FIELD OF THE INVENTION

The present invention relates to the technical field of solid inorganic foams, methods for manufacturing said solid inorganic foams, and uses of said solid inorganic foams.

BACKGROUND OF THE INVENTION

By 2030, modern thermal insulation materials are expected to reduce the total energy costs up to 20%. However, today’s modern solutions must also adapt to continuously- changing regulations, such as the use of non-flammable, non-toxic and environmental-friendly materials. Currently, many of the major industrial actors do not meet at least one of these requirements. Polymeric solutions such as Expanded Polystyrene (EPS) or Polyurethane (PU) are most frequently used because of their low thermal conductivity in spite of being made through toxic processes and being highly flammable. Other solutions, such as glass wool or mineral wool are not flammable, but they are energy intensive during manufacturing and they may lead to human health issues. Recent inorganic solutions, such as aerogels tend to reach extremely low thermal conductivities, while being flame resistant. However, aerogels still remain very expensive. Lightweight AAC (autoclaved aerated concrete) panels cannot currently provide thermal conductivity values lower than 0.04 mW/mK. As a result, there is a gap in the building insulation market that is not filled by currently available solutions.

A rather simple and relatively inexpensive production method for lighter density products is achieved by foamed concretes, which typically consist of a slurry of cement, sand and water, which is then further blended with an aqueous foam. The foam is created using a foaming agent, such as proteins.

Generally, mineral foams or inorganic foams are very advantageous for many applications due among others to their thermal and sound insulation properties. Mineral foam is a material in the form of a foam. This material is more lightweight than the bulk counterpart due to its porosity or empty spaces. These pores or empty spaces are due to the presence of air or other gases in the mineral foam and they may be in the form of bubbles. With 1 m ³ of raw material it is possible to produce approximately 5 m ³ of a finished product if the porous body is composed by 20% of material and 80% of air (this is valid for raw materials with bulk density of 2000 kg/m ³ and a final density of approximately 400 kg/m ³ ).

US2017158568 A1 discloses a method for producing an ultra-light mineral foam, wherein a slurry of Portland cement and an aqueous foam comprising water and a foaming agent are mixed. Thereby, a slurry of foamed cement is obtained, which is then subjected to casting, followed by setting for several days and drying at 45 °C. Although the ultra-light mineral foam hardens at temperatures close to the room temperature, the production of the Portland cement, which typically requires firing limestone at temperatures of about 1450 °C, results in high carbon dioxide emissions.

W02020007784 describes an alternative method of preparing foams. The method relies upon the provision of a suspension comprising an aqueous liquid, particles, such as fly ash particles and earth particles, and at least one surfactant, wherein the at least one surfactant at least partially hydrophobizes a surface of the particles. Following foaming, the foam can be subjected to sintering or hardening. The sintering process requires exposure of the foam to high temperatures from about 800 °C to about 1800 °C for several hours depending on the samples size and material thermal conductivity. Sintering is per se energy-intensive and associated with high carbon dioxide emissions. When applied to thermal insulating foams, which hinder the heat diffusion across the foam, sintering reduces the process efficiency becoming even more energetically costly. The hardening process requires the addition of hardening agents, such as sodium silicate, to the suspension and a maturation period. The low temperature hardening is more environmentally friendly than the methods including sintering. Nevertheless, the foams obtained via said methods can still embody high carbon footprint depending on the used raw materials and their concentrations.

In consequence, there is a need for solid mineral or inorganic foams, having a low or even neutral embodied carbon footprint without impacting the thermal and mechanical properties of said foams. Further, there is a need for improved methods for manufacturing of such foams, specifically in regard to manufacturing time, energy consumption and carbon dioxide emissions.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a method for manufacturing an ultralight solid, inorganic foam, wherein the foam has a low embodied carbon footprint due to mineral carbonation and improved properties, particularly mechanical properties. This object is achieved by the method for manufacturing according to claim 1.

Also claimed and described herein is a solid, inorganic foam comprising an inorganic material entrapping nano-size pores, meso-size pores and optionally macro-size pores in the shape of bubbles, wherein the inorganic material contains on a surface facing the meso-size pores and the macro-size pores at least one of a crystalline sodium sesquicarbonate, a crystalline potassium sesquicarbonate and a crystalline calcium carbonate, and the foam has a density from about 20 kg/m ³ to about 300 kg/m ³ . The ultralight solid, inorganic foam claimed and described herein has a low embodied carbon dioxide footprint, and improved mechanical properties, in particular improved mechanical strength.

A further aspect according to the present invention is directed to a use of the foam claimed and described herein as a thermal insulating material, a construction or building material, a filtering material, a catalyst support material, a sound insulating material, or a fire resistant material.

An additional aspect according to the present invention relates to a use of an industrial grade carbon dioxide gas and/or of an exhaust gas comprising at least 4 vol% CO2 for manufacturing the solid, inorganic foam, particularly the solid, inorganic foam claimed and described herein. SHORT DESCRIPTION OF THE FIGURES

Figure 1 shows a micrograph of a pore present in a foam according to the present invention. Needle-shaped crystalline sodium sesquicarbonate are present on surfaces facing the pore. The micrograph was obtained with the foam according to example 4.

Figure 2 shows a close-look micrograph of a pore present in a foam according to the present invention. The surface of the bubble-shaped pore is decorated with needle-shaped crystalline sodium sesquicarbonate. The micrograph was obtained with the foam according to example 4.

Figure 3 shows a comparison of the compressive strength of foams according to the present invention (empty bars) that were obtained by the process according to the present invention versus foams that were not subjected to a curing step (d) as described herein (patterned bars). The compressive strength was measured on foams prepared as described in example 2.

DETAILED DESCRIPTION OF THE INVENTION

It is an object of the present invention to address the need for solid mineral or inorganic foams, preferably ultralight (density lower than 300 kg/m ³ ), having a low or even neutral embodied carbon footprint without impacting the thermal and mechanical properties of said foams and for improved methods for manufacturing such foams, specifically in regard to manufacturing time, energy consumption and carbon dioxide emissions. The objective is achieved by the method for manufacturing a solid, inorganic foam according to claim 1 , a solid, inorganic foam comprising an inorganic material entrapping pores according to claim 13, a use of the solid, inorganic foam according to claim 16, and a use of an industrial grade carbon dioxide gas and/or of an exhaust gas comprising at least 4 vol% CO2 according to claim 17. Preferred embodiments are disclosed in the specification and the dependent claims.

The present invention will be described in more detail below.

Where the present description refers to “preferred” embodiments/features, combinations of these “preferred” embodiments/features are also deemed to be disclosed as long as the specific combination of the “preferred” embodiments/features is technically meaningful.

Unless otherwise stated, the following definitions shall apply in this specification:

As used herein, the term "a", "an", "the" and similar terms used in the context of the present invention (especially in the context of the claims) are to be construed to cover both the singular and plural unless otherwise indicated herein or clearly contradicted by the context.

As used herein, the term “and/or” means that either all or only one of the elements of said group may be present. For example, “A and/or B” means “only A, or only B, or both A and B”. In the case of “only A”, the term also covers the possibility that B is absent, i.e. “only A, but not B”.

As used herein, the terms "including", "containing" and "comprising" are used herein in their open-ended, non-limiting sense. It is understood that the various embodiments, preferences and ranges may be combined at will. Thus, for instance a solution comprising a compound A may include other compounds besides A. However, the term “comprising” also covers, as a particular embodiment thereof, the more restrictive meanings of “consisting essentially of” and “consisting of, so that for instance “a solution comprising A, B and optionally C” may also (essentially) consist of A and B, or (essentially) consist of A, B and C. As used herein, the transitional phrase “consisting essentially of” (and grammatical variants) is to be interpreted as encompassing the recited materials or steps and those that do not materially affect the basic and novel characteristic(s) of the claimed invention. Thus, the term “consisting essentially of” should not be interpreted as equivalent of “comprising”.

As used herein, the term "about" means that the amount or value in question may be the specific value designated or some other value in its neighborhood. Generally, the term "about" denoting a certain value is intended to denote a range within ± 5 % of the value. As one example, the phrase "about 100" denotes a range of 100 ± 5, i.e. the range from 95 to 105. Preferably, the range denoted by the term "about" denotes a range within ± 3 % of the value, more preferably ± 1 %. Generally, when the term "about" is used, it can be expected that similar results or effects according to the invention can be obtained within a range of ±5 % of the indicated value.

Surprisingly, it has been found that a method comprising the steps:

(a) providing an aqueous suspension, the suspension comprising, or consisting of:

■ inorganic particles,

■ at least one surfactant, which at least partially hydrophobizes a surface of the inorganic particles,

■ water soluble silicates,

■ optionally one or more additives; and

(b) foaming the suspension of step (a) to thereby obtain a wet, inorganic foam;

(c) optionally drying, or partially drying the foam of step (b) to thereby obtain a dry or partially dry inorganic foam; and

(d) curing the inorganic foam of step (b) or (c) by exposing it to a CO2 containing gas with a CO2 concentration of at least 4 vol-%, and a relative humidity from about 5% to about 95 %, provides a solid, inorganic foam with a low embodied carbon footprint, very low density, and improved mechanical properties, in particular improved mechanical strength. The solid inorganic foam presents a marked closed-cell morphology. The use of step (d) in combination with the water soluble silicates in the manufacturing method claimed and described herein significantly reduces the hardening time and the carbon footprint of the obtained solid, inorganic foam.

In one embodiment, the method comprises the steps of:

(a) providing an aqueous suspension, the suspension comprising, or consisting of:

■ inorganic particles,

■ at least one surfactant, which at least partially hydrophobizes a surface of the inorganic particles,

■ water soluble silicates,

■ optionally one or more additives;

(b) foaming the suspension of step (a) to thereby obtain a wet, inorganic foam; (c) drying or partially drying the foam of step (b) to thereby obtain a dry or partially dry inorganic foam; and

(d) curing the dry or partially dry inorganic foam of step (c) by exposing it to a CO2 containing gas having a CO2 concentration of at least 4 vol% and a relative humidity from 5% to 95% to thereby obtain a cured, inorganic foam.

The aqueous suspension may consist of inorganic particles, at least one surfactant, which at least partially hydrophobizes a surface of the inorganic particles, water soluble silicates, optionally one or more additives, and water.

The term “solid, inorganic foam” refers to a hardened or cured foam made of a solid inorganic material surrounding gas-filled voids or cells. Preferably, the solid, inorganic foam is a closed-cell foam i.e. a foam wherein the gas forms discrete cells, each completely surrounded by the solid material. Open-cell foams are porous foams having a large number of interconnected pockets or voids or cells within their solid material. The solid, inorganic foam claimed and described herein has preferably a water content lower than 50 wt-%, more preferably lower than 20 wt-% e.g. lower than 10 wt-% based on the total weight of the solid, inorganic foam.

The wet, inorganic foam obtained at step (b) is a soft foam that can be subjected to casting, extruding or additive manufacturing, in particular 3D-printing.

The inorganic material contained by the solid, inorganic foam may contain up to about 10 wt-%, preferably up to about 5 wt-% of organic compounds.

As used herein, the term “partially dry inorganic foam” relates to a foam having a water content lower than the water content present in the suspension provided at step (a) and higher or equal to about 5 wt-%, the wt-% being based on the total weight of the partially dry inorganic foam. In some embodiments, the partially dry inorganic foam has a water content from about 5 wt-% to about 70 wt-%, in particular from about 5 wt-% to about 60 wt-%, wherein the wt-% are based on the total weight of the partially dry inorganic foam. Knowing the total water content in the suspension and the weight of the suspension used at step (a), the water content of the partially dry inorganic foam can be easily determined by weighing. As used herein, the term “dry inorganic foam” relates to a foam having a water content lower than about 5 wt-% with the wt-% being based on the total weight of the dry inorganic foam as determined by weighing the dry inorganic foam and knowing the total water content in the suspension and the weight of the suspension used at step (a). The dry inorganic foam has preferably a water content higher than about 0.1 wt-%, more preferably higher than about 1 wt-% with the wt-% being based on the total weight of the dry inorganic foam as determined by weighing knowing the total water content in the suspension and the weight of the suspension used at step (a).

In the manufacturing method claimed and described herein, preferably

• the inorganic particles are selected from the group consisting of secondary raw material particles, naturally occurring mineral particles, and mixtures thereof; and/or

• the at least one surfactant has a backbone chain comprising at least nine carbon atoms, preferably the at least one surfactant is an amphiphilic molecule consisting of a tail coupled to a head group, wherein the tail comprises the backbone chain comprising at least nine carbon atoms; and/or • the water soluble silicates are selected from the group consisting of sodium silicates, potassium silicates, calcium silicates and combinations thereof, in particular sodium silicates, potassium silicates and combinations thereof; and/or

• the additives are selected from the group consisting of stabilizers, plasticizers, superplasticizer, retarders, accelerators, binding agents, wetting agents, gas generating agents and catalysts thereof, hardening agents, and rheology modifiers; and/or

• the CO2 containing gas is selected from an industrial grade CO2 gas, an exhaust gas comprising at least 4 vol% CO2, a gas containing at least 4 vol% captured CO2, and a combination thereof.

The inorganic particles used herein comprise an amorphous part containing Al ³⁺ cations (e.g. amorphous aluminum oxide; amorphous aluminum silicate). The water-soluble silicates react with the amorphous parts of the inorganic particles leading to aluminosilicate gel network, including tetrahedral Al ³⁺ sites charge balanced by alkali metal cations.

As used herein, the term “secondary raw material” includes, but is not limited to, excavated materials, filter cakes from excavated materials, expanded perlite, fly ash, metakaolin, mineral processing tailings, catalyst residues, coal bottom ash, rice husk ash, palm oil ash, waste glass, paper sludge ash, paper ash, sludge from water treatments, ground granulated blast-furnace slags (GGBS), microsilica (silica fume) calcium carbonate and ceramic waste material and combination thereof, in particular excavated materials, filter cakes from excavated materials, fly ash, mineral processing tailings, catalyst residues, coal bottom ash, rice husk ash, palm oil ash, waste glass, paper sludge ash, paper ash, sludge from water treatments, ground granulated blast-furnace slags (GGBS), calcium carbonate, ceramic waste material and combinations thereof.

In certain embodiments, the secondary raw materials include expanded perlite, fly ash, metakaolin, mineral processing tailings, catalyst residues, coal bottom ash, rice husk ash, palm oil ash, paper sludge ash, paper ash, sludge from water treatments, ground granulated blast-furnace slags (GGBS), microsilica (silica fume) and combinations thereof. As well known, fly ash, or flue ash, or coal ash, or pulverized coal ash is a heterogeneous material with silicon dioxide (SiO2), aluminum oxide (AI2O3), iron oxide (Fe2Oa) and occasionally calcium oxide (CaO) being the main chemical components.

Examples of suitable naturally occurring minerals include, but are not limited to, phyllosilicates, such as serpentine, clay, and mica, feldspar, silica, perlite, calcium carbonate and combinations thereof, in particular, phyllosilicates, such as serpentine, clay, and mica, feldspar, silica, perlite, and combinations thereof. Preferably, the naturally occurring mineral is selected from serpentine, clay, mica, perlite, calcium carbonate and combinations thereof, in particular from serpentine, clay, mica, perlite, and combinations thereof. More preferably, the naturally occurring mineral selected from clay, perlite, calcium carbonate and combinations thereof, in particular from clay, perlite and combinations thereof. As well known, clay is a fine-grained natural soil material containing one or more clay minerals (hydrous aluminum phyllosilicates) with possible traces of quartz (SiO2) and metal oxides, such as aluminum oxide (AI2O3) and magnesium oxide (MgO). Preferably, the clay mineral is selected from kaolin, montmorillonite-smectite, illite, chlorite, vermiculite, talc, pyrophyllite, halloysite, sepiolite, palygorskite and mixtures thereof. In certain embodiments, the inorganic particles include phyllosilicates, such as serpentine, clay, and mica, feldspar, silica, calcium carbonate, fly ash, metakaolin, microsilica (silica fume), and combinations thereof.

In other embodiments, the inorganic particles used in the aqueous suspension described herein are selected from the group consisting of fly ash particles, clay particles, metakaolin particles, perlite particles, including expanded perlite particles, and combinations thereof.

The inorganic particles have preferably a particle size from about 1 nm to about 500 pm, more preferably from about 50 nm to about 200 pm, and even more preferably from about 200 nm to about 65 pm as determined by scattering methods and/or image analysis. The particle size corresponds to the mean particle size measured for the largest dimension and depends on the origin of the particles. In the case of fly ash, for example, the fly ash particles are generally spherical in shape and range in size from about 0.5 pm to about 300 pm. If desired, the particle size can be adjusted by sieving or ball milling techniques as commonly known in the present field of technology.

The suspension described herein contains preferably from about 10 wt-% to about 70 wt-%, more preferably from about 10 wt-% to about 50 wt-%, much preferably from about 15 wt-% to about 40 wt-% inorganic particles, the wt-% being based on the total weight of the suspension.

The suspension provided at step (a) contains at least one surfactant, which at least partially hydrophobizes a surface of the inorganic particles. The use of inorganic particles in combination with at least one surfactant, which at least partially hydrophobizes a surface of the inorganic particles leads to a significant increase in the stability from a few minutes to months of the resulting wet, inorganic foam. Here, the stability of the wet, inorganic foams results from i) the adsorption of the particles having the at least partially hydrophobized surface at the liquid-gas interface, i.e. interface stabilization due to the adsorption of the surface- modified particles on the surface of the bubbles and from ii) bulk stabilization due to the formation of a percolating network of particles throughout the aqueous liquid.

As well-known, a surfactant or a surface-active agent is an amphiphilic compound i.e. a compound containing at least one hydrophilic group (head) and at least one hydrophobic group (tail). By adding an amphiphilic surfactant to the suspension comprising the inorganic particles, depending on the type of surfactant, an initially hydrophobic or lyophobic particle surface can be rendered more hydrophilic or lyophilic, and an initially hydrophilic or lyophilic particle surface can be rendered more hydrophobic or lyophobic, respectively. The meaning of the terms “hydrophobic”, “lyophobic”, “hydrophilic” and “lyophilic” as used herein corresponds to the generally known meaning of these terms. For example, hydrophilic/lyophilic means readily dispersed by water/a solvent or readily absorbing water/a solvent, whereas hydrophobic/lyophobic means the opposite. Preferably, the at least one surfactant present in the suspension described herein has a backbone chain comprising at least nine carbon atoms. Compared to surfactants having a lower backbone chain, a surfactant having a backbone chain comprising at least nine carbon atoms, also referred herein as a long-chain surfactant, significantly reduces the amount of surfactant required for hydrophobizing the inorganic particles and creating the foam. Preferably, the long-chain surfactant has a molecular weight higher than around 300 g/mol. Such surfactant further increases the stability of the foam upon hydrophobic interactions-induced suspension gelation.

In the present context, the term “backbone chain” refers to the longest series of covalently bonded atoms that together create a continuous chain of the molecular structure of the surfactant. Hence, the backbone chain of a surfactant having a backbone chain comprising at least nine carbon atoms can comprise at least nine carbon atoms being covalently connected with each other (linear backbone chain) or at least nine carbon atoms some of which being covalently connected to other atoms (branched backbone chain), or at least nine carbon atoms part of them forming a cyclic ring. Due to the fact that the surfactants have a backbone chain of at least nine carbon atoms, the particles will be hydrophobized upon adsorption of the surfactant.

The at least one surfactant is preferably an amphiphilic molecule consisting of a tail coupled to a head group, wherein the tail comprises the backbone chain comprising at least nine carbon atoms. The at least one surfactant is preferably selected from the group consisting of polyelectrolytes, proteins, polysaccharides, glycerols, glycerides such as monoglycerides, diglycerides and triglycerides, fatty acids such as oleic acid or linoleic acid, ammonium compounds, alkyl compounds, or combinations thereof. That is, the surfactant used to partially hydrophobize the surface of the particles in the suspension can be a polyelectrolyte and/or a protein and/or a polysaccharide and/or a glycerol and/or a glyceride and/or a fatty acid and/or an ammonium compound and/or an alkyl compound, which in each case comprises a backbone chain comprising at least nine carbon atoms.

A suitable polysaccharide surfactant is chitosan. A suitable triglyceride surfactant is Miglyol® 812. Imwitor® 988 is an example of a conceivable monoglyceride surfactant and diglyceride surfactant, respectively. Examples of suitable polyelectrolyte surfactants include polyacrylic acid, polystyrene sulfonate and polyallylamine hydrochloride.

The polyelectrolytes in the suspension can be anionic, cationic, or zwitterionic, and/or the proteins in the suspension can be anionic, cationic, non-ionic or zwitterionic, and/or the polysaccharides in the suspension can be anionic, cationic, non-ionic or zwitterionic.

The polyelectrolytes and/or the proteins and/or the polysaccharides and/or the glycerols and/or the glycerides and/or the fatty acids and/or the ammonium compounds and/or the alkyl compounds preferably have at least one group selected from bromides, chlorides, amines, phosphates, phosphonates, sulfates, amides, carboxylic acids, pyrrolidines, betaines or gallates or corresponding salts thereof. In this case, the group selected from bromides, amines, phosphates, phosphonates, sulfates, amides, carboxylic acids, pyrrolidines, betaines or gallates or corresponding salts thereof corresponds to the above-mentioned head of the amphiphilic surfactant.

A suitable gallate-compound is lauryl gallate, a suitable betaine-compound is cocamidopropyl betaine, and a suitable amine-compound is a primary, a secondary or a tertiary amine compound, wherein the one or more substituents are preferably alkyl or aryl groups, respectively.

The polyelectrolytes correspond to molecules bearing anionic or cationic or zwitterionic dissociable groups as they are known in the state of the art. These groups dissociate in aqueous solutions such as water, making the molecules charged. The polyelectrolytes can be classified as either weak or strong types, wherein a strong polyelectrolyte is one that carries a charge independent of the pH-value of the aqueous solution, whereas a weak polyelectrolyte is one whose degree of dissociation depends from the pH-value of the aqueous solution. Here, it is conceivable to use both types of polyelectrolytes as surfactants.

The at least one surfactant is preferably a glycerol monostearate-based compound, such as Cremodan SE 019 available from Danisco ingredients, a quaternary ammonium salt, preferably a quaternary ammonium salt, or a nonylamine. Suitable quaternary ammonium salts include cetrimonium bromide (CTAB), and tetradecyltrimethylammonium bromide (TTAB).

The at least one surfactant can be present in amounts from 0.001 wt-% to 5 wt-% based on the amount of inorganic particles, preferably from 0.01 wt-% to about 2 wt- % based on the amount of inorganic particles. Hence, regarding the particularly preferred surfactants above it is to be noted that preferred amounts are from 0.001 wt-% to about 5 wt-% of nonylamine based on the amount of inorganic particles, from 0.001 wt-% to 5 wt-% of CTAB based on the amount of inorganic particles, from 0.001 wt-% to 5 wt-% of TTAB based on the amount of inorganic particles, and from 0.001 wt-% to about 5 wt-% of Cremodan based on the amount of inorganic particles, respectively. The particularly preferred amounts depend on the nature and composition of the inorganic particles. For example, it is particularly preferred to use about 0.09 wt-% of TTAB based on the amount of inorganic particles. It is likewise particularly preferred to use about 1.15 wt-% of Cremodan per total weight of the inorganic particles, from about 0.1 wt-% to about 1.5 wt-% of CTAB per total weight of the inorganic particles, and from about 0.05 wt-% to about 0.5 wt-% of nonylamine per total weight of the inorganic particles.

Preferably, the water soluble silicates are selected from the group consisting of sodium silicates, potassium silicates calcium silicates, and combinations thereof, in particular from the group consisting of sodium silicates, potassium silicates, and combinations thereof. The water soluble silicates may be solubilized in hydroxide (e.g. NaOH, KOH, LiOH) prior to addition to the suspension. The water-soluble silicates react with the amorphous parts of inorganic particles leading to aluminosilicate gel network, including tetrahedral Al ³⁺ sites charge balanced by alkali metal cations. Furthermore, the water-soluble silicates react with the carbon dioxide in presence of water to form the corresponding carbonate forms. Water soluble silicates are preferably present in the suspension described herein in an amount from about 10 wt-% to about 30 wt-%, more preferably from about 15 wt-% to about 30 wt-%, much preferably from about 20 wt-% to about 30 wt-%, the wt-% being based on the total weight of the suspension.

Preferably, the sodium silicates are of general formula (Na2O) (SiC>2)x, with x being from 1 to 3.5. More preferably, the sodium silicates are selected from Na2SiOs (sodium metasilicate), Na2SiOs, Na4SiC>4 (sodium orthosilicate), NaeSi2O? (sodium pyrosilicate), and mixtures thereof.

Preferably, the potassium silicates are of general formula (K2O) (SiO2) _y , with y being from 1 to 3. More preferably, the potassium silicates are selected from K2SiOa (potassium metasilicate), K ₂ SiC>5, K4SiO4 (potassium orthosilicate), KeSi2O? (potassium pyrosilicate), and mixtures thereof. Preferably, the calcium silicates are of general formula (CaO) (SiC>2)z, with z being from 0.33 to 2. More preferably, the calcium silicates are selected from CaSiCh (wollastanite), Ca2SiC>4 (calcium orthosilicate), CasSiOs (alite), CasSiO?, and mixtures thereof. Calcium silicates can originate from precipitation of dissolved silicates in solution in presence of calcium cations and/or from dissolution of solid silica in highly alkaline solutions in presence of calcium ions. In a preferred embodiment, the water-soluble silicates are sodium silicates as described herein.

The suspension described herein may optionally contain one or more additives, which are preferably selected from the group consisting of stabilizers, plasticizers, superplasticizers, retarders, accelerators, binding agents, wetting agents, gas generating agents and catalysts thereof, hardening agents, and rheology modifiers. In some embodiments, the one or more additives are selected from the group consisting of stabilizers, plasticizers, superplasticizers, retarders, accelerators, binding agents, wetting agents, gas generating agents, hardening agents, and rheology modifiers.

The total amount of the additives in the suspension does not exceed preferably 25 wt-%, more preferably 10 wt-%, based on the total weight of the suspension.

Suitable stabilizers for limiting cracks in the solid, inorganic foams are cellulose- compounds, such as cellulose-based microfibers, methyl cellulose, hydroxypropyl cellulose, or microcrystalline cellulose such as e.g. Vivapur™, a mixture of microcrystalline cellulose and sodium carboxymethylcellulose. Moreover, these agents also serve the purpose of increasing the foam stability. The stabilizers may be present in an amount of up to about 10 wt-% per total weight of the inorganic particles, preferably up to about 4 wt-% by weight per total weight of the inorganic particles, more preferably up to about 2 wt-% per total weight of the inorganic particles, particularly preferably up to about 0.1 wt-% per total weight of the inorganic particles.

In order to reduce the viscosity of the suspension, both organic and inorganic plasticizers and/or superplasticizers can be used. The former comprises lignosulfonates, naphthalene, melamine or polycarboxylate compounds, for example sulfonate-based naphthalene or sulfonate-based melamine or polycarboxylate ether, can be used. The latter comprises sodium compounds as sodium hexametaphosphates.

The setting time can be adjusted by the use of retarders such as lignosulphonates, hydroxycarboxylic acid and their salts, phosphonates, saccharides, phosphates, gluconates, borates and salts of lead, zinc, arsenic or antimony, for example and/or by the use of accelerators. Examples of suitable accelerators include calcium chloride, potassium chloride, alkali hydroxides, calcium aluminate, sodium aluminate, aluminum sulfate, aluminum hydroxide, nitrite, lime, quick lime, anhydrite, and combinations thereof.

Addition of binding agents such as polymers, in particular polyvinyl acetate (PVAc), polyvinyl alcohol (PVA), polyethylene glycol (PEG), or starch may be useful to prevent shrinkage or cracking of the solid, inorganic foam.

Wetting agents such as alcohols, oils, silicones, silanes, siloxanes or glycols, in particular alcohols, oils, silicones, or glycols, can be used for modifying a contact angle and serve the purpose of tuning the water adsorption behavior of the foam.

If an in-situ foaming is desired, gas generating agents such as aluminum powder or hydrogen peroxide can be added to the suspension described herein. The gas generating agent may be present in the suspension described herein for example in an amount from about 0.1 wt-% to about 5 wt-%, in particular from about 1 wt-% to about 5 wt-%, such as about 3 wt-% or about 2 wt-% , with the wt-% being based on the total weight of the suspension. The gas generating agent may be used in combination with a catalyst. Examples of catalysts include iron oxides, iron hydroxides, iron(lll) oxide hydroxide (geothite), iron chlorides, copper (I) oxide, manganese dioxide, titanium (IV) dioxide, iron (II) carbonate, potassium permanganate, potassium iodide, and nickel (II) oxide.

Where required, the aqueous suspension may further contain a hardening agent. If present, the amount of the hardening agent shall not exceed 20 wt-% based on the total weight of the suspension. As used herein, the term hardening agent refers to a hydraulic binder, such as cements, quicklime (CaO), stucco, calcium sulfate, calcium sulfate hemi-hydrated, calcium aluminate, portlandite (Ca(OH)2), Mg(OH)2, MgO and combinations thereof, in particular cements, quicklime (CaO), stucco, calcium sulfate, calcium sulfate hemi-hydrated, calcium aluminate, and combinations thereof. Examples of cements include cements CEM l-V, calciumsulfoaluminates (CSA), calciumaluminate cements (CaC). In certain embodiments, the aqueous suspension is free of hardening agent.

Moreover, a rheology modifier such as fumed silica, cellulose, (e.g. cellulose fibers), salt insensitive superabsorbers, such as poly(acrylamide-co-acrylic acid), glass (e.g. glass fibers), carbon (e.g. carbon fibers) or combinations thereof, in particular fumed silica, cellulose (e.g. cellulose fibers), salt insensitive superabsorbers, such as poly(acrylamide-co-acrylic acid), or combinations thereof can be used. This is in particular useful if the wet, inorganic foam is subsequently subjected to extrusion or 3D-printing. In this context the rheology modifier serves the purpose of improving the printability of the wet, inorganic foam, wherein said printabilityimproving agent can be added to the suspension, or to the wet, inorganic foam. The rheology modifier may be present in an amount up to about 30 wt-%, preferably up to about 15 wt-% based on the total weight of the inorganic particles, more preferably up to about 10 wt-% based on the total weight of the inorganic particles. If ash particles are used as inorganic particles and the rheology modifier is fumed silica, the amount of said fumed silica is preferably from about 1 wt-% to about 5 wt-% based on the total weight of the inorganic particles. If ash particles are used as inorganic particles and the rheology modifier is cellulose, the amount of said cellulose is preferably from about 0.1 wt-% to about 3 wt-% based on the total weight of the inorganic particles.

The CO2 containing gas used at step (d) of the inventive manufacturing method is preferably selected from an industrial grade CO2, an exhaust gas comprising at least 4 vol% CO2 and a gas containing at least 4 vol% captured CO2. Preferably, the CO2 containing gas contains at least 10 vol% CO2, more preferably at least 20 vol% CO2, most preferably at least 50 vol% CO2. In a preferred embodiment, the gas contains at least 80 vol% CO2, preferably at least 90 vol% CO2, more preferably at least 95 vol% CO2. The exhaust gas comprising at least 4 vol% CO2 may originate from a combustion engine, a coal- or gas- or petroleum- or wood-fired power plant, a kiln or a volcanic activity. The gas containing at least 4 vol-% captured CO2 may be obtained by direct CO2 capture from the air using the known adsorption/desorption technologies, or by CO2 capture using the known adsorption/desorption technologies from large point sources, such as large fossil fuel or biomass electricity power plants, industries with major CO2 emissions, natural gas processing, synthetic fuel plants and fossil fuel-based hydrogen production plants. In a preferred embodiment, the CO2 containing gas used at step (d) of the inventive manufacturing method is an exhaust gas comprising at least 4 vol-% CO2 as described herein, preferably an exhaust gas from a combustion engine, or a coal- or gas- or petroleum- or wood-fired power plant or a kiln. In another preferred embodiment, the CO2 containing gas used at step (d) of the inventive manufacturing method is an industrial grade CO2 gas.

Advantageously, the solid, inorganic foam obtained by the manufacturing process claimed herein is characterized by one or more of the following properties:

• a density from about 20 kg/m ³ to about 300 kg/m ³ , preferably from about 20 kg/m ³ to about 100 kg/m ³ , as determined by volumetric calculation;

• a thermal conductivity from about 20 mW/(mK) to about 100 mW/(mK) as determined with a thermal conductimeter, preferably a heat flow meter;

• a compressive strength from about 1 kPa to about 4 MPa as determined with a mechanical tester;

• a porosity from about 80 vol% to about 99 vol%, preferably from about 80 vol-% to about 98 vol-% as determined by the complement to one of the porous material’s specific density;

• a surface area as measured by Brunauer-Emmett-Teller method of at least about 1 m ² /g;

• a bubble size for the macro-size pores lower than 1.5 mm.

The manufacturing method claimed and described herein enables the production of ultra-light, solid, inorganic foams, and in particular having a density from about 20 kg/m ³ to about 300 kg/m ³ , preferably from about 20 kg/m ³ to about 100 kg/m ³ . Such foams can be advantageously used as construction or building materials.

The manufacturing method claimed and described herein enables the production of solid, inorganic foams having excellent thermal properties, and in particular very low thermal conductivity from about 20 mW/(mK) to about 100 mW/(mK). Such foams can be advantageously used as thermal insulating material. Thermal conductivity (also called lambda N) is a physical value characterizing the behaviour of materials during the transfer of heat by conduction. Thermal conductivity represents the quantity of heat transferred per unit of surface and per unit of time submitted to a gradient of temperature. In the international system of units, thermal conductivity is expressed in watts per metre Kelvin, (W/mK).

Furthermore, the manufacturing method claimed and described herein enables the production of solid, inorganic foams having good mechanical properties, in particular good mechanical strength from about 1 kPa to about 4 MPa.

Moreover, the manufacturing method claimed and described herein enables the production of solid, inorganic foams having a surface area as measured by Brunauer- Emmett-Teller (BET) method of at least about 1 m ² /g and/or a bubble size for the macro-size pores lower than 1.5 mm. This is particularly advantageous because foams having a surface area as measured by BET method of at least about 1 m ² /g present lower thermal conductivities for similar mechanical strength compared to foams with lower surface area (less than 1 m ² /g).

The suspension provided at step (a) preferably contains ■ up to 70 wt-% water,

■ 10-70 wt- % inorganic particles as described herein,

■ 0.001 - 5 wt-% surfactants as described herein based on the amount of inorganic particles,

■ 10-30 wt-% water soluble silicates as described herein, and

■ optionally 0.001-25 wt-%, particularly 0.001-10 wt-%, more particularly 0.001-5 wt-% additives as described herein, with the wt-% being based on the total weight of the suspension.

In a preferred embodiment, the suspension provided at step (a) preferably contains

■ 35 - 60 wt-% water,

■ 15 - 40 wt- % inorganic particles as described herein,

■ 0.001 - 5 wt-% surfactants as described herein based on the amount of inorganic particles,

■ 20-30 wt-% water soluble silicates as described herein, and

■ optionally 0.001-25 wt-%, particularly 0.001-10 wt-%, more particularly 0.001-5 wt-% additives as described herein, with the wt-% being based on the total weight of the suspension.

The surface modification of the particles, i.e. the hydrophobization of the inorganic particles, is achieved by the physical and/or chemical adsorption of the surfactant described herein on the surface of the inorganic particles. The pH value of the suspension depends on the type and amount of inorganic particles used. Moreover, dependent upon the charge of the surface to be coated by the surfactants, as well as dependent upon the charge of the surfactants, either lower or higher pH conditions are preferred. It might therefore be desirable to adjust the pH value of the suspension to create an optimal chemical environment for surface modification, foaming or further processing of the foams. Thus, in an embodiment, step (a) further comprises adjusting the suspension to a pH value from about 3 to about 14, preferably from about 8 to about 14. Depending on the composition of the particles, a particular pH value will yield a better adsorption of the surfactant on the particles’ surface. For example, alumina (AI2O3) particles have a positive charge at a pH-value of 3-7. Under these conditions it is therefore preferred to use negatively-charged surfactants since the adsorption of a negatively- charged surfactant on positively-charged particles is enhanced. Likewise, an improved interaction is obtained between e.g. silica particles and a positively-charged surfactant. In other words, a preferred electrostatic adsorption of the surfactants to the particle surface is achieved if the surfactants and the particles have opposite charges. A preferred pH value is then a pH value at the pKa value, i.e. the logarithmic acid dissociation constant of the surfactant. The pH value can be adjusted by means of adding a basic or acidic compound or solution to the suspension. In doing so hydrochloric acid (HCI) and sodium hydroxide (NaOH) are commonly used for adjusting the pH value.

In the manufacturing method claimed and described herein, step (b) preferably comprises:

■ mechanically foaming the suspension of step (a), and/or

■ in situ-foaming the suspension of step (a) by adding a gas generating agent to the suspension, and/or ■ injecting and/or bubbling a gas in the suspension of step (a).

Mechanically foaming the suspension of step (a) may be achieved for example by subjecting the foamable suspension to a high-speed agitation while the foamable suspension is exposed to a gas, such as air (exposure to ambient atmosphere). The agitation can be earned out by a mixer and for a sufficient period of time, during which time bubbles of a gas, such as air, are introduced into the foamable suspension until a desired expansion has reached. Other ways of introducing the gas into the foamable suspension are for example by means of bubbling a gas through a filter into the foamable suspension or by means of injecting pressurized gas through a nozzle into the foamable suspension. Through the choice of the pore size of the filter or the diameter of the ejection nozzle it is possible to adjust the pore size of the solid, inorganic foams.

In situ-foaming the suspension of step (a) requires adding a gas generating agent, such as hydrogen peroxide (H2O2) or aluminium powder, wherein the generated gas foams the foamable suspension. The gas generating agent may be present in the suspension described herein for example in an amount from 0.1 wt-% to about 5 wt-%, in particular from about 1 wt- % to about 5 wt-%, such as about 3 wt-%. The gas generating agent may be used in combination with a catalyst.

It should be understood that the more gas is incorporated into the foamable suspension or generated in the foamable suspension the more porous the thus generated foam is, wherein the porosity levels reached are also dependent on the inorganic particle size, the inorganic particle type and the inorganic particle concentration. It should furthermore be noted that the foams can be generated within a few minutes only, wherein there is no need for any special treatment beyond the surface modification, i.e. the hydrophobization, of the inorganic particles with the surfactants described herein.

The wet, inorganic foams obtained at step (b) are typically soft i.e. they can be subjected to casting, extruding or additive manufacturing, in particular 3D-printing prior to drying and curing. If the foams are subjected to printing, it is conceivable, but not compulsory, to add a rheology modifier or a printability-improving agent, respectively, such as fumed silica to the suspension prior to foaming. Subsequently, the suspension can be foamed and discharged out of an orifice or nozzle. This can be obtained among others upon loading the foam in a cartridge and then pushing it out through an orifice or nozzle provided on the cartridge. The printing speed depends on the size of the orifice or nozzle and material flow rate. Preferably, an orifice size or nozzle size is in the range of about 0.2 mm to about 200 mm, particularly preferably in the range of about 10 mm to about 100 mm. Depending on the nozzle diameter different printing speeds can be obtained. In the case of a smaller nozzle diameter of about 0.4 mm, for example, printing speeds of about 1 mm/s to 15 mm/s, preferably of about 4 mm/s can be reached. In the case of a larger nozzle diameter of about 200 mm, for example, printing speeds of about 1 cm/s to 20 cm/s, preferably of about 5 cm/s can be reached.

In the manufacturing method claimed and described herein, step (c) is preferably performed at a temperature lower than about 80 °C, in particular at room temperature, and I or the dry or partially dry inorganic foam obtained at step (c) has a water content lower than about 50 wt-%, preferably lower than about 40 wt-%. Part of the drying is preferably conducted at room temperature without requiring additional heat. Preferably, at step (d) the dry or partially dry inorganic foam of step (c) or the wet, inorganic foam of step (b) is exposed to the CO2 containing gas for a time period of 0.25 to 100 hours, preferably for less than 50 hours, more preferably for less than 40 hours and/or step (d) is conducted at a temperature lower than about 80 °C, preferably without applying additional heat, more preferably at room temperature, and a relative humidity from about 5% to about 95%, preferably from about 20% to about 90%. In certain embodiments, the CO2 containing gas used at step (d) originates from a combustion engine, a coal- or gas- or petroleum- or wood-fired power plant, a kiln or a volcanic activity and is used at the temperature at which it is generated without any cooling step. Advantageously, the water soluble silicates present in the suspension and the exposure of inorganic foam to CO2 containing gas with a CO2 concentration of at least 4 vol-%, preferably of at least 10 vol-%, more preferably of at least 20 vol-% and a relative humidity from about 5% to about 95%, preferably from about 20% to about 90%, results in the expedient curing of the inorganic foam with formation of solid, inorganic foam. Without being bound by the theory, it is believed that the exposure of the dry or partially dry foam or of the wet, inorganic foam to a CCh-rich gas at a relative humidity from about 5% to about 95% leads to an acidification of the aqueous solution present in the pore interstice and fosters the polycondensation of silicates present in said aqueous solution. In certain embodiments, the curing is conducted at room temperature or ambient temperature, without requiring additional heat.

As used herein, the term “room temperature” or “ambient temperature” refers to a temperature from about 20 °C to about 35 °C.

A preferred embodiment according to the present invention is directed to a manufacturing method as claimed and described herein, wherein the method further comprising steps (e) and (f) conducted after step (d):

(e) washing the cured, inorganic foam obtained at step (d) with an aqueous solution and/or water to obtain a foam with a reduced carbonate content;

(f) drying the foam obtained at step (e).

Step (e) enables the washing of water-soluble carbonates, such as sodium carbonates and the potassium carbonates from the foam, and thereby the production of a solid, inorganic foam with a reduced carbonate content.

Step (f) is preferably performed at a temperature lower than about 80 °C, more preferably at room temperature without requiring additional heat. The solid, inorganic foam with a reduced carbonate content obtained at step (f) has preferably a water content lower than 20 wt-%, more preferably lower than 10 wt-%, with the wt-% being based on the total weight of the solid, inorganic foam.

An alternative preferred embodiment according to the present invention related to a manufacturing method as claimed and described herein, wherein the method further comprises step (g) conducted after step (d):

(g) drying the cured, inorganic foam obtained at step (d). Step (g) is preferably performed at a temperature lower than about 80 °C, more preferably at room temperature without requiring additional heat. It may be advantageous to conduct step (g) if the cured, inorganic foam obtained at step (d) has a water content higher than about 20 wt-%, so as to obtain a solid, inorganic foam having a water content lower than 20 wt-% more preferably lower than 10 wt-%, with the wt-% being based on the total weight of the solid, inorganic foam. Hence, a preferred method according to the present invention comprises the steps (a), (b), (c), (d) and (g) as described herein.

In a second aspect, the invention relates to a new solid, inorganic foam comprising an inorganic material entrapping nano-size pores, meso-size pores and optionally macro-size pores, wherein

■ the inorganic material contains on a surface facing the meso-size and macro-size pores at least one of a crystalline sodium sesquicarbonate, a crystalline potassium sesquicarbonate, and a crystalline calcium carbonate, and

■ the foam has a density from about 20 kg/m ³ to about 300 kg/m ³ , preferably from about 20 kg/m ³ to about 100 kg/m ³ .

The solid, inorganic foam may further contain one or more of the following: a crystalline phase selected from crystalline natrolite, crystalline calcite, and crystalline quartz, a sodium-calcium-alumino-silicate-hydrate gels, a salt and/or its hydrated form, wherein the salt is selected from sodium carbonate, sodium bicarbonate, calcium bicarbonate, sodium silicate, potassium carbonate, potassium bicarbonate, and potassium silicate.

As used herein, the term “crystalline sodium sesquicarbonate” or “crystalline trisodium hydrogendicarbonate” encompasses the anhydrous crystalline sodium sesquicarbonate and its hydrated forms, such as crystalline sodium sesquicarbonate monohydrate and crystalline sodium sesquicarbonate dihydrate. As used herein, the term “crystalline potassium sesquicarbonate” encompasses the anhydrous crystalline potassium sesquicarbonate and its hydrated forms. As used herein, the term “crystalline calcium carbonate” encompasses the anhydrous crystalline calcium carbonate and its hydrated forms.

Advantageously, the foam claimed and described herein is characterized by one or more of the following properties:

■ a thermal conductivity from about 20 mW/(mK) to 100 mW/(mK) as determined with a thermal conductimeter, preferably a heat flow meter,

■ a compressive strength from about 1 kPa and about 4 MPa as determined with a mechanical tester,

■ a porosity from about 80 vol% to about 99 vol%, preferably from about 80 vol-% to about 98 vol-% as determined by the complement to one of the porous material’s specific density,

■ a surface area as measured by Brunauer-Emmett-Teller method of at least about 1 m ² /g,

■ a bubble size for the macro-size pores lower than 1 .5 mm.

As used herein, the term “nano-size pores” refers to pores having a size of between 1 nm and 80 nm as determined Brunauer-Emmett-Teller (BET) method using a density functional theory (DFT) analysis based on a Non Local DFT (NLDFT) calculation model for nitrogen at 77 K on cylindrical pores in silica. As used herein, the term “meso-size pores” refers to pores having a size from 80 nm to 1 mm as determined by scanning electron microscopy i.e. includes both submicron-size (80 nm - 1 pm) pores and micro-size (1 pm - 1 mm) pores. As used herein, the term “macro-size pores” refers to pores having a size greater than 1 mm as determined by light microscopy using an inverted digital microscope in reflection mode (VHX 6000 with VH-K20 attachment, Keyence. SEM analysis was conducted using LEO 1530, instrument from Zeiss GmbH, Germany using a working distance of 4 mm and an acceleration voltage of 5 kV.

Preferably, the solid, inorganic foam claimed and described herein is obtained by the manufacturing method claimed and described herein.

Also claimed and described herein is a solid, inorganic foam having a density from 20 kg/m ³ to about 300 kg/m ³ , preferably from about 20 kg/m ³ to about 100 kg/m ³ , obtained by the manufacturing method claimed and described herein.

A third aspect according to the present invention relates to a use of the foam claimed and described herein as a thermal insulating material, a construction or building material, a filtering material, a catalyst support material, a sound insulation material or a fire-resistant material.

A fourth aspect according to the present invention is directed to a use of an industrial grade CO2 gas and/or an exhaust gas comprising at least 4 vol% CO2 for manufacturing a solid, inorganic foam, particularly the solid, inorganic foam claimed and described herein. Preferably, the exhaust gas is an exhaust gas from a combustion engine, a coal- , gas-, petroleum- or wood- fired power plant, a kiln, or a volcanic activity, in particular from a combustion engine, a coal- , gas-, or petroleum- fired power plant, or a kiln.

The present invention may be further summarized by reference to the following clauses #1 - #16

#1 A method for manufacturing a solid, inorganic foam, wherein the method comprises the steps of:

(a) providing an aqueous suspension, the suspension comprising:

■ inorganic particles,

■ at least one surfactant, which at least partially hydrophobizes a surface of the inorganic particles,

■ water soluble silicates,

■ optionally one or more additives;

(b) foaming the suspension of step (a) to thereby obtain a wet, inorganic foam;

(c) drying or partially drying the foam of step (b) to thereby obtain a dry or partially dry inorganic foam; and

(d) curing the dry or partially dry inorganic foam of step (c) by exposing it to a CO2 containing gas having a CO2 concentration of at least 4 vol% and a relative humidity from 5% to 95% to thereby obtain a cured, inorganic foam.

#2 The method according to #1 , wherein

■ the inorganic particles are selected from the group consisting of secondary raw material particles, naturally occurring mineral particles, and combinations thereof; and/or ■ the at least one surfactant has a backbone chain comprising at least nine carbon atoms, preferably the at least one surfactant is an amphiphilic molecule consisting of a tail coupled to a head group, wherein the tail comprises the backbone chain comprising at least nine carbon atoms; and/or

■ the water-soluble silicates are selected from the group consisting of sodium silicates, potassium silicates, calcium silicates and combinations thereof; and/or

■ the additives are selected from the group consisting of stabilizers, plasticizers, superplasticizer, retarders, accelerators, binding agents, wetting agents, gas generating agents, hardening agents, and rheology modifiers; and/or

■ the CO2 containing gas is selected from an industrial grade CO2 gas, an exhaust gas comprising at least 4 vol-% CO2, a gas containing at least 4 vol-% captured CO2, and a combination thereof.

#3 The method according to #1 or #2, wherein the solid inorganic foam is characterized by one or more of the following properties:

■ a density from about 20 kg/m ³ to about 300 kg/m ³ ;

■ a thermal conductivity from about 20 mW/(mK) to about 100 mW/(mK);

■ a compressive strength from about 1 kPa to about 4 MPa.

#4 The method according to any one of #1 to #3, wherein the suspension of step (a) contains

■ 40 - 60 wt-% water,

■ 15 - 40 wt- % inorganic particles,

■ 0.001 - 5 wt-% surfactants based on the amount of inorganic particles,

■ 20 - 30 wt-% water soluble silicates, and

■ optionally 0.001 - 5 wt-% additives.

#5 The method according to any one of #1 to #4, wherein step (a) further comprises adjusting the suspension to a pH value from about 3 to about 14.

#6 The method according to any one of #1 to #5, wherein step (b) comprises

■ mechanically foaming the suspension of step (a), and/or

■ in situ-foaming the suspension of step (a) by adding a gas generating agent to the suspension, and/or

■ injecting and/or bubbling a gas in the suspension of step (a).

#7 The method according to any one of #1 to #6, wherein step (c) is performed at a temperature lower than 80 °C and I or the dry or partially dry inorganic foam obtained at step (c) has a water content lower than about 50 wt-%.

#8 The method according to any one of #1 to #7, wherein at step (d) the dry or partially dry inorganic foam of step (c) is exposed to the CO2 containing gas for a time period from 15 minutes to 100 hours, preferably for less than 50 hours, more preferably for less than 40 hours and/or step (d) is conducted at a temperature lower than 80°C, preferably without applying additional heat, more preferably at room temperature.

#9 The method according to any one of #1 to #8 further comprising step (e) and (f) conducted after steps (d):

(e) washing the cured, inorganic foam obtained at step (d) with an aqueous solution and/or water to obtain a foam with a reduced carbonate content;

(f) drying the foam obtained at step (e).

#10 The method according to any one of #1 to #8 further comprising step (g) conducted after step (d):

(g) drying the cured, inorganic foam obtained at step (d).

#11 A solid, inorganic foam comprising an inorganic material entrapping nano-size pores, meso-size pores and optionally macro-size pores, wherein

■ the inorganic material contains on a surface facing the meso-size and macro-size pores at least one of a crystalline sodium sesquicarbonate, a crystalline potassium sesquicarbonate and a crystalline calcium carbonate, and

■ the foam has a density from 20 kg/m ³ to about 300 kg/m ³ .

#12 The foam according to #11 , characterized by one or more of the following properties:

• a thermal conductivity from about 20 mW/(mK) to about 100 mW/(mK),

• a compressive strength from about 1 kPa to about 4 MPa,

• a porosity from about 80 vol-% to about 99 vol-%.

#13 A solid inorganic foam obtained by the manufacturing process according to any one of #1 to #10.

#14 Use of a foam according to any one of #11 to #13 as a thermal insulating material, a construction material, a filtering material, a catalyst support material, a sound insulation material, or fire resistant material.

#15 Use of an industrial grade CO2 gas and/or exhaust gas comprising at least 4 vol% CO2 for manufacturing a solid, inorganic foam, particularly the solid, inorganic foam according to any one of #11 to #13.

#16 The use of #15, wherein the exhaust gas is an exhaust gas from a combustion engine, a coal-, gas-, or petroleum- fired power plant, or a kiln. To further illustrate the invention, the following examples are provided. These examples are provided with no intend to limit the scope of the invention.

Example-1 :

An aqueous solution of sodium silicate (37 wt-% of sodium silicate) was mixed with clay particles, expanded perlite particles and an aqueous solution containing CTAB (cetyltrimethylammonium bromide) to provide a slurry. An aqueous solution of hydrogen peroxide (30 wt-% solution) was added to the slurry to provide a suspension having the composition depicted in the table below:

A wet foam was generated upon in situ foaming. The wet foam was dried under ambient conditions until a water content of about 15wt-% was obtained. This foam has a density of 68 kg/m ³ determined by volumetric calculation. Subsequently, the foam was cured by exposure for 13-62 h to a relative humidity of 5% and a gas having a CO2 concentration of 95 vol% at 1 bar pressure. The cured inorganic foam can capture up to 2.1g of CO2 per gram of foam.

Example-2

An aqueous solution of sodium silicate (37 wt-% of sodium silicate) was mixed with clay particles, expanded perlite particles and an aqueous solution containing CTAB (cetyltrimethylammonium bromide) to provide a slurry. An aqueous solution of hydrogen peroxide (30 wt-% solution) was added to the slurry to provide a suspension having the composition depicted in the table below:

A wet foam was generated upon in-situ foaming. The wet foam was dried under ambient conditions until a water content of about 15 wt% to 60 wt% was obtained. Foams with 15 wt% water content were subsequently cured by exposure for 1080 min (empty bar in sample 2 of Figure 3) and 2220 min (empty bar in sample 1 of Figure 3), respectively to a relative humidity of 5% and a gas having a CO2 concentration of 95 vol% at 1 bar pressure. The compressive strength of the cured foams and uncured counterparts was tested with a universal testing machine according to DIN826. The results are depicted in Figure 3. Compared to the uncured counterparts, cured foams according to the present invention display up to 45% higher values of compressive strength (see Figure 3). Additionally, Figure 3 shows that the compression strength can be increased by longer curing the foam under the conditions described herein. The cured inorganic foams can capture up to 3.1g of CO2 per gram of foam.

Example-3

An aqueous solution of sodium silicate (38 wt-% of sodium silicate) was mixed with clay particles, metakaolin particles, fly ash particles, expanded perlite particles and an aqueous solution containing CTAB (cetyltrimethylammonium bromide) to provide a slurry. An aqueous solution of hydrogen peroxide (30 wt-% solution) was added to the slurry to provide a suspension having the composition depicted in the table below:

A wet foam was generated upon in-situ foaming. The wet foam was dried at 40°C until a water content of 40-15 wt% was obtained. This foam has a dry density of 60 kg/m ³ determined by volumetric calculation, compressive strength of 56 kPa and a surface area of 37 m ² /g determined by BET. Subsequently, the foam was cured by exposure for 15 min and 900 min, respectively to a relative humidity of 5% and a gas having a CO2 concentration of 95 vol% at 1 bar pressure. The cured inorganic foam can capture up to 1.6 g of CO2 per gram of foam. Example-4

An aqueous solution of sodium silicate (37 wt-% of sodium silicate) was mixed with clay particles, and nonylamine to provide a suspension having the composition below:

A wet foam was generated upon mechanical stirring. The wet foam was dried under ambient conditions until a partially dry foam having a 15 wt% water content was obtained and subsequently cured by exposure for 900 min to a relative humidity of 5% and a gas having a CO2 concentration of 95 vol% at 1 bar pressure. The cured inorganic foam can capture up to 2.7 g of CO2 per gram of foam. The cured foam has a dry density of 99 kg/m ³ determined by volumetric calculation, a compressive strength of 177 kPa and a surface area of 8.5 m ² /g determined by BET. The obtained foam was analyzed by microscope. Figure 1 and Figure 2 show micrographs obtained on said foam.

Example-5

An aqueous solution of sodium silicate (38 wt-% of sodium silicate) was mixed with clay particles, metakaolin particles, fly ash particles, expanded perlite particles and an aqueous solution containing CTAB (cetyltrimethylammonium bromide) to provide a slurry. An aqueous solution of hydrogen peroxide (30 wt-% solution) was added to the slurry to provide a suspension having the composition depicted in the table below:

A wet foam was generated upon in-situ foaming. The wet foam was kept in a saturated environment for 3 days. Thereafter, the wet foam was dried at 20°C until a moisture content of 44 wt% was obtained. In the subsequent curing step, samples of said foam were exposed for 60 min at 20°C, 1 bar pressure and 80% relative humidity to either a gas having initially a CO2 concentration of 94 vol%, or a and a gas having initially a CO2 concentration of 23 vol%. The average CO2 concentration over the period of exposure was 85 vol% and 21 vol%, respectively. The compressive strength of the cured foams is greater than the uncured counterparts. Foams cured at higher initial CO2 concentration (85 vol%) depict a larger increase in compressive strength compared to the foams cured at lower CO2 concentration (21 vol%). The cured inorganic foam can capture up to 1.6 of CO2 per gram of foam.