GB2082641A - Fibre-cement board - Google Patents

Abstract

Non-asbestos compression- moulded heat-resistant board of density at least 1500 kg/m<3> having a matrix of cured hydraulic cement which is reinforced by both synthetic inorganic fibres and organic web-forming fibres and incorporates a micaceous mineral, the board having a crushing strength of at least 50 MPa, a shear strength of at least 15 MPa, and a cross-breaking strength of at least 15 MPa. The board is made by forming a slurry of at least 30% of the solids in water, dewatering under pressure, e.g. in a ram-operated moulding press, and curing.

Description

SPECIFICATION Heat resistant board This invention relates to heat-reistant board having a matrix of cured hydraulic cement reinforced with fibre and being of density at least 1500 kg/m3.

A A well known form of such board, used for example in circumstances where a load has to be supported at elevated temperature, as in a pneumatic press with a heated platen, takes the form of cured Portland cement reinforced with asbestos fibres and is made by compression-moulding an aqueous fibre/cement slurry. 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 3-100 mm, which is then removed from the mould and set aside for the cement to cure or treated to cure the cement quickly.

The present invention provides a non-asbestos alternative to the board just described, having crushing strength, shear strength and cross-breaking strength approaching those of the known asbestos product.

According to the invention, heat-resistant non-asbestos compression-moulded board of density at least 1500 kg/m3 having a matrix of cured hydraulic cement contains both synthetic inorganic fibres and organic web-forming fibres as reinforcement, and incorporates a micaceous mineral, the board having a crushing strength of at least 50 MPa, a shear strength of at least 15 MPa and a cross-breaking strength of at least 15 MPa. These strengths are measured according to British Standard 3497 (1967).

The weight proportions of the various ingredients are suitably as follows: Hydraulic cement 50-85% Synthetic inorganic fibres 1-15% Organic web-forming fibres 2-7% Micaceous mineral 1-15% A preferred range for the content of hydraulic cement is 60-80% 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 reinforcement of the cured cement matrix is provided by both kinds of fibres specified, although the organic fibres have, as indicated, the additional function of forming a web during formation of the board by compression moulding, and so preventing the escape of an unduly high proportion of hydraulic cement from the water-permeable mould. The 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.

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 board is not required for electrical application. The carbon fibres employed are preferably those of low modulus in tension, suitably of not more than 4% elongation at break.

The function of the micaceous mineral ingredient of the board is to confer improved heat-resistance. The mineral may be a mica proper (such as muscovite, or the phlogopite mica recommended for fire-retardant and heat-insulation applications by Locke at al in their paper "Suzorite Mica" delivered at the First Industrial Minerals International Congress held in London in July 1974). Alternatively, it may be a delaminated exfoliated vermiculite, or a chlorite. All these minerals contain infinite two-dimensional negatively charged complex metal-silicate ions. The mineral is employed in the ordinary finely divided state, and should not be eso further ground that the plate-like structure of its particles is destroyed.

The board is preferably made with a content of clay to assist formation during compression moulding. The 'clay, which is preferably ball clay, may form 2-30% by weight of the board.

To provide reinforcement at high temperature, there may be included in the board a proportion of the naturally occurring fibrous material known as wollastonite, which is a calcium silicate mineral stable up to and beyond 1000 C. The mineral is suitably used in its commercially available form, with fibre lengths in the range 200-1000 Fm, and may be employed in an amount of 2% or more by weight of the board, provided that, together with any clay concurrently employed, it does not form more than 30% by weight of the board.

According to another aspect of the invention, there is provided a method of making a board as set forth above, in which a slurry of water-settable hydraulic cement, synthetic inorganic fibres, organic web-forming fibres and micaceous mineral and containing at least 30% by weight of solids, is dewatered under a pressure of at least 10 MPa, and the dewatered product is cured.

The invention is further illustrated by the following Examples: Example 1 This Example illustrates the manufacture of board with good resistance to heat shock, and suitable for use for example as furnace support blocks. Because of its content of carbon fibre, which is electrically conductive, the board is not suitable for electrical applications.

1. Bleached softwood sulphate pulp (1.65 kg) was made into an aqueous slurry of freeness 20 Schopper Rieglerwith 31 litres of water.

2. Low modulus carbon fibre (0.33 kg; 2.5 % elongation at break; fibre length 6 mm) was added to the slurry of (1) and dispersed in it.

3. Mineral wool (2.3 kg) was added to the slurry of (2) and dispersed in it 4. The slurry of (3) was put in a ribbon mixer, and mica (4 kg; 75% passing a sieve of aperture 500 um), ball clay (3.30kg; 90% passing a sieve aperture of 5 Fm) and ordinary Portland cement (21.4 kg) were added, with further water to bring the total volume to 50 litres of water. The mixer was run for 5 minutes after addition of these materials had been completed.

5. 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.

6. The dewatered board thus formed (thickness about 12 mm) was allowed to cure at ambient humidity and temperature for 21 days. The properties of this non-combustible board were: Density 1900 kg/m3 Crushing strength 70 MPa Cross-breaking strength 17 MPa Shear strength 19 MPa.

Example 2 Following generally the procedure of Example 1, a board suitable for use in the construction of arc chutes for air-circuit breakers was made from the following ingredients in the following proportions by weight: Portland cement 65% Alkali-resistant glass fibres 3% Cellulose fibres 5% Mica 5% Ball clay 5% Wollastonite 17% The non-combustible board obtained (again of thickness 12 mm) had crushing strength 70 MPa, cross-breaking strength 17 MPa, and shear strength 19 MPa. Its density was 1900 kg/m3, and its arc resistance (British Standard 3497 (1967)) was 120 seconds. 1)

Claims (16)

1. Non-asbestos compression-moulded heat-resistant board of density at least 1500 kg/m3 having a matrix of cured hydraulic cement which is reinforced by both synthetic inorganic fibres and organic web-forming fibres and incorporates a micaceous mineral, the board having a crushing strength of at least 50 MPa, a shear strength of at least 15 MPa, and a cross-breaking strength of at least 15 MPa.

2. Board according to Claim 1, in which the weight proportions of the various ingredients are: Hydraulic cement 50-85% Synthetic inorganic fibres 1-15% Organioweb-formingfibres 2-7 % Micaceous mineral 1-15%

3. Board according to Claim 2, in which the weight proportion of the hydraulic cement is 60-80%.

4. Board according to Claim 1, 2 or 3, in which the organic web-forming fibres are cellulose fibres.

5. Board according to any one of Claims 1 to 4, in which the synthetic inorganic fibres are vitreous fibres.

6. Board according to Claim 5, in which the vitreous fibres are of mineral wool.

7. Board according to Claim 5, in which the vitreous fibres are alkali-resistant glass fibres.

8. Board according to any one of Claims 1 to 4, in which the synthetic inorganic fibres are carbon fibres.

9. Board according to any one of Claims 1 to 8, which contains a clay in an amount forming from 2 to 30% by weight of the board.

10. Board according to Claim 9, in which the clay is ball clay.

11. Board according to any one of Claims 1 to 10, containing fibres of the mineral wollastonite, which are present in an amount of at least 2% by weight and which, together with any clay present, form not more than 30% by weight.

12. Board according to any one of Claims 1 to 11, in which the hydraulic cement is Portland cement.

13. A method of making a board as set forth in any preceding claim, in which a slurry of water-settable hydraulic cement, synthetic inorganic fibres, organic web-forming fibres and micaceous mineral and containing at least 30% by weight of solids is dewatered under a pressure of at least 10 MPa, and the dewatered product is cured.

14. Board obtained by the method of Claim 13.

15. Board substantially as described with reference to Example 1.

16. Board substantially as described with reference to Example 2.