WO1985000035A1

WO1985000035A1 - Ceramic material

Info

Publication number: WO1985000035A1
Application number: PCT/AU1984/000105
Authority: WO
Inventors: Gordon Wallace Collett; Henry Wynands; Bruce Gordon Small; Kathleen Marjorie Day
Original assignee: Monier Limited
Priority date: 1983-06-17
Filing date: 1984-06-15
Publication date: 1985-01-03
Also published as: EP0148869A4; ES533902A0; IT1199142B; ES8604044A1; IT8448413A0; EP0148869A1

Abstract

A tile or other building product is manufactured by firing a green product from a coarse component such as metallurgical slag or crushed rock and a fine component such as fly ash, with sodium silicate or other alkali metal compound which reacts with at least the fine component to form a binding matrix.

Description

CERAMIC MATERIAL

TECHNICAL FIELD

This invention relates to the manufacture of building products such as tiles or bricks, from inexpensive materials such as slag, fly-ash, crushed rock and the like. BACKGROUND ART

The published art reveals a great many attempts to produce satisfactory and economic bricks or tiles from waste materials, particularly from mixtures of slag and fly-ash. Brown U.S. patent 2,576,565 describes the production of ceramic articles such as bricks, tiles and pipes, in which particles of coal-ash slag are bound in a continuous matrix consisting of. thermall merged fly- ash. A similar product is described in Minnick, L.J. , "New Ply Ash and Boiler Slag Uses", Technical Association of the Pulp and Paper Industry, January 1949, Vol. 32, No. 1, pp. 21-28, and in Minnick, L.J. and Bauer, W.H., "Utilization of Waste Boiler Fly Ash and Slags", Ceramic Bulletin, Vol. 29, No. 5 (1950), pp. 177-180.

The production of bricks from a mixture of fly- ash and slag in which a high fly-ash content is used, is described in Shafer et al "Status Report on Bricks from Fly Ash", Bureau of Mines Information Circular 8348, Fly Ash Utilization, pp. 195-203, and in Cockrell et al U.S. patent 3,573,940.

In Smith U.S. patent 4,120,735, inorganic, non- ferrous residue from municipal incinerators is substituted

O PI for blast furnace slag, and lower firing temperatures are used due to the fact that the incinerator residue forms the binding matrix, rather than the fly-ash.

In the case of each of the above prior art techniques, the fly ash and slag components are mixed, usually with a binder such as bentonite or sodium silicate which provides green strength, formed into shape and fired at elevated temperatures, generally in the region of 1100°C. At the firing temperatures employed, the fly-ash (or, in the case of Smith 4,120,735, the incinerator residue) softens to such an extent as to form a glassy- matrix and to merge with the aggregate particles, and the strength of the fired product depends upon the firing temperature being sufficiently high for this bonding mechanism to occur. SUMMARY OF THE INVENTION

The high firing temperatures required in the prior art, involve several disadvantages. Firstly a considerable amount of energy is required to raise the ■ kiln to these temperatures, so the advantage otherwise gained by the shorter firing regimens which may be employed with fly-ash/slag products compared with the use of terracotta, is diluted. Secondly, considerable problems may be experienced with bloating and other surface effects which arise from moisture and the release of gases from materials trapped upon quenching of the slag. One object of the present invention is therefore to provide a composition and method of manufacture of such building products, which enables lower firing temperatures to be employed while still achieving adequate compressive strength and acceptable lack of porosity.

In the present invention, a building product such as a tile, brick or pipe is manufactured by mixing with a coarse and a fine aggregate component, an effective amount of an alkali metal compound and firing the product at a temperature at which, it is believed, there occurs a surface reaction between the particles at least of the fine component and the alkali metal compound such as to form a substantially fully reacted binding matrix, preferably withoutsigni icant merging of the particles of the fine component or the aggregate.

The preferred alkali metal compound employed is sodium silicate, but it is possible instead to employ other alkali metal silicates such as lithium or potassium silicate, or alkali metal compounds such as caustic soda or other suitable source of alkali metal such as anhydrous sodium carbonate, together with silica, such that the alkali metal silicate component (or possibly an alkali metal aluminosilicate, other metal-rich glasses or ternary silicates) is formed in situ during the drying and firing operations.

In many examples of prior art products employing blast furnace slag and fly-ash, sodium silicate is mentioned, amongst other materials such as bentonite, as a

OMPI constituent for the purpose of providing adequate green strength. In all such cases, the amount of sodium silicate is inadequate to provide the binding matrix achieved in products of the present invention, and the firing temperatures employed in the prior art are such that the subject bonding process, to the extent that it can occur at all in those formulations, is overridden by the conventional ceramic bond.

The coarse component may comprise materials, exemplified by metallurgical slags, crushed fine-grained mafic rock such as basalt containing substantially no free silica, and ground calcined clay, which are -believed to react with the sodium silicate and are referred to herein as reactive coarse component materials, and may also comprise materials, exemplified by silica, which do not react with the sodium silicate or other alkali metal compound and are referred to herein as non-reactive coarse component materials. The coarse component may comprise mixtures of such'materials.

It is found that the firing temperature at which the bonding process of the present invention occurs is significantly lower than the firing temperatures used in the prior art referred to above. For example, in the case of a tile made from blast furnace slag and fly-ash, the firing temperature is in the region of 850°C. With less reactive coarse component materials higher firing temperatures may be required to achieve optimum strength. as is discussed below, but such temperatures will be less than those required to achieve conventional ceramic bonding of the materials in question.

Suitable materials for the fine component include apart from fly-ash, such materials as mineral dust, precipitator dust from ore processing, and finely ground material selected from those described above in relation to the reactive coarse component materials.

In one broad form therefore, the invention resides in a method of manufacturing a building product comprising the steps of

1. A method of manufacturing a building product comprising the steps of:

1. forming a green product from a mixture of:

(a) a course component selected from the class comprising:

(i) blast furnace slag, other metallurgical slags, crushed fine-grained mafic or other rock, ground calcined clay, particulate materials capable of reacting with sodium silicate chemically to form a reaction product thereof at the surface of a particle, herein referred to as "reactive course component materials" (ii) particulate silica and siliceous materials including sand, and other materials which are stable at the firing temperature employed and which exhibit substantially no reaction with alkali metal compounds up to the firing temperature, herein referred to as "unreactive coarse component materials", and (iii) mixtures of such materials,

(b) a fine component selected from the class comprising finely ground or otherwise finely sized particles of reactive coarse component material and fly-ash, mineral dust from ore processing and mixtures thereof, and

(c) an alkali metal compound capable of reacting with silica and metal oxides present 'in the fine or reactive course component material to form a binding matrix, and

2. firing the green product at a temperature at which there occurs a surface reaction between the particles of the fine component and the alkali metal compound such as to form a substantially fully reacted binding matrix. Where the coarse component includes reactive

OMPI coarse component materials, the firing temperature may be chosen such that a surface reaction also occurs between the sodium silicate and the particles of the coarse component.

Preferably, the relative proportions of the coarse component and the fine component are chosen as those providing substantially maximum packing density. Thus, the optimum proportion of the fine component will vary with the particle size distribution of the coarse component. Where, for example, blast furnace slag is employed having a relatively large proportion of fine particles, less fine component may be used, providing adequate reactive material and reactive surface area is available for the formation of a sufficiently strong matrix. Experience suggests that the most satisfactory range for the fine component is from 10 to 20% by weight of the unfired mixture. Economic considerations will also affect the choice of these proportions.

The quantity of sodium silicate or other alkali metal compound is chosen in relation to the total surface area of the fine component, and the coarse component sufficient to provide coverage of a sufficient portion of this total surface area to achieve adequate bond strength in the fired product without leaving a significant quantity of unreacted alkali metal compound in the product after firing. A "significant quantity" of unreacted sodium silicate will be revealed by the leaching of an

OM?I unacceptable quantity of soda from the fired product.

Where the product does not require great strength, smaller amounts of sodium silicate or other alkali metal compound may be used, and it has been found that amounts down to 10% sodium silicate solution can be employed, while greater strengths are obtained with up to 20%, with optimum results from 12 to 18%.

The reactions believed to occur in the formation of products according to the invention are discussed below.

The particle size distribution of the fine component material must be chosen to provide adequate plasticity in the green mix, sufficient packing density with the coarse component to achieve the required degree of impermeability in the fired product, and sufficient surface area to enable complete reaction of the sodium silicate.

A suitable particle size distribution of the fine component is that contained in the Australian Standard 1129-1971 for fly-ash for use in concrete, which requires a distribution which is such that when wet sieved on a 150 um sieve the amount retained shall not exceed 10% and when wet sieved on a 45 um sieve the amount retained shall not exceed 50%.

It will be appreciated that larger particle sizes may be employed where the tolerable physical characteristics of the green product and of the fired product allow.

Similar considerations apply to the particle size distribution of the coarse component. While for the manufacture of roof tiles the particle size of the coarse component should preferably not exceed 3 mm, this may be varied for other applications.

It will be appreciated that the particle size distribution of the coarse component will also effect the properties and composition of the green and fired product. For example, where the coarse component is relatively deficient in smaller particles, a greater proportion of fine component will be required to achieve a given bulk density.

The choice between a reactive or non-reactive coarse component will naturally depend on the properties sought in the fired product and the relative cost of the raw materials, which will vary with geographical location and access to deposits or sources of material. In general it is believed that products of greater strength will be obtained from the use of reactive coarse component materials such as slag or basalt, but the lower strength provided by the use of unreactive fillers such as beach or river sand may be acceptable in some applications.

The alkali metal compound, typically sodium silicate, will normally be added in aqueous solution, and the quantity of additional water, which may also be required to achieve correct plasticity, will be determined by experiment. Naturally the total initial water content of the unfired mixture will depend on the process by which the article is to be formed, whether by extruding or pressing, for example. The green product is, of course, dried before firing, to minimize the quantity of free water.

In the description of examples of the invention, attention will be concentrated on the use of reactive coarse component materials. EXAMPLES OF EMBODIMENTS OF THE INVENTION

Example I

A mixture was prepared containing blast furnace slag, fly-ash and sodium silicate solution in the following proportions:

Slag 69

Fly-ash 13

Sodium silicate solution 18

The blast furnace slag had a maximum particle size of 3 mm and the following approximate composition:

SiO. 37. 6%

TiO. 0. 5%

^Al2°3 16. 8% Fe₂0₃ 2. 0%

MgO 3. 3%

CaO 39. 3%

PI κ₂o 0. 6% 0. 6%

The fly-ash had the following composition:

sio₂ 62% τio₂ 1.2%

A1₂0₃ 25.5%

Fe₂0₃ 3.3%

MgO 1%

CaO ^■ 1%

Na₂0 0.4% κ₂o 2.2%

S0₃ ^• 0.6%

with the remainder being made up of small quantities of MnO, ZnO, Li₂0 and P₂0₅.

The sodium silicate solution was that with a silica-soda weight ratio of the order of 2.25 and a density of the order of 1.56 with a solid content of approximately 46%.

The proportion of fly-ash was determined by adding fly-ash to a sample of slag until maximum bulk density was achieved, so that the proportions employed represent the maximum packing of the two ingredients for the particle size range of the two components.

The ingredients were mixed for three minutes in a planetary mixer and tiles were formed on pallets in a conventional concrete tile making machine. The tiles were then dried for two hours at approximately 90°C. Immediately after the tiles left the drier, a low temperature acid resistant ceramic glaze was applied by spray applicator.

The glazed tiles were stripped from the pallets and fired over a period of three hours, the maximum temperature being approximately 820°C, and the tiles being gradually reduced from this temperature to a kiln exit temperature of approximately 130°C.

Microscopic examination of the fired tiles showed some evidence of dissolution of fine particles of fly-ash in the sodium silicate, and the glassy matrix formed by the reacted sodium silicate appeared well bonded to slag particles. No distinct reaction zone between the matrix and the slag could be observed, but it is believed that a surface reaction between the matrix and the slag particles contributed to the bonding between those components.

Pieces of sample tile were tested for water absorption by being stood on end in approximately 1 cm of water for 24 hours, then totally immersed in water for a further 24 hours, and then boiled in water for 45 minutes. The following table shows the water absorption ⁽%⁾ measured after each of these treatments, respectively designated as A, B and C:

OMPI A_ JB C.

6. 2 6. 6 11. 9 ,

The quality of glaze on the fired tile was excellent, with a strong bond and very little gas bubble formation.

Example II

Samples were prepared from a mix having the following composition:

Blast furnace slag 70 Fly-ash 15

Sodium silicate solution 15 In order to examine the physical behaviour of the tile during a firing cycle, a bar of this material was taken through a cycle of heating to 1000°C in a dilatometer followed by cooling, and the same sample was subsequently subjected to a further heating and cooling cycle in the dilatometer.

Fig. 1 illustrates the dilatometer curve produced in the first "firing" of the sample, while Fig. 2 shows the curve obtained in the second cycle.

In Figs. 1 and 2, the percent change in length is plotted against temperature in °C. The dilatometer was operated on a normal cycle with a rate of temperature change of 3°C per minute, the sample being in an air atmosphere.

The major effects revealed in Fig. 1 occur between 600°C and 850°C.

Prior to 620°C, the bar expands linearly with rising temperature. In the region of 620-630°C softening of the material commences, and the dilatometer records a sharp contraction through the temperature zone of 700- 800°C where the material is in a pyroplastic state, while beyond approximately 850°C linear expansion is resumed, although the expansion of the sample recorded in this region is assumed to be in fact bloating, as the slag particles soften sufficiently to release entrapped gases.

The curve of Fig. 2 shows the material behaving quite linearly throughout the temperature range, with no contraction occurring in the region of 600-800°C. The linear behaviour of the sample in Fig. 2 indicates that the reaction occurring in the unfired sample in the 600- 800°C region is not repeated, and was completed in the course of the first dilatometer cycle.

In view of the faster firing times desirable in practice, it would not be expected that the reaction of the components which is believed to occur in the pyroplastic region, would be completed. This is in fact demonstrated in Fig. 3, which shows the dilatometer curve of a tile sample previously fired over a 4-hour cycle to 860°C. In this curve the scale of the vertical axis is magnified for the purpose of clarity. The sample shows the characteristic form of Fig. 1, but the contraction which occurs in the pyroplastic zone is only of the order of 0.3%, compared with 2% in the unfired specimen of Fig. 1.

If the dilatometer cycle is repeated on this material, it will be quite inert and show almost linear expansion and contraction throughout the temperature range in question in the same manner as that illustrated in Fig. 2. It therefore appears that the curve of Fig. 3 shows a residual reaction between unreacted sodium silicate and the other components.

The coefficient of thermal' expansion revealed by the dilatometer curves is of the order of 10 x 10 , and is comparable with those of terracotta tiles or common bricks.

In order further to investigate the nature of the product, experiments have been conducted with reduced proportions of sodium silicate, and these have shown that the strength of the fired tile is progressively reduced' with the sodium silicate content.

While the reactions occurring during firing have not been definitely identified, the evidence of microscopic examination and dilatometry clearly points to a reaction between the sodium silicate and the fly-ash and slag components, with the sodium silicate being fully reacted when the firing temperature exceeds some 830 to 850°C. It is in this characteristic that the present invention is radically distinguished from prior art fly- ash/slag bricks and tiles, where the products relied for their final strength upon merging of the slag and fly-ash components by firing at considerably higher temperatures than those employed in the present invention.

Where fly-ash and blast furnace slag are employed, it is believed that a reaction occurs at the surface of the fly-ash particles which essentially involves the dissolution of Al₂0₃ to form a sodium aluminosilicate. At the interface with the slag particles, it is believed that a similar reaction takes place, accompanied by the formation of a lime soda alumina silica glass.

On this view of the reactions involved, it is clear that other material may be employed which will provide for similar or functionally equivalent reactions by which the sodium silicate is converted to a reacted glassy matrix. Suitable substituents for the fly-ash and for the coarse component will therefore generally extend to materials capable of reacting with the sodium silicate at the particle surfaces, to form metal-rich glasses and/or ternary silicates.

Example III

An example of a tile composition employing a substitute for the fly-ash as the fine component material was prepared from a mixture of slag, manganese mud and sodium silicate solution in the following proportions:

Rod milled slag 70

Manganese mud 15

Sodium silicate 15

Samples were prepared and subjected in the unfired state to dilatometry under the same conditions as those of Example II. The dilatometer curve of Fig. 4 shows the behaviour of the unfired specimen, and that of Fig. 5, the behaviour of the specimen on a second run in the dilatometer.

It will be observed that the curves of Figs. 4 and 5 exhibit properties similar to those of Figs. 1 and 2.

Example IV

As a further example of a fly-ash substitute, a sample was prepared employing crushed and ground limonite, having the following composition:

Blast furnace slag 70 Limonite 15

Sodium silicate 15

This sample was fired in a dilatometer as in the previous examples, and the result is shown in Fig. 6.

An attractive alternative to blast furnace slag for the purpose of the coarse component in products according to the present invention, is fine-grained mafic rock such as basalt. Such a material contains no significant quantities of free silica, and therefore does not exhibit a quartz inversion. It has a low coefficient of thermal expansion and is capable of short firing times. Mafic rocks also contain no hydrated water, and the green product is therefore not subject to drying shrinkage, which cannot be tolerated in the production of tiles on pallets. Basalt furthermore contains suitable quantities of metal oxides to enable a surface reaction with the sodium silicate.

Experiments have been conducted with compositions in which the fly-ash content lies between 15 and 20%, sodium silicate between 13 and 16% with the balance crushed basalt of varying particle sizes but all passing 7 mesh.

The following approximate composition is typical of dolerite from Prospect, New South Wales, used in the trials referred to:

sio₂ 51

A1₂0₃ 17

Fe₂0₃ 1

FeO 3

MgO 2

CaO 12

' OM Na₂0 2 κ₂o 6

H₂0 4

TiO- 1

Basalt-based tiles have been produced with strengths approximating those of the slag-based tiles (3000 Newtons) but with superior characteristics in greater acid insolubility, and in the shorter firing cycle which is possible due to the fact that the tiles can be raised to the maximum firing temperature and cooled from this temperature more quickly without the cracking which would occur when using blast furnace slag. Example V

A sample having the following composition: Crushed dolerite 70

Fly-ash 15

Sodium silicate 15 was prepared and subjected to a standard dilatometer cycle at 3°C per minute to a maximum of 1000°C. Fig. 7 shows the curve obtained.

The sample was subjected to a second cycle and the result is recorded in Fig. 8.

Reference to Figs. 7 and 8 will show that the behaviour of the fly-ash and basalt material is similar to that of the previous examples, although the temperature at which the onset of softening and contraction occurs is

O PI raised to approximately 700°C, and the contraction of the material through the pyroplastic region is more gradual.

The curve of Fig. 8 shows no secondary reaction on re-heating.

It will be observed that the heating of the sample in this example ceased at 1000°C while contraction was still occurring. Unlike the slag-based samples, dilatometry reveals no reversion to expansion with further softening, apparently due to the absence of gas bubble formation or bloating. The broken line A in Fig. 7 indicates the curve typically obtained by dilatometry on such a sample heated beyond 1000°C.

As with the preceding examples, it is found that the reaction between the components is substantially complete when the tile is fired to a temperature which is within the pyroplastic zone as indicated by the dilatometer curve. Thus satisfactory tiles of the composition of Example IV have been produced by firing to approximately 950°C.

It will be appreciated that the rock employed for the production of the coarse component should contain no significant free lime, since this will be calcined to the oxide at approximately 920°C, and with subsequent contact with water this will convert to the hydroxide, causing expansion and damage to the body.

A further alternative aggregate material which has been referred to above is calcined clay, where satisfactory tiles have been produced by use of fly-ash in proportions of 15 to 25% and sodium silicate solution between 15 and 18%. In this case, higher firing temperatures again are required to produce the necessary bond, and in these cases firing temperatures may exceed 1000°C and be in the region of 1000°C to 1100°C, typically 1050°C.

Example VI

Fig. 9 shows the dilatometer curve obtained under the same conditions of dilatometry as in the preceding example, from a sample bar produced from a mixture of crushed calcined Maryborough yellow clay, fly- ash and sodium silicate in the following proportions:^'

Calcined clay 69

Fly-ash 15

Sodium silicate 16

It will be observed that the behaviour of this sample is similar to that of the basalt. In the particular sample there is an initial drying shrinkage in the region of 100°C, and evidence of some quartz inversion at 550°C - 600°C. Softening commences at approximately 900°C and the dilatometer indicates rapid shrinkage in the pyroplastic region. Based on this curve it would seem that a suitable firing temperature for products of this composition would be in the region of 1050°C, and this is found to be correct in practice. As was explained above, it is possible to substitute for the sodium silicate in the starting materials, caustic soda and a source of silica. It is found that this produces a strong well-bound product, and this approach should therefore be considered where the cost of materials is appropriate.

Example VII

A sample having the following composition:

Slag 77

Fly-ash 15

Caustic soda 3^'

100 mesh silica 5

was prepared and fired at 850°C. A hard insoluble product resulted, similar to the material obtained using sodium silicate.

Remarkably, it is found that the strength of the product after the drying stage is much greater than would be expected, and it is believed that this results from some of the reaction with the caustic soda occurring during the drying phase.

The precise nature of the reactions occurring during drying and firing in this form of the invention is not yet known. It is possible that sodium silicate is formed as such, but it is also likely in cases such as examples VII and VIII, that the alumina present in the slag and fly-ash enters a complex reaction so that direct formation of a binding matrix of sodium aluminosilicate occurs. Where the coarse or fine components contain significant quantities of other metal oxides, corresponding complex reactions may occur in the formation of the matrix.

It is also possible, in a further alternative form of the present invention, to substitute for the caustic soda, other alkali metal compounds capable of producing the binding matrix by reacting with the slag and alumina or other metal oxides. Soda ash is an example of such alternative material, as is potash, although the latter is unlikely to be economically attractive.

In example VII the slag will no doubt contribute silica to the reaction with the caustic soda, and fly-ash may also act as a source of silica.

It will have been observed from the dilatometer curves discussed above that the temperature at which materials of the present invention move into the temperature range over which they are pyroplastic and in which, it is believed, the major reactions occur between the sodium silicate and the fine component, and where applicable between the sodium silicate and the coarse component, varies with the materials used, as does the amount of contraction which occurs in that zone, and the width of that zone in °C. In all cases, however, it appears that the most desirable firing temperature for the

f OMPI product will be a temperature which is within, and preferably well into, that zone.

While considerable information is provided herein in connection with the proportions of the components employed in the manufacture of building products according to the present invention, it will be appreciated that complex factors affect the choice of these proportions in given applications, and experimentation will always be necessary to achieve optimum results for given raw materials and production techniques. For example, while the use of a 46% solids sodium silicate solution is preferred, preparations of lower density may be used if the consequent reduction in product strength is acceptable. Reduction of the sodium silicate content may be acceptable from the point of view of the strength of the finished product, but may result in a green mixture of inadequate plasticity for the pressing or other moulding techniques to be employed prior to drying. This cannot readily be overcome by the addition of free water, as this leads to consequential problems of shrinkage, bloating and other mechanical defects.

O PI

Claims

1. A method of manufacturing a building product comprising the steps of:

1. forming a green product from a mixture of:

(a) a coarse component selected from the class comprising: (i) blast furnace slag, other metallurgical slags, crushed fine-grained mafic or other rock ground calcined clay, particulate materials capable of reacting with alkali metal compounds chemically to form a reaction product thereof at the surface of a particle, herein referred to as "reactive coarse component materials" (ii) particulate silica and siliceous materials including sand, and other materials which are stable at the firing temperature employed and which exhibit substantially no reaction with alkali metal compounds up to the firing temperature, herein referred to as "unreactive coarse component materials", and (iii) mixtures of such materials, (b) a fine component selected from the class comprising finely ground or otherwise finely sized particles of reactive coarse component material and fly-ash, mineral dust from ore processing and mixtures thereof, and (c) an alkali metal compound capable of reacting with silica and metal oxides present in the fine or reactive coarse component material to form a binding matrix, and 2. firing the green product at a temperature at which there occurs a surface reaction between the particles of the fine component and the alkali metal compound such as to form a substantially ^* fully reacted binding matrix.

2. A method according to claim 1, wherein said coarse component material is a reactive coarse component material, said product being fired at a temperature at which a surface reaction occurs between said coarse component material and the alkali metal compound.

3. A method according to claim 1 or claim 2, wherein said firing temperature is chosen such that there is substantially no merging of the particles of the coarse component.

4. A method according to claim 1 wherein the quantity of alkali metal compound is chosen in relation to the total surface area of the fine and coarse components sufficient to cover in aqueous solution in the mix substantially all of said total surface area.

5. A method of manufacturing a building product according to claim 1 or claim 4, wherein said alkali metal compound is selected from the group comprising sodium hydroxide, sodium carbonate and potassium carbonate and said fine component includes silica.

6. A method according to claim 1 or claim 4, wherein said alkali metal compound is an alkali metal silicate.

7. A method according to claim 6 wherein said alkali metal silicate is sodium silicate.

8. A method according to claim 1 wherein the fine component is fly-ash.

9. A method according to claim 8 wherein the coarse component is blast furnace slag.

10. A method according to claim 8 in which the coarse component is crushed rock.

11. A method according to claim 8 in which the coarse component is ground calcined clay.

12. A method according to claim 1 wherein the fine component is a mineral dust derived from ore processing.

13. A method according to claim 1 wherein said fine component is a material capable of reacting with alkali silicate at the surface of particles of said component, to form a metal-rich glass.

14. A method according to claim 1 wherein said fine component is a material capable of reacting with alkali silicate at the surface of particules of said component, to form ternary silicates. gUREA OMPI - 23 -

15. A method according to claim 1 in which the content of the fine component is from 10 to 20% by weight of the unfired mixture.

16. A method according to any preceding claim wherein the said alkali metal compound is sodium silicate in aqueous solution in a quantity of from 10 to 20% by weight of the green product.

17. A method according to claim 16 wherein the quantity of sodium silicate solution is from 12 to 18% by weight of the green product.

18. A method according to claim 1 in which the relative proportions of the fine and coarse components are chosen to provide maximum packing density of the mix.

19. A method of manufacturing a building product comprising the steps of:

1. forming a green product from a mixture of:

(a) a coarse component selected from the class comprising: (i) blast furnace slag, other metallurgical slags, crushed fine-grained mafic or other rock, ground calcined clay, particulate materials capable of reacting with sodium silicate chemically to form a reaction product thereof at the surface of a particle, herein referred to as "reactive coarse component materials" (ii) particulate silica and siliceous materials including sand, and other materials which are stable at the firing temperature employed and which exhibit substantially no reaction with alkali metal compounds up to the firing temperature, herein referred to as "unreactive coarse component materials", and (iii) mixtures of such materials,

(c) an alkali metal compound capable of reacting with silica and metal oxides present in the fine or reactive coarse component material to form a binding matrix, and

2. firing the green product at a temperature at which the product exhibits pyroplastiσity.

20. A method according to claim 19, wherein said coarse component material is a reactive coarse component material, said product being fired at a temperature at which a surface reaction occurs between said coarse component material and the alkali metal compound.

21. A method according to claim 19, wherein said firing

OMPI temperature is chosen such that there is substantially no merging of the particles of the coarse component.

22. A method according to claim 19, wherein the quantity of alkali metal compound is chosen in relation to the total surface area of the fine and coarse components sufficient to cover in aqueous solution in the mix substantially all of said total surface area.

23. A method of manufacturing a building product according to claim 19 or claim 22, wherein said alkali metal compound is selected from the group comprising sodium hydroxide, sodium carbonate and potassium carbonate and said fine component includes silica.

24. A method according to claim 19 or claim 22, wherein said alkali metal compound is an alkali metal silicate.

25. A method according to claim 24, wherein the quantity of sodium silicate solution is from 12 to 18% by weight of the green product.

26. A method according to claim 19 wherein the fine component is fly-ash.

27. A method according to claim 26 wherein the coarse component is blast furnace slag.

28. A method according to claim 26 in which the coarse component is crushed rock containing substantially no free silica.

29. A method according to claim 26 in which the coarse component is ground calcined clay.

30. A method according to claim 19 wherein the fine component is a mineral dust derived from ore processing.

31. A method according to claim 19 wherein said fine component is a material capable of reacting with alkali silicate at the surface of particles of said component, to form a metal-rich glass.

32. A method according to claim 19 wherein said fine component is a material capable of reacting with alkali silicate at the surface of particles of said component, to form ternary silicates.

33. A method according to claim 19 in which the content of the fine component is from 10 to 20% by weight of the unfired mixture.

34. A method according to claim 19 wherein the said alkali metal compound is sodium silicate in aqueous solution in a quantity of from 10 to 20% by weight of the unfired mixture.

35. A method according to claim 34 wherein the quantity of sodium silicate solution is from 12 to 18% by weight of the green product.

36. A method according to claim 19 in which the relative proportions of the fine and coarse components are chosen to provide maximum packing density of the mix.