GB2132598A - Building material strengthened by fibres - Google Patents

Abstract

A building material which contains no asbestos is constituted mainly by an hydraulic binder, for example cement, strengthened by fibres having a diameter of between 3 and 8 microns and a breaking- strength of at least 800 N/mm<2>. The fibres may be of viscose or polyvinylalcohol. The material may also contain active or inert fillers. The manufacture of the material takes place on machines for the manufacture of asbestos-cement material, and can be formed into pipes or sheets.

Description

SPECIFICATION Building material based on a hydraulic binder strengthened by fibres and its method of manufacture The present invention relates to a building material constituted mainly by a hydraulic binder strengthened by fibres as well as to the method for the manufacture of said material.

In building products based on cement or a hydraulic binder and asbestos, it is known to replace asbestos fibres by fibres of another type, namely organic and/or minerai. Since the manufacture of products from asbestos-cement gradually takes place from a mixture of asbestos fibres and cement fibres suspended in water, then filtered in order to obtain a layer which is then transformed into sheets or pipes, it is necessary to use replacement fibres which allow filtration, on the one hand and which ensure strengthening of the finished product, on the other hand. Filtration is intended to mean retention of the fibres on a filtering wall, which fibres thus form a lattice and thus prevent the passage of particles of cement through said wall. Similarly, the purpose of the strengthening fibres is to remedy the brittleness inherent in a product which would be constituted by pure cement.If the asbestos fibres satisfy these two conditions simultaneously, the same is not true for the replacement fibres presently used and it is therefore necessary to make use of two different types of fibre, one of which facilitates filtration and the other of which gives the finished product its mechanical strength by virtue of the strengthening effect.

Thus, cellulose fibres, which impart little mechanical strength to the finished product, improve the filter-ability of the suspensions containing other strengthening fibres. But these fibres are sensitive to humidity which causes swelling of the latter generating microfissures and they are subject to degradation by micro-organisms, so that it is preferable to avoid laying a product containing cellulose fibres in humid atmospheres or outside buildings.

An object of the invention is to remedy or at least mitigate these drawbacks. According to a first aspect of the invention there is provided a building material which does not contain asbestos, said material being constituted mainly by a hydraulic binder strengthened by fibres, and said fibres having a diameter of between 3 and 8 microns and a tensile strength of at least 800 N/mm2.

Surprisingly it has been found that since the fibres replacing asbestos fulfil this double function, they are able to satisfy the two requirements of filtration and adequate mechanical strengthening.

As stated, the fibrous used have an average diameter of between 3 and 8 microns. It is possible that a diameter of fibres less than 3 microns may be suitable, but present technology does not make it possible to obtain such fibres. If the diameter exceeds 8 microns, the lattice of fibres retained has too large a mesh, which thus allows an excessive fraction of particles of cement to pass during filtration.

The nature of the fibres is immaterial, provided that they withstand the alkaline action of the hydraulic binder and that they have a tensile strength greater than 800 N/mm2. This value has been determined after examining the mechanical characteristics of the finished product, which according to the use envisaged, either a pipe or covering sheet, must be adequate, at least in the longitudinal direction, corresponding to the direction of travel of the felt on which the filtration cake is received. Synthetic organic fibres are quite suitable, in particular polyethyiene, polypropylene, polyacrylonitrile, polyacrylamide, polyvinyl alcohol. Mineral fibres may also be used to advantage, on condition that they have an adequate alkali-resistant composition.

Fine fibres are used either in the form sold by the supplier, or after surface treatment which improves their anchorage in the cement matrix. These fibres generally have a circular section, but the shape of a single or multiple lobe or of a tape will also be satisfactory. In a case of this type, the diameter of the fibres will be replaced by the equivalent diameter:

in which S is the section.

The length of the fibres is at least 1 mm, in order that the connection between the fibre and the cement matrix is ensured, even if microfissures were to appear in the finished product. It is less than 10 mm, a length beyond which difficulties as regards use occur.

The hydraulic binder, may be cement which may or may not have an additive. The cement may be an aluminous cement, a slag cement or a blast furnace cement. Similarly, plaster, gypsum or lime may be suitable.

The material can also contain inert or active fillers which give the finished product the desired characteristics, such as heat retention or inhibition of the lime liberated when the cement sets.

It is also advantageous to provide flocculating agents of known type, such as those based on soluble polymers, the function of which is to increase the retention of solid particles and dispersing agents, whose action prevents the formation of accumulations or tufts of fibres in the suspension subject to filtration.

According to a second aspect of the invention there is provided a method of manufacturing the material according to the second aspect, wherein said fibres, and hydraulic binder are mixed in water, the mixture is filtered with a view to obtaining one or more basic layers forming a single layer which is wound, then cut, shaped and dried in an oven, the specific proportion of the fibres, or ratio between the percentage of the weight of the fibres with respect to the total dry materials and the specific mass of the fibres expressed in g/cm3, being between 1 and 5.

If filler material is used it may be mixed with the fibres and hydraulic binder in water before the mixture is filtered.

The fibres are first of all put into suspension in water, in a quantity which is determined by the conditions for satisfactory operation of the machine. This quantity is between 1 and 5% by volume of fibres with respect to the finished product; nevertheless, in order to take into account the various natures of the fibres which can be used, a parameter is introduced which deals with the density of fibres, namely the "specific proportion" Pf/p in which Pf is the percentage by weight of fibres with respect to the total weight of dry materials and p is the mass of fibres expressed in g/cm3. This specific proportion is advantageously comprised between 1 and 5.

Cement is then added, at the rate of 60 to 98 parts by weight with respect to the total of dry materials. When the products are manufactured on machines of the "board machine" type, the active or inert fillers do not in principle exceed 30% by weight of cement, which represents 0 to 30 parts by weight of the total of dry materials. The proportion of filler may be greater when the manufacturing technique differs: casting, moulding, centrifugation, extrusion for example.

After making the mixture homogeneous by stirring the aqueous suspension, the latter is introduced into machines whereof the two main types are those carrying out the method with a screen, such as the HATSCHEK machine and those using the method without a screen, such as the MAGNANI machine.

In the method with a screen, a vat receives the aqueous suspension, which is filtered through a metal gauze disposed on a rotating cylinder. A fine layer of fibres and cement is thus deposited on the screen, which, after transfer to the felt, forms a basic layer. The superimposition of several basic layers, in the case of several vats with screens, produces a single layer which is wound on a cylinder until the desired thickness of product is obtained. When the latter is reached, a device for cutting along a generatrix makes it possible to unroll the paste in the form of a flat sheet which is then matured in a flat, undulating or profiled form. Another current use of this mixture is the manufacture of pipes.

In both cases, the products obtained are matured either in the open air, or in water, or in an oven for several hours in an atmosphere saturated with humidity.

Several mixtures are prepared under the conditions according to the invention, for the purpose of making flat test pieces which are subjected to rupture tests, such as those provided by the standard ASTM D-38 80-80. The attached final table combines the results obtained with several types of fibres, depending on the different parameters of machines, and by way of reference, the results obtained on a test piece containing asbestos.

Example 1 According to a first production example, a mixture is used containing cement and viscose fibres sold by Rhone Poulenc Textiles under the registered trade mark "Fibranne", the diameter of which is 6.5 microns, the length 5 mm and the breaking-strength 800 N/mm2. The mixture is constituted by 5% by weight fibres and a flocculating agent of known type (E 31 8) is added thereto at the rate of 3 g/litre. The flat sheet obtained from the single layer resulting from filtration is compressed at a pressure of 1 00 bars, then left to mature at ambient temperature. One then proceeds with breaking tests, after 14 days, in a saturated state and one notes the resistance to bending, both in the direction of travel L of the screen of the machine as well as in the perpendicular direction T to this travel.

Example 2 The same type of fibres as in example 1 is used under the same conditions, but the tests are carried out after 21 days, in the dry state. The density is thus 1.65.

Example 3 The fibres used are of high strength viscose, having a diameter of 6 microns, a length of 5 mm and a breaking-strength of 930 N/mm2. The mixture subjected to filtration contains 2% fibres and 98% by weight cement and maturing takes place at ambient temperature, in a confined environment. In this case, the density of the plate subject to breaking tests after 1 4 days is 1.75.

Example 4 Polyvinyl alcohol (PVA) fibres, having a diameter of 7 microns, a length of 6 mm and breakingstrength of 1 100 N/mm2, are mixed with the cement at the rate of 4 parts by weight to 96 parts of cement. The sheet is not compressed after manufacture. It undergoes maturing in a saturated environment and in this case the breaking tests are carried out after 1 4 days. The density of the sheet is 1.55.

Example 5 Fibres of the same type as in example 4 are added to the cement at the rate of 4 parts to 96 parts by weight of cement. The sheet is compressed at a pressure of 200 bars after manufacture, but the other conditions of maturing and tests remain identical to those of example 4.

Example 6 A quantity of 1.5% by weight polyvinyl alcohol fibres, having a diameter of 1 2 microns, a length of 6 mm and a breaking-strength of 1100 N/mm2 is added to 98.5% cement and the mixture is filtered.

During this operation, it is found that it is impossible to obtain a suitable basic layer of composition on the screen, since 80 to 90% of the cement particles are thrown away with the filtration water and the felt receives almost solely fibres.

Example 7 The reference member is constituted by asbestos, of the chrysotile type in the form of a mixture of 4-5-6 grade; 80% of the fibres have a diameter comprised between 1 and 1 0 microns. As previously, a sheet is prepared, without compression after manufacture but with maturing at ambient temperature in a confined environment. This sheet then undergoes the breaking tests.

On reading the table it will be seen that after hardening, the material is distinguished from asbestos-cement by a greater deformation capacity, which provides increased safety during use. In fact, it is possible to allow the elastic limit to be exceeded accidentally one or more times, without cracks or a dangerous rupture necessarily occurring.

It will also be noted that the density of the filtrate or the density of the waste material, during manufacture, is representative of the satisfactory filtration of the mixture in suspension, achieved by the fibres according to the invention.

The use of these fibres provides an appreciable simplification of the preparation of mixtures and suspensions, since they require no previous refining treatments or other treatments which consume a considerable amount of energy.

As a variation, the building material according to the invention may be prepared from a mixture of different fibres and hydraulic binder, the only conditions to be respected being those relating to the dimensional characteristics of the fibres (diameter, length), to the mechanical strength and to the resistance to alkalies.

Examples 1 2 3 4 5 6 7 Nature of Fibre Viscose Viscose High strength PVA PVA PVA A.N.

viscose Diameter of Fibres ( ) 6.5 6.5 6 7 7 12 1 to 10 Length (mm) 5 5 5 6 6 6 Breaking-strength 800 800 930 1100 1100 1100 (N/mm) Composition of mixture (% 5 5 2 4 4 1.5 11 weight fibres/total) Screen vat density 1.020 1.020 1.048-1.036 1.070 1.070 - 1.050-1.060 Waste material density 1.003-1.004 1.003-1.004 1.006-1.012 1.003-1.026 1.003-1.026 1.003-1.004 Compression after Yes(100 bars) Yes(100 bars) No No Yes(200 bars) - No manufacture Maturing Water Water Confined environment Saturated Saturated - Confined Density of finished 1.63 1.65 1.75 environment environment - environment product 1.55 1.80 - 1.58 Breaking conditions 14 days 21 days 14 days in 14 days in 14 days in - 14 days saturated dry the present the present the present saturated state state state Resistance to L 17.7 22.8 18.2 27.3 35.5 - 29 bending (MPa) T 14.1 16.5 12.0 18.5 25.2 - 22 Bending L 1.8 1.9 2.61 32.6 17.9 - 2.92 Deformation (x 10-3) T 1.3 1.3 1.48 13.8 3.7 - 2.28 Bending L 12,500 14,600 12,700 14,400 21,900 - 13,850 Modulus (MPa) T 10,800 12,100 11,500 13,200 19,300 - 12,800

Claims (11)

Claims

1. A building material which does not contain asbestos, said material being constituted mainly by a hydraulic binder strengthened by fibres, and said fibres having a diameter of between 3 and 8 microns and a tensile strength of at least 800 N/mm2.

2. A material as claimed in claim 1, in which the section of the fibre is not circular, its equivalent diameter, given by equiv.=2s where S is the section, is comprised between 3 and 8 microns.

3. A material as claimed in claim 1 or claim 2, in which the length of the fibres is between 1 and 10 mm.

4. A material as claimed in any one of claims 1 to 3, in which the fibres used are synthetic organic fibres.

5. A material as claimed in claim 4, in which the fibres used are polyvinyl alcohol fibres.

6. A material as claimed in any one of claims 1 to 3, in which the fibres are of mineral origin.

7. A method of manufacturing the material claimed in any one of claims 1 to 6, wherein said fibres and hydraulic binder are mixed in water, the mixture is filtered with a view to obtaining one or more basic layers forming a single layer which is wound, then cut, shaped and dried in an oven, the specific proportion of the fibres, or ratio between the percentage of the weight of the fibres with respect to the total dry materials and the specific mass of the fibres expressed in g/cm3, being between 1 and 5.

8. A method as claimed in claim 7, in which filler material is mixed with the fibres and hydraulic binder in water before the mixture is filtered.

9. A method as claimed in claim 7 or claim 8, in which the hydraulic binder is cement which amounts to 60 to 98% by weight of the total weight of dry materials.

10. A method as claimed in claim 8, in which active or inert fillers added to the mixture of fibres and cement in suspension in water amount to up to 30% by weight of the total weight of dry materials.

11. A building material which does not contain asbestos, said material being as claimed in claim 1 or claim 2, and substantially as hereinbefore described in any one of Examples 1 to 5.