GB2053185A - Preparing fibre-reinforced binders - Google Patents

Abstract

A hydraulic binder reinforced with polyvinyl alcohol fibres, is prepared by distributing the fibres in the binder using a dispersing agent.

Description

SPECIFICATION Preparing fibre-reinforced binders Many fibrous reinforcing media have been used for cement-like materials and hydraulic binders. Examples of such media are natural, metallic and synthetic fibres of materials such as asbestos, glass, steel, carbon, aramide and polyolefins. Disadvantages are associated with the use of all these materials, and many of them have only a small effect on the mechanical and flexural strength of the product.

British Patent Publication No. 2,009,276A discloses the use of polyvinyl alcohol (PVA) fibres in cement products. However, it has proved difficult to achieve satisfactory and uniform distribution of the fibres in the water/cement mix. When using the mixing ratios conventional in the asbestos industry, before the excess water of the original composition is drawn off on sieves or filters, the fibres can only be distributed in the water as separate fibres by special techniques. Agglomeration of the fibres is common, resulting in products of low strenght and poor finish, with fibres insufficiently covered by the binder.

Hitherto, it has usually been necessary to work the desired amount of fibre into the binder in such a way that much of the water intended to facilitate the mixing must be removed after mixing. For example, more than 80 to 85% of the water must be removed.

According to the present invention, a method for preparing a hydraulic or water-setting binder reinforced with PVAfibres involves using fibres having a high modulus and distributing them in the binder with a dispersing agent.

It has been found that PVA fibres can be distributed in a relatively small amount of water, and that no more water is needed for this purpose than for setting a shaping the mix, if dispersing or wetting agents are added. While it is true that more water is required for thinner fibres and/or an increased number of fibres, the amount of water which is needed can generally be kept within the bounds needed for the working-up steps.

For example, 2% by weight of PVA fibres with a cut length of 6 mm can be worked up in a water-Portland cement mix in a weight ratio of 0.35:1, and 5% by weight of PVA fibres 2 mm long in a weight ratio of 0.42:1.

Various dispersing agents or surfactants of different chemical composition may be used in the method of the invention. More than one such distribution aid may be used, possibly with a synergic effect. Examples of suitable wetting agents are surfactants conventionally used in the manufacture of paper and wet non-wovens from synthetic fibres, e.g. ethoxylated fatty alchols or carboxylic acids, ethoxylated alkylphenols, propylene oxide ethylene oxide copolymers and formaldehyde-melamine resins.

The dispersing aid is preferably applied after spinning the fibres, together with conventional materials used in the fibre stretching. It is also possible, however, to apply the aid after stretching, to treat the cut fibres with a suitable aqueous liquor or to spray them with a diluted form of the aid. It is of course necessary to ensure compatability between the form of the fibres and the aid, in order that the desired effect can be achieved.

It is not critical whether the fibres are added to a mixture of the binder and water or the binder to a mixture of the fibres and water. Undesirable foaming can often be avoided if the fibres are first introduced into the desired volume of water. The total amount of the necessary binder can then be stirred into the aqueous mix after short time. Conventional aggregates and additives, including conventional liquefying agents, gravel, fillers, improving agents etc, may be added then or later. It is also unimportant whether the fibres prepared for distribution in water are used in a moist or dried state.

The amount of the aid which is used can be determined according to the desired effect. Experience has shown that satisfactory distribution can be achieved using 0.5% by weight of an aid, based on the weight of the fibres. It may be undesirable to use more than 3% w w of the aid, prior to stretching, since, under certain conditions, such an addition may have a detrimental effect on the desired high modulus properties of the fibres. When adding the aid by immersion, it has been observed that, if as much as 10% w w of the aid is used, hard lumps may be produced when the fibres are dried. Such lumps require undesirably long swelling times when the fibres are mixed in.

Suitable hydraulic binders which can be used in the method of the invention include cements such as Portland cement, trass cement, blast-furnace Portland cement and gypsum. While all or almost all PVA fibres may be used in the method of the invention, it is preferred that these fibres have a high A modulus, i.e. a steep gradient on the first linear rise in the curve representing a stress-strain experiment. This gradient is usually determined by extending the tangent at the appropriate curve up to 100% breaking load and is calculated from the formula: 100 x (breaking load) (thickness x elongation value) Using a suitable stretching process, strengths of 8-9 pdtex oi m3re can be achieved for an elongation at break of only 4.6%.The corresponding stress-strain curves give A modulus values of 180-300 pdtex (2350-3900 daN mm2) or more. These values. which are dependent on thickness, show that the high A modulus of such PVA fibres, higher than that which is easily obtained for other synthetic fibres, should be superior or at least similar to the modulus of elasticity of the matrix to be reinforced. Accordingly, the reinforcing fibres are able to exert their action after break-down of the matrix.

Another advantage of the use of PVA fibres lies in their chemical structure, i.e. in the existence of the hydroxyl groups which are able to provide hydrogen bonds between the fibres and the matrix.

It is preferred to use PVA fibres with a sufficiently high A modulus in all thickness ranges, the reinforcing action increasing with increasing area, i.e. decreasing thickness.

It is advantageous if the length of the fibres which are used can be produced easily. it has been found that withdrawal of the fibres on fracture can be avoided if the fibres are 1 to 2 mm long. A distinct reinforcing action is achieved with fibres 2 to 3 mm long. Longer cut fibres can be used without any problems, particularly for the coarser fibres. For example, 10 dtex fibres having a cut length of 12 mm can be used satisfactorily. The fibres are preferably 1 to 50, and more preferably 1 to 25, mm long.

As indicated above, the upper limit on the amount of fibres which can be used with respect to the binder depends on whether the binder can wholly envelop the fibres. Thus while, in general, a reinforcing effect can be achieved using 1.5% v/v, i.e. about 1% w/w fibres, the effect will depend on the surface area of the fibres.

For example, a mix containing 3.5% w/w of 6.6 dtex PVA fibres with a cut length of 12 mm is strawy in appearance, whereas 6% by weight of the same fibres 1.2 mm long gives a plastic mix. 1 to 5, and more preferably 1 to 2.5,% w/w of PVA fibres are used for reinforcement. These amounts mean that the products of the invention can be cheaper to prepare than comparable products using other reinforcing fibres.

It has generally been considered that the particular reinforcing fibres disclosed in the prior art have been the reason for any particular advantages of the products obtained using those fibres. However, when PVA fibres are used, comparable green strength, mould-release properties, low micro-crack formation and other properties are obtained with respect to known products.

An important advantage of the process of the invention is that it allows the production of an easily workable mix particularly suitable for use, for example, as a surface filler, plaster, finish or gap-filling cement. For these purposes, the reinforcing action of the fibres is particularly important, because the elasticity and deformability of the mix when set are considerably greater than for non-reinforced mixes. To this end, from 0.5 to 3% wtw of fibres in the hydraulic binder may be sufficient, since an increase of 10 to 20% in strength can be observed in such cases.

It is known to reinforce gunite with steel fibres. If, for example, gunite is reinforced with 4 to 6% w/w steel fibres, a large increase in green strength can be obtained although a only a modest improvement in compression strength is observed after, for example, 28 days. It is, however, an expensive and labour-intensive disadvantage that an undesirably high proportion, up to 25-30%, of steel fibres is present in the concreteifibre mixture in the so-called "rebound" from subsoil during injection. In contrast, no more than 1.2 .o w;w of PVAfibres in such a mix can effect a substantial part of the reinforcing action of the steel fibres without their effect being lost on "rebound".The deformability of a shaped concrete part obtained in accordance with the invention is of considerable importance in such an application, e.g. in underground mining, where early take up of the load by the reinforced concrete body is desirable.

In general, an increased working capacity, i.e. the area beneath the stress-strain curve, of up to 5-fold can be observed.

It will be apparent that hydraulic binders, and in particular all types of cement and gypsum, can be so reinforced with PVA fibres that increased strength, and particularly flexural, tensile, cracking and compression strength, and increased deformability or increased working capacity, can be achieved by the method of the invention.

The following Examples illustrate the invention.

Example 1 A mortar is prepared from 1000 parts by weight of Portland cement and 350 parts by weight of water with a disc stirrer. 18 parts by weight of PVA fibres which have been immersed in a 10% solution of a propylene oxide,ethylene oxide copolymer with a molecular weight of 3300 and thereby provided with a coating of 3% by weight of the dispersing agent are stirred into the mortar within a few minutes. As soon as a plastic, lump-free mix has been formed, the strirrer is turned off and the mortar is worked.

The PVA fibres have the following characteristics: Diameter: about 25 tt (corresponds to about 6.5 dtex) Cut length: 6mm Tenacity: 100 daN mm' Elongation at break: about 4.5 mO A modulus: about 3500 daN mm The density of the prepared fibrous mortar is about 1.86.

Test bodies (100 x 30 x 10 mml produced by casting in a suitable mould show bending compression strength values of 14.4 N mm2. This is about 146% of the strength (9.9 N mm2) of a like test body which is produced without adding fibres. In contrast to a deformability of 0.18% in the test body without PVA reinforcement, the deformability of the test body reinforced in accordance with the invention rises to about 0.7%.

Example 2 25 parts by weight of PVA fibres which have been provided with a coasting of 5% by weight of dispersing agent by spraying with a 5% solution of nonylphenol ethoxylate are added to 380 parts by weight of water.

After waiting for a short time, 1000 parts by weight of cement are mixed in by means of a pug mill.

The PVA fibres have the following characteristics: Diameter: 12 F (corresponding to 1.6 dtex) Cut length: 6mm Tenacity: 110 daN/mm2 Elongation at break: 4.5% A modulus: 3200 daN/mm2 The density of the prepared raw fibrous mortar is 1.84.

For test bodies produced in accordance with Example 1, a bending compression strength of 14.5 N/mm2 is measured, which is about 156% compared with 9.3 N/mm2 for corresponding test bodies without any reinforcement. The deformability is more than 0.8%.

Example 3 30 parts by weight of PVA fibres with a diameter of about 6 ss (corresponding to 0.6 dtex) and having a coating of 1.5% by weight of an ethoxylated fatty alcohol of the formula C36H74011 (applied simultaneously with the stretching preparation) are worked into a mortar with a water/cement ratio of 0.35-1. A fibre mixture with cut lengths of 1-3 mm is used as the fibre. The density of the fibrous mortar obtained is 1.83 and the resulting strength values of the relevant test bodies are about 136% of the unreinforced matrix.

Example 4 20 parts by weight of PVA fibres with a diameter of 32 LL (corresponding to 10.0 dtex) are stirred into 100 parts by weight of gypsum and 650 parts by weight of water. The fibres are provided with a coating of 1% by weight of formaldehyde-melamine resin and worked up in accordance with Example 1.

The test bodies produced show an improved bending compression strength similar to Example 1.

In a comparable manner, it is possible to work up to 70 parts by weight of PVA fibres with mixed cut lengths of 1.3 mm and 1.5% by weight of dispersing agent and a water/cement ratio of 0.4:1 into a workable fibrous mortar which can be applied as a highly deformable coating or overlay material to a cracked surface.

Example 5 0.6 parts by weight of PVA fibres (diameter 25 tt, length 6 mm, A modulus 3300 daNimm2) are mixed into a dry repair mortar mix consisting of 600 parts by weight of sand and 400 parts by weight of cement. After adding 150 parts by weight of acrylic resin dispersion, a plastic, slightly thixotropic paste is formed which is workable with a trowel.

The paste obtained has a strength increased by 15% compared with unreinforced material after only 7 days and, even with extreme stress due to heat and'or wind at the place of use, shows no cracking or tendency to shrink.

Example 6 3 parts by weight of PVAfibres (diameter 25 tt, length 6 mm, A modulus 3300 daN/mm2) which have been provided with 1.2% by weight of dispersing agent are mixed by means of a cement-mixing drum into 1000 parts by weight of a moist of low water-content transit-mixed or ready-mixed concrete with a maximum grain size of 8 mm. Thus, 3% by weight of PVA fibres are added in place of the usual 6% by weight of steel fibres.

By adding water, the mix obtained is thereafter sprayed in known manner in a water,cement ratio of 0.48:1. The rebounding material is about 60% of a comparable concrete containing 6% by weight of steel fibres. It contains practically no PVAfibres. The green or early strength (measured in accordance with DIN 1048 and 18551) is around 50% above that of steel fibre-reinforced concrete. The strength values after 28 and 56 days are about 35% higher.

Claims (17)

1. A method for preparing a hydraulic binder reinforced with polyvinyl alcohol fibres, in which the fibres are distributed in the binder using a dispersing agent.

2. A method according to claims 1 in which the fibres and the dispersing agent are mixed in an amount of water which is no more than that needed in subsequent shaping of the mixture.

3. A method according to claim 1 or or claim 2 in which the fibres have a A modulus of 180 to 300 p/dtex.

4. A method according to any preceding claim in which the thickness of the fibres is from 0.25 to 25 dtex.

5. A method according tQ claim 4 in which the thickness of the fibres is from 0.5 to 10 dtex.

6. A method according to any preceding claim in which the fibres are form 1 to 50 mm long.

7. A method according to claim 6 in which the fibres are from 1 to 25 mm long.

8. A method according to any preceding claim in which from 0.5 to 20% by weight of the the despersing agent, based on the weight of the fibres, is used.

9. A method according to claim 8 in which the amount of the dispersing agent is from 1 to 10% by weight.

10. A method according to any preceding claim in which the dispersing agent is selected from ethoxylated fatty alcohols or carboxylic acids, ethoxylated alkylphenols, propylene oxide/ethylene oxide copolymers and formaldehyde-melamine resins.

11. A method according to any preceding claim in which the dispersing agent is applied to the fibres after spinning thereof.

12. A method according to any preceding claim in which from 0.5 to 10% by weight of polyvinyl alcohol fibres, based on the weight of the hydraulic binder, are used.

13. A method according to claim 12 in which from 1 to 5% by weight of polyvinyl alcohol fibres are used.

14. A method according to claim 1 substantially as described in any of the Examples.

15. A fibre-reinforced hydraulic binder prepared by a method according to any preceding claim.

16. A product according to claim 15 containing from 0.5 to 3% by weight of polyvinyl alcohol fibres.

17. A product according to claim 16 containing from 0.5 to 2% by weight of polyvinyl alcohol fibres.