WO2025011738A1 - Method for producing a cement stone - Google Patents

Abstract

The invention relates to a method for producing a cement stone, comprising the steps of: providing and comminuting a starting product to a fineness corresponding to a BET surface of 0.1 m2/g, wherein the starting product contains at least 20% by weight of magnesium silicate hydrate and has an iron content of at least 1% by weight; homogenising the starting product; and at least partly dewatering the starting product of bound water by means of thermal treatment in a unit for thermal treatment; bringing into contact and mixing the dewatered starting product with water at a ratio of water to dewatered starting product of 1:2; and subsequently allowing the mixture to harden to produce the cement stone to a compressive strength of at least 10 MPa.; and bringing the hardened cement stone into contact with CO2.

Description

METHOD FOR PRODUCING A CEMENT STONE

The invention relates to a method for producing a cement stone.

Cement is an inorganic, non-metallic building material and is a binding agent. It is finely ground for processing and is in powder form. It is used to manufacture components and structures.

It hardens through a chemical reaction with water, known as hydration. To produce building materials such as mortar and concrete, additional water, formerly known as mixing water, and other substances such as sand and aggregate are added to the cement. 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 commonly used building material.

Cement mixed with water is called cement stone. The cement stone envelops the rock particles of the aggregate, fills the cavities and makes the fresh concrete workable. The cement stone is created when the cement stone hardens. The nature of the cement stone determines the strength of the concrete. In the context of the invention, cement stone refers to any solidified material that can be produced by mixing a binding agent with water and hydrating.

Limestone and clay are used to make cement, which often occur as a natural mixture and are then referred to as marl. If necessary, additives such as quartz sand and iron oxide-containing substances are added. The raw materials are ground into raw meal and then heated to around 1,450 °C until they partially fuse together at the grain boundaries (sintering) and the so-called cement clinker is formed. The now spherical material is cooled and ground into the final product, cement. In order to produce types of cement with certain properties, To obtain the desired properties, granulated blast furnace slag, fly ash, limestone and gypsum can be added in different dosages and fineness of grinding before grinding.

The impact of cement production on the climate is now considered problematic. The cement industry is one of the main emitters of carbon dioxide, which contributes to global warming. In relation to annual global production, the CO2 released during the burning of lime, more precisely calcium carbonate, corresponds to an emission of almost three billion tonnes of CO2, or about 6 to 8% of annual CC emissions, which is three to four times the magnitude of all air traffic.

Carbon dioxide (CO2) acts as a greenhouse gas in the atmosphere and is considered one of the main causes of man-made global warming. In addition to the general reduction of carbon dioxide emissions, attempts are being made to bind CO2 that is already in the atmosphere. Binding CO2 can also be used to make manufacturing processes that produce large amounts of CO2, such as cement production, CO2-neutral.

Sequestration of CO2 means removing CO2 from the atmosphere, ideally in such a way that the CO2 is combined with other substances so that it cannot escape again. Various proposals are known for this. One possibility is to use olivine.

Olivine is a mineral with the general composition A2[SiO4], where A can contain various divalent ions 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.

The invention is based on the object of providing an efficient method for producing a cement stone in which CO2 can additionally be sequestered.

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.

According to claim 1, it is provided that a starting product is first provided which contains at least 20 mass% magnesium silicate hydrate (Mg ₃ Si2Os(OH)4, Mg ₃ Si40io(OH)2), has an iron content of at least 1 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.

The magnesium silicate hydrate content is preferably at least 40% by mass, more preferably at least 60% by mass, even more preferably at least 80% by mass. An example of this is serpentinite. Serpentinite is a metamorphic rock that is formed by natural transformation, in particular weathering, of ultramafic rocks. The starting material should advantageously not contain any 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 and thereby reduce the product quality.

An important mineral in ultramafic rocks is olivine. This is a solid solution series between fayalite (Fe2SiO4), forsterite (Mg2SiO4), tephroite (Mn2SiÜ4) and other minerals of the form A2[SiÜ4]. Natural olivine deposits are documented and the olivine is often a magnesium-rich material with iron components.

The reactions occurring during weathering are as follows, which are given here in a simplified form, starting from forsterite (Mg2SiO4)

(2) 3 Mg ₂ SiO ₄ + 5 SiO ₂ + 2 H ₂ O 4 2 Mg ₃ Si ₄ Oio(OH) ₂

Magnesium silicate hydrate (Mg ₃ Si2Os(OH)4, Mg ₃ Si40io(OH)2) can be present in the form of lizardite, antigorite, talc and other forms. 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 Mass% to 20 mass%. This can be explained by the fact that some of the materials are so fine that water can adhere to their surface.

In a similar way, 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 can contain iron, carbonate and alkalis or other foreign ions.

The iron content is given here converted to an elemental iron content, since iron (Fe) can exist in many different forms and bonds. Examples of this are: magnetite, hematite, iron in olivine solid solution or as a foreign ion in other compounds.

This starting product is comminuted 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 the specific surface area of solids by gas adsorption" using the BET method. A BET surface area of 0.5 m ² /g is advantageous, even more preferably a BET surface area of 1.0 m ² /g or even finer. This comminution can be carried out by grinding. Depending on the source of the magnesium silicate hydrate, comminution in the sense of the invention can also take place during or through mining, extraction or, more generally, production.

After the raw material has been prepared, it is homogenized if necessary. The serpentinite used as an example is a material from natural deposits. Experience has shown that it is neither in pure form nor homogenized. Homogenization can be carried out, for example, with a mixer or at the same time as crushing to the desired fineness.

The homogenized starting product is then at least partially dewatered of bound water by thermal treatment in a thermal treatment unit. The proposed Thermal treatment 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 great effort. According to the invention, the water content of bound water should be reduced by at least 50%, preferably at least 70%, even more preferably by at least 90%.

For thermal treatment, the starting product can be heated to a temperature between 180 °C and 1000 °C. Depending on the fineness, heating for 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.

In this step, dewatering converts magnesium silicate hydrate (Mg3Si2Os(OH)4, Mg3Si40io(OH)2) present in the starting product at least partially into dewatered magnesium silicate hydrate, which can be simplified as xMgO SiC y^O. Dewatering refers to the reduction of the crystalline water or the water of crystallization in the converted starting product.

The underlying chemical processes are, again simplified, as follows:

₍ 3) _Mg3Si2O5 ( _OH ) ₄ ^ _{2xMgOSiO2yH2O} + _zH2O

(4) Mg ₃ Si ₄ Oio(OH)2^ 2 aMgO SiO ₂ bH ₂ O + c H ₂ O

(5) Mg(OH) ₂ -> MgO + H ₂ O, whereby in (3) a largely amorphous reaction product is formed with a Mg to Si ratio of 1.5 to 2 and a water content of about 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. After dewatering, the water content of the bound water in the converted, dewatered starting product is preferably below 10% by mass, advantageously below 5% by mass, more preferably below 3.5% by mass, even more preferably below 2.5% by mass.

The dehydrated starting product is therefore a multiphase product. Other possible secondary phases are hematite, magnetite, enstatite, feldspars, pyroxenes and amorphous phases.

After dewatering, the dewatered starting product is mixed with water in a water to dewatered starting product ratio of 1:2 or less. This mixture is then allowed to harden to a compressive strength of at least 10 MPa. The dewatered starting product hardened in this way is called cement paste.

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 found that a higher ratio, i.e. a higher water content, prolongs the hardening process and reduces the strength. In principle, the bonding process can be accelerated by heat or pressure treatment.

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 high enough that further processing, such as stripping, can begin without damaging or destroying the cement paste, which is still hardening.

Before hardening, no SiO2 should be added or contained, especially no unbound or free SiO2, as otherwise too much MgO or other additives will be needed to bind the SiO2. Also or alternatively, no substances containing alite or belite should be added, as this can have a negative effect on hardening.

A reaction that causes permanent solidification of the dehydrated starting product that has been contacted and mixed with water is the hydration of the dehydrated magnesium silicate hydrate. This produces phases such as antigorite, talc and lizardite, which are usually present in amorphous form. This hydration reaction is comparable to the hardening of cement stone with the difference that MSH is formed instead of CSH.

After the dehydrated starting product, which can now be called cement paste, has been allowed to harden, it is contacted with CO2. The CO2 is mainly bound in the resulting magnesium carbonate (MgCOs) and/or magnesium carbonate hydrate (MgCOs mFhO). Magnesium carbonate is referred to by the mineral name magnesite. Magnesium carbonate hydrates include, for example, barringtonite (m=2), nesquehonite (m=3), landsfordite (m=5). There are also basic magnesium carbonate hydrates such as artinite, hydromagnesite and dypingite.

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

Furthermore, the CO2 is partially bound by M-S-H, which is formed during hydration, whereby the extent of the CC bond 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.

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 which can be converted into cement stone and which 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 at most 750°C and if the starting product is thermally treated for at least 5 minutes, advantageously 15 minutes, preferably 30 minutes, even more preferably at least 60 minutes. It is advantageous to maintain the dewatering temperature characteristic of a particular material as precisely as possible with a deviation of less than 20°C. If the temperature during the thermal treatment is too low, no or only insufficient dewatering of the magnesium silicate hydrate, such as serpentinite, occurs. If the temperature is too high, the magnesium silicate hydrate is largely converted into olivine, which has poor reactivity. Dewatering only occurs in a narrow temperature range with little or no olivine formation. Instead, the dewatered starting product forms in the form of an X-ray amorphous phase with a low residual water content of between 2 and 5% by mass. This phase has high reactivity and is the target product of the thermal treatment.

It is therefore preferred if the thermal treatment unit has a substantially homogeneous temperature distribution. This enables good dewatering to be achieved without the formation of unwanted by-products. It is therefore advantageous if the furnace is not heated directly with a flame in order to be able to maintain the desired dewatering temperature in the furnace as precisely as possible and over a large part of the material's residence time. 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. It is therefore advantageous if a rotary kiln, in particular an indirectly heated rotary kiln without open flames in the reaction chamber, is used as the thermal treatment unit. The temperature during the thermal treatment, which can also be referred to as the firing temperature, can be controlled particularly precisely in electrically heated furnaces. Electrical heating should be used in particular to precisely maintain the target temperature in the furnace chamber. It is advantageous to heat the furnace with electrical energy from renewable energy sources, as this does not produce any CO2 emissions or exhaust gases and does not consume any fuel. In contrast, preheating is also possible using other heat sources, in particular a heat exchanger, which takes some of the heat from the dehydrated starting product, such as the fired serpentinite, and thereby cools it, while at the same time supplying this heat to unfired serpentinite. The use of flue gas from combustion Processes should be prevented if possible, as this could lead to, for example, uncontrolled binding of CO2.

In order to achieve a sufficient residence time at the target temperature, rotary kilns are preferred because they have a large volume and thus a high throughput can be achieved. Rotary kilns are also characterized by good thermal efficiency.

Thermal treatment is particularly efficient for small particle sizes of magnesium silicate hydrate, such as ground serpentinite, since 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 for binding the CO2. A low water vapor partial pressure can be achieved by flushing the furnace chamber with air.

The ratio of water vapor volume to purge gas volume should be less than 1:1, more advantageously less than 1:2, even more advantageously less than 1:4, even more advantageously less than 1:8, ideally less than 1:10. Purge gas in the sense of the invention is, for example, air that can be blown in to remove the water vapor. More generally, it is a gas that is used to replace the water vapor.

After thermal treatment, the dehydrated starting product, such as tempered serpentinite, is usually in the form of a powder. This powder can be mixed directly with water to produce cement stone. Further crushing is not necessary and it has even been shown that grinding can have a negative effect on hardening.

Preferably, the dewatered starting product brought into contact with water and mixed is filled into a mold or formwork to harden there. The dewatered starting product brought into contact with water and mixed, which can also be referred to as cement paste, has a sludge-like consistency depending on the ratio of dewatered starting product and water. If less water is used, the consistency is firmer. However, in order to form the cement stone or the concrete in which the cement stone is used into a desired shape, can be provided for the concrete or cement stone to be 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 are, for example, normal weather influences and in particular no forces that are explicitly aimed at causing damage. An example of this is stripping the formwork.

The cement stone or the dewatered starting product according to the invention can be used with water alone, i.e. pure. However, it is also possible to produce a concrete-like building material. To do this, sand and/or an 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 it is subjected to further forces. Reinforcements, e.g. made of steel, carbon or glass fibers, can also be added before, during or after the addition of water.

It is preferred that the dewatered 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. Experience shows that a compressive strength of at least 10 MPa is achieved after this time. Advantageously, the dewatered starting product contacted and mixed with water is allowed to harden for longer than, for example, 24 hours or even 48 hours. This increases the strength of the cement paste.

The dewatered, in particular hardened starting product can advantageously be brought into contact with CO2 in a treatment unit such as a closed container, in particular in an autoclave, or in a container subjected to excess pressure. In this way, the CC bonding process can be optimized, for example, by adjusting the CC partial pressure, the prevailing temperature and/or the water content of the atmosphere in the autoclave. In principle, however, contact can also be made at ambient pressure. This has the advantage that components or elements made from or with the cement stone according to the invention can absorb CO2 from the ambient air. The binding of CO2 is particularly efficient when the dehydrated, hardened starting product 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 value of the pore solution drops too much.

It is advantageous if the hardened cement stone is brought into contact with CO2 at a temperature above room temperature, as this accelerates the chemical reactions. If the contact is carried out in a treatment unit, the heating can be carried out, 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 work with very low CO2 partial pressures, which are preferably below 0.5 bar.

It is advantageous to add organic additives, such as flow agents, to the dewatered starting product before or during contact and mixing of the dewatered starting product with water. This facilitates further processing of the cement paste and can influence the 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 to a high degree. 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 or silicon slows down the reactions or sometimes stops them completely. It is therefore not insignificant to shift the corresponding molar ratios 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.

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 product is subjected to grinding, particularly wet grinding. The starting product is at least partially - even if it is already very fine due to natural weathering - not available in a higher fineness. This fineness can be easily increased by grinding. Wet grinding is also preferred here, as this is often more energy efficient than dry grinding.

The invention further relates to a cement stone which contains magnesium carbonate hydrate and/or magnesium carbonate and is produced according to the method according to the invention. This has the advantage that it can bind CO2 from the ambient air in the chemical processes explained above. It preferably consists of magnesium silicate hydrate (M-S-H) and advantageously contains no amorphous SiO2. Furthermore, it can have a minimum strength of 10 MPa, preferably 20 MPa, even better 30 MPaMPa when tested as mortar or concrete.

The invention is explained in more detail below using exemplary embodiments.

To verify the invention, the investigations described in more detail below were carried out.

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 ^m2 /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 dewatering of the magnesium silicate hydrate to purge gas volume was 1:5. The material was then used as a binding agent for mortar production in accordance with 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 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.

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 heated at 700°C for 60 minutes and used as a binder for the production of concrete with grading curve AB16, 3% plasticizer (based on the binder), water/binder ratio=0.30, binder:aggregate=1:5. After 7 days, the compressive strength of a cube with an edge length of 10 cm was 55 MPa.

The material from example 1 was alternatively fired in an electrically heated rotary kiln, where the temperature was also around 675°C. The volume ratio of water vapor to purge gas was 1:8. The mortar was produced according to the same recipe as in example 1 and the compressive strength when removing the formwork after 2 days was 12 MPa. After 7 days, a compressive strength of 32 MPa was achieved. CO2 binding was also demonstrated after air storage.

Claims

Method for producing a cement stone, comprising the steps: a) providing and crushing, 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% by mass of magnesium silicate hydrate, has an iron content of at least 1% by mass, and can contain magnesium hydroxide, b) homogenizing the starting product, c) at least partially dewatering the starting product of bound water by means of thermal treatment in a thermal treatment unit, wherein during thermal treatment the converted starting product is treated at a temperature between 180°C and 1000°C, 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 and dewatered magnesium silicate hydrate, d) contacting and mixing the dewatered starting product with water in the ratio of water to dewatered Starting product 1:2 or less, and then allowing the mixture to harden to produce the cement stone to a compressive strength of at least 10 MPa, e) contacting the hardened cement stone after step d) with CO2, whereby the CO2 in cement stone is bound in magnesium carbonate hydrate and/or magnesium carbonate formed there. Process according to claim 1, characterized in that 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 and that the starting product is thermally treated for at least 15 minutes, preferably 30 minutes, more preferably at least 60 minutes. 3. Method according to claim 1 or 2, characterized in that the unit for thermal treatment has a substantially homogeneous temperature distribution. 4. Process according to one of claims 1 to 3, 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. 5. Process according to one of claims 1 to 4, characterized in that the dehydrated starting product contacted and mixed with water is filled into a mold or formwork in order to allow it to harden there. 6. Process according to one of claims 1 to 5, characterized in that sand and/or an aggregate is added to the dewatered starting product before, during or after contact with water. 7. Process according to one of claims 1 to 6, characterized in that the dehydrated starting product contacted and mixed with water is allowed to harden for at least 8 hours before the cement stone is subjected to external forces. 8. Process according to one of claims 1 to 7, characterized in that the contacting of the dewatered starting product with CO2 is carried out in a closed container, in particular in an autoclave, a scrubber or a container with excess pressure. 9. Process according to one of claims 1 to 8, characterized in that the contacting of the hardened cement stone after step d) with CO2 is carried out with a CC partial pressure of maximum 1000 ppm. 10. Process according to one of claims 1 to 9, characterized in that the contacting of the hardened cement stone with CO2 is carried out at a temperature of at least 30°C, preferably above 50°C. 11. Process according to one of claims 1 to 10, 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. 12. Process according to one of claims 1 to 11, 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. 13. Cement stone produced using a method according to one of claims 1 to 14, comprising magnesium carbonate hydrate and/or magnesium carbonate, which is designed to bind CO2.