A Construction element and method of manufacture thereof

Abstract

A method of forming a construction element is disclosed. The method comprises the steps of selecting a monomer material, the monomer being preferably a bio-based monomer. The monomer is mixed with a p

Description

Technical Field of the Invention The present invention is concerned with a construction element for the building industry such as a brick, tile, or slate. The element is based on a biochar material and a bio-based binder material. The invention is further concerned with a method of manufacture thereof. The elements manufactured are suitable for recycling at the end of their useful life. Background to the Invention The building and construction industry is estimated worldwide to contribute to around 39% of carbon dioxide emissions for which about 28% are operational and 11% embodied carbon emissions. Although a large proportion of the embodied carbon emissions are due to the production of concrete, other building materials also make a significant contribution. For example, bricks, roof tiles, slabs etc. typically utilise clay-based materials which require significant heat treatment in order to be functional. Attempts are known in the art to reduce the amount of building materials used, but the volumes used are still significant. For example, methods are known of producing concrete structures which leave voids within the bulk material, so also reducing the mass of the structure. There are however risks associated with this approach as the recent issues with Reinforced Autoclaved Aerated Concrete (RAAC) have shown. Amongst the replacement materials for clay which have been suggested, some are based on the use of renewable materials such as wood. Apart from wood itself, treatment of the wood material can produce materials such as charcoal and biochar. Charcoal predominantly consists of carbon itself in particulate form and in which the carbon atoms are primarily in a zero or negative oxidation state: most of the hydrogen and oxygen atoms having been removed during processing. In order to prepare a building element, the charcoal is mixed with a binder. Biochar has similarities to charcoal and is formed from a biomass but tends to have a larger surface area than charcoal and be more porous. This feature means, that although it is known to be utilised in forming construction elements, its main use is in the agricultural and horticultural areas as improving soil quality and structure. As with charcoal, when used to form a construction element, a binder is present to bind the biochar particles to each other. So, for example, CN110395941 discloses binding biochar with an epoxy resin, but does not disclose a pressurisation step. It is an object of the current invention to provide a method of manufacture of a composite construction element formed by binding a biochar material with a bio-based polymeric material. It is a further object of the invention to provide a construction element formed of a biochar bound together by means of a bio-based polymeric material. Summary of the Invention According to a first aspect of the invention, there is provided a method of forming a construction element, the method comprising the steps of selecting a monomer material, the monomer being a bio-based monomer, mixing the monomer with a polymerisation catalyst to form a pre-binder mixture, mixing the pre-binder mixture into biochar, to form a preformed mixture, transferring the preformed mixture into a forming volume and allowing to at least partially harden to form the construction element. Preferably, the monomer is a furan-based monomer and further preferably furfuryl alcohol. Alternatively preferably, the monomer is selected to form a bio-epoxy resin, a polyester or a polyurethane or a polyurea or an unvulcanised rubber or a vinyl ester or a bismaleimide or a polyimide or a cyanate ester or a phenolic or a phenol-formaldehyde or a urea-formaldehyde or a melamine resin or an in-situ polymerising thermoplastic resin (e.g. Elium (RTM)). Optionally, a mixture of two or more polymeric materials can be utilised as a binder. Preferably, the monomer is mixed with the polymerisation catalyst to form a mixture having a viscosity of <3.1 Pascal second, further preferably a viscosity of <2.0 Pascal second and yet further preferably a viscosity of <1.0 Pascal second. Preferably, the biochar is provided as a powder having a grain size of 1pm to 5mm, preferably 2pm to 2mm, and further preferably 5pm to 600pm. Preferably, the biochar has been pyrolysed at a temperature of from 350°C to 2,500°C prior to use, further preferably from 500°C to 800°C and yet further preferably 500°C or higher. Preferably the ratio of resin binder to biochar is within the range of 1:2 - 1:16 by weight, further preferably 1:3 -1:16 by weight, and especially preferably 1:3 - 1:6 by weight. Preferably, pressurisation of the resin / biochar mixture is carried out to increase the density of the mixture. The overpressure applied is preferably from 0.1 MPa to 20.0 MPa, further preferably the range is from 0.1 to lOMPa, yet further preferably from 0.5 to 5.0 MPa, and still yet further preferably from 1.0 to 5.0MPa. Preferably, pressurisation is carried out from Is to 10 minutes, further preferably 30s to 7 minutes, and particularly preferably 30s to 5 minutes to ensure good compaction. Preferably, the density achieved by pressurisation is from 0.70 to 1.40 g / cm3 and further preferably 0.80 to 1.30 g / cm3 to give a good building product. Preferably, the resin / biochar mixture is subjected to an elevated temperature of from 20 to 180°C, further preferably 60 to 120°C to complete the hardening process. The mixture is preferably held at the elevated temperature for 0.5 to 120.0 minutes. Optionally a layer of decorative material is added to the mixture prior to pressurisation. Preferably, the mixture includes fibres to reinforce the construction element. The fibres are further preferably incorporated into the outer region of the element. Preferably, overpressure is applied using a double belt press or calendar roller / calendar press, for example where a sheet element is to be formed. According to a second aspect of the invention, there is provided a construction element, the construction element being formed in accordance with the above method. Detailed Description of the Invention There is an urgent need to reduce the carbon dioxide (CO2) emissions of the construction industry as it is one of the biggest contributors to man-made carbon dioxide emissions. Although the main contributor to the emissions is the production of concrete, other construction elements such as bricks, slates, tiles, and the like, formed primarily from clay, also require large energy inputs to manufacture. The present invention seeks to utilise a material, biochar (also trivially referred to in the art as biomass charcoal and biomass coke), which has a negative carbon dioxide emission in the formation of construction elements, or masonry units. The method can also be utilised to make other articles to replace the use of structural foam, drywall or plasterboard and MDF board. Biochar has been trialled in various construction materials but in combination with cement (in concrete) or bitumen (in asphalt) and to replace sand as an aggregate. In broad terms, biochar is a product of the low-oxygen pyrolysis of biomass. The form in which the biochar is used is as a finely divided particulate, which is then bound using a suitable binder. The porous nature of the biochar material enables it to absorb and release moisture more readily than conventional clay, and so allows a measure of control of humidity within a structure, reducing the action of a construction element as a cold bridge. The elements formed are also lighter than comparable ones formed of clay. Further, the compression strength of the articles manufactured according to the current invention have a higher compression strength than comparable articles made in accordance with CN110395941 above. In broad terms, the current invention contemplates mixing biochar with a suitable binder material and then applying pressure to the mixture. The pressurisation is carried out in a mould or other forming volume to produce the finished article. Optionally, the moulded article can be subject to a period of raised temperature to finalise the hardening of the finished article. Hardening of the article can take place fully in the mould, or at least partially once the article has been removed from the mould. The use of biochar and bio-based binders makes the overall product potentially carbon negative. In this document, the use of a mould can include extrusion. The material mixture suitable for the current invention has a pyrolysed biomass of 60 to 93%w / w, preferably 75 to 86%w / w. In order to provide a high surface area to engage the binder material, the biochar is provided as a powder having a grain size of 1pm to 5mm, preferably 2pm to 2mm, and further preferably 5pm to 600pm. A material milled to around 30 mesh (< 595pm) has been found to be particularly useful. Although the above describes the predominant particle size, particles of size less than 5pm can be present. In one embodiment, a mixture of grades of particle sizes across a range can be utilised in a single sample. For example, in the further preferable range, the material can include material in each of the following 3 particle size categories: 1 - 89pm, 90 -180pm, 181 - 355pm and 356 - 710pm. The biochar is preferably produced such that it has a low to zero volatile content. To this end, biochar which has been pyrolysed from 350°C to 2,500°C can be used, preferably from 500°C to 800°C; 500°C or higher is preferable to that which has been pyrolysed below 500°C. Further, biochar from any biomass source can be used e.g. animal manures, agricultural residue and forestry residue (e.g. crop straw and residues, fruit pits, twigs, leaf litter, forestry wastes), food leftover, bagasse, sewage sludge, packaging waste, construction waste, wood; if wood-based biochar is employed then biochar formed from a hardwood source is preferable to that formed from a softwood source. Alternatively, biochar from incineration processes can be employed, e.g. biochar residue from tire recycling. The biochar content is selected as such to offset all other production emissions, including binder CO2 emissions from the formation of the binder. Life-Cycle-Assessment (LCA) cradle-to-grave, indicates that, depending on the binder choice, a value of from -1.5 to 0.05 kg CO2 / kg using current energy mixture data (data UK 2023) is obtained. In preferred embodiments, the CO2 emissions of the final product are from -0.5 to -1.5 kg CCh / kg (using current energy mix data). The binder utilised is selected to be a bio-base polymeric material. Currently used binder materials have a 2 to 5 kg CO2 / kg emission, which adds significantly to the overall CO2 emissions. Preferably, the polymers selected are thermoset materialsand based on a furan, and especially furfuryl alcohol. It has been found that, when using furan as a monomer, the porous nature of the biochar absorbs water formed during the polymerisation reaction and reduces any foaming which might otherwise occur. As an alternative to polyfurfuryl alcohol, then a bio-epoxy resin (for example Sicomin resin SR GreenPoxy33) can be used. As further alternatives, a polyester or a polyurethane or a polyurea or an unvulcanised rubber or a vinyl ester or a bismaleimide or a polyimide or a cyanate ester or a phenolic or a phenol-formaldehyde or a urea-formaldehyde or a melamine resin can be utilised as a binder. In a further embodiment, a mixture of two or more polymeric materials can be utilised as a binder. In an alternative embodiment, an in-situ polymerising thermoplastic resin such as Elium (RTM) can be used. Although the viscosity can be higher, to facilitate manufacture, the resin selected preferably has a viscosity of <3100cP (3.1 Pascal second), further preferably a viscosity of <2000cP (2.0 Pascal second) and yet further preferably a viscosity of <1000cP (1.0 Pascal second). In order to achieve best mixing, the resin material, along with the polymerisation catalyst is added to the biochar rather than adding the biochar to the resin mixture. Further processing should take place before the gel time of the mixture is reached. This facilitates the engagement of the resin with the surface of the biochar particles, so facilitating mixing whilst also reducing airborne dust generation which would be harmful to those preparing the mixture. Where a furan-based resin is used, a water mist can be introduced to minimise the airborne particles, as water is in any event added as a viscosity control and can be removed from the finished product during a curing / drying step. For the furan-based resins a water content, prior to drying, of up to 150% of the resin content can be utilised, some of the water possibly being absorbed into the biochar. The polymerisation catalyst added to the resin mixture acts to cause the polymer chain to be formed. As a suitable example of such a polymerisation catalyst for furan -based polymers, Alton HM1448 can be cited. The ratio of resin binder to biochar is preferably within the range of 1:2-1:16 by weight, preferably 1:3 - 1:16, and especially preferably 1:3 -1:6. Once the biochar and the resin have been mixed together, they are transferred into a mould. The pressure applied to the resin / biochar mixture is raised at this point to increase the density of the mixture and to increase the engagement of the resin with the surface of the biochar and the pores of the biochar. The pressure is the compression force exerted by the press on the surface of the material (open / movable mould side), The overpressure applied is preferably from 1 to 200.0 bar (0.1 MPa to 20.0 MPa) A further preferable range is from 1.0 to 100.0 bar (0.1 to lO.OMPa), yet further preferably from 5.0 to 50.0 bar (0.5 to 5.0 MPa), and still yet further preferably 10.0 to 50.0 bar (1.0 to 5.0MPa). The overpressure is applied until the mixture is suitably dense, but typically from Is to 10 minutes, especially preferably 30s to 7 minutes, and particularly 30s to 5 minutes. In a further embodiment, a pre-pressure can be applied, applying a lower pressure initially followed by a higher pressure. This is suitable for applying additional, optionally decorative, layers. In an alternative embodiment, the mixture can be extruded. In yet an alternative embodiment, the mixture can be pressed using double belt presses or calendar rollers / calendar press. This embodiment is suitable for the manufacture of, for example, sheet materials. At this stage the material density of the article produced is from 0.70 to 1.40 g / cm3 and preferably 0.80 to 1.30 g / cm3. It has been noted that the use of a finer biochar starting material, a higher binder content a greater applied overpressure or combination thereof leads to a denser product. Once formed, an additional curing step can be undertaken in which the compressed article is maintained at an elevated temperature, for example from 20 to 180°C, preferably from 60 to 120°C for a defined period, 0.5 to 120.0 minutes. In this step the polymerisation takes place, including any cross-linking. The conditions are chosen to suit the resin constituents. For example, for furan-based resins an elevated temperature is utilised to drive off water produced in the polymerisation reaction, whereas for an epoxy-based resin the elevated temperature may not be required. The use of heat is preferable as this improves the cross-link density and the mechanical properties resulting therefrom. Example 1 In the following example, the general production of a brick slip is described. A 50g batch of biochar, ground to a nominal particle size of 30 mesh is selected. Resin, including a polymerisation catalyst is prepared as a catalyst premix. The premixed resin is added to the biochar to a ratio of 1:2 -1:16 resin: biochar as required. The resin / biochar is mixed, either by hand or using a suitable mixing device until homogeneous. Typically, where a standard mixing machine known in the art is used, a low mixing speed (~50 r.p.m.) is used. Once the mixture is near homogeneous, then a higher mixing speed can be introduced. It is important during the initial mixing phase that biochar particles are not rendered airborne. When mixing is complete, the mixture is decanted to a mould cavity and a top pressure plate added. A decorative brick-dust layer can be added to the top of the mixture. If this is the case, then a prepressure of approximately 50% of the final intended pressure is applied to the mixture before the brick-dust layer is added, typically using a brush. The top-plate is then re-secured in position and the final, desired pressure applied. This can be form (0.1 - 20.0 MPa). For brick slip samples of length 101.5mm, width 50.8 mm and a resulting (pressed) thickness ~14mm, a pressure of 2.7t (5 Mpa) is used. The pressure was applied for 5 minutes. The pressure is released and the top-plate removed. The moulded product is removed using a plug of the same size as the product to avoid damage to the product. The product is placed on a tray to dry. Heating can be applied to assist this process. Where the resin is furan-based for example, a temperature of 100°C for 120 minutes can be used. Example 2 A furan-based resin mixture was prepared comprising (parts by weight): Furolite 050915 RF2ST (100.0), Phosphoric acid (as 85%w / w) (4.0) and HM1448 (1.0). The resin mixture was then added to 50.0g biochar in different amounts to give a different resin: biochar weight ratio: 25.0g for a 1:2 ration, 16.6g for a 1:3 ratio, 12.5g for a 1:4 ratio, and 8.3g for a 1:6 ratio. The actual mass of Furolite was adjusted depending on the water content of the sample (varied from 6% to 14%) to give the required ratio of resin: biochar. The viscosity of samples, varying with water content and temperature, is shown in Table 1. § Temperature rq 6,3 « HaO H-0 % 11,6¾ HsO lie 5^75 1353 $36 IM1 111® €45 352: [ 40 7C.7' 514 w W :341, 244 JaS. W ss 183 : 15$ 103 $7 j 70 BS 50 37 Table 1: Viscosity of resin samples produced In addition to the above step, further, optional steps which can be undertaken are as follows. First, prior to the full pressure being applied to compact the article, a lower pressure is or is not applied to pre-compact and pre-shape the product if desired, a decorative surface can be added to the article to be incorporated into the final article. For example, a cover layer of ground, recycled bricks (or other materials such as a veneer) can be applied to provide a different appearance. This is then followed by the full pressurising step and is then permanently bound through the curing process. The water absorption of the materials formed, determined on samples immersed in water for 24h at 21°C, depends on the binder, compaction pressure, resin to biochar ratio and resin type. Without compaction, the water absorption was determined to be between 32% to 44% (for ratios 1:2,1:2.5 and 1:3 using epoxy binder and coarse biochar). With 50 bar compaction pressure, the water absorption dropped between 3% to 21% (for ratios 1:2,1:3,1:4 and 1:6); the water absorption was significantly lower for epoxy-based samples (3% to 7%) compared to Furan based samples (13% - 21%). Once the material has been formed and cured, the surface of the material can be coated and polished if desired. This can be applied to reduce or eliminate the water absorption of the material, which otherwise can be up to 44%w / w without compaction or up to 21% when suitably compacted due to the porous nature of the biochar, or as a visual effect. If required, the biochar can be substituted, at least partially with a recycled composite material. The composite material can be for example, derived from a wind turbine or boat hull ground to a fine powder, such as having a particle size similar to that of the biochar particle size. The recycled composition is typically a mixture of glass fibres (sometime other fibres such as carbon, aramid etc are present) and cured polyester or epoxy resins (or other thermoset resins). This can also contain less desired material such as paint fragments, small metal pieces, parts of cores (balsa, foams or others) or contaminants. Alternatively, the biochar can be substituted with mineral fillers, e.g. recycled construction waste and / or mining by-products and / or fly ash and / or salts (e.g. Calcium carbonate) and / or glass and / or geopolymers. The fillers can replace biochar in the range of 0% to 100%, most interestingly 0% - 50%. The filler is incorporated in a similar particle size to the biochar, more preferably a coarser size than the biochar when recyclates are used together. With biochar, incorporating fibre reinforcements in the material increases tensile and compressive strength for use in more load-bearing solutions such as lintels. The fibres can be present as individual fibres, fibre mats or fabrics. Preferably, the fibres will be in the outer region and especially on the outside of the article formed and act as a sandwich material. Alternatively, fibres can be distributed throughout the material. Ideal fibre types are natural fibres e.g. flax and hemp which can also act as carbon sinks. Example 3 Samples were produced using a hardened high strength steel pellet press die (from Pellet Press Dies UK) with 50mm diameter to produce samples for characterisation of resulting material density and later testing of compressive properties. Sample mixtures with resin to biochar ratios of 1:2, 1:3, 1:4 and 1:6 were prepared as described above using biochar ground to nominal 30 mesh. For samples with ratio 1:2, 26g biochar was weighted and for all other ratios 39g. The required amount of premixed resin with catalyst (and hardener), trialled here Epoxy (SR GreenPoxy 33 with slow hardener from Sicomin Composites) and Furan (Furolite 050915 from Trans Furans Chemicals) resin was added by weight to achieve the desired mixing ratios; for Furan resins, the resin viscosity was adjusted to be <1000cP by adding additional water and the water content was disregarded to determine the resin : biochar ratio. The biochar and resin mixture was carefully mixed by hand in a mixing cup using a lollipop stick until the mixture was homogenous and no resin was stuck to the wall of the container and no agglomerations were present. The resin was then carefully filled into the pallet die, the mixture was slightly compacted with a spoon to ensure the mixture was below the upper rim of the die. The plunger was inserted, and the filled die set placed in the centre of the press platens (Carver 3891 CEB Bench Top Press). The material inside the die press was then compacted using the press with target load of 0.2t, 0.5t, It, 2t, 3t and 4t pressure (equates to lObar, 25bar, 50bar, lOObar, 150bar and 200bar respectively), pressure was read at the press' pressure gauge. The material remained under pressure for 5 min after which the die was removed from the press. The die was placed upside down on a sturdy surface and the die's bottom support plate removed. The plunger was then used to push out the compacted material by holding the die sleeve and sliding it down along the plunger. Once fully outside the die sleeve, the compacted sample was moved manually (taking care not to deform the sample) onto a tray and left to cure (room temperature curing epoxy) or placed into an oven for up to 2h at 120C (Furan resins). The fully cured sample was then labelled and packed for later testing. Mechanical Testing Mechanical tests were performed on an Instron 5980 series universal test machine equipped with a 250 kN load cell and flat compression plates in a controlled laboratory environment at 20°C. The calibrated system was tared and samples placed between the compression plates. The cylindric test sample diameter was 50 mm and the thickness was between 16 mm to 30 mm, with 16 mm to 19 mm measured for samples with resin to biochar ratio of 1:2 and 23 to 30 mm for all other test ratios. Test speed selected was 1 mm / min in line with EN 772-1 1 Methods of test for masonry units. Determination of compressive strength. The tests were stopped once the maximum sample load was exceeded, and the sample started to disintegrate. Maximum load from these tests were used to determine maximum compression strength according to stress = force / surface area. 1.20 CH b log | 0.80 0.60 M -1:6 — • -fine bio 0.40 10 IS 20 Pressure, MPa 5 Figure 1: Resulting material density as function of compaction pressure used. Material coarse and fine (30 mesh) biochar, resin used Epoxy. Density determined from dimensions of produced cylindrical samples with 50 mm diameter. Fine biochar content determined from displacement of pallet press die after pressure released at given pressure. 10 max. test machine limits exceeded 120 « 100 Q. s x: 80 s so 40 E 20 10 15 Pressure, MPa 20 10 Figure 2: Measured compression strength of samples using Epoxy and Furan (indicated with an F) as binder at different mixing ratios. Compression tests performed at 1 mm / min. Sample diameter 50 mm and height 1:2 ratio samples 16 mm - 19 mm, height other ratios 23 mm - 30 mm. Measurements taken on an Instron universal test machine with 250 kN loadcell. - Density Figure 3: Ashby chart positioning the "biochar" material with ranges 1:2 to 1:6 using epoxy and furan resin in comparison to selected other building materials.

Claims

Claims1. A method of forming a construction element, the method comprising the steps of selecting a monomer material, the monomer being a bio-based monomer, mixing the monomer with a polymerisation catalyst to form a pre-binder mixture,mixing the pre-binder mixture into biochar, to form a preformed mixture, transferring the preformed mixture into a forming volume and allowing to at least partially harden to form the construction element.

2. A method according to Claim 1, wherein the monomer is a furan-based monomer.

3. A method according to Claim 2, wherein the monomer is furfuryl alcohol.

4. A method according to Claim 1, wherein the monomer is selected to form a bio-epoxy resin, a polyester or a polyurethane or a polyurea or an unvulcanised rubber or a vinyl ester or a bismaleimide or a polyimide or a cyanate ester or a phenolic or a phenol-formaldehyde or a urea-formaldehyde or a melamine resin or a in-situ polymerising thermoplastic resin e.g. Elium (RTM)on polymerisation.

5. A method according to any preceding claim, wherein a mixture of two or more polymeric materials is utilised as a binder.

6. A method according to any preceding claim, wherein the monomer is mixed with the polymerisation catalyst to form a mixture having a viscosity of <3.1 Pascal second.

7. A method according to Claim 6, wherein the mixture has a viscosity of <2.0 Pascal second.

8. A method according to Claim 7, wherein the mixture has a viscosity of <1.0 Pascal second.

9. A method according to any preceding claim, wherein the biochar is provided as a powder having a grain size of 1pm to 5mm.

10. A method according to Claim 9, wherein the biochar is provided as a powder having a grain size of 2pm to 2mm.

11. A method according to Claim 10, wherein the biochar is provided as a powder having a grain size of 2pm to 600pm.

12. A method according to any preceding claim, wherein the biochar has been pyrolysed at a temperature of 350°C to 2,500°C prior to use.

13. A method according to Claim 12, wherein the biochar has been pyrolysed at a temperature of from 500°C to 800°C prior to use.

14. A method according to Claim 13, wherein the biochar has been pyrolysed at a temperature of greater than 500°C prior to use.

15. A method according to any preceding claim, wherein the ratio of resin binder to biochar is within the range of 1:2 - 1:16 by weight.

16. A method according to Claim 15, wherein the ratio of resin binder to biochar is within the range of 1:3 - 1:16 by weight.

17. A method according to Claim 16, wherein the ratio of resin binder to biochar is within the range of 1:3 - 1:6 by weight.

18. A method according to any preceding claim, wherein an overpressure is applied of from 0.1 MPa to 20.0 MPa.

19. A method according to Claim 18, wherein the overpressure applied is from 0.1 to 10.0 MPa.

20. A method according to Claim 19, wherein the overpressure applied is from 0.5 to 5.0 MPa.

21. A method according to Claim 20, wherein the overpressure applied is from 1.0 to 5.0 MPa.

22. A method according to any preceding claim, wherein pressurisation is carried out from Is to 10 minutes.

23. A method according to Claim 22, wherein pressurisation is carried out from 30s to 7 minutes.

24. A method according to Claim 23, wherein pressurisation is carried out from 30s to 5 minutes.

25. A method according to any preceding claim, wherein the density achieved by pressurisation is from 0.70 to 1.40 g / cm3.

26. A method according to Claim 25, wherein the density achieved by pressurisation is from 0.80 to 1.30 g / cm3.

27. A method according to any preceding claim, wherein the resin / biochar mixture is subjected to an elevated temperature of from 20 to 180°C.

28. A method according to Claim 27, wherein the resin / biochar mixture is subjected to an elevated temperature of from 60 to 120°C.

29. A method according to Claim 27 or Claim 28, wherein the mixture is held at the elevated temperature for 0.5 to 120.0 minutes.

30. A method according to any preceding claim, wherein a layer of decorative material is added to the mixture prior to pressurisation.

31. A method according to any preceding claim, wherein the mixture includes fibres to reinforce the construction element.

32. A method according to Claim 31, wherein the fibres are incorporated into the outer region of the element.5 33. A method according to Claims 18 - 32, wherein overpressure is applied using a double beltpress or calendar roller / calendar press.

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