Process for producing a cement block and binder constituent for producing a cement block

Abstract

The invention relates to a process for producing a cement block from asbestos fibre-containing serpentinite, - wherein the serpentinite has an asbestos content of at least 0.008% by mass, - wherein the process comprises the steps of: a) providing a starting material which contains at least 40% by mass of magnesium silicate hydrate and has an iron content of at least 1% by mass, where the starting material is provided at, or comminuted to, a Blaine fineness of 40000 cm2 / g or coarser, b) homogenizing the starting material, c) subjecting the starting material to thermal treatment in a unit for thermal treatment at a treatment temperature between 600°C and 1000°C, wherein, after step c), magnesium silicate hydrate present in the thermally treated starting material is at least partly dewatered and can be converted to dehydrated magnesium silicate hydrate, d) recomminuting the thermally treated starting material in a comminuting system to achieve an increase in fineness by at least 5% relative to the fineness of the thermally treated starting material, e) contacting and mixing the thermally treated comminuted starting material with water in a ratio of water to thermally treated comminuted starting material of 1:2 or less, and subsequently hardening the mixture to produce a cement block up to a compressive strength of at least 10 MPa, with the possibility of hardening to 10 MPa without further additives. The binder constituent produced by the process is free of asbestos fibres that are hazardous to health. Fragments of tempered, crushed and dewatered chrysotile fibres that are shorter than 5 µm and / or have a diameter greater than 3 µm and / or have a length / diameter ratio of less than 3:1 are detectable in the binder constituent.

Description

[0001] II* Wunderlich's home

[0002] the patent attorney

[0003] 0 0530

[0004] Method for producing a cement stone and binder component for producing a cement stone

[0005] The invention relates to a method for producing a cement block from asbestos-containing serpentinite and to a binder component for producing a cement block. The serpentinite has an asbestos content of at least 0.008% by mass.

[0006] Conventional cement, also known as Portland cement, is an inorganic binder used in the production of mortar and concrete to create a strong bond between aggregates. The hardening of cement is based on a reaction between the cement and water, known as the hydration reaction. In this reaction, the hydraulically active phases of the cement dissolve in the water, and hydrate phases precipitate out of the solution. The primary strength-imparting phase in the hydration of conventional Portland cement is called the CSH phase and is a calcium silicate hydrate.

[0007] Conventional Portland cement is made from the raw materials limestone and clay. Quartz sand, iron oxide-containing substances, and other materials can also be added. The raw materials are typically homogenized, ground, and fired at 1450 °C in a rotary kiln. The resulting cement clinker is cooled and, with the addition of other materials, ground to produce the final product, cement.

[0008] Due to the global availability of raw materials, as well as the strength and durability of concrete, cement is one of the most important binding agents worldwide. With a global production of 4.1 billion tons in 2017, cement is the most widely used building material.

[0009] Cement mixed with water is called cement paste. The cement paste coats the aggregate particles, fills the voids, and makes the fresh concrete workable. As the cement paste hardens, cement stone is formed. Hardened concrete thus consists of cement stone and embedded aggregate. The properties of the cement stone determine the strength of the concrete. Within the scope of the invention, cement stone refers to any hardened material that can be produced by mixing a binder with water and hydrating it.

[0010] Cement production uses limestone and clay, which often occur as a natural mixture known as marl. If necessary, aggregates such as quartz sand and iron oxide-containing materials are added. 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 cement clinker. This now spherical material is cooled and ground into the final product, cement. To obtain cement types with specific properties, blast furnace slag, fly ash, limestone, and gypsum can be added as main components in varying proportions and degrees of fineness before grinding.

[0011] Both the use of fossil fuels in the firing process and the deacidification of the limestone release a significant amount of CO2 during the production of Portland cement. While the CO2 from the firing process can be reduced through renewable energy sources, the CO2 from the deacidification of the limestone is considered difficult to reduce. Alternative, CO2-free raw material sources for the required CaO are not available in sufficient quantities to produce the large amounts of cement needed worldwide. Over 4 billion tons of cement are produced annually, releasing approximately 2 billion tons of CO2, about 8% of annual anthropogenic CO2 emissions, which is roughly three times the amount emitted by the entire aviation industry.

[0012] 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 fundamental reduction of CO2 emissions, efforts are also being made to capture CO2 already present in the atmosphere. CO2 capture can also be used to make manufacturing processes that generate large amounts of CO2, such as cement production, carbon-neutral.

[0013] As an alternative to calcium silicate-containing Portland cement, binders based on magnesium oxide (MgO) and silicon dioxide (SiCh) are known. During hydration, these binders form the strength-providing MSH phase, analogous to the aforementioned CSH, a magnesium silicate hydrate. Such a binder is described, for example, in EP 2508496 A1. In some cases, magnesium carbonate (MgCOs) is described as a raw material for these binders; however, a similar CO2 release occurs during the necessary deacidification as in the Portland cement manufacturing process described above.

[0014] An alternative raw material without bound CO2 is magnesium silicate-rich rocks. A corresponding process is described, for example, in WO 2023 / 134849 A1. Ultramafic rocks with a high olivine mineral content or serpentinites transformed from these rocks through metamorphism are suitable examples. Olivine is the name for a group of minerals with the chemical composition A₂[SiO₄], where A represents various divalent ions, such as magnesium (forsterite, Mg₂SiO₄), iron (Fe₂SiO₄, fayalite), and manganese (Mn₂SiO₄, tephroite) found in ultramafic rocks. Serpentinites consist mainly of the serpentine minerals antigorite, lizardite, and chrysotile with the chemical formula Mg₃Si₂Os(OH)₄ and differences in crystal structure.

[0015] The mineral chrysotile is a layered silicate, also known as fibrous serpentine or white asbestos. Its crystal structure causes the individual layers to curl cylindrically, forming long fibers. Fibers that, according to the WHO definition of respirable asbestos fibers, are longer than 5 pm and have a diameter of less than 3 pm and a length-to-diameter ratio greater than 3:1, pose a high health risk (Convention concerning Safety in the Use of Asbestos (C162 Asbestos Convention), entered into force on June 16, 1989, International Labour Organization (ILO), Geneva 1986). Exposure to, and especially inhalation of, these fibers increases the risk of developing asbestosis and lung cancer. Particularly in the 20th century, asbestos fibers were widely used due to their high tensile strength and fire resistance.However, the use of asbestos fibers is now banned in many countries due to the aforementioned health risks, including the USA and the European Union. Industrial mining for chrysotile, which still takes place in some countries today, generates large quantities of chrysotile-containing waste rock that is stored in landfills and is available as a potential raw material for further use.

[0016] The content of respirable asbestos fibers (according to the WHO definition mentioned above) in material samples can be determined using various analytical methods. For mass contents < 1 wt%, scanning electron microscopy analysis is suitable, as described, for example, in method BIA (IFA) 7487 ("Method for the analytical determination of low mass contents of asbestos fibers in powders, dusts and other dusts using SEM / EDX", reference number 7487). It has been shown that acid treatment is not necessary for determining the content of respirable asbestos fibers in serpentinite, as well as in the product of the invention. The quantitative analysis is based on counting fibers in a powdered sample. The asbestos fibers are chemically identified by their morphology and by energy-dispersive X-ray analysis.The volume of the identified fibers in a sample is summed up, the total mass of the fibers is calculated using the density and compared to the total mass of the analyzed sample.

[0017] EP 2508496 A1 mentions serpentinite as a possible raw material for the production of an MSH binder; however, according to the description, the serpentinite must be treated in such a way that the crystal lattice is destroyed and free MgO and SiO₂ are present. For this purpose, the starting material must be digested in acid according to methods commonly found in the literature and precipitated as MgCO₃ and SiO₂.

[0018] WO 2023 / 134849 A1 also mentions the processing of serpentinite into a building material.

[0019] Due to their metamorphic formation conditions, almost all known economically viable serpentinite deposits contain varying amounts of chrysotile as a secondary phase. Therefore, when using this raw material, care must be taken to ensure that the chrysotile fibers contained within do not enter the environment as respirable asbestos fibers (as defined by the WHO) during the manufacturing process and are not present in the final product.

[0020] Various methods are known for rendering asbestos fibers ineffective, including heating, grinding, or dissolving them in acid. EP 0344563 B1, for example, describes the treatment of chrysotile waste by temperature treatment above 580 °C. The method described therein aims to convert the chrysotile into non-reactive olivine, thus making further use possible only as a filler. EP 1 277 527 A1 also describes a method for converting chrysotile waste, particularly asbestos cement products, by means of temperature treatment at 600 °C - 1000 °C and using the product as a binder in the cement industry, whereby the hydraulic activity is based on dicalcium silicate (Ca₂SiO₄).

[0021] The processes mentioned have in common that chrysotile-containing waste materials are used as starting materials.

[0022] The invention is based on the task of providing an efficient method for producing a cement block from asbestos-containing serpentinite, as well as a binder component for producing a cement block that does not entail any asbestos-related health risks.

[0023] This problem is solved according to the invention by a method having the features of claim 1 and a binder component for producing a cement stone having the features of claim 14.

[0024] Further advantageous features are specified in the dependent claims and the further description.

[0025] According to claim 1, it is intended to provide a starting product which contains at least 40 wt% magnesium silicate hydrate (Mg3Si2Os(OH)4, Mg3Si4O (OH)2) and has an iron content of at least 1 wt%.

[0026] Preferably, the magnesium silicate hydrate content is at least 40 wt%, more preferably at least 60 wt%, and even more preferably at least 80 wt%. Serpentinite is an example of this. Serpentinite is a metamorphic rock formed by the natural alteration, particularly weathering, of ultramafic rocks. Advantageously, the starting material should not contain any, especially reactive, silicon dioxide (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.

[0027] Olivine is an important mineral in ultramafic rocks. It is a solid solution series consisting of fayalite (Fe₂SiO₄), forsterite (Mg₂SiO₄), tephroite (Mr⁻SiC₆), and other minerals of the form A₂[SiO₄]. Natural olivine occurrences are documented, and the olivine is often a magnesium-rich material with iron content.

[0028] The reactions occurring during weathering are as follows, although these are simplified here, starting with forsterite (Mg2SiO4).

[0029]

[0030] Magnesium silicate hydrate (Mg3Si2Os(OH)4, Mg3Si4O(OH)2) can exist in the form of lizardite, antigorite, chrysotile, talc, and other forms. It should be noted that the stoichiometric water content is sometimes lower—around 13 wt% for antigorite—than the value determined experimentally—ranging from 16 wt% to 20 wt%. This can be explained by the fact that some of these materials are so fine that water can adhere to their surface.

[0031] The mineral chrysotile, when incorporated into the structure of the parent rock serpentinite, is not harmful to health. Only the release of the chrysotile in fibrous form during the grinding of the rock produces harmful asbestos fibers. Therefore, for the transport and processing of the potentially asbestos-containing raw material, it is advantageous if it is not finely ground into powder but rather used in a crushed, coarse form. The magnesium silicate hydrate content and its distribution among the individual mineral phases antigorite, lizardite, and chrysotile in the raw material can be determined, for example, by X-ray diffraction with quantitative Rietveld phase analysis.

[0032] Similarly, deviations from stoichiometry also apply to the ratio between Mg and Si. Furthermore, foreign ions, such as Fe, can be incorporated into the reaction products. Other reaction products, such as hydromagnesite, hematite, magnetite, or gibbsite, can also be formed. This depends on the exact composition of the starting material. All or some of the reaction products may contain iron, carbonate, alkalis, or other foreign ions.

[0033] The iron content is given here in converted form to 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.

[0034] The starting product is already produced with a fineness of 40000 cm. 2 / g Blaine or coarser, or ground to a maximum of this size. Preferably, the fineness is between 4 cm. 2 / g Blaine and 40000 cm 2 / g Blaine. A fineness of 10000 cm is even more preferred. 2 / g or coarser, even more preferred at 1000 cm 2 / g Blaine or coarser. The Blaine surface can be determined according to the standard DIN EN 196-6 "Test methods for cement - Part 6: Determination of fineness of grinding" using the Blaine air permeability method. A mean particle size distribution of dso = 5 mm corresponds approximately to a fineness of 4 cm. 2 / g Blaine for the starting material used: serpentinite, since the density of the material also influences the conversion. Such a coarse fineness of 4 cm 2The Blaine fineness cannot be reliably determined in practice using classical Blaine measurement methods due to the high air permeability of the sample. Therefore, within the scope of the invention, this Blaine fineness is still referenced to enable comparability of finenesses.

[0035] Due to its rapid sample preparation and low investment costs, the Blaine method is the most common method in the building materials industry for characterizing the fineness of a powder. It determines the external surface area of ​​the sample. Using gas adsorption with the BET method according to the standard DIN ISO 9277:2003-05 "Determination of the specific surface area of ​​solids by gas adsorption," the internal porosity of the sample is additionally determined. Therefore, the BET method typically yields higher measured values ​​for the specific surface area compared to the Blaine method. For the characterization of mill fineness described here, the Blaine method is very well suited.

[0036] Comminution can be achieved by crushing and / or grinding. Depending on the source of the magnesium silicate hydrate, comminution according to the invention can also occur during or through mining, extraction, or, more generally, production. After providing the starting material, homogenization follows if necessary. Due to its geological formation process, the serpentinite rock used as the starting material is neither pure nor homogenized. Homogenization can be carried out, for example, with a mixer or simultaneously during comminution to the desired fineness. Subsequently, the homogenized starting material is thermally treated in a unit at a treatment temperature between 600°C and 1,000°C.

[0037] Thermal treatment can also be called tempering or calcining. In this process, the magnesium silicate hydrate present in the starting product is at least partially dehydrated of bound water and converted into dehydrated magnesium silicate hydrate (MgO SiO2 yH2O). Bound water is sometimes also referred to as water of crystallization. It must be distinguished from unbound water, which can be considered free H2O.

[0038] The residual water content in the magnesium silicate hydrate after thermal treatment depends on various factors, in particular the temperature and burning time in the thermal treatment unit and the fineness of the starting material.

[0039] Complete dehydration can be achieved with considerable effort. According to the invention, it is preferred if the water content of bound water in the magnesium silicate hydrate is reduced by at least 50%, preferably by at least 70%, and even more preferably by at least 80%.

[0040] For thermal treatment, the starting material is heated to a temperature between 600°C and 1000°C. Depending on the desired fineness, heating for just a few minutes is sufficient. Temperatures between 600°C and 900°C are preferred, and temperatures between 650°C and 850°C are even more advantageous.

[0041] During thermal treatment, it is important that the starting material is not burned until olivine and / or free MgO and free SiO2 are formed, as this would reduce the proportion of partially dehydrated magnesium silicate hydrate in the converted starting product, which would decrease the product quality.

[0042] The underlying chemical processes are, again simplified, as follows: (1) Mg3Si2O5(OH)4-> 2 xMgO SiO2yH2O + z H2O

[0043] (2) Mg3Si4Oio(OH)2^ 2 aMgO SiO2bH2O + c H2O where (1) a largely amorphous reaction product with a Mg to Si ratio of 1.5 to 2 and a water content of about 3% is formed. The reaction product formed in equation (2) has an even lower Mg to Si ratio. Hence the variables a, b, c, x, y and z. This depends in each case on the exact composition of the starting material and the treatment parameters.

[0044] After dewatering, the content of bound water in the converted, dewatered starting product is preferably below 10 wt%, advantageously below 5 wt%, even more preferably below 3.5 wt% and above 2.5 wt%.

[0045] The dehydrated starting material is therefore a multiphase product. Other possible secondary phases include hematite, magnetite, enstatite, feldspars, pyroxenes, and amorphous phases.

[0046] The chrysotile mineral, present in the raw material and harmful to health in its fibrous form, is transformed by dehydration and is no longer chrysotile in the mineralogical sense. Therefore, it is not definitively defined whether the fibers contained in the thermally treated raw material still qualify as asbestos fibers according to the WHO definition mentioned above. Furthermore, it is not sufficiently understood whether these dehydrated fibers are still harmful to health. Analysis using a scanning electron microscope according to method BIA (IFA) 7487 will identify these fibers as asbestos fibers, since the morphology and chemical composition, particularly the Mg / Si ratio, do not change significantly as a result of the thermal treatment.

[0047] According to the invention, the thermally treated starting product is again crushed in a crushing plant, whereby an increase in fineness, in particular in the form of the specific surface area, of at least 5% is achieved with respect to the fineness of the thermally treated starting product.

[0048] This essentially ensures that the thermally treated and partially dewatered starting material no longer contains asbestos fibers. Through re-crushing, the cryotile or mineral fibers, already weakened in their structure by the thermal treatment, are reduced to such an extent that they no longer qualify as harmful fibers according to the WHO definition. The treated starting material is considered asbestos-free if the fiber content is below 0.008% by weight. Re-crushing takes place in a suitable grinding plant, for example, a mill. According to the invention, the increase in fineness after re-crushing compared to the starting material after thermal treatment is used as a measure of the energy input required to break down the mineral fibers contained in the thermally treated starting material. An increase in fineness of at least 5% is desired.It should be noted that the fineness of the comminuted starting material can be reduced during thermal treatment due to the onset of sintering processes. Therefore, it can become coarser after thermal treatment.

[0049] It is advantageous if the fineness is increased by 10%, preferably by 20%, and even more preferably by 30%.

[0050] The thermally treated, crushed raw material, which is dewatered and asbestos-free, is then mixed with water to produce a mortar or concrete, specifically according to DIN EN 196 or 206. The ratio of water to thermally treated crushed raw material is 1:2 or less. The mixture is then allowed to harden to a compressive strength of at least 10 MPa to produce a cement block. This hardening to 10 MPa is possible without additives. The compressive strength can be tested according to DIN EN 196. While other aggregates, such as Portland cement, can be added to increase strength, the 10 MPa strength is achieved solely by the thermally treated, crushed raw material.The hardened, dehydrated starting product can be called cement stone and used to hold together aggregates in concrete or mortar.

[0051] It is advantageous if the ratio of water to thermally treated, comminuted starting material is 1:2.22, preferably 1:2.5, ideally 1:2.86, and even better 1:3.33 or lower. 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 further accelerated by heat or pressure treatment.

[0052] It has been shown that a compressive strength of at least 10 MPa can be achieved relatively quickly, depending on environmental variables. This strength is already so high that further processing, such as demolding, can begin without damaging or destroying the still-hardening cement stone. "Quickly" in the context of the invention can be understood as 28 days, preferably 2 days.

[0053] Before hardening, no reactive silicon dioxide (SiO₂), especially unbound or free SiO₂, should be added or present, as this would require excessive amounts of magnesium or other additives for binding the SiO₂. Alternatively, substances containing alite or belite should also be avoided, as these can negatively affect the hardening process.

[0054] One reaction that causes the permanent solidification of the dehydrated starting material when it comes into contact with and 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 present in amorphous form. This hydration reaction is comparable to the hardening of Portland cement, except that MSH is formed instead of CSH.

[0055] In principle, any comminution device can be used as a comminution plant for re-combining the thermally treated feedstock. However, a vertical roller mill, a ball mill, a jet mill, and / or a roller bowl mill have proven to be the most energy-efficient.

[0056] By re-combining the thermally treated starting material, an absolute increase in fineness is advantageously achieved in relation to the fineness of the thermally treated starting material, determined as specific surface area by 1000 cm². 2 / g Blaine, preferably around 2000 cm 2 / g instead. The fineness or the associated specific surface area is determined according to Blaine (DIN EN 196-6), since the porosity of the re-comminuted thermally treated starting product does not influence the measurement result in this measuring method.

[0057] It is preferred that at least the thermal treatment and the subsequent grinding are carried out in facilities that operate under negative pressure from the ambient air. This prevents asbestos fibers from being released into the environment. It is advantageous if the thermal treatment of the starting material is carried out at a temperature of at least 700°C and / or a maximum of 850°C, and if the starting material is thermally treated for at least 5 minutes, preferably 15 minutes, preferably 30 minutes, and even more preferably at least 60 minutes. A shorter time of 2 to 3 minutes may also be used if the starting material is very fine and good heat absorption is possible.

[0058] It is advantageous to maintain the characteristic dehydration temperature for a specific natural raw material as precisely as possible, with a deviation of less than 20°C. If the temperature during thermal treatment is too low, no or insufficient dehydration of the magnesium silicate hydrate / serpentinite occurs. At excessively high temperatures, the magnesium silicate hydrate is largely converted into olivine, which exhibits poor reactivity. Dehydration occurs with little or no olivine formation only within a narrow temperature range. Instead, the dehydrated starting material forms as an X-ray amorphous phase with a low residual water content of between 2 and 5 wt%. This phase exhibits high reactivity and is the target product of the thermal treatment.

[0059] 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 through the conversion of the magnesium silicate hydrate into these two phases. Highly specialized treatments that cause the separation of the magnesium silicate hydrate into MgO and SiO2 are not the subject of the present invention and are therefore to be avoided. The starting material may contain small amounts of free, unbound MgO and Mg(OH)2 (brucite). During thermal treatment, brucite may be converted into periclase. Accordingly, any MgO content present in the binder is attributable 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 through the conversion of the magnesium silicate hydrate into periclase and SiO2.

[0060] Therefore, it is preferable for the thermal treatment unit to have a substantially homogeneous temperature distribution. This allows for effective dewatering without the formation of unwanted byproducts. To maintain the desired dewatering temperature in the furnace as precisely as possible and for a large portion of the material's residence time, it is advantageous if the furnace is not directly heated by 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, particularly an indirectly heated rotary kiln without open flames in the reaction chamber, as the thermal treatment unit.The temperature during thermal treatment, also known as firing temperature, can be controlled particularly precisely in electrically heated furnaces. Electric heating should be used especially for maintaining the exact target temperature in the furnace chamber. Heating the furnace with electricity from renewable energy sources is advantageous, as this produces no CO2 emissions or exhaust gases and consumes no fuel. In contrast, preheating is also possible using other heat sources, particularly a heat exchanger. This exchanger extracts some of the heat from the dehydrated starting material, such as the fired serpentinite, thereby cooling 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 uncontrolled CO2 absorption, for example.

[0061] To achieve a sufficient residence time at the target temperature, rotary kilns are preferred because they have a large volume and thus allow for a high throughput. Furthermore, rotary kilns are characterized by good thermal efficiency. Indirectly heated rotary kilns without open flames in the reaction chamber, such as electrically heated rotary kilns, are particularly advantageous. Thermal treatment is especially efficient with small particle sizes of magnesium silicate hydrate or ground serpentinite, as in this case the water bound in the particles can be driven off more quickly and efficiently, and the asbestos fibers are heat-treated sufficiently to be destroyed. At the same time, a low water vapor partial pressure in the thermal treatment unit, such as the kiln chamber, facilitates the formation of the reactive phase. A low water vapor partial pressure can be achieved by purging the kiln chamber with air.

[0062] The ratio of water vapor volume to purge gas volume should be less than 1:1, advantageously less than 1:2, even more advantageously less than 1:4, still more advantageously less than 1:8, ideally less than 1:10. Purge gas within the meaning of the invention is, for example, air that can be blown in to carry away the water vapor. More generally, it is a gas that is used to replace the water vapor. As described, the heat-treated material is then ground again.

[0063] Preferably, the water-mixed, dewatered, and re-ground starting material can be filled into a mold or formwork to harden. This water-mixed, dewatered, and re-ground starting material, which can also be called cement paste, has a slurry-like consistency depending on the ratio of dewatered starting material to water. A firmer consistency is achieved with less water. However, to be able to shape the cement paste or the concrete in which the cement paste is used, it may be possible to fill the concrete or cement paste alone into a mold or formwork and allow it to harden 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 conditions and, in particular, no forces explicitly aimed at causing damage. An example of normal environmental influences is formwork stripping.

[0064] The dewatered starting material according to the invention can be used alone with water, i.e., in its pure form, to produce a cement block. However, it is also possible to produce a concrete-like building material. For this purpose, sand and / or one or more aggregates can be added to the dewatered starting material before, during, or after contact with water. Ideally, the concrete-like building material should also be allowed to harden to a compressive strength of at least 10 MPa before being 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.

[0065] It is preferred that the dewatered starting material, which has been in contact with and mixed with water, be allowed to harden for at least 8 hours before the cement paste is subjected to external forces. Experience has shown that a compressive strength of at least 10 MPa is achieved after this time. Advantageously, the dewatered starting material, which has been in contact with 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.

[0066] It is advantageous to add organic additives, such as plasticizers, to the dewatered, re-milled raw material before or during contact and mixing with water. This facilitates further processing of the cement paste and can influence its hardening. Other additives can improve frost resistance or reduce shrinkage.

[0067] The starting materials provided according to the invention are usually not pure substances, so 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, respectively, slows down the reactions in relation to magnesium and silicon, respectively, or even brings them to a complete standstill. Therefore, it is important to shift the corresponding molar ratios significantly in the direction of magnesium and silicon, respectively. 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.

[0068] Following the hardening of the dehydrated, re-milled, and water-mixed starting material, which can now be called cement stone, it can be contacted with CO2. The CO2 is primarily bound in the resulting magnesium carbonate (MgCOs) and / or magnesium carbonate hydrate (MgCOs mFhO). 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). There are also basic magnesium carbonate hydrates such as artinite, hydromagnesite, and dypingite.

[0069] 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 is present in the microstructure and forms during hydration. This is essentially based on a reaction of Mg(OH)2 with water, as described in equation 3.

[0070]

[0071] Another reaction for binding CO2 in cement paste occurs through the conversion of magnesium silicate hydrate with a Mg / Si ratio of 1.5 to magnesium silicate hydrate with a lower Mg / Si ratio, with the simultaneous formation of hydromagnesite or other carbonate phases, exemplified in equation (4) for a resulting magnesium silicate hydrate with a Mg / Si ratio of 0.75. (4) 10 Mg3Si2O5(OH)4 + 12 CO25 Mg3Si4O10(OH)2 + 3 Mg5(CO3)4(OH)2 + 12 H2O In addition, other binding forms of CO2 are possible in other phases. Detection can be carried out, among other things, by determining the carbonation depth using an indicator solution such as phenolphthalein or by measuring the inorganic carbon content (TIC).

[0072] The presence of water as a reaction medium is highly advantageous for the reaction with CO2. This reaction occurs after the binder is mixed with water and via the pore solution of the cement paste. The pore solution remains throughout the service life of the cement paste and is in constant exchange with the environment. It can even be extracted from concrete samples for analysis after decades.

[0073] Accordingly, as shown in equation (4), 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.

[0074] The contact of the dehydrated, especially hardened, starting material 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 atmospheric moisture content in the autoclave. However, contact can also be carried out at ambient pressure. This has the advantage that components or elements made from or containing the cement paste according to the invention can absorb CO2 from the ambient air.

[0075] The binding of CO2 is particularly efficient when the dehydrated, hardened starting material is contacted with CO2 at a CO2 partial pressure of no more than 1000 ppm. Higher CO2 partial pressures should be avoided, as they would cause the pH of the pore solution to drop too drastically.

[0076] It is advantageous to contact the hardened cement paste with CO2 at a temperature above room temperature, as this accelerates the chemical reactions. If the contact 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 partial pressures of carbon dioxide (CCh), preferably below 0.5 bar.

[0077] To provide the starting product with a fineness corresponding to a Blaine surface of 4 cm 2For products with a fineness of 1 / g or finer, it is preferred that the starting material undergoes milling, particularly wet milling. The starting material, even if it is already very fine in part due to natural weathering, is not present at a higher fineness level. This fineness can easily be increased by milling. Wet milling is also preferred here, as it is often more energy-efficient than dry milling.

[0078] Furthermore, the invention relates to a binder component for the production of a cement block, wherein the binder component comprises processed serpentinite formerly containing asbestos fibers, the serpentinite having an asbestos content of at least 0.008 wt.%. The binder component is miscible with conventional cements and / or their main components or can also be used separately.

[0079] Cement consists of main components, minor components, and additives. Main components are selected inorganic substances whose mass fraction exceeds 5% of the total of all main and minor components. Examples of main components include granulated blast furnace slag, silica fume, pozzolan, fly ash, burnt shale, or limestone.

[0080] Minor constituents are selected inorganic substances whose mass fraction does not exceed 5% of the total sum of all major and minor constituents. These substances can originate from clinker production or be major constituents according to DIN EN 197-1, provided they are not already present as major constituents in the cement. Minor constituents serve to improve the physical properties of the cement (e.g., workability or water retention capacity) by optimizing the particle size distribution.

[0081] Additives according to DIN EN 197-1 are added to improve the production or properties of cement. The total amount of additives must not exceed 1.0% by mass based on the cement. The amount of organic additives in the dry state must not exceed 0.2% by mass based on the cement or binder.

[0082] The binder component itself comprises at least 40 wt% thermally treated dehydrated magnesium silicate hydrate (Mg3Si2Os(OH)4, Mg3Si4O(OH)2) and at least 1 wt% iron content, with the remaining components not contributing significantly to the strength. The thermally treated dehydrated magnesium silicate hydrate is amorphous and has a Mg to Si ratio of 2 or less. Furthermore, the water content of bound water in the thermally treated dehydrated magnesium silicate hydrate is preferably less than 10 wt%. The binder component contains detectable fragments of annealed, broken, and dehydrated chrysotile fibers that are shorter than 5 µm and / or have a diameter greater than 3 µm and / or a length-to-diameter ratio of less than 3:1. These fragments constitute more than 0.008 wt%, and in particular more than 0.1 wt%, of the binder component.In other words, it can be proven that the starting material of the binder component contained asbestos fibers.

[0083] Using only the binder component – ​​without any additional strength-contributing substances – MSH phases can form upon contact and mixing with water at a water-to-binder ratio of 1:2 or less, followed by hardening at temperatures below 30°C, thus producing a solid cement stone. This cement stone, produced using only the binder component, exhibits a compressive strength of at least 10 MPa after 28 days. The test is conducted according to DIN EN 196-1:2016 with a reduced water / B average value of 0.35 and the addition of a superplasticizer, with hardening to 10 MPa occurring without additives. The premixing time for this measurement can be approximately 3 minutes.

[0084] The proportion of dehydrated magnesium silicate hydrate in the binder component is preferably more than 60% by mass, and even more preferably more than 80% by mass. The higher the proportion, the greater the compressive strength of the resulting cement paste.

[0085] Depending on the composition of the starting material, olivine, which is non-reactive, can unintentionally form during the thermal dewatering process of producing the binder component according to the invention. The olivine content can be up to 30% by mass. Furthermore, the starting material often contains 5-10% by mass of iron phases such as magnetite and hematite. Sometimes the starting material also contains garnets, enstatite, or during mining, it can be mixed with other rocks that cannot be separated during extraction. Thus, the content of dewatered magnesium silicate hydrate in this example can be around 60% by mass.

[0086] It is advantageous if the dehydrated magnesium silicate hydrate has a Mg to Si ratio of 1.8 to 1.5. Faster hardening and / or higher strength can be achieved if the water content of bound water in the dehydrated magnesium silicate hydrate is below 5 wt%, preferably below 3 wt%.

[0087] The strength achievable with the binder component according to the invention refers to the setting of the pure, undiluted binder component. This means that no further additions or additives, such as alkalis, acids, or salts, particularly 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 them. Examples of such substances include quicklime and / or slaked lime, gypsum hemihydrate, water glass, cement, magnesium sulfate, and magnesium chloride. If such substances are included, their content in the binder should not exceed 8%. Higher concentrations of, for example, quicklime, slaked lime, or cement would worsen the CO2 balance of the binder due to the CO2 emissions associated with their production.Higher concentrations of substances such as water glass result in highly alkaline mixing water, making handling more difficult. The addition of magnesium sulfate and magnesium chloride is detrimental to the durability of the manufactured components.

[0088] Furthermore, it is advantageous if the binder component has a fineness corresponding to a Blaine surface area of ​​5000 cm². 2 / g, advantageous 10000 cm 2 / g, even more advantageous 15000 cm 2 / g or finer. The finer the binder component, the faster the hydration and hardening process.

[0089] It is advantageous if, upon contact with CO2 in the hardened cement paste produced using the binder component, the CO2 in the cement paste can be bound in the resulting magnesium carbonate hydrate and / or magnesium carbonate. This is based on the chemical processes explained in more detail above. The invention is explained in more detail below with reference to exemplary embodiments. The investigations described in more detail below were carried out, among other things, to verify the invention.

[0090] Example 1

[0091] Serpentinite from a quarry with a magnesium silicate hydrate content of 68 wt.% and an iron content of 7.1 wt.% was mined to a fineness of 10700 cm. 2 / g Blaine ground and homogenized. The content of respirable asbestos fibers according to the WHO definition was 2.1% by mass after this grinding.

[0092] The starting material has a molar Mg / Ca ratio of 122:1 and a molar Si / Al ratio of 19:1. The starting material was thermally treated in a rotary kiln at 780°C for 3 minutes. After thermal treatment, the content of respirable asbestos fibers was 1.044% by mass, and the fiber fineness was 8100 cm⁻¹. 2 / g Blaine. Subsequently, the thermally treated starting material was milled in a ball mill for two hours, achieving a 32% increase in fineness compared to the thermally treated starting product. The Blaine fineness of the final product was 10,700 cm. 2 / g. The content of respirable asbestos fibers determined by scanning electron microscopy using the method according to BIA (IFA) 7467 was below the detection limit of this method < 0.008 wt.%.

[0093] The material was used as a binder for mortar production according to DIN EN 196, whereby the water-to-binder ratio (w / b) was reduced to 0.33 and 1% by mass of PCE superplasticizer (based on the binder) was added. Before adding sand according to the standard, the binder, water, and PCE superplasticizer were mixed first slowly for 30 seconds and then rapidly for 180 seconds. The subsequent mixing process was carried out as described in the standard. The compressive strength of the standard prisms was 13 MPa after 2 days and 22 MPa after 7 days, and remained constant thereafter.

[0094] Example 2

[0095] In another experiment, serpentinite from a quarry with a magnesium silicate hydrate content of 68 wt.% and an iron content of 7.1 wt.% was ground to a fineness of 15300 cm. 2 / g Blaine ground and homogenized. The content of respirable asbestos fibers according to the WHO definition was 2.1 wt% after this grinding. The starting material has a molar Mg / Ca ratio of 122:1 and a molar Si / Al ratio of 19:1. The starting material was thermally treated in a rotary kiln at 745°C. The content of respirable asbestos fibers after thermal treatment was 0.64 wt%, and the fineness was 16400 cm⁻¹. 2 / g Blaine. Subsequently, the thermally treated starting material was ground in a ball mill for 120 minutes, achieving a 10% increase in fineness compared to the thermally treated starting product. The Blaine fineness of the final product was 18000 cm⁻¹. 2 / g. The content of respirable asbestos fibers determined by scanning electron microscopy using the method according to BIA (IFA) 7467 was below the detection limit of this method < 0.008 wt.%.

[0096] The material was used as a binder for mortar production according to DIN EN 196, whereby the water-to-blank ratio was reduced to 0.35 and 2.7% by mass of PCE superplasticizer (based on the binder) was added. Before the standard mixing program was completed, the binder, water, and PCE superplasticizer were mixed in the mixer, first slowly for 30 seconds and then quickly for 180 seconds. The compressive strength of the standard prisms was 50.0 MPa after 2 days, 54.9 MPa after 7 days, and 57.7 MPa after 28 days.

[0097] Example 3

[0098] The material from Example 2 was mixed as a binder component with limestone flour, a major component known from the production of Portland cement. Mixtures with the following ratios of binder component to limestone flour were prepared: 100 / 0, 90 / 10, 80 / 20, and 70 / 30. The development of the compressive strength of these mixtures was measured on mortar prisms according to DIN EN 196 with the adjustments described in Example 2. The following table shows the development of the compressive strength up to 28 days.

[0099]

[0100]

[0101] Table 1: Development of compressive strength for mixtures of binder component and limestone flour.

[0102] Example 4

[0103] The material from Example 1 was milled in a ball mill for different milling times to determine the influence of milling after thermal treatment on the content of respirable chrysotile fibers. The results are shown in Table 2. As the milling time increases, the content of respirable chrysotile fibers decreases due to the energy input and the comminution of the particles. After 120 minutes of milling, the content is below the detection limit of the method according to BIA (IFA) 7467 of < 0.008 wt%, and the thermally treated starting material is therefore asbestos-free.

[0104] <

[0105]

[0106] Table 2: Influence of the grinding time in a ball mill on the content of respirable asbestos fibers in the thermally treated starting product serpentinite.

Claims

II* wunderlich& heim the patent attorney - 23 - 0 0530 PATENT CLAIMS 1. A method for producing a cement block from asbestos-containing serpentinite, wherein the serpentinite has an asbestos content of at least 0.008 wt%, the method comprising the steps: a) Providing a starting material containing at least 40 wt% magnesium silicate hydrate (Mg3Si2Os(OH)4, Mg3Si4O (OH)2) and having an iron content of at least 1 wt%, the starting product has a fineness of 40000 cm 2 / g Blaine or coarser, provided or crushed, b) Homogenizing the starting product, c) Thermal treatment of the starting product in a thermal treatment unit at a treatment temperature between 600°C and 1,000°C wherein, after step c), magnesium silicate hydrate present in the thermally treated starting material is at least partially dehydrated and can thereby be converted into dehydrated magnesium silicate hydrate (xMgO SiO2 yH2O), d) re-commination of the thermally treated starting material in a comminution plant, whereby an increase in fineness of at least 5% with respect to the fineness of the thermally treated starting material is achieved, e) Contacting and mixing the thermally treated crushed starting material with water in a ratio of water to thermally treated crushed starting material of 1 : 2 or less, and subsequent hardening of the mixture to produce a cement stone up to a compressive strength of at least 10 MPa, whereby hardening to 10 MPa is possible without further additives.

2. Method according to claim 1 , characterized by that a vertical roller mill, a ball mill, a jet mill and / or a roller bowl mill is used as a crushing plant for further crushing.

3. Method according to claim 1 or 2, characterized by that by re-combining the thermally treated starting material, an absolute increase in fineness of 1000 cm is achieved in relation to the fineness of the thermally treated starting material. 2 The surface area achieved per gram of a gram of water is determined as the specific surface area using the Blaine method.

4. Method according to claims 1 to 3, characterized by that at least steps c) and d) are carried out in facilities that have negative pressure on the ambient air.

5. Method according to any one of claims 1 to 4, characterized by that the thermal treatment of the starting product is carried out at a temperature of at least 700°C and / or a maximum of 850°C and that the starting product is thermally treated for at least 5 min, preferably 15 min, even more preferably at least 30 min.

6. Method according to any one of claims 1 to 5, characterized by that the thermal treatment unit has a substantially homogeneous temperature distribution.

7. Method according to any one of claims 1 to 6, characterized in that a rotary kiln, in particular an indirectly heated rotary kiln without open flames in the reaction chamber, is used as the unit for thermal treatment.

8. Method according to any one of claims 1 to 7, characterized by that the thermally treated crushed starting material, which has been contacted with water and mixed, is filled into a mold or formwork to allow it to harden there. Method according to any one of claims 1 to 8, characterized by that sand and / or an aggregate is added to the dewatered starting product before, during or after contact with water. 10 Methods according to any one of claims 1 to 9, characterized by that the dewatered starting product, which has been contacted with water and mixed, is allowed to harden for at least 8 hours before the cement stone is exposed to external forces.

11. Method according to any one of claims 1 to 10, characterized by that organic additives, such as flow agents, are added to the dehydrated starting product before or during contact and mixing with water. 12 Methods according to one of claims 1 to 11 , characterized by that the starting material 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 Method according to one of claims 1 to 12, characterized by that the hardened cement stone is contacted with CO2 after step d), whereby the CO2 is bound in the cement stone in the magnesium carbonate hydrate and / or magnesium carbonate formed there.

14. Binder component for the production of a cement stone, the binder component contains processed serpentinite that formerly contained asbestos fibers, wherein the serpentinite had an asbestos content of at least 0.008 wt%, wherein the binder component is miscible with conventional cements and / or their main components or can also be used separately, • wherein the binder component comprises at least 40 wt.% thermally treated dehydrated magnesium silicate hydrate (Mg3Si2Os(OH)4, Mg3Si4O(OH)2) and at least 1 wt.% iron content, wherein the remaining components do not make a significant contribution to strength, • wherein the thermally treated dehydrated magnesium silicate hydrate is amorphous, o has a Mg to Si ratio of 2 or less, and o wherein the water content of bound water in the thermally treated dehydrated magnesium silicate hydrate is preferably below 10 wt.%, • wherein the binder component contains detectable fragments of tempered, broken and dehydrated chrysotile fibers that are shorter than 5 pm and / or have a diameter greater than 3 pm and / or have a length-to-diameter ratio of less than 3:1, • whereby, by means of the binder component alone, MSH phases can be formed upon contact and mixing with water in a water-to-binder ratio of 1:2 or less, and subsequent hardening at temperatures below 30°C, thereby enabling the production of a solid cement stone, • wherein the cement stone produced using only the binder component has a compressive strength of at least 10 MPa at an age of 28 days, tested according to DIN EN 196-1:2016 and the test is carried out with reduced water / B mean value of 0.35 with the addition of superplasticizer, whereby the hardening to 10 MPa takes place without further additives.

15. Binder component according to claim 14, characterized in that, in the hardened cement stone produced by means of the binder component, upon 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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