Building material mixtures containing one or more alkaline-earth metal compounds and method for the production of molds and cores

Abstract

The invention relates to building material mixtures comprising a primary building material, water glass, amorphous silicon dioxide and one or more alkaline-earth metal compounds, and to the production of molds and cores, in particular for metal casting.

Description

[0001] Building material mixtures containing one or more alkaline earth metal compounds and processes for the production of molds and cores

[0002] The invention relates to building material mixtures, inter alia, for the foundry industry, comprising one or more alkaline earth metal compounds in combination with refractory building materials, amorphous silicon dioxide, and water glass, in particular for the production of aluminum castings. Furthermore, the invention relates to a building material mixture comprising the above components, excluding the water glass-based binder, as well as a kit comprising, separately from one another, a liquid component with the water glass-based binder and a solid component comprising at least the amorphous silicon dioxide, wherein the alkaline earth metal compound is part of the liquid or solid component.

[0003] State of the art

[0004] Casting molds essentially consist of cores and molds, which represent the negative molds of the casting to be produced. These cores and molds consist of a refractory material, for example quartz sand, and a suitable binder, which gives the casting mold sufficient mechanical strength after removal from the mold. For the sake of simplicity, cores and molds are referred to individually and collectively as casting mold or casting molds. The building material mixture, which consists of the building material and binder, is preferably in a flowable form so that it can be poured into a suitable hollow mold and compacted there. The binder creates a strong bond between the particles of the building material, giving the casting mold the necessary mechanical stability. Insofar as the building material mixture, building material mixture or the building material is poured into a mold ora mold is inserted to produce the casting mold, these are also called molding material mixture, mold base material mixture or mold base material. In this case, the terms building material mixture, building base material mixture and building base material were chosen because the structure or casting mold for metal casting can also be produced directly using 3D printing. In this respect, the term building material mixture includes the term molding material mixture, the term building base mixture includes the term mold base material mixture and the term building base material includes the term mold base material. Casting molds must meet various requirements. During the casting process itself, they must first have sufficient strength and temperature resistance to be able to hold the liquid metal in the cavity formed from one or more casting (part) molds.Once the solidification process begins, the mechanical stability of the casting is ensured by a solidified metal layer that forms along the walls of the mold. The mold material must then decompose under the influence of the heat released by the metal, causing it to lose its mechanical strength, thus breaking down the cohesion between individual particles of the refractory material. Ideally, the mold will disintegrate into a fine sand that can be easily removed from the casting.

[0005] The term "rapid prototyping" refers to various methods for producing three-dimensional objects (structures) by layering them together. One advantage of these processes is the ability to produce even complex, one-piece objects with undercuts and cavities. With conventional methods, these objects would have to be assembled from several individually manufactured parts. Another advantage is that 3D printers are capable of producing the objects directly from CAD data without the need for molds.

[0006] 3-dimensional (3D) printing processes result in new requirements for binders that hold the casting mold together when the binder or a binder component is to be applied through the nozzles of a print head. The binders must not only provide a sufficient level of strength and good disintegration properties after metal casting, as well as exhibit sufficient thermal and storage stability, but must also be "printable," meaning that the nozzles of the print head must not become clogged with the binder, and the binder should not be able to flow directly out of the print head, but rather form individual droplets. Furthermore, both process variants require that emissions, e.g.in the form of CO2 or hydrocarbons to protect the environment and limit the odor nuisance caused by hydrocarbons, primarily aromatic hydrocarbons. To meet these requirements, inorganic binders have been developed. Their use can prevent or at least significantly minimize emissions of CO2 and hydrocarbons during the production of metal molds. However, the use of inorganic binders is often associated with other disadvantages.

[0007] Compared to organic binders, inorganic binders have the disadvantage, for example, that the molds produced from them exhibit relatively low strength. This is particularly evident immediately after the mold is removed from the tool. However, good strength at this point is particularly important for the production of complex and / or thin-walled molded parts and their safe handling. Resistance to humidity is also significantly reduced compared to organic binders.

[0008] EP 1802409 B1 discloses that higher instant strengths and greater resistance to atmospheric humidity can be achieved by using a refractory base material, a water glass-based binder, and the addition of particulate amorphous silicon dioxide. This additive ensures safe handling even of complex casting molds because the addition of particulate amorphous silicon dioxide achieves high hot strengths.

[0009] DE102013111626 A1 discloses the use of oxide boron compounds to improve the strength of the molded core, both after production and during extended storage, as well as the core disintegration properties after casting. Problems of the prior art and task

[0010] The inorganic binders known to date for foundry purposes still have room for improvement. Above all, it is desirable to improve the strength of the known inorganic binders:

[0011] (a) in particular with regard to hot strength - also known as immediate strength - and strength after storage in contact with atmospheric moisture and

[0012] (b) leads to improved thermal stability, so that the produced cores remain dimensionally stable even during casting and high thermal stress. Increased thermal stability is essential, especially for delicate cores.

[0013] The invention was therefore based on the object of providing a building material mixture, in particular for the production of building bodies or casting molds for metal processing, which achieves a level of strength which is necessary in the automated production process, comprising the production of the casting molds, their handling and, if necessary, intermediate storage up to the removal of the used casting mold components after casting.

[0014] Furthermore, the production of structural elements or casting molds with complex geometries, which may, for example, include thin-walled sections, should be enabled. The casting mold should also exhibit high storage stability and remain stable even at higher temperatures and humidity.

[0015] Summary of the invention

[0016] The above objects are achieved by the building material mixture, the building material mixture, and the multi-component system in the form of a kit, or the method, with the features of the independent patent claims. Advantageous further developments are the subject of the dependent patent claims and are described below. Surprisingly, it has been discovered that by adding certain alkaline earth metal compounds to the building material mixture, in particular to the binder or the building material mixture, casting molds based on inorganic binders can be produced that exhibit high strength both immediately after production and during extended storage. Even very small addition amounts are sufficient.

[0017] The alkaline earth metal compound is selected from the group consisting of calcium chloride, calcium oxide, calcium hydroxide, calcium sulfate, magnesium sulfate, and mixtures thereof. The aforementioned alkaline earth metal compounds also include their hydrates. These compounds are referred to individually and collectively as alkaline earth metal compounds.

[0018] Furthermore, due to the alkaline earth metal compound in the casting molds, especially in the cores, an improvement in deformation stability (e.g. during metal casting) was observed, especially at a loading temperature of 650°C.

[0019] The mixtures according to the invention comprise as a building material mixture at least:

[0020] (A) a refractory building material; and

[0021] (C) a solid additive or a solid additive package comprising at least particulate amorphous silicon dioxide, and in the building material mixture produced therewith also the binder (B) comprising water glass, wherein the alkaline earth metal compound is part of the solid additive or the binder.

[0022] The building material mixture according to the invention (mixture as building material mixture) comprises:

[0023] - at least 90% by weight of a refractory building material, based on the building material mixture,

[0024] - 0.1 to 2 wt.%, preferably 0.2 wt.% to 1.0 wt.%, of an amorphous silicon dioxide, based on the refractory building material; and - 0.001 to 0.2 wt.%, preferably 0.005 to 0.1 wt.%, of an alkaline earth metal compound, based on the refractory building material, wherein the alkaline earth metal compound is selected from the group:

[0025] Calcium chloride, calcium oxide, calcium hydroxide, calcium sulfate, magnesium sulfate and mixtures thereof.

[0026] The building material mixture according to the invention (mixture as building material mixture) comprises:

[0027] - at least 80% by weight of a refractory building material, based on the building material mixture,

[0028] - 0.1 to 2% by weight, preferably 0.2% by weight to 1.0% by weight, of an amorphous silicon dioxide, based on the refractory building material;

[0029] - 0.001 to 0.2% by weight, preferably 0.005 to 0.1% by weight, of an alkaline earth metal compound, based on the refractory building material, wherein the alkaline earth metal compound is selected from the group:

[0030] Calcium chloride, calcium oxide, calcium hydroxide, calcium sulfate, magnesium sulfate and mixtures thereof; and

[0031] - a binder comprising water glass.

[0032] The mixtures can be prepared using a kit which separately includes at least the following components:

[0033] (B1) a binder, wherein the binder is an aqueous binder comprising at least water glass; and

[0034] (C1 ) a solid additive package, wherein the solid additive package (C1 ) comprises:

[0035] - an amorphous silicon dioxide and

[0036] - an alkaline earth metal compound, wherein the alkaline earth metal compound is selected from the group:

[0037] Calcium chloride, calcium oxide, calcium hydroxide, calcium sulfate, magnesium sulfate and mixtures thereof in a weight ratio of amorphous silicon dioxide to the alkaline earth metal compound of 1:2 to 2000:1, preferably 2:1 to 200:1, wherein the amorphous silicon dioxide and the alkaline earth metal compound are each present as particulate solids in free-flowing form in the solid additive package (C1) or

[0038] (B2) a binder, wherein the binder is an aqueous binder comprising at least - water glass and

[0039] - 0.05 to 10 wt.%, preferably 0.25 to 5 wt.%, of an alkaline earth metal compound, based on the binder, wherein the alkaline earth metal compound is selected from the group: calcium chloride, calcium oxide, calcium hydroxide, calcium sulfate, magnesium sulfate and mixtures thereof; and

[0040] (C2) a solid additive package comprising an amorphous silicon dioxide, wherein the amorphous silicon dioxide is present as a particulate solid in free-flowing form in the solid additive package C2.

[0041] For example, silicon dioxide and alkaline earth metal compound, in relation to the binder, can also be used as part of the kit, so that:

[0042] - 5 to 100% by weight, preferably 10 to 50% by weight, of an amorphous silicon dioxide, based on the binder; and

[0043] - 0.05 to 10 wt.%, preferably 0.25 to 5 wt.%, of the alkaline earth metal compound, based on the binder.

[0044] The binder preferably consists of water glass (as a solid) or consists of water glass (liquid) including diluent or solvent, in particular water.

[0045] In addition, the kit may include the refractory base material as a separate component. If a substance is included in one component of the kit, it is preferable that the substance is not additionally included in any other component of the same kit.

[0046] According to one embodiment of the invention, the building material mixture contains organic components in a proportion of up to a maximum of 0.49 wt.%, in particular up to a maximum of 0.2 wt.%, so that only very small amounts of emissions of CO2 and other pyrolysis products are generated

[0047] Detailed description of the invention

[0048] Common and well-known materials can be used as refractory base materials for the production of casting molds. Suitable examples include quartz, zircon, or chrome ore sand, olivine, vermiculite, bauxite, fireclay, as well as synthetic base materials and their mixtures, especially mixtures containing more than 50% quartz sand by weight based on the refractory base material. It is not necessary to use exclusively virgin sand. To conserve resources and avoid landfill costs, it is even advantageous to use as much reclaimed sand as possible, such as that obtained from used casting molds through recycling.

[0049] A refractory building material refers to materials that have a high melting point. The melting point of the refractory building material is preferably greater than 600°C, preferably greater than 900°C, particularly preferably greater than 1200°C, and especially preferably greater than 1500°C, which means that the components of the refractory building material each have a melting point greater than 600°C.

[0050] The refractory building material makes up at least 80 wt.%, in particular at least 90 wt.%, particularly preferably at least 95 wt.%, of the building material mixture.

[0051] A suitable sand is described, for example, in WO 2008 / 101668 A1 (= US 2010 / 173767 A1). Reclaimed materials obtained by washing and subsequently drying crushed used casting molds are also suitable. As a rule, the reclaimed materials can make up at least approximately 70 wt.% of the refractory building material, preferably at least approximately 80 wt.%, and particularly preferably greater than 90 wt.%.

[0052] The average particle size of the refractory building materials is generally between 100 and 600 pm, preferably between 120 and 550 pm and particularly preferably between 150 and 500 pm. Particular preference is given to particle geometries with a ratio of greatest length to smallest length (at right angles to one another and in each case for all spatial directions) of 1:1 to 1:5 or 1:1 to 1:3, i.e. those which are not fibrous, for example. In particular, the particle size of the refractory building materials is less than or equal to 600 pm, preferably less than or equal to 500 pm, determined by sieving in accordance with DIN 66165 Part 2. The refractory building material used is preferably free-flowing, in particular in order to be able to process the building material mixture according to the invention in conventional core shooting machines.

[0053] The water glass contains dissolved alkali silicates and can be produced by dissolving lithium, sodium, and / or potassium silicates in water. The water glass preferably has a molar modulus SiO2 / M2O (cumulative with M = Li, Na, K) in the range from 1.6 to 4.0, in particular 2.0 to less than 3.5. Furthermore, the water glass can also comprise multivalent ions, such as the aluminum-modified water glass described in EP 2305603 A1 (= WO 2011 / 042132 A1). According to a particular embodiment, a proportion of lithium ions, in particular amorphous lithium silicates, lithium oxides, and lithium hydroxide, or a ratio [Li2O] / [M2O] or [Li2O active] / [M2O] as described in DE 102013106276 A1 is used.

[0054] The water glass in particular has a solids content in the range of 25 to 65 wt.%, preferably 30 to 55 wt.%, in particular 30 to 50 wt.%, particularly preferably 30 to 45 wt.%, wherein the solids content refers to the amount of SiO2 and M2O contained in the water glass. Depending on the application and the desired strength level, between 0.5 wt.% and 5 wt.% of the binder comprising water glass, preferably between 0.75 wt.% and 4 wt.%, particularly preferably between 1 wt.% and 3.5 wt.%, and especially preferably 1 to 3 wt.%, in each case based on the building material, are used in the building material mixture. This information can also refer to the total amount of water glass, including the (particularly aqueous) solvent or diluent and the (possible) solids content (together = 100 wt.%). For the purposes of calculating the preferred total amount of water glass, a solids content of 35 wt.% (see examples), regardless of the solids content actually used.

[0055] "Powdery" or "particulate" refers to solid powder (including dust) or granules that are pourable and thus also sievable. According to one embodiment, the building material mixture according to the invention contains one or more powdered alkaline earth metal compounds. "Powdery" or "particulate" refers to solid powder or granules that are pourable and thus also sievable.

[0056] According to another embodiment, the alkaline earth metal compound can also be dissolved in the binder. The binder is then an aqueous water glass solution.

[0057] The compounds of the invention include calcium oxide, calcium hydroxide, calcium sulfate, calcium chloride, and magnesium sulfate (both anhydrous and hydrated). Calcium oxide, calcium hydroxide, and calcium sulfate are preferred. Calcium sulfate in the anhydrous form (anhydrite) is particularly preferred.

[0058] The average particle size can be determined using sieve analysis. According to one embodiment, the sieve residue on a sieve with a mesh size of 1.00 mm is less than 5 wt.%, more preferably less than 2.0 wt.% and especially preferably less than 1.0 wt.%. Particularly preferably, regardless of the above information, the sieve residue on a sieve with a mesh size of 0.5 mm is preferably less than 30 wt.%, preferably less than 15 wt.%, more preferably less than 10 wt.% and especially preferably less than 5 wt.%. Particularly preferably, regardless of the above information, the sieve residue on a sieve with a mesh size of 0.25 mm is preferably less than 50 wt.%, preferably less than 25 wt.% and especially preferably less than 10 wt.%.The sieve residue is determined according to the machine sieving method described in DIN 66165 (Part 2), with a chain ring additionally being used as a sieving aid.

[0059] According to one embodiment, the average particle size of the alkaline earth metal compounds is preferably less than 1 mm, preferably less than 0.75 mm, particularly preferably less than 0.5 mm. The particle size of the alkaline earth metal compounds is preferably greater than 0.1 μm, preferably greater than 1 μm, and particularly preferably greater than 5 μm. Surprisingly, it has been found that only very small additions to the building material mixture significantly improve the storage stability of the casting mold. The proportion of the alkaline earth metal compound, based on the refractory building material, is 0.001 to 0.2 wt.%, preferably 0.005 to 0.1 wt.% based on the refractory building material. Or to put it another way, preferably less than 0.2 wt.%, preferably less than 0.1 wt.%, particularly preferably less than 0.075 wt.%, particularly preferably less than 0.05 wt.%, and especially particularly preferably less than 0.03 wt.%. The lower limit is preferably greater than 0.001 wt.%, preferably greater than 0.005 wt.% and particularly preferably greater than 0.01 wt.%.

[0060] In addition, it was shown that the addition of alkaline earth metal compounds has a positive effect on the thermal stability of the produced cores.

[0061] The building material mixture according to the invention contains a proportion of amorphous silicon dioxide to increase the strength level of the casting molds or structures produced with such building material mixtures. Increasing the strength of the casting molds, particularly increasing the hot strength, can be advantageous in automated manufacturing processes. Synthetically produced amorphous silicon dioxide is particularly preferred.

[0062] The particle size of the amorphous silicon dioxide is preferably less than 300 pm, more preferably less than 200 pm, and particularly preferably less than 100 pm. The sieve residue of the particulate amorphous SiO2 when passing through a sieve with a mesh size of 125 pm (120 mesh) is preferably no more than 10 wt.%, more preferably no more than 5 wt.%, and most preferably no more than 2 wt.%. Irrespective of this, the sieve residue on a sieve with a mesh size of 63 pm is less than 10 wt.%, preferably less than 8 wt.%. The sieve residue is determined according to the machine sieving method described in DIN 66165 (Part 2), with a chain ring additionally used as a sieving aid.

[0063] The amorphous silicon dioxide preferably used according to the present invention has a water content of less than 15 wt.%, in particular less than 5 wt.%, and particularly preferably less than 1 wt.%. The amorphous SiO2 is used as a solid and is thus in particulate and free-flowing form before being added to the building material mixture or base material mixture. The particulate amorphous SiO2 is used as a powder (including dusts).

[0064] Both synthetically produced and naturally occurring silicas can be used as amorphous SiO2. The latter are known, for example, from DE 102007045649, but are not preferred because they generally contain significant crystalline fractions and are therefore classified as carcinogenic. Synthetic is understood to mean non-naturally occurring amorphous SiO2, i.e. its production involves a deliberately performed chemical reaction such as that initiated by humans, e.g. the production of silica sols by ion exchange processes from alkali silicate solutions, precipitation from alkali silicate solutions, flame hydrolysis of silicon tetrachloride, or the reduction of quartz sand with coke in an arc furnace during the production of ferrosilicon and silicon. The amorphous SiO2 produced by the last two processes is also referred to as pyrogenic SiO2.

[0065] Occasionally, synthetic amorphous silicon dioxide refers only to precipitated silica (CAS No. 112926-00-8) and flame-hydrolytically produced SiO2 (pyrogenic silica, fumed silica, CAS No. 112945-52-5), while the product resulting from ferrosilicon or silicon production is referred to simply as amorphous silicon dioxide (silica fume, microsilica, CAS No. 69012-64-12). For the purposes of the present invention, the product resulting from ferrosilicon or silicon production is also understood as amorphous SiO2.

[0066] Precipitated silicas and pyrogenic silicon dioxide, i.e., silicon dioxide produced by flame hydrolysis or arc annealing, are preferred. Particularly preferred materials are amorphous silicon dioxide produced by thermal decomposition of ZrSiO4 (described in DE 102012020509 A1) and SiO2 produced by oxidation of metallic Si using an oxygen-containing gas (described in DE 102012020510 A1). Also preferred is quartz glass powder (mainly amorphous silicon dioxide), which is produced by melting and rapidly cooling crystalline quartz, so that the particles are spherical and not splintery (described in DE 102012020511 A1). The average primary particle size of the particulate amorphous silicon dioxide can be between 0.05 pm and 10 pm, in particular between 0.1 pm and 5 pm, particularly preferably between 0.1 pm and 2 pm. The primary particle size can be determined, for example, with the aid of dynamic light scattering (e.g.Horiba LA 950) and examined by scanning electron microscope images (SEM images using, for example, a Nova NanoSEM 230 from FEI). Furthermore, the SEM images made it possible to visualize details of the primary particle shape down to the order of 0.01 pm. For the SEM measurements, the silicon dioxide samples were dispersed in distilled water and then applied to an aluminum holder covered with copper tape before the water was evaporated.

[0067] Furthermore, the specific surface area of ​​the particulate amorphous silicon dioxide was determined using gas adsorption measurements (BET method) according to DIN 66131. ​​The specific surface area of ​​the particulate amorphous SiO2 is between 1 and 200 m 2 / g, especially between 1 and 50 m 2 / g, particularly preferably between 1 and 30 m 2 / g. If necessary, the products can also be mixed, e.g., to obtain mixtures with specific particle size distributions.

[0068] Depending on the manufacturing method and the producer, the purity of the amorphous SiO2 can vary considerably. Types with a silicon dioxide content of at least 85 wt.% have proven suitable, preferably at least 90 wt.%, and particularly preferably at least 95 wt.%. Depending on the application and the desired strength level, between 0.1 wt.% and 2 wt.% of particulate amorphous SiO2 are used, preferably between 0.1 wt.% and 1.8 wt.%, particularly preferably between 0.2 wt.% and 1.0 wt.%, in each case based on the building material.

[0069] The ratio of binder to amorphous silicon dioxide can be varied within wide limits. This offers the advantage of significantly improving the initial strength of the cores, i.e., the strength immediately after removal from the tool, without significantly affecting the final strength. This is of particular interest in light metal casting. On the one hand, high initial strengths are desirable to enable easy transport of the cores after production or to enable assembly into complete core packages. On the other hand, the final strengths should not be too high to avoid problems with core disintegration after casting, i.e., the basic material should be easy to remove from the cavities of the casting mold after casting.

[0070] Based on the total weight of the binder (water glass including diluents or solvents such as water), the amorphous silicon dioxide is preferably present in a proportion of 1 to 80 wt.%, preferably 2 to 60 wt.%, particularly preferably 3 to 55 wt.%, and especially preferably between 4 and 50 wt.%. Or, independently of this, based on the ratio of solids content of the water glass (based on the oxides, ie, total mass of alkali metal oxide and silicon dioxide) to amorphous silicon dioxide, from 10:1 to 1:1.2 (parts by weight) is preferred.

[0071] According to EP 1802409 B1, the amorphous silicon dioxide can be added directly to the refractory material both before and after the binder is added. However, as described in EP 1884300 A1 (= US 2008 / 029240 A1), a premix of the amorphous silicon dioxide with at least a portion of the binder or sodium hydroxide solution can also be prepared first, and this can then be mixed into the refractory material. Any binder or binder portion still present but not used for the premix can be added to the refractory material before or after the premix is ​​added, or together with it. The amorphous silicon dioxide is preferably added to the refractory material before the binder is added.

[0072] In a further embodiment, barium sulfate may be added to the building material mixture to further improve the surface of the casting, especially if it is made of aluminum.

[0073] The barium sulfate can be synthetically produced or natural barium sulfate, i.e., added in the form of minerals containing barium sulfate, such as barite or baryte. This, as well as other features of the suitable barium sulfate and the building material mixture produced with it, are described in more detail in DE 102012104934 A1, and the disclosure content thereof is therefore incorporated by reference into the disclosure of the present patent. The barium sulfate is preferably added in an amount of 0.02 to 5.0 wt.%, particularly preferably 0.05 to 3.0 wt.%, especially preferably 0.1 to 2.0 wt.%, or 0.3 to 0.99 wt.%, in each case based on the total building material mixture.

[0074] In a further embodiment, at least aluminum oxides and / or aluminum / silicon mixed oxides in particulate form or metal oxides of aluminum and zirconium in particulate form can be added to the building material mixture according to the invention in concentrations between 0.05 wt.% and 4.0 wt.%, preferably between 0.1 wt.% and 2.0 wt.%, particularly preferably between 0.1 wt.% and 1.5 wt.% and especially preferably between 0.2 wt.% and 1.2 wt.%, in each case based on the building base material, in particular via the additive component (A), as described in more detail in DE 102012113073 A1 or DE 102012113074 A1.

[0075] In this respect, these documents are also claimed by reference as disclosure for the present patent. Such additives can be used to obtain castings, particularly made of iron or steel, with very high surface quality after metal casting, so that only minimal or even no post-processing of the surface of the casting is required after removal of the mold.

[0076] In a further embodiment, the building material mixture according to the invention can comprise a phosphorus-containing compound. This additive is preferred for very thin-walled sections of a casting mold. These are preferably inorganic phosphorus compounds in which the phosphorus is preferably present in the +5 oxidation state.

[0077] The phosphorus-containing compound is preferably in the form of a phosphate or phosphorus oxide. The phosphate can be present as an alkali metal or alkaline earth metal phosphate, with alkali metal phosphates, and in particular the sodium salts, being particularly preferred. Both orthophosphates and polyphosphates, pyrophosphates, or metaphosphates can be used as phosphates. The phosphates can be produced, for example, by neutralizing the corresponding acids with a corresponding base, for example an alkali metal base such as NaOH, or optionally also an alkaline earth metal base, although not all of the negative charges of the phosphate necessarily have to be saturated by metal ions. Both metal phosphates and metal hydrogen phosphates and metal dihydrogen phosphates can be used, such as NasPO4, Na2HPO4, and NaH2PO4. Anhydrous phosphates and hydrates of the phosphates can also be used.The phosphates can be incorporated into the building material mixture in both crystalline and amorphous form.

[0078] Polyphosphates are understood to mean, in particular, linear phosphates that comprise more than one phosphorus atom, with the phosphorus atoms being linked to one another via oxygen bridges.

[0079] Polyphosphates are obtained by condensation of orthophosphate ions with elimination of water, resulting in a linear chain of PCM tetrahedra, each connected by vertices. Polyphosphates have the general formula (O(PO3)n) (n+2)_where n corresponds to the chain length. A polyphosphate can comprise up to several hundred PO4 tetrahedra. However, preference is given to using polyphosphates with shorter chain lengths. n preferably has values ​​from 2 to 100, particularly preferably from 5 to 50. Higher-condensed polyphosphates can also be used, i.e., polyphosphates in which the PO4 tetrahedra are connected to one another via more than two vertices and therefore exhibit polymerization in two or three dimensions.

[0080] Metaphosphates are cyclic structures composed of PÜ4 tetrahedra, each connected by vertices. Metaphosphates have the general formula ((PO3)n) n- where n is at least 3. Preferably, n has values ​​from 3 to 10.

[0081] Both individual phosphates and mixtures of different phosphates and / or phosphorus oxides can be used. The preferred proportion of the phosphorus-containing compound, based on the refractory building material, is between 0.05 and 1.0 wt.%. The proportion of the phosphorus-containing compound is preferably between 0.1 and 0.5 wt.%. The phosphorus-containing, inorganic compound preferably contains between 40 and 90 wt.%, particularly preferably between 50 and 80 wt.% phosphorus, calculated as P2O5. The phosphorus-containing compound can be added to the building material mixture in solid or dissolved form. The phosphorus-containing compound is preferably added to the building material mixture as a solid.

[0082] According to an advantageous embodiment, the building material mixture according to the invention contains a proportion of platelet-shaped lubricants, in particular graphite or M0S2. The amount of the added platelet-shaped lubricant, in particular graphite, is preferably 0.05 to 1 wt.%, particularly preferably 0.05 to 0.5 wt.%, based on the building material.

[0083] The graphite preferably has a particle size of less than or equal to 300 pm, preferably less than or equal to 200 pm, particularly preferably less than or equal to 100 pm, determined by sieving according to DIN 66165 Part 2.

[0084] According to a further advantageous embodiment, surface-active substances, in particular surfactants, which improve the flowability of the building material mixture can also be used. Suitable representatives of these compounds are described, for example, in WO 2009 / 056320 A1 (= US 2010 / 0326620 A1). Anionic surfactants are preferably used for the building material mixture according to the invention. Surfactants with sulfuric acid or sulfonic acid groups, such as C6- to C18-alkyl sulfates and / or C6- to C18-alkyl sulfonates, may be mentioned here. In the building material mixture according to the invention, the pure surface-active substance, in particular the surfactant, is preferably present in a proportion of 0.001 to 1 wt.%, particularly preferably 0.01 to 0.2 wt.%, based on the weight of the refractory building material.

[0085] The building material mixture according to the invention represents an intensive mixture of at least the aforementioned components. The particles of the refractory building material are preferably coated with a layer of the binder. By evaporating the water present in the binder (approximately 40-70% by weight, based on the weight of the binder), a strong bond between the particles of the refractory building material can be achieved.

[0086] Despite the high strengths achievable with the binder according to the invention, the casting molds produced with the building material mixture according to the invention surprisingly exhibit high storage stability. The use of the molded articles produced from the building material mixture according to the invention is not limited to light metal casting and / or non-ferrous metal casting. The casting molds are generally suitable for casting metals, such as non-ferrous metals or ferrous metals. However, the building material mixture according to the invention is particularly suitable for casting aluminum.

[0087] The invention further relates to a method for producing casting molds for metal processing, using the building material mixture according to the invention. The method according to the invention comprises the following steps:

[0088] Providing the building material mixture described above by bringing together and mixing at least the mandatory components mentioned above;

[0089] Forms of the building material mixture;

[0090] Curing of the shaped building material mixture, whereby the cured casting mold is obtained.

[0091] When producing the building material mixture according to the invention, the general procedure is to first add the refractory building material (component A) and then add the binder or component B or B1 or B2 and the additive or component C or C1 or C2 while stirring. The additives described above can be added to the building material mixture in any form. They can be added individually or as a mixture, in particular as a solid additive package C1 or C2. According to a preferred embodiment, the binder is provided as a two-component system, wherein a first liquid component contains the water glass and optionally a surfactant (see above) (components B orB1 or B2) and a second but solid component of one or more alkaline earth metal compounds and the particulate amorphous silicon dioxide (components (C1)) as well as all other above-mentioned solid additives, with the exception of the building materials, and in particular the platelet-shaped graphite and optionally a phosphate, optionally barium sulfate and / or optionally other components as described.

[0092] When preparing the building material mix, the refractory base material is placed in a mixer, and then the solid component(s) of the binder are preferably added first and mixed with the refractory base material. The mixing time is selected to ensure thorough mixing of the refractory base material and the solid binder component. The mixing time depends on the quantity of building material mix to be produced and the mixing equipment used. The mixing time is preferably between 1 and 5 minutes.

[0093] The liquid binder is then added, preferably while continuing to move the mixture, and the mixture is then further mixed until a uniform layer of the binder has formed on the grains of the refractory building material.

[0094] Here too, the mixing time depends on the amount of building material mixture to be produced and the mixing unit used. The mixing time is preferably selected between 1 and 5 minutes. A liquid component is understood to mean both a mixture of different liquid components and the totality of all individual liquid components, whereby the latter can also be added individually. Likewise, a solid component is understood to mean both the mixture of individual or all of the solid components described above and the totality of all individual solid components, whereby the latter can be added to the building material mixture together or one after the other. According to another embodiment, the liquid binder can first be added to the refractory building material and only then is the solid component added to the mixture. According to a further embodiment, 0.05 to 0.3 wt.% water, based on the weight of the building material, is added to the refractory building material and only then are the solid and liquid components of the binder added. With this embodiment, a surprisingly positive effect on the processing time of the building material mixture can be achieved. The inventors assume that the water-removing effect of the solid components of the binder is reduced in this way and the curing process is thereby delayed. The building material mixture is then formed into the desired shape. The usual shaping processes are used. For example, the building material mixture can be shot into the mold using a core shooter with the aid of compressed air. The building material mixture is then cured, whereby all processes can be used that are known for binders based on water glass, e.g. hot curing, gassing with CO2 or air.a combination of both, as well as curing with liquid or solid catalysts. Hot curing is preferred. During hot curing, water is removed from the building material mixture. This presumably also initiates condensation reactions between silanol groups, resulting in crosslinking of the water glass.

[0095] Heating can take place, for example, in a mold, which preferably has a temperature of 100 to 300 °C, particularly preferably a temperature of 120 to 250 °C. It is possible to fully cure the casting mold in the mold. However, it is also possible to cure the casting mold only in its edge region so that it has sufficient strength to be removed from the mold. The casting mold can then be fully cured by removing further water. This can be done, for example, in an oven. The water can also be removed, for example, by evaporating the water under reduced pressure.

[0096] The curing of the casting molds can be accelerated by blowing heated air into the mold. This embodiment of the process achieves rapid removal of the water contained in the binder, whereby the casting mold is solidified within a time period suitable for industrial application. The temperature of the blown-in air is preferably 100°C to 180°C, particularly preferably 120°C to 150°C. The flow rate of the heated air is preferably adjusted so that curing of the casting mold occurs within a time period suitable for industrial application. The time periods depend on the size of the casting molds produced. The aim is to cure within a time period of less than 5 minutes, preferably less than 2 minutes. However, longer times may be necessary for very large casting molds.

[0097] The removal of water from the building material mixture can also be achieved by heating the building material mixture with microwaves. For example, it would be conceivable to mix the building material with the solid, powdered component(s), apply this mixture layer by layer to a surface, and print the individual layers using a liquid binder component, in particular a water glass. The layer-by-layer application of the solid mixture is followed by a printing process using the liquid binder.

[0098] At the end of this process, i.e. after the last printing operation, the entire mixture can be heated in a microwave oven.

[0099] The methods according to the invention are suitable for the production of all conventional casting molds for metal casting, such as cores and molds. Particularly advantageously, casting molds comprising very thin-walled sections can also be produced.

[0100] According to another embodiment, the molds and cores or building bodies are produced by layer-by-layer construction of the bodies using 3D printing, comprising at least the following steps: a) spreading at least one layer of the building material mixture with a layer thickness of 0.05 mm to 3 mm, preferably 0.1 mm to 2 mm, and particularly preferably 0.1 mm to 1 mm of the building material mixture, b) printing selected areas of the at least one layer with the binder comprising water glass, and c) repeating at least steps a) and b several times; wherein the alkaline earth metal compound is part of the building material mixture and / or the binder. The binder is then a printing fluid with all its components as it is transported through the nozzle(s) of a print head. Preferably, all components of the binder are in dissolved or liquid form according to this embodiment.

[0101] Printing is carried out, for example, with a print head having a plurality of nozzles, wherein the nozzles can preferably be selectively controlled individually. According to a further embodiment, the print head is moved at least in one plane under computer control, and the nozzles apply the liquid binder layer by layer and selectively according to the available data. The print head can be, for example, a drop-on-demand print head with bubble jet or, preferably, piezo technology.

[0102] In another embodiment of 3D printing, an inorganic hardener for waterglass-based binders is optionally added to the building material mixture before the binder is added. Examples of such inorganic hardeners include phosphates such as Lithopix P26 (an aluminum phosphate from Zschimmer und Schwarz GmbH & Co. KG Chemische Fabriken) or Fabutit 748 (an aluminum phosphate from Chemische Fabrik Budenheim KG). Other inorganic hardeners for waterglass-based binders include calcium silicates and their hydrates, calcium aluminates and their hydrates, aluminum sulfate, magnesium carbonate, and calcium carbonate.

[0103] The ratio of hardener to binder can vary depending on the desired properties, e.g., processing time and / or demolding time of the building material mixtures. The hardener content (weight ratio of hardener to binder and, in the case of water glass, the total mass of the silicate solution or other binders absorbed in solvents) is advantageously greater than or equal to 5 wt.%, preferably greater than or equal to 8 wt.%, particularly preferably greater than or equal to 10 wt.%, in each case based on the binder.

[0104] Once the strength permits, the unbound building material mix (this is essentially the basic building material mix) can be removed from the casting mold (or core), and the mold can be subjected to further processing, e.g., preparation for metal casting. The unbound and bound building material mixes can be separated, for example, by means of a spout, allowing the unbound building material mix to trickle out. The bound building material mix (casting mold) can be freed of any residue of the unbound building material mix using compressed air or by brushing.

[0105] The casting molds produced from the building material mixture according to the invention or using the method according to the invention have a high level of strength immediately after production, without the strength of the casting molds being so high after curing that difficulties arise when removing the casting mold after the casting has been produced. Furthermore, these casting molds have a high level of stability at elevated air humidity, i.e. the casting molds can surprisingly be stored for extended periods of time without any problems. As an advantage, the casting mold has a very high level of stability under mechanical stress, so that even thin-walled sections of the casting mold can be realized without them being deformed by the metallostatic pressure during the casting process. The invention therefore further relates to a casting mold which was obtained by the method according to the invention described above.

[0106] The invention will be explained in more detail below using examples, without being limited to them. The fact that only hot curing is described as the curing method does not constitute a limitation.

[0107] Examples

[0108] 1) Influence of various powdered alkaline earth metal compounds on the flexural strength

[0109] So-called Georg Fischer test bars were produced to test a building material mixture. Georg Fischer test bars are cuboid-shaped test bars with the dimensions 180 mm x 22.36 mm x 22.36 mm. The compositions of the building material mixtures are given in Table 1. The Georg Fischer test bars were produced as follows: • The components listed in Table 1 were mixed in a laboratory paddle mixer (Hobart Manufacturing UK, Model HSM10). First, the quartz sand was added and the water glass was added while stirring. The water glass used was sodium water glass, which contained potassium. In the following tables, the modulus is therefore given as SiO2:M2O, where M is the sum of sodium and potassium.

[0110] After the mixture was stirred for one minute, the amorphous silicon dioxide, the platelet-shaped graphite, and, if present, the powdered alkaline earth metal compound were added while stirring. The mixture was then stirred for another minute.

[0111] • The building material mixtures were transferred into the storage bunker of a core shooter from Laempe & Mössner GmbH, whose mold was heated to 180 °C;

[0112] • The building material mixtures were introduced into the mold using compressed air (5 bar) and remained in the mold for a further 35 seconds;

[0113] • To accelerate the curing of the mixtures, hot air (2 bar, 100 °C when entering the tool) was passed through the mold during the last 20 seconds;

[0114] • The mold was opened and the test bars removed.

[0115] To determine the flexural strength, the test bars were placed in a 3-point test device from Morek Multiserv, and the force that caused the test bars to break was measured. The flexural strengths were measured according to the following scheme:

[0116] • 10 seconds after removal (hot strength)

[0117] • 24 hours after removal (cold strength)

[0118] • 24 hours after storage of the kernels in the climate cabinet of Weiss Umwelttechnik at 30°C and 60% relative humidity, whereby the kernels were only placed in the climate cabinet after cooling down (1 hour after removal).

[0119] To determine the processing time, the finished building material mixture was covered and stored at room temperature for 2 hours. The material was then processed as described above. The flexural strengths were measured again according to the following schedule: • 10 seconds after removal (hot strengths)

[0120] • 24 hours after removal (cold strength)

[0121] Table 1 Compositions of the building material mixtures

[0122] The indices in Table 1 have the following meaning: a) Alkali water glass with a molar modulus SiO2:M2O of approximately 2.2; based on the total water glass. Solids content of approximately 35%, the remainder being water. b) amorphous silicon dioxide (Microsilica POS BW 90 LD supplier Possehl Erzkontor) c) platelet-shaped graphite (85% C, supplier Luh) d) Calcium oxide (Magnesia 7258, supplier MAGNESIA) e) Calcium oxide (Nekafin 0, supplier KFN)

[0123] Calcium oxide (Precal 30 S, supplier SCHAEFER KALK) g ) Calcium oxide (Micronized Calcium oxide, supplier Euro Minerals) h) Calcium hydroxide (supplier Alfa Aeser)

[0124] 0 Calcium sulfate (supplier Thermo Scientific) j)Magnesium sulfate (IMAG AS, supplier Imagine) k) Calcium chloride (supplier Thermo Scientific)

[0125] The measured hot and cold bending strengths are summarized in Table 2. In addition, the respective strengths were measured after a processing time of 2 hours to reflect the storage stability of the building material mixture, since immediate processing of the building material mixture in the series process is not always possible.

[0126] Example 1.01 contains only the base material, amorphous silicon dioxide (according to EP 1802409 B1 and DE 102012020509 A1), platelet-shaped graphite, and an alkali water glass. The results serve as a reference in the following test series. The strength levels before and after the processing time are comparable, and the strength delta is correspondingly low.

[0127] In Examples 1.02 to 1.07, the addition of an alkaline earth metal compound was continuously increased. The increase in hot strength of up to 20% is clearly visible. The highest possible instantaneous strength is important for high-speed machining of the produced cores and is necessary to reduce the reject rate of defective cores during subsequent core processing.

[0128] After a two-hour processing time, an influence on the strength level is also evident depending on the addition amount. While low addition amounts (1.02 to 1.05) have little effect on the processing time (low strength delta), both the initial hot strength and the strength after the processing time are significantly reduced at higher addition amounts (mixtures 1.06 and 1.07) (strength delta increases). Table 2

[0129] Influence of alkaline earth metal compounds on hot strength, cold strength and strength after defined processing time nb = not determined

[0130] Table 3 summarizes the climatic strengths of the produced bending beams. In general, a relative strength reduction of 50% was defined as the evaluation limit in this test. Strength reductions <50% were subsequently evaluated as positive, and strength reductions >50% as negative. Mixture 1.01 again serves as a reference. The influence of the extreme climatic conditions (temperature of 30°C and humidity of 60%) on the strengths is particularly clear in this example. After 24 hours of storage, the cold strength is reduced by approximately 70%. By continuously increasing the addition quantity (mixtures 1.02 - 1.06) of the alkaline earth metal compound, a significant improvement in climatic resistance results. The strength reduction is reduced to as little as 25%. As with the processing time, the effect of overdosing an alkaline earth metal compound in the building material mixture is also evident here. For mixture 1.07 the strength loss increases to 36%, but is still significantly lower than the reference.

[0131] Table 3 Influence of alkaline earth metal compounds on strength after climatic storage 2) Improvement of thermal stability

[0132] The influence of various powdered alkaline earth metal compounds on thermal stability was investigated. For this purpose, the following procedure was used:

[0133] • Georg Fischer test bars of the manufactured building material mixtures were examined with regard to the flexural strengths (analogous to example 1 - no differences were found compared to the values ​​listed in Table 2).

[0134] • The bending pins were then positioned on a holder at their outermost edges in a muffle furnace, and a weight (407 g) was loaded in the center of the core. This setup resulted in a three-point load at a temperature of 650°C (approximately equivalent to the casting temperature for aluminum) for a total of 2 minutes. After a cooling phase of 2 minutes, the deformation was measured using a caliper.

[0135] The values ​​listed in Table 4 are the mean values ​​of a quadruple determination.

[0136] The increase in deformation stability with increasing amounts of the alkaline earth metal compound is clearly visible. Depending on the added amount, the deformation of the bending bar could be reduced by up to 62%.

[0137] Table 4

[0138] Influence of alkaline earth metal compounds on deformation behavior

Claims

Patent claims 1. Mixture as a building material mixture for the production of shaped bodies comprising: - at least 90% by weight of a refractory building material, based on the building material mixture, - 0.1 to 2 wt.%, preferably 0.2 wt.% to 1.0 wt.%, of an amorphous silicon dioxide, based on the refractory building material; and - 0.001 to 0.2 wt.%, preferably 0.005 to 0.1 wt.%, of an alkaline earth metal compound, based on the refractory building material, wherein the alkaline earth metal compound is selected from the group: Calcium chloride, calcium oxide, calcium hydroxide, calcium sulfate, magnesium sulfate and mixtures thereof.

2. Mixture as a building material mixture for the production of shaped bodies, comprising: - at least 80% by weight of a refractory building material, based on the building material mixture, - 0.1 to 2% by weight, preferably 0.2 to 1.0% by weight, of an amorphous silicon dioxide, based on the refractory building material; - 0.001 to 0.2 wt.%, preferably 0.005 to 0.1 wt.%, of an alkaline earth metal compound, based on the refractory building material, wherein the alkaline earth metal compound is selected from the group: Calcium chloride, calcium oxide, calcium hydroxide, calcium sulfate, magnesium sulfate and mixtures thereof; and - a binder comprising water glass.

3. Kit for the preparation of a building material mixture comprising the following components separately: (B1) a binder, wherein the binder is an aqueous binder comprising at least water glass; and (C1 ) a solid additive package, wherein the solid additive package (C1 ) comprises: - an amorphous silicon dioxide and - an alkaline earth metal compound, wherein the alkaline earth metal compound is selected from the group: Calcium chloride, calcium oxide, calcium hydroxide, calcium sulfate, magnesium sulfate and mixtures thereof in a weight ratio of amorphous silicon dioxide to alkaline earth metal compound of 1:2 to 2000:1, preferably 2:1 to 200:1, wherein the amorphous silicon dioxide and the alkaline earth metal compound are each present as particulate solids in free-flowing form in the solid additive package (C1). or (B2) a binder, wherein the binder is an aqueous binder comprising at least - Water glass and - 0.05 to 10 wt.%, preferably 0.25 to 5 wt.%, of an alkaline earth metal compound, based on the binder, wherein the alkaline earth metal compound is selected from the group: calcium chloride, calcium oxide, calcium hydroxide, calcium sulfate, magnesium sulfate and mixtures thereof; and (C2) a solid additive package comprising an amorphous silicon dioxide, wherein the amorphous silicon dioxide is present as a particulate solid in free-flowing form in the solid additive package C2.

4. Kit according to claim 3 comprising separately further (A) a refractory building material; 5. Mixture according to at least one of the preceding claims 1 or 2, wherein - the refractory building material, the amorphous silicon dioxide and the alkaline earth metal compound are each present or added as particulate solids in free-flowing form.

6. Mixture or kit according to at least one of the preceding claims, wherein the components of the refractory building material each have a melting point of greater than 600°C.

7. Mixture or kit according to at least one of the preceding claims, wherein the refractory building material has a particle size of less than or equal to 600 pm, preferably less than or equal to 500 pm, determined by sieving according to DIN 66165 Part 2.

8. Mixture or kit according to at least one of the preceding claims, wherein the alkaline earth metal compound has a particle size of less than or equal to 1 mm, preferably less than or equal to 0.5 mm, particularly preferably less than or equal to 0.25 mm, determined by sieving according to DIN 66165 Part 2.

9. Mixture or kit according to at least one of the preceding claims, wherein the amorphous silicon dioxide has a particle size of less than 300 pm, preferably less than 200 pm, determined by sieving according to DIN 66165 Part 2.

10. Mixture or kit according to at least one of the preceding claims, wherein the amorphous silicon dioxide has an average primary particle size D50 of 0.05 pm to 10 pm, in particular of 0.1 pm to 5 pm, preferably of 0.1 pm to 2 pm, determined by dynamic light scattering.

11. Mixture or kit according to at least one of the preceding claims, wherein the amorphous silicon dioxide has a specific surface area of ​​1 to 200 m 2 / g, preferably from 1 to 50 m 2 / g, particularly preferably from 1 to 30 m 2 / g, has.

12. Mixture or kit according to at least one of the preceding claims, wherein the mixture or kit further contains graphite, preferably between 0.1 wt.% and 1.0 wt.%, preferably between 0.15 wt.% and 2 wt.%, based on the refractory building material; and / or between 0.1 wt.% and 1.0 wt.%, preferably 0.15 wt.% and 2 wt.%, based on the binder.

13. Mixture or kit according to claim 12, wherein the graphite has a particle size of less than or equal to 300 pm, preferably less than or equal to 200 pm, particularly preferably less than or equal to 100 pm, determined by sieving according to DIN 66165 Part 2.

14. Mixture or kit according to at least one of the preceding claims, wherein the mixture or kit contains a maximum of 0.2 wt.% organic compounds.

15. Mixture or kit according to at least one of the preceding claims, wherein the water glass has a molar modulus SiO2 / M2O of 1.6 to 4.0, preferably 2.0 to less than 3.5, with M being lithium, sodium and potassium.

16. Mixture or kit according to at least one of the preceding claims, wherein the mixture or kit contains 0.5 to 5 wt.% water glass, preferably 1 to 3.5 wt.% water glass, based on the building material, wherein the solids content of the water glass is 25 to 65 wt.%, preferably 30 to 60 wt.%.

17. Mixture or kit according to at least one of the preceding claims, wherein the mixture or kit further contains surfactants, preferably anionic surfactants and in particular a C6 to C18 alkyl sulfate and / or C6 to C18 alkyl sulfonate.

18. Mixture or kit according to claim 17, wherein the surfactant is present in a proportion of 0.001 to 1 wt.%, particularly preferably 0.01 to 0.2 wt.%, based on the weight of the refractory building material.

19. Mixture or kit according to at least one of the preceding claims, wherein the mixture or kit further contains at least one phosphorus-containing compound, preferably between 0.05 and 1.0% by weight, particularly preferably between 0.1 and 0.5% by weight, based on the weight of the refractory building material.

20. A method for producing casting molds or cores comprising: • Providing the mixture or a kit from which the mixture is produced, optionally with the addition of building material, in each case according to at least one of the preceding claims, • Placing the building material mixture into a mold, and • Curing of the building material mixture.

21. The method according to claim 20, wherein the building material mixture is introduced into the mold by means of a core shooter with the aid of compressed air and the mold is a molding tool and the molding tool is flowed through with one or more gases, in particular comprising CO2.

22. The method according to claim 20 or 21, wherein the building material mixture is exposed to a temperature of at least 100°C for less than 5 minutes for curing and independently thereof the building material mixture is heated, particularly preferably by blowing air at a temperature of 100°C to 180°C into the mold.

23. A method for the layered construction of bodies comprising at least the following steps: a) spreading at least one layer of a building material mixture comprising at least the building material and the solid additive comprising amorphous silicon dioxide, each according to at least one of claims 1 and 5 to 19, with a layer thickness of 0.05 mm to 3 mm; b) printing selected areas of the at least one layer with the binder comprising at least water and water glass; c) repeating at least steps a) and b) several times, wherein the solid additive or the binder or both comprise an alkaline earth metal compound and the alkaline earth metal compound is selected from the group: calcium chloride, calcium oxide, calcium hydroxide, calcium sulfate, magnesium sulfate and mixtures thereof.

24. A mold or core producible according to at least one of claims 20 to 23.

25. A process for aluminum casting comprising • Providing the building material mixture or a kit from which the building material mixture is produced, optionally with the addition of building material, in each case according to at least one of claims 1 to 19, • Placing the building material mixture into a mold, • Curing the building material mixture to produce a casting mould or core, and • Pouring aluminum or an aluminum alloy into the mold or bringing the liquid aluminum or a liquid aluminum alloy into contact with the core.

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