Porous inorganic granular material eg foamed ceramic material produced from calcined aluminosilicate material is known from various prior art references, eg GB-B-2067174, GB-A-2271987, EP-A- 758,633 and EP-A-764617.

Such material may be employed as a lightweight granular filler material, eg for use in resin compositions, or in inorganic constructional materials such as concrete. The material may not have ideal properties for use in the composition in which it is to be employed. For example, the material may have a surface porosity which absorbs too much water from the surrounding environment of a structured composition.

In certain applications, it is desirable to minimise the ingress of water into the pores of a porous inorganic material. Various methods have been described in the prior art, eg. in GB-A-1482180 and US-A-4873145, for providing a coating which minimises the ingress of water into the pores of the underlying material. However, we have found that for some applications although providing a water barrier coating is necessary, it is desirable for the coated porous material still to be wettable. For example, where the porous material is to be employed as a lightweight filler or property modifier in a matrix comprising a cementitious composition formed (prior

to setting) as an aqeuous composition, it is desirable for the porous material to be wettable and thereby able to form a good bond with the matrix material.

According to the present invention in a first aspect there is provided a ceramic foam material having a water resistant coating thereon provided by a first coating material comprising a hydrophobic polymeric material and, deposited on the first coating material, a second coating material which renders the ceramic foam material coated by the first coating material more water wettable.

The first coating material may comprise a silicon containing hydrophobic compound or composition, eg a silane or siloxane. Alternatively, the compound or composition may comprise a melamine aldehyde, eg melamine formaldehyde.

The second coating material may comprise a latex polymeric material.

We have found that lightweight foamed ceramic materials, especially pellets or prills thereof produced by calcining foamed aluminosilicate material, coated in accordance with the invention can retain their low density because substantial water ingress into the pores thereof is reduced or eliminated but such materials can retain a wettability which is required for good contact with the aqueous medium. For example, the aqueous medium may comprise a cementitious composition, eg for use in concrete and the like, which is to be set and hardened with the foamed ceramic material embedded

therein with a good bonding between the material and the matrix composition.

The hydrophobic first coating material may be applied to the ceramic foam material in a suitable known manner. For example, the hydrophobic material, if for example a silane or siloxane, may be applied as an aerosol spray using the hydrophobic material diluted in a simple carrier liquid. The carrier liquid may comprise an organic solvent, eg acetone.

Alternatively, the hydrophobic first coating material may be applied in the form of an aqueous emulsion using a suitable emulsifier, eg a cationic surfactant, eg a quaternary ammonium salt. Such an emulsion may be applied by spraying to coat the granules. The latex coating may be applied in the manner described earlier to the granules coated with the hydrophobic layer optionally after drying.

Where the first coating material comprises a melamine aldehyde polymer it may be applied in one of the ways described in GB-A-2314324.

The latex polymer coating may be applied by contacting the ceramic foam material already coated with the hydrophobic first coating material with liquid containing a latex polymeric material such as natural rubber, a natural rubber which has been substituted with functional groups, a synthetic rubber, an acrylic copolymer, a poly (vinyl acetate) or a copolymer of vinyl acetate. Poly (vinyl acetate) is especially preferred. The liquid may comprise an aqueous emulsion or colloidal suspension or an organic solution depending on the particular latex

employed. The contacting may be by immersion, spraying or other known processes. The active amount of the latex polymer solid material which is deposited on the surface of the ceramic foam material is preferably in the range of from 1% to 10% by weight, more preferably in the range of from 2% to 5% by weight, based on the weight of the ceramic foam material.

Provision of a latex coating on the surface of a ceramic foam material allows the resultant product to have improved properties, such as reduced surface porosity and/or increased water resistance as illustrated later.

The latex may be applied during or after, preferably after, the application of the first coating material.

Various methods are known in the prior art for the production of foamed ceramic material and the foamed ceramic material to be employed to form the latex coated material according to the present invention may be prepared by one of these various methods. Thus, the foamed ceramic material may be produced by one of the methods described in prior patent specifications GB-A-986,635, GB-B-2,067,174, GB-A-2,271,987, EP-A-758,633 and EP-A-764,617.

The foamed ceramic material may advantageously be prepared by a known method which includes: (a) preparing a foam from a slurry of a particulate ceramic forming material, eg a clay; (b) shaping and optionally breaking or dividing the foam into discrete pellets or prills;

(c) optionally drying the pellets or prills; and (d) calcining the pellets or prills at an elevated temperature to cause sintering of the particles thereof.

The foamed ceramic material employed to form the coated material according to the present invention itself may be produced directly or indirectly from closed cell ceramic foam granules which comprise bubbles or cells. The bubbles are desirably lOOum or less in size, especially 10pm to 60pm in size. Such granules are described for example in Applicants' GB-B-2,0,271,987. The bubbles produced by the method described therein are polyhedral bubbles of varying sizes bounded by thin walls, the walls and junctions between walls generally bounding two or more bubbles.

The material which is employed to form the foamed ceramic material (from which the coated material according to the present invention is obtained) may comprise any one or more of the known minerals and/or synthetic materials from which ceramics may be formed, eg as described later.

The foamed ceramic material employed in the material according to the invention may comprise initially after calcination granules, pellets or prills, eg having a length of from 0.5mm to 20mm, especially lmm to 5mm, and may have a bulk density less than 1000kg. m-3 (lg. cm-3), eg in the range 80kg. m-3 to 700kg. m-3. The solid material of that material will, following calcination, comprise inorganic particles, eg of aluminosilicate, which

have been sintered and fused together. The granules etc may be made finer prior to coating by comminution as described later.

Where the inorganic particulate material employed to produce foamed ceramic material for production of the coated material according to the present invention the material may comprise one or more naturally occurring aluminosilicate compounds.

The compound may be a naturally-occurring mineral, such as a clay mineral, e. g. a mineral of the kandite and/or smectite type. Clay minerals of kandite group, for example kaolinite, dickite, nacrite and halloysite, have been found to be particularly advantageous."Kaolinite"is the main constituent of kaolin type clays, ball clays, fire clays and China clays which may be used to form the ceramic foam material. Such clays occur in nature. The kandite clay mineral may be used in its natural, hydroxylated or hydrous state. Where the aluminosilicate comprises a smectite clay it may comprise for example one or more of bentonite, hectorite and saponite.

The particulate material employed to produce foamed ceramic material will generally be employed as particulate material incorporated in a suitable liquid medium in which a suitable suspension or dispersion can be formed. Suitable liquid media are known in relation to the formation of ceramic materials from the various classes of known material.

In many cases, especially where the particulate material comprises a mineral, a suitable liquid medium comprises water or an aqueous solution. Foam

may be made from the liquid medium by a process involving incorporating a gas in the liquid. The liquid may contain a surface active agent or surfactant to form a stable froth.

Examples of suitable surface active agents include known cationic, anionic, non-ionic and amphoteric surface active agents.

The gas may for example be air incorporated by agitating the liquid medium to form a froth. The gas may be added to the liquid medium before or after the particulate material (and other optional additives) is added thereto.

Conveniently, as described in GB-B-2,067,174, an aqueous foam containing a surface active agent may be formed prior to addition to the ceramic forming particulate material. The aqueous foam may be added to a paste or slurry containing the particulate material. The addition may conveniently be carried out in an extrusion machine from which foamed ceramic material is to be extruded. The machine may be a screw extruder, eg a co-rotating twin screw extruder.

The machine may extrude foamed ceramic material into a plurality of individual elongate portions. The portions may be divided by a divider or by allowing extrudate to fall onto a moving belt which by the action of carrying away the portions causes lengths or portions to break from the extruding material. In any event, the granules, eg pellets or prills so formed may be collected and optionally sized by one or more screen meshes, eg so that only lengths

greater than a chosen minimum length, eg a minimum in the range lmm to 5mm, are selected.

The selected granules are converted into a sintered ceramic form by calcining as described hereinafter.

Before calcining, the granules are preferably pre-dried to avoid possible damage to the granules in the calciner which might result from the rapid evolution of water if the water content of the foamed granules is excessively high. The granules of foam may therefore be dried to a water content of not more than 1% by weight as a separate step, before being introduced into the calciner. Alternatively, the foam granules may be introduced directly into a calciner which has a preliminary drying zone. If the drying is performed as a separate step, the granules are preferably dried to a water content of not more than 0.5% by weight. Drying may be carried out in a heated atmosphere, eg an oven, at a temperature of from 50°C to 200°C.

The foamed ceramic material produced in the manner described may incorporate one or more additive materials added at one or more of the stages of producing such material or after its production. The foamed ceramic material may, for example, incorporate one or more of a fluxing material, for example forming from 5 per cent to 50 per cent by weight of the mixture with the particulate material (mineral and/or synthetic material), the fluxing agent comprising for example mica or feldspar, which subsequently reduces the temperature at which the

material may be calcined, a biocide, eg forming up to 1 per cent by weight of the solids portion of the foamed ceramic material, or an organic or inorganic binder or filler or a combustible material, eg forming up to 30 per cent by weight of the solids portion of the foamed ceramic material.

Calcining may be carried out in a known manner.

The temperature and time of the calcining will depend on the material being calcined and the amount of fluxing agent present but, for example, material comprising clay may be calcined at a temperature typically in the range 800C to 1600C for a period of 5 minutes to 24 hours.

A preferred method of forming the coated ceramic foamed material according to the present invention comprises the following steps: (a) preparing a foam from an aqueous suspension of a particulate ceramic forming material, eg in a mixture with a fluxing agent; (b) forming granules by shaping and optionally dividing or breaking the foam; (c) drying the granules; (d) calcining the granules at an elevated temperature to form a calcined foamed ceramic material; (e) optionally comminuting the calcined granules of foamed ceramic material; and (f) latex coating the calcined granules.

The optional comminution which may be employed to reduce the size of the calcined foamed granules may be performed by a device which exerts a gradual

pressure or controlled squeezing action on the calcined foamed granules. This action causes the foamed ceramic to fracture at its weakest points, which are generally the thin cell walls nearest the outside surfaces of the granule. The device requires an adjustable discharge gap by which the crushing surfaces are spaced apart during the comminution.

Suitable comminuting devices which have such an adjustable discharge gap include roll crushers, cone crushers and gyratory crushers.

The granules or particles produced by comminution may have an average size in the range 50pm to 10OOpm, eg 100pm to 500pm. Granules or particles in a particular preferred size range may be separated by a known size fractionation procedure, eg screening.

Following optional comminution, the particles of the ceramic foam material to be coated to produce material according to the present invention may be separated into size ranges, eg by screening, prior to coating.

As stated earlier, the coated ceramic foam material according to the invention may be prepared to be suitable for use as a lightweight filler for a cementitious, eg constructional material such as concrete, which is to be used in an environment in which some ingress of water into the material is possible. The purpose of the hydrophobic coating of the ceramic foam material in such an application is to provide a barrier which is resistant to the ingress of water into the pores of the foam material.

However, in this form, the coated ceramic foam material needs to be sufficiently wettable to enable the granules of the foam material to be efficiently bonded within the matrix of the cementitious material.

Embodiments of the present invention will now be described by way of example with reference to the following illustrative Examples.

EXAMPLE 1 Granules of ceramic foam granular filler were prepared in the manner described in GB-A-2271987. The granules had sizes in the range of from 3mm to 5mm.

This product was further treated by crushing in a roller crusher and screening the crushed product to provide a fraction consisting predominantly of granules of size in the range of from 250pm to 400pm.

These granules were then coated with an octyltriethoxysilane supplied under the trade designation A137 by Dow Corning. The coating was applied to the granules by spraying a diluted suspension of the silane in acetone. The silane coated granules were subsequently coated with a poly (vinyl alcohol) latex and dried in an oven. The granules were observed to be water resistant but did not have the very hydrophobic nature of the granules after coating with the silane alone. The resultant granules were suitable for use as a lightweight water resistant filler in a cementitious, eg concrete composition.

EXAMPLE 2 Granules for use in concrete were prepared as in Example 1, except that the latex was a styrene- butadiene rubber latex.

EXAMPLE 3 Granules for use in concrete were prepared as in Example 1, except that octylsiloxane was used (instead of octyltriethoxysilane in Example 2).

EXAMPLE 4 Granules for use in concrete were prepared as in Example 1, except that the spraying medium was an aqueous emulsion of octyltriethoxylsilane containing a quaternary ammonium salt as cationic emulsifier.

EXAMPLE 5 Granules for use in concrete were prepared as in Example 4, except that the hydrophobic material was octylsiloxane.

EXAMPLE6 Granules for use in concrete were prepared as in Example 4, except that the latex was a styrene- butadiene rubber latex.

EXAMPLE 7 Granules for use in concrete were prepared as in Example 5, except that the latex was a styrene- butadiene latex.