GB2081762A

GB2081762A - Board making

Info

Publication number: GB2081762A
Application number: GB8123936A
Authority: GB
Original assignee: Amfu Ltd
Current assignee: Amfu Ltd
Priority date: 1980-08-07
Filing date: 1981-08-05
Publication date: 1982-02-24

Abstract

A process for the preparation of an aqueous slurry of solids content at least 30% by weight, whose predominant solids content is a hydraulic cement and which contains synthetic inorganic reinforcing fibres to reinforce the cement when cured, said process comprising mixing with water a felt comprising organic web- forming fibre and said synthetic inorganic fibre, to form an aqueous dispersion of mixed fibres which contains said organic web-forming fibres in an amount forming at least 1% by weight of the water present, and thereafter adding the hydraulic cement to the dispersion of mixed organic and inorganic fibres to produce a slurry of cement content at least 15% by weight. Incorporating the ingredients according to this procedure reduces bailing up of the synthetic inorganic fibres and so improves the properties of composite articles made by compression- moulding the slurry.

Description

SPECIFICATION Preparation of aqueous slurries of hydraulic cement and inorganic reinforcing fibres This invention relates to the preparation of aqueous siurries of hydraulic cement and inorganic reinforcing fibres. Such slurries are required for use in the production of composite articles having a matrix of cured hydraulic cement reinforced with inorganic fibre and being of high density (density at least 1 500 kg/m3).

A well known form of composite article of the kind just defined is asbestos board, which is made by compression-moulding an aqueous slurry of ordinary Portland cement and asbestos fibres. This entails subjecting the slurry, of initial solids content at least 30% by weight, to a pressure of the order of 10 MPa while it is in a mould permeable to water under pressure. The slurry is thus dewatered to form a moulded product (usually of thickness in the range 3-1 00 mm) which is then removed from the mould and set aside for the cement to cure or treated to cure the cement quickly.

in attempting to make a high-density cured cement board without the use of asbestos, we have prepared slurries of solids content 30% or more from cement and a mixture of synthetic inorganic fibres (e.g. vitreous fibres) with organic web-forming fibres such as cellulose, intending that the inorganic fibre should perform the reinforcing function of asbestos fibres in the cement when cured and that the organic fibre should perform the web-forming function which asbestos performs during dewatering, i.e. should trap the relatively dense particles of cement and prevent them from escaping from the mould, while permitting quick draining of water therefrom.

However, we have found that the properties of board obtained from such slurries vary unacceptably, particularly in respect of crushing strength. Our further investigations have shown that these variations are due to intertwining or balling up of the synthetic inorganic reinforcing fibres during preparation of the slurry, so that not all such fibres function effectively in the cured cement product.

We have further found, and this forms the basis of the present invention, that intertwining or balling up of the synthetic inorganic reinforcing fibres can be very much reduced by making them into a felt with the organic web-forming fibres, then mixing that felt with water to form an aqueous dispersion of mixed fibres, and then adding the hydraulic cement to that aqueous dispersion.

The felt can be made in large quantities at water content 50-80% by weight, dried for storage to below 10% water content, and when required for use can be very readily dispersed in water to form an aqueous dispersion containing both web-forming fibres and inorganic reinforcing fibres, to which dispersion the hydraulic cement and any other ingredients can then be added.

According to the invention there is provided a process for the preparation of an aqueous slurry of solids content at least 30% by weight, whose predominant solids content is a hydraulic cement and which contains synthetic inorganic reinforcing fibres to reinforce the cement when cured, said process comprising mixing with water a felt comprising organic web-forming fibres and said synthetic inorganic fibres, to form an aqueous dispersion of fibres which contains said organic web-forming fibres in an amount forming at least 1% by weight of the water present, and thereafter adding the hydraulic cement to the dispersiol. of mixed organic and inorganic fibres to produce a slurry of cement content at least 1 5% by weight.

It will usually be desired to form an aqueous dispersion whose content of synthetic inorganic reinforcing fibres is at least 0.5% by weight.

The invention can be performed without the use of any specially added surface-active agent to disperse the synthetic inorganic reinforcing fibres.

The invention can readily be applied to the production of heat-resistant high density board of crushing strength at least 50 MPa, shear strength at least 1 5 MPa and cross-breaking strength at least 15 MPa.

The weight proportions of the various ingredients forming the solids content of the slurry that is prepared are suitably as follows: Hydraulic cement 5085% Synthetic inorganic 115% reinforcing fibres Organic web-forming 27% fibres A preferred range for the content of hydraulic cement is 6080% by weight. The hydraulic cement may, for example, be ordinary Portland cement, fine-ground (and therefore rapid hardening) Portland cement or high alumina cement.

The organic web-forming fibres are preferably of cellulose (for example, wood, jute or sisal) but may be polyethylene or polypropylene fibres of the kind commercially available under the trademark Pulpex. They are suitably employed at a freeness in the range 20--300 Schopper Riegler.

Preferably, the felt is employed in an amount such that its content of organic web-forming fibres forms 2% or more by weight of the water with which the felt is mixed.

The synthetic inorganic fibres, which are suitably employed in lengths up to about 25 mm, may be vitreous, for example mineral wool, or the so-called alkali-resistant glass fibres such as those made with a content of combined zirconia.

Alternatively, or additionally (particularly where mineral wool is used), carbon fibres may be used as synthetic inorganic fibre reinforcement, although only in those instances where the composite product is not required for electrical applications. The carbon fibres employed are preferably those of low modulus in tension, suitably of not more than 4% elongation at break.

Materials in addition to the hydraulic cement may for special purposes be included in the aqueous dispersion of organic web-forming fibres and synthetic inorganic fibres. These include a micaceous mineral to confer improved heatresistance on the eventual composite product, a clay to assist formation of the compressionmoulded material, and natural fibrous wollastonite to provide improved reinforcement at high temperature.

According to a further aspect of the invention, there is provided a process for the production of a composite article, in which a slurry prepared according to the procedure already described is dewatered in a water-permeable mould under a pressure of at least 10 MPa and the dewatered material is formed into a composite article of density at least 1 500 kg/m3 having a matrix of cured hydraulic cement reinforced with synthetic inorganic fibres.

This invention is further illustrated by the following Examples: EXAMPLE 1 This Example illustrates the preparation of an aqueous slurry via a felt of organic web-forming fibres and synthetic inorganic reinforcing fibres.

1. Bleached softwood sulphate pulp (5 kg) was made into an aqueous dispersion of freeness 20--300 Schopper Riegler with 4,500 litres of water.

2. Alkali resistant glass fibres (3 kg; length 12 mm) were then added to the dispersion of (1) and dispersed in it.

3. The dispersion of (2) was dewatered on the wire mesh of an entireiy conventional Foundrinier paper making machine to form a felt of water content 70% by weight. The felt thus produced was rolled up and stored for 2 days. (If not required until a longer period has elapsed, the felt should be dewatered to water content below about 10%.) 4. When required, the felt was shredded into sections measuring about 6 cm square and agitated with water (1 part by weight of water for each part by weight of solids content in the felt) in a ribbon mixer for 2 minutes to form an aqueous dispersion of the web-forming fibres and the synthetic inorganic reinforcing fibres.

5. Fibrous wollastonite (fibre length, 200-1000 ftm), mica (75% passing a sieve of aperture 500 hum), ball clay (90% passing 5 Mm) and ordinary Portland cement were then incorporated with additional water in proportions such as to give a solids content made up as follows: Portland cement 65 Wollastonite 17 Cellulose fibres 5 Mica 5 Ball clay 5 Alkali resistant glass 3 fibres 6. The slurry was then transferred to an entirely conventional ram-operated moulding press, and pressure was progressively applied to it during 2 minutes to a maximum of 20 MPa to force water from the slurry through the gauze of the press.

7. The dewatered board thus formed (thickness about 12 mm) was allowed to stand at ambient humidity and temperature for 21 days. The properties of this board were as follows: Density - 1900 kg/m3 Crushing Strength 70 MPa Cross-breaking Strength 1 7 MPa Shear Strength 19 MPa Arc Resistance 120 secs EXAMPLE 2 This Example illustrates the preparation of an aqueous slurry via a felt which contains not only organic web-forming fibres and synthetic inorganic reinforcing fibres but also natural reinforcing fibre, in particular wollastonite.

The procedure of Example 1 was followed generally, but with the difference that carbon fibre (1 kg; length 6 mm) was substituted for the glass fibre, and the wollastonite (17 kg) was added with that carbon fibre in stage 2, not in stage 5. The felt thus formed had water content 57% by weight.

Claims

1. A process for the preparation of an aqueous slurry of solids content at least 30% by weight, whose predominant solids content is a hydraulic cement and which contains synthetic inorganic reinforcing fibres to reinforce the cement when cured, said process comprising mixing with water a felt comprising organic web-forming fibres and said synthetic inorganic fibres, to form an aqueous dispersion of fibres which contains said organic web-forming fibres in an amount forming at least 1% by weight of the water present, and thereafter adding the hydraulic cement to the dispersion of mixed organic and inorganic fibres to produce a slurry of cement content at least 15% by weight.

2. Process according to Claim 1, in which the organic web-forming fibres are cellulose fibres.

3. Process according to Claim 1 or 2, in which the synthetic inorganic fibres are vitreous fibres.

4. Process according to Claim 3, in which the vitreous fibres are fibres of mineral wool.

5. Process according to Claim 3, in which the vitreous fibres are of alkali-resistant glass.

6. Process according to Claim 1 or 2, in which the synthetic inorganic fibres are carbon fibres.

7. Process according to any one of Claims 1 to 6, in which the hydraulic cement is Portland cement.

8. Process according to any one of Claims 1 to 7, in which the weight proportions of the various ingredients forming the solids content of the aqueous slurry are: Hydraulic cement 5085% Synthetic inorganic 115% fibres Organic web-forming 27% fibres

9. Process according to Claim 8, in which said weight proportion of the hydraulic cement is 60-80%.

10. An aqueous slurry of hydraulic cement, synthetic inorganic reinforcing fibres and organic web-forming fibres, when obtained by the process of any preceding claim.

11. A process for the production of a composite article, in which a slurry prepared according to any one of Claims 1 to 9 is dewatered in a waterpermeable mould under a pressure of at least 10 MPa, and the dewatered material is formed into a composite article of density at least 1 500 kg/m3 having a matrix of cured hydraulic cement reinforced with synthetic inorganic fibres.

12. A composite article when obtained by the process of Claim 11.