CA1226589A

CA1226589A - Sheet material of fibre-reinforced cement

Info

Publication number: CA1226589A
Application number: CA000465986A
Authority: CA
Inventors: James W. Smith; Rodney P. Knowles; Everard J. Smith; Ian R.K. Greig
Original assignee: Pilkington Brothers Ltd
Current assignee: Pilkington Group Ltd
Priority date: 1983-10-31
Filing date: 1984-10-19
Publication date: 1987-09-08
Also published as: IT8468082A0; IT8468082A1; PH20044A; IT1179807B; DK519184D0; FI844239A0; GB2148871B; PT79433B; BR8405520A; GB2148871A; ES8702219A1; GB8426002D0; ES537240A0; NO844294L; FI844239L; DE3439813A1; DK519184A; FR2559760B1; PT79433A; NL8403247A

Abstract

ABSTRACT OF THE DISCLOSURE

Sheet material, which may be flat or of profiled (e.g.
corrugated) cross-section, suitable for use as building panels, e.g.
roofing slates, is formed of a fibre reinforced cement composition comprising, in weight percentages:-Ordinary Portland cement 50 to 71%
Pulverised fuel ash 14 to 40%
Volatilised silica (containing at least 86% SiO2) 5 to 12%
Filamentised chopped glass fibre strands 2 to 7%
these components constituting at least 90% by weight of the composition, the material having a minimum average modulus of rupture of 16 N.mm-2 and a minimum density of 1.3 g.cm-3. The balance of the composition may include up to 4% by weight cellulose pulp and up to 5%
of a plasticiser such as ball clay, bentonite or talc, together with small quantities of known dispersing agents and flocculating agents.

Description

I

SHEET MATERIAL OF FIBRE-REINFORCED CEMENT

This invention relates to sheet material of fibre-reinforced cement, which Jay be flat or of profiled cross-section, en corrugated. Such material is suitable for use as building panels, for roofing. The flat material is particularly suitable for use as roofing slates.

Sheet materials of cement reinforced with asbestos fires have been known Pro many years and provide a lightweight roofing material which is both weather-resistant and fire-resis~ant. There is now a need to replate asbestos as a component of such materials, but the product obtained as a result of such replacement must be substantially equivalent to the existing asbestos-cement materials in all their desirable properties. The materials must also be capable of being made on existing asbestos-cement equipment so as to avoid the capital cost of replacing or drastically altering such equipment.

The use of glass fire as a replacement for asbestos fire in such equipment has been the subject of considerable research over the past 10 to lo years. The problems originally encountered, which are discussed for example in our US Patent Specification No. 1,543,951, have been overcome to the extent that it is now possible to formulate glass fibre-con~aining cement slurries which can be run on asbestos cement machines, by the measures descried on Specification No. 1,543,~51. This ability is not necessarily accompanied by the production of a product with adequate properties for a particular purpose.

Sheets or panels for use in building generally require to have a mean modulus of rupture (bending strength of at lest 16 No and a density of at least 1.3 g.cm~3. For roofing slates, the requirements : ' , .. .. . . . ...

~;22~5~3~

are more stringent. Presently available asbestos cement roofing slates, otherwise known as compressed asbestos cetnent slaves, have a smooth surface which generally carries an acrylic coating of the desired killer and are required by current US standard specifications to have an average minimum mean modulus of rupture MY
of 22.5 N.r~n~2 A minimum density of 1.8 g.cm~3 is advisable to avoid porosity which could result in frost damage in roofing slates.

Simple replacement of the asbestos content of asbestos-cement lo sheet materials by glass fire in chopped strand form does not produce materials of equivalent appearance, because the chopped strands tend to be visible in or to project from the surface, resulting in a surface finish which is either unacceptable when the usual amount of coating has been applied or requires so much decorating material to produce an acceptable coating as to cause an unacceptable increase in cost.
Furthermore such simple replacement ox the asbestos by chopped strand lass fire does not enable one to achieve equivalent strengths with economic amounts of glass fire, due to uneven distribution of the glass fire in the cement and attack on the glass fire by the alkalis in the cement. In our US Patent Specification No. 1,543,951 we have described a method of manufacturing an asbestos-free fibre-reinforced cement composite material from an aqueous cement slurry incorporating both chopped strands of glass fire and single filaments of inorganic non-crystalline material, which latter Jay be produced from continuous filament chapped glass fire strands which separate or fulminates on contact with the cement slurry. If glass fire in such single filament phony produced from filamentised chopped strands is used as the sole fibrous reinforcement, a smooth surface finish can be obtained. The strength problems are aggravated, however, because the single filaments are open to attack by the alkalis in the cement and there is also ; difficulty in achieving an adequate bond between the fire and the matrix.

I, , . ,.. .. ..

~2~5~

To improve the strength of glass fire reinforced cement products, various proposals have been made for the incorporation ox reactive silica in different forms. For example, US Patent Specification No. 1,402,~5 (N.R.D.G.) relates to a glass fire reinforced pozzolanic cement product comprising a cement matrix containing at least 10~ by weight of a pozzolana which is a silicate glassy material capable of reacting with calcium hydroxide and thereby setting into a hard strong material) and fires of an alkall-resistant silica/zirconia glass containing at least 6 mol. % ZrO2. The spectfiçation states that a very desirable increase in the strength of the composites may be obtained by controlled heat treatment which accelerates the attainment of stable properties and ultimate strength and which may take the form of at least two days under water at a temperature of e 60C to ~0C. One of the preferred pozzolanas described is pulverized fuel ash PEA used in amounts from 15~ to 40 by weight. US Patent Specification No. 1,421,556 TOKYO Construction Materials Limited) describes the production of cement board products reinforced with a mixture of long and short staple alkali resistant vitreous ~ibres, I chopped glass fire strands and milled mineral fires, with cellulose fire and sufficient silica, in the form of diatomite, to react with the free lime liberated during the hydration of the cement. The board products are autoclave to assist the reaction. European Patent Application No. 68,742 (Cape Boards Panels Limited) relates Jo shaped articles such as boards and sheets for cladding and roofing, and describes a manufacturing process using an aqueous slurry comprising, on a dry weight basis, SO to 90~ cement, UP
to 40% highly reactive pozzolanic silica and I to 15~ cellulose fires. Reaction occurs between the cement and the silica by air curing. The specification states that mixtures of highly reactive pozzolanic silicas, e.g. volatilized silica and diatomite~ may be used and that glass fires may be incorporated in addition to the cellulose fires. US Patent Specification No. 2,~48~330B (Rockwell International A/S) describes a method ox preparing a fire reinforced so cementitious plate from an aqueous slurry of cement and fires selected from the group consisting of synthetic mineral fires, natural organic fires and inures thereof, wherein a fine material (specifically a filter dust produced as a product in the manufacture of silicon or silicon alloys by an electrothermal process and consisting essentially of volatilized silica) having an average particle s key lower than that of the cement particles is incorporated in the slurry to reduce the loss of fine cement particles and to neutralize alkaline products in the mixture, thus inhibiting alkaline degradation of the mineral lo fires. It is said that the amount of fine particles should preferably not exceed 15~ by weight based on the weight of the cement because high concentrations of fine particles reduce the rate at which excess water is removed from the slurry. Finally in our own US Patent Specification Nut 1,565,823 we disclose the use of reactive silica in the form of pulverized fuel ash PEA or a fine silted flour such as diatomite or ELK EM silica which is a volatilized silica, which were found to have a synergistic effect with the particular size compositions described in that specification for application to alkali-resistant glass fires for reinforcement of cement. Proportions of 10~ to 40~ of the reactive silica were shown to produce a further improvement in durability of the sized glass fire strands incorporated in the cement We believe that the reactive silica assists in bonding the glass fire into the cement matrix as well as reacting with alkalis in the matrix.

Our own recent investigations indicate that the various forms of reactive silica have different effects on the early strength and on the long term strength of the composite materials. This is particularly the oases where slays fire In single filament form is used for reinforcement. Our investigations have shown that if one uses filamentised dispersible glass fire strands as the sole fibrous reinforcing material, it is essential to use a mixture ox two different reactive silicas, namely pulverized fuel ash and vslatilised silica, within specific ranges, Jo order to be able to produce flat sheet 5~3~

material suitable for use as building panels on existing asbestos-cement machinery, with both a good start strength and good long term durability, which can oven be accompanied by an increase of strength beyond the initial strength. Use of the volatilized silica without the pulverized fuel ash gives a good start strength but on subjection to simulated aging tests the material shows a marked falling off in strength. Polarized fuel ash on its own without the volatilized silica gives a poor start strength and a poor overall matrix strength.

According to the invention, therefore, a sheet material is provided, formed of a fibre-reinforced cement composition comprising, in percentages by weigh of solids:-Ordinary Port land cement 50 to 71P
Pulverized fuel ash 14 to 40 Volatilized silica containing at least 86 by weight Sue) 5 to 12~
Filamentised chopped glass fire strands to 7X

in which these components constitute at least 90~ by weight of the solid constituents of the composition and in which, when the cement composition comprises less than I by weight of volatilized silica, the volatilized silica is of a grade containing more than 86~ by weight of Sue, and when the cement composition comprises only I by weight vola~ilised silica, the volatilized silica is of a grade containing at least 94~ by weight Sue, the sheet material having a minimum average modulus of rupture of 16 N.mm~2 and a minimum density of 1.3 g,cm~3.

The combination of polarized fuel ash and volatilized silica in the proportions set out above results in a product with a satisfactory initial strength and long term durability, accompanied by an increase of strength beyond the initial strength after curing. The use of filamentised glass fire produces a smooth surface, as required where a coating is to be applied, by avoiding the occurrence of projecting strand ends as would occur if integral chopped strands were used.

I

The term "volatilized silica" is used herein to describe the material (otherwise known as micro silica or silica fume) which is commercially produced as a by-product during the manufacture of silicon or silicon/metal alloys by an electro-metallurgical process. Its chemical composition can vary slightly according to the characteristics of the process and of the main product, but it normally contains at least 86~ by weight Sue and 0.1 to 0.7~ Sick Grades of lower purity are not suitable for use in the present invention because the quantity then required causes problems in de-watering the cement sheet material.
Purer grades of relatively carbon-free volatilized silica are available, containing at least 94~ Sue and around 0.1~ Sick It has been found that these purer grades have a higher surface activity and hence can be used in smaller quantities than the normal grades, in the present invention. If the proportion of volatilized silica used in the sheet material of the present invention is less than I by weight, the volatilized silica must therefore be of a grade containing more than 86% by weight Sue and where the minimum proportion of 5g of volatilized silica is used, the latter material must contain at least 94~ by weight Sue-The preferred percentage of filamentised chopped glass fire strands in the sheet material is from 3 to I Although the percentage may be as low as 2%, problems can then arise in handling the green (i.e. uncured) sheet. With percentages between I and I difficulties can be encountered in ensuring uniform dispersal of the glass fires in the cement matrix.

The balance of the composition may comprise up to I by weight of cellulose pulp, which is included as a processing aid to assist drainage. Up to 5% by weight of a plasticizer, for example ball clay, bentonite or talc, may also be incorporated to improve the plasticity of the material prior to curing. Small quantities of conventional dispersing and flocculating agents may also ye used.

The ~ilamentised strands are preferably made of an i5~3g alkali-resistant glass composition containing a least 6 mol. ZrO2.
For example, the glass fire strands may be of the type described and claimed in our US Patent Specification No. 1,290,528 and sold by Fjbreglass Limited under the Trade Mark Cem-FIL, and their composition may be substantially, in milks Sue 69 Zra2 9 Noah 15.5 Coo 6.5~

The invention further provides a flat sheet material suitable for use as roofing slates, formed of a fibre-reinforced cement composition comprising, in percentages by weight of solids:-Ordinary Port land cement 50 to 70 Pulverized fuel ash 20 to 40 ; Volatilized silica (containing at least 86% by weight Sue) 8 to 12~
Filamentised chopped glass fire strands 2 to 5%

these components constituting at least 98~ by weight of the solid : 20 constituents of the composition, the balance (if any) consisting of compatible constituents, the sheet material having a minimum average modulus of rupture of 20 N.mm~2 and a minimum density of 1.8 g.cm~3 and a smooth surface for reception of a coating.

Tests on roofing slates made from such material have shown that they have a considerably better resistance to repeated freezing and thawing and general weathering than conventional asbestos cement slates.
::
the invention also resides in a method of making a sheet material : of fibre-reinforced cement comprising the steps of mixing an aqueous slurry whose solid contents i include:-Ordinary Purloined cement 50 to 71 ' I_ .. . .. . . . ..

1;~2 Pulverized fuel ash 14 to 40 Volatilized silica (containing at least 86~
by wet go So Ox) 5 to 1 2X
Dispersible chopped glass fire strands 2 to I

in which these components constitute at least 90% by weight of the solid contents of the slurry and in which, when the slurry comprises less than 8% by weigh: of volatilized silica, the volatilized silica is so a trade containing more Han 86~ by weight sj2, and when the slurry comprises only I by weight of volatilized silica, the volatilized lo silica is of a grade containing at least 94g by weight Sue, the mixing being carried out so as to disperse the chopped glass fire strands into swig filaments, depositing a famine from the slurry on to a pheromones surface, superimposing a plurality of said laminate on one another to build up a sheet of cementitious material, and curing the sheet.

In order to avoid mechanical damage to the glass fires during processing, the slurry mixing may be carried out by first mixing the cement pulverized fuel ash and silt cay with water in a hi go shear mixer and then adding the dispersible chopped glass fire strands under low 20 shear mixing conditions. Alternatively the slurry mixing may be carried out by first mixing the cement and pulverized fuel ash and optionally some of the silica with water in a high shear mixer and then adding the silica or the remet nuder of the silica with the dispersible chopped glass fire strands under low shear mixing conditions.

The deposition of the famine and the building up of the sheet can be carried out on an asbestos-cement machine of the Bell or Hatschek type.

The sheet may be shaped to a desired cross-sectional profile before curing, eye. so as to provide it with contiguous or spaced 30 corrugations. The sheet may, for example, be shaped by application of a vacuum profiling head of known type, or by placing it on a former , .

_ . .... . . . . . . . .. . ., ... .. . .. , . ...

~2~5~3~

plate whose upper surface has the desired cross-sectional profile and allowing the sheet to collapse into contact with said surface. In the latter case, a second former plate may be placed over the sheet to complete the shaping.

The sheet may be cured at a temperature in the range from 6~C to 90C, preferably from 70C to 80C, for 24 hours and then stored at ambient temperature for at least 7 days to complete the cure. Such a high temperature initial cure assists in ensuring the long term durability of the finished product, while the presence of the pulverized fuel ash and vola~ilised silica enables the filamentised glass fire to survive the cure in sufficient quantity to provide adequate reinforcement.

The sheet may be pressed to de-water it before curing, eye. by placing a stack of such sheets with interleaving plates in a press and subjecting the stack to pressure to expel water.

For making roofing slates, the shapes of the roofing slates are preferably stamped out from the sheet before it is pressed and cured and the slates are separated after curing.

Specific embodiments of the invention will now be described in more detail by way of example.

Sheet material in accordance with the invention, suitable for making roofing slates, can be made on conventional asbestos cement machines ox either the Bell type, as described in Us Published Patent Application No. AYE, or of the well-known Hatschek type. In both these machines, several laminate (typically eater superimposed to on a sheet of glass ire reinforced cement with 3 thickness of 4 to S mm after pressing. In the case of the jell machine, a glass fire containing aqueous cement slurry is transferred through a doctor roll system on to a moving web or belt where it is de-watered to around 25~
water content by suction. The famine so formed is transferred from the * Trade Mark (Bell Maschinen Fiberlike AGO. ) 65~39 belt on to a rotating drum. When sufficient l~minae have been superimposed on one another on the drum to build up a sheet of the desired thickness, the sheet is cut off the drunk. In the case of the Hatschek machine the famine is deposited on a rotating sieve and then transferred to a belt and thence to a drum. Again the sheet is cut off the drum once a suffusion number of laminate hale been superimposed to build up the required thickness.

The subsequent processing is virtually the same in each case. A
cutter us used to stamp out the shape of several slat rectangular roofing slates with their fixing holes on the sheet. The states my be of the standard dimensions of 600 x 300 mm, or 500 x 25G my, for example. The sheet is placed on a steel interleaving plate and transferred to a stack of similar sheets. When a sufficient number of stamped-out sheets have been accumulated in the stack, it is passed to a press where the sheets are further de-watered by pressure to a final water content of about 20X. The interleaving plates ore then removed and the sheets are cured at 80C for 24 hours and finally stored at ambient temperature for seven days. The individual roofing slates can then be detached from one another and are ready for coating if desired.

The following specific Examples I and II relate to production of roofing slates in the above manner on a Bell machine.

Example I
An aqueous slurry was formulated having the fang composition expressed as parts by weight of solids:-Ordinary Paranoid cement 61 Pulverized fuel ash Yolatilised silica (86~ by weight Sue) 9 Filamentised chopped strands of alkali-resistant glass fire as sold under the Trade Mark "Cem-FIL"
by Fiberglass Limited I
Cellulose pulp 2X

:., 65~39 Conventional dispersing agents, flcccula~ing agents etc. may also be incorporated in small quantities (less than 0.1~ in known manner.

The glass fire strands were substantially 3 mm in length, made up of filaments substantially 20~ in diameter sized with a size composition designed to ensure that the strands disperse or fulminates in contact with the slurry. Examples of such sizes are disclosed in US. Patent Specification No. 3,948,673. The composition of the glass fire was, in mol. X:-Sue 69%
Zrû2 9 Noah 15.5 Coo 6.5~

After curing as described above at 80C for 24 hours and storage for seven days at ambient temperature, the slates were tested to ascertain their bending strengths in both the longitudinal and transverse directions relative to the direction of movement of the belt of the Bell machine; they were also tested for impact strength.
Further sample slates were subjected to accelerated aging by total immersion in water at 70C for sixteen days, simulating approximately 30 years natural weathering, and then retested. The results obtained we-e as follows--Table 1 _ . . . _ _ __ Long mu- Trays- Mean do net verse __ _ Limp t of Propriety on-Immediately amity ~N.mln~2) 22.9 12.6 Modulus of Rupture after (N.n~n~2~ 25.8 15.1 2û.5 Impact strength cur ( Nmm. no 3 . 2 Limit of Proportion-After amity (N.mm~~ 28.7 16.0 Modulus of Rupture accelerated (N.n~n~2) 29.7 17.3 23.5 Impact Strength age no ) _ - 2. 4 Density of slates: 1.95 y.cm~3 It will be seen that the mean modulus of rupture was over 20 N.mm-2 and increased after aging.

Example II
Roofing slates were made up as described above from an aqueous cement slurry having the following composition expressed as percentages by weight of the solids content:-Ordinary Port land cement 61 Pulverized fuel ash 24,5 Volatilized silica (8Ç~ by weight Sue) 9 Filamentised chopped glass fire strands (as in Example 1) 3.5X
Cellulose pulp 2g Dispersing agents, flocculating agents etc.~O.l~

The slates produced were tested as in Example I with the following results:-Tall e 2 . Long tug Trays- Mean Jo _ dial _ verse Limit of Proportion-Immediately amity (N.mm-2)24.6 15.9 Modulus of Rupture after ~N.mm~2) 25.8 16.5 21.2 Impact Strength cure 2. 9 Density of slaves: 1.90 g.cm~3 ....

Swahili ROQfjng slates were made up as in Examples I and II from an aqueous slurry having the following solids contents, by weight;-Ordinary Port land cement 7 Pulverized fuel ash 14 Volatilized silica (86~ by weight Sue) 11 Filamentised chopped glass flare strands (as in Example I) 3.
Processing aids cellulose pulp, dispersing agent, flocculating agent) 1.5 Comparative Example IV
For comparison, a further set of slates were made up from slurry made up as in example III but with the pulverized fuel ash replaced by limestone flour tCaC03).

Comparative Example Y

For further comparison, roofing slates were made from a slurry whose solids contents were, by weight:-Ordinary Port land cement 69 Pulverized fuel ash 6 Volatilized silica ~86~ by weight Sue) 20 Filamentised chopped lass fire strands (as in Example I) 3 Processing aids (cellulose pulp, dispersing agent flocculating agent) I

: Results of tests on the roofing slates of Example Ill and : Comparative Examples IV and Y were as follows:-, ..... , _.. .. . . .. . . .. . .... .

5~9 Table 3 .
Density LOP MOW Impact Specific MOW
III Immediately after cure 1.7 12.3 15.1 3.8 8~9 After accelerated aging 1~7 16.0 17.0 2.7 1~.1 -- _ _ IVY In~ediately after cure 1.7 12.6 15.4 4.0 Sol After accelerated aging 1.7 11.1 11.4 1.0 6.7 V Immediately after cure 1.5 8.7 11.8 I 7.g ....~

: It will be seen that the replacement of the pulverized fuel ashby limestone flour in Example IVY gave much reduced strength after j aging, while Example Y with too much silica and insufficient - 20 pulverized fuel ash had poor strength even immediately after curing.

Example VI
This example relates to the production of a flat sheet of : glass-fibre reinforced cement on a Hatschek motion or use as a building panel, for which a minimum mean modulus of rupture (MOW) of 16 N.~n~2 and a minimum density of 1.3 g.cm~3 may be specified.

The aqueous slurry had the following composition, in parts by weight:-Ordinary Port land Cement 60,5 Pulverized fuel ash 24.5 Yolatilised Silica (86~ by weight Sue Dispersible chopped glass fire strands 3.5 Processing Aids (cellulose pulp, dispersing lo agent, flocculating agent) 3.5~

The glass fire strands were as described above in detail in Example I.

Sheets were formed in conventional manner on the Hatschek machine. Some of these sheets were profiled (i.e. corrugated) by use of a conventional profiling head and deposited on a correspondingly shaped former plate for curing. Other sheets were pressed to de-water them to a water content of 20~ before curing. In each case the sheet was then cured at ~0C for 24 hours and stored for seven days thereafter at ambient temperature. Samples cut from the sheet were tested as described above to ascertain their bending strengths, in both the longitudinal and transverse direction relative to the movement of the cylindrical sieve and belt of the Hatschek machine. They were also tested for impact strength. The results obtained were:-i8~3 Tall e 4 Profiled unpressed sheet: Dry density 1.4 g.cm-3 Longitudinal Transverse Mean LOP 13.8 12.6 13.2 MOW 22.4 16.7 19.6 ¦ Pressed flat sheet. Dry density 1.7 g.cm~3 I Longitudinal Transverse Mean ! LOP 18.7 16.9 17.8 I MOW 31.8 19.8 25.8 ¦ Impact - - 3-5 Further trials have been made in the laboratory to investigate the range of possible compositions for the sheet material for making roofing slates. In such laboratory trials, with normal equipment, it is difficult Jo produce as thin a product as on the full scale machines and it is not possible to build up the slate as a series of laminate or to obtain the same product density. The results obtained are useful in a comport sense though the absolute values obtained cannot be translated directly into values which would be obtained using similar formulations in full scale operation on a Bell or Hatschek mashing.
The laboratory trials were carried out by forming a slate, 30 cm square and 8 mm thick, and de-watering it by simultaneous application ox pressure and suction. The slate was then cured a 60C for 24 hours and the strengths measured as in the preceding examples. The results are set out in Table 5, which also includes the composition of each slate, the length of the glass filaments employed and a figure for the modulus of rupture corrected for the lower density of the slate as opposed Jo that to be expected for a slate produced from the same composition in full scale operation. In all these samples, volatilized silica of at least 86S by weigh S10z content was used.

Table S

Sample OPT PEA Vale- Glass Film- Solely- Den- LOP MOW Imp- Specs No. X lilt- Fire mint lose, sty act ific sod % 1 length do super sire- MOW
So 1 i cay slants, ngth ( Con-phlox- feat-tents Ed for & C. Dens-ivy I_ : 10 1 I 25 3.5 6 mm ~.~ 1.68 7.3 13.1 2.'j7.~

2 59 14.5 ills 3.5 6 nllR 1.5 1.72 1~.9 16.93.~ I

3 69 14.5 11.5 3.5 3 on 1.5 1.72 18.3 2~.34.0 11.7

4 62 28.5 5.0 3.0 3 no 1.5 1.59 11.4 12.5~.3 7.9 So 24 9.5 3.0 3 no 1.66 15.4 l~.B3.3 lû.l 6 57 29 9.5 3.0 3 no 1.5 1.64 15.3 16.73.5 10.2 7 I 27 11 3.5 3 mm 1.5 1.67 15.2 16.64.7 9.9 8 I 32 11 I 3 nun 1.5 1.63 13.7 15.B4.9 9.7 9 71 14 11 2.5 6 nun 1.5 1.68 14.g lb.l3.4 9.6 81 0 lo 3.0 3 errs 6.0 1.55 1~.0 14.53.2 9.4 Jo In the above Table, OPT = Ordinary Port land cement, PEA = Pulverized fuel ash, LOP = Limit of Proportionality and MOW = Modulus of rupture.

Of the ten samples tested samples 1 and 10 are included only as comparative samples which are outside the scope of the invention because they contain no volatilized silica or pulverized fuel ash, respectively. Sample 4 is also outside the scope of the invention because the convent of volatilized silica is too low for use of a material of only 86~ by weight Sue content. The strengths obtained could be increased by increasing the temperature of cure and would certainly be increased in full scale operation due Jo the better distribution of the glass fire which arises prom the formation of individual laminate on the jell and Hatschek machines.
* Trade Mark -l 9-Table 5 demonstrates what if the volatilized silica is not present (sample 1) the overall strength of the matrix is poor compared with those samples containing o'er 8% of the volatilized silica.
Sample 10 shows what in the absence of pulverized fuel ash, the initial strength is low as compared with those samples containing equivalent amounts of volatilized silica; products made from such a composition, when subjected to artificial aging, show a further falling off in strength. The strength of sample 4 would be substantially increased of volatilized silica of 94~ Sue convent were used in accordance with the invention.

To demonstrate the importance of using volatilized silica of 94Z
Sue content rather Han 86~ Sue content when the proportion of volatilized silica in the material is relatively low, a series of samples were made in the laboratory differing only in the type and proportion of volatilized silica, and tested for LOP and MORN with the following results. Two grades of Yolatilised silica containing 86~
Sue and one containing 94~ Sue, identified in the Tables as Grades 1, 2 and 3, were used.

Table PA

! LOP MOW
I

owe Volatilized Silica (86~ So?) (Grade 1) 12.6 15.3 6 2 ED I 11 11 1 1 . 1 1 3 . 4

5-~ if if if 10.4 12.4 table 6B
,, _ _ _ _ _ ............. .. . . . .. _ _ _ .
owe Yolatilised Silica (86~ Sue) (Grade 1) 10.6 13.2 81D 11 11 grade 2) 10.8 13.6 owe ,. ., ,. 10.3 12.5 Lo Table 6C
- . _ LOPMOR
_ I
owe Volatilized Silica (86~ Sue) (Grade 1) 10.6 12~2 8' Volatilized Silica (94Z Sue) grade 3) 9.6 12.5 aye ., ;. 9.4 12.0 Lowe .. 9.5 12.0 It will be seen that reduction of the silica content from owe Jo owe was accompanied by a substantial loss of strength where 86Z Sue content volatilized silica was used, but that with the 94Z Sue content volatilized silica no appreciable loss of strength resulted.

Claims

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

1. A sheet material formed of a fibre-reinforced cement composition comprising, in percentages by weight of solids:-Ordinary Portland cement 50 to 71%
Pulverised fuel ash 14 to 40%
Volatilised silica (containing at least 86% by weight SiO2) 5 to 12%
Filamentised chopped glass fibre strands 2 to 7%
in which these components constitute at least 90% by weight of the solid constituents of the composition and in which, when the cement composition comprises less than 8% by weight of volatilised silica, the volatilised silica is of a grade containing more than 86% by weight of SiO2, and when the cement composition comprises only 5% by weight volati1ised silica, the volatilised silica is of a grade containing at least 94% by weight SiO2, the sheet material having a minimum average modulus of rupture of 16 N.mm-2 and a minimum density of 1.3 g.cm-3.

2. A sheet material according to Claim 1 wherein the weight percentage of filamentised chopped glass fibre strands is from 3 to 4%.

3. A sheet material according to Claim 1 or 2, wherein the balance of the composition comprises up to 4% by weight of cellulose pulp.

4. A sheet material according to Claim 1, wherein the filamentised chopped glass fibre strands are made of an alka1i-resistant glass composition containing at least 6 mol.% ZrO2.

5. A sheet material according to Claim 4, wherein the composition of the glass fibre strands is substantially, in mol. %:-Si02 69%
Zr02 9%
Na20 15.5%
CaO 6.5%

6. A flat sheet material suitable for use as roofing slates, formed of a fibre-reinforced cement composition comprising, in percentages by weight of solids:-Ordinary Portland cement 50 to 70%
Pulverised fuel ash 20 to 40%
Volatilised silica (containing at least 86% by weight Si02) 8 to 12%
Filamentised chopped glass fibre strands 2 to 5%
these components constituting at least 98% by weight of the solid constituents of the composition, the balance (if any) consisting of compatible constituents, the sheet material having a minimum average modulus of rupture of 20 N.mm-2 and a minimum density of 1.8 g.cm-3, and a smooth surface for reception of a coating.

7. A method of making a sheet material of fibre-reinforced cement comprising the steps of mixing an aqueous slurry whose solid contents include:-Ordinary Portland cement 50 to 71%
Pulverised fuel ash 14 to 40%
Volatilised silica (containing at least 86% by weight SiO2) 5 to 12%
Dispersible chopped glass fibre strands 2 to 7%
in which these components constitute at least 90% by weight of the solid contents of the slurry and in which, when the slurry comprises less than 8% by weight of volatilised silica, the volatilised silica is of a grade containing more than 86% by weight SiO2, and when the slurry comprises only 5% by weight of volatilised silica, the volatilised silica is of a grade containing at least 94% by weight SiO2, the mixing being carried out so as to disperse the chopped glass fibre strands into single filaments, depositing a lamina from the slurry on to a foraminous surface, superimposing a plurality of said laminae on one another to build up a sheet of cementitious material, and curing the sheet.

8. A method according to Claim 7, wherein the slurry mixing is carried out by first mixing the cement, pulverised fuel ash and silica with water in a high shear mixer and then adding the dispersible chopped glass fibre strands under low shear mixing conditions.

9. A method according to Claim 7, wherein the slurry mixing is carried out by first mixing the cement and pulverised fuel ash and optionally some of the silica with water in a high shear mixer and then adding the silica or the remainder of the silica with the dispersible chopped glass fibre strands under low shear mixing conditions.

10. A method according to Claim 7, wherein the deposition of the lamina and the building up of the sheet are carried out on an asbestos-cement machine of the Bell or Hatschek type.

11. A method according to Claim 7, wherein the sheet is shaped to a desired cross-sectional profile before curing.

12. A method according to Claim 11, wherein the sheet is shaped by application of a vacuum profiling head of known type.

13. A method according to Claim 11, wherein the sheet is shaped by placing it on a former plate whose upper surface has the desired cross-sectional profile and allowing the sheet to collapse into contact with said surface.

14. A method according to Claim 13 wherein a second former plate with the said profile is placed over said sheet.

15. A method according to Claim 7, wherein the sheet is cured at a temperature in the range from 60°C to 90°C for 24 hours and then stored at ambient temperature for 7 days to complete the cure.

16. A method according to Claim 15, wherein the sheet is cured at a temperature in the range from 70°C to 80°C.

17. A method according to Claim 7, wherein the sheet is pressed to de-water it before curing.

18. A method according to Claim 17, for making roofing slates, wherein the shapes of the roofing slates are stamped out from the sheet before it is pressed and cured and the slates are separated after curing.