Method for producing a hydrated cement

Abstract

The invention relates to a method for producing a hydrated cement, with the steps of: - providing and comminuting a starting material to a fineness corresponding to a BET surface area of 0.1 m2 / g, where the starting material contains at least 20% by mass of magnesium silicate hydrate and has an iron content of at least 1% by mass; - homogenizing the starting material, and - at least partly dewatering the starting material to remove bound water, by means of -- thermal treatment in an assembly for thermal treatment -- reactive grinding, or -- microwave treatment, - contacting and mixing the dewatered starting material with water in a ratio of water to dewatered starting material of 1:2, and - subsequently allowing the mixture to harden to produce the hydrated cement, until the compressive strength is at least 10 MPa, and - contacting the hardened hydrated cement with CO2.

Description

[0001] METHOD FOR PRODUCING A CEMENT BLOCK

[0002] The invention relates to a method for producing a cement stone.

[0003] Cement is an inorganic, non-metallic building material and a binding agent. It is finely ground for processing and is available in powder form. It is used in the manufacture of building components and structures.

[0004] It hardens through a chemical reaction with water, known as hydration. To produce building materials such as mortar and concrete, cement is mixed with additional water, formerly known as mixing water, and other substances such as sand and aggregate. Due to the global availability of raw materials and the strength and durability of concrete, cement is one of the most important binding agents worldwide. With global production of 4.1 billion tons in 2017, cement is the most widely used building material.

[0005] Cement mixed with water is called cement paste. The cement paste coats the aggregate particles, fills the voids, and makes the fresh concrete workable. Hardening of the cement paste creates cement paste. The composition of the cement paste determines the strength of the concrete. For the purposes of this invention, cement paste refers to any hardened material that can be produced by mixing a binder with water and hydrating it.

[0006] Cement production uses limestone and clay, which often occur naturally and are then referred to as marl. Aggregates such as quartz sand and iron oxide-containing materials are added as needed. The raw materials are ground into raw meal and then heated to approximately 1,450 °C until they partially fuse together at the grain boundaries (sintering), forming what is known as cement clinker. The now spherical material is cooled and ground into the final product, cement. To obtain cement types with specific properties, granulated blast furnace slag, fly ash, limestone, and gypsum can be added in varying doses and finenesses before grinding.

[0007] The impact of cement production on the climate is considered problematic today. The cement industry is one of the main sources of carbon dioxide, which contributes to global warming. Relative to annual global production, the CO2 released during the burning of lime, or more precisely calcium carbonate, corresponds to an emission of almost three billion tons of CO2, or approximately 6 to 8% of annual carbon dioxide emissions, which is three to four times the magnitude of all air traffic.

[0008] Carbon dioxide (CO2) acts as a greenhouse gas in the atmosphere and is considered one of the main causes of human-induced global warming. In addition to the general reduction of CO2 emissions, parallel efforts are being made to capture CO2 already present in the atmosphere. Capture CO2 can also be used to make manufacturing processes that generate large amounts of CO2, such as cement production, carbon-neutral.

[0009] CO2 sequestration refers specifically to the removal of CO2 from the atmosphere or process gases, ideally by combining the CO2 with other substances so that it cannot escape again. Various proposals have been proposed for this. One possibility involves the use of olivine.

[0010] Olivine is a mineral with the general composition A2[SiO4], where various divalent ions can occur for A, such as magnesium (forsterite, Mg2SiO4), iron (Fe2SiO4, fayalite), manganese (Mn2SiO4, tephroite) as well as other ions and a combination of the various cations, since olivine is a solid solution series.

[0011] The invention is based on the object of providing an efficient method for producing a cement stone.

[0012] This object is achieved according to the invention by a method having the features of claim 1 and a cement stone having the features of claim 13. Further advantageous embodiments are specified in the dependent claims and the further description.

[0013] According to claim 1, it is provided that a starting product is first provided which contains at least 20 mass% magnesium silicate hydrate (Mg3Si2Os(OH)4, Mg3Si4O(OH)2), has an iron content of at least 1 mass%, and may contain magnesium hydroxide (Mg(OH)2). This starting product is refined to a fineness corresponding to a BET surface area of ​​0.1 m 2 / g or finely crushed, for example ground.

[0014] In principle, the presence of magnesium hydroxide is not necessary for the invention. While it is present in many natural sources for the starting product, it has no significant influence on the strength-giving reactions in the hardening cement paste. Furthermore, Mg(OH)2 has a higher water content than antigorite, which consumes more thermal energy during firing. Also, in the presence of Mg(OH)2, more superplasticizer may be required due to the increased surface area.

[0015] The magnesium silicate hydrate content is preferably at least 40 mass%, more preferably at least 60 mass%, and even more preferably at least 80 mass%. An example of this is serpentinite. Serpentinite is a metamorphic rock formed by natural transformation, particularly weathering, of ultramafic rocks. Advantageously, the starting material should not contain SiO2, and no substance should be added that releases SiO2 during thermal treatment. SiO2 could react with the magnesium silicate hydrate during subsequent thermal treatment, thereby reducing product quality.

[0016] An important mineral in ultramafic rocks is olivine. This is a solid solution series between fayalite (Fe2SiO4), forsterite (Mg2SiO4), tephroite (Mn2SiO4), and other minerals of the form A2[SiO4]. Natural olivine deposits have been documented, and the olivine is often a magnesium-rich material with iron components.

[0017] The reactions occurring during weathering are as follows, which are simplified here, starting with forsterite (Mg2SiO4)

[0018] (2) 3 Mg2SiO4+ 5 SiO2+ 2 ^ 2 Mg3Si4Oio(OH)2

[0019] Magnesium silicate hydrate (Mg3Si2Os(OH)4, Mg3Si4O(OH)2) can occur in the form of lizardite, antigorite, talc, and other minerals. It should be noted that the stoichiometric water content is sometimes lower—in the range of 13 mass% for antigorite—than that determined by experiments—in the range of 16 to 20 mass%. This can be explained by the fact that some of the materials are so fine that water can adhere to their surfaces.

[0020] Similarly, such deviations from stoichiometry also apply to the ratio between Mg and Si. Furthermore, foreign ions, such as Fe, can also be incorporated into the reaction products. However, other reaction products such as hydromagnesite, hematite, magnetite, or gibbsite can also be formed. This depends on the exact composition of the starting product. All or some of the reaction products may contain iron, carbonate, alkalis, or other foreign ions.

[0021] The iron content is expressed here as an elemental iron content, since iron (Fe) can exist in many different forms and bonds. Examples include magnetite, hematite, iron in olivine solid solution, or as a foreign ion in other compounds.

[0022] This starting product is refined to a fineness corresponding to a BET surface area of ​​0.1 m 2 / g or finer, whereby the BET surface area can be determined according to the standard DIN ISO 9277:2003-05 "Determination of specific surface area of ​​solids by gas adsorption" using the BET method. A BET surface area of ​​0.5 m is advantageous. 2 / g, more preferably a BET surface area of ​​1.0 m 2 / g or even finer. This comminution can be achieved by grinding. Depending on the source of the magnesium silicate hydrate, comminution within the meaning of the invention can also occur during or through mining, extraction, or, more generally, production.

[0023] After the raw material has been prepared, it is homogenized, if necessary. The serpentinite cited as an example is a material from natural deposits. Experience has shown that it is neither pure nor homogenized. Homogenization can be achieved, for example, with a mixer or simultaneously with grinding to the desired fineness.

[0024] Subsequently, the homogenized starting product is at least partially dewatered of bound water. This can be achieved by thermal treatment in a thermal treatment unit, whereby the homogenized starting product is at least partially dewatered of bound water. The thermal treatment proposed here can also be referred to as tempering or calcining. Bound water is sometimes also referred to as crystal water. It must be distinguished from unbound water, which can be regarded as free H2O. Complete dewatering can be achieved with considerable effort. According to the invention, the water content of bound water should be reduced by at least 50%, preferably by at least 70%, even more preferably by at least 90%.

[0025] For thermal treatment, the starting material can be heated to a temperature between 180 °C and 1000 °C. Depending on the fineness of the material, heating for just a few minutes is sufficient. Temperatures between 300 °C and 800 °C are preferred, and between 500 °C and 700 °C are even more advantageous.

[0026] Alternative processes for at least partially dewatering the homogenized starting product are reactive milling or microwave treatment. Reactive milling involves a rearrangement of the crystal structures in the starting product. Microwave treatment allows for targeted heat generation in the starting product to alter the crystal structure and / or cause partial dewatering. Combinations of these or other processes can also be used.

[0027] During this dehydration step, the magnesium silicate hydrate (Mg3Si2Os(OH)4, Mg3Si4O(OH)2) present in the starting product is at least partially converted into dehydrated magnesium silicate hydrate, which can be simplified as xMgOSiO2 yH2O. Dehydration refers to the reduction of the crystalline water or the water of crystallization in the converted starting product. It is important that during thermal treatment, the starting material, such as serpentinite, is not calcined to the point where olivine and / or free MgO and free SiO2 are formed, as otherwise the proportion of partially dehydrated magnesium silicate hydrate in the converted starting product would be reduced, which would impair product quality.

[0028] The underlying chemical processes are, again simplified, as follows:

[0029] (3) Mg3Si2O5(OH)4^ 2 xMgO SiO2yH2O + z H2O

[0030] (4) Mg3Si4Oio(OH)2^ 2 aMgO SiO2bH2O + c H2O

[0031] (5) Mg(OH)2-> MgO + H2O, where (3) produces a largely amorphous reaction product with a Mg to Si ratio of 1.5 to 2 and a water content of approximately 3%. The reaction product formed in equation (4) has an even lower Mg to Si ratio. Hence the variables a, b, c, x, y, and z. This depends on the exact composition of the starting product and the treatment parameters.

[0032] After dewatering, the water content of the bound water in the converted, dewatered starting product is preferably below 10 mass%, advantageously below 5 mass%, more preferably below 3.5 mass% and above 2.5 mass%.

[0033] The dehydrated starting product is thus a multiphase product. Other possible secondary phases include hematite, magnetite, enstatite, feldspars, pyroxenes, and amorphous phases.

[0034] Following dewatering, the dewatered starting product is mixed with water in a water to dewatered starting product ratio of 1:2 or less to produce a mortar, particularly in accordance with DIN EN 196. This mixture is then allowed to harden to a compressive strength of at least 10 MPa, with testing taking place in particular in accordance with DIN EN 196. The hardened dewatered starting product is called cement paste and can be used to hold the aggregate of the concrete or mortar together. It is advantageous if the ratio is 1:2.22, preferably 1:2.5 and ideally 1:2.86 and even better 1:3.33 or less. It has been shown that a higher ratio, i.e. a higher water content, delays hardening and reduces strength. In principle, the setting process can be accelerated further through heat or pressure treatment.

[0035] It has been shown that a compressive strength of at least 10 MPa can be achieved relatively quickly, depending on environmental variables. However, this is already sufficiently high that further processing, such as demolding, can begin without damaging or destroying the still-hardening cement paste. "Rapid," as used herein, is understood to mean 28 days, preferably 2 days.

[0036] Before hardening, no SiO2 should be added or contained, especially no unbound or free SiO2, as otherwise too much Mg or other additives would be required to bind the SiO2. Also, or alternatively, no substances containing alite or belite should be added, as this can have a negative impact on hardening.

[0037] One reaction that causes permanent hardening of the dehydrated starting product when it is mixed with water is the hydration of the dehydrated magnesium silicate hydrate. This process forms phases such as antigorite, talc, and lizardite, which are usually in amorphous form. This hydration reaction is comparable to the hardening of cement paste, with the difference that MSH is formed instead of CSH.

[0038] After allowing the dehydrated, water-mixed starting product, which can now be called cement paste, to harden, it is contacted with CO2. The CO2 is primarily bound in the resulting magnesium carbonate (MgCO3) and / or magnesium carbonate hydrate (MgCO3 mH2O). Magnesium carbonate is known by the mineral name magnesite. Examples of magnesium carbonate hydrates include barringtonite (m=2), nesquehonite (m=3), and landsfordite (m=5). In addition, there are basic magnesium carbonate hydrates such as artinite, hydromagnesite, and dypingite.

[0039] The carbon dioxide diffuses into the cement paste and can react with various phases or be stored in the pores. The binding of CO2 occurs, among other things, through a reaction with magnesium hydroxide, which forms during hydration. This is essentially based on a reaction of MgO with water, with equation 5 running in reverse. Magnesium hydroxide present in the microstructure can react with CO2 according to equation (6).

[0040] A further reaction for binding CO2 in the cement paste occurs through the conversion of magnesium silicate hydrate with a Mg / Si ratio of 1.5 to a magnesium silicate hydrate with a lower Mg / Si ratio with simultaneous formation of hydromagnesite or other carbonate phases exemplified in equation

[0041] (7) for a resulting magnesium silicate hydrate with a Mg / Si ratio of 0.75.

[0042] In addition, other forms of CO2 binding are possible in other phases. Detection can be achieved, among other things, by determining the carbonation depth using an indicator solution such as phenolphthalein or by measuring the inorganic carbon content (TIC).

[0043] The presence of water as a reaction medium is very advantageous for the reaction with CO2. This reaction takes place after the binder is mixed with water and via the pore solution of the cement paste. The pore solution remains throughout the lifetime of the cement paste and is in constant exchange with the environment. It can be extracted from concrete samples for analysis even decades later by squeezing it out.

[0044] Accordingly, as shown in equation (7), CO2 is partially bound by MSH, which is formed during hydration, whereby the extent of CO2 binding depends on the chemical composition of the starting material and the treatment parameters, as well as the addition of other substances when mixing with water.

[0045] According to the invention, it was recognized that by expelling crystal water from natural materials, such as weathered ultramafic rocks, for example serpentinite, an intermediate material can be produced that can be converted into cement stone and is suitable for binding CO2. It is preferred if the thermal treatment of the starting product is carried out at a temperature of at least 550°C and / or a maximum of 750°C to 800°C, and if the starting product is thermally treated for at least 5 minutes, advantageously 15 minutes, preferably 30 minutes, and even more preferably at least 60 minutes.

[0046] It is advantageous to maintain the dehydration temperature characteristic of a specific material as precisely as possible, with a deviation of less than 20°C. If the temperature during thermal treatment is too low, no or only insufficient dehydration of the magnesium silicate hydrate, such as serpentinite, occurs. At temperatures that are too high, the magnesium silicate hydrate is largely converted to olivine, which has poor reactivity. Dehydration occurs only within a narrow temperature range with little or no olivine formation. Instead, the dehydrated starting product forms in the form of an X-ray amorphous phase with a low residual water content of between 2 and 5 mass%. This phase exhibits high reactivity and is the target product of the thermal treatment.

[0047] In the process according to the invention, no formation of MgO (periclase) and / or unbound SiO2 occurs during the thermal treatment of the starting material due to the conversion of the magnesium silicate hydrate into these two phases. Very specific treatments that cause the separation of the magnesium silicate hydrate into MgO and SiO2 are not the subject of the present invention and should therefore be avoided. Small amounts of free, unbound MgO and Mg(OH)2 (brucite) may be present in the starting material. During the thermal treatment, a conversion of brucite to periclase may occur according to equation 5. Accordingly, any MgO contents present in the binder are due to the presence of MgO in the starting material, the dehydration of brucite, and possibly the addition of MgO, but not to the formation of MgO by conversion of the magnesium silicate hydrate into periclase and SiO2.

[0048] It is therefore preferred if the thermal treatment unit has a substantially homogeneous temperature distribution. This allows good dewatering to be achieved without the formation of unwanted by-products. Therefore, in order to maintain the desired dewatering temperature in the furnace as precisely as possible and over a large part of the material's residence time, it is advantageous if the furnace is not directly heated with a flame. In this case, the material would be temporarily exposed to very high temperatures, which would lead to olivine formation. For example, the temperature distribution in a directly heated rotary kiln is too uneven. Therefore, it is advantageous to use a rotary kiln as the thermal treatment unit, in particular an indirectly heated rotary kiln without any open flames in the reaction chamber.The temperature during thermal treatment, which can also be referred to as firing temperature, can be controlled particularly precisely in electrically heated kilns. Electric heating should be used in particular to precisely maintain the target temperature in the kiln chamber. It is advantageous to heat the kiln with electrical energy from renewable energy sources, as this does not produce any CO2 emissions or exhaust gases and no fuel is consumed. In contrast, preheating is also possible using other heat sources, in particular a heat exchanger, which extracts some of the heat from the dehydrated starting product, such as the fired serpentinite, and thereby cools it, while simultaneously transferring this heat to unfired serpentinite. The use of flue gas from combustion processes should also be avoided if possible, as this can lead to, for example, uncontrolled binding of CO2.

[0049] To achieve a sufficient residence time at the target temperature, rotary kilns are preferred because of their large volume and thus high throughput. Furthermore, rotary kilns are characterized by good thermal efficiency.

[0050] Thermal treatment is particularly effective for small particle sizes of magnesium silicate hydrate, such as ground serpentinite, because in this case, the water bound in the particles can be expelled more quickly and efficiently. At the same time, a low water vapor partial pressure in the thermal treatment unit, such as a furnace chamber, facilitates the formation of the reactive phase. A low water vapor partial pressure can be achieved by purging the furnace chamber with air.

[0051] The ratio of steam volume to purge gas volume should be less than 1:1, preferably less than 1:2, even more preferably less than 1:4, even more preferably less than 1:8, and ideally less than 1:10. Purge gas within the meaning of the invention is, for example, air, which can be injected to remove the steam. More generally, it is a gas used to replace the steam.

[0052] After thermal treatment, the dehydrated starting product, such as tempered serpentinite, is usually in powder form. This powder can be mixed directly with water to produce cement paste. Further crushing is unnecessary, and it has even been shown that grinding can negatively affect hardening.

[0053] Preferably, the dewatered starting product that has been brought into contact with water and mixed is filled into a mold or formwork to harden there. The dewatered starting product that has been brought into contact with water and mixed, which can also be referred to as cement paste, has a slurry-like consistency depending on the ratio of dewatered starting product to water. If less water is used, the consistency is firmer. However, in order to be able to bring the cement paste or the concrete in which the cement paste is used into a desired shape, it can be provided that the concrete or the cement paste alone is filled into a mold or formwork and left to harden there until it is so stable that it can no longer be damaged by normal environmental influences. Normal environmental influences or external forces within the meaning of the invention include, for example, normal weather influences and in particular not forces that are explicitly intended to cause damage. One example of this is formwork removal.

[0054] The cement paste or dewatered starting product according to the invention can be used alone with water, i.e., in its pure form. However, it is also possible to produce a concrete-like building material. For this purpose, sand and / or aggregate can be added to the dewatered starting product before, during, or after contact with water. Ideally, the concrete-like building material should be allowed to harden to a compressive strength of at least 10 MPa before subjecting it to additional forces. Reinforcements, e.g., made of steel, carbon, or glass fibers, can also be added before, during, or after the addition of water.

[0055] It is preferred that the dewatered starting product, which has been contacted with water and mixed, be allowed to harden for at least 8 hours before subjecting the cement paste to external forces. Experience shows that a compressive strength of at least 10 MPa is achieved after this time. Advantageously, the dewatered starting product, which has been contacted with water and mixed, is allowed to harden for longer than, for example, 24 hours or even 48 hours. This increases the strength of the cement paste.

[0056] Bringing the dewatered, particularly hardened, starting product into contact with CO2 can advantageously take place in a treatment unit such as a closed container, particularly an autoclave, or in a pressurized container. In this way, the CO2 binding process can be optimized, for example, by adjusting the CO2 partial pressure, the prevailing temperature, and / or the water content of the atmosphere in the autoclave. In principle, however, contacting can also take place at ambient pressure. This has the advantage that components or elements made of or containing the cement paste according to the invention can absorb CO2 from the ambient air.

[0057] CO2 binding is particularly effective when the dehydrated, hardened starting material is brought into contact with CO2 at a maximum CO2 partial pressure of 1000 ppm. Higher CO2 partial pressures should be avoided, as otherwise the pH of the pore solution would drop too much.

[0058] It is advantageous to contact the hardened cement paste with CO2 at a temperature above room temperature, as this accelerates the chemical reactions. If contacting is carried out in a treatment unit, heating can be achieved, for example, by introducing hot flue gas. The temperature in the treatment unit should be at least 30°C, preferably 50°C. However, the solubility of CO2 decreases with increasing temperature, and the temperature increase should be limited to 70°C. The process can operate with very low CO2 partial pressures, preferably below 0.5 bar.

[0059] It is advantageous to add organic additives, such as superplasticizers, to the dewatered starting product before or during contact and mixing with water. This facilitates further processing of the cement paste and can influence hardening. Other additives can improve frost resistance or reduce shrinkage. The starting products provided according to the invention are usually not pure substances, so that impurities are present in high quantities. However, it is advantageous if at least the molar ratio of Mg to Ca is 10:1 or greater and / or the molar ratio of Si to Al is also 10:1 or greater. It has been shown that the presence of calcium and aluminum, in relation to magnesium and silicon, respectively, slows down the reactions or sometimes stops them completely.Therefore, it is important to shift the corresponding molar ratios significantly toward 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.

[0060] To provide the starting product with a fineness corresponding to a BET surface area of ​​0.1 m 2 / g or finer, it is preferred if the starting product is subjected to grinding, especially wet grinding. The starting product is at least partially not available in a higher fineness – even if it is already partly very fine due to natural weathering. This fineness can easily be increased by grinding. Wet grinding is also preferred here, as it is often more energy-efficient than dry grinding.

[0061] The invention further relates to a cement paste comprising magnesium carbonate hydrate and / or magnesium carbonate and produced according to the process of the invention. This paste has the advantage of being able to bind CO2 from the ambient air according to the chemical processes explained above. It preferably consists of magnesium silicate hydrate (MSH) and advantageously contains no amorphous SiO2. Furthermore, it can have a minimum strength of 10 MPa, preferably 20 MPa, and even better 30 MPa when tested as mortar or concrete.

[0062] Furthermore, the invention relates to a binder component for producing a cement stone, in particular concrete. The binder component can be mixed with conventional cements, such as Portland cement, cement mixtures, such as Portland composite cement, or used separately.

[0063] The binder component according to the invention comprises at least 20 mass % dehydrated magnesium silicate hydrate (xMgO SiO2 yH2O), with its remaining components making no significant contribution to strength. The dehydrated magnesium silicate hydrate is amorphous, has a Mg to Si ratio of 2 or less, and the water content of bound water in the dehydrated magnesium silicate hydrate is preferably below 10 mass %. By means of the binder component, MSH phases in the form of a cement paste can be produced simply by contacting and mixing with water in a water to binder ratio of 1:2 or less, and then allowing to harden at temperatures below 30°C. The producible cement paste has a compressive strength of at least 10 MPa at an age of 28 days, tested as standard mortar according to DIN EN 196-1:2016, whereby the test is carried out at a reduced water / binder ratio of 0.35 with the addition of superplasticizer.In the hardened cement paste of the mortar or concrete, when it comes into contact with CO2, the CO2 in the cement paste can be bound in the resulting magnesium carbonate hydrate and / or magnesium carbonate.

[0064] The proportion of dehydrated magnesium silicate hydrate in the binder component is preferably more than 40% by mass, more preferably more than 60% by mass, and even more preferably more than 80% by mass of the binder component. The higher the proportion, the more compressive strength the resulting cement paste will be.

[0065] Depending on the composition of the starting material, non-reactive olivine may be unintentionally formed during thermal dewatering during the production of the binder component according to the invention. The olivine content can be up to 30 mass%. Furthermore, the starting product often contains 5-10 mass% iron phases such as magnetite and hematite. Sometimes the starting product also contains garnets, enstatite, or during mining, the starting product may be mixed with other rocks that cannot be separated during mining. Thus, the content of dewatered magnesium silicate hydrate in this example can be around 60 mass%.

[0066] It is advantageous if the dehydrated magnesium silicate hydrate has an Mg to Si ratio of 1.8 to 1.5. Faster hardening and / or greater strength can be achieved if the water content of bound water in the dehydrated magnesium silicate hydrate is below 5 mass%, preferably below 3 mass%, advantageously below 2 mass%. The strength achievable by means of the binder component according to the invention relates to the setting of the pure, undiluted binder component. This means that no further additions such as alkalis, acids or salts, in particular magnesium salts, are necessary to achieve the strength according to the invention. In principle, such substances can be added, but sufficient hardening is also possible without the additions. The substances are, for example, burnt and / or slaked lime, gypsum hemihydrate, water glass, cement, magnesium sulfate, magnesium chloride.If such substances are present, their concentration in the binder should not exceed 8%. Higher concentrations of, for example, burnt lime, slaked lime, or cement would worsen the binder's carbon footprint due to the CO2 emissions associated with their production. Higher concentrations of, for example, water glass result in highly alkaline mixing water, making handling difficult. The addition of magnesium sulfate and magnesium chloride is detrimental to the durability of the manufactured components.

[0067] The strength test according to DIN EN 196-1:2016, whereby the test is carried out at a reduced water / binder ratio of 0.35 with the addition of superplasticizer.

[0068] It is advantageous if the water to binder ratio is 1:2.22, preferably 1:2.5, and ideally 1:2.86, and even better 1:3.33 or less. It has been shown that a higher ratio, i.e., a higher water content, delays setting and reduces strength. In principle, setting can be accelerated further through heat or pressure treatment.

[0069] The binder component may have an iron content of at least 1% by mass.

[0070] Furthermore, it is advantageous if the binder component has a fineness corresponding to a BET surface area of ​​0.5 m 2 / g, preferably 1 m 2 / g, even more advantageous 2 m 2 / g or finer. The finer the binder component, the faster the hydration and hardening process.

[0071] The invention is explained in more detail below using exemplary embodiments. To verify the invention, the tests described in more detail below were carried out, among others.

[0072] Serpentinite with a magnesium silicate content of 90% and a brucite content of 2% was ground in a ball mill to a specific surface area of ​​1.1 m 2 / g. The molar ratio Mg / Ca was 15:1, the molar ratio Si / Al was 12:1. According to chemical analysis, the starting material contained an iron content of Fe=2.1%. The powder was then fired in a muffle furnace at 675°C for 90 minutes. Air was used as the purge gas and the ratio of water vapor volume from the dehydration of the magnesium silicate hydrate to purge gas volume was 1:5. The material was then used as a binder for mortar production according to DIN EN 196, whereby the w / b value was reduced to 0.30 and 15 g of PCE superplasticizer were added. The standard prisms were demolded after 2 days and showed a compressive strength of 23 MPa. After 7 days the compressive strength was 58 MPa, the same after 28 days. The binding of CO2 was demonstrated by X-ray diffraction after storage in ambient air and it was shown that nesquehonite was present in the microstructure.

[0073] Serpentinite was ground in a ball mill to a specific surface area of ​​1.3 m 2 / g according to BET (wet grinding). This material was fired at 700°C for 60 minutes and used as a binder in the production of concrete with a grading curve AB16, 3% superplasticizer (based on the binder), a water / binder ratio of 0.30, and a binder:aggregate ratio of 1:5. After 7 days, the compressive strength of a 10 cm cube was 55 MPa.

[0074] The material from Example 1 was alternatively fired in an electrically heated rotary kiln, where the temperature was also approximately 675°C. The volume ratio of steam to purge gas was 1:8. The mortar was prepared according to the same recipe as in Example 1, and the compressive strength upon removal after 2 days was 12 MPa. After 7 days, a compressive strength of 32 MPa was reached and remained constant for 28 days. CO2 binding was also verified after air storage by determining the carbonation depth with phenolphthalein.

Claims

A method for producing a cement stone comprising the steps of: 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% magnesium silicate hydrate (Mg3Si2Os(OH)4, Mg3Si4O(OH)2), an iron content of at least 1 mass.-%, and may contain magnesium hydroxide (Mg(OH)2), b) homogenizing the starting product, c) at least partially dewatering the starting product of bound water, wherein after step c) magnesium hydroxide and / or magnesium silicate hydrate present in the dewatered starting product is at least partially dewatered and can be converted into magnesium oxide (MgO) and dewatered magnesium silicate hydrate (xMgO SiO2 yH2O), d) contacting and mixing the dewatered starting product with water in a ratio of water to dewatered starting product of 1:2 or less, and then allowing the mixture to harden to produce the cement paste up to a compressive strength of at least 10 MPa, e) contacting the hardened cement paste after step d) with CO2, wherein the CO2 in the cement paste is bound in the magnesium carbonate hydrate and / or magnesium carbonate formed there.Process according to claim 1, characterized in that the at least partial dewatering of the starting product from bound water is carried out by means of one or more of the following processes:. ■ thermal treatment of the starting product in a thermal treatment unit, whereby the starting product is treated at a temperature between 180°C and 1000°C during the thermal treatment., ■ Reaction milling, ■ Microwave treatment.

3. Process according to claim 2, characterized in that the thermal treatment of the starting product is carried out at a temperature of at least 550°C and / or at most 750°C and that the starting product is thermally 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 thermal treatment unit has a substantially homogeneous temperature distribution.

5. Process according to one of claims 2 to 4, characterized in that a rotary kiln, in particular an indirectly heated rotary kiln without open flames present in the reaction chamber, is used as the unit for the thermal treatment.

6. A method according to any one of claims 1 to 5, characterized in that the dehydrated starting product contacted and mixed with water is filled into a mold or formwork to allow it to harden there.

7. A process according to any one of claims 1 to 6, characterized in that sand and / or aggregate is added to the dewatered starting product before, during or after contact with water.

8. Method according to one of claims 1 to 7, characterized in that the dehydrated starting product contacted and mixed with water is allowed to harden for at least 8 hours before the cement paste is subjected to external forces.

9. Method according to one of claims 1 to 8, characterized in that the contacting of the hardened cement stone with CO2 is carried out in a closed container, in particular in an autoclave, a scrubber or a container with excess pressure.

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

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

12. Process according to one of claims 1 to 11, characterized in that organic additives, such as flow agents, are added to the dewatered starting product before or during the contacting and mixing of the dewatered starting product with water.

13. Process according to one of claims 1 to 12, characterized in that the starting product has a molar ratio of Mg to Ca of 10:1 or greater and / or a molar ratio of Si to Al of 10:1 or greater.

14. Binder component for producing a cement stone • containing at least 40 mass% dehydrated magnesium silicate hydrate (xMgO SiC yF ), the remaining components not making a significant contribution to strength, • wherein the dehydrated magnesium silicate hydrate o is amorphous, o has a Mg to Si ratio of 2 or less, and o wherein the water content of bound water in the dehydrated magnesium silicate hydrate is preferably below 10 mass%, • whereby MSH phases can be formed by means of the binder component when contacting and mixing with water in a water to binder ratio of 1:2 or less, and then allowing to harden at temperatures below 30°C, whereby a solid cement stone can be produced, • where the cement stone produced has a compressive strength of at least 10 MPa at the age of 28 days, tested according to DIN EN 196-1:2016 and the test is carried out at a reduced water / water average value of 0.35 with the addition of superplasticizer • where in the hardened cement stone, when it comes into contact with CO2, the CO2 in the cement stone can be bound in the resulting magnesium carbonate hydrate and / or magnesium carbonate.

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