Method for preparing set cement

Abstract

The invention relates to a method for producing set cement, comprising the following steps:-providing and comminuting a starting product to a fineness corresponding to the BET surface area of 0.1 m2 / g, the starting product containing at least 20 mass% of magnesium silicate hydrate and having an iron content of at least 1 mass%,-homogenizing the starting product, and-dehydrating the starting product at least partially in order to remove bound water by means of heat treatment in a heat treatment unit,-reaction grinding, or-microwave treatment,-contacting and mixing the dehydrated starting product with water at a ratio of water to dehydrated starting product of 1: 2, and-subsequently hardening the mixture, the hardened set cement is brought into contact with CO2 in order to produce set cement until the compressive strength reaches at least 10 MPa, and the hardened set cement is brought into contact with CO2.

Description

[0001] The present invention relates to a method for producing cement stone.

[0002] Cement is an inorganic non-metallic building material and belongs to the group of binders. For processing, it is finely ground and present in powder form. It is used for the manufacture of components and buildings.

[0003] Cement hardens by chemical reaction with water, called hydration. To produce building materials such as mortar and concrete, additional water, previously called mixing water, is added to the cement, as well as other substances such as sand and aggregates. Due to the worldwide availability of raw materials and the strength and durability of concrete, cement has become one of the most important binders worldwide. With a global production of 4.1 billion tons in 2017, cement is the most widely used building material.

[0004] Cement mixed with water is called cement paste. The cement paste coats the stone particles in the aggregates, fills the voids, making fresh concrete workable. By hardening of the cement paste, cement stone is formed. The properties of the cement stone determine the strength of the concrete. In the present invention, all kinds of setting materials that can be produced by mixing a binder with water and hydrating are referred to as cement stone.

[0005] For the production of cement, limestone and clay are used, which are often present in the form of a natural mixture, then called marl. If necessary, additives such as quartz sand and iron oxide-containing substances are mixed in. These raw materials are ground into raw meal, then heated at about 1450°C until they partially fuse at the grain boundaries (sintering), forming the so-called cement clinker. The now spherical material is cooled and ground into the final product, cement. To obtain a cement type with specific properties, different dosages and grinding finenesses of granulating sand, fly ash, limestone and gypsum can be added before grinding.

[0006] Today, the impact of cement production on the climate is considered problematic. The cement industry is one of the main causes of carbon dioxide, which leads to global warming. The CO2 released when lime, more precisely calcium carbonate, is burned corresponds to an amount of almost 3 billion tons or about 6 to 8 percent of the annual CO2 emissions, which corresponds to three to four times the amount of all air traffic, calculated on the basis of the global annual production.

[0007] Carbon dioxide (CO2) in the atmosphere as a greenhouse gas and is considered one of the main causes of global warming by humans. In addition to fundamentally reducing CO2 emissions, attempts are also being made to bind CO2 that is already present in the atmosphere. The binding of CO2 can also be used to achieve CO2 neutralization for production methods that produce large amounts of CO2, such as cement production.

[0008] Sequestration of CO2 is understood in particular to mean removal of CO2 from the atmosphere or from process gases, ideally in such a way that the CO2 is combined with other substances so that it cannot escape again. Various different approaches are known at present. One possible way is that in which olivine is used.

[0009] Olivine is a mineral with the general composition A2[SiO4], in which A can be various divalent ions, such as magnesium (forsterite, Mg2SiO4), iron (fayalite, Fe2SiO4), manganese (manganese olivine, Mn2SiO4), and combinations of other ions and different cations, since olivine is a mixed crystal series.

[0010] The object of the present application is to provide an efficient method for producing cement stone.

[0011] According to the application, this object is achieved by the method having the features of claim 1 and the cement stone having the features of claim 13.

[0012] Further advantageous embodiments are given in the dependent claims and the further description.

[0013] According to the provision of claim 1, a starting product is first provided, which contains at least 20% by mass of magnesium silicate hydrate (Mg3Si2O5(OH)4, Mg3Si4O 10 (OH)2)) with an iron content of at least 1% by mass and can contain magnesium hydroxide (Mg(OH)2). This starting product is comminuted, for example ground, to a fineness corresponding to a BET surface area of 0.1 m 2 / g or finer.

[0014] In principle, the present application does not necessarily require the presence of magnesium hydroxide. Although this is the case in many natural sources of the starting product, this does not have a significant influence on the reactions which give strength to the hardened cement stone. Furthermore, the water content of Mg(OH)2 is higher than that of antigorite, so that more thermal energy is consumed in the combustion process. In addition, more flow aids are sometimes required in the presence of Mg(OH)2 due to the increased surface area.

[0015] The magnesium silicate hydrate content is preferably at least 40% by mass, more preferably at least 60% by mass, and still more preferably at least 80% by mass. An example of this is serpentinite. Serpentinite is a metamorphic rock which is formed from ultramafic rocks by natural conversion, in particular weathering. It is advantageous if the starting material should not contain SiO2 and no substances which release SiO2 on thermal treatment are added. SiO2 can react with the magnesium silicate hydrate on subsequent thermal treatment and thereby reduce the product quality.

[0016] An important mineral in ultramafic rocks is olivine. It is a mixed crystal series between fayalite (Fe2SiO4), forsterite (Mg2SiO4), manganolivine (Mn2SiO4) and other A2[SiO4] form minerals. The natural origin of olivine is documented in the literature. Olivine is usually a magnesium-rich mineral which also contains iron.

[0017] The reactions which take place on weathering are as follows, where they are given here in simplified form starting from forsterite (Mg2SiO4): (1) 2 Mg2SiO4+ 3 H2O → Mg3Si2O5(OH)4+ Mg(OH)2 (2) 3 Mg2SiO4+ 5 SiO2+ 2 H2O → 2 Mg3Si4O 10 (OH)2 The magnesium silicate hydrates (Mg3Si2O5(OH)4, Mg3Si4O 10 (OH)2) can occur in the form of serpentine, antigorite, talc and other forms. It should be noted here that the stoichiometric water content is sometimes (13 mass% in antigorite) lower than that which can be determined by testing (16 mass% to 20 mass%). This can be explained by the fact that some materials are very fine, so that water can also adhere to their surface.

[0018] Similarly, this deviation from stoichiometry also applies to the ratio between Mg and Si. In addition, foreign ions such as Fe can also be incorporated into the reaction products. However, other reaction products can also form, such as hydromagnesite, hematite, magnetite or gibbsite. This depends on the exact composition of the starting product. All or part of the reaction products can contain iron, carbonate and alkali ions or other foreign ions.

[0019] The iron content is given here converted to elemental iron content, since iron (Fe) can exist in different forms and bonding. Examples of this are: magnetite, hematite, iron in the olivine mixed crystal, or in the form of foreign ions in other compounds.

[0020] The starting product is comminuted to a fineness corresponding to a BET surface area of 0.1 m 2 / g or finer, where the BET surface area can be determined according to the standard DIN ISO 9277:2003-05 "Determination of the specific surface area of solids by gas adsorption: method using the Brunauer-Emmett-Teller (BET) theory" using the BET method. It is advantageous here for the BET surface area to be 0.5 m 2 / g, more preferably for the BET surface area to be 1.0 m 2 / g or finer. This comminution can be achieved by grinding. Depending on the source of the magnesium silicate hydrate, the comminution in the sense of the present application can also have occurred during the decomposition, the acquisition or more generally the production or by them.

[0021] After providing the starting product, the starting product is homogenized, if necessary. The serpentinite listed as an example is a natural deposit material. Experience shows that these components are neither in pure form nor homogenized. The homogenization can be carried out, for example, using a mixer or at the same time when comminuting to the desired fineness.

[0022] Subsequently, the homogenized starting product is at least partially dehydrated to remove the bound water. This can be achieved by heat treatment in a heat treatment device, in which the homogenized starting product is at least partially dehydrated to remove the bound water. The heat treatment presented here can also be referred to as tempering or calcination. The bound water is sometimes also referred to as crystal water. It is to be distinguished from the non-bound water, which can be regarded as free H2O. Complete dehydration can be achieved by high expenditure. According to the present application, the water content of the bound water should be reduced by at least 50%, preferably by at least 70%, more preferably by at least 90%.

[0023] For the heat treatment, the starting product can be heated to a temperature of 180°C to 1000°C. In this case, depending on the fineness present, heating for a few minutes is sufficient. A temperature of 300°C to 800°C is preferred, more advantageously 500°C to 700°C.

[0024] An alternative to at least partially dehydrating the homogenized starting product is reactive grinding or microwave treatment. In the case of reactive grinding, the crystal structure of the starting product is rearranged. In the case of microwave treatment, heat can be generated in the starting product in a targeted manner, thus changing the crystal structure and / or achieving partial dehydration. Combinations of these or other methods can also be used.

[0025] By the dehydration, the magnesium silicate hydrate (Mg3Si2O5(OH)4, Mg3Si4O 10 (OH)2) present in the starting product in this step is at least partially converted into dehydrated magnesium silicate hydrate, which can be simplified as xMgO-SiO2-yH2O. Here, dehydration means a reduction in the water of crystallization or crystal water in the converted starting product. It is important here that, during the heat treatment, the starting material, for example the serpentinite, does not burn until olivine and / or free MgO and free SiO2 are formed, because otherwise the content of the partially dehydrated magnesium silicate hydrate in the converted starting product would be reduced, thus reducing the product quality.

[0026] The basic chemical processes here are again simplified as follows: (3) Mg3Si2O5(OH)4→ 2 xMgO-SiO2-yH2O + z H2O (4) Mg3Si4O 10 (OH)2→ 2 aMgO·SiO2·bH2O + c H2O (5) Mg(OH)2→ MgO + H2O wherein (3) produces a reaction product which is substantially amorphous, has a Mg to Si ratio of 1.5 to 2 and a water content of about 3%. The reaction product formed in formula (4) has an even lower Mg to Si ratio. Accordingly, the variables a, b, c, x, y and z are so too. These depend on the exact composition of the starting product and the processing parameters, respectively.

[0027] After the dehydration, the water content of the bound water in the converted and dehydrated starting product is preferably below 10 mass%, advantageously below 5 mass%, still more preferably below 3.5 mass% and above 2.5 mass%.

[0028] Thus, the dehydrated starting product is present in the form of a multiphase product. Other possible secondary phases are hematite, magnetite, enstatite, feldspar, pyroxene and amorphous phases.

[0029] After the dehydration, the dehydrated starting product is mixed with water in a ratio of water to dehydrated starting product of 1 :2 or less, to make a mortar therefrom, in particular according to DIN EN 196. This mixture is then hardened until the compressive strength reaches at least 10 MPa, wherein this test is carried out in particular according to DIN EN 196. The hardened and dehydrated starting product is called cement stone and can be used to bind the aggregates in concrete or mortar together.

[0030] Advantageously, the ratio is 1 :2.22, preferably 1 :2.5, ideally 1 :2.86, still better 1 :3.33 or less. It has been found that higher ratios, i.e. higher water contents, prolong the hardening time and reduce the strength. In principle, the binding can be further accelerated by a thermal or pressure treatment.

[0031] It has been found that, depending on the environmental variables, a compressive strength of at least 10 MPa can be reached relatively quickly. However, this strength is already so high that further processing, such as demoulding, can be started without damaging or disturbing the further hardening of the cement stone. "Quickly" in the present invention is to be understood as 28 days, preferably 2 days.

[0032] Before the hardening, no SiO2, in particular no non-bound or free SiO2, should be added or contained, because otherwise too much Mg or other additives would need to be added to bind the SiO2. In addition or alternatively, no substances containing tricalcium silicate (Alit) or dicalcium silicate (Belite) should be added either, because these would have a detrimental effect on the hardening.

[0033] The reaction that leads to the persistent setting of the dehydrated starting product in contact with and mixing with water is the hydration of the dehydrated magnesium silicate hydrate. Here phases such as antigorite, talc and lizardite are formed, which are usually present in amorphous form. This hydration reaction is similar to the hardening of cement stone, with the difference that M-S-H is formed instead of C-S-H.

[0034] After the dehydrated starting product mixed with water has hardened (now referred to as cement stone), it is brought into contact with CO2. The CO2is bound mainly in magnesium carbonate (MgCO3) and / or magnesium carbonate hydrate (MgCO2-mH2O) that is formed. The mineral name for magnesium carbonate is magnesite. The magnesium carbonate hydrates include, for example, brucite (m = 2), nesquehonite (m = 3) and lansfordite (m = 5). In addition, there are also some basic magnesium carbonate hydrates, such as artinite, hydromagnesite and botallackite.

[0035] The carbon dioxide diffuses into the cement stone and can react with the different phases or be stored in the pores. The binding of SiO2is achieved, inter alia, by reaction with the magnesium hydroxide that is formed during the hydration process. This is essentially based on the reaction of MgO with water, in which equation 5 proceeds in reverse. The magnesium hydroxide present in the microstructure can react with CO2according to equation (6).

[0036] (6) Mg(OH)2+ CO2+ (m-1) H2O → MgCO3-mH2O A further reaction of the CO2binding in the cement stone is achieved by the conversion of magnesium silicate hydrates with a Mg / Si ratio of 1.5 into magnesium silicate hydrates with a lower Mg / Si ratio and the formation of hydromagnesite or other carbonate phases, for example as shown in equation (7), in which a magnesium silicate hydrate with a Mg / Si ratio of 0.75 is formed.

[0037] (7) 10 Mg3Si2O5(OH)4+ 12 CO2→ 5 Mg3Si4O 10 (OH)2+ 3 Mg5(CO3)4(OH)2+ 12 H2O In addition, other binding forms of CO2in other phases are also possible. This can be demonstrated, inter alia, by measuring the carbonation depth using indicator solutions such as phenolphthalein or by measuring the inorganic carbon content (TIC).

[0038] The presence of water as a reaction medium is very advantageous for the reaction with CO2. The reaction takes place via the pore solution of the cement stone after the binder has been mixed with water. The pore solution is retained in the cement stone over the service life and is exchanged with the environment. Even after decades, the pore solution can be obtained by pressing out of a concrete sample for analysis.

[0039] Thus, as shown in formula (7), a partial binding of CO2 is achieved by the MSH produced upon hydration, wherein the degree of binding of CO2 depends on the chemical composition of the starting material and the processing parameters and further substances added upon mixing with water.

[0040] It has been found according to the present application that by expelling the crystal water from natural materials, such as weathered ultramafic rocks, such as e.g. serpentinite, an intermediate material can be produced which can be converted into cement stone and which is suitable for binding CO2.

[0041] Preferably, the starting product is heat treated at a temperature of at least 550 °C and / or at most 750 °C to 800 °C and the starting product is heat treated for at least 5 minutes, advantageously 15 minutes, preferably 30 minutes, more preferably at least 60 minutes.

[0042] Advantageously, for a particular material, the characteristic dehydration temperature is followed as precisely as possible, wherein the deviation is less than 20 °C. If the temperature during the heat treatment is too low, the magnesium silicate hydrate, such as e.g. serpentinite, cannot be dehydrated or only insufficiently dehydrated. At too high temperatures, the magnesium silicate hydrate is essentially converted into olivine which is poorly reactive. Only in a narrow temperature range, the dehydration occurs without the formation of olivine or only with the formation of small amounts of olivine. Instead, the dehydrated starting product is formed in the form of an X-ray amorphous phase which has a low residual water content of 2 to 5 mass%. This phase has a high reactivity and is the target product of the heat treatment.

[0043] In the process according to the present application, during the heat treatment of the starting material, no MgO (periclase) and / or free SiO2 is formed as a result of the conversion of the magnesium silicate hydrate into these two phases. A very specific process for the separation of the magnesium silicate hydrate into MgO and SiO2 is not the subject of the present application and should therefore be avoided. A low content of free, unbound MgO and Mg(OH)2 (brucite) can be present in the starting material. Upon heat treatment, a conversion of brucite into periclase can occur according to formula 5. Thus, the MgO content which can be present in the binder should be attributed to the presence of MgO in the starting material, the dehydration of brucite and optionally the admixture of MgO and not to the production of MgO from the conversion of the magnesium silicate hydrate into periclase and SiO2.

[0044] It is therefore advantageous to use a rotary kiln, in particular an indirectly heated rotary kiln without open flame in the reaction chamber, as a heat treatment device. For electrically heated kilns, the temperature during the heat treatment, also referred to as the combustion temperature, can be controlled particularly precisely. In particular, an electric heating should be used in order to precisely follow the target temperature in the furnace. It is advantageous to use electric energy generated from renewable sources for heating the kiln, as this does not result in CO2 emissions and exhaust gases and does not consume fuel. In contrast, other heat sources can also be used for preheating, in particular using a heat exchanger, which absorbs part of the heat from the dehydrated starting product, for example the combusted serpentinite, and cools it thereby, wherein at the same time heat is supplied to the uncombusted serpentinite. Furthermore, the use of flue gases resulting from the combustion process should be avoided as far as possible, as this leads, for example, to uncontrolled incorporation of CO2.

[0045] In order to achieve a sufficient residence time at the target temperature, a rotary kiln is preferred, as it has a large volume and thus a high throughput can be achieved. Furthermore, a rotary kiln is characterized by a good thermal efficiency.

[0046] For magnesium silicate hydrates with a small particle size, for example ground serpentinite, the heat treatment is particularly effective, as the water bound in the particles can be expelled more quickly and effectively in this case. At the same time, the heat treatment device, for example a low water vapor partial pressure in the furnace, favors the formation of reactive phases. A low water vapor partial pressure can be achieved by air purging of the furnace.

[0047] The ratio of water vapor volume to purging gas volume should be less than 1 : 1, advantageously less than 1 :2, still more advantageously less than 1 :4, still more advantageously less than 1 :8, and ideally less than 1 : 10. The purging gas in the present invention is, for example, air, which can be blown in to expel water vapor. More generally, it is a gas which is used to replace the water vapor.

[0048] After the heat treatment, the dehydrated starting product, for example the tempered serpentinite, is usually present in the form of a powder. This powder can be mixed directly with water to produce a cement stone. No further comminution is necessary, and it has even been found that grinding has a detrimental effect on the hardening.

[0049] Preferably, the dehydrated starting product, which is brought into contact with and mixed with water, is injected into a mold or a casing, in order to harden it there. The dehydrated starting product, which is brought into contact with and mixed with water, also referred to as cement paste, has a paste-like consistency, which depends on the ratio of the dehydrated starting product to water. The less water is used, the thicker the consistency. However, in order to be able to produce the cement stone or the concrete produced using the cement stone into the desired shape, it can be provided that the concrete or the cement stone is injected into the mold or the casing alone and hardened there until it is so stable that it can no longer be damaged due to normal environmental influences. Normal environmental influences or external forces in the present case are, for example, normal weathering influences, in particular not including forces which are explicitly intended to cause damage. An example of this is stripping.

[0050] The cement stone or the dehydrated starting product according to the application can be used alone with water, i.e. in pure form. However, it is also possible to produce a concrete-like building material. To this end, sand and / or aggregates can be added to the dehydrated starting product before, during or after contact with water. Ideally, this concrete-like building material should also harden until the compressive strength reaches at least 10 MPa, and then be subjected to further forces. Reinforcing materials, for example made of steel, carbon fibers or glass fibers, can also be added before, during or after the addition of water.

[0051] It is preferred that the dehydrated starting product, which is brought into contact with and mixed with water, hardens for at least 8 hours, and then the cement stone is subjected to external forces. Experience has shown that at this time the compressive strength reaches at least 10 MPa. It is advantageous if the dehydrated starting product, which is brought into contact with and mixed with water, hardens for longer than, for example, 24 hours or even 48 hours. This increases the strength of the cement stone.

[0052] The contact of the dehydrated, in particular hardened, starting product with CO2 can advantageously take place in a treatment apparatus, for example a closed container, in particular an autoclave, or in a container in which an overpressure is applied. In this way, the process of CO2 binding can be optimized, for example by adjusting the CO2 partial pressure, the temperature present in the autoclave and / or the water content of the atmosphere. In principle, the contact can also take place at ambient pressure. The advantage of this is that the cement stone according to the application or a component or element using the cement stone according to the application can absorb CO2 from the ambient air.

[0053] The CO2 binding is particularly effective when the contact of the dehydrated, hardened starting product with CO2 takes place at a CO2 partial pressure of at most 1000 ppm. Higher CO2 partial pressures should be avoided, because otherwise the pH value of the pore solution drops too much.

[0054] Advantageously, the contact of the hardened cement stone with CO2 is carried out at a temperature above room temperature, since the chemical reaction is thereby accelerated. If the contact is carried out in a treatment device, the heating can be carried out, for example, by introducing hot flue gases. The temperature within the treatment device should be at least 30°C, preferably 50°C. However, the solubility of CO2 decreases with increasing temperature, and the increase in temperature should be limited to 70°C. The method can be operated at very low CO2 partial pressure, which is preferably below 0.5 bar.

[0055] Advantageously, an organic additive, for example a flow aid, is added to the dehydrated starting product before or during the contact and mixing with water. This facilitates the subsequent processing of the cement paste and can influence the hardening. Other additives can improve the frost resistance or reduce the shrinkage.

[0056] The starting product according to the present application is usually not a pure substance and thus contains a large number of impurities. However, it is advantageous if the molar ratio of Mg to Ca is at least 10:1 or higher and / or the molar ratio of Si to Al is also at least 10:1. It has been shown that the presence of calcium and aluminium, respectively, relative to magnesium or silicon, slows down the reaction or sometimes even stops it completely. It is therefore important to shift the respective molar ratio significantly in the direction of magnesium or silicon. Preferably, the molar ratio of Mg to Ca is at least 20:1 and / or the molar ratio of Si to Al is at least 20:1.

[0057] In order to achieve a fineness of the provided starting product corresponding to a BET surface area of 0.1 m 2 / g or finer, the starting product is preferably ground, in particular wet ground. The starting product is not present in a higher fineness at least in part, even if it is very fine due to natural weathering at times. By grinding, the fineness can be easily increased. Here, wet grinding is preferred, since it is generally more energy-efficient than dry grinding.

[0058] The present application also relates to a cement stone comprising magnesium carbonate hydrate and / or magnesium carbonate and produced according to the method of the present application. The advantage is that it can absorb CO2 from the ambient air according to the chemical process described above. It preferably consists of magnesium silicate hydrate (M-S-H) and preferably does not contain amorphous SiO2. Furthermore, its minimum strength can be 10 MPa, preferably 20 MPa, more preferably 30 MPa, when tested as a mortar or concrete.

[0059] The present application also relates to a binder component for the production of cement stone, in particular concrete. The binder component can be mixed with conventional cement, for example Portland cement, a cement mixture, for example a Portland-composite cement mixture, or can also be used alone.

[0060] The binder component of the present invention comprises at least 20 mass% of dehydrated magnesium silicate hydrate (xMgO-SiO2-yH2O), wherein the remainder of its components does not contribute significantly to the strength. The dehydrated magnesium silicate hydrate is amorphous, the ratio of Mg to Si is 2 or less, and the water content of the bound water in the dehydrated magnesium silicate hydrate is preferably less than 10 mass%. With this binder component, an M-S-H phase in the form of a cement stone can already be produced when contacted and mixed with water at a water to binder ratio of 1 :2 or less and then hardened at a temperature below 30°C. The compressive strength of the producible cement stone at 28 days of aging is at least 10 MPa, as tested as a standard mortar according to DIN EN 196-1 :2016, wherein the test is carried out with the addition of a flow aid at a reduced water / binder value of 0.35. In the hardened cement stone of the mortar or concrete, CO2 in the cement stone can be bound in the generated magnesium carbonate hydrate and / or magnesium carbonate upon contact with CO2.

[0061] In the binder component, the content of dehydrated magnesium silicate hydrate is preferably more than 40 mass%, more preferably more than 60 mass%, more preferably more than 80 mass% of the binder component. The higher this content, the greater the compressive strength of the resulting cement stone.

[0062] Depending on the composition of the starting product, non-reactive olivine can be generated in an undesirable manner in the thermal dehydration when producing the binder component of the present invention. The olivine content can be at most 30 mass%. Furthermore, 5-10 mass% of iron phases, such as magnetite and hematite, are usually also present in the starting material. The starting product sometimes also contains garnet, enstatite, or the starting product can be mixed with other rocks during mining, which cannot be separated during mining. Therefore, the content of dehydrated magnesium silicate hydrate can be about 60 mass% in this case.

[0063] It is advantageous if the ratio of Mg to Si of the dehydrated magnesium silicate hydrate is 1.8 to 1.5. Faster hardening and / or higher strength can be achieved if the water content of the bound water in the dehydrated magnesium silicate hydrate is less than 5 mass%, preferably less than 3 mass%, more advantageously less than 2 mass%.

[0064] The strength achievable with the binder component according to the application is based on the setting of the pure, undiluted binder component. This means that no other additives, such as bases, acids or salts, in particular magnesium salts, are required in order to achieve the strength according to the application. In principle, such substances can be added, but sufficient hardening is achieved even without the addition. These substances include, for example, slaked lime and / or quicklime, hemihydrate gypsum, water glass, cement, magnesium sulfate, magnesium chloride. If such substances should be contained, the content in the binder should be a maximum of 8%. Greater contents of, for example, slaked lime, quicklime or cement would worsen the CO2 balance of the binder due to the CO2 emissions produced during their production. Greater contents of, for example, water glass would lead to strongly alkaline mixing water, which makes handling more difficult. The addition of magnesium sulfate and magnesium chloride is not advantageous for the durability of the components produced.

[0065] The strength test was carried out according to DIN EN 196-1 :2016, wherein the test was carried out with the addition of a flow aid at a reduced water / binder value of 0.35.

[0066] Advantageously, the ratio of water to binder component is 1 :2.22, preferably 1 :2.5, ideally 1 :2.86, still better 1 :3.33 or lower. It has been found that higher ratios, i.e. higher water contents, slow down the hardening and reduce the strength. In principle, the setting can be further accelerated by thermal or pressure treatment.

[0067] The iron content of the binder component can be at least 1 mass %.

[0068] Furthermore, it is advantageous if the fineness of the binder component corresponds to a BET surface area of 0.5 m 2 / g, advantageously 1 m 2 / g, even more advantageously 2 m 2 / g or finer. The finer the binder component, the faster the hydration and hardening.

[0069] The application will be explained in more detail below with reference to examples.

[0070] In order to verify the application, inter alia, the investigations explained in more detail below were also carried out.

[0071] A serpentinite with a magnesium silicate content of 90%, a brucite content of 2% was ground in a ball mill to a specific surface area of 1.1 m 2 / g. The Mg / Ca molar ratio was 15:1 and the Si / Al molar ratio was 12:1. According to chemical analysis, the starting material contained an iron content of Fe = 2.1%. Subsequently, the powder was combusted in a muffle furnace at 675 °C for 90 minutes. Air was used as sweep gas and the ratio of water vapor volume produced by the dehydration of the magnesium silicate hydrate to the volume of sweep gas was 1 :5. The material was then used as binder for the preparation of a mortar according to DIN EN 196 with a w / b value reduced to 0.30 and with the addition of 15 g PCE fluidifier. The standard prisms were demoulded after 2 days with a compressive strength of 23 MPa. After 7 days the compressive strength was 58 MPa and also after 28 days. The incorporation of CO2 after storage in ambient air was confirmed by X-ray diffraction and showed the presence of brucite in the microstructure.

[0072] The serpentinite was ground in a ball mill to a BET specific surface area of 1.3 m 2 / g (wet grinding). The material was combusted at 700 °C for 60 minutes and used as binder for the preparation of concrete with a sieve size of AB 16, a fluidifier of 3% (based on binder), a water / binder ratio = 0.30, binder: aggregate = 1 :5. The compressive strength of cubes with an edge length of 10 cm was 55 MPa after 7 days.

[0073] The material of example 1 was alternatively combusted in an electrically heated rotary furnace with a temperature of about 675 °C as well. The ratio of water vapor to sweep gas was 1 :8. The preparation of the mortar was carried out with the same formulation as in example 1 and the compressive strength at demoulding after 2 days was 12 MPa. After 7 days a compressive strength of 32 MPa was achieved and remained stable until 28 days. The incorporation of CO2 after storage in air was also confirmed by the determination of the carbonation depth using phenothalin.

Claims

1. Method for producing a cement stone, comprising the following steps: a) providing and comminuting, in particular grinding, a starting product to a fineness corresponding to a BET surface area of 0.1 m 2 / g or finer, wherein the starting product contains at least 20 mass% of magnesium silicate hydrate (Mg3Si205(OH)4, Mg3Si4O 10 (OH)2)) with an iron content of at least 1 mass% and can contain magnesium hydroxide (Mg(OH)2), b) homogenizing the starting product, c) at least partially dehydrating the starting product to remove bound water, wherein after step c) the magnesium hydroxide present in the dehydrated starting product and / or the magnesium silicate hydrate present is at least partially dehydrated and can be converted here into magnesium oxide (MgO) and dehydrated magnesium silicate hydrate (xMgO-SiO2-yH2O), d) contacting and mixing the dehydrated starting product with water at a ratio of water to dehydrated starting product of 1 :2 or less, followed by hardening the mixture to produce a cement stone until a compressive strength of at least 10 MPa is achieved, e) after step d) contacting the hardened cement stone with CO2, wherein the CO2 is bound in the cement stone here as magnesium carbonate hydrate and / or magnesium carbonate is formed.

2. Method according to claim 1, characterized in that the starting product is at least partially dehydrated to remove bound water by one or more of the following methods: • heat treatment of the starting product in a heat treatment device, wherein the starting product is treated at a temperature of from 180 °C to 1000 °C during the heat treatment, • reactive milling, • microwave treatment.

3. Method according to claim 2, characterized in that the heat treatment of the starting product is carried out at a temperature of at least 550 °C and / or at most 750 °C, and the starting product is heat treated for at least 15 minutes, preferably 30 minutes, more preferably at least 60 minutes.

4. Method according to claim 2 or 3, characterized in that the heat treatment device has a substantially uniform temperature distribution.

5. Method according to any one of claims 2 to 4, characterized in that a rotary furnace, in particular an indirectly heated rotary furnace without open flame in the reaction chamber, is used as the heat treatment device.

6. Method according to any one of claims 1 to 5, characterized in that the dehydrated starting product that is contacted and mixed with water is injected into a mold or a shell, where it is hardened.

7. Method according to any one of claims 1 to 6, characterized in that sand and / or aggregates are added to the dehydrated starting product before, during or after the contact with water.

8. Method according to any one of claims 1 to 7, characterized in that the dehydrated starting product that is contacted and mixed with water is hardened for at least 8 hours, and the cement stone is then subjected to an external force.

9. Method according to any one of claims 1 to 8, characterized in that the contact of the hardened cement stone with CO2 is carried out in a closed container, in particular an autoclave, a washing machine or an overpressure vessel.

10. Method according to any one of claims 1 to 9, characterized in that the contact of the hardened cement stone with CO2 after step d) is carried out at a CO2 partial pressure of at most 1000 ppm.

11. Method according to any one of claims 1 to 10, characterized in that the contact of the hardened cement stone with CO2 is carried out at a temperature of at least 30 °C, preferably above 50 °C.

12. Method according to any one of claims 1 to 11, characterized in that An organic additive, such as a flow aid, is added to the dehydrated starting product before or during the contacting and mixing of the dehydrated starting product with water.

13. The method according to any one of claims 1 to 12, characterized in that the molar ratio of Mg to Ca of the starting product is 10:1 or more, and / or the molar ratio of Si to Al is 10:1 or more.

14. Binder composition for the production of a cement stone • which comprises at least 40 mass% of dehydrated magnesium silicate hydrate (xMgO SiO2 yH2O), wherein the remaining components do not contribute significantly to the strength, • wherein the dehydrated magnesium silicate hydrate o is amorphous, o has a ratio of Mg to Si of 2 or less, and o wherein the water content of the bound water in the dehydrated magnesium silicate hydrate is preferably below 10 mass%, • wherein an M-S-H phase can be formed when the binder composition is contacted and mixed with water at a water to binder ratio of 1 :2 or less and subsequently hardened at a temperature below 30°C, so that a solid cement stone can be produced, • wherein the compressive strength of the producible cement stone at 28 days of aging is at least 10 MPa, tested according to DIN EN 196-1 :2016, and this test was carried out with the addition of a flow aid at a reduced water / binder value of 0.35, • wherein in the hardened cement stone, upon contact with CO2, CO2 can be bound in the cement stone in the formation of magnesium carbonate hydrate and / or magnesium carbonate.