METHOD FOR PRODUCEING A BUILDING MATERIAL

Description

[0001] The invention relates to a method for producing a building material which can be used, among other things, as a concrete substitute.

[0002] Concrete is produced by mixing sand, gravel, water, and cement, sometimes with the addition of additives and admixtures. Cement consists of one or more main components. The most important of these is Portland cement clinker. This material is produced by burning limestone, clay, marl, and other materials in a high-temperature process. Portland cement clinker has an average chemical composition of 55–65% CaO, 18–22% SiO₂, 1–5% Al₂O₃, 1–5% Fe₂O₃, and other oxides in small concentrations.

[0003] The production of Portland cement clinker leads to carbon dioxide (CO2) emissions, which are due, among other things, to the deacidification of the limestone or marl. Approximately 0.8 tons of carbon dioxide are released during the production of one ton of Portland cement clinker, considering the entire technological process.

[0004] Currently, alternatives for cement production are being sought that do not rely on the deacidification of limestone or marl and thus release less CO2. One possible alternative would be the excitation of ultramafic rocks. These are rocks with a high concentration of magnesium oxide, iron oxide, and silicon dioxide. However, current statements from the cement industry point out that there are currently no viable research approaches in this area and that the excitation of ultramafic rocks is not considered a realistic option for a sustainable reduction in the cement industry's carbon dioxide emissions.

[0005] The chemical reaction of ultramafic rocks with water is very slow, as studies on the chemical weathering of these rocks have shown. For this reason, no possibility has yet been found to produce binders for concrete directly from these rocks.

[0006] Olivine is an important mineral in ultramafic rocks. It is a solid solution series consisting of fayalite (Fe₂SiO₄), forsterite (Mg₂SiO₄), tephroite (Mn₂SiO₄), 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. The analogy between forsterite and belite (Ca₂SiO₄), a mineral of Portland cement clinker, has led to the suggestion that binders could be produced from forsterite. This is also based on observations of the hydration of magnesium oxychlorides, known as Sorel cements. In some cases, the chlorides in Sorel cements are replaced by other ions such as sulfate or carbonate ions. The presence of free MgO in the binder is always a prerequisite for the hardening of these cements.However, MgO is rarely found in nature and must be produced from forsterite (Mg 2 SiO 4 ) or other magnesium-containing raw materials.

[0007] In this regard, processes are known in which forsterite is separated into MgO and SiO2.

[0008] One such synthesis method for producing MgO from forsterite and other magnesium silicates is described in WO 2012 / 168176 A1. In this process, the oxides of forsterite are separated into MgO and SiO₂ in a high-temperature process at temperatures of 800°C to 1000°C. Alkalis are added to the reaction to enable the separation of the SiO₂ in the form of sodium or potassium silicate. The MgO is precipitated as MgCO₃. After the separation of the two reaction products, the MgCO₃ can be thermally treated again in a second step, yielding MgO as a starting material for binder production. The carbon dioxide released in the second process step can be recycled back into the first process step along with the alkalis from the alkali silicate.

[0009] The magnesium oxide obtained in this way can be used to produce binders. For this purpose, the MgO is mixed with magnesium carbonate, for example, and this mixture can harden at room temperature after being mixed with water.

[0010] An alternative method for producing magnesium oxide-based cements, described in WO 2009 / 156740 A1 and WO 2012 / 028418 A1, involves the production of a mixture of MgO, MgCO₃, and optionally Mg(OH)₂, as well as silicates. This mixture does not contain reactive Mg₂SiO₄; instead, the forsterite is first processed to MgCO₃ and SiO₂ in an additional process. A reactor is described in which forsterite can react with CO₂ at relatively high pressures in the presence of water and dissolved salts to form SiO₂ and magnesium carbonate. The magnesium carbonate can be processed to MgO by calcination, and the CO₂ released in this process can be used to decompose new forsterite.

[0011] Using the two methods mentioned, cements with a high magnesium content can be produced. However, each requires an intermediate technological step involving the conversion of forsterite to MgO or MgCO3 with the release of SiO2, making the overall processes very complex and time-consuming.

[0012] US Patent 4,274,881 relates to a high-temperature cement. Example 7 describes a cement system consisting of Class G cement, finely divided quartz, calcined chrysotile, and deionized water, which recrystallizes under hydrothermal conditions (180°C - 400°C, 68.9 MPa) to diopside and, in some cases, wollastonite, xonotlite, and serpentinite.

[0013] The invention therefore lies in the Task The aim is to specify a method for producing a building material that is more efficient and environmentally friendly than conventional methods.

[0014] This problem is solved according to the invention by a method having the features of claim 1.

[0015] Advantageous embodiments of the invention are specified in the dependent claims and in the description.

[0016] According to the invention, a method for producing a building material with the features of claim 1 is provided.

[0017] An autoclave is generally understood to be a gas-tight, sealable pressure vessel used for the thermal treatment of materials under pressure. However, for the purposes of this invention, the term can also refer more generally to a vessel or device for enclosing a volume that prevents the comminuted and homogenized starting material from drying out during heating. This can be achieved either by sealing the volume or, for example, by ensuring that sufficient moisture is present or supplied within the device. For instance, it is also within the scope of this invention to heat the comminuted and homogenized starting material in a special chamber where the humidity is maintained at a consistently high level. This is particularly feasible at temperatures below 100°C, and especially below 60°C.

[0018] The invention is based on the surprising finding that forsterite reacts with SiO₂ and water to form magnesium silicate hydrate (MSH), and that these reactions can be accelerated by elevated temperatures. Forsterite is a mineral with the chemical composition Mg₂SiO₄. The reaction underlying the invention can be described as follows: Mg₂SiO₄ + SiO₂ + 2 H₂O → 2 MgO-SiO₂-H₂O

[0019] Consequently, forsterite reacts with SiO2 and water to form magnesium silicate hydrate.

[0020] Therefore, according to the invention, it is proposed to provide a forsterite source together with a SiO2 source as a starting material, to crush and homogenize it.

[0021] According to the invention, the required water can be supplied in various ways, which can also be combined. Firstly, it is possible to add water to the starting material before, after, or simultaneously with grinding and homogenization. Secondly, it is also possible to introduce steam into the autoclave in which the starting material is treated, thus adding the necessary amount of water to the process.

[0022] Of course, both methods can also be combined.

[0023] By treating the starting material in an autoclave at temperatures above 30°C, the reaction described above is accelerated, so that the inventive method can produce a solidified building material, in particular a prefabricated component, which can be used similarly to known concrete blocks or precast concrete elements. It is also possible, for example, to produce tiles that can be used for cladding buildings or the like. The essential point here is that a solid building material can be produced that can be shaped as desired and therefore can be used for a variety of purposes.

[0024] The inventive method thus makes it possible to produce a building material which has a significantly better CO2 balance than conventional cement, since no limestone needs to be deacidified.

[0025] Furthermore, the process or procedure described here is significantly more energy-efficient, as no high-temperature processes are required, as is the case with the production of Portland cement clinker.

[0026] According to the invention, a natural olivine source is used as the forsterite source.

[0027] It is preferred that amorphous and / or crystalline SiO₂ in pure and / or impure form is used as the SiO₂ source. Accordingly, quartz, tridymite, and / or cristobalite, for example, can be added as the SiO₂ source. Impure silicon carriers include, among others: thermally treated clays, pozzolans, cement, blast furnace slag, steel slag, carbonated cement paste from concrete recycling, feldspars, glass, especially recycled glass, and other silicate materials with an SiO₂ concentration above 10%, preferably 30%, advantageously above 50%. Not according to the invention, the comminuted, homogenized starting material is treated at above 60°C, particularly above 80°C, whereby a corresponding pressure is established in the autoclave, which is below atmospheric pressure. It is advantageous to set the water vapor partial pressure in the autoclave to above 3 bar; this can be achieved, for example, with higher temperatures.This prevents the resulting building material from drying out during heat treatment.

[0028] According to the invention, the crushed and homogenized starting material is treated in the autoclave at a temperature above 150°C and at a water vapor partial pressure above 5 bar for at least 12 hours. A starting material treated in this way results in a building material with high strength.

[0029] These parameters also represent a good compromise between the energy and time required. It is, of course, equally possible to carry out the process at a lower temperature for a longer period, and vice versa.

[0030] Preferably, a CO₂ source can be present in the autoclave. The presence of CO₂ can accelerate the hardening process of the building material. Furthermore, it allows carbon dioxide to be bound, thus enabling a further reduction in its emissions. The underlying reaction can be described as follows: Mg₂SiO₄ + 2 CO₂ → 2 MgCO₃ + SiO₂

[0031] The uptake of carbon dioxide can be accelerated by setting higher CO2 partial pressures and medium humidity.

[0032] According to the invention, the forsterite source is calcined after the optional addition of corrective agents and / or the addition of an oxidizing agent to the starting material. Not according to the invention, the comminuted, homogenized starting material can be placed in the autoclave in a pressure-resistant, dimensionally stable container.

[0033] When natural olivine is used, pore formation and volume expansion occur during autoclave treatment. It is assumed that, due to the Fe₂SiO₄ present in natural olivine, the following reaction also occurs during autoclave treatment, parallel to the previously described reaction: Fe₂SiO₄ + 3 H₂O + Mg₂SiO₄ → Fe₂O₃ + H₂ + 2 MgO-SiO₂ → H₂O

[0034] In this process, the hydrogen produced after the reaction is in a gaseous state. The formation of this gas creates porosity, or volume increase and pore formation, in the building material being produced. While the presence of gas is also known from the production of aerated concrete, there the gas is generated before the hardening process, unlike in this case, where gas release occurs continuously throughout the entire process, thus reducing the strength.

[0035] To prevent this, the forsterite source can be pretreated, for example by calcination. However, during the calcination process, a partial conversion of the forsterite to enstatite (MgSiO₃) also occurs. Unlike forsterite, however, enstatite is unable to react with SiO₂ and water to form magnesium silicate hydrate. Therefore, corrective agents, such as CaO sources, can be added optionally.

[0036] Alternatively, an oxidizing agent such as MnO₂ can be used during autoclave treatment, which can be added, for example, during the preparation of the starting material. This measure also prevents the formation of hydrogen gas.

[0037] Another option is to place the starting material in a pressure-resistant, dimensionally stable container before entering the autoclave. This prevents the expansion of hydrogen gas by the pressure-tight container, thus also reducing outgassing. It is also possible to combine two or all three of these methods.

[0038] The preferred ratio of forsterite to SiO₂ by mass is between 30% to 70% and 85% to 15%. Additional, sometimes inert, additives may also be included, such as extenders like limestone flour, corundum, sand, plastics, by-products like ash and slag, or metal reinforcement.

[0039] It is also possible to add substances that influence and control the reaction. Examples include calcium hydroxide, nucleating agents, and organic additives. Often, low concentrations are sufficient. Nucleating agents accelerate the reaction progress, and organic additives such as flow agents can reduce the water requirement and thus improve the porosity and strength of the hardened material.

[0040] Preferably, the molar ratio between forsterite and SiO₂ is 1:1. At this ratio, a particularly good reaction is achieved in which both starting materials are essentially completely consumed.

[0041] It is further preferred if sand is added to the starting material as an aggregate, and the crushed, homogenized starting material is placed in a mold with the sand and treated in this mold in an autoclave. The resulting product can be used as a solidified building material, in particular as a prefabricated component. In other words, the starting material is used analogously to cement, whereby the addition of sand allows the production of a concrete-like product that has similar properties and can also be used in a similar way.

[0042] The following describes some comparative experiments which are not in accordance with the invention. These show: Fig. 1 shows a comparative representation of the phase composition of different experimental products.

[0043] Initially, comparative tests were carried out in which forsterite and SiO2 were stored once at 25°C for 28 days (product 2), forsterite without the addition of SiO2 was treated for 28 days at 60°C (product 3), and forsterite with SiO2 was treated for 28 days at a temperature of 60°C (product 1).

[0044] The result is in Fig. 1 shown, with the corresponding curves marked.

[0045] Forsterite was prepared in detail by milling 40.8 g of magnesium hydroxide carbonate (KMF 12-091.1000) and 12.9 g of SiO₂ (Merck 1.13126.0500) in ethanol for 2.5 hours in a planetary ball mill, followed by drying and firing at 1100°C for 2.5 hours. The resulting product was analyzed by QXRD and contained 98% Mg₂SiO₄ and 2% MgO.

[0046] This material (5.0 g) was milled with 2.13 g of amorphous SiO₂ (Merck 1.13126.0500) for one minute in a disc mill. 2.0 g of this mixture were prepared with 2.0 g of deionized water and stored in a sealed container at 60°C for 28 days. The sample (product 1) was then dried at 60°C and analyzed. The phase composition determined by thermal analysis and 29Si MAS NMR was: 37% forsterite, 17% amorphous SiO₂, and 46% magnesium silicate hydrate. This clearly demonstrates that a chemical reaction occurred between forsterite (69% in the initial mixture) and SiO₂ (30% in the initial mixture), forming magnesium silicate hydrate (0% in the initial mixture).

[0047] In contrast, no reaction occurred when the forsterite-SiO2 mixture was stored at room temperature (product 2) or when a reference sample of pure forsterite without the addition of SiO2 was stored at 60°C for the same period (product 3).

[0048] Obviously, a significant reaction of the forsterite after 4 weeks is only possible if reactive SiO2 is added and the temperature is higher than room temperature.

[0049] For comparison, a further experiment demonstrated that an increased temperature further accelerates the reaction rate. In this experiment, the forsterite produced as described above was again ground with 2.13 g of amorphous SiO₂ (Merck 1.13126.0500) in a disc mill for one minute. 2.0 g of this mixture were then mixed with 2.0 g of deionized water and stored in a sealed container at 90°C for 28 days.

[0050] The sample was then dried at 60°C and the phase composition was analyzed by thermal analysis and 29<Si MAS NMR spectroscopy. This revealed that the sample contained 20% forsterite, 8% SiO₂, and 71% magnesium silicate hydrate. The degree of conversion was therefore higher than that of product 1.

[0051] In comparison to product 1, it is clearly visible in this sample that significantly more magnesium silicate hydrate was formed, indicating that the reaction proceeded more quickly. Consequently, the reaction between forsterite and SiO₂ can be accelerated by higher temperatures.

[0052] Furthermore, a strength test of the building material according to the invention was carried out. The starting material for this test was produced by homogenizing olivine (50 wt%), Elkem microsilica 940 (10 wt%), and ground recycled glass (40 wt%). The olivine used originated from a natural deposit in Aheim, Norway, and had the following chemical composition: 49.9 wt% MgO, 41.9 wt% SiO₂, 0.6 wt% Al₂O₃, 6.9 wt% Fe₂O₃, and 0.1 wt% CaO.

[0053] The raw material was homogenized by grinding it together in a disc mill for 30 seconds at 700 rpm. Concrete was then produced from the raw material with the following composition: 110 g raw material, 110 g standard sand 0 / 2, 110 g gravel 2 / 8, 27.5 g water, and 0.7 g superplasticizer. After mixing, the concrete was poured into a cube-shaped steel mold with an edge length of 5 cm and compacted. The mold was sealed and treated underwater in an autoclave for 42 hours at a steam partial pressure of approximately 15 bar and a temperature of approximately 200°C.

[0054] After autoclave treatment, the hardened cube was cooled, demolded, and its strength determined. A compressive strength of 25.5 MPa was found, which is comparable to concrete made from Portland cement.

[0055] The inventive method thus makes it possible to produce a building material that can be used flexibly in a more environmentally friendly way.

Claims

1. Method for producing a building material, wherein a starting material is provided which has a forsterite source in the form of a natural olivine source and a SiO2 source, wherein the starting material is crushed and homogenized, wherein the crushed, homogenized starting material is treated in an autoclave at a temperature of above 150°C, at above 5 bar and at least above 12 hours and wherein water is added to the starting material before, after or at the same time with the crushing and homogenization and is mixed with the starting material and / or water steam is introduced into the autoclave, wherein the forsterite source is calcined after optional addition of corrective agents to prevent enstatite formation and / or an oxidizing agent is added to the starting product.

2. Method according to claim 1, characterized in that pure, amorphous and / or contaminated SiO2 is used as the SiO2 source.

3. Method according to any one of claims 1 to 2, characterized in that a CO2 source is present in the autoclave.

4. Method according to any one of claims 1 to 3, characterized in that the ratio in mass% of forsterite to SiO2 is 30% to 70% up to 85% to 15%, wherein additionally additives can be added.

5. Method according to any one of claims 1 to 4, characterized in that the ratio at the molar level between forsterite and SiO2 is in the proportion of 1:1.

6. Method according to any one of claims 1 to 5, characterized in that sand is added to the starting material, in that the crushed, homogenized starting material to which sand has been added is placed in a mold and is treated in this mold in an autoclave.

7. Use of the product resulting from the method according to claim 6 as a solidified building material, in particular as a prefabricated component.