ENERGY-SELF-FORCE REACTOR CONTAINER FOR THE PRODUCTION OF LOW-CARB BUILDING PRODUCTS

Description

[0001] The present invention relates to a reaction vessel for the production of a porous geopolymer that can be used as a sustainable building material.

[0002] Geopolymers (also known as alkali-activated materials or alkali-activated substances) are inorganic polymers formed by polycondensation at a high pH of approximately 13-14. Aluminum and silicon oxides are first activated and cleaved by alkalis such as sodium or potassium hydroxide. This results in the formation of a three-dimensional network, i.e., a geopolymer consisting primarily of silicon and aluminum tetrahedra. Geopolymers exhibit advantageous physical properties, such as easy and rapid shaping, good bonding properties, the ability to modify product properties through additives, and high chemical and thermal resistance. Therefore, geopolymers, especially foamed, porous geopolymers, are a promising alternative to conventional building materials.

[0003] In state-of-the-art processes, the production of geopolymers is carried out in a multi-stage procedure. This can lead to drying shrinkage, which in turn can cause stress cracking. DE 102015121553 A1, for example, describes a process for producing aerated concrete, in which a pourable, aqueous mixture of a hydrothermally reacting calcium oxide component and a hydrothermally reacting silicon dioxide component is mixed with a pore-forming blowing agent and / or a foam and poured into a mold in which the mixture expands. Hydrothermal curing then takes place in an autoclave.

[0004] To produce aerated concrete, steam curing in an autoclave at 170-200 °C for 14-18 hours and a pressure of up to 10 bar can be carried out. This process is very energy-intensive, requiring approximately 2000 MJ / m³. This is accompanied by a significant global warming potential (GWP) of approximately 200 kg CO₂ equivalent / m³. Furthermore, current technology rarely uses recycled materials in the production of geopolymers, which is detrimental to resource conservation and sustainability. The production of porous geopolymers is also complex, as curing and pore creation typically occur in separate process steps.

[0005] Further methods for producing aerated concrete or gas concrete are known from CN 211 104 613 U, EP 0 677 363 A1 and CN 206 748 680 U, wherein CN 211 104 613 U discloses a reaction vessel comprising an outer vessel with a bottom, a pressure valve, an intermediate space and a lid for hermetically sealing the outer vessel, and wherein the pressure valve is arranged to discharge pressure and water vapor from the intermediate space.

[0006] It is therefore an object of the present invention to provide a reaction vessel with which a porous geopolymer can be produced from a starting material comprising recycled material in just one step and by a resource-saving, energy-efficient process, whereby drying shrinkage and the associated stress cracking are avoided, so that a porous geopolymer with high strength, homogeneous pore structure (in particular pore distribution and pore size) and low residual moisture content is obtained, which can be used as a sustainable building material.

[0007] This problem is solved according to the invention by a reaction vessel and a method according to the claims.

[0008] In the present invention, a geopolymer – also called an “alkali-activated material” or “alkali-activated substance” – is understood to be an inorganic polymer that can be formed from a starting material containing silicon dioxide and aluminum oxide by cleavage reactions and a polycondensation reaction.

[0009] The reaction vessel according to the invention comprises an outer container with a bottom, an inner container with a bottom, a spacer, a pressure valve, an intermediate space formed by the spacer between the outer container and the inner container, and a lid for hermetically sealing the outer container and the inner container, wherein the bottom of the inner container is diffusion-open and forms a barrier for liquids, and wherein the pressure valve is arranged to discharge pressure and water vapor from the intermediate space.

[0010] The spacer according to the present invention can be arranged perpendicular to the flow direction and have a recess in the flow direction of the steam. For a good balance between high rigidity and minimal resistance to steam, the spacer can have a large recess, preferably located centrally. Alternatively, the spacer can also have several smaller recesses. The spacer can be positively and / or force-fitted to the outer and inner containers, for example by clamping or screwing, so that distortion of the spacer due to a compressive force acting upon it is prevented. Preferably, the reaction vessel has at least four, and more preferably at least eight, spacers.These spacers can be arranged in the space between a side wall of the inner container and a side wall of the outer container, and / or between the bottom of the inner container and the bottom of the outer container. This allows for high rigidity of both the inner and outer containers, thus preventing or at least reducing pressure-induced deformation. Instead of individual spacers, a spacer rail, preferably perforated, can also be used.

[0011] The spacer according to the invention can also be designed as a guide plate to increase the flow resistance for water vapor in the space, thus extending its residence time in the space. Since the water vapor can act as a heat transfer medium, this can reduce the cooling rate of a mixture present in the inner container. The guide plate can be arranged at an angle to the flow direction, e.g., at an angle of 45° to 60°.

[0012] In a preferred embodiment, the bottom of the inner container is perforated. This facilitates handling, particularly easy cleaning. For example, the bottom of the inner container can be designed as a perforated sheet, ensuring sufficient temperature and pressure stability. The perforation can cover 10 to 75% of the bottom surface. Preferably, the perforation covers 30 to 60% of the bottom surface to allow for easy diffusion of water vapor. The diameter of the holes in the perforated bottom is preferably in the range of 1.5 to 40 mm, more preferably in the range of 3 to 8 mm. This also facilitates the diffusion of water vapor through the perforated bottom. At the same time, the perforated bottom provides a good barrier against liquids. A liquid is understood to be a flowable substance (or mixture, component), such as a highly viscous, pasty mixture or water.While a small portion of the still-flowable mixture may flow through the perforated bottom when filling the inner container with the mixture from which the porous geopolymer is produced, depending on the hole size, this material loss is negligible. The holes in the perforated bottom can be sealed by this mixture, creating a diffusion-open bottom that forms a barrier to liquids. This prevents further flow of the mixture, or, as the reaction progresses during the production of the porous geopolymer, the mixture solidifies, rendering it non-flowable under the conditions in the inner container (especially pressure and temperature). Alternatively, a grid-like bottom can be provided, with recesses comprising 10 to 75% of the grid-like bottom area, preferably 30 to 60%.Furthermore, each recess can have a side length in the range of 1.5 to 40 mm, preferably 3 to 8 mm.

[0013] Furthermore, according to the invention, it is preferred if the bottom of the inner container is covered with a nonwoven fabric or a membrane, and if the bottom is perforated. This prevents water or a mixture that is still flowable after filling from reaching the bottom of the inner container, thus preventing it from flowing into the space between the containers. In a preferred embodiment, the bottom of the inner container is covered with a glass or basalt nonwoven fabric, as this provides an optimal barrier effect against water while allowing water vapor to pass through. In an alternative, preferred embodiment, the bottom of the inner container is covered with a membrane made of expanded polyethylene terephthalate (PTFE). This also provides good water vapor permeability while simultaneously providing a barrier effect against water.Both a glass fleece or basalt fleece and a PTFE membrane exhibit good temperature resistance, so that their use in the inner container according to the invention does not lead to thermal aging and thus no impairment of the barrier properties and the quality of the porous geopolymer. Alternatively, a frit, in particular a glass frit, can also be used.

[0014] If, when using the reaction vessel according to the present invention, no fleece or membrane is provided to cover the possibly perforated or grid-like bottom of the inner vessel, a small portion of the still-flowable mixture may flow through the perforated bottom of the inner vessel during filling, potentially clogging the holes. This can create a diffusion-open layer that acts as a barrier to liquids, so that subsequently (i.e., during several successive filling processes), even without a fleece or membrane, the bottom of the inner vessel will already provide a good barrier to liquids and be diffusion-open due to this layer when the reaction vessel is refilled.

[0015] According to a preferred embodiment of the present invention, a section of a side wall of the inner container adjoining the bottom of the inner container (as well as the bottom of the inner container) is diffusion-permeable and forms a barrier to liquids. This allows water vapor to diffuse from the inner container into the space between the inner and the surrounding container, both through the bottom and through this section in the side wall. This embodiment is particularly advantageous for large-volume reaction vessels to ensure a uniform moisture distribution in the mixture. A side wall is understood to be, for example, the outer surface of a cylindrical inner container or the side wall of a rectangular inner container. Preferably, this section comprises one-third of the height of the side wall, and more preferably, two-thirds. The section can also comprise the entire side wall.This allows for efficient diffusion of water vapor through the side wall. In the present invention, it is further preferred that the section of the inner container's side wall adjoining the bottom of the inner container is perforated and covered with a nonwoven fabric or a membrane. This is preferably a glass nonwoven fabric or a membrane made of expanded polyethylene terephthalate (PTFE). The perforation preferably comprises 10 to 75%, more preferably 30 to 60% of the section to facilitate the diffusion of water vapor. The diameter of the holes provided in the perforated section is preferably in the range of 1.5 µm to 40 mm, more preferably in the range of 3 to 8 mm. This also facilitates the diffusion of water vapor.For a homogeneous moisture distribution around the circumference of an inner container with multiple side walls, such as a rectangular inner container, it is preferred if the sections of all side walls adjoining the bottom of the inner container are designed in such a way that water vapor from the inner container can diffuse through the side walls into the space between them, and these sections of the side walls simultaneously form a diffusion barrier for water. Alternatively to a perforated section, a grid-like section of the side wall can also be provided, with recesses comprising 10 to 75% of the grid-like section, preferably 30 to 60%. Furthermore, each recess can have a side length in the range of 1.5 to 40 mm, preferably 3 to 8 mm.

[0016] In a preferred embodiment of the present invention, a bottom region of the intermediate space is designed to absorb 10 to 95 wt%, preferably at least 70 wt%, of the amount of water present in the inner container. The bottom region is defined as the space between the bottom of the outer container and a plane coinciding with the bottom of the inner container. Since the inner container can have a higher temperature than the surrounding intermediate space during the production of the porous geopolymer, partial condensation of the water vapor diffusing through the bottom of the inner container can occur in the intermediate space. Condensed water can then accumulate in the intermediate space at the bottom of the outer container.This preferred embodiment ensures that water vapor diffusing into the space can reach the pressure valve throughout the entire production of the porous geopolymer, even if some of this water vapor condenses in the space. To increase the base area without increasing the distance between the bottom of the inner container and the bottom of the outer container across the entire cross-sectional area, one or more depressions can be provided in the base of the outer container to collect water. This prevents excessive expansion of water vapor and the associated cooling (and potentially even condensation).

[0017] The reaction vessel according to the invention has a pressure relief valve through which pressure and water vapor diffused into the space between the containers can be released from the reaction vessel. The pressure relief valve enables the controlled release of pressure and water vapor. This ensures that water released during the polycondensation reaction (i.e., curing) required for the formation of the porous geopolymer remains entirely within the inner container and that the foamed mixture does not dry out prematurely. Otherwise, the polycondensation reaction could be completely or partially interrupted by a shift in the reaction equilibrium, which would subsequently lead to a loss of strength in the porous geopolymer being produced. In particular, according to the invention, a moisture equilibrium can be maintained in an air space between the lid and the foamed mixture during the polycondensation reaction.the developing porous geopolymer is produced in the inner container. During this process, the foamed mixture absorbs the same number of water molecules as evaporate from it. Furthermore, the inventive removal of water vapor allows both the core and the outer zones of the foamed mixture to be uniformly supplied with heat and moisture. This ensures uniform conditions for the polycondensation reaction across the entire cross-section and height of the inner container, resulting in a porous geopolymer with homogeneous properties, particularly homogeneous strength and pore structure (especially pore size and pore distribution).

[0018] Preferably, according to the invention, the pressure valve opens at a predetermined pressure value in the inner container and then remains open. This allows the porous geopolymer formed in the inner container to dry by releasing water vapor. According to the invention, the pressure valve also remains open if the pressure in the inner container drops below the predetermined value during drying, as otherwise sufficient drying would not be achieved. For example, it is also possible to release water vapor only after a predetermined time has elapsed since the pressure in the inner container has reached the predetermined value, e.g., after 3 minutes or after 5 minutes. This allows the degree of curing (i.e., degree of cross-linking) of the porous geopolymer and, consequently, its strength to be further increased.

[0019] According to the invention, the predetermined pressure value in the inner container, at which the pressure valve opens and then remains open, can be at least 1180 mbar, preferably at least 1500 mbar, even more preferably at least 1800 mbar, and particularly preferably at least 2000 mbar. The ambient pressure (i.e., atmospheric pressure) can be 1013 mbar. A pressure increase in the inner container up to this value indicates a high water vapor concentration in the inner container and thus the end of the polycondensation reaction. Such a pressure increase can therefore be used as an indicator to initiate controlled, uniform drying of the porous geopolymer by opening the pressure valve. This prevents stress cracking (due to stress differences between the core and the outer zone of the geopolymer, which would otherwise occur due to inhomogeneous drying) as well as deformation or destruction of the pores formed.

[0020] Alternatively, according to the present invention, the pressure valve can also open and remain open as soon as the temperature in the inner container drops to a predetermined value below the maximum temperature reached in the inner container during the production of the porous geopolymer. This predetermined value is preferably 5 °C, more preferably 10 °C. The maximum temperature reached is understood to be the temperature that is reached at its maximum during the production of the porous geopolymer in the inner container (due to heat released by exothermic reactions and, if applicable, additional external energy input). A temperature drop to the predetermined value indicates the end of the polycondensation reaction and can also be used as an indicator to initiate controlled, uniform drying of the porous geopolymer by opening the pressure valve.

[0021] According to a preferred embodiment, the pressure valve is provided in a region of the lid that defines the space between the inner and outer containers, or in a section of a side wall of the outer container immediately adjoining the lid. This arrangement ensures that the water vapor diffusing from the inner container into the space between the inner and outer containers flows not only along, for example, an outer surface of the bottom of the inner container, but also along an outer surface of a side wall of the inner container before being discharged from the reaction vessel via the pressure valve. The water vapor thus acts as a heat transfer medium and contributes (in addition to the heat released by exothermic reactions in the inner container) to further heating of the mixture, or slows down its cooling after the polycondensation reactions have ceased, so that subsequent drying can be carried out more efficiently.This also prevents any premature condensation of water vapor in the inner container. The section of the outer container's side wall immediately adjoining the lid preferably comprises one-fifth of the side wall's height.

[0022] Furthermore, according to the invention, it is preferred if the reaction vessel comprises at least two pressure valves, preferably four. This allows for better control of the steam discharge. For example, instead of one pressure valve with a comparatively high flow rate, two pressure valves with smaller flow rates can be used, enabling finer adjustment of the flow rate. Preferably, the pressure valves are arranged evenly around the circumference of the space between the two walls, so that a uniform steam discharge can occur. Particularly preferred are four pressure valves arranged evenly distributed around the circumference of the space in a region of the lid that defines the space between the two walls, in order to achieve a particularly uniform steam discharge. In addition, four further pressure valves can be arranged in a section of a side wall of the outer vessel immediately adjoining the lid (e.g.,in one fifth of the height of the side wall immediately adjacent to the lid; for example in the corner areas of a rectangular reaction vessel), wherein the four additional pressure valves are preferably evenly distributed around the circumference of the space.

[0023] According to a preferred embodiment, the outer container and the lid include thermal insulation. This slows down the cooling of the mixture, allowing for better utilization of the heat released by exothermic reactions in the inner container. The thermal insulation can comprise a foamed plastic (e.g., polystyrene, polyurethane), an inorganic insulating material (e.g., mineral wool (such as glass or rock wool), foamed glass, perlite, calcium silicate), vacuum panels, a natural insulating material (e.g., wood wool, hemp fibers, cork, reed), or a recycled material (e.g., waste paper).

[0024] The present invention also relates to a method for producing a porous geopolymer, comprising the steps of: (a) Providing the reaction vessel according to the present invention, (b) Filling the inner vessel with a mixture comprising comminuted recycled material, an alkaline activator, blowing agent, and water, wherein the recycled material includes silicon dioxide and aluminum oxide, (c) Foaming the mixture to a maximum foam height and hermetically sealing the inner vessel and the outer vessel with the lid, (d) Initiating exothermic cleavage reactions of amorphous components of the recycled material with the alkaline activator and heating the mixture to a temperature in the range of 70 to 100 °C, preferably 75 to 85 °C, by the cleavage reactions, (e) Forming the porous geopolymer by further cleavage reactions and a polycondensation reaction with the release of water vapor, (f) Drying the porous geopolymer by discharging the released water vapor into the space and out of the reaction vessel via the pressure valve.and (g) demolding of the porous geopolymer.

[0025] According to the invention, a starting material comprising shredded recyclate is mixed with a solution of an alkaline activator in water, a blowing agent, and optionally an additive and / or a filler. This mixture is then filled into the inner container of the reaction vessel. The inner container can have a filling capacity of 0.5 to 10,000 liters. Before filling the inner container with the mixture, it can be coated with a release agent to facilitate subsequent demolding of the porous geopolymer.

[0026] Various recyclates can be used according to the invention, for example, combustion residues from thermal power plants or waste incineration plants, fly ash, slag (e.g., from metallurgical processes), mine waste from mining or iron smelting, mineral and glass fiber waste, waste glass, rock wool, ceramic grinding dust, foundry sand, and mineral waste or by-products from industry or building demolition. Mineral waste is preferably used in the present invention because its alkaline earth metal content is comparatively high. The recyclate can be comminuted, for example, by grinding, so that a powdered recyclate is obtained. According to the invention, the recyclate contains amorphous silicon dioxide and amorphous aluminum oxide. Preferably, recyclates with a proportion of amorphous components of 5 to 95 wt% are used.The silicon dioxide content in the recycled material can range from 5 to 99 wt%, and the aluminum oxide content from 5 to 95 wt% (both based on the weight of the recycled material). In the mixture, the silicon dioxide content can then range from 5 to 70 wt%, while the aluminum oxide content can range from 2.5 to 35 wt% (both based on the total weight of the mixture). The ratio of silicon dioxide to aluminum oxide has a significant influence on the quality of the resulting geopolymer. Preferably, the ratio of silicon dioxide to aluminum oxide in the recycled material is in the range of 15:8 to 60:30 (values ​​in wt%). This results in a geopolymer with particularly high strength, especially compressive strength. Furthermore, preferably, the recycled material has a total silicon oxide and aluminum oxide content in the range of 25 to 75 wt%, so that a geopolymer with high homogeneity can be formed.Furthermore, other components, such as phosphorus pentoxide, boron trioxide, iron(III) oxide, calcium oxide, magnesium oxide, or mixtures thereof, may be present in the recycled material, particularly to improve certain properties of the porous geopolymer (e.g., improved flame retardancy in the presence of boron trioxide). Other components may also be present as impurities that can be incorporated into the geopolymer but do not significantly affect its homogeneity and properties.

[0027] In the present invention, a water-soluble alkali metal hydroxide, an alkali metal silicate, an alkali carbonate, an alkali metal aluminate, or a mixture thereof, or sodium or potassium hydroxide, can be used as an alkaline activator. The alkaline activator can initiate cleavage reactions in the reaction vessel to produce a geopolymer at a high pH, ​​particularly in the range of 13 to 14. According to the invention, sodium hydroxide is preferably used as the alkaline activator because it causes a more active (i.e., particularly effective) cleavage reaction, thus requiring a lower total alkali content. Sodium hydroxide is also readily available and economically viable on the market. Preferably, the concentration of the alkaline activator in an aqueous solution is in the range of 5 to 15 wt%, as this allows for an efficient reaction with the recycled material.A co-activator can also be added to the mixture to further accelerate the reaction, for example an inorganic acid, a hydroxycarboxylic acid, a triethanolamine or a mixture thereof.

[0028] According to the invention, a blowing agent is also added to increase the rate of pore formation and the number of pores in the mixture. Hydrogen peroxide is preferably used as the blowing agent, since its decomposition products are water and oxygen, with oxygen acting as an efficient blowing agent and water already present in the mixture. This avoids undesirable byproducts such as those formed during the decomposition of other peroxides. Furthermore, the use of hydrogen peroxide as a blowing agent allows for the formation of very fine, uniformly distributed pores. Preferably, 1 to 6 wt% hydrogen peroxide is added to the mixture, based on the weight of the recycled material.To simplify dosing, hydrogen peroxide can be added in the form of an aqueous solution, wherein the concentration of hydrogen peroxide in the aqueous solution is preferably in the range of 25 to 40 wt%, more preferably 33 to 37 wt%, and particularly preferably 35 wt%. Manganese dioxide or another peroxide can be used to catalyze the hydrogen peroxide reaction.

[0029] According to the invention, a ratio of recycled material to water in the range of 85:15 to 70:30 (values ​​in wt%) is preferably used to ensure complete conversion and to obtain a geopolymer with high strength and good pore structure. The water content is to be considered as the total water content present in the mixture (e.g., the sum of the water content of the aqueous solution of the alkaline activator, the water content of the blowing agent solution, and the separately added water).

[0030] If an additive is added to the mixture which is filled into the inner container according to the invention, this additive can be selected from the group consisting of surfactants, dispersants, wetting agents, stabilizers, complexing agents, or a mixture thereof. A filler can also be added to the mixture to further reduce shrinkage; for example, perlite, pumice, vermiculite, or foamed glass.

[0031] To produce a geopolymer according to the invention, the mixture is filled into the inner container of the reaction vessel. After filling, the mixture foams up to a maximum foam height. The maximum foam height is preferably in the range of three-fifths to five-fifths of the height of the inner container, more preferably in the range of four-fifths to five-fifths of the height of the inner container. When the maximum foam height is reached, the inner and outer containers are hermetically sealed with the lid, for example by screwing or using quick-release fasteners. The reaction vessel, and in particular the lid, must not have any leaks, as otherwise the water vapor generated during the reaction could escape. This could subsequently lead to the mixture drying out in the upper area near the lid, while the mixture in the lower area near the bottom could become too moist.This imbalance in moisture content could then lead to stress cracking.

[0032] According to the invention, the mixture in the inner container is heated to a temperature in the range of 70 to 100 °C, preferably 75 to 85 °C, by exothermic reactions. These exothermic reactions consist of cleavage reactions of the amorphous components of the recycled material (in particular, the amorphous silicon dioxide and the amorphous aluminum oxide) followed by a polycondensation reaction. Naturally, heat is released during these reactions, which heats the mixture. The exothermic cleavage reactions are initiated by the alkaline activator, resulting in the formation of monomers such as Si(OH)₄ and Al(OH)₄. Subsequently, curing takes place under hydrothermal conditions, whereby the exothermic polycondensation reaction, releasing water, forms a covalent, three-dimensional network, i.e., a geopolymer (or an alkali-activated material).If the amorphous components of the recyclate consist of silicon dioxide and aluminum oxide, an aluminosilicate network is formed, composed of [SiO₄]⁴⁻ and [AlO₄]⁵⁻ tetrahedra linked by oxygen atoms. The negative charge of the [AlO₄]⁵⁻ tetrahedron is balanced by the incorporation of monovalent metal cations, such as sodium or potassium ions. Since the mixture in the inner container can thus be heated by the exothermic reactions (and the heat released thereby), the process according to the invention can be carried out energy-autonomously, i.e., without an external energy supply, which offers enormous advantages in terms of energy efficiency and sustainability.

[0033] According to the present invention, the heating of the mixture in step (d) of the process according to the invention can additionally be achieved by an exothermic energy input. This allows the process according to the invention to be carried out in a shorter time. The external energy input can be applied for a duration of 10 to 1,000 minutes. Preferably, the external energy input is applied for 5 to 30 minutes in order to accelerate the reactions at the beginning of the process and thus achieve faster heating. No further external energy input is required for subsequent cleavage reactions and the polycondensation reaction. The heat released by the exothermic reactions can be used for further reactions taking place in the inner container. The temperature of the mixture can rise to up to 90 °C, or even up to 130 °C, which can result in a total reaction time of up to 20 hours, preferably 8 to 12 hours.The actual total reaction time depends, among other things, on the composition and density of the recycled material. The heat released by the exothermic reactions can also be used for subsequent drying of the synthesized porous geopolymer. Thus, the present invention provides a resource-efficient method for producing a porous geopolymer.

[0034] According to the invention, a pressure increase can occur in the inner container during the process relative to the ambient pressure (i.e., atmospheric pressure). This pressure increase is primarily due to the release of water during the polycondensation reaction, which, due to the high temperature in the inner container, can be present as water vapor to at least 90 wt%, preferably at least 95 wt%, and most preferably entirely. Furthermore, additional decomposition reactions of remaining propellant, particularly hydrogen peroxide, may further increase the pressure in the inner container. Due to the pressure increase in the inner container, water vapor diffuses through the mixture towards the bottom of the inner container, partly due to gravity and partly because the bottom of the inner container allows the water vapor to diffuse into the space between the two containers.

[0035] During the process according to the invention, particularly during the polycondensation reaction with the release of water vapor, the pressure in the inner container can increase relative to the ambient pressure. This enables a uniform supply of moisture to the mixture by establishing a moisture equilibrium in the air space between the lid and the foamed mixture. This allows the polycondensation reaction to proceed under uniform conditions throughout the entire mixture, resulting in a porous geopolymer with homogeneous properties, especially homogeneous strength and pore structure (particularly pore size and pore distribution).

[0036] According to the invention, the pressure in the inner container can rise to up to 1180 mbar, preferably up to 1500 mbar, even more preferably up to 1800 mbar, and particularly preferably up to 2000 mbar. The ambient pressure (i.e., atmospheric pressure) can be 1013 mbar. At this pressure, the pressure valve preferably opens and then remains open to initiate the subsequent drying of the porous geopolymer formed. Alternatively, a drop in temperature in the inner container of at least 5 °C, or at least 10 °C, can indicate the end of the polycondensation reaction. In this case, the pressure valve preferably opens at this temperature and then remains open to initiate the subsequent drying of the porous geopolymer formed. This prevents stress cracking (due to stress differences between the core and the outer zone of the porous geopolymer, which would otherwise occur due to inhomogeneous drying) as well as deformation or...Destruction of the formed pores is avoided. Due to the heat released in the exothermic reactions according to the invention and the controlled removal of water vapor, the porous geopolymer can be dried to a residual moisture content of a maximum of 15 wt%, preferably a maximum of 10 wt% (based on the initial water content present in the mixture). In comparison, the residual moisture content of geopolymers in conventional manufacturing processes is in the range of 20 to 30 wt%. Because of the low residual moisture content achievable according to the invention, volume changes following demolding of the geopolymer can be significantly reduced, thus preventing associated cracking. The residual moisture content is determined using a DAB-200-2 type moisture analyzer (Kern & Sohn GmbH, Germany).In this process, 50 g of the mixture are dried using the drying parameters defined according to the factory settings for standard drying at 120 °C with automatic shut-off.

[0037] After drying according to the invention, the porous geopolymer can be demolded to obtain a geopolymer block. Depending on the geometry of the inner container, this block can have a rectangular or cylindrical shape. The geopolymer block can then undergo further processing steps, such as cutting and / or packaging. Depending on the intended application, the geopolymer block can also be further solidified in an autoclave, which can further increase the degree of cross-linking.

[0038] The total energy requirement of the inventive process for producing a porous geopolymer is only about 1 / 3 of the energy required for the production of aerated concrete and brick building materials using methods common in the prior art. For example, instead of 2000 MJ / m³, only 700 MJ / m³ is required to produce a porous geopolymer with a density of 500 kg / m³ – this corresponds to a CO₂ reduction of up to 70 wt%.

[0039] The present disclosure further relates to a porous geopolymer (also referred to as a porous, alkali-activated material or porous, alkali-activated substance) obtainable by the process according to the invention, with a residual moisture content after demolding of a maximum of 15 wt%, preferably a maximum of 10 wt% (determined with a moisture analyzer as described above). Preferably, the porous geopolymer has a closed-cell structure with pores of 0.1 to 2.5 mrr diameter. The pores are isotropic, resulting in direction-independent properties. More preferably, the porous geopolymer consists of up to 36 vol%, more preferably up to 80 vol%, and particularly preferably up to 93 vol% air, making it ideal for lightweight construction. Good thermal insulation properties can also be achieved by means of air trapped in the pores. The porous geopolymer can have a density of 80 to 1000 kg / m³.

[0040] The present disclosure also relates to the use of the porous geopolymer as a building material, for example as a fire-resistant, sound-absorbing, and / or heat-insulating foam. Due to the low overall energy consumption of the inventive process by which the porous geopolymer is produced, it can be used as a sustainable, resource-conserving building material. Preferably, the porous geopolymer is used as a building block, component, or insulating material, since its isotropic pore structure, due to its direction-independent properties, allows for flexible use compared to conventional aerated concrete (in which anisotropic pores are present). The invention is further explained below with reference to descriptions of figures illustrating preferred embodiments, to which, however, it is not limited.

[0041] This shows Fig. 1-3 Aerial views of reaction vessels according to the invention during the drying of a porous geopolymer.

[0042] Fig. 1 Figure 1 shows a reaction vessel 1 according to the present invention, comprising an outer vessel 2, an inner vessel 3, a lid 4, and pressure valves 5. A foamed mixture is contained in the inner vessel 3. A space 6 is provided between the outer vessel 2 and the inner vessel 3. The bottom 7 of the inner vessel 3 is perforated to allow water vapor to diffuse through it. Spacers 8, each with a centrally arranged recess, are provided in the space 6 and are perpendicular to the bottom 7 of the inner vessel. The pressure valves 5 are arranged in a region of the lid 4 that defines the space 6 and in a section of a side wall of the outer vessel 2 that is directly adjacent to the lid 4.

[0043] Furthermore, it is from Fig. 1 It can be seen that during the drying process according to the invention, water vapor diffuses towards the bottom 7 of the inner container 3 due to the pressure increase (indicated by arrows 9), so that zones AD with different moisture contents are formed, with zone A having the lowest moisture content in the inner container 3 and zone D the highest. In particular, it is possible with the present invention to establish a moisture equilibrium in zone A (the air space between the lid 4 and the foamed mixture). After the water vapor has diffused through the perforated bottom 7 of the inner container 3 into the space 6, it flows towards the pressure valves 5 (indicated by arrows 10), through which it is discharged.

[0044] Fig. 2 Figure 1 shows another reaction vessel 1 according to the invention. The structure essentially corresponds to the structure of reaction vessel 1 of the Fig. 1 , except that sections of the side walls of the inner container 3, which adjoin a base 7 of the inner container 3, are also perforated.

[0045] In Fig. 3 A further reaction vessel 1 according to the present invention is shown, the structure of which is essentially the same as the structure of reaction vessel 1 of the Fig. 1 This corresponds, except that the outer container 2 and the lid 4 include thermal insulation 11. Furthermore, pressure valves 5 are only provided in an area of ​​the lid 4 that limits the space 6.

Claims

1. Reaction vessel (1) for producing a porous geopolymer, comprising an outer vessel (2) with a bottom, an inner vessel (3) with a bottom (7), a spacer (8), a pressure valve (5), an interspace (6) formed by the spacer (8) between the outer vessel (2) and inner vessel (3), and a lid (4) for hermetically sealing the outer vessel (2) and the inner vessel (3), wherein the bottom (7) of the inner vessel (2) is diffusion-permeable and forms a barrier for liquids, and wherein the pressure valve (5) is arranged to discharge pressure and water vapor from the interspace (6).

2. Reaction vessel (1) according to claim 1, characterized in that the bottom (7) of the inner vessel (3) is perforated.

3. Reaction vessel (1) according to claim 2, characterized in that the bottom (7) of the inner vessel (3) is covered with a fleece, preferably a glass fleece or basalt fleece, or with a membrane, preferably consisting of stretched polyethylene terephthalate (PTFE).

4. Reaction vessel (1) according to any one of claims 1 to 3, characterized in that a section of a side wall of the inner vessel (3) adjoining the bottom (7) of the inner vessel (3) is diffusion-permeable and forms a barrier for liquids.

5. Reaction vessel (1) according to any one of claims 1 to 4, characterized in that a bottom region of the interspace (6) is formed such that it can accommodate 10 to 95 wt%, preferably at least 70 wt%, of an amount of water present in the inner vessel (3).

6. Reaction vessel (1) according to any one of claims 1 to 5, characterized in that the pressure valve (5) is configured such that it opens at a predetermined value of pressure in the inner vessel (3) and then remains open.

7. Reaction vessel (1) according to claim 6, characterized in that the predetermined value of pressure in the inner vessel (3) is at least 1180 mbar.

8. Reaction vessel (1) according to any one of claims 1 to 7, characterized in that the pressure valve (5) is provided in a region of the lid (4) delimiting the interspace (6) or in a section of a side wall of the outer vessel (2) directly adjoining the lid (4).

9. Reaction vessel (1) according to any one of claims 1 to 8, characterized in that the reaction vessel (1) comprises at least two pressure valves (5), preferably four.

10. Reaction vessel (1) according to any one of claims 1 to 9, characterized in that the outer vessel (2) and the lid (4) include thermal insulation (11).

11. Method for producing a porous geopolymer, comprising the steps: (a) providing a reaction vessel (1) according to any one of claims 1 to 10, (b) filling the inner vessel (3) with a mixture comprising comminuted recyclate, an alkaline activator, blowing agent and water, wherein the recyclate includes silicon dioxide and aluminum oxide, (c) foaming the mixture up to a maximum foam height and hermetically sealing the inner vessel (3) and the outer vessel (2) with the lid (4), (d) initiating exothermic cleavage reactions of amorphous components of the recyclate with the alkaline activator, and heating the mixture to a temperature in the range of 70 to 100°C by means of the cleavage reactions, (e) forming the porous geopolymer through further cleavage reactions and a polycondensation reaction with release of water vapor, (f) drying the porous geopolymer by discharging the released water vapor into the interspace (6) and via the pressure valve (5) from the reaction vessel (1), and (g) demolding the porous geopolymer.

12. Method according to claim 11, characterized in that the ratio of silicon dioxide to aluminum oxide in the recyclate is in the range of 15:8 to 60:30 (data in wt%).

13. Method according to claim 11 or 12, characterized in that the ratio of recyclate to water in the mixture is in the range of 85:15 to 70:30 (data in wt%).