WO1998057905A1

WO1998057905A1 - Aqueous suspensions of metakaolin and a method of producing cementitious compositions

Info

Publication number: WO1998057905A1
Application number: PCT/GB1998/001758
Authority: WO
Inventors: Anthony Hales Asbridge; Thomas Richard Jones; David Robert Skuse
Original assignee: Ecc International Ltd.
Priority date: 1997-06-17
Filing date: 1998-06-16
Publication date: 1998-12-23
Also published as: GB9712479D0; EP0991605A1; JP2002504882A; NO996245D0; NO996245L; ZA985252B; CA2294372A1; CN1260769A; AU8222498A

Abstract

An aqueous suspension suitable for use in the production of cementitious compositions which includes, prior to production of a cementitious composition, an aqueous medium and a particulate inorganic material suspended in the aqueous medium, the particulate inorganic material comprising metakaolin and being substantially lime-free and having a pozzolanic activity, the suspension having a Ph of at least 7.5, the suspension being in a stable form having a rheology which allows it to be flowable as a wet slurry and wherein the suspension includes a thickening agent which inhibits sedimentation of the inorganic particulate material in the suspension.

Description

AQUEOUS SUSPENSIONS OF METAKAOLIN AND A METHOD OF PRODUCING CEMENΗΗOUS COMPOSITIONS

The present invention relates to aqueous suspensions of inorganic particulate materials. 5 Aqueous suspensions or slurries of so-called silica fume comprising silica particles are known for incorporation together with hydraulic cement in settable cementitious compositions for use in building, construction and like industries . Such suspensions

10 provide a ready mixed product which can be easily transported and added to the other ingredients required to form the cementitious composition. The silica present is known to react chemically with basic material such as lime present as a product of the

15 hydration of the cement which assists setting and hardening. The inclusion of the silica as an additive in the cementitious composition improves the properties of the composition, especially its compressive strength when it is allowed to set and harden.

20 Preparation and use of silica fume suspensions or slurries is not ideal. Thorough and intensive mixing is required to produce the suspension. Complete dispersion of the silica particles may be difficult or impossible to achieve and the formation of reactive

25 aggregates or agglomerates may result. Such aggregates can reduce the effectiveness of the silica content of the suspension and can, seriously, lead to gellation, expansion and cracking in the cementitious material when allowed to set and harden. Additionally, the maximum dose of the silica particles which can be incorporated in a practical cementitious composition is limited to about 10 weight per cent (based on the dry weight of cement present) . According to the present invention in a first aspect an aqueous suspension suitable for use in the production of cementitious compositions includes, prior to production of a cementitious composition, an aqueous medium and a particulate inorganic material suspended in the aqueous medium, the particulate inorganic material comprising metakaolin and being substantially lime-free and having a pozzolanic activity, the suspension having a pH of at least 7.5, the suspension being in a stable form having a rheology which allows it to be flowable as a wet slurry and wherein the suspension includes a thickening agent which inhibits sedimentation of the inorganic particulate material in the suspension. The flowability of the suspension allows it to be pumped, stirred, metered etc. and to be transported to a user site for addition to a cementitious composition at the user site.

Metakaolin having a substantial pozzolanic activity is known per se for use as an additive to cementitious compositions. However, such material is known and used in a dry powdered form. Use of such material in an aqueous suspension as in accordance with the first aspect of the present invention has not previously been known for use in such an application. In GB-A-2294259 it is envisaged that addition of water to metakaolin produces a material having the appearance of a dry powdered product. However, this prior specification does not contemplate the production of a wet, stable, non-sedimenting flowable slurry of metakaolin as described herein.

Calcined kaolin pigments are used in low concentrations in aqueous suspensions in the paper industry. Such pigments are quite different in several respects from metakaolin to be used for the inorganic particulate material used in the compositions of the present invention. Such differences may be summarised as follows.

Calcined kaolin pigments are produced by calcining kaolin at temperatures substantially greater than 1000°C, eg 1050°C to 1150°C. On cooling after calcining at these higher temperatures, recrystallisation in crystal phases quite different from the original occurs. Such pigments have no substantial pozzolanic activity and would not therefore be useful as additives for use in cementitious materials as described herein.

In contrast, metakaolin is produced by calcining kaolin at lower temperatures. The kaolin is dehydroxylated and the structure becomes amorphous but no recrystallisation occurs. Metakaolin may be characterised as the product of calcining kaolin between the temperature of 400°C at which dehydroxylation starts to occur and 980°C at which an exothermic transition occurs. Desirably, the metakaolin to be employed in the suspension according to the first aspect of the present invention contains less than 5% more particularly less than 1%, especially less than 0.5%, by weight of chemically bound water (as measured as loss of weight by ignition at 1000°C for 2 hours) .

Metakaolin when formed into an aqueous suspension by addition to water or an aqueous solution normally has a pH of between 6 and 7. When the solids concentration (on a dry weight basis) of such a suspension is 40 per cent or more, especially 50 per cent or more, the viscosity of the suspension is unduly high, eg greater than 2000 mPa.s. However, we have found unexpectedly and beneficially that if the pH of the suspension is increased by addition of a basic substance the viscosity can be reduced to a level whereby the suspension is usable in applications similar to those in which silica fume suspensions are used. Furthermore, incorporation in the suspension of a thickening agent allows the suspension to retain a suitable stable, non-sedimenting form which benefits its suitability for transport to and use by a user.

Desirably, the pH of the suspension according to the first aspect of the present invention is in the range 7.5 to 9.5 especially in the range 8.5 to 9.0. The pH of the suspension may be adjusted by simple addition of a suitable alkali or base other than lime which does not substantially react with the metakaolin present. The alkali or base is preferably a material which is readily available commercially, has minimal effect on the end use of the aqueous suspension and gives suitable pH adjusting effect for minimum dose. The alkali or base may be selected from hydroxides and conjugate base salts of alkali metals, eg hydroxides, silicates, or carbonates of sodium or potassium. Sodium hydroxide in dry or solution form is preferred as pH adjuster.

The amount of alkali or base added will depend upon the particular alkali or base used and its concentration. In general, active alkali amounts of much less than 1% are likely to be required to achieve the required pH. For example, to achieve a pH in the desired range 8.5 to 9.0, about 0.2% by weight of sodium hydroxide added in dry form is required (based on the dry weight of inorganic particulate material) .

Desirably, the aqueous suspension according to the first aspect of the present invention has a solids concentration of at least 40 per cent by weight, desirably at least 50 per cent by weight although it may be 60 per cent or more, based on the dry weight of solids (the inorganic particulate material) contained in the aqueous medium. The maximum solids concentration will be that at which the suspension is still a wet, flowable, stable slurry which may be determined by experimentation. The maximum solids concentration of the suspension may be about 65% by weight or in some cases even from 70% to 80% by weight. We have found that if a wet flowable slurry of an inorganic particulate material comprising metakaolin is produced, it is necessary to avoid sedimentation of the material, otherwise the suspension when obtained by a user requires substantial reworking or is not useful at all. The suspension according to the first aspect of the present invention is such that substantial visible sedimentation of the inorganic particulate material does not occur in the aqueous medium. Potential sedimentation is beneficially deterred by incorporation in the aqueous suspension of a thickening agent, eg in a concentration of not more than 10% by weight on a dry weight basis. Desirably, no sedimentation occurs in the suspension for at least two weeks after its preparation or, at least, if a limited degree of sedimentation occurs in this period, it is easily reworked, eg by simple mechanical agitation and/or pumping and/or mixing. Desirably, the amount of thickening agent employed is such that no gellation occurs in the suspension for a period of at least two weeks or, at least, if a limited degree of gellation occurs it is easily broken down by simple mechanical agitation and/or pumping and/or mixing.

Suitable thickening agents which may be incorporated in the aqueous suspension according to the first aspect to deter sedimentation (without substantial gellation) include materials known as thickening/gelling agents especially for use in aqueous media, eg selected from hydrophilic polymers such as polysaccharides, eg starches or gums, eg xanthan gum or guar gum, alginates, cellulose ethers, modified celluloses such as carboxy methyl cellulose and the like known to make an aqueous phase more viscous, absorbent clays such as attapulgite and smectite clays, eg bentonite, which absorb water and give a thickening (water-reducing) effect, ultrafine siliceous particulate materials, eg silica, which form a siliceous polymer in the aqueous phase thus increasing viscosity, particulate minerals, eg hydrotalcite, having a charged (cationic) mineral surface which interacts with particle surfaces of the inorganic particulate material to create a network-like structure which limits sedimentation. Gums such as xanthan gum or guar gum are preferred.

As noted earlier, the amount of thickening agent employed is preferably such that substantial gellation does not occur in the suspension or occurs only to an extent that any gel formed can be broken down by simple mechanical agitation. The maximum amount of thickening agent depends on the agent and the suspension to which it is applied. The maximum amount may be determined by experimentation. Generally, the maximum amount will be not greater than 5% by weight, in many cases less than 2% by weight. Where the thickening agent comprises a gum, the amount of gum employed (on a dry weight basis) is preferably in the range 0.05% to 0.5% of the suspension by weight. In ASTM C 618, a pozzolan is defined as a siliceous material or siliceous and aluminous material which, in itself, possesses little or no cementitious value but which will, in finely divided form in the presence of moisture, react chemically with calcium hydroxide at ordinary temperature to form compounds possessing cementitious properties. The reactivity is determined using the known Chapelle test wherein a dilute slurry of the pozzolan is reacted with an excess of calcium hydroxide at a temperature of 95°C for 15 hours. After this period, the unreacted calcium hydroxide is measured by titration.

Preferably the pozzolanic activity of the inorganic particulate material employed in the suspensions of the invention is such that the reactivity of the material with calcium hydroxide is at least 100 mg of calcium hydroxide per gramme, especially at least 400 mg of calcium hydroxide per gramme. Preferably, the inorganic particulate material having a reactivity with calcium hydroxide of at least 700 mg calcium hydroxide per gramme.

Production of metakaolin having the required pozzolanic activity is known, eg as described in the article by Walters, G V and Jones, T R, 2nd International Conference on Durability of Concrete, Canada, ed V H Malhotra, 1991, pp 941-953.

Essentially, to produce a suitable metakaolin, kaolin is calcined for a period of at least a few minutes, eg between a few minutes and up to 10 hours, at a temperature in the temperature range defined earlier, eg in the range 600°C to 950°C, eg 650°C to 850°C.

The particles of the inorganic particulate material employed in the suspension according to the first aspect of the present invention may have the following particle size properties: not less than 70 per cent having a particle size (equivalent spherical diameter or "esd" as measured by sedimentation) less than 10μm; not less than 40 per cent having a particle size less than 5μm; and not less than 20 per cent having a particle size less than 3μm. We have found that aqueous suspensions in accordance with the first aspect of the present invention unexpectedly and beneficially can be fluid, eg have a viscosity of 1000 mPa . s or less, over a reasonable period of time eg 7 days or more, often 10 days or more, eg two weeks or more, and can, as noted above, therefore be flowable and pumpable and can be stirred and/or mixed and conveniently transported and used as a flowable ready mixed additive for addition to cementitious or cement -forming materials and concrete- making compositions and the like. Such suspensions may advantageously be prepared and used without the use of added dispersing agents, although dispersing agents may optionally be employed if desired. Desirably, the kaolin from which the metakaolin employed in the suspension according to the first aspect of the invention is produced is relatively pure, ie has an impurity metallic oxide content (ie other than Al₂0) and Si0₂) of less than 10 weight per cent desirably less than 6 weight per cent with an alkali metal oxide content of less than 3 weight per cent . Such suspensions can be suitable for delivery to and use at a user site without the user encountering serious problems, eg substantial deposit of solid material at the base of the storage tank in which the delivered suspension is contained, the deposit being difficult to rehomogenise, or strong gellation giving difficulty in stirring or pumping.

The aqueous suspension according to the first aspect of the present invention may show the following viscosity properties:

(i) an initial (makedown) viscosity of <500 mPa.s to aid makedown;

(ii) a viscosity increase which is minimal, eg not more than 500 mPa.s, over the first three days after makedown;

(iii) at all ages of up to at least one month after makedown a viscosity of not more than 1500 mPa.s.

Preferably, the viscosity of the suspension is not greater than 1000 mPa . s over a one week period, preferably over a two week period, after makedown where the solids concentration of the suspension is at least 60 per cent by weight and is not greater than 700 mPa . s over a one week period, preferably over a two week period, after makedown where the solids concentration of the suspension is at least 50 per cent by weight.

In this specification, references to viscosity of a suspension or slurry are to be understood as meaning viscosity as measured at ambient temperature (20°C) using a Brookfield Viscometer operating with a spindle speed of 100 rpm.

In the preparation of aqueous suspensions according to the first aspect of the present invention it is preferred that the suspension is made fluid by pH adjustment prior to addition of thickening agent. However, partial or complete pH adjustment may be made during or after addition of thickening agent. A preferred sequence of addition of ingredients to water, eg in an efficient mixer, is pH adjuster followed by metakaolin containing inorganic particulate material and then thickener.

Surprisingly and beneficially, aqueous suspensions in accordance with the first aspect of the present invention provide excellent materials suitable for use in known applications for aqueous suspensions of pozzolanic particulate materials, eg as additives to cementitious compositions or as additives to alkali activators to form cements and cement-containing products. Pozzolanic particulate materials for additives to cementitious compositions or to form cements or cement products are preferred by users in some applications to be in slurry rather than dry form, eg because this offers ease of handling and ease of mixing. Such suspensions in accordance with the first aspect of the present invention can, as noted above, be produced in high solids form, eg having a solids content of 40 weight per cent or more, eg 50 per cent or more, and they can be flowable, pumpable, stirrable and transportable and can show no sedimentation and no or little gellation for at least ten days, even two weeks or more after preparation. Furthermore, they may be stable to excessive temperature variations, eg over the range -50°C to +100°C, and may show no substantial dilatancy of the particles of the inorganic particulate material in the suspension.

The suspensions according to the first aspect of the present invention may be prepared more easily than known silica fume suspensions for use in the same applications. The suspensions according to the invention require thorough but less intensive mixing and show less tendency to produce aggregates or agglomerates than silica fume suspensions. This allows the suspensions according to the first aspect of the present invention to be used over a wider range of concentrations than those of silica fume to give a wider range of potential beneficial effects in cementitious compositions, cements and cement- containing products.

Additives in the various known classes of materials incorporated in aqueous particulate suspensions for use by addition to cementitious compositions or formation of cements or cement- containing products may be incorporated in the suspension according to the first aspect. Such optional additives include, for example, (i) dispersing agents, eg water soluble polymers such as polycarboxylates, eg polyacrylic acids and their salts, lignosulphonate salts and sulphonated melamine or naphthalene formaldehydes and also alkali silicates; (ii) water repellents, eg stearates of calcium, zinc or aluminium or other chemicals with hydrophobic imparting properties;

(iii) other inorganic particulate material having pozzolanic properties, eg fine siliceous materials; (iv) other additives deemed appropriate or necessary for the final product application, eg micaceous materials and other mineral extenders or property modifiers;

(v) colouring agents, eg pigments such as iron oxides; (vi) shrinkage control agents;

(vii) fibres, whiskers, rods, strands, foils, beads and the like to play a part in the cementitious material produced from the suspension, eg as a shrinkage control agent and/or as a tensile strength improver; such additives may be made of materials known for this application, eg fibres may be made of glass and/or thermoplastic materials, (viii) biocides.

Desirably, in the suspension according to the first aspect of the present invention, the metakaolin forms at least 70 per cent by weight, eg at least 85 per cent by weight of the particulate solids present in the suspension.

The suspensions according to the first aspect of the present invention may be employed in the same known applications as silica fume suspensions. Thus, the suspensions according to the first aspect of the present invention may be employed to form settable cementitious compositions in known materials incorporating hydraulic cements, eg for use in the following applications:

(a) concrete (optionally including metal or other reinforcements) - ready mixed;

(b) concrete (optionally including metal or other reinforcements) - preset; (c) paving products - slabs, bricks, pavers;

(d) tiles - eg for roofs or floors;

(e) renders - cement and/or lime based materials;

(f) mortars - jointing, grouting and sealant materials ; (g) sprayed concrete/mortars/render;

(h) flooring systems including self-levelling systems; (i) fibre reinforced systems/composites; (j) oil-well cement (class G and H portland cement) applications . The suspension according to the first aspect of the present invention may also be added to alkaline materials known as alkali activators to form hardenable cementitious compositions or cements. Such alkali activators are known per se in the art, eg for use to form alkali activated slags, and will in general be added in greater amounts than that previously added to establish the pH of the suspension to facilitate flowability etc. as described hereinbefore. For example, the relative amounts by weight of metakaolin to alkali activator present in the cementitious composition formed may be in the range of from 1% to 99% especially 30% to 70%. The following materials are suitable for use as alkali activators for forming cementitious compositions in this way: hydroxides and conjugate base salts of alkali metals especially sodium and potassium hydroxide and sodium and potassium silicate .

The cementitious compositions formed by use of alkali activators may be employed in any of the applications as described hereinbefore (for compositions including an hydraulic cement) although the production of paving products, tiles and the like is likely to be a particularly suitable application for such compositions.

The settable cementitious composition may incorporate other known optional additives, eg: (i) any of the additives specified above for optional addition to the aqueous suspension according to the first aspect of the present invention;

(ii) commercially available chemicals to retard or accelerate setting of cement;

(iii) chemicals to entrain air in cement or composites thereof to impart freeze-thaw resistance; (iv) chemicals such as calcium nitrate to inhibit corrosion of materials employed to reinforce cement composites, eg steel employed to reinforce concrete.

Hardenable cementitious compositions, eg concrete compositions, produced by addition of suspensions according to the first aspect of the present invention to cement or cement forming material, eg alkali activator, and optionally other ingredients, eg sand and aggregate to form concrete, show various advantages over those produced using known silica fume suspensions. The presence of aggregates and agglomerates in the composition can be avoided thereby avoiding the problems due to such aggregates and agglomerates described hereinbefore. The inorganic particulate material of the suspension according to the first aspect of the present invention may be incorporated at higher dose levels, eg up to about 25% on a dry weight basis of the settable cementitious composition to be prepared. Additionally, the pozzolanic activity of metakaolin can be greater than that of silica fume and this allows better bonding into the settable cementitious composition thereby providing improved mechanical (compressive) strength when the composition eventually sets and hardens. Furthermore, the hardened material (compared with materials formed from silica fume suspensions) can have a better capacity to resist chemical degradation. It can have a better capacity to resist ingress of contaminants such as chlorides because it can provide a hardened material having a more limited pore structure and it can also have an improved chemical resistance resulting from a better conversion of chemically reactive calcium hydroxide . According to the present invention in a second aspect there is provided a method of producing a settable cementitious composition which includes addition of a pozzolanic material to an hydraulic cement or an alkali activator in the presence of water wherein the pozzolanic material comprises an aqueous suspension according to the first aspect.

Water may be added to the cement and/or alkali activator prior to and/or during and/or after addition of the said suspension to the hydraulic cement and/or alkali activator.

The settable composition may, depending on the intended use, include further additives, eg ingredients and reinforcements etc. as described hereinbefore to form concrete or reinforced concrete . The preparation and properties of aqueous suspensions embodying the present invention will now be described with reference to the following illustrative Examples which are given by way of example only.

Example 1

A slurry was prepared by addition of a metakaolin to a dilute (lime-free) solution of sodium hydroxide to give a solids content on a dry weight basis of 60 per cent . The metakaolin employed had the following properties :

(i) about 12% by weight of particles having an esd of greater than 10μm; (ii) about 37% by weight of particles having an esd of less than 2μm;

(iii) surface area 14m . g^" ;

(iv) Si0₂ content 52wt%;

(v) Al₂0₃ content 41wt%; (vi) content of other metallic oxides 5.5wt%;

(vii) pozzolanic reactivity (Chapelle Test) 1050mg

Ca(OH)₂ per gramme.

The suspension was thoroughly mixed after which the pH was adjusted to 9.4 (as measured by a pH meter) by further sodium hydroxide addition after which 0.125 per cent (on a dry weight basis) of xanthan gum thickening agent sold under the trade name Kelzan was added. Also 200 parts per million by weight of a isothioazolinone biocide (sold under the trade name MetaSol 150 by Calgon Corporation, Pittsburgh, USA) was added. Further thorough mixing was applied. The resulting slurry SI was found to have the viscosity properties listed in the Table 1 as follows.

Table 1

In addition, no sedimentation of the particulate material contained in the slurry SI took place within a period of 14 days after production (makedown) of the slurry SI. A limited amount of gelling in the slurry SI occurred after 6 days following production but the gelling could easily be reversed by simple stirring.

Example 2 A slurry S2 was produced in a manner similar to that given in Example 1 except that the metakaolin employed had the following properties:

(i) about 4% by weight of particles having an esd greater than lOμm; (ii) about 58% by weight of particles having an esd less than 2μm;

(iii) surface area 15m .g^" ;

(iv) Si0₂ content 55wt%;

(v) Al₂0₃ content 40wt%; (vi) content of other metallic oxides 3.6wt%;

(vii) pozzolanic reactivity (Chapelle test) 1050mg

Ca(OH)₂ per gramme; and the final pH of the slurry S2 was 8.4.

The final slurry S2 produced showed the following properties :

(i) an initial viscosity of 150 mPa . s ;

(ii) no sedimentation in the period of 21 days after production; (iii) limited easily reversible gelling in the period of 6 days to 21 days after production.

Example 3 Samples of the slurry SI prepared in Example 1 were added to standard portland cement whereby 10% by weight and 20% by weight of the solids present respectively in the composition formed consisted of the metakaolin present. Water was added to give a standard consistency as determined by the well known VICAT test (BS EN 196-3:1995) thereby forming cement slurries CI and C2 respectively. Further cement slurries C3 and C4 respectively having the same compositions as CI and C2 were made using dry metakaolin instead of the slurries SI and S2. The initial setting time and the final setting time of each of the cement compositions CI to C4 was measured. The results obtained are given in Table 2 as follows:

Table 2

As is seen in Table 2 the metakaolin form, ie slurry or dry powder form, does not significantly affect the final setting time of the cement composition.

Example 4

Slurry SI produced as in Example 1 and slurry S2 produced as in Example 2 were each separately added together with 0.5% (by dry weight relative to the solids present) biocide as used in Example 1 to standard portland cement and water to give respectively cement compositions C5 and C6. 15 per cent by weight of the cementitious binder present in each case was metakaolin. A similar composition C7 comprising only portland cement (ie no metakaolin included as part of the binder composition) was prepared as a control for comparative purposes. The compositions C5 , C6 and C7 were each allowed to set. The following properties of the resulting hardened materials produced by setting of compositions C5 to C7 were measured in a known way:

(i) compressive strength and density 7 days after production of the settable composition;

(ii) compressive strength and density 28 days after production of the settable composition;

(iii) compaction factor (workability/fluidity) of the still fluid composition.

The results obtained are listed in Table 3 as follows :

Table 3

The results given in Table 3 show that the addition of a slurry of a metakaolin to a composition of portland cement and water improves the compressive strength of the resulting hardened material without significantly affecting its density and compaction factor.

Example 5 A sample C5a of cement composition C5 prepared as in Example 4 was cooled to a temperature of about -5°C and remained at this temperature for a period of 72 hours. The temperature of the sample was then allowed to rise naturally to ambient temperature (20°C) . The compressive strength and density after 7 hours and after 28 hours were measured. The results obtained are shown in Table 4 below.

A further sample C5b of composition C5 prepared as in Example 4 was heated to a temperature of 90 °C for a period of 24 hours. The sample was then allowed to cool slowly to ambient temperature (20°C) . The same properties as measured for sample C5a were measured and the results are shown in Table 4 as follows.

In Table 4, the results obtained for properties of the sample of composition C5 measured as in Example 4 are repeated. This sample, which was not subjected to temperature cycling, is referred to as C5c in Table 4.

Table 4

The results given in Table 4 show that the aqueous suspension of metakaolin shows no undesirable effects after prolonged heating or cooling/freezing.

Example 6

A metakaolin slurry was prepared as in Example 1. A silica fume slurry of known composition having the same solids concentration as the metakaolin slurry was also prepared for comparison. Various cement and concrete compositions were made from the silica fume slurry and separately from the metakaolin slurry (each composition formed from silica fume slurry having a corresponding composition formed from metakaolin slurry) , by adding the slurry in each case to portland cement forming 85 to 95 per cent by weight of the solids present in each case and water forming 50 per cent by dry weight of the solids present and thoroughly mixing the compositions produced.

Photomicrographs of the hardened products in each case were taken. For the compositions produced using the silica fume slurry the photomicrographs showed discrete agglomerates present which were identified as agglomerates of silica fume particles. In contrast, the corresponding compositions produced using the metakaolin slurry showed no distinct agglomerates.

Example 7

Concrete compositions were made using the metakaolin and silica fume slurries as produced in

Example 6, to each concrete composition having the following properties: binder: metakaolin or silica fume (15% weight) plus portland cement; water: 50% by dry weight of binder; aggregate : present with the binder in an aggregate to binder weight ratio of 4.65 to 1.

The compressive strength of the concrete produced in each case was measured after 7 days and after 28 days. The results which were obtained are as shown in

Table 5 as follows. Table 5

As seen in Table 5 superior compressive strength concrete is obtained by use of metakaolin slurry instead of conventionally used silica fume slurry.

Claims

1. An aqueous suspension suitable for use in the production of cementitious compositions which includes, prior to production of a cementitious composition, an aqueous medium and a particulate inorganic material suspended in the aqueous medium, the particulate inorganic material comprising metakaolin and being substantially lime- free and having a pozzolanic activity, the suspension having a pH of at least 7.5, the suspension being in a stable form having a rheology which allows it to be flowable as a wet slurry and wherein the suspension includes a thickening agent which inhibits sedimentation of the inorganic particulate material in the suspension.

2. A suspension as claimed in claim 1 and wherein the pH is in the range 7.5 to 9.5.

3. A suspension as claimed in claim 1 or claim 2 and wherein at least 70 per cent by weight of the inorganic particulate material is metakaolin.

4. A suspension as claimed in claim 1, claim 2 or claim 3 and wherein the metakaolin has been produced from a kaolin having a metallic oxide impurity content of less than 10 weight per cent.

5. A suspension as claimed in any one of the preceding claims and wherein the metakaolin employed in the suspension has a pozzolanic activity of at least 100 mg of calcium hydroxide per gramme.

6. A suspension as claimed in claim 5 and wherein the metakaolin has a pozzolonic activity of at least 400mg of calcium hydroxide per gramme.

7. A suspension as claimed in any one of the preceding claims and which has a solids concentration of at least 40 per cent by weight and not greater than 80 per cent by weight.

8. A suspension as claimed in any one of the preceding claims and which shows no substantial visible sedimentation in the first 14 days after production.

9. A suspension as claimed in claim 8 and wherein the suspension contains up to 5 per cent by weight of the thickening agent .

10. A suspension as claimed in claim 8 and wherein the thickening agent is selected from polysaccharides, alginates, starches, modified celluloses, absorbent clays, and particulate material other than the said inorganic particulate material which provides a thickening effect in the suspension.

11. A suspension as claimed in any one of the preceding claims and which in the first 14 days after production shows no gellation or only reversible gellation which can be converted to a fluid by simple mechanical agitation.

12. A method as claimed in claim 11 and wherein the thickening agent comprises a gum and is present in the suspension in an amount of from 0.05% to 0.5% by weight .

13. A suspension as claimed in any one of the preceding claims and which also contains up to 2000 parts per million by weight of a biocide.

14. A suspension as claimed in any one of the preceding claims and wherein the suspension has a viscosity which is not above lOOOmPa.s for at least one week after production.

15. A method of producing a settable cementitious composition which includes addition of a pozzolanic material to an hydraulic cement or an alkali activator in the presence of water wherein the pozzolanic material comprises an aqueous suspension as claimed in any one of claims 1 to 14.