GB1604736A - Multi-layer board - Google Patents

Abstract

This comprises a core layer (10) of an inorganic material, substantially consisting of a cured inorganic binder and, if appropriate, additives, and two single-sided, fibre-reinforced outer layers (11, 13), which are joined in a force-transferring manner to the core layer. Between the core layer and each outer layer, consisting of plastic, there is provided a fibre-reinforced intermediate layer (12, 14) of cured natural or synthetic resins, which are water-dilutable in the uncured state. <IMAGE>

Description

(54) MULTI-LAYER BOARD (71) We, PORTLAND ZEMENT WERKE HEIDELBERG AG, of 6 Berliner Strasse, Heidelberg, Germany, a company organised and existing under the laws of the Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a multi-layer loadbearing board and a process of preparing the same.

Multilayer boards composed of laminated layers are known. U.S. Patent No. 2 806 811, for example, describes a paper-covered gypsum board wherein the paper layers are bonded to the gypsum board with a resin adhesive.

Laminated boards taking advantage of the good qualities of inorganic materials used for the core layer and organic plastics for the cover layer or layers are very useful. However, considerable difficulties have been encountered in providing a true and lasting bond between layers of such different materials since not only the adhesion but also the mechanical properties of the materials cause problems.

Glueing or bonding with adhesive does not provide strong enough lamination.

Attempts to bond the two layers to each other while their materials were still wet and the layers were, therefore, in a plastic condition, failed because when an organic plastics layer was cast on a fresh core layer of concrete or a like watercontaining cementitious binder material, which was not yet hardened, a water layer formed between the layers as the core layer hardened, and prvented formation of a bond between the layers. Similar disadvantages are found when a material in a plastic condition was applied to a hardened layer.

The invention consists in a process of preparing a multi-layer load-bearing board, which comprises separately forming a basic layer consisting of a cementitious inorganic material, and a cover layer of a fibre-reinforced plastics, providing an intermediate layer consisting of a hydrophilic resin dilutable or miscible with water or a cementitious binder material containing said resin, superposing the layers with the intermediate layer positioned between the basic layer and the cover layer while the material of at least one of the two last mentioned layers is still not hardened, and per mining the said material to harden at or above room temperature.

The invention also consists in a multi-layer load-bearing board when prepared by the abovedefined process of the invention.

The binder of the cementitious inorganic material is illustrated by such materials as cement, lime, gypsum, magnesite, and/or mixtures of magnesia and magnesium chloride, and may be fibreireinforced. In a preferred embodiment, the basic layer is of mortar or concrete.

In a preferred embodiment, the board has two cover layers, the basic layer being disposed betewen the two cover layers, with the interposition of said intermediate layers.

The material of the intermediate layer and accordingly the intermediate layer itself differs in kind, properties and effect from the adjacent layers. The intermediate layer is a bonding layer.

By suitably adjusting the properties of the individual layers, a substantially stress-free and load-bearing product is obtained which constitutes a very advantageous laminated plate combining the advantages of inorganic materials with those of organic plastics.

The basic layer (also referred to herein as core) has a high rigidity or stiffness and there is no danger of ageing since progressive hydration of the cementitious binder material will actually improve it. Subsequent shrinkage and cracks caused thereby are avoided and there is no decrease in rigidity because evaporation of the water is impeded.

The core has a resistance to deformation which imparts to the multi-layer board the characteristics of a single-layer board, or imparts to a pipe made therefrom the characteristics of homogeneous layer pipe. The cover layers are also load-carrying and contribute to the high quality of the board and enhance it. Heretofore, no sheet material of this type was known which was so well adapted for the production of round bodies and imparted to them the required rigidity. Being resistant to abrasion and wear, for instance by corrosion, the cover layers operate as protective layers and simultaneously provide a desired surface configuration.

They also absorb maxima of tensile forces.

Using an intermediate layer removes difficulties arising from surface irregularities at the interfaces. An intermediate layer transmits shearing forces, prevents the spreading of cracks and imparts great stability to the multilayer board. The connecting layer constitutes an advantageous transition between the core and the cover layers.

In contrast to the cover layers in known multi-layer boards, the cover layers in the boards of the invention are load-carrying layers, whereby the load-carrying capability of the board is increased. Since the board is a substantially integral structure, it combines this mechanical advantage wit the advantages obtained by the use of inorganic and organic structural materials.

The multi-layer board of the present invention is far superior in its mechanical properties to known laminates of this type and will find particular application in the construction industry, in water pipe systems, and in building tanks or like containers.

The binder material of the core layer may be cement, particularly a fast-hardening type of cement, or a magnesite type material, or a mixture of magnesia and magnesium chloride.

Other cementitious binder materials useful for the board of the invention are burnt lime and gypsum, or solutions of siliceous materials which may be rapidly hardened by the admixture of suitable additives. If desired, the cementitious binder material may contain fillers of natural or synthetic substances, such as sand, slag, bauxite or corundum and the like, as well as fibers of inorganic substances, including glass fibers, asbestos fibers and/or other mineral or metallic fibers, and fibers of organic substan es. It is also possible to add to the cementitious binder material hydrophilic, self-hardening plastics, such as a vinyl ester resin, a phenolic resin and/or other plastics with or without suitable curing agents. Such plastics additions to the cementitious binder material will improve the extensibility of the inorganic binder material matrix.

The cover layer comprises strong plastics and operates not only as a protective layer but forms, in fact, an integral part of the core layer in the multi-layer board, the core layer reinforcing the core at its weak points, in addition to being resistant to abrasion, wear and corrosion, as well as being capable of imparting a desired surface configuration to the board. It consists, at least primarily of a fiber-reinforced plastics material. The cover layer may contain inorganic fillers and/or carbon fibers. The fibers may be filaments, staple fibers, fibrous webs or rovings arranged in parallel. Similiar reinforcing fiber may be arranged in the core in one or several superposed layers and/or in the intermediate bonding layer(s).

If desired, the cover layers may consist of several plies which differ from each other in their mechanical properties.

The favourable properties of the multilayer board of the present invention are further enhanced by the use of the intermediate layers between the core and cover layers, the intermediate layer being comprised of hydrophilic plastics which may be cold hardening or thermosetting resins of the type of natural or synthetic elastomers. The intermediate layer may have a thickness of about 0.1 to about 5 mm or more. It may be reinforced with inorganic fibers, fabrics or webs.

Such an intermediate layer does not only add to the rigidity of the multi-layer board but it also transmits shearing forces, reduces such forces and impedes tears and cracks, thus increasing the stability of the board and shaped structures made therefrom. The intermediate layers also operate as bonding layers which provide a favourable interface between the core and cover layers. The reason for the strong bonding provided by the intermediate layer is the fact that it is a hydro philic, water-dilutable or water-miscible resin connecting a cementitious material (inorganic) and a plastics or other resinous material, i.e.

it has the properties of both layers it interconnects, thus tending to interface the core and cover layers with each other.

As indicated, we have found that the multilayer board may be prepared wet-on-wet, i.e.

the core layer with a matrix of a cemet;titjous binder material may not yet be hardened when it is laminated with an intermediate layer and with a cover layer of plastics not yet polymerized. The process of the invention has overcome the well known difficulties of binding inorganic and organic materials in their still workable condition, so that it is now possible to particularly deposit organic layers in the workable condition on inorganic layers in the workable condition, that is wet-on-wet and also wet-on-dry.

In order to make the invention clearly understood, reference will now be made to the accompanying drawings which are given by way of example, and wherein: FIG. 1 is a cross sectional view of a portion of a fiat board composed of a core faced by two cover layers given by way of illustration and not forming part of the invention; FIG. 2 is a similar cross section of such a board further comprising intermediate layers between the core and cover layers, and forming part of the invention; FIG. 3 is a similar cross section showing a modification of the embodiment of FIG 2; FIG. 4 is a side view of a portion of a multi-layer board constructed according to any of the preceding embodiments of the invention but being arcuately curved and indented, rather than extending rectilinearly;; FIG. 5 is an end view of such a board shaped into a pipe; and FIGS. 6 to 9 are end views of variously shaped boards incorporating the structure of the embodiments of FIGS. 1, 2 or 3.

The multi-layer board of FIG. 1 is comprised of core layer 1 and two cover layers 2 and 3 and is illustrative of known prior art.

The board of FIG. 2 is comprised of core layer 10, a cover layer 11 bonded to the core layer by intermediate layer 12, and another cover layer 13 bonded to the core layer by intermediate layer 14.

In the modification of this board shown in FIG. 3, the core layer has two plies 20 and 22 wherebetween there extends a fibrous layer 21, cover layer 24 being bonded to the two-ply core by intermediate layer 23 and cover layer 26 being bonded to the core by intermediate layer 25.

In FIG. 6, the board has a U-shaped section, in FIG. 7 it has a truncated V-shape with longitudinal flanges, the board of FIG.

8 is corrugated, and FIG. 9 is a rectangular hollow cross section. Other shapes may obviously be fabricated.

The following specific examples further illustrate the practice of this invention, all parts being by weight unless otherwise indicated.

Example 1.

(Board according to FIG. 2).

A flat one-square-meter multi-layer board according to FIG. 2 was produced in the following manner. Core 10 was glass fiberreinforced concrete having a thickness of 20mum and consisting of a very rapidly harden ing modified Portland cement having a water cement value (ratio of water to cement) of 0.4 and containing 5%, by volume, of alkali resistant glass staple fibers.Each cover layer 11 and 12 had the following composition: One hundred grams of a styrene-containing vinyl ester resin (a polymerized adduct of an epoxy resin and acrylic acid dissolved in styrene, with a styrene content of 45--50u,,) were dissolved with two grams of 500, methyl ethyl ketone peroxide in a plasticizer, 0.125 g of cobalt actoate (6 ,o Co in styrene) and 1.2 g of dimethyl aniline (10% in styrene) being added as catalysts and activators. A glass fiber web weighing 450 g/m was disposed in the vinyl ester resin.

The core layer was aged in a mold for 48 hours and the two cover layers were applied to the aged, hardened core layer before they were cured, the intermediate layers 12 and 14 being disposed between the core layer and the cover layers.

The parameters of the layers calculated according to the above equation were as follows: Modulus of elasticity of the cover layers, Et = 260,000 kp/cm2 Modulus of elasticity of the core layer, Ee = 200,000 kp/cm2 Thickness of the cover layer, t = 0.1 cm Thickness of the core layer, c = 2.00 cm Thickness of the structural part, b = 1.00 cm Sg = 190,707 kp/cm Each of said intermediate layers 12 and 14 had the following composition:: One hundred parts of "Beckopox" (Registered Trade Mark) VEP 22 ("Beckopox" being liquid or solid epoxy resins of Farbwerke Hoechst, Germany, which may be cured with the addition of commercially-available curing agents or special "Beckopox" curing agents and/or in conjunction with phenolic or amino resins at ambient or elevated temperatures), 80 parts of "Beckopox" special curing agent (also available from Farbwerke Hoescht, as a variety of modified polyamines and polyamide amines capable of imparting to the "Beckopox" epoxy resins different curing conditions and properties of the cured product), and 10 parts of alkali-resistant glass fibers having a length of 5 to 10 mm.

The surfaces of the cover layers facing the intermediate layer were roughened before the cover material was cured. The components of the intermediate layer were mixed and the mixture was deposited as intermediate layer 12 onto the not yet cured but roughened cover layer 11. Then the preformed core 10 was pressed onto the intermediate layer 12.

Then the following intermediate layer 14 was deposited between the cores 10 and the cover layer 13.

Modifications of the compositions were made by replacing the rapidly hardening modified Portland cement by an ordinary cement to which an accelerator was added, the specific accelerating agent used being calcium chloride.

In either cement formulation, the vinyl ester resin was replaced by an unsaturated polyester or an epoxy resm.

As special curing agents, "Beckopox" VEH 29 or VEH 14 were used, as well as the aliphatic polyamine "H 105 B" sold by "Rfltger- swerke h4eiderich, Germany (curing period 20 to 40 hours at 25"C). The "Beckopox" resins or curing agents were replaced by epoxy resins and curing agents therefor, sold by Ciba, of Basle, Switzerland, with substantially the same results.

Any suitable thermosetting plastics containing suitable curing agents may be used. Also, in addition to the mentioned glass fibers, it is possible to use resin-covered glass fibers, carbon fibers, graphite fibers, steel fibers or organic fibers.

The residual content of water in the cementitious binders of the core layer had no perceptible influence on the bonding quality of the intermediate or cover layers to the core.

No delamination occured under mechanical stress and the mechanical quality of the multilayer boards was excellent. Even higher qualities were obtained by adding to the cementitious binder material of the core layer about 5 to 10 percent by weight of the plastics used in the cover or intermediate layers.

Example 2.

(Board according to FIG. 2) The composition of the core layer was as follows: 100 parts of magnesia, 6 parts of a urea-formaldehyde condensation product, 142 parts of a 20% aqueous solution of magnesium chloride, 0.6 parts of glycerol or butyl glycol as a plasticizer. All components were thoroughly mixed and the mixture was placed into a mold for hardening.

The composition of the cover layer was as follows: 100 parts epoxy resin "Ciba X20", (CIBA is a Registered Trade Mark), 90 parts of "Ciba HT 907" epoxy resin curing agent, 10 parts of "DY 040" (an accelerator sold by Ciba), 1 part of Ciba's DY 062 epoxy resin accelerator, 50 parts of hydrocarbon resin E ("Lithoplast") and 100 parts of a glass fiber web. The components were mixed, the mixture was molded into a plate and cured at a temperature of 1300C for 90 minutes.

"Lithoplast" is a dark brown resin with a softening point of 1000C and a melting temperature of about 1200C to 1400C, having a molecular weight of 1000 to 2000. It is a hydrocarbon resin of aromatic character which contains hydrocarbons condensed in a ring, direct C-C bonds, secondary and tertiary C-atoms, and 2 to 3 double bonds per molecule. It is weakly polar.

The composition of the intermediate layer was as follows: 100 parts of Ciba's hydantoin resin, 100 parts of Ciba's curing agent for hydantoin resin, 20 parts of glass fibers and 1 part of a polyester fiber unwoven web ("KT 1751" of the firm Freudenberg, Weinheim, Germany).

The intermediate layer composition is poured in the liquid state over the shaped core and the formed cover layer was placed there over, and the laminate was subjected to a tem perature of 80"C for two hours.

A water-soluble epoxy resin, with a suitable curing agent therefor, was used instead of the hydantoin resin with the same results.

Example 3.

(Board according to FIG. 2).

The core layer had the following com position: 100 parts of Portland cement, 20 parts of mineral aggregates having a maxi mum dimension of 2 mm, 50 parts of water, 0.06 parts of liquefier, 6 parts of zirconium glass fibers, 0.1 parts of 100, sodium silicate solution.

As is well known, the mineral aggregates used in cement include such materials as aranaceous quartz, granite, diorite, quartz, quartz-phorphyry, basalt, quartzite, quartzi tic sandstone, other sandstones, dense limestone, other limestones and blast-furnace slag, as well as mixtures thereof, in grain sizes of 0.1 to 30 mm, preferably 0.8 to 8 mm. Such aggregate additions may also be used with advantage up to about 10 . by weight, in the cover and intermediate layer materials, fine cement also having been used as an advantageous additive in the intermediate layers.

The cover layer had the following composition: 100 parts of unsaturated, highly reactive polyester ("P 8" of BASF) of medium viscosity, having a double bond value of 0.20 0.3 parts of a cobalt accelerator solution con taining 1 ' Co, 2 parts of a catalyst paste (methyl ethyl ketone peroxide), and 100 parts of a roving fabric, the rovings consisting of short staple glass fibers.

The intermediate layer had the following composition: 100 parts of "Beckopox" VEP 22 epoxy resin, 50 parts of "Beckopox" VEH 14 curing agent, and 1 part of a polyester cotton fabric, the denier of the polyester fibers being 5 to 10 mm.

The multi-layer board of FIG. 2 was pro duced wet-on-wet from the core, intermediate and cover layers of Examples 3 and 4, i.e.

cover layer 11 was the lowest layer and the subsequent layers were superimposed thereon in the illustrated sequence.

Instead of the "P 8" polyester, mixtures of this resin with resin "E 200" of BASF, can be used with the same result. Also useful for this purpose were the alkali-resistant product "A 410" of BASF as well as such resins as "W 41" or "W 45" of Bayer Leverkusen or similar resins of Hoechst.

By preparing the laminates in the wet-on wet process, i.e. by superimposing the layers in the given sequence before the individual layers are hardened, the mechanical properties and resistance to peeling of the board are con siderably improved. In this connection, it has proven particularly useful to place a polyester or polyethylene web or fabric in the inter mediate layer, which contains wool or cotton fibers, i.e. fibers which well absorb the resin and produce a defined intermediate layer. Very good results are obtained using a web having randomly projecting fibers.

The thicknesses of the layers may be freely chosen to suit the end use of the multi-layer board, practical ranges encompassing 3 to 20 mm for the core layer, 2 to 10 mm for the cover layer, and 0.5 to 2 mm for the intermediate layer.

In the wet-on-wet process, several plies of the cover layer may be applied by providing superposed plies of plastics, and/or the core itself may consist of a plurality of superposed plies. In the latter case, as shown in FIG.

a fibrous layer may be disposed centrally in the core layer, which will prevent any propagation of cracks from ply to ply.

In providing multi-ply cover layers, the outer ply composition may be so selected as to make it resistant to chemical reactions and/ or this ply may be mixed with sand to make the board useful in an abrasive environment.

The surface layer of the outer cover layer in a multi-ply cover layer can be formed of a thermoplastic material, such as polyethylene, polypropylene, polyvinyl chloride, polyvinylidene fluoride, or other thermoplastics or also polyimides. For a good adhesion between the surface layer and the adjacent thermosetting resins, e.g. the resins of the cover layers (Example 2), it is preferred to press a thin fibrous reinforcement - in the main glass fiber - into the thermoplastic material so that upon curing a strong bonding is obtained.

With polyvinyl chloride a known binder may be used for applying the thin glass fiber fabric.

With such surface film or layer of the above thermoplastics an excellent corrosion-resistant layer is obtained. At the same time said thermoplastics - which may have a thickness of preferably 0.1 to 10 mm - are useful in sealing shapes as shown in Figures 4 to 9.

As illustrated, the board may be shaped into any desired form, including tubes. They may be molded into the desired shapes at the time of manufacture and various methods may be used in preparing tubes or pipes, including a centrifugal method in which layer after layer is consecutely applied in a continuous process from nozzles supplying the compositions of the respective layers. The multi-layer tube is then cured by means of warm air or infrared radiation, at a maximum temperature of about 80"C.

The tube may also be produced by winding the cover layers over the core layer which is produced on a mandrel which receives a ribbon of the core layer composition wound about the mandrel.

After the multi-layer board has been finished, it may be subjected to desired surface treatments, for instance a plastic coating and/or polishing.

WHAT WE CLAIM IS:- 1. A process of preparing a multi-layer load-bearing board, which comprises separately forming a basic layer consisting of a cementitious inorganic material and a cover layer of a fibre-reinforced plastics, providing an intermediate layer consisting of a hydrophilic resin dilutable or miscible with water or a cementitious binder material containing said resin, superposing the layers with the intermediate layer positioned between the basic layer and the cover layer while the material of at least one of the two last mentioned layers is still not hardened, and permitting the said material to harden at or above room temperature.

2. A process as claimed in claim 1, wherein the layers are formed by molding.

3. A process as claimed in claim 1 or 2, comprising the further step of coating the multi-layer board with a synthetic resin.

4. A process as claimed in claim 1, 2 or 3, comprising the further step of polishing at least one surface of the multi-layer board.

5. A multi-layer load-bearing board when made by the process of claim 1.

6. A multi-layer load-bearing board as claimed in claim 5, wherein the cementitious inorganic material has a binder selected from cement, lime, gypsum, magnesite and/or mixtures of magnesia and magnesium chloride.

7. A multi-layer load-bearing board as claimed in claim 6, wherein the cementitious inorganic material contains aggregates and/or fillers.

8. A multi-layer load-bearing board as claimed in claim 5, wherein the core or basic layer is comprised of mortar or concrete.

9. A multi-layer load-bearing board as claimed in claim 5, wherein the cementitious inorganic material contains admixtures.

10. A multi-layer load-bearing board as claimed in claim 5 or 9, wherein the cementitious inorganic material is fiber reinforced.

11. A multi-layer load-bearing board as claimed in claim 10, wherein the reinforcing fiber is selected from filaments, staple fibers, fibrous mats and rovings.

12. A multi-layer load-bearing board as claimed in claim 5, wherein the intermediate layer is fiber-reinforced.

13. A multi-layer load-bearing board as claimed in claim 12, wherein the reinforcing fiber is selected from filaments, staple fibers, fibrous mats and rovings.

14. A multi-layer load-bearing board as claimed in claim 5 or 12, wherein the intermediate layer contains fillers.

15. A multi-layer load-bearing board as claimed in claim 5, wherein the synthetic resin of the cover layer is a thermosetting resin.

16. A multi-layer load-bearing board as claimed in claim 5, wherein the resin of the cover layer is a thermoplastic.

17. A multi-layer load-bearing board as claimed in claim 15 or 16, wherein the cover layer contains fillers.

18. A multi-layer load-bearing board as claimed in claim 15, wherein the reinforcing fiber of the cover layer is selected from filaments, staple fibers, fibrous mats and rovings.

19. A multi-layer load-bearing board as claimed in claim 5, comprised of two of said cover layers and two of said intermediate layers interconnecting the core and respective ones of the cover layers in a force-transmitting manner, the core layer being interposed between the intermediate layers.

20. A multi-layer load-bearing board as claimed in claim 5 or 17, further comprising

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (21)

**WARNING** start of CLMS field may overlap end of DESC **. the core layer, which will prevent any propagation of cracks from ply to ply. In providing multi-ply cover layers, the outer ply composition may be so selected as to make it resistant to chemical reactions and/ or this ply may be mixed with sand to make the board useful in an abrasive environment. The surface layer of the outer cover layer in a multi-ply cover layer can be formed of a thermoplastic material, such as polyethylene, polypropylene, polyvinyl chloride, polyvinylidene fluoride, or other thermoplastics or also polyimides. For a good adhesion between the surface layer and the adjacent thermosetting resins, e.g. the resins of the cover layers (Example 2), it is preferred to press a thin fibrous reinforcement - in the main glass fiber - into the thermoplastic material so that upon curing a strong bonding is obtained. With polyvinyl chloride a known binder may be used for applying the thin glass fiber fabric. With such surface film or layer of the above thermoplastics an excellent corrosion-resistant layer is obtained. At the same time said thermoplastics - which may have a thickness of preferably 0.1 to 10 mm - are useful in sealing shapes as shown in Figures 4 to 9. As illustrated, the board may be shaped into any desired form, including tubes. They may be molded into the desired shapes at the time of manufacture and various methods may be used in preparing tubes or pipes, including a centrifugal method in which layer after layer is consecutely applied in a continuous process from nozzles supplying the compositions of the respective layers. The multi-layer tube is then cured by means of warm air or infrared radiation, at a maximum temperature of about 80"C. The tube may also be produced by winding the cover layers over the core layer which is produced on a mandrel which receives a ribbon of the core layer composition wound about the mandrel. After the multi-layer board has been finished, it may be subjected to desired surface treatments, for instance a plastic coating and/or polishing. WHAT WE CLAIM IS:-

1. A process of preparing a multi-layer load-bearing board, which comprises separately forming a basic layer consisting of a cementitious inorganic material and a cover layer of a fibre-reinforced plastics, providing an intermediate layer consisting of a hydrophilic resin dilutable or miscible with water or a cementitious binder material containing said resin, superposing the layers with the intermediate layer positioned between the basic layer and the cover layer while the material of at least one of the two last mentioned layers is still not hardened, and permitting the said material to harden at or above room temperature.

2. A process as claimed in claim 1, wherein the layers are formed by molding.

3. A process as claimed in claim 1 or 2, comprising the further step of coating the multi-layer board with a synthetic resin.

4. A process as claimed in claim 1, 2 or 3, comprising the further step of polishing at least one surface of the multi-layer board.

5. A multi-layer load-bearing board when made by the process of claim 1.

6. A multi-layer load-bearing board as claimed in claim 5, wherein the cementitious inorganic material has a binder selected from cement, lime, gypsum, magnesite and/or mixtures of magnesia and magnesium chloride.

7. A multi-layer load-bearing board as claimed in claim 6, wherein the cementitious inorganic material contains aggregates and/or fillers.

8. A multi-layer load-bearing board as claimed in claim 5, wherein the core or basic layer is comprised of mortar or concrete.

9. A multi-layer load-bearing board as claimed in claim 5, wherein the cementitious inorganic material contains admixtures.

10. A multi-layer load-bearing board as claimed in claim 5 or 9, wherein the cementitious inorganic material is fiber reinforced.

11. A multi-layer load-bearing board as claimed in claim 10, wherein the reinforcing fiber is selected from filaments, staple fibers, fibrous mats and rovings.

12. A multi-layer load-bearing board as claimed in claim 5, wherein the intermediate layer is fiber-reinforced.

13. A multi-layer load-bearing board as claimed in claim 12, wherein the reinforcing fiber is selected from filaments, staple fibers, fibrous mats and rovings.

14. A multi-layer load-bearing board as claimed in claim 5 or 12, wherein the intermediate layer contains fillers.

15. A multi-layer load-bearing board as claimed in claim 5, wherein the synthetic resin of the cover layer is a thermosetting resin.

16. A multi-layer load-bearing board as claimed in claim 5, wherein the resin of the cover layer is a thermoplastic.

17. A multi-layer load-bearing board as claimed in claim 15 or 16, wherein the cover layer contains fillers.

18. A multi-layer load-bearing board as claimed in claim 15, wherein the reinforcing fiber of the cover layer is selected from filaments, staple fibers, fibrous mats and rovings.

19. A multi-layer load-bearing board as claimed in claim 5, comprised of two of said cover layers and two of said intermediate layers interconnecting the core and respective ones of the cover layers in a force-transmitting manner, the core layer being interposed between the intermediate layers.

20. A multi-layer load-bearing board as claimed in claim 5 or 17, further comprising

a coating over at least one of the cover layers.

21. A multi-layer board as claimed in claim '0, wherein a reinforcing material is embedded in the protective coating.