CA1093729A - Fibre cement compositions - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
This fibre reinforced cementitious composite having improved post-cracking strength comprise an inorganic cementitious matrix having embedded therein a mixture of chopped fibres, one of the fibres being brittle, highly stiff fibre selected from glass, asbestos and mineral fibres, and another of the fibres comprising a tough, flexible fibre selected from polypropylene, polyethylene, polyamide, polyimide and polyester. The use of such fibre mixtures enables stronger composites to be produced, containing larger total quantities of fibres without experiencing the processing difficulties of requiring very large amounts of water for their preparation.

Description

This invention relates to fibre r~inforced cement compositions, of the type which utilize short, chopped fi~res for reinforcement purposesr and which are to be used as general construction materials.
Short-length-fibre reinforced compositions of cement based materials of various strengths are known.
Examples include asbestos-cement sheets, glass fibre reinforced cement board, and short steel wire reinforced concreteO One method of making them is the simple mixing of short reinforcing fibres, for example asbestos or chopped glass s-trands, with the matrix material in liquid formt for example a slurry of portland cement, and subsequently allowing -the mixture to harden. The strength of such composites is generally relatively low, because insufficient shor~ length fibres ~usually below 1% by volume) can be added without increasing the viscosity of the matrix materiaI when in its liquid form to such an extent that it becomes unhandleable.
As the viscosity increases, it becomes increasingly difficult to coat the fibres properly. Further dilution of the cement slurry with watex will reduce the viscosity, but at the same time impairs the strength of the final, hardened material.
The slurry can be concentrated after mixing in the fibres, e.g. by removing excess water under vacuum, by centrifuging or by pressing, but thi~ adds an expensive processing step.
In another method, a continuous fibre to be used for reinforcement is chopped ln a cut-ter and sprayed in a sieparate stream simultaneously with the cementitious slurry, to form the liquid composite ready for hardening. This process rPquires comp1Px equipmene, and does not ensure ~3~

uniform distribution of the fibres within the cementitious matrix~
In another method, the fibre cement slurry (for ex~lple asbes~os cement slurry with a very low solids content) is poured onto a conveyor belt, and the thin sheet of wet asbestos cement composite is picked up and wound on-to a drum. A flat sheet of the material is produced by cutting the material on the drum and placing it flat, with or withou-t subsequent pressing, to remove moisture from the lQ asbestos cemen~ composite. This process requires heavy and complex machinery, and is no-t suitable for small-scale operations~
These methods of the prior art all suffer from the co~mon disadvantage, that in order to incorporate therein a sufficient amount of short length fibres to provide good strength characteristics (5-6% by volume of the composite), high water content mixes have to be preparedO This is necessary in order to provide a liquid cementitious slurry of sufficiently low viscosity to be workable r to permit ZO.shaping, casting, etc~ Howeverl the use of large amounts of water in preparing cementitious composites leads to undes--irable properties in the final~ hardened compos.ite, notably lower strength. This is at least partly due to poor bonding between the fibres and the matrix, and increased porosity of the cured, hardened composite, which also has the- effect of lowering the strength ~thereof~
The present invention provides methods ~y which chopped fihre reinforced cementitiou5 composite5 may be prepared which incorporate larger amounts of chopped fibres ~3~
so as to increase the strength of the resulting composites, which methods also use smaller amounts of water so as to produce final hardened composites of improved propertles, whilst using simple process steps and apparatus. The invention also provides novel fibre reinforced cementitious composites.
According to one aspect of the present invention, there is provided a fibre reinforced composite having improved post-cracking strength, said composite comprising:
a matrix of inorganic cementitious material having chopped fibres embedded therein, said chopped fibres comprising first chopped fibres of a brittle, highly stiff nature consisting essentially of mineral fibres;
and second chopped fibres of a tough, flexible nature and selected Erom the group consisting of polypropylene fibres, polyethylene fibres, polyamide fibres, polyimide fibres and polyester fibres;
the average length of said first chopped fibres and said second chopped fibres being not greater than about 2.0 inches.
Preferred mineral fibres are glass fibres and asbestos fibres. It has been found that the use of mixtures of chopped fibres as described above, one of the fibres being a shor-t high stiffness, brittle fibre and the other being a low stiffness, tough fibre, enables cementitious composites of improved strengths to be prepared whilst using smaller volumes of water in their preparation. Such composites have high ultimate strength and high post-cracking strength. By the term ~'post-cracking strength" there is meant the strength ~3 by which the fibre cement composite is held together, even after serious cracking has occurred in the composite. In the composites according to the present invention, after reachlng their ultimate tensile st~el~gth and cracking or breaking, -the composites exhibit a substantial residual stress resistance, and the broken pieces remain held together by the tough flexible second chopped fibres~ In this way, a catastrophic failure of the fibre cement composite may be avoided~
One of the reasons why so much water has previously had to be used in preparing chopped glass fibre reinforced cement compositions is that the glass fibre absorbs large quantities of water. Sufficient water must, therefore, be added to saturate the glass fibres, and then to make a slurry of sufficiently low viscosity to be handleableO This use of large amounts of water leads to substantial reductions in strength of the final, cured composite, as previously discussed. In the present invention, however, a large proportion of the glass fibres can be replaced with a hydrophobic or at least less water absorbent fibre such as chopped polypropylene. ~y this meansl less water i5 used in preparing the composite, and a laxger amount of fibres can be incorporatedl leading to a stronger finishedS cured compositeO In addition, as noted above, there is obtained the extra advantage of improved post~cracking strength, when using combination of fibres according to the present invention.
The reinforcing of cement compositions with fibres results from several different effects and interactions, depending upon the na~ure of pxoperties of ~he cement~ and of the chosen fibres. In all cases, a bond is formed between the cementitious material and the fibres. IE this bond at the fibre-cement interface is not very strong initlally, a tough composite may be Eormed, but the composite may become brittle on aging and under natural weather conditions. This happens, for example, in the case of cementitious composites reinforced with asbestos fibres, due to the type of manufacturing processe~ commonly used (involving dewatering and pressing), and the chemical affinity between cement and asbestos fibres. In the case of glass fibre cement composites, the bond de~elGps over some-what longer periods to time, due to continou~ fusion of the glass fibre and the cement, but brittleness sets up with natural weatheringO Composites of cement reinforced with glass fibres alone, according to the prior art, have been reported to become undesirably brit~le af~er 3-5 years weathering in a natural environment. When such composites fail, the failure may be catastrophic, as discussed above, hecause of their lack of post-cracking strength. The brittle nature of asbestos fibres and glass fibres themselves tends to contribute to the problem of brittleness developing during natural weathering. On the other hand, r~in~orcement of cement composites solely with low stiffness, tough non-brittle fibres results in composites of very low ultimate strength, since such fibres weaken the cement matrix and al~o have lower stiffness and ultimate strength than the glass fibres.
The use of mixture~ of the two types of fibres~ e.g.
mixtures of chopped glass fibres and chopped polypropylene fibres, leads to fibre reinforced composites of much improved~

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over-all, all-round strength properties r and processes for their manufacture.
The term "cement" as used herein refers to inorganic cements such as portland cements, phosphate cements, high alumina cements, high gypsum cements or gypsum-~ree cements, or combinations thereof, where these cements are distinguished by their ability to cure in the presence of, or immersed in, water. The term also embraces such cements which have added thereto inorganic and organ~c compounds for improvement of 10 their 5~tting, strength gain, chemical resistance, permeability and other properties, as known in the art.
Preferably according to the present invention, the first and second fibres are present in the composite in total in an amount of from about 0.5 to about 10 parts by weight, per 100 parts by weight of inorganic components o~ said matrix.
Also/ it is preferred to use a weight ratio of first chopped fibre to second chopped fibre of from about 5:1 to about lo~.

It is also preferred that the fibres he randomly oriented in all directions, within the cementitiows matrix.
The most preferred fibres for use in the present invention are chopped glass fibres as the first fibres, ar.d chopped polypropylene fibres as the second fibres. The vas~
majority of su~h fibres should have a length not greater than 20 0 inches and most preferably the fibre length o~ both fibre component~ should be about Q.5 inches~

~ccording to another aspect of the present invention, the chopped glass ~i~res used in reinforcing the cementitious composites are precoated with a hydrophobic composition, prior to being incorporated in the inorganic cementitiolls matrix.

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Typical compounds which may be ~lsed for procoating for this purpose are polyvinyl ace-tates, polyacrylics, (i.e. polymers and copolymers o~ acrylic acid, acrylic acid ester or acrylonitrite) polyvinyl chlorides, styrene-butadiene resins, silicone resins, polyethylene, nylon, polystyrene and paraffinic hydrocarbo~ waxes. Preferred precoating compounds for use in the present invention are polyvinyl acetate, polyacrylics and silicones~ The use of such hydrophobic, water-repellent coatings has the effect of drastically xeducing the amount of water which the chopped glass fibres will absorb. This allows mixes containing high amounts of absorbent fibres such as glass to be made with lower water contents in the mixes, to produce a mix of workable viscosity, and a final composite of improved strengthu In addition to the provision of water resistance to the glass fibres, such coatings also provide the glass fibres with protection against alkali attack. Ordinary glass is liable to attack by alkalis commonly contained in cement. Special alkali resistant glasses are available but are comparatively expensive. By using the coatings described above, ordinary glass fibres can be given a degree of protection against alkali attack and hence used in the present invention with ordinary alkali content cements.
According to a further aspect of the present invention, the cementitious composite may include various plasti~izing additives, such as sulfona~ed naphthalene, melamine-formaldehyde condensates~ sulfonated melamine~
formaldehyde condensates, modified ligno sul~onates and polymeric additives such as acrylate based latexes and solid -- 7 ~

water dispersible polymeric additives which can be re-em~llsified to form a latex-like material in the cement slurry.
These plasticizing additives have the furlction of dispersing the solids, including the cement, fibres and any aggregates that may be used in the liquid mixture at relatively low water/cement ratios, and provide sufficient workability for direct casting or spraying of such composites. This improved solids dispersion ensures the formation of more fluid mixtures even at high solids contents, and improves the evenness and random orientation of the fibres.
One preferred embodiment of the present invention is light weight fibre reinEorced cementitious composites, in which a mixture of first chopped fibres of a brittle, highly stiff nature and second chopped fibres of a tough Elexible nature as previously defined are used as the reinforcement.
Such light weight composites have densities in the range from about 30 to about 130 pounds per cubic foot. Normal fibre cement co~posites previously available on the market have densities in the 130 - 160 pounds per cubic foot range.
There is a definite demand in the marketplace fcr strong, tough, non-combustible light weight materials such as these light weight reinforced composites, for example in light weight partition walls, or light weight modular houses. Low density fLbre cement composites have not previvusly been availa~le~ mainly due to a lack of a suitable method for their fabrication~ Attempts have previously been made to prepare light weight cements by incorporation of light weight aggregates into the cements, such as perlite, vermiculite or fly ashO Due to their high porosity, however, these 3'7~

aggregates absorb large amounts of water, usually three to five times their dry weight, even when they are treated with water repellent materials. Consequently, the amount of water required for mixing is very large, and the amount of fibres which can be incorpora-ted in any such fibre-cement composite is limited due to the bulk occupied by the aggregatPs. The large amounts of water required for mixing these small volume of fibres and the poor bond between the matrix and fibres results in poor propexties of the light weight fibre cements.
It has also been previously proposed to make light ~eight cements by i~parting a uniform cellular structure into the cement. This has been attempted using high speed mixing during which air entrainment is effected by vigorous mixing of water with a foaming agent. The second method employed in producing cellular cements is by using gas forming chemicals suoh as aluminum powder or hydrogen peroxide and calcium hypochlorite. The cement foam produced by this method however is not s~able in its newly formed state. Thus, the foam changes its volume while the gas forming reaction takes place, which in production of precast products is a disadvantage, since the products which are mainly blocks or slabs have to be cut to required dimensions. All these light weight cement composites previously prepared have low strength and low fracture toughness, but with incorporation of fibres according to the present invention, th~s strength and fracture toughness can be significantly pro~ed.
In one process according to the present invention~
firstly a stable, uniform air cell s~ructure foam is pxepared by vigorous mixing of 40 to 80 parts of water and ~3~

0.20 to 0.~5 parts per 100 parts of cement by weight, of a suitable air entraining agentO I~ is preferred to use air entraining agents which produce a s~able, uniform foam by using standard mortar or concrete mixlng equipment. Suitable such air entraining agents are hydrolyzed protein and keratin compounds, sodi~n isopropyl naphthalene sulfonate, petrolewn naphthalene sulfate, sodium secondary alkyl sulate, saponin, sodium alkyl aryl sulfate and highly stabilized saponified rosin and resin compounds. The amount of air entraining agent and the mixing procedure controls the amount of foam produced and consequently the final density of the fibre cement compositeO The reinforcing fibres, as previcusly described, are then incorporated into the foam in amounts varying from about 0.1 parts to 10 parts by weight per 130 parts by weight of cement. Alternatively, the fibres can under some circumstances be added to the foam along with or prior to the addition of the cement and aggregates.
It is preferred ~o use a combination of chopped glass and chopped polypropylene fibres to obtain both ~0 strength improvement and post-cracking strength~ After the fibre air cell foam is prepared, the cement ingredients in the amount of 100 parts by weight, such as portlan~ cements (including any of the types I - V), regulated set port7and cements, gypsum cements, pozzolapic cements, magnesium oxysulfate, magnesium oxychloride, zinc oxysulfate~ zinc oxychlorid~, magnesium oxyphosphate, ~ZiDC oxyphosphate, methyl silicates such as calcium sil.ica~e and alumina silicates are then added. It is preferred to use portland cements and regulated set portland cements as the cementitious binder.

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Other aggregates, for example sand such as common or silica sand, or light weight aggregat~s such as perlite, ~ermiculite, fly ash, pumice, expanded clay, or polystyrene or carb3n beads can also be added if desired, further to control the density of the light weight fibre cement composite, and consequently its compressive stren~th. Sand-can be added in amounts varying from 0 - 250 par-ts, per 100 parts of cement. The light weight aggregates can be added in amounts varying from 0 - 50 parts by weight, per 100 parts of cement. It is preferred to use sand for densities above 30 pounds per cubic foot composites, and perlite for densities below 30 pounas per cubic foot composites.
Further improvements in the strength of light weight cement composites in accordance with the present invention can be achieved by adding polymeric modifiers to the cement formulation. Such modifiers are suitably added in amounts from about 6.5 to about 27 parts by weight of dry polymeric solids, per 100 parts by weight of cement. Commonly~ the polymeric modi~iers are added in liquid, latex form.
Suitable such modifiers include acrylate polymers and copolymers such as acrylonicrile polymers and copolymers, acrylic-methacrylic acid copolymers, acrylic-styrene copolymers, polyvinyl acetatet polyvinyl acetate modified with versatic acid, vinyl propionate polymers, vinyl ch1oride copolymers, polyvinyl chlorlde, vinyl propionate -vinyl chloride ~opolymer, vinyl chloride - vinylidene chloride copolymer and butadiene- styrene copolymer latexes.
It is preferred to use acrylate copolymers a~ modifiers for '7~

the light weight fibre cement composites, since such composites have good long term durability and superior mechanical and chemical properties.

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The.strength.of light ~eight Ei~re'cement composites according to the invention can be'further i~.proved by addiny plasticizing addit.ives' such as sulfo.nated naphthalene, melam.ine-formaldehyde condensates, sulfonated melamine~
formaldehyde condensates and modified lignosulfonates', as previously described~ It is preferred to use these plasticizing additives in amounts varying from about'O.l to about 6 par~s by weight, per lQ~ parts by weight of cement.
It is preferred to add both the polymeric modifiers and the .plasticizers into the stable air cell foam~
A typical composition of a ligh.t weight fibre cement mix according to the present invention is as follows:
Portland cemen~, Type III100.0 parts Water 40.0 parts Plast.icizing additive1.5 parts Glass fibres, 0.5 inches long 5.0 parts Polypropylene, chopped monofilament ' fibre, O ~ 5 inches long 2.0 parts Air entraining agent0.37 parts The invention is further illustrated in the following specific examples:
In all of the following specific examples, portland cement type III was obtained from St. Lawxence Cement Company, Toronto; silica sand was prepared and supplied by Cana~ian Foundaries htd., Toronto plast.icizer Melment L-10~ a sulfonated melamin~-formaldehyde condensate, was supplied by ;

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Sternson Ltd., Brantford, Ontario. The ~R glass fibres were those available from Fiberglas Canada Ltd., Guelph, Ontario, and are alkaline-resistant glass fibres, designa-ted AR
chopped strand 385 CD, 0.5 inches long. The E glass fibres were also obtained Erom Eiberglas Canada I,td., Guelph, Ontario, and are tnose designated E glass chopped strand 01-956 899-00, 0.5 inches longO The polypropylene monofilament fibre, 0.5 inches long and 0.006 inches in diameter, was supplied by Whiting Co., Burlington, Vermont, U.S.A.
EXA~LE 1 Two types of fibre cement composites were prepared to illustrate the advantages of the present invention. The composite referred to hereinafter as A is a control and contains glass fibre as so~e reinforcement. The composite referred to hereinafter as B is reinforced with the combination of glass and polypropylene fibres, in accordance with the present invention. The mixes had the following cornpositions:
20 A. Portland cement, type III 100 parts Sillca sand, mesh 40 50 parts Superplasticizer Melment L-105 parts Chopped AR glass fibres, 0.5 inches long 5 parts Water 30 parts B~ Portland cement, type III 100 parts Silica sand, mesh 40 50 parts Superplasticizer~ Melment L-lQ5 parts Chopped AR glass fi~res, Q.5 inches long 5 parts Chopped polypropylene monofilament fi~re~ 2 parts .

~ater 30 parts 7Z~

Both compositions were hand mixed and a layer about 29 1/4 inch.es thick was cast. After one week curing at 1~0 relative humidity, the~strip~ 1 inches ~ide and 7 inche~
long were tested in tension using the Universal Instron Testiny Machine. The average ultimate'strengths of composites A and B were 640 psi and 700 psi respectivelyD Whils~
the ultimate strengths of both composites are'comparable, the fracture modes differ. Composite A does' not exhibit any residual strength after the ultimate tensile strength i5 10 reached, the stress in the composite is zero, and the composite is broken into two separate pieces. Composite B, after reaching its ultimate tensile strength, exhibits a residual stress of 250 psi and the two broXen pieces are held together by the polypropylene fibres.
The example demonstra-tes the advantaye of using a combination of high and low stiffness fibres as rein-forcement for cement, since such a combination prevents a catastrophic failure of the fibre cement composite.

Two types of light weight fibre cement composites were prepared tv illustrate the advantages of the present ,invention. The compvsite referrea to hereinafter as C
is a control and contains glass fibre as sole reinforcement.
I'he composite referred to hereinafter as D i5 reinforced with the combination of glass and polypropylene fibres, in accordance with the present invention. The mixes have the following compositions:

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C. Portland cementr. type III . lOO.parts ~ater 60 parts Celluchem air entraining agent 0.37 parts Chopped AR glass fibres, 0.5 inches long 5 pa~ts -D. Portland cement~. type III lQ~ parts Water 60 parts Celluchem air entraining agent 0.37 parts Chopped AR glass fibres, 0~5 inches long 2.5 parts Chopped polypropylene monofilament fibres 1 part ~ .. . . . . . . . . ....... . . . .
Celluchem air entraining agent~ which is a mixture of an air entraining and a non-air entraining agent in an inert base was supplied by Bowaine Ltd., Boxmoor, Hemel Hempstead, England.
In preparation of both composites, the air cell foam was formed first by mixing water and the air entraining agent for 3 minutes, using a Hobart food mixer~ The fibres were added into the air cell foam and mixed for 1 minute and then the cement was added and mixed for an additional

2 minutesO
Specimens measuring 2 inches by 2 inches by 2 inches were cast from bo-th mixes and air cured for 7 days. The average compresive strength of both composites was 580 psi's;
however their behavior after reaching the ultimate compresive strength was different~ Sample D, containing the combination of glass~and polypropy1ene fibres~ exhibited more ductile fractuIe behavior and also when gross cracking occurred, the composite was still held together, whereas the sample C was broken to individual pieces. The densi~y of the sample5 ranged fronl 33 pounds per cubic foot to ,:

37 pounds per cubic foot.

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Two types of fibre cement composites were prepared. The composite referred to hereinafter as E was rein~`orced with treated chopped glass fibre and chopped polypropylene fibre. The composite referxed to hereinafter as F contained non-treated glass fibre in combination with chopped po~yprop~lene fibre. The mixes had the following compositions:
E. Portland cement, type III 100 parts Silica sand, mesh 40 50 parts Latex E--330 30 parts Chopped E glass fibres, 0~5 inches longl treated 3 parts Chopped polypropylene, monofilament fibre 0.5 parts Water 17 parts Anti-foaminy agent AF-60 0.1 parts F. Portland cement~ type III100 parts Silica sand, mesh 40 50 parts Latex E-330 30 parts Chopped E glass fibres, 0.5 inches long~
non-treated 9 parts Chopped polypropylenej mono~ilament fibre 0.5 parts Water 27 parts Anti-foaming agent AF-60 ~0.1 parts Lat~x E-330, which is an acrylate copolymer latex, was supplied by Rohm and ~aas Corpora~ion ~imited, Toronto;
the anti-foaming agent AF-60, which is a silicone base~

material, was supplied by Canadian General Electric, Toronto.
Ordinary tap water was used in preparing the mixes~

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The glass fibres of mix E were txeated by soaking them for several minutes in a water~repellent compound comprising a soluti.on of a paraffinic hydrocarbon wax and a silicone in a suitable solvent, and hot ai.r dried.
Both mixes were hand mixed b~ first mixing the solids, i.e. the cement and sand into the liquids, i.e. water, latex and anti-foaming agent, and then the chopped polypropylene and glass fibres were addedO Flat sheets, 1/4 inch thick, were cast and air cured for one week. Samples 7 inches long and 1 inch wide were cut and tested in tension using the Instron. The specimens of composite E
exhibited an average ul~imate tensile strength of 1150 psi;
the specimens of composite F exhibited an average ultimate tensile strenth of 8~0 psi.

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The lower strength of composite F was primarily due to the higher water content in the composite (27 parts as opposed to 17 parts of water in composite E), which was required to obtain similar workabilities of both mixes.
The higher water xequirement in composite F is consistent with the higher water abscrption of non-treated fibres. It was found that the treated fibres absorbed, on average, 42% water relative to their dry weight, whereas :: : :
the non-treated fibres absorbed an average of 142~ by weight ..of water.

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The composite prepared in this exàmple is a typical : example o~ polymer-modified~cement, but reinforced with a :: :
combination of chopped polypropylene and gIass fi~res in accordance with the invention The mlx had the following - composition: :

~ 17 Portland cement, type III100 parts Silica sand, mesh 40 50 parts Latex E-330 30 parts Chopped AR glass fibre, 0 5 inches 10n~ 4 parts Chopped polypropylene monofilament fibre, 0.5 inches long 0~7 parts Anti-foaming agent AF-60 0.1 parts The composition was hand mixed by first mixing the solids, i.e. the cement and sand into the liquids~ i.e.-the water, latex and anti-foaming agent, and then the chopped polypropylene and glass fibres were added.
A flat sheet, 1/4 inch thick, was cast and air cured for 1 week. The samples, 7 inches long and 1 inch wide were ... ... . ... . . . . . . . .
cut and tested in bending using the Instron. The average modulus of rupture was 3950 psi.
E~AMPLE 5 In this example, the plasticizing effect which is desirable in the absence of a latex modifier in the cement, is shown. The following mix was prepared:
Portland cement, type III100 parts Silica sand, mesh 40 50 parts Plasticizer Melment L-10 5 parts Chopped AR ~lass fibre, 1.0 inches long 5 parts Chopped polypropylene, monofilament fibreJ
0.5 inches ~ong 0.6 parts Water 33 parts The presence of the plasticizer Melment L 10 not only makes the cement slurry castable at given amounts of water, but also allo~s mixing in o the required amount of -~18 -fibres necessary for obtaining a composite of sufficient strength.
An attempt was made to prepare a composite having ~he same water content as that of the mix given ahove in this example, without adding the plasticizing agent Melment L-10. The viscosity of the prepared cement slurry was so high that it did not allow mixing in of any significant amount of chopped fibres.

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Claims (11)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A fibre reinforced composite having improved post-cracking strength, said composite comprising:
a matrix of inorganic cementitious material having chopped fibres embedded therein, said chopped fibres comprising a mixture of first chopped fibres of a brittle, highly stiff nature and consisting essentially of mineral fibres;
and second chopped fibres of a tough, flexible nature and selected from the group consisting of polypropylene fibres, polyethylene fibres, polyamide fibres, polyimide fibres and polyester fibres;
the average length of said first chopped fibres and said second chopped fibres being not greater than about 2.0 inches;

2. The composite of claim 1 wherein said first chopped fibres are selected from glass fibres and asbestos fibres.

3. The composite of claim 2 wherein the first and second chopped fibres are present in total in an amount of from about 0.5 to about 20 parts by weight, per 100 parts by weight of inorganic cementitious material.

4. The composite of claim 3 wherein the weight ratio of first chopped fibre to second chopped fibre is from about 5:1 to about 1:5.

5. The composite of claim 4 wherein the first chopped fibres are chopped glass fibre, and the second chopped fibres are chopped polypropylene fibre.

6. The composite of claim 3 wherein the first chopped fibres are precoated with a hydrophobic composition prior to being incorporated in the inorganic cementitious matrix.

7. The composite of claim 6 wherein the hydrophobic composition comprises polyvinyl acetate, a polyacrylic resin, a silicone resin, or a paraffinic hydrocarbon wax.

8. The composite of claim 3 including a plasticizing additive selected from the group consisting of sulfonated naphthalene, melamine formaldehyde condensates, sulfonated melamine formaldehyde resin, modified lignosulfonates, polymeric latex and a solid polymeric additive.

9. The composite of claim 3 having a density of from about 30 to about 100 pounds per cubic foot.

10. The composite of claim 9 including an air entraining agent selected from the group consisting of hydrolysed protein and keratin compounds, sodium isopropyl naphthalene sulfonate, petroleum naphthalene sulfate, sodium secondary alkyl sulphate, saponin, sodium alkyl aryl sulphate, and highly stabilized saponified rosin and resin compounds, in an amount of from about 0.10 to about 1.00 parts by weight, per 100 parts by weight of cement.

11. The composite of claim 1, claim 4 or claim 8 wherein the first and second chopped fibres are randomly oriented in three dimensions within the matrix of inorganic cementitious material.