articles or components that need to be connected or bonded.

[0016] The terms “carbonate bonded article” and “carbonated article” are used interchangeably in the present disclosure, and refer to an article obtained by carbonation of a carbonatable binder composition.

[0017] The terms “inert phase” and “non-reactive phase” are used interchangeably in the present disclosure, and refer to phases which do not react with CO2, for example upon carbonation. Consequently, such phases are considered as noncontributing to the strength, in particular the compressive strength, of the obtained carbonated article.

[0018] According to a first aspect of the present disclosure, there is provided a method for producing a carbonatable binder composition as set out in the appended claims.

[0019] The carbonatable binder composition produced by the method of the first aspect comprises a mixed Ca-Mg silicate, i.e. one single type of mixed Ca-Mg silicate or a plurality of different types of mixed Ca-Mg silicates.

[0020] The reactive mixture comprises or substantially consists of a mineral source material and a Ca-comprising compound. In other words, the reactive mixture may comprise further compounds in addition to the mineral source material and the Ca- comprising compound. Whether the reactive mixture comprises further compounds depends, amongst others, on the composition of the mineral source material. Advantageously, the reactive mixture consists of the mineral source material and the Ca- comprising compound, i.e. the sum of their respective amounts equals the weight of the reactive mixture, and the sum of their respective % by weight equals 100 %.

[0021] The reactive mixture comprises between 40 and 90 % by weight, preferably between 45 and 85 % by weight, more preferably between 50 and 80 % by weight of a mineral source material, based on the total weight of the reactive mixture.

[0022] The reactive mixture comprises between 60 and 10 % by weight, preferably between 55 and 15 % by weight, more preferably between 50 and 20 % by weight of a Ca-comprising compound, based on the total weight of the reactive mixture.

[0023] The mineral source material comprises or substantially consists of a Mg-comprising silicate, i.e. one or a plurality of Mg-comprising silicate(s).

[0024] In the light of the present disclosure, the % by weight of each compound of the reactive mixture refers to the weight of the respective compound prior to composing the reactive mixture, based on the total weight of the reactive mixture. For example, a reactive mixture of 100 g and comprising 75 % by weight of a mineral source material and 25 % by weight of a Ca-comprising compound comprises 75 g of the mineral source material and 25 g of the Ca-comprising compound, wherein these respective masses are measured prior to combining them to form the reactive mixture.

[0025] The molar ratio of elemental Mg to elemental Si in the mineral source material is between 5:1 and 1 :5, preferably between 2:1 and 1 :2, more preferably between 1.5:1 and 1 :1.5.

[0026] The method comprises heating a reactive mixture to a temperature between 800 °C and 1400 °C, preferably between 1000 °C and 1375 °C, more preferably between 1200 °C and 1375 °C, thereby forming a mixed Ca-Mg silicate and obtaining the carbonatable binder composition.

[0027] More particularly, heating the reactive mixture results in the formation of at least akermanite (Ca2MgSi2O?) as mixed Ca-Mg silicate, i.e. the obtained carbonatable binder composition comprises akermanite. The inventors have surprisingly discovered that akermanite shows a high reactivity towards CO2 in the carbonation reaction, contrary to what is generally assumed.

[0028] The Ca-comprising compound is capable of reacting with the Mg- comprising silicate, advantageously upon heating the reactive mixture to a temperature as described hereinabove.

[0029] Advantageously, the Ca-comprising compound comprises or substantially consists of a Ca-comprising carbonate. The Ca-comprising carbonate advantageously comprises or substantially consists of one or more of calcite, aragonite, chalk, limestone or dolomite. Advantageously, the Ca-comprising carbonate comprises or substantially consists of CaCCh.

[0030] Advantageously, the Mg-comprising silicate is MgSiO3or Mg2SiO4 or diopside. In other words, the mineral source material advantageously comprises or substantially consists of one or more of MgSiO3and / or Mg2SiO4 and / or diopside.

[0031] The mineral source material may further comprise iron (Fe), i.e. anFe-comprising compound. Advantageously, the mineral source material comprises equal to or lower than 20 % by weight, preferably equal to or lower than 15 % by weight, more preferably equal to or lower than 10 % by weight, most preferably equal to or lower than 5 % by weight, such as at most 4 % by weight of Fe, based on the total weight of the mineral source material, wherein the amount of Fe is expressed as Fe oxide in the mineral source material.

[0032] Advantageously, the mineral source material further comprises a calcium-comprising silicate, for example a Ca-silicate. Advantageously, the mineral source material comprises equal to or lower than 20 % by weight, preferably equal to or lower than 15 % by weight, more preferably equal to or lower than 10 % by weight, such as at most 5 % by weight of Ca-comprising silicate, based on the total weight of the mineral source material.

[0033] The inventors have found that the presence of such an amount of aCa-comprising silicate in the mineral source material allows to reduce the amount of the Ca-comprising compound to be incorporated in the reactive mixture, i.e. the reactive mixture can comprise a higher percentage by weight of mineral source material and a lower amount of the Ca-comprising compound.

[0034] Advantageously, the mineral source material comprises a pyroxene having the general formula XY(Si,AI)2O6, whereinX individually is Ca, Na, Fe(ll) or Mg; andY individually is Cr, Al, Mg, Co, Mn, Sc, Ti, V, Fe(ll) or Fe(lll).

[0035] Advantageously, X and / or Y is / are Mg and the pyroxene comprisesSi, i.e. the pyroxene advantageously is a Mg-comprising silicate. Advantageously, in particular when the pyroxene is a Mg-comprising silicate, the pyroxene is diopside(CaMgSi20e; X=Ca and Y=Mg) or enstatite (MgSiCh; X=Y=Mg), of which diopside is particularly preferred.

[0036] Alternatively or additionally, yet advantageously, the mineral source material comprises an amphibole having the general formula ZySisC^OH^, wherein Z individually is selected from the group consisting of: Ca, Mg, Na, Cr, Al, Co, Mn, Sc, Ti, V, Fe(ll) and Fe(lll).

[0037] According to a second aspect of the present disclosure, there is provided a carbonatable binder composition as set out in the appended claims.

[0038] The carbonatable binder composition comprises or substantially consists of a mixed Ca-Mg silicate. The carbonatable binder composition comprises at least 10 % by weight, preferably at least 15 % by weight, more preferably at least 17.5 % by weight, most preferably at least 20 % by weight, for example at least 25 % by weight, or at least 30 % by weight, or at least 50 % by weight of akermanite (Ca2MgSi2O?), based on the total weight of the carbonatable binder composition.

[0039] In the present disclosure, the weight percentage (% by weight) of phases in the carbonatable binder composition are measured and obtained by means of X-ray diffraction (XRD).

[0040] Advantageously, when the carbonatable binder composition is produced by means of the method of the first aspect of the invention, akermanite is formed during heating the reactive mixture, i.e. the formed akermanite is a result of the reaction of the Ca-comprising compound with the mineral source material, in particular with the Mg-comprising silicate comprises therein. It will be understood that the amount of akermanite in the carbonatable binder composition depends, amongst others, on the composition of the reactive mixture, and in particular on the composition of the mineral source material comprised in the reactive mixture.

[0041] Advantageously, the carbonatable binder composition comprises equal to or lower than 20 % by weight, preferably equal to or lower than 15 % by weight, more preferably equal to or lower than 10 % by weight, of other phases from the melilite group (i.e. phases from the melilite group excluding akermanite), based on the total weight of the carbonatable binder composition, wherein the % by weight is measured as described hereinabove, i.e., by using XRD. A particular example of such a phase is gehlenite (Ca2AhSiOy). The inventors have surprisingly discovered that the reactivity of such other phases from the melilite group have a very low, even substantially negligible, reactivity towards CO2 in mild carbonation conditions, i.e. mild temperature (10 - 100 °C) and CO2 pressure (at most 2 MPa), contrary to akermanite which has a surprisingly elevated reactivity towards CO2 in such carbonation conditions. In other words,carbonatable binder compositions comprising a significant amount of such phases, i.e. more than 20 % by weight, based on the total weight of the carbonatable binder composition, require a higher temperature and / or a higher CO2 pressure for any carbonation to take place.

[0042] Contrary to what is disclosed in the prior art, and thus known, the inventors have surprisingly discovered that akermanite can be carbonated (i.e. is not inert during carbonation). The inventors further surprisingly discovered that akermanite present in the binder composition strongly contributes to the mechanical strength, in particular the compressive strength, of the obtained carbonate bonded articles. The inventors further surprisingly discovered that small amounts of akermanite are capable of imparting a higher mechanical strength, in particular the compressive strength, of the obtained carbonated articles that cannot be obtained by the use of Ca silicates.

[0043] The inventors have further surprisingly discovered that contrary to akermanite, other phases of the melilite group do not seem to contribute to a large extent to the mechanical strength of the obtained carbonate bonded article, i.e. these phases seem to be more “inert” (i.e. non-reactive with CO2 during carbonation).

[0044] Advantageously, the carbonatable binder composition comprises equal to or lower than 20 % by weight, preferably equal to or lower than 15 % by weight, more preferably equal to or lower than 10 % by weight, of merwinite (Ca3MgSi20s), monticellite (CaMgSiOt) and diopside (CaMgSi2O6), based on the total weight of the carbonatable binder composition. With “equal to or lower than 20 % by weight of merwinite, monticellite and diopside" is meant in the present disclosure the sum of the respective amounts of each one of merwinite, monticellite and diopside.

[0045] According to a third aspect of the present disclosure, there is provided a construction material composition as set out in the appended claims.

[0046] The construction material composition comprises the carbonatable binder composition according to the second aspect of the present disclosure or the carbonatable binder composition obtained by the method of the first aspect of the present disclosure.

[0047] The construction material composition advantageously further comprises typical compounds used in construction material compositions, such as sand, pebbles, aggregates, fibres, inert fillers and additives such as limestone filler, fine quarts and quartz flower.

[0048] The construction material composition is advantageously applied, i.e. added to or used in, construction materials, in particular concrete articles. Suchconcrete articles are advantageously used in applications including, but not limited to, infrastructure, construction, pavement, and landscaping industries.

[0049] According to a further aspect of the present disclosure, there is provided a method of reacting a carbonatable binder composition with CO2, i.e. a carbonation reaction, as set out in the appended claims.

[0050] The carbonatable binder composition is according to the second aspect of the present disclosure or obtained by the method of the first aspect of the present disclosure.

[0051] The carbonatable binder composition is contacted with (exposed to, reacted with) CO2 at a temperature between 10 °C and 100 °C and at a CO2 pressure equal to or lower than 2 MPa. Contacting the carbonatable binder composition with CO2 is advantageously performed by means and methods known in the art, such as passing a flow of CO2 through the binder composition, or placing the binder composition in an environment comprising or substantially consisting of CO2.

[0052] The present disclosure further relates to a carbonate bonded article obtained by this carbonation method. Advantageously, the carbonate bonded article has a compressive strength of at least 5 MPa, preferably at least 10 MPa.

[0053] An advantage of the foregoing methods of producing a carbonatable binder material is that the reactive mixture used to obtain the carbonatable binder composition can comprise a high amount, up to 90 % by weight, of Mg-comprising silicate comprising by-products or waste products from industrial processes. More, these byproducts and waste products can be obtained from a wide variety of industrial processes. The inventive methods thus have a high valorisation potential of industrial by-products and waste products, while the valorisation possibilities of such waste streams have always been limited up to now.

[0054] An advantage of the carbonatable binder compositions of the present invention is that they can be carbonated at moderate conditions of temperature (between 10 and 100 °C) and CO2 pressure (equal to or lower than 2 MPa), thereby reducing the energy consumption of the carbonation process. Without wishing to be bound by any theory, the inventors believe that this is realised due to the high amount of akermanite present in the binder composition.

[0055] Further, despite the mild carbonation conditions, the carbonate bonded articles obtained from the inventive carbonatable binder compositions show a high mechanical strength, in particular a compressive strength, which was surprisingly significantly higher than the mechanical strength of prior art carbonate bonded articles obtained by carbonation, in particular articles obtained by carbonation of other Mg-silicates, other mixed CaMg-silicates, or, surprisingly, even Ca-silicates at similar conditions, and equal to and even higher than the mechanical strength of prior art carbonate bonded articles obtained by carbonation at high temperature and / or high CO2 pressure. Without wishing to be bound by any theory, the inventors believe that this is realised due to the akermanite present in the binder composition. The inventors surprisingly discovered that the presence of a limited amount of akermanite already suffices to achieve a high increase of the compressive strength upon carbonation, the increase being higher than expected from carbonation of corresponding Ca-silicates.Brief description of the figures

[0056] Aspects of the invention will now be described in more detail with reference to the appended drawings, wherein same reference numerals illustrate same features and wherein:

[0057] Figure 1 shows the compressive strength of carbonate bonded articles obtained from two binder compositions of the invention, carbonated at varying pressure and temperature.

[0058] Figure 2 shows the compressive strength of carbonate bonded articles obtained from binder compositions comprising various amounts of CaCCh, in function of the % by weight of CaCCh.

[0059] Figure 3 shows the theoretical simulation of the phase composition of a binder composition of the invention in function of the amount of CaCCh.

[0060] Figure 4 shows the theoretical simulation of the phase composition of another binder composition of the invention in function of the amount of CaCCh.

[0061] Figure 5 shows the compressive strength of carbonate bonded articles obtained from inventive binder compositions comprising mine tailings and various amounts of CaCCh, in function of the % by weight of CaCCh.

[0062] Figure 6 shows the theoretical simulation of the phase composition of yet another binder composition of the invention in function of the amount of CaCCh.

[0063] Figure 7 shows the compressive strength of carbonate bonded articles obtained from inventive binder compositions comprising commercial Mg-silicate and various amounts of CaCCh, in function of the % by weight of CaCCh.Description of embodiments

[0064] The reactive mixture heated to form (i.e. produce or manufacture) a carbonatable binder composition comprises or substantially consists of between 40 and 90 % by weight of a mineral source material comprising or substantially consisting of aMg-comprising silicate, and between 60 and 10 % by weight of a Ca-comprising compound, based on the total weight of the reactive mixture.

[0065] A wide variety of materials or compositions can be used as the Ca- comprising compound. Advantageously, the Ca-containing (or Ca-comprising) compound comprises or substantially consists of an industrial waste product, preferably originating from mining, quarrying, or cement sectors. The inventors believe that the chemical composition of the Ca-containing compound, i.e. the presence of calcium, and the reactivity with the mineral source material are the essential features of the Ca- comprising compound in the reactive mixture. In other words, the exact composition of the phase and the physical properties of the Ca-comprising material are considered less important for providing an optimal reactive mixture.

[0066] Advantageously, the Ca-comprising compound comprises or substantially consists of an inorganic Ca-comprising compound. Non-limiting examples of inorganic Ca-comprising compounds include Ca oxide, Ca nitrate, Ca sulphate, Ca carbonate, Ca phosphate, Ca chloride. A particularly preferred Ca-comprising compound is Ca-carbonate, in particular CaCCh. The inventors have found that when the Ca- comprising compound comprises or substantially consists of CaCCh, the formation of phases that can react with CO2 is accelerated during the heat treatment.

[0067] Alternatively, yet also advantageously, the Ca-comprising compound comprises or substantially consists of an organic Ca-comprising compound. Non-limiting examples of organic Ca-comprising compounds include Ca citrate, Ca oxalate, and Ca acetate.

[0068] Advantageously, the Ca-comprising compound is a particulate material, i.e. comprises solid particles. Advantageously, the particulate material has a particle size distribution having a dso value between 0.5 pm and 500 pm, preferably between 1 pm and 200 pm, more preferably between 2 pm and 150 pm, or between 5 pm and 100 pm, as measured by laser diffraction, in particular laser diffraction performed according to standard ASTM B822-20 (2020). It will be understood that the particle size and the particle size distribution can vary, depending on the source from which the Ca- comprising compound is provided, as well as any optional pre-treatments to obtain a certain average size or particle size distribution. For example, the reaction rate and yield of the heating operation can be improved by using a particulate Ca-comprising compound having a particle size distribution having a dso value between 5 pm and 75 pm, preferably between 10 pm and 40 pm, as measured by laser diffraction, in particular laser diffraction performed according to standard ASTM B822-20 (2020).

[0069] The molar ratio of elemental Mg to elemental Si in the mineral source material may vary depending on the mineral source material’s composition, but is between 10:1 and 1 :10, preferably between 5:1 and 1 :5, more preferably between 2:1 and 1 :2, most preferably between 1.5:1 and 1 :1.5.

[0070] An advantage of the present invention is that a wide variety of materials or compositions can be used as the mineral source material, as long as the molar ratio of elemental Mg to elemental Si in the mineral source material is between 10:1 and 1 :10, preferably between 5:1 and 1 :5, more preferably between 2:1 and 1 :2, most preferably between 1.5:1 and 1 :1.5.

[0071] The inventors have surprisingly discovered that the relative amount, for example the molar ratio, of Mg to Ca in the reactive mixture is not an essential parameter for the present disclosure, in particular the methods of the first aspect, as long as the reactive mixture comprises the mineral source material and the Ca-comprising compound in an amount as described hereinabove. It was discovered that it is the molar ratio of elemental Mg to elemental Si in the mineral source material, together with the % by weight of the mineral source material and the % by weight of the Ca-comprising compound which must be within the ranges mentioned hereinabove, that impacts the characteristics and composition of the carbonatable binder compositions obtained by the methods of the first aspect.

[0072] Advantageously, the molar ratio of Mg to Ca in the reactive mixture is comprised between 1 :20 and 20:1 , preferably between 1 :10 and 10:1 , more preferably between 1 :5 and 5:1 , for example between 1 :4 and 4:1 , or between 1 :4 and 2:1.

[0073] Non-limiting examples of suitable mineral source materials include naturally occurring minerals, industrial mineral waste streams and industrial mineral side products. Examples of suitable waste streams and side products (i.e. by-products) include, without being limited thereto, metallurgical slags (for example from ferronickel production), mine tailings (for example Mg-silicate comprising mine tailings) and leaching residues. It will be understood that mixtures of two or more of the foregoing can also be used as the mineral source material.

[0074] For example, mining operations targeting mafic and ultramafic rocks(which contain high levels of magnesium) may generate mixed Mg-silicate or Ca-Mg- silicate-comprising residues. For example, mining of mafic rocks can release a substantial amount of pyroxene and amphibole minerals (Mg-silicate or mixed CaMg- silicate) into tailings. Merwinite (3Ca0 MgO2Si02) is commonly found in steel slags. Olivine (2MgO 2FeO SiO2) is another mineral present in ferronickel slags or mine tailings that contributes to the magnesium silicate content.

[0075] Advantageously, the mineral source material is a particulate material, i.e. comprises solid particles. Advantageously, the particulate material has a particle size distribution having a dso value between 0.5 pm and 500 pm, preferably between 1 pm and 200 pm, more preferably between 2 pm and 150 pm, or between 5 pm and 100 pm, as measured by laser diffraction, in particular laser diffraction performed according to standard ASTM B822-20 (2020). It will be understood that the particle size and the particle size distribution can vary, depending on the source from which the mineral source material is provided, as well as any optional pre-treatments to obtain a certain average size or particle size distribution. For example, the reaction rate and yield of the heating operation can be improved by using a particulate mineral source material having a particle size distribution having a dso value between 5 pm and 75 pm, preferably between 10 pm and 40 pm, as measured by laser diffraction, in particular laser diffraction performed according to standard ASTM B822-20 (2020).

[0076] Advantageously, the mineral source material comprises at least 10% by weight, preferably at least 15 % by weight, more preferably at least 20 % by weight of a pyroxene, based on the total weight of the mineral source material, wherein the pyroxene has the general formula XY(Si,AI)2O6, whereinX individually is Ca, Na, Fe(ll), Mg, Zn, Mg or Li, preferably Ca, Na, Fe(ll) or Mg; andY individually is Cr, Al, Mg, Co, Mn, Sc, Ti, V, Fe(ll) or Fe(lll).

[0077] Advantageously, Y (i.e. the Y ion) has a size equal to or lower than the size of X (i.e. the X ion).

[0078] Advantageously, at least one of X and Y is Mg, i.e. the pyroxene is a Mg-comprising pyroxene. When at least one of X and Y is Mg, the Mg-comprising silicate of the mineral source material advantageously comprises or substantially consists of the Mg-comprising pyroxene. It is known that Mg-comprising pyroxenes show a high reactivity towards Ca-comprising compounds, in particular CaCCh, resulting in the formation of akermanite. Non-limiting examples of preferred pyroxenes include diopside, CaMgSi2C>6 (X=Ca and Y=Mg) and enstatite, MgSiCh (X=Y=Mg), of which diopside is particularly preferred.

[0079] Alternatively or additionally, yet advantageously, the mineral source material comprises at least 10 % by weight, preferably at least 15 % by weight, more preferably at least 20 % by weight of an amphibole, based on the total weight of the mineral source material, wherein the amphibole has the general formula ZySisC^OH^, wherein Z individually is selected from the group consisting of: Ca, Mg, Na, Cr, Al, Co, Mn, Sc, Ti, V, Fe(ll) and Fe(lll).

[0080] Advantageously, at least one of Z is Mg, i.e. the amphibole is a Mg- comprising amphibole. It is known that Mg-comprising amphiboles show a high reactivity towards Ca-comprising compounds, in particular CaCCh, resulting in the formation of akermanite.

[0081] Amphiboles are generally considered as being detrimental to human health, but are often disposed of, despite their health risk. Consequently, the methods of the present disclosure allow to convert these products into a useful product that is no longer detrimental to human health, thereby providing a valorisation of such hazardous products.

[0082] Advantageously, the mineral source material further comprises equal to or lower than 20 % by weight of a Ca-comprising compound, based on the total weight of the mineral source material. Non-limiting examples of such a Ca-comprising compound include one or more of calcite, aragonite, chalk, limestone and dolomite.

[0083] The inventors have found that the presence of a Ca-comprising compound in the mineral source material allows to reduce the amount of the Ca- comprising compound that needs to be added to the mineral source material in the reactive mixture to form mixed Ca-Mg silicate(s).

[0084] However, the mineral source material advantageously comprises equal to or lower than 20 % by weight, preferably equal to or lower than 15 % by weight, more preferably equal to or lower than 10 % by weight of a Ca-silicate, based on the weight of the mineral source material. The inventors have found that the presence of a Ca-silicate in the mineral source material tends to limit the formation of akermanite, and thereby reduce the reactivity towards CO2.

[0085] The mineral source material can further comprise an Fe-comprising compound. Non-limiting examples of Fe-comprising compounds include mixed Mg-Fe silicates such as olivine (general formula (Mg,Fe)2SiO4), mixed Ca-Fe silicates (general formula (Ca,Fe)SiC>4), or mixed Ca-Mg-Fe silicates (general formula Ca,Mg,Fe)2SiO4). A particular example of an olivine is fayalite (Fe2SiC>4).

[0086] The presence of Fe-comprising compounds may favour the formation of compounds such as magnetite (FesOt) or hematite during the heating operation. However, magnetite and hematite are known to have a low reactivity towards CO2, and thus a low carbonization rate.

[0087] Consequently, and advantageously, to favour the formation of mixed Ca-Mg-silicates during the heating operation, and in particular the formation of akermanite, the mineral source material comprises equal to or lower than 20 % by weight, preferably equal to or lower than 15 % by weight, more preferably equal to or lower than10 % by weight, such as at most 5 % by weight of Fe, based on the total weight of the mineral source material, wherein the amount of Fe is expressed as Fe oxide in the mineral source material.

[0088] Advantageously, the Ca-comprising compound is added to the mineral source material, thereby obtaining a reactive mixture. Advantageously, the reactive mixture has a homogeneous composition, i.e. the mineral source material and the Ca-comprising compound are evenly distributed in the reactive mixture. The Ca- comprising compound can be added by means known in the art, such as by mixing.

[0089] Advantageously, the reactive mixture is heated in an atmosphere comprising at least 1 % of oxygen, preferably at least 5 %, more preferably at least 10 %. Non-limiting examples include air and a substantially pure O2 atmosphere. Preferably, the heating operation is performed in air.

[0090] The carbonatable binder composition of the present disclosure, in particular the carbonatable binder composition obtained by methods of the present disclosure, comprises at least 10 % by weight, preferably at least 15 % by weight, more preferably at least 20 % by weight, most preferably at least 25 % by weight, such as at least 50 % by weight or at least 75 % by weight of akermanite (Ca2MgSi2O?), based on the total weight of the carbonatable binder composition. It is known in the art that akermanite is a crystalline compound. The % by weight of akermanite is advantageously measured by means of X-ray diffraction (XRD).

[0091] The inventors have surprisingly discovered that akermanite shows a high reactivity towards CO2 in the carbonation reaction. In particular, akermanite shows a high CO2 uptake at a temperature equal to or lower than 100 °C and at a CO2 pressure equal to or lower than 2 MPa (20 bar). Consequently, the presence of akermanite in the carbonatable binder composition was found to allow carbonation at moderate temperature and at moderate CO2 pressure.

[0092] The inventors have further surprisingly discovered that the presence of even a small amount of akermanite in the carbonatable binder composition has a positive impact on the mechanical strength, in particular the compressive strength, imparted by the binder composition upon carbonation, as well as on the resistance against water of the carbonated article.

[0093] The carbonatable binder composition can further comprise an iron oxide, in particular one or more of FeO, Fe2Oa, or a mixed Fe(l l)-Fe(l 11) oxide. It is known that iron oxide in the carbonatable binder composition does not contribute to an improved the mechanical strength, in particular compressive strength, of carbonated articles upon carbonation of the carbonatable binder composition, the binder compositionadvantageously comprises equal to or lower than 20 % by weight, preferably equal to or lower than 15 % by weight, more preferably equal to or lower than 10 % by weight, most preferably equal to or lower than 5 % by weight of iron oxide, based on the total weight of the carbonatable binder composition. The % by weight of iron oxide is advantageously measured by means of XRD.

[0094] The carbonatable binder composition can further comprise a Mg silicate, such as MgSiCh and Mg2SiC>4. It is however known that such Mg silicates tend to have a rather low reactivity towards CO2, and consequently the amount of Mg silicates in the binder composition is limited. Advantageously, when the binder composition comprises a Mg silicate, it comprises at most 40 % by weight, preferably at most 30 % by weight, more preferably at most 20 % by weight, most preferably at most 10 % by weight, such as 5 % by weight or less of the Mg silicate, based on the total weight of the carbonatable binder composition. The % by weight of the Mg silicate is advantageously measured by means of XRD.

[0095] The binder composition can further comprise one or more of merwinite, monticellite and diopside. It is known that they have a limited reactivity towards CO2 and show a lower contribution to the strength development upon carbonation of the binder composition when compared to akermanite, and consequently their presence in the carbonatable binder composition is equal to or lower than 20 % by weight, preferably equal to or lower than 15 % by weight, more preferably equal to or lower than 10 % by weight, most preferably equal to or lower than 5 % by weight, based on the total weight of the carbonatable binder composition. The % by weight of the one or more of merwinite, monticellite and diopside is advantageously measured by means of XRD.

[0096] Advantageously, the carbonatable binder composition is a particulate material, i.e. comprises solid particles. The method can optionally comprise a particle size reduction operation, wherein the particle size is reduced, e.g. by means of milling, to enhance the reactivity of the binder composition towards CO2. Advantageously, the particles have a particle size distribution having a dso value equal to or lower than 50 pm, preferably equal to or lower than 40 pm, more preferably equal to or lower than 20 pm, as measured by means of laser diffraction , in particular laser diffraction performed according to standard ASTM B822-20 (2020). It will be understood that the desired or optimal particle size distribution, including the dso value, depends on the application or use of the carbonatable binder composition, i.e. on the carbonate bonded article to be obtained.

[0097] The present invention further relates to a method for producing a carbonate bonded article by carbonating the carbonatable binder composition of the invention. The method comprises a carbonation operation, and optionally a shaping operation prior to the carbonation operation.

[0098] The carbonation operation comprises contacting the carbonatable binder composition with CO2 (i.e. exposing to), so that the binder composition reacts with the CO2. Advantageously, upon exposure to CO2, the akermanite and any other compounds that are reactive to CO2 in the binder composition convert into a carbonate bonded article.

[0099] The carbonation operation is advantageously performed at a temperature between 10 °C and 100 °C, preferably between 20 °C and 90 °C, more preferably between 30 °C and 80 °C, such as between 50 °C and 70 °C.

[0100] The carbonation operation is advantageously performed at a CO2 pressure equal to or lower than 10 MPa, preferably equal to or lower than 5 MPa, more preferably equal to or lower than 2 MPa.

[0101] Advantageously, the carbonation operation is performed at a temperature between 10 °C and 100 °C, preferably between 20 °C and 90 °C, and at a CO2 pressure equal to or lower than 5 MPa, preferably equal to or lower than 2 MPa.

[0102] Advantageously, the carbonatable binder composition is exposed to a gas comprising at least 15 % by volume, preferably at least 30 % by volume, more preferably at least 50 % by volume, for example 75 % by volume or substantially 100 % by volume of CO2.

[0103] Advantageously, the carbonation operation is performed by means and methods known in the art, such as passing a gas flow comprising or substantially consisting of CO2 through the binder composition, or placing the binder composition in an environment comprising or substantially consisting of CO2. A preferred method comprises placing the binder composition in a CCh-comprising environment.

[0104] Advantageously, the carbonation reaction is performed without the active incorporation of water into the carbonatable water composition, i.e. the carbonation operation is advantageously a non-hydraulic binding operation.

[0105] The optional shaping operation comprises adding further compounds to the binder composition, such as - without being limited thereto - fillers such as limestone filler, fine quarts, quartz flower, nucleation agents and workability improvers. Advantageously, no hydraulic binder additives are added during the shaping operation. It will be understood that the selection of the further compounds and theiramount depends on the purpose of the carbonate bonded article obtained after the carbonation operation.

[0106] In a particular embodiment, a construction material composition is prepared during the shaping operation by adding construction material additives to the carbonatable binder composition. Upon exposure of the construction material composition to CO2 during the carbonation operation, a (carbonated) construction material is obtained. Non-limiting examples of suitable construction material additives include sand, pebbles, aggregates, fibers, inert fillers and additives such as limestone filler, fine quartz and quartz flower.

[0107] The present disclosure further relates to a carbonated (i.e. carbonate bonded) article obtained by the carbonation method of the invention. Advantageously, the carbonate bonded article has a compressive strength of at least 5 MPa, preferably at least 10 MPa.

[0108] Non-limiting examples of carbonate bonded articles include carbonate bonded concrete, stone, clinkers, bricks and tiles. It will be understood that the article obtained depends on the composition of the binder composition, and more particularly of any compounds added during the optional shaping operation.

[0109] The carbonate bonded articles, and in particular the articles obtained from carbonation of the construction material composition as described hereinabove, are advantageously used in applications including, but not limited to, infrastructure, construction, pavement, and landscaping industries.Comparative example 1

[0110] Two slags remaining from nickel or iron-nickel production were provided. The chemical composition of both slags was analysed by means of Energy Dispersive X-ray Fluorescence (ED-XRF) analysis on powder of the slags. The % by weight of the elements are shown in Table 1 for elements present for at least 2 % by weight, based on the total weight of the slag, and are represented in the form of the oxide of the respective elements. Minor elements, i.e. elements being present in the slags in amounts below 2 % by weight, are mentioned under “Others” in Table 1. Slag 1 comprised significant amounts of FeO and SiO2, while slag 2 mainly comprised SiO2 and MgO.Table 1: chemical composition of two slags

[0111] First, both slags were used “as received”, i.e. without any further treatments or preparations, as reference binder compositions. Thereto, mixtures comprising slag 1 or slag 2, respectively, and 10 % by weight of moisture, based on the total weight of the mixtures, were prepared. The mixtures were shaped into cylinders of 23 mm diameter and 20 mm height, and were then carbonated by contacted them with a gaseous CO2 atmosphere for 16 hours at six CO2 pressure-temperature combinations. Three different pressures were tested - 10, 20 and 40 bar, and two temperatures were tested - 60 °C and 100 °C. Afterwards, the compressive strength was measured.

[0112] Figure 1 shows the compressive strength for the carbonate bonded articles obtained from slag 2. It is clear that low compressive strengths well below 0.5 MPa were measured. No compressive strength could be measured for the carbonate bonded article obtained by carbonation at 10 bar and 100 °C.Example 1

[0113] Carbonatable binder compositions were prepared from each of the two slags from Comparative example 1 . For each slag, three mixtures were prepared by adding 10, 20 or 30 % by weight of CaCCh, based on the total weight of the respective mixture. The mixtures were then heated to 1300 °C in air at a heating rate of 3 °C / min and were then maintained at 1300 °C in air for 30 minutes. Afterwards, there were cooled at a cooling rate of 3 °C / min.

[0114] The obtained carbonatable binder compositions were then carbonated by contacted them with a gaseous CO2 atmosphere for 16 hours at 10 bar and 60 °C, and their compressive strength was measured.

[0115] Figure 2 shows the compressive strength for the carbonate bonded articles in function of the amount of CaCCh added, wherein 0 wt.% CaCCh refers to the reference carbonate bonded articles obtained in comparative example 1 by carbonation at the same conditions. It is clear that for the carbonate bonded articles obtained from slag 2 a significant increase of the compressive strength was obtained by the addition of CaCCh. The increase is larger with increasing amount of CaCCh, reaching values close to 40 MPa for 30 wt.% of CaCCh. For slag 1 only a slight improvement was found, reaching compressive strengths of approx. 3 MPa for articles obtained with 30 wt.% of CaCO3.

[0116] To better understand the impact of the addition of CaCCh, the phase composition of the binder composition and carbonate bonded articles comprising slag 2was investigated using XRD. A PANalytical Empyrean was used with Rietveld for quantification and an external rutile standard for calculating the quantity of amorphous. HighScore 5.0 Plus software and the PDF-4 database supported the analyses.

[0117] Table 2 shows the results, the values being the % by weight based on the total weight of the carbonatable binder composition or carbonated article, respectively.Table 2: Phase composition of carbonatable binder compositions and carbonated articles comprising slag 2

[0118] It was noticed that the composition of the binder comprising noCaCo3 (reference of comparative example 1) and 10 wt.% CaCO3 was not changed significantly upon carbonation, which explains the low compressive strengths measured, indication that almost no carbonation took place. Further, for all phases except akermanite there was no significant change in amount upon carbonation.

[0119] However, it was surprisingly noticed that only from 20 wt.% CaCCh on the binder compositions did comprise akermanite, up to 18 wt.% for the binder composition obtained by adding 30 wt.% CaCCh to slag 2. Further, upon carbonation, the amount of akermanite significantly decreased while the obtained articles showed high compressive strengths, the highest strength being obtained from the binder composition comprising the highest amount of akermanite.

[0120] Further analysis of the results learned that akermanite presented in the thermally treated mixture had reacted for about 2 / 3, while other phases are within the error of the XRD measurement. Dolomite is formed as main reaction product in the binder after carbonation, although in other cases also minor quantities of aragonite were found, while the silica present in the akermanite was transformed into amorphous silica.

[0121] The binder composition obtained from slag 2 was further optimized to increase its amount of akermanite by optimizing the amount of CaCCh added.

[0122] Further optimization of the akermanite content was calculated theoretically using FactSage thermodynamic simulations in the Equilibrium module of FactSage 8.0 using the Fact Pure Substances and FT Oxid databases. A constant oxygen partial pressure of 0.1 was implemented to simulate the conditions in a clinkering furnace. The phase composition as function of CaCCh addition is shown in Figure 3. A maximal akermanite content is simulated at a CaCCh addition of 40-45 wt%.

[0123] The optimization was then experimentally validated by preparing mixtures comprising 40, 45 and 50 wt.% of CaCCh. The mixtures were then heated in the same way as the thermal treatment of Example 1. The phase composition of the obtained binder compositions was then investigated in the same way as described in Example 1 (Table 3).Table 3: Phase composition of carbonatable binder compositions comprising slag 2

[0124] From Table 3 it is clear that the highest wt.% of akermanite were effectively obtained when 45 to 50 wt.% CaCCh was added to slag 2 prior to the thermal treatment to obtain the carbonatable binder composition. This, together with the amounts of the other phases, confirms the theoretical calculation represented in Figure 3.

[0125] The binder compositions were then also carbonated in the same way as described in Example 1 (60 °C and 10 bar), and the compressive strength was measured. The carbonated article obtained from the binder composition obtained from a mixture comprising 45 wt.% CaCCh (the binder composition itself comprising 66 wt.% akermanite) has a compressive strength of almost 80 MPa, or twice as high as the binder composition obtained from a mixture comprising 30 wt.% CaCCh (the binder composition itself comprising 18 wt.% akermanite). Such high compressive strengths of 80 MPa are higher than what is typically required for construction materials. This means that thebinder composition obtained by thermal treatment of a mixture of the slag 2 and CaCCh could be diluted with filler material to obtain an economic construction material.Example 2

[0126] A vast supply of source material can also be found in tailings, which often contain pyroxenes and / or amphiboles. By mixing pyroxenes and amphiboles with CaCCh and heat treating the mixture, akermanite can be formed. From Example 1 it is known that akermanite in the binder composition significantly contributes to the compressive strength of articles obtained after carbonation of the binder composition at moderate conditions of 10 bar CO2 and 60 °C.

[0127] To investigate the use of such tailings, a phase composition simulation was performed for tailings mixed with a varying amount of CaCCh (from 0 to 40 wt.%) and subsequently heat treated in air at 1200 °C. Figure 4 shows the theoretical simulation, from which it is clear that substantial wt.% of akermanite can be formed, starting from 10 wt.% of CaCCh in the mixture, and with highest akermanite concentrations for 30 wt.% CaCCh.

[0128] This means that the binder composition obtained by thermal treatment of a mixture of the tailings and CaCCh could be diluted with filler material to obtain an economic construction material.

[0129] The optimization was then experimentally validated by preparing mixtures comprising 20, 25, 30 and 35 wt.% of CaCCh. The mixtures were then heated to 1200 °C in air, thereby obtaining carbonatable binder compositions. The phase composition of the binder compositions was then investigated in the same way as described in Example 1 (Table 4).Table 4: Phase composition of carbonatable binder compositions from tailings

[0130] From Table 4 it is clear that when mine tailings are used as mineral source material, an optimal akermanite content of 53-46 wt.% was reached in thecarbonatable binder compositions with the addition of 25-30 wt.% CaCCh to the reactive mixture. This, together with the amounts of the other phases, confirms the theoretical calculation represented in Figure 3.

[0131] The binder compositions were then also carbonated in the same way as described in Example 1 (60 °C and 10 bar), and the compressive strength was measured. Figure 5 shows that the carbonatable binder compositions with the highest akermanite content provide the highest compressive strength, with values of 60 MPa to even 70 MPa. Surprisingly, the carbonatable binder composition with a large amount of merwinite (35 wt.% CaCCh addition, 33 wt.% of merwinite and 36 wt.% of akermanite) shows a lower compressive strength (50 MPa) - although still high - than the high akermanite carbonatable binder compositions (25-30 wt.% CaCCh). Similarly, the carbonatable binder composition with a large amount of diopside (20 wt.% CaCCh addition, 46 wt.% of diopside and 37 wt.% of akermanite) also shows a lower compressive strength (approx. 54 MPa) - although still high - than the high akermanite carbonatable binder compositions (25-30 wt.% CaCCh). This confirms that akermanite is the main carbonation-reactive phase responsible for the strength development in the carbonate bonded articles.Example 3

[0132] A commercially available Mg-silicate was also tested. Table 5 shows the chemical composition as analysed by means of ED-XRF analysis on powder of the Mg-silicate. The % by weight of the elements are shown in Table 5 for elements present for at least 2 % by weight, based on the total weight of the slag, and are represented in the form of the oxide of the respective elements. Minor elements, i.e. elements being present in the slags in amounts below 2 % by weight, are mentioned under “Others" in Table 5. The loss on ignition (LOI) comes from the decomposition of hydrous Mg- silicates. The Mg / Si molar ratio was above 1 .Table 5: Chemical composition of the commercial Mg-silicate

[0133] To investigate the use of the Mg-silicate in obtaining carbonatable binder compositions allowing to provide carbonate bonded articles having a high compressive strength, a phase composition simulation was performed for the Mg-silicate mixed with a varying amount of CaCCh (from 0 to 30 wt.%) and subsequently heat treatedin air at 1200 °C. Figure 6 shows the theoretical simulation, from which it is clear that substantial wt.% of akermanite can be formed, starting from 10 wt.% of CaCCh in the mixture, and with highest akermanite concentrations for 20 wt.% CaCCh.

[0134] The theoretical simulation was then experimentally validated by preparing mixtures comprising the Mg-silicate and 10, 15, 20, 25 and 30 wt.% of CaCCh. The mixtures were then heated in the same way as the thermal treatment of Example 1. The phase composition of the obtained binder compositions was then investigated in the same way as described in Example 1 (Table 6).Table 6: Phase composition of carbonatable binder compositions comprising the commercial Mg-silicate

[0135] From Table 6 it is clear that the highest wt.% of akermanite were effectively obtained when 20 to 25 wt.% CaCCh was added to the Mg-silicate prior to the thermal treatment. This, together with the amounts of the other phases, confirms the theoretical calculation represented in Figure 6.

[0136] The binder compositions were then carbonated in the same way as described in Example 1 (60 °C and 10 bar), and the compressive strength was measured (Figure 7). From Figure 7 it is clear that the carbonate bonded article obtained from the binder composition obtained from a mixture comprising 20 wt.% CaCCh (the binder composition itself comprising 24 wt.% akermanite) has a compressive strength of 18 MPa, whereas even with less optimal wt.% of CaCCh compressive strengths of at least 10 MPa were obtained, with the exception of the addition of 10 wt.% CaCCh - which was expected since from Table 6 it is clear that adding 10 wt.% CaCCh to the Mg-silicate did not result in any akermanite being formed.

[0137] Surprisingly, further increasing CaCCh addition to 30 wt.% was not beneficial, despite the formation of well-known carbonatable phases such as merwinite (19 wt.%), belite (2 wt.%), and periclase (4 wt.%). The 30 wt.% of CaCCh addition resulted in a decrease in the akermanite content in the binder composition and in lower compressive strength in the carbonated cylinder. This confirms that akermanite is the main carbonation-reactive phase responsible for the strength development in the carbonated articles.

Claims

CLAIMS1. A method for producing a carbonatable binder composition comprising a mixed Ca-Mg silicate, wherein the method comprises forming a mixed Ca-Mg silicate by heating a reactive mixture to a temperature between 800 °C and 1400 °C, thereby obtaining the carbonatable binder composition comprising the mixed Ca-Mg silicate, wherein the reactive mixture comprises: between 40 and 90 % by weight of a mineral source material comprising a Mg- comprising silicate, and between 60 and 10 % by weight of a Ca-comprising compound, wherein the percentage by weight of each component of the reactive mixture is based on the total weight of the reactive mixture, characterized in that a molar ratio of elemental Mg to elemental Si in the mineral source material is between 5:1 and 1 :5, and in that the Ca-comprising compound is capable of reacting with the Mg-comprising silicate.

2. The method according to claim 1 , wherein the molar ratio of elemental Mg to elemental Si in the mineral source material is between 2:1 and 1 :2.

3. The method according to claim 1 or claim 2, wherein the Ca-comprising compound comprises a Ca-comprising carbonate, preferably one or more of calcite, aragonite, chalk, limestone or dolomite.

4. The method according to claim 3, wherein the Ca-comprising carbonate comprises CaCCh.

5. The method according to any one of the preceding claims, wherein the Mg- comprising silicate is MgSiO3or Mg2SiC>4.

6. The method according to any one of the preceding claims, wherein the mineral source material further comprises equal to or lower than 20 % by weight of Fe, based on the total weight of the mineral source material, wherein the amount of Fe is expressed as Fe oxide in the mineral source material.

7. The method according to any one of the preceding claims, wherein the mineral source material comprises equal to or lower than 20 % by weight of a Ca-comprising silicate, based on the total weight of the mineral source material.

8. The method according to any one of the preceding claims, wherein the mineral source material comprises a pyroxene having the general formula XY(Si,AI)2O6, whereinX individually is Ca, Na, Fe(ll) or Mg; andY individually is Cr, Al, Mg, Co, Mn, Sc, Ti, V, Fe(ll) or Fe(lll).

9. The method according to claim 8, wherein the pyroxene is diopside (CaMgSi2Oe) or enstatite (MgSiCh).

10. The method according to any one of the preceding claims, wherein the mineral source material comprises an amphibole having the general formula ZySisC^OH^, wherein Z individually is selected from the group consisting of: Ca, Mg, Na, Cr, Al, Co, Mn, Sc, Ti, V, Fe(ll) and Fe(lll).

11. A carbonatable binder composition comprising a mixed Ca-Mg silicate, characterized in that the carbonatable binder composition comprises at least 15 % by weight of akermanite (Ca2MgSi2O?), based on the total weight of the carbonatable binder composition.

12. The carbonatable binder composition according to claim 11 , comprising at least 17.5 % by weight of akermanite (Ca2MgSi2O?), based on the total weight of the carbonatable binder composition.

13. The carbonatable binder composition according to claim 11 or claim 12, further comprising equal to or lower than 20 % by weight of other phases from the melilite group, based on the total weight of the carbonatable binder composition.

14. The carbonatable binder composition according to any one of claims 11 to 13, comprising equal to or lower than 20 % by weight of merwinite (Ca3MgSi20s), monticellite (CaMgSiOt) and diopside (CaMgSi2O6), based on the total weight of the carbonatable binder composition.

15. A construction material composition, comprising the carbonatable binder composition according to any one of claims 11 to 14.

16. A method of producing a carbonate bonded article, comprising contacting a composition comprising the carbonatable binder composition obtained with the process of any one of claims 1 to 10 or the carbonatable binder composition of any one of claims 11 to 14 with CO2 at a temperature between 10 °C and 100 °C and at a CO2 pressure equal to or lower than 2 MPa.