Decomposition of organic constituents in recycled gypsum by ozone oxidation

Abstract

The present application pertains to a process for the treatment of recycled gypsum, which contains siloxanes with Si-H bonds and / or organic constituents, wherein the gypsum is treated with ozone in amount of at least 1 mg / kg recycled gypsum. By means of such treatment, the siloxanes with Si-H bonds and organic constituents can be degraded to respective oxidized compounds, which do not affect the foaming characteristic or processing behavior of the recycled gypsum. The present application further pertains to a process for preparing a foamed gypsum composition and gypsum boards, wherein the process comprises a process as indicated above, correspondingly prepared gypsum and gypsum boards, the use of ozone for the decomposition of siloxanes with Si-H bonds and / or for the oxidation of organic constituents, in particular gypsum setting retarders, in gypsum recyclates and the use of ozone for the improvement of foaming characteristics of recycled gypsum.

Description

[0001] DECOMPOSITION OF ORGANIC CONSTITUENTS IN RECYCLED GYPSUM BY OZONE OXIDATION

[0002] The present application concerns a process for the treatment of recycled gypsum, which contains siloxanes with Si-H bonds and / or organic constituents, where the gypsum is treated with ozone in amount of at least 1 mg / kg recycled gypsum. The present application further concerns a process for preparing a foamed gypsum slurry and gypsum boards, wherein the process comprises a process as indicated above, correspondingly prepared gypsum and gypsum boards, the use of ozone for the decomposition of siloxanes with Si-H bonds and / or for the oxidation of organic constituents, in particular gypsum setting retarders, in gypsum recyclates and the use of ozone for the improvement of foaming characteristics of recycled gypsum.

[0003] State of the art

[0004] A building material that is readily and cheaply available today is so-called FGD gypsum (i.e., gypsum obtained from the residues of flue gas 1 esulphurization plants). This material is obtained from the combustion of coal by reacting the waste gases from the combustion with a calcium oxide or calcium carbonate suspension, whereby the sulphur dioxide (SO2) contained in the flue gas is first converted to sulphur trioxide (SO3) in the presence of oxygen and then bound as calcium sulphate (CaSCH).

[0005] However, many governments have decided to phase out coal-fired power generation in the coming years, so it is to be expected that FGD gypsum will become an increasingly scarce raw material. In order to still be able to meet the existing demand for gypsum building materials, it can therefore be assumed that the recycling of gypsum products will play an increasingly important role over time. One problem with the use of gypsum products to make gypsum recyclates is the fact that gypsum in many products (such as not least gypsum boards for use in drywall construction) is treated with water-repellent additives to reduce the material’s ability to absorb water (e. g. for the production of hydrophobized boards). Such a treatment is described, for example, in DE10220659 A1 , in which a mixture of "H-siloxane" (i.e., organosiloxane containing Si-bonded hydrogen atoms) and starch ethers is added for the production of plaster bodies. Such H-siloxane has a disruptive effect on the production cycle for making new building material boards, as it has a foam-destroying effect and thus leads to fluctuating building material and board properties in the production process of the boards (which are made of foamed gypsum slurries for weight reduction and better thermal insulation).

[0006] In addition, other constituents which are used as additives for the production of gypsum boards, also affect the processing behavior of a gypsum slurry which is prepared from gypsum, which has been recycled from a previous use in a gypsum board. Such additives specifically include additives, which affect the curing behavior of gypsum (e. g. retarders), thickeners, e. g. methyl celluloses, which may thicken the gypsum slurry, liquefiers, which may influence the fluidity of the gypsum slurry, and foaming agents, which may lead to uncontrolled foaming. Due to the absence of effective means to remove such agents from recycled gypsum, recycled gypsum now is used only in admixture with fresh gypsum, which does not comprise respective additive residues, as thereby the unwanted effects as noted above can be kept at a minimum.

[0007] Against this background, there is a need for the provision of gypsum recyclates from gypsum products in which siloxanes with Si-H bonds (“H-siloxanes”) are contained, or which have been produced by including such siloxanes, which are as comparable as possible with conventional sources of gypsum (e.g. natural gypsum, FGD-gypsum (flue-gas desulfurization), titano-, phospho- or citrogypsum) in terms of their foamforming properties, and which can be processed analogously to such gypsum into products with properties that are as consistent as possible. Likewise, there is a need for the provision of recycled gypsum, wherein the effects of the processing additives used in a previous gypsum article (such as a gypsum board or plaster) are minimized.

[0008] In a previous application (not yet published) the applicants have described a process, where siloxanes with Si-H bonds in the gypsum are degraded in an alkaline aqueous environment. However, in this process the gypsum is first treated with dissolved alkaline solution, which prior to processing of the gypsum to the respective hemihydrate has to be fully removed (to allow a later dehydration of the gypsum dihydrate to gypsum hemihydrate). As is apparent to the skilled practitioner, such process has room for improvement in terms of the energy balance.

[0009] Accordingly, there is a need for a process, which provides effective elimination of siloxanes with Si-H bonds and conventional gypsum board additives at minimal costs and energy input.

[0010] The present application addresses these needs.

[0011] Description of the invention

[0012] In the investigations, which are underlying the present invention, it has unexpectedly been found that a treatment of the recycled gypsum with ozone can provide an effective oxidative conversion of siloxanes with Si-H bonds and processing additives, which are present in recycled gypsum, to oxidized species thereof, which no longer provide the same effect than the original additive (without being bound by theory, it is believed that on treatment with ozone Si-H bonds can be converted to Si-OH bonds, and siloxanes can be further degraded to SiC ; SiO2 may have no impact on the stability of a foam, which is prepared in a slurry of gypsum in water). Likewise, alcohol functionalities in additives such as malic acid can be oxidized to the respective ketone or carboxylic acid function, from which the compounds can be further oxidized with release of CO2 and polysaccharides in the mixture can be oxidatively degraded. The treatment in addition has the benefit, that the ozone can be applied to the recycled gypsum powder directly, so that the addition of water or aqueous solutions, which would later have to be removed for the conversion of the treated gypsum to hemihydrate is not necessary.

[0013] Accordingly, in a first aspect, the present invention is directed to a process for the treatment of recycled gypsum, which contains siloxanes with Si-H bonds and / or organic constituents, wherein the gypsum is treated with ozone in amount of at least 1 mg / kg recycled gypsum.

[0014] In the invention, the term “H-siloxane” or siloxane with Si-H bonds designates silicone oligomeric or polymeric compounds, which have [SiR1R2-O] repeating units and at least one Si-H group. R1and R2in the above repeating unit is regularly an alkyl radical and can be the same or different in R1and R2. In a preferred embodiment, at least one of R1and R2is CH3 and in a particularly preferred embodiment both of R1and R2are CH3. It is possible and within the scope of this invention that R1and / or R2over the entire length of the oligomer or polymer can be different.

[0015] In the invention, the “organic constituents” in the recycled gypsum are organic constituents, which are regularly used in gypsum boards or gypsum plasters, including liquefiers, setting retarders, surfactants for foam formation, wax-paraffin- emulsions, starch, or cellulose products, e. g. methyl cellulose. In a preferred embodiment, the organic constituents are selected from one or more of these alternatives. In a particularly preferred embodiment, the organic constituents comprise a setting retarder, in particular in the form of a hydroxycarbocylic acid such as tartaric acid, malic acid or citric acid or in the form of a protein-based retarder. In the invention, the total amount of such organic constituents in the recycled gypsum may be in the range of from 0.1 wt.-% to 5.0 wt.-%, preferably from 0.5 wt.-% to 4.0 wt.-%, and more preferably from 1 wt.-% to 3.0 wt.-%.

[0016] The recycled gypsum in the invention, is a recycled gypsum which is derived from the recycling of gypsum articles such as gypsum boards or gypsum plaster. In general, the recycled gypsum may contain at least 60 wt.-% of calcium sulfate dihydrate (CaSCH • 2 H2O), preferably at least 80 wt.-%. Further, recycled gypsum may contain other calcium sulfate phases (hemihydrate (CaSCH • 0.5 H2O (“hemihydrate”) or anhydrite (CaSCH without H2O)) as well as carbonates or other minerals in small amounts. Accordingly, the recycled gypsum contains organic constituents and / or siloxanes which comprise Si-H bonds, and which may originate from hydrophobized boards. The recycled gypsum in most cases will not have a content of combined organic constituents and siloxanes containing Si-H Bonds, which is more than 5 wt.- %, preferably, the total content of such constituents is less than 2 wt.-% and in particular less than 1 wt.-%. If the recycled gypsum contains siloxanes comprising Si- H bonds, the content thereof in the recycled gypsum is preferably in the range of 1 to 0.001 % by weight, more preferably 0.5 to 0.01 % by weight and even more preferably from 0.2 to 0.05 % by weight. In addition, if the recycled gypsum contains setting retarders, it is preferred if the content of such retarders in the recycled gypsum is in the range of 0.5 to 0.001 wt.-%, more preferably from 0.2 to 0.005 wt.-% and even more preferably in the range of 0.1 to 0.01 wt.-%.

[0017] In the treatment with ozone, the amount of ozone, which can be applied to the recycled gypsum is not subject to any relevant restrictions, and the skilled practitioner will be aware that for increasing the amount of ozone, which is applied to the gypsum, the gypsum can simply be treated with a continuous stream of ozone for a longer time. On the other hand, since more time usually makes a process more expensive (and the content of organic constituents and siloxanes having Si-H groups is usually not excessive), a treatment in an amount of 150 mg / kg of ozone on the recycled gypsum is usually sufficient. A particularly suitable amount range of ozone for the treatment of the recycled gypsum is in the range of from 3 to 100 mg / kg of ozone / recycled gypsum, and especially in the range of from 4 to 80 mg / kg.

[0018] As noted above, the treatment with ozone has the advantage, that the ozone can be applied to the dry recycled gypsum, so that the inventive process is preferably a dry process. Such process can suitably be conducted such, that the ozone is introduced into a solid bed of material containing the recycled gypsum. In the inventive process, the recycled gypsum is usually treated with the ozone directly (i.e., prior to a later mixing of the treated recycled gypsum with fresh gypsum). It is also possible to apply ozone treatment to a mixture of recycled gypsum and fresh gypsum (i.e. gypsum, which is not contaminated with either of siloxanes having Si-H bonds or organic constituents), however, for such processing, the reaction of ozone with the organic compounds in the gypsum mixture may not be equally effective as when the recycled gypsum is treated directly, so that such processing is less preferred in the context of the invention.

[0019] The inventive process can be run in a batch mode or in a continuous mode. A continuous mode may be preferred in a process, where a mixture of fresh and recycled gypsum is treated with ozone, whereas a batch mode may be preferred in a process, where only recycled gypsum is treated with ozone. The time, during which the treatment is applied is not of relevant importance, provided that the time is sufficient to deactivate or eliminate the siloxanes with Si-H groups and / or the organic components. Here the skilled practitioner will appreciate that the time also depends on the ozone concentration which is used in the treatment. In most cases, however, the ozone treatment time will be in the range of from 30 Min to 120 Min and especially in the range from 45 Min to 90 Min.

[0020] In the inventive process it is furthermore preferred that the gypsum is treated to an extend with ozone that at least 10 % (by weight) of the siloxanes comprising Si-H groups therein are oxidized and / or that at least 10 % (by weight) of the organic constituents in the recycled gypsum are oxidized. More preferably, the treatment is such that at least 20 % and even more preferably at least 30 % of siloxanes comprising Si-H groups or of the organic constituents are oxidized. Usually, it is not necessary to oxidize all of the siloxanes comprising Si-H groups or of the organic constituents as the oxidation only has to be carried out to the extent that remaining siloxanes or organic constituents no longer harmfully affect the processing characteristics of the recycled gypsum. Thus, in one aspect, the treatment may be such that from 30% to 95%, in particular from 40 to 85% of the respective compounds are oxidized.

[0021] The process as described above provides recycled gypsum comprising a lower quantity of components, which may affect the processing of the gypsum to new products. Thus, in a further aspect, the invention also relates to a gypsum which is produced by this process. Whereas a respective gypsum aims at being similarly pure as a fresh gypsum in its composition, it will regularly comprise detectable amounts of oxidation products from the reaction of the siloxane and / or organic constituents with the ozone, via which a correspondingly processed gypsum can be distinguished from a fresh gypsum.

[0022] As noted above, one benefit of the inventive treatment of recycled gypsum with ozone is that components therein, which may affect the processing of the recycled gypsum to new products are oxidized, thereby improving the processing characteristics and in particular the stability of a foam, which is prepared with the recycled gypsum. Therefore, in a further aspect, the present application is also directed to a process for preparing a foamed slurry comprising mixing calcined gypsum, a foaming agent and water and foaming the mixture to provide a foamed slurry, or generating a foam from a foaming agent and water and adding the foam to a slurry of calcined gypsum and water to obtain a foamed slurry, wherein as the calcined gypsum a calcined gypsum is included in the process which has been subjected to a process as described above and a subsequent calcination.

[0023] Accordingly, this process comprises several steps, namely treating a gypsum as in the foregoing process, calcining it and including the calcined gypsum thus treated in a process for producing a foamed slurry. This procedure avoids the disadvantageous effects of reduced foam stability observed when gypsum recyclate containing siloxanes with Si-H bonds, or gypsum contacted with such siloxane is processed to include a foam. Since this stability depends on the stability of the gas bubbles in the slurry, it is irrelevant whether the foamed slurry is prepared by foaming a mixture of all later ingredients in water, or whether an aqueous foam is first prepared from a foaming agent and water, which is then mixed with calcined gypsum and water or a calcined gypsum / water slurry to produce the foamed gypsum slurry in this way.

[0024] Further, it generally has to be mentioned that gypsum (calcium sulfate dihydrate) contains water of crystallization and has a chemical formula of CaSC • 2 H2O. By calcination the water of crystallization may be removed to obtain calcined gypsum. However, several modifications of calcined gypsum are possible:

[0025] - a-Hemihydrate (CaSC •1 / 2 H2O) is obtained when heating gypsum in an autoclave in wet atmosphere or by treatment with acids or aqueous salt solutions. Due to its higher density, it is used for gypsum having a higher hardness. For setting less water is required but an increase setting time is needed.

[0026] - [3- He mi hydrate (CaSC •1 / 2 H2O) is obtained by calcining gypsum in an open atmosphere. Upon addition of water the p-hemihydrate is transformed into the dihydrate within minutes. It is used for gypsum having lower hardness and is used for example in construction industry.

[0027] - Anhydrite III (CaSC • 0.x H2O; 0<x<0,5) is obtained upon calcination at temperatures of up to 300 °C. In the presence of water, e.g., air moisture, a transformation to the hemihydrate or dihydrate occurs very quickly.

[0028] - Anhydrite Ils (CaSCH) is obtained upon calcination at temperatures of 300 to 500 °C. It is sparingly soluble in water. In the presence of water, a slow hydration occurs within hours or days.

[0029] - Anhydrite Hu (CaSCH) is obtained upon calcination at temperatures of 500 to 700 °C. This gypsum modification is insoluble in water.

[0030] Anhydrite I (CaSCH) is the high temperature modification of gypsum and is formed at temperatures of above 1180 °C (dead burned gypsum). In the present invention, the term “calcined gypsum” mainly represents the calcination of calcium sulfate dihydrate to p-hemihydrate. However, other calcium sulfate phases (most notably dihydrate (CaSCH • 2 H2O) or anhydrite (CaSC )) as well as carbonates or other minerals may be present in the calcined gypsum as well.

[0031] For the sake of completeness, it has to be mentioned that any gypsum (not only recycled gypsum) can be treated with the inventive process. Even if the gypsum does not contain siloxanes with Si-H bonds or the organic constituents as mentioned above, there is the possibility of other unwanted (chemical and / or natural) compounds that can be destroyed or transformed into other not disturbing compounds with the inventive process.

[0032] The foamed gypsum slurry prepared by this process preferably has a ratio of solid components to gas such that, after curing and drying of the slurry, a bulk density in the range of 400 to 900 kg / m3and in particular in the range of 550 to 800 kg / m3is obtained.

[0033] Advantageously, the above process for preparing a foamed slurry may further comprise a step of casting the foamed slurry onto a planar substrate and allowing the slurry to cure. By these steps, a planar gypsum product can be obtained. A further advantageous embodiment arises here in that a carrier paper or veil is arranged between the planar substrate and the slurry. The carrier paper or veil allows the slurry to be conveyed further on the carrier, for example into a drying unit, while the carrier is continuously applied onto the substrate. This allows the foamed gypsum product to be produced continuously. In a preferred embodiment, in the curing slurry this process is covered with a carrier paper or veil to thereby provide a paper or Veil cover on both sides of the cured slurry.

[0034] The method as described above allows the production of gypsum boards with a higher content of recycled gypsum and preferably with less weight (due to the increased foam stability provided by the recycled gypsum according to the inventive process). Accordingly, in a further aspect, the present application also pertains to a gypsum board which is produced by the above-described process. Such gypsum board preferably comprises recycled gypsum in an amount of at least 3 % by weight, more preferably at least 10 % by weight, even more preferably 20 % by weight and still even more preferably at least 40% by weight. In addition, it is preferred that the gypsum board has bulk density in the range of 400 to 900 kg / m3and in particular in the range of 550 to 800 kg / m3.

[0035] A still further aspect of the present invention relates to the use of ozone for the decomposition of siloxanes with Si-H bonds and / or for the oxidation of organic constituents, in particular gypsum setting retarders, in gypsum recyclates, wherein gaseous ozone is contacted with gypsum recyclate powder. In general, gypsum recyclate powder can be dry or can have up to 10 wt.-% moisture.

[0036] Gypsum recyclates differ from conventional gypsum in that they contain components that are usually included in finished gypsum products. According to the invention, the gypsum recyclate contains at least one of siloxanes with Si-H and organic constituents.

[0037] Finally, a still further aspect of the present invention relates to the use of ozone for the improvement of foaming characteristics of recycled gypsum, wherein gaseous ozone is contacted with dry gypsum recyclate powder and siloxanes and / or organic constituents are at least partially oxidized.

[0038] The respective processes and uses as described above provide the particular benefit that the content of compounds in recycled gypsum, which affect the processing of the gypsum to new products such as gypsum boards, can significantly be reduced so that is becomes possible to use higher quantities of recycled gypsum or even exclusively recycled gypsum for the production of respective products without affecting the production process.

[0039] The present invention is further described with reference to a number of examples of embodiments which, however, are intended solely to illustrate the invention and are not in any way to be construed as limiting the scope of protection of the application. with ozone) a) treatment alternative 1 :

[0040] Approximately 25 g of test material from a milled H1 board (as defined in DIN EN 520 (2009), 4.10: total water uptake less or equal to 5 %) was placed in a box with a lid on a perforated Plexiglas platform. To prevent the powder from falling through the perforation, a layer of filter paper was placed between them. The ozone was introduced via a hose that was inserted from above past the lid. The ozone flows through the test material from bottom to top. The gassing duration was 60 minutes. b) treatment alternative 2:

[0041] To ensure better mixing of the test material and ozone, the powder was stirred in the second test setup. A wash bottle with a flat bottom and two openings was chosen. Approximately 35 g of the test material was filled into the bottle. The hose for the ozone was passed through one opening and the other opening was closed with a stopper. The ozone flows through the test material from above. Stirring was carried out using a 6 cm long stirring rod and a magnetic stirrer. The gassing duration was 50 minutes. c) treatment alternative 3:

[0042] This experiment was used to simulate a fluidized bed treatment. To this end, a plastic drinks bottle was cut in half and put back together with the screw cap facing down. A hole was cut in the lower part for the ozone generator hose. The screw cap was also perforated, and the tube was secured in the hole with a metal sleeve and sealed with silicone. In the upper part, a porous separating layer made of material was used, onto which the powder was added. The experimental setup allowed the ozone to flow completely through the powder bed from bottom to top to allow for maximum contact. The gassing duration was 60 minutes.

[0043] To determine the foam-destroying effect, a test for foam yield and stability was carried out as follows: 0.5 g of the powder was weighed into the test vessel. 150 ml of tap water (room temperature) were then poured in and 150 pl of the foam concentrate Vinapor GYP 10 (BASF) was added. The mixture was stirred at the highest level for 30 s using an IKA stirrer with an inclined star stirrer insert. At the end of the stirring time, the foam yield was read on the scale on the outside of the test vessel. The foam breaks down in such a way that more water forms from below over time. The foam yield towards the top remains almost unchanged. To determine the foam stability, the times within which 50 ml or 100 ml of water settled at the bottom of the vessel were noted. For comparison, a zero test without gypsum powder and a reference test with a gypsum board (GB) without H-siloxane were carried out. The results of these tests are provided in the following table 1 :

[0044] Table 1 The GB showed no changes compared to the zero test, which indicates that it does not have any foam-destroying properties. With the untreated H1 plate, the foam yield and stability is reduced to about half of that of the zero test. After ozone treatment there is still a foam-destroying effect, but this is significantly reduced compared to the untreated H1 plate. Treatment with condition a) provided the best results. The foam yield was 425 ml compared to 333 ml for the untreated H1 plate. The time to the 100 ml mark was 1 :23 Min compared to 1 :05 Min with the untreated H1 plate. Treatment with conditions b) showed slightly less favorable results with 417 ml foam yield and 1 :14 Min to reach the 100 ml mark. The reason for this is probably an accumulation of powder on the edge of the bottle over the course of the test time, which was then no longer mixed. Treatment with conditions c) also shows less favorable results than treatment with condition a) with 408 ml foam yield and 1 :13 Min to reach the 100 ml mark. of ozone on malic acid as a

[0045] To test the influence of ozone on retarders, 0.04% malic acid was mixed with rotary kiln calcined gypsum (WGW 0.58). The ozone treatment was carried out analogously to the treatment according to condition c) in claim 1.

[0046] To check whether the retarder was eliminated by the ozone, the setting times were tested with the following method: 100 g calcined gypsum are weighed into a bowl and water according to the water gypsum ratio (V (ml) = 100 x WGR) is weighed into a porcelain bowl. Now a stopwatch is started and the gypsum is sprinkled into the water within 15 seconds. The mixture is left to soak for 45 seconds and then stirred for 1 minute. Two patties (diameter about 10 - 12 cm / thickness about 0.5 -1 cm) from this mixture are poured on a plastic sheet or glass plate. To determine the initial setting, a knife is drawn through a patty and the coalescence of the edges is observed. The initial setting (IS) is defined as the time at which there is no flow-back. To determine final setting (FS), it is pressed into the patty with a thumb applying a pressure of approx. 5 kg. The final setting is defined as the time at which no surface water is released upon pressing. The results are provided in the following table 2: Table 2

[0047] As is apparent from the above table 2, the setting time after ozone treatment was significantly shortened. The start of setting was 45 minutes in the original state and 30 minutes after the ozone treatment. The end of setting was between 65 and 85 minutes in the original state and only 49 minutes after the ozone treatment. On average, thus, about 44% of the initial retarder were degraded by the ozone treatment.

[0048] These results were achieved at a bed height of 2.4 cm, an inserted gypsum mass of 100 g, a volume of 100 cm3(under assumption of a bulk density of 1000 g / l) and a flow rate (mass flow) of ozone of 500 mg / h.

Claims

Claims1 . Process for the treatment of recycled gypsum, which contains siloxanes with Si-H bonds and / or organic constituents, where in the gypsum is treated with ozone in amount of at least 1 mg / kg recycled gypsum.

2. Process according to claim 1 , wherein the recycled gypsum is treated with an amount of from 3 to 100 mg / kg of ozone, preferably in an amount of from 4 to 80 mg / kg.

3. Process according to claim 1 or 2, wherein the recycled gypsum as an organic constituent contains a setting retarder, which is preferably selected from tartaric acid, citric acid and malic acid.

4. Process according to any one of claims 1 to 3, wherein the process is a dry process, preferably, wherein the ozone is introduced into a solid bed of material containing the recycled gypsum.

5. Process according to any one of the preceding claims, wherein the treatment is provided in a batch wise or continuous manner for a time of from 30 Min to 120 Min and preferably from 45 Min to 90 Min.

6. A process for preparing a foamed slurry comprising mixing calcined gypsum, a foaming agent and water to calcined gypsum and foaming the mixture to provide a foamed slurry, or generating a foam from a foaming agent and water and addingthe foam to the calcined gypsum to obtain a foamed slurry, wherein as the calcined gypsum a gypsum is included in the process which has been treated according to a process according to any one of claims 1 to 5 and a subsequent calcination.

7. The process according to claim 6, wherein the foamed slurry contains gas bubbles in an amount to provide a bulk density in the range of 400 to 900 kg / m3and preferably in the range of 550 to 800 kg / m3after drying.

8. A process according to claim 6 or 7, further comprising a step of casting the foamed slurry onto a planar substrate and allowing the slurry to cure.

9. A process according to claim 8, wherein a carrier paper or veil is arranged between the planar substrate and the slurry, and wherein preferably the curing slurry is covered with a carrier paper or veil.

10. Gypsum produced by a process according to any one of claims 1 to 5.11 .A gypsum board produced by a process according to any one of claims 6 to 9.

12. Use of ozone for the decomposition of siloxanes with Si-H bonds and / or for the oxidation of organic constituents, in particular gypsum setting retarders, in gypsum recyclates, wherein gaseous ozone is contacted with dry gypsum recyclate powder.

13. Use of ozone for the improvement of foaming characteristics of recycled gypsum, wherein gaseous ozone is contacted with dry gypsum recyclate powder and siloxanes and / or organic constituents are at least partially oxidized.

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