POROUS CERAMIC SHAPES, COMPOSITIONS FOR THE PREPARATION THEREOF, AND METHOD FOR PRODUCING SAME

Abstract: Disclosed herein is a foamable, self-setting ceramic composition which can be poured into molds or injection-molded or extruded to achieve desired shapes, and thereafter optionally fired to produce shaped, porous ceramic articles useful as filter elements, insulation, kiln furniture, molds, furnace linings, building blocks, or other like articles, the composition containing agents capable of developing an alkali metal aluminosilicate hydrogel and capable of generating gas to produce porosity within the hydrogel bound molded shape.

Description: POROUS CERAMIC SHAPES, COMPOSITIONS FOR THE PREPARATION THEREOF, AND METHOD FOR PRODUCING SAME BACKGROUND OF THE INVENTION The present invention relates to ceramic articles, and more particularly to porous ceramic shapes, made from compositions which can be cast into molds or injection molded or extruded in desired configuration, which shapes find utility for a wide variety of purposes, including filters, kiln furniture, molds, furnace linings, insula¬ tion, and other similar products such as lightweight building blocks. It is known in the art to prepare ' porous ceramic articles for a number of uses. For example, ceramics possessing open-celled porosity can be employed as filters for removing impurities from molten metals or for removing particulates from gaseous streams or other like filtration uses, while ceramics of open- or closed-cell porosity can find use as insulation or for any other purpose where low density structures are advantageous. While the . structural and dimensional properties of ceramics theoretically lend themselves well to utili¬ zation as the materials from which filtering elements, molds, furnace linings, kiln furniture, building blocks and the like can be constructed, their practical use for these purposes has been limited by consideration of cost (i.e., difficulty in economic attainment of desired porosity) and lack of design flexibility (i.e., diffi¬ culty in producing desired, sometimes complex, shapes and various surface treatments or configurations) . It is the primary object of the present invention to provide compositions and processes which enable the economic manufacture of porous ceramic shapes of any desired configuration and any desired type or degree of porosity. SUMMARY OF THE INVENTION According to the present invention, there is described a ceramic composition which is capable of being molded (e.g., cast, injection molded, extruded) into a desired configuration, and which develops stable porosity as a result of internal reactions and interactions between and among deliberately present elements of the composition. Among the significant advantages of the composition is the ability to manipulate its elements and/or the amounts thereof so as to controllably achieve a wide variety of characteristics in the final ceramic article. The " composition is self-setting, again con¬ trollably, and in the set state is then further pro- cessable (e.g., by drying, sintering or firing) to pro- duce a final ceramic article having a porous structure. Porous ceramic articles made from the compositions of the present invention can be arranged to have a predominantly open or a predominantly closed cell network depending upon desired end use. The open-cell configurations are especially useful for the production of filtering ele¬ ments (such as elements for filtering particulate soot suspended in diesel exhaust gases), while both open and closed-cell configurations exhibit utility as insulation, furnace linings, kiln furniture, molds, building blocks and the like. The composition of the present invention has the important characteristic of being moldable into any desired configuration, thereby greatly expanding the potential porous ceramic shapes which are achievable and which may be required for a wide variety of functional and/or aesthetic purposes. Generally, the composition will be poured or cast into a mold of desired configura¬ tion or injection-fed into an appropriate mold. However, it also is possible to directly extrude the composition into particular porous shapes or into gross porous shapes which can then be cut to a particular desired shape. According to the present invention, there is pro¬ vided a composition comprised of an admixture of an aluminosilicate hydrogel and suitable particulate ceramic materials which are chosen depending upon the desired end use, e.g., for articles which will be subject to high temperature processes or thermal cycling, such as exhaust filters, kiln furniture or the like, particulate refractory ceramics such as refractory oxides, carbides, nitrides, borides, suicides and the like, such as alumina, chromia, zirconia, magnesia, titania, silica and mixtures thereof (either as admixtures per se or admixed as part of a compound itself, e.g., mullite, cordierite, calcined kyanite and the like), silicon carbide, silicon nitride, boron carbide, boron nitride, and the like, and, for applications such as building blocks, inorganic ceramic clays, silica sand and other like particulate aggregates. Also included as part of the composition are a particulate metal, a surfactant system and a viscosity- modifying and gel strengthening agent, these latter ingre¬ dients being present in essential yet relatively minor proportions relative to the hydrogel and ceramic compo¬ nents. In addition, inorganic fibers may be included in the composition to attain yet additional strength in the eventual porous ceramic where needed. • The foregoing composition is described with refer¬ ence to generally identifiable constituents of the com¬ position at the time of its molding or extrusion and setting, but the general process of preparing the composi- tion utilizes more fundamental components which, upon admixture, result in the formation of the described hydrogel (i.e., rather than addition of the hydrogel as an identifiable separate ingredient). The hydrogel is formed from a water soluble source of silicate and a water soluble source of aluminate, and the remaining components of the castable composition (e.g., ceramic materials, surfactant, gel strengthening agent, particu¬ late metal, fibers) can be added to or distributed between one or both of the aluminate or silicate composi¬ tions. Upon admixture of these two separately prepared and maintained aqueous compositions or slurries, there is formed an aluminosilicate hydrogel which serves to bind together all components of the composition. The hydrogel binder is self-setting at ambient conditions and is capable of setting and binding the composition to a generally self-supporting structure within a brief but controllable time. According to the present invention, the component parts of t the ceramic composition are admixed to. form a moldable composition, generally adding the silicate slurry to the aluminate slurry. Mixing is done in this fashion because the aluminate slurry acts to retard the gelation time while the silicate slurry acts as an acce¬ lerator. Therefore, if the aluminate slurry is added to the silicate slurry, the possibility exists for a rapid, but partial gelation to occur which would, in effect, result in an incompletely set mix. Before any substan¬ tial self-setting of the composition occurs, it is cast or molded or .extruded to the desired shape, taking into account, of course, that the composition is intended to foam and expand to attain the final desired configura¬ tion. During the self-setting reaction, additional reaction takes place within the composition in which the particulate metal reacts with alkali (e.g. sodium) com¬ pounds in the composition to produce, inter alia, hydro- gen gas. By arranging the self-setting hydrogel reaction to be of suitable duration, the composition increases in volume as a consequence of the internal gas generation and takes on a porous nature as the gas evolves within and from the composition. Then, as close as possible to the cessation of gas evolution, the in-situ hydrogel formation causes the composition to set in the desired porous molded configuration. Where the intended ultimate use of the porous ceramic article is in high temperature environments and/or environments involving substantial temperature cycling, it generally is necessary to further treat the self-supporting, molded structure to substantially remove therefrom or therein compounds which would inhibit the thermal shock resistance of the ceramic article, e.g., compounds which lead to formation of glassy phases (e.g., compounds of alkali metals or phosphorus or boron, etc. which might be present as a consequence of the need to employ water-soluble forms of the silicates and alumi- nates used to form the hydrogel or by virtue of other added components) . Such treatment can include removal of such compounds per se and/or alteration of such compounds by ionic exchange. Thereafter, the structure is dried to remove any water therefrom, and is then fired at suitable conditions to result in the ultimately-desired shaped porous ceramic article. Also according to the invention, processes are pro¬ vided ' for arranging for a porous ceramic membrane layer to be present on one or more surfaces of the article. This porous ceramic membrane layer can be utilized, for example, to provide on selected surfaces of an article such as kiln furniture a smooth skin, or for providing on outlet surfaces of a filter element a thin porous layer of smaller average pore size than exists throughout the remainder of the filter so as to act as a final filter for increasing the overall filtering ability of the element. x. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a process flow chart illustrating the generalized steps of the process of the'present invention for arriving at shaped porous ceramic articles, including certain preferred process and compositional aspects. FIG. 2 is a graphical depiction of the effect of a particular viscosity-modifying and gel strengthening agent, silica fume, on the viscosity of the foamable composition. FIG. 3 is a graphical depiction of the effect of silica fume content on the green modulus of rupture (MOR) strength of porous shapes cast according to the inven¬ tion. FIG. 4 is a graphical depiction of the effect of aluminum metal content on the pore size of shapes made according to the present invention. FIG.. 5 is a graphical depiction of the effect of aluminum metal content on the density of porous shapes made according to the invention. FIG. 6. is a graphical depiction of the rate of removal of sodium from a porous ceramic shape during water rinsing in accordance with the invention. FIG. 7 is a graphical depiction of the rate of sodium removal from a porous ceramic shape utilizing a combined- rinsing/ion exchange cycle according to the invention. FIG. 8 is a schematic sectional view of an appara¬ tus for effecting rinsing and ion exchange in a porous ceramic shape according to the present invention. FIG. 9 is a graphic depiction of the rate of sodium removal from porous ceramic shapes utilizing the appara¬ tus of FIG. 8. FIG. 10 is a graphical depiction of the effect of solids content on viscosity for a number of aggregate materials. FIG. 11 is a graphic depiction of the effect of silica fume on viscosity for one of the aggregates shown in FIG. 10. DETAILED DESCRIPTION OF THE INVENTION The present invention provides a moldable ceramic composition, and thus enables the preparation of final ceramics of the shapes and sizes most suitable for any particular end use and for optimized geometric configura¬ tion in such end use. Moreover, the nature of the compo- sition permits the eventual ceramic to retain its as- molded condition yet possess porosity making it suitable for a wide variety of end uses requiring such porosity, such as particulate filters, kiln furniture, lightweight building blocks, and the like. In this regard, the com- position and process of the invention offer substantial advantages in ease of processing and economics as com- pared to other known methods for preparing porous cera¬ mics of some defined shape, such as the impregnation of foamed polyurethane or other resinous structures with thixotropic ceramic compositions followed by firing to decompose the carrier foam (see, e.g., British Patent Specification No. 1,596,446 and U.S. Patent Nos. 3,833,386 and 4,332,753) or other methods known in the art (see, e.g., U.S. Patent Nos. 3,419,495; 3,466,221; and 3,450,547). In the description provided hereinafter, details are given first as to the foamable composition per se, thereafter as to certain processing features, applicable to some or all of the potential end uses, such as alkali metal removal or formation of a porous ceramic membrane layer, and thereafter details as to particular end uses. Foamable Ceramic Composition The aluminosilicate hydrogel portion of the composi¬ tion of the present invention is, in essential respects, as described in commonly-assigned U.S. Patent Nos. 4,357,165 and 4,432,798, both of which are expressly incorporated herein by reference. As described in those patents, the hydrogel results from the admixture of water soluble sources of both silicate and aluminate (typical¬ ly, sodium silicate and sodium aluminate), which admix- ture then self-sets at ambient temperatures in times which can be exceedingly short (e.g., on the order of as little as a few seconds but typically on the order of a few minutes) , but nevertheless can be controlled by pre¬ determined choice of molar ratio between aluminate and silicate, concentration of water, and temperature. The ability to exercise control over setting times for the hydrogel binder leads to important advantages with respect to attainment in the present invention of molded ceramic articles of both desired geometry and desired porosity. Also described in the above-noted patents is the utilization of the hydrogel components along with granular refractory particles to produce, e.g., molds, by virtue of the self-setting hydrogel serving to bind the granular materials into a self-supporting structure. According to the present invention, the separately prepared and admixed components for forming the alumino¬ silicate hydrogel have added to them and/or distributed between them the remainder of the components which will make up the moldable foamable ceramic composition and the eventual porous ceramic shaped article. As earlier noted, the essential elements of this composition, be¬ sides the hydrogel-forming constituents, are particulate ceramic materials, particulate metal powder, a viscosity- modifying and gel strengthening agent such as silica fume, and a surfactant component, with ceramic fibers or other conventional materials optional. The particulate ceramic materials generally will be present in the over¬ all composition in a weight percentage of from about 50% to about 90%, preferably from about 60% to about 70%. In a preferred embodiment of the invention for end uses requiring refractoriness, the ceramic materials included in the composition will be chosen from cordierite, cal¬ cined kyanite and mixtures thereof, with most preferred compositions containing nearly equal weight proportions of both cordierite and calcined kyanite, e.g., from about 30 to 35% of each ceramic. According to the invention, the requisite porosity in the final ceramic article is provided as a consequence of in situ reaction between metal powder and alkali com¬ pounds (e.g., sodium hydroxide) present in the moldable composition, . resulting in development of hydrogen gas as a reaction by-product. As a consequence of this internal gas production and evolution, the composition will expand in volume in the mold (or during extrusion as the case may be) and develop porosity, the quantity of the composi- tion obviously being regulated to take into account the expected (and predetermined) degree of expansion within the mold or during extrusion to arrive at the desired final density and size of the article. At the same time, the surfactant present in the composition serves to break up the bubbles of evolving gas in the aqueous composition to achieve, controllably, suitably small bubbles and to assure that the porosity developed in the structure will be predominantly of the open-celled type, e.g., as required for filtration uses, or predominantly of the closed-cell type as useful, e.g., for applications where lower thermal conductivity, higher strength, buoyancy or the like is the ultimate criterion. The preferred particulate metal is aluminum, although other metals or metal alloys such as silicon or ferrosilicon which similarly will react with alkali com¬ pounds present in the composition to produce hydrogen gas also can be employed. For most generalized compositions, the amount of surfactant and metal (e.g. aluminum) powder will be rela¬ tively small compared to the other components of the system, with the typical levels of addition of the surfac- tant being in the range of from about 0.05 to 1.0 percent by weight of the total composition and the metal powder being in the range of from about 0.05 to 0.5 percent by weight of the total composition. Preferred ranges of addition for these materials are 0.4 to 0.8 percent by weight for the surfactant (most preferably about 0.6%) and 0.1 to 0.2 percent by weight for the metal powder (most preferably about 0.15%), and a preferred ratio between the surfactant and metal powder is generally from about 2:1 to 8:1, most preferably about 4:1. Among the preferred class of surfactants (which may be used alone or in combination) for use in the invention are the silicone glycols such as are available from the Dow Chemical Company for use, e.g., in producing poly- urethane foams. These surfactants have a stabilizing effect on the gaseous by-products produced and are avail¬ able in a variety of customized formulations (based upon the silicone glycol chemistry) that are designed to con¬ trol bubble (or cell) size as well as to control or dic¬ tate whether the cells are mostly open or mostly closed. For example, the surfactants from Dow Chemical known by the tradenames DC 190, DC 198, Q2 5160 and Q2 5125, pro¬ vide a mostly open cell structure in the present inven¬ tion, ' while other surfactants from Dow Chemical, such as the tradename products DC 193, DC 197, DC 5103 and DC 5098, provide a mostly closed cell structure in the pre¬ sent invention. In addition, still other Dow Chemical silicone glycol surfactants are available to further customize a foamed cell structure with a controlled or limited amount of cells opened, such as the tradename products Q2 5243, DC 5043, Q2 5169, X2 5256, X2 5258 and Q2 5244. Although the silicone glycol type surfactants are preferred, a variety of other non-silicone surfactant types also may be employed, such as those available from Air Products & Chemicals, Inc. under tradename LK-221 and LK-443. * 5 With respect to the aluminum or other particulate metal, the average particle size of the powder employed generally will be in the range of from about 1 to 44/mτι, and preferably about 6-9nrι, with the understanding that the larger the surface area of the metal present in the 10 composition, the more vigorous and extensive will be the foaming reaction. Another essential ingredient of the composition of the invention is a viscosity-modifying and gel strengthen¬ ing agent, preferably silica fume, although other suit- 15 able agents may be employed. Silica fume is a by-product collected in the airstream during the reduction of silica sand by coal or coke in an arc furnace to make metal- lurical-grade silicon metal. The particulates are .hollow spheres, roughly 0.25 micron in diameter, composed of 20 about 96% silica and having a light carbonaceous layer on their surface. Although the mechanism by which silica fume operates in the compositions of the invention is not entirely understood, its addition brings about a number of advantages, such as lowering the viscosity of the 25 composition -for a given solids content (thus permitting higher solids loading) and reinforcing the gel network (without increasing viscosity) to give greater green strength. Without the presence of the silica fume, the hydrogel bonded aggregate structure appears more prone to 30 cracking during drying operations. By reinforcing the gel structure, the silica fume reduces shrinkage as the molded article is dried. Generally, it has been found that the silica fume is effective at levels of from about 0.25 to about 10 percent by weight of the total composi- 35 tion, preferably from about 1 to 4 percent by weight, and most-preferably from about 1 to 2% by weight. As noted, viscosity-modifying and gel strengthening agents other than silica fume can be employed, such as fly ash, manganese oxide fume, ferrosilicon fume and the like. Based upon experimentation to date, the chief characteristic required to be possessed by the gel strengthening agent is the small, spherical shape enab¬ ling it to react readily with the matrix binder and/or aggregate constituents. As earlier noted, the moldable ceramic composition may advantageously further comprise ceramic fibers, such as Kaowool™, Fiberfax™ and Fiberkal™. type aluminosilicate fibers, Saffil™ alumina fibers, sili¬ con carbide whiskers and calcium silicate fibers, to give further rigidity to the molded structure. Typically, these fibers can be present in an amount up to as much as about 60 percent by weight of the composition, but most typically are, employed in amounts from about 1 to 4% by weight. In the present invention, the components of the foamable ceramic composition are selected to yield a particular setting time (e.g., by variation in aluminate/silicate ratio and/or solids content, and taking into account the temperature at which the composi¬ tion will be molded) , consistent with the anticipated duration of the foaming process in the mold or during extrusion. As noted earlier, a distinct advantage of the invention is that the setting time can be arranged to achieve a particular dimensionally stable degree of. gela¬ tion at or very near the time when the gassing reaction ceases, thus insuring retention of the developed porosity in the molded and eventually fired article. If gelation occurs too soon, the composition lacks the freedom to develop and accommodate the desired degree of porosity and/or may result in cracking of the set structure as gas continues to be evolved, while if gelation is delayed too long, the developed porosity will have a tendency to break down before the structure can be firmed up. While this latter problem might be curable by excess utiliza¬ tion of surfactant and/or aluminum, cure in this way may introduce into the article too substantial amounts of components making control more difficult and which may adversely affect final product characteristics. The effect of particular compositional aspects upon certain product characteristics is graphically depicted in FIGS. 2 through 5. Thus, as shown in FIG. 2, the presence of silica fume in the composition results in substantial reduction of the viscosity of the composi¬ tion, the measured reduction being greater at higher spindle speeds on the measuring device and also greater with increasing amount of silica fume. As shown in FIG. 3, the green strength (as measured by the modulus of rupture or MOR) of the molded shapes increases, generally linearly within the region shown, with increasing silica fume content. FIGS. 4 and 5 graphically depict the interrelation- ship among the amount of aluminum powder and amount of surfactant upon the pore size and density of the porous shapes. Thus, increase in the amount of surfactant or increase in available surface area of aluminum (increase in amount or also, e.g., by using either a flaked alumi- num or smaller grain size) increases the number of pores per linear inch in the molded product. As shown in FIG. 5, increase in slurry temperature or other means to decrease set time results in an increase in density of the molded product, while a decrease in the available surface area of aluminum powder also increases the density. As noted, the foamable composition can be cast or injection-molded into a mold of desired configuration, in which the foaming/expansion of the composition and the aluminosilicate hydrogel formation then take place to form a self-supportable shape in the configuration of the mold. Alternatively, the foamable composition can be extruded, with foaming/expansion and hydrogel formation occurring during the extrusion process, into a foamed self-supportable shape which, if necessary, can then be cut into, the desired configuration. Depending upon end use, the shaped porous ceramic article can be dried for use as is or, alternatively (and essentially for most applications) will be dried and then sintered or fired. Alkali Metal Removal Particularly for those end uses where elevated tem¬ perature and/or wide-ranging temperature cycling will be encountered (e.g., diesel exhaust filters, kiln furni¬ ture, furnace linings) , firing is required and it general¬ ly is necessary to treat the porous molded article prior to firing to substantially remove or eliminate alkali or other fluxing and/or glass-forming ingredients therefrom or therein in order to increase the refractoriness of the eventual fired porous ceramic article. Typically this will be achieved by means of a leaching and/or ion- exchange treatment, and preferably is accomplished by a first water leach followed by ion exchange. In the preferred embodiment of this invention, the dealkalizing of the porous shaped ceramic is effected by treatment of the porous shape with water to remove leach- able alkali compounds, followed by treatment with ' a dilute aqueous solution of a non-alkali metal salt to effect exchange of the cation thereof for alkali metal cation in the porous article. Thereafter, the porous article can be further rinsed to insure that any residual anions (e.g., chlorides) of the non-alkali metal salt are removed. In the most preferred embodiment of the invention, the non-alkali metal salt is an ammonium salt. The prior art contains reference to a number of techniques for dealkalizing of silicate compositions. although not strictly in the context of preparing refrac¬ tory ceramic articles. For example, in a paper entitled "Durable Glass By Reconstitution Of Hydratable Sodium Silicate Glasses," published by the American Chemical Society in 1982, Bartholomew, et al. discuss a procedure utilizing either sodium or ammonium nitrate solutions to reduce the alkali content of 12-21% Na 2 0-containing hydrated sodium silicate glasses by 50% over a several hour treatment period, as well as a more efficient pro- cess utilizing a 0.6 molar solution of ammonium nitrate at 80°C to reduce the alkali content of a 2mm thick hydrated sodium silicate glass, containing 21% sodium oxide, to about 100 ppm sodium in a 24-hour period. In U.S. Patent No. 4,340,642 to Netting, there is disclosed a process wherein aluminum sulfate is used to enrich the surface layer of sodium silicate-derived spherical particles. In U.S. Patent . No. 3,653,864 to Rothermal, alkali metal oxide-containing glasses are dealkalized by first hydrating the alkali to a hydroxide, preferably by auto- claving, and then by contacting the glass surface with a non-acidic, non-aqueous • solvent having a high dielectric constant, such as formamide or dimethyl formamide. With reference to the particular aluminosilicate- based foamable ceramic composition previously described, and by way of further explanation, it should be under¬ stood that, as formed, for example from sodium aluminate and sodium silicate, the sodium aluminosilicate bond con¬ tains an excess of sodium ions (as much as 55%) over the assumed stoichiometric chemical structure (one mole of sodium oxide to one mole of aluminum oxide and two moles of silicon oxide, i.e., Na2θ:Al2θ3:2Siθ2:H2θ) . To remove a sufficient amount of this excess sodium in order to improve the refractoriness of the fired article, a three step process was developed which in¬ cludes: 1) water leaching/rinsing to remove about 55 percent of the sodium present as free sodium hydroxide; 2) an ion exchange process utilizing an ammonium salt solution to remove additional sodium linked to the hydro¬ gel network; and 3) an additional, although optional, water rinse to remove residual anion content. Two techni¬ ques were evaluated following the above procedure. The first method simply used a closed system in which the liquids in each of the above three steps were recycled in a constant volume container, changing the entire solution of one step before going on to the next, thereby allowing the removal of sodium via a diffusion or migration from the molded porous part to the recirculating, i.e., flow¬ ing, solution. The second method used a specially design¬ ed apparatus, as shown in FIG. 8, in which the water and ion exchange solutions were forced through the molded part with no recirculation. The advantage of the second procedure is in the time required for effecting sodium removal. The .constant volume system was used initially to evaluate the effectiveness of the various sodium removal solutions involving just water, acids (such as hydro¬ chloric and acetic, etc.), other soluble salts (such as aluminum chloride, aluminum sulfate, magnesium chloride, etc.) and ammonium salts. Table I summarizes these ini- tial experiments that were obtained for a porous ceramic plate component with an initial 5 percent sodium oxide content at 0.9 g/cc foamed wet density. As can be seen from the .data (see FIG. 7), rinsing/leaching in deionized water for 72 hours without changing the water reduces the sodium oxide content down from the initial 5% level to 2.3-2.4%, i.e., approximately 50%. At this point, the solution was changed to the particular water, acid or salt solution being evaluated to further remove the sodium. Based on the results tabulated in Table I, it was determined that ammonium chloride salt solution is the most effective material evaluated in removing sodium either by direct dissolution or ion exchange. TABLE I Summary of Ion Exchange Experiments in a Constant Volume Bath to Remove Sodium from Sodium Aluminosilicate Hydrogel Bonded Porous Plates FIG. 6 illustrates the sodium removal rate during the water leach/rinse cycle, showing that after about 30 hours, the rate of sodium removal approaches zero, i.e., no further sodium removal occurs. Similarly, in FIG. 7, it can be seen that when the water leach solution is then followed by an ammonium chloride solution, a rapid in¬ crease in the rate of sodium removal is achieved, follow¬ ed by a decrease in rate after about 30 hours. It also should be noted that heating the ion exchange solution significantly reduces the time required for essentially complete sodium removal as compared to use with a room temperature solution. A series of further experiments was also performed to determine the optimum amount of ammonium chloride and water in these constant volume tanks with respect to the total amount of sodium being removed. In the work referred to above, 2.6 pounds of ammonium chloride were used to remove (or more precisely ionically exchange NH 4 + for Na + ) one pound of sodium oxide. This amount was determined by balancing the following equa¬ tion: - Na2θ+3NH4Cl+xH2θ-_>3NH 4 OH+2NaCl+HCl+(x-2)H2θ To determine whether this 2.6:1 ratio was optimum, experiments were performed comparing a 1.3:1 and 5.2:1 ammonium chloride to sodium oxide ratio to the 2.6:1 ratio. While the lower ratio system displayed slower exchange rates, the higher ratio only provided slightly greater exchange rates and, therefore, was not considered to provide substantial benefit commensurate with the added cost. In a similar fashion, the amount of water or sodium oxide dilution level was also varied from 75:1 (water to sodium oxide) to 300:1. As before, the lower level was not quite as effective as a 150:1 ratio employed in the work referred to * above, with little improvement seen by going to the 300:1 ratio. The flow through design using a type of apparatus as that illustrated in FIG. 8 has been found to be con¬ siderably faster on sodium removal. In this device, shown in open section, a liquid-tight housing 10 having a perforated bottom portion 15, has arranged within it (so as to rest on bottom portion 15) a porous support 20 (e.g., urethane) on which is placed the ceramic foam element 30 to be leached.. A rubber seal 40 is arranged to insure that all liquid flow passes through the foam element 30. Table II summarizes the results obtained using this design. The procedure in this case is to pour a given amount of pure water (again, deionized preferred) through the porous cast component until about 55% of the original sodium content is removed i.e., down to the stoichiometric hydrogel composition. After the 45% remaining sodium level is achieved, another quantity of a 1% NH 4 C1 solution is poured through the same part until essentially a 100% sodium removal level is obtained. After this ionic exchange step, pure water is again poured through the foam to remove any residual chloride ions. At this point, the sodium content in the part should be less than about 200 ppm as shown in Table II and schematically represented in FIG. 9. The entries in Table II represent sequential steps and the times set forth in Table II are for each particular step, i.e., total time is the total for all sequential steps. TABLE II Effect of Leaching (DI Water) and Ion Exchange (1% NH4CI Solution) on Residual Sodium ' Oxide Content in Final Fired Porous Ceramic Plate (1) Based on 10 inch porous plate at 0.86 g/cc wet density (846 g wet weight) with essentially 37.7 g a2θ initial content. (2) Based on expected final part weight of 621 g after firing at 2425°F. It will be noted that this aspect of the present inven¬ tion also has applicability to treatment of any unfired supportable porous ceramic shape containing excess and/or ionically exchangeable alkali irrespective of the fundamen- tal composition of the shape or the process used to produce the supportable porous shape. The invention obviously has particular applicability to treatment of supportable porous shapes made ' by molding or extrusion of foamable ceramic compositions which rely upon an aluminosilicate hydrogel as the binder for the composition or shape as described herein, this particular applicability being by reason of the fact that such compositions generally make deliberate and substan¬ tial use of alkali metal (e.g., sodium) aluminates and sili¬ cates as percursors for forming the ultimate aluminosilicate bond and, hence, exhibit limited refractoriness and thermal shock properties in the fired state by reason of the signifi¬ cant level of alkali. However, the invention has applica¬ bility beyond this particular system, as for example in clay- bonded aggregate systems in which alkali metal is present in the clays used to form the adhesive bond (e.g., kaolin, bentonite, etc.), or any other system where the presence of alkali resulting from use of particular components would otherwise lead to limited refractory or thermal shock proper¬ ties in the fired porous article. In providing a means for safely, efficiently and effectively removing alkali from porous ceramic shapes and thereby produce fired porous ceramic. articles of low alkali content, this aspect of the present invention offers the significant advantage of permitting one to make liberal use of alkali compounds in forming the unfired shape without concern for the ultimate effect of the alkali on the refrac¬ tory or thermal shock properties of the fired ceramic. In this way, greater flexibility is provided to the compounder as well as opportunities for improving economics by liberal use of inexpensive starting materials. Following this removal of alkali or other fluxing or glass-forming ingredients, the molded article is dried to remove any water therefrom and is then fired in any suitable furnace at the temperatures required (e.g., 2200° F to 2600°F) to form the shaped porous ceramic article. Depending upon the composition of the moldable ceramic composition and the processing conditions, sin¬ tered ceramic refractory articles can be prepared having a broad range of porosity, surface area and the like. Formation Of Porous Ceramic Membrane Layer According to another aspect of the present inven¬ tion, particularly suitable for production of filters, kiln furniture and the like, provision is made for a thin porous ceramic membrane layer on one or more surfaces of the molded porous ceramic article. By "thin" is meant a layer which is thin relative to the body portion and, in most embodiments, thin per " se as in the nature of a skin. By "porous" is meant that the layer contains pores, which may be connected or unconnected depending upon particular applications, and which may be in the form of generally spherical voids or cylindrical channels or the like. By "membrane" is meant to connote a thin layer and is not necessarily an indication that the layer is permeable, although in certain applications it will be. According to one. application of this aspect of the invention, there is described a porous ceramic article for use in particular as a trap for diesel exhaust parti- culates, comprised of a porous ceramic body portion hav¬ ing pores whose average diameter is designed to assure relatively high permeability throughout the body. Inte¬ gral with the body, at the exhaust discharge surfaces thereof, there is provided a thin porous ceramic membrane layer having pores whose average size is less than that of the pores within the body portion. It has been found that by proceeding in this manner a larger amount of particulates can be trapped since the smaller particu- lates that could normally pass all the way through the filter are trapped just below the membrane layer surface, causing more particulates to be trapped in the larger pore cavities on the inlet side of the filter, i.e., in the body portion of the porous ceramic. Because the region or length of the membrane surface is short as com¬ pared to the body portion, it is found that no signifi¬ cant increase in clean back pressure is encountered while trapping efficiency for particulates is greatly enhanced. More generally, this aspect of the invention has applicability to any use of porous ceramic shapes wherein it is desired that one or more surfaces thereof possess a relatively smooth layer or skin as can be provided accord- ing to the invention, irrespective of whether the body portion of the ceramic is of the open- or closed-cell type. Notable in this regard is the provision of smooth- surfaced kiln furniture and molds according to the inven¬ tion. In its broadest aspects, then, this aspect of the present invention provides a porous ceramic article hav¬ ing a body portion of any desired shape and configura¬ tion, the body portion having associated with it either open- or closed-cell porosity characterized by pores of generally predetermined average diameter. On one or more predetermined surfaces or external areas of the body por¬ tion there is provided, integral with the body portion at that area, a thin porous ceramic membrane layer or skin having pores whose average diameter is less than that of the pores within the body portion and on any remaining surfaces or areas of the body portion. This aspect of the present invention provides a variety of techniques for efficient and economical forma¬ tion of this thin porous ceramic membrane layer on select- ed surfaces of a porous ceramic shape. All of the techni¬ ques are applicable to use with the foamable ceramic composition which is the primary subject of this inven¬ tion, while some are uniquely suited for such use and others are suitable for use with other systems for making porous ceramic shapes. With reference to the particular porous shapes produced by the foamable compositions of the present invention, the following techniques can be employed. In one method, which is also applicable to porous ceramic body portions made by any number of processes, such as by extrusion of compositions containing fugitive components or by processes employing polyurethane sub¬ structures or by processes utilizing foamable ceramic casting or injection-moldable or extrudable compositions, the body portion, after formation but prior to firing, is treated by applying to one or more surfaces or areas thereof a ceramic paste or slurry containing a fugitive constituent capable of leaving a small pore when removed during a drying or firing operation. The fugitive constituent can be a sublimable' compound or a burnable (e.g., carbonaceous) compound, utilized in a size and an amount which will, result in pores having an average diameter smaller than that of the pores which will be present in the body portion ■ or at untreated surfaces. During the firing operation, the ceramic paste or slurry becomes integrally associated with (fused to) the porous body portion. Where the formation of porosity within the body portion is itself dependent upon burn-out or sublima¬ tion of a fugitive constituent, the firing operation will, thus, simultaneously develop a porous body portion and an integral porous ceramic membrane layer on the treated areas. In another method, applicable to all processes wherein the body portion is prepared from a foamable, castable or injection-moldable ceramic composition, surfaces of the mold corresponding to the areas on the body portion where the ceramic membrane is desired to be formed, are treated by application thereto (generally onto the mold release agents already present) of a mix¬ ture of ceramic powder and fugitive constituent. The castable or moldable composition is then poured or inject- ed into the mold and, after setting and removal from the mold, is found to have associated with it at the areas corresponding to the pre-treated mold surfaces, a thin skin of ceramic material which is rendered porous during the firing step. In this embodiment, it is also possible to eliminate use of fugitive constituents by choosing for the ceramic powder ingredients which are more refractory than those of the underlying body portion, such that during firing, the greater refractoriness of these grains prohibits sintering thereby leaving a partially-sintered, i.e., porous, membrane layer on the preselected areas of the body portion. Among the preferred methods are those specifically applicable to processes wherein the underlying porous ceramic body portion is formed through use of a foamable, castable or injection-moldable ceramic composition which can be poured or injected into a mold and set therein to assume the mold configuration while at the same time developing porosity of the closed- or open-cell type by virtue of reactions and interactions between and among components of the composition, e.g., involving release of a gaseous constituent, such as the composition which is the primary subject of this invention. One method of forming a porous ceramic membrane layer on preselected surfaces of such body portions involves the application of a ceramic paper (either woven, air-laid, or the like) atop the release agent on one or more predetermined mold surfaces prior to molding the ceramic composition. In this manner, the composition, during foaming, expands into the ceramic paper, thereby laminating or bonding the systems together. On firing, there is developed a porous body portion having on one or more of its surfaces a thin porous ceramic membrane layer by reason of the now integrally-bonded ceramic paper whose pores are on the average smaller than those of the underlying " body portion. In the most preferred methods, for use with foam¬ able, moldable ceramic compositions, formation of a porous ceramic membrane layer is accomplished integral with the formation of the underlying porous body. In situ processing in this manner offers significant advantage in the economics of manufacture of the final ceramic article. According to one of these preferred methods, the release agent used in the mold, at those preselected areas corresponding to where on the body portion the ceramic membrane is desired, consists of or contains a defoaming surfactant (i.e., a foam suppressor). During the internal development of porosity in the cast or injected composition by virtue of gas-generating reac¬ tions therein, the defoaming agent acts to sufficiently suppress the reaction to keep the pores at these surfaces controllably small, i.e., smaller than those within the body portion and at surfaces not in contact with the foam suppressor. Since the surfactant is per se a release agent or is associated with a release agent, no problems are encountered in- demolding the part. The choice of defoaming surfactant is dependent upon the chemistry of the fundamental foaming reaction. Thus, in the case of air entrapment type foaming (i.e., in situ gas generation and surfactants to size and stabilize the bubbles so formed) , commonly used surfactants for the defoaming of detergents, paints, varnishes and the like are eminently suitable. In the specific context of the foamable ceramic compositions which are the primary subject of this inven- tion, a number of uniquely applicable techniques have been developed for in situ formation of a porous ceramic membrane layer or skin on preselected surfaces or areas of the porous ceramic body.portion. According to one such method, there is used, as the release agent per se or along with a release agent, a 5 foam suppressing agent consisting of an organic compound having an unhindered hydroxyl group (i.e., an OH- "tail"), such as common alcohols, polyethylene glycol, polyvinyl alcohol, and the like. By provision of such agents on mold surfaces corresponding to those areas of 10 the body portion where the porous ceramic membrane layer is desired, the hydroxyl group apparently absorbs the outgassing hydrogen molecules at these surfaces, thereby restricting their growth. A porous ceramic membrane is attained by virtue of the underlying foaming reaction and i5 the fact that hydrogen gas bubbles at the desired sur¬ faces are kept small. In another method applicable to this alumino¬ silicate system, mold surfaces ' corresponding to those where a porous ceramic membrane is desired to be formed 20 are provided with a gel accelerating agent, preferably along with a release agent, and most preferably along with a release agent consisting of or containing an OH- tail as above described. The gel accelerating agent serves to locally set the aluminosilicate hydrogel prior '.-5 to reaction between the particulate metal and alkali com¬ pounds in the composition, with the result that in the fired article, the pores at these treated surfaces are smaller than those in the underlying body portion or at other surfaces. 30 As earlier noted, a characteristic of the alumino¬ silicate hydrogel system is that gelation can be accele¬ rated by variation in concentration of soluble silicate and soluble aluminate. The nature of the system is such that more rapid gel formation will occur both at condi- 5 tions of lower soluble silicate concentration (i.e., by dilution of the system) as well as at conditions of high concentrations of soluble silicate and aluminate (e.g., by minimizing water in the system). Thus, additional methods to achieve localized rapid gelation of the aluminosilicate system at surfaces where a porous ceramic membrane is desired include incorporation of water along with the release agent at the desired mold surfaces, the water being in an amount such that the combined, but not yet set, silicate and aluminate mixture absorbs a suffi¬ cient portion of this water to locally dilute the origi- nal amounts of soluble silicate and soluble aluminate, thereby locally reducing the gel time at these surfaces as compared to that occurring throughout the remainder of the composition. In another method, it can be arranged that water is iocally removed from surfaces where a porous ceramic membrane is desired so as to bring about more rapid gelation of the aluminosilicate system at those areas (by virtue of increased silicate and alumi¬ nate solids content). This can be achieved, for example, by treating the corresponding mold surfaces with a hydro- scopic release agent (or a release agent composition containing a hydroscopic agent) or by arranging a layer of dry paper at the required mold surface or by localized heating of the required mold surface. Another method applicable to the aluminosilicate hydrogel system as well as with other catalyzed silicate systems is to bring about a change in pH on the surface where the porous ceramic membrane is required. For example, -incorporation of an acidic component in the release agent such as acetic acid or dilute hydrochloric acid will locally accelerate the gelation prior to the onset of foaming. It should be pointed out that the process of using a gel accelerator to cause a membrane effect on a cast- able or moldable foam also can be extended to other catalyzed silicate systems as indicated above, as well as to other self setting binder systems, both inorganic and organic in nature. For example, this same process can be used to form a membrane in a phosphoric acid containing binder system in which the acid reacts with alumina materials (or compounds) to form an aluminum phosphate inorganic bond. Addition of metal powders to such a system will generate gaseous by-products that will foam the mixture to produce a porous ceramic body portion. If a ceramic membrane is desired, then the incorporation of a gel accelerator such as a stronger concentration of phosphoric acid in the release agent would produce this result. Similarly, in an organic binder system such as a phenolic thermosetting resin system in which the phenolic resin is catalyzed with an acidic solution, gaseous by- products could again be generated with metal powders to bring about a porous ceramic body portion, and again by incorporating a stronger acid in the release agent the membrane effect is accomplished. Applications Although the foamable ceramic compositions descri¬ bed herein, as well as the techniques for reduction of alkali metal amounts and for formation of thin porous ceramic membrane layers, are useful for producing dried unfired, sintered, or fired ceramics for any number of uses as heretofore discussed, a number of particularly, important applications bear additional discussion. "Diesel exhaust traps" are filtering devices designed to remove particulate material (e.g., soot) from the exhaust of automobile or truck diesel engines, a need dictated to a large extent by increasingly stringent governmental regulations in the United States and European countries regarding maximum allowable particu¬ lates in automotive emission gases. Generally, soot trapped by the filtering devices is then periodically combusted in the filter so as to regenerate the filtering • surfaces, the combustion being initiated, for example, by electrical means or fuel burner devices associated with the overall trap design, or by variable operation of the engine itself or other means to provide to the filter an exhaust stream sufficiently hot to initiate the combus¬ tion process. As a consequence of the generalized designs for particulate traps of this type, the filtering element is required to have a number of properties. Obviously, it is essential that the filtering element exhibit porosity which permits trapping of particulates, but at the same time it is essential that the construction of the filter be such that exhaust gases can travel relatively easily therethrough without creation of any significant degree of back pressure. Moreover, it is necessary that the filtering element present a substantial filtering surface per unit . length, area or volume so as to permit the ele¬ ment to be fabricated in an overall.size consistent with the constraints imposed by the size of the exhaust sys- terns of diesel-powered automobiles and trucks. Because of its exposure to hot exhaust gases, and even higher temperatures during a combustion/regeneration cycle, the filtering element also necessarily must possess struc¬ tural and dimensional stability under such conditions. The prior art has sought to provide filtering elements for diesel particulate traps possessing these physical characteristics by resort to a variety of materials. Early efforts relied upon stainless steel mesh or coils of fibrous metallic wire as filtering materials, as evidenced ' by U.S. Patent Nos. 3,937,015 and 4,270,936, respectively. More recent efforts have con¬ centrated upon ceramic materials since they generally possess excellent structural and dimensional stability under stringent (i.e., high temperature) operating conditions, with the requirement that the trap exhibit porosity effective to filter soot from exhaust gases being accomplished by various compositional and process¬ ing techniques. Most notable in these efforts has been the utilization of so-called ceramic monolithic honeycomb filtering elements as described, for example, in U.S. Patent Nos. 4,276,071 and 4,364,761 assigned to General Motors Corp.; U.S. Patent Nos. 4,329,162; 4,415,344; 4,416,675; 4,416,676; 4,417,908; 4,419,108 and 4,420,316 assigned to Corning Glass; and U.S. Patent Nos. 4,283,210; 4,293,357; 4,307,198; 4,340,403; and 4,364,760 assigned to NGK Insulators. Essentially, these elements consist of a monolithic ceramic having a multitude of internal parallel chambers separated by thin porous ceramic internal walls, with a number of the chambers being end-sealed so as to force particulate-containing exhaust gas to traverse across a porous wall before exit¬ ing the element. Generally, these elements are formed by an extrusion process, and materials are included in the compositions which are burned out during the firing pro¬ cess so as to provide the requisite porosity in the inter- nal filtering surfaces. In another process, as reflected in U.S. Patent No. 4,363,644 assigned to Nippon Soken, foamed, structural polyurethane systems are utilized in admixture with inorganic materials in processes wherein the polyurethane burns out during firing so as to leave behind a ceramic structure having a variety of inter¬ connected open cells for trapping- particulates. While the structural and dimensional properties of ceramics generally lend themselves well to utilization as the material from which filter elements for diesel traps are constructed, it is not an easy or inexpensive matter to achieve from ceramic materials elements possessing the porosity needed to effectively and efficiently filter soot as well as permit exhaust gas flow without substan¬ tial build-up of back pressure. For example, in the highly permeable reticulated foam filters in the art, a condition can occur ("blow-off") in which soot already collected in the filter can be dislodged as a consequence either of excessive build-up or sudden increase in the velocity of the exhaust gas stream. Of additional import¬ ance, efforts toward optimizing the geometry of diesel filter trap designs (so as to facilitate inclusion of the .filter in the exhaust area of a vehicle, or to maximize filtration, or to facilitate regeneration or removal of the filter element) can be severely limited by the inability to produce such shapes efficiently (or at all) utilizing ceramics. Through use of the compositions and processes of the present invention, excellent diesel particulate traps of any desired geometry can be prepared. To this end, the foamable ceramic composition containing alkali metal aluminate, alkali metal silicate, metal powder, surfac¬ tant and viscosity-modifying and gel-strengthening agent, all as earlier described, contains suitable refractory * ceramic materials, also as earlier"described, and either cast or injection molded to a suitable expanded porous shape of desired configuration, or extruded as a porous expanded shape to either the desired configuration or a gross configuration from which the desired shape can be cut. In each case, the surfactant is chosen to provide an open-cell porosity suitable for filtering applica- tions. Also, in the preferred embodiment of the diesel particulate trap, the porous unfired shape is treated to reduce the alkali metal content thereof so as to increase refractoriness in the fired filter, and also is arranged to have on at least the outlet filter surface an integral thin porous ceramic membrane layer, also of open-cell porosity, whose pores have an average size less than that of the pores in the remaining portions and surfaces of the filter. After alkali removal, the porous self- supportable shape is dried to remove any water therefrom and is then fired in any suitable furnace at the tempera¬ tures required (e.g., 2200° F to 2600°F) to form the shaped porous ceramic filter article. Depending upon the composition of the moldable ceramic composition and the processing conditions, sintered ceramic refractory arti¬ cles can be prepared having a broad range of porosity, surface area and the like. A wide range of refractory foam compositions can be achieved using the basic procedures outlined above depend¬ ing on the • specific requirements of the final ceramic filter product. For example, if thermal shock resistance is of paramount importance, refractory compositions that result in low thermal expansion can be incorporated such as those containing lithium aluminosilicate, cordierite (a magnesium aluminosilicate) and/or aluminum titanate. In addition, if strength and toughness are more impor- tant, then such materials as mullite, zirconia-toughened ceramics and ceramic composites may be incorporated. If high thermal conductivity -is important, then the use of silicon carbide or silicon nitride is recommended. If high refractoriness is important, pure alumina can be used. If long term durability is required in both ther¬ mal and mechanical shock conditions, then low expansion, strong and tough type systems will be utilized. Another, application of significant interest is low mass, porous refractory ceramic articles useful as kiln furniture. Kiln furniture refers to the refractory shapes used to support ceramic ware during the firing process and includes shelves, contoured supports and spacers which are referred to in the industry as batts, setters (box, plate, yoke-ring) and the like. These refractory shapes must not only be able to withstand the high temperatures encountered in firing ceramics, but must also be strong enough to support the necessary load of the ceramic at firing temperature without deforming. Also of great importance, the kiln furniture must be reusable, a pro¬ perty demanding highly thermal shock resistant material. Recent advances in high speed or fast-firing kilns in the ceramics industry have led to the possibility of improved economics in the manufacture of ceramic ware as a direct result of potential for fuel reduction during the firing cycle. In order to take ' maximum advantage of these potentially available process cost reductions, how¬ ever, it is necessary to employ kiln furniture which is of low mass and yet which possesses extremely high ther¬ mal shock resistance in order to withstand the thermal cycling which characterizes the newer kiln designs and firing processes. Added to this is the need to provide kiln furniture of a variety of shapes and configurations as needs may dictate, as well as furniture having smooth surfaces suitable for supporting shapes to be fired into ceramics. To date, a cost-effective kiln furniture article meeting all these requirements has not been realized. However, such goals are attainable using the compositions and processes of the present invention. In most respects, this application is quite similar to that earlier-described for a diesel filter trap, in terms of the refractory ceramic materials used in the foamable ceramic composition, the reduction of alkali metal in the unfired porous shape, the firing of the article, and the preferred provision of a thin porous ceramic membrane layer on selected surfaces (average pore size less than that of remaining body or other surfaces). The major difference is that the porosity of the porous main body or any porous ceramic membrane layer can, for kiln furni- ture, be of predominantly open-cell structure or predomi¬ nantly closed-cell structure as may be desired for parti¬ cular applications. With particular regard to the use of a thin porous ceramic membrane layer to provide smooth surfaces on a ceramic body, such as above-described for kiln furniture, other applications include furnace linings, mold materials for conventional slip casting of ceramic ware, and other like uses. Smooth-surfaced furnace linings are advantageous in reflecting more heat than is absorbed, for example. Another advantageous use of the thin porous ceramic membrane layer is in foundry molds or cores where the body portion can be made from inexpensive materials and of a porosity which permits easy removal of core or mold material after the casting has been made, while a membrane layer can be provided using the more expensive inert materials required for the particular metal being cast, and in this way greatly reducing overall cost and processability as compared to a mold or core made entirely of the expensive inert materials. Particular advantage is found in applications, such as those above, where functional considerations require the provision of porous ceramic articles of complex shape. The use of moldable, foamable ceramic compositions is ideally suited for manufacture of articles of complex geometry, and the present invention, in providing means for arranging such compositions to produce smooth porous membrane layers on selected surfaces of the overall porous body portion, greatly expands the potential- uses and functionality for such castable compositions. As noted above, a particular application requiring a smooth porous ceramic membrane surface over a porous ceramic substrate material is in the area of mold materials for conventional slip casting of ceramic ware. Currently such molds are made with plaster materials that have been slightly dried so that they are extremely absorbent to the moisture in the ceramic slip. Typical ceramic slips used in the production of dinnerware, for example, contain 20 to 30 percent water. Once cast against a dried plaster mold, water is gradually "sucked" out of the slip and into the plaster. As the moisture is removed, the green strength of the slip increases until a strong, leather-like consistency is obtained. Once suffi¬ cient water is removed and strength obtained, the cast dinnerware can be removed from the plaster mold and further dried prior to firing to the required tempera¬ ture. The plaster mold is then redried and reused. The number of times a particular plaster mold can be reused depends on the level of detail required in the slip cast part. With continued use, the plaster surface will deteriorate and the mold will have to be discarded. Currently, the technology does not exist to allow the reuse of the spent plaster. As a result, huge waste piles of this plaster are collecting in landfills. Dis¬ posal is becoming a definite part of the production cost and alternatives to plaster molds are therefore being actively pursued. One method being employed more and more often is the pressure cast and/or the compression molding of simple shapes in permanent molds. Difficult shapes, i.e., those with complex configurations cannot be produced by these methods / however, and the traditional plaster molding process must therefore be used. The ability to produce smooth porous ceramic mem- brane surface layers according to the present invention provides porous ceramic products which are an ideal sub¬ stitute for the plaster molds described above. The strength and durability of such ceramic membrane porous molds is considerably higher than that of the plaster system. This aspect, coupled with the ability of using vacuum assisted dewatering of the ceramic slip material being cast against the mold, implies a considerable improvement in the cost of producing ceramic ware parts. In addition, the disposal of the ceramic mold is not as critical a problem as the plaster. If desired, the material can be ground up, sized and reused to make more molds. The principles and compositional features of the present invention are also eminently suited to the production of lightweight building materials, such as building blocks for homes and other structures. The art has long been aware of the theoretical desirability of replacing building materials such as concrete blocks as typically used in the U.S. or fired, extruded clay-type blocks typically used in Europe, with blocks based upon bound aggregate systems which afford more rapid curing and, hence, increased productivity in block manufacture. It is also a most desirable goal to produce concrete block replacements which are lightweight (i.e., low density) so as to facilitate handling and reduce materials costs as well as to improve insulating properties. Despite knowledge of these goals, the art hereto¬ fore has been unsuccessful in attaining them to any practical degree. In order to compete with concrete or extruded clay-type blocks, these rapid-curing bound aggregates must be produceable at very low cost, some¬ thing that is impossible for many systems relying upon rather expensive binding techniques unless the binder is used at extremely low levels. At such low binder levels, however, the solids content of the composition is so high as to make the composition extremely viscous and impracti¬ cal for casting or injection molding. Although it may be possible to extrude such highly viscous slurries, that alone is not a solution to the problem where the original slurry is made up by mixing together separate aggregate- loaded slurries such as occurs in the present invention where an aggregate-loaded alkali metal aluminate slurry is mixed with an aggregate-loaded alkali metal aluminate slurry to form a composition in which the aluminate and silicate react to form the alkali metal aluminosilicate hydrogel binder. At high solids contents, the separate slurries are so viscous as to make intimate intermixing difficult if not impossible, leading to lack of homo¬ geneity and lack of intermixing needed to achieve the proper hydrogel formation. Thus, even though the resul¬ tant composition (if formable at all) may be extrudable. it " is not capable of producing the desired bound aggregate. This difficulty in molding or mixing is greatly exacerbated when low density products are required (e.g., 40 lbs./ft. 3 or less, most desirably, 30 lbs./ft. 3 or less) since, in order to meet this goal, porous blocks are required and it is generally necessary to have extremely thin cell wall thicknesses in the open-or closed-cell pores, as the case may be (e.g., on the order of 200 microns or less, and 100 microns or less for very low density products). In turn, then, the aggregate materials (i.e., inexpensive inorganic ceramic materials such as clays and other natural materials) must have grain sizes predominantly less than 200 or 100 microns. Because such small grain size aggregates have extremely high surface areas, the viscosity of the composition increases even further, making casting or injection mold¬ ing or even mixing essentially impossible. In such cases, solids contents must be drastically reduced in order to arrive at a processable composition, leading to blocks of inadequate strength and increased cost because of greater non-solids usage. The present invention provides a unique solution to these problems in providing a foamable ceramic composi- tion which can be heavily loaded with solids of predomi¬ nantly fine grain size (i.e., 70% or greater solids con¬ tent, 60% or greater aggregate content) and yet which, by reason of the viscosity-reducing and gel-strengthening agent (e.g., silica fume), can be made at a starting viscosity of 40,000 cps and less, and even below 30,000 cps (Brookfield viscosity) . According to the invention, then, a foamable cera¬ mic composition is provided containing an aqueous admix¬ ture of alkali metal aluminate, alkali metal silicate, metal powder and surfactant, all as earlier described, and further containing suitable particulate aggregate material such as clays (which may be raw or partially or wholly calcined) or other like inorganic ceramic particu¬ lates. The particulate aggregate materials are present in the composition in an amount of at least about 60% by weight, and the total solids content of the composition is at least about 70% by weight. The particle size of the aggregate material is predominantly quite small, and generally at least about 70% thereof is 200 microns or less. A preferred particle size distribution for a suit- able partially calcined clay is as follows: % Micron Range 8 - 10 100 - 200 8 - 10 75 - 100 15 - 20 50 - 75 40 - 50 1 - 50 5 - 10 < 1 Using * clays of this particle size distribution, which can be augmented up to 30% by weight with coarser aggregates, foamed alkali metal aluminosilicate hydrogel-bound aggre- gate blocks can be prepared having a bulk density of 40 lbs./ft. 3 or less. For making even lower density pro¬ ducts (e.g., 30 lbs./ft. 3 or less), at least about 70% of the aggregate material will have a particle size less than about 100 microns. A preferred grain size distribu- tion for preparing a low density block is as follows: Micron Range 100 - 150 75 - 100 50 - 75 1 - 50 < 1 In all such cases, the viscosity-reducing and gel strengthening agent is utilized in an amount necessary to reduce the viscosity of the starting foamable composition (and the separate slurries used to prepare it) to 40,000 cps or less, preferably less than 30,000 cps, and most preferably between about 25,000 and 30,000 cps. At these viscosities, the composition is easily and readily mixed and then cast or injection molded or extruded to produce quick-curing blocks of substantial solids content at very low cost. FIGS. 10 and 11 illustrate the effects of the viscosity-reducing and gel-strengthening agent (here, silica fume) on the viscosity of the foamable composi¬ tion. In FIG. 10, three different partially calcined clays (HC-1, HC-2 and HC-3), each having a particle size distribution such that about 80% of the particles are 74 microns or less, were.used as the aggregate portion of a foamable composition containing alkali metal aluminate, alkali metal silicate, metal powder, surfactant and water. As shown, * viscosities below 30,000 cps could be achieved only at aggregate contents of 62 - 64% by weight, and higher contents of aggregate led to dramatic increases in viscosity, at which the composition was unprocessable (in contrast, it can be seen that standard calcined kyanite permits high loading levels at process- able viscosities). In FIG. 11, the HC-2 aggregate- containing foamable composition was augmented with 5% silica fume in one case and with 5% silica fume and 9% coarse aggregates. As can be seen, the silica fume results in lower composition viscosity at equal HC-2 loading levels, to the point where viscosities of 30,000 cps or less can be achieved at up to 67% aggregate load¬ ing for HC-2 alone, and up to 70% aggregate loading for HC-2 and 9% coarse aggregate. The foamable composition is either cast or injection-molded into a mold of desired (e.g., block-like) configuration such that, after the foaming/expansion, the self-supportable aluminosilicate hydrogel-bound aggregate is in the desired final dimen¬ sions (e.g., 9 x 18 x 6 inches). Alternatively, the composition can be extruded as a foam in a bulk shape and then cut to size. In each case, either in the mold or extrusion chamber as the case may be, the composition foams and expands by reason of reaction between the metal powder and alkali in the composition to produce hydrogen gas bubbles which, in conjunction with the surfactant, produces open-or closed-cell pores in the eventually bound product," while the aluminate and silicate compon¬ ents of the composition react to form the alkali metal aluminosilicate hydrogel binder which self-sets and binds the aggregates together in a porous, self-supporting shape. The self-supporting hydrogel-bound aggregate shape is then dried to reduce the water content thereof. De¬ pending upon the end use, the dried shapes may be employ¬ ed as such (preferably with a first water treatment to remove any unbound alkali metal compounds which might, over time, migrate out to the block surface and form dis- colorations) or, as is typically the case, will be sub¬ jected to higher temperature conditions to effect sinter¬ ing. Here again, if firing is in excess of 1000°C, it may be desirable to first remove unbound alkali metal compounds. Also if desired, the so-produced shapes (e.g., blocks) can be treated on all or particular sur¬ faces to render the surface fluid or liquid impervious or to produce aesthetic smoothing effects. Generally speaking, the so-produced blocks, in the initially dried state, will have a bulk density of less than about 40 lbs./ft. 3 , a compressive strength of 2 - 3 N/mπ.2, a modulus of rupture of about 1 - 2 N/mm 2 , and a loss on. ignition of 15% or less. When fired at 1000°C, the properties of the block improve to a bulk density of about 28 - 32 lbs./ft. 3 , compressive strength of 4 - 5 N/mm 2 , modulus of rupture of 3 - 4 N/mm 2 , and essentially no loss on ignition. Preferred compositions of the foamed product in¬ clude about 28 - 34% hydrogel (28% - 34% solids, 66% - 72% water), 0.1% of surfactant (e.g., silicone glycol), 4 - 6% silica fume (0.25 micron), and the remainder particu- late aggregate (e.g., for very low density products of cell wall thickness less than 100 microns, 60 - 65% of partially calcined clay having a predominant particle size of 74 microns or less, and for low density products of cell wall thickness less than 200 microns, 55 - 60% of the above clay and 8 - 10% of coarser aggregates). To further describe the present invention and its various aspects, a number of examples are presented in the following section illustrating a variety of potential compositions, processing techniques, applications and the like. In accumulating the data set forth in some of the examples, density, three-point modulus of rupture (MOR) and the coefficient of thermal expansion were measured by • - standard ASTM techniques. The pore structure (number of complete pore cells per linear inch) was measured using both Scanning Electron Microscope (SEM) and stereographic light microscope photographs. The predominant micro- structural phases were determined using X-ray diffraction techniques as well as light microscope observation of polished and etched surfaces. The relative permeability, where applicable, was obtained using a turbine type air blower and recording the backpressure associated with the open celled structure as 100 scfm (standard cubic feet per' minute) of air was forced through. Thermal shock resistance was interpreted as the percent of initial MOR strength the ceramic foam retained after exposing the material to 100 thermal cycles between room temperature and 1250°F. EXAMPLE 1 A particularly effective ceramic foam filter for high temperature filtering applications was prepared using fused cordierite and calcined kyanite (mullite) refractories in the following manner. Initially, two slurries were prepared, one containing sodium silicate and the other, sodium aluminate. The slurries were pre¬ pared to a specific gravity of 2.1 g/cc at a viscosity of 25,000 cps at 70°F. Sodium Silicate Slurry sodium silicate grade 50 (44.1% solids) 27.2% additional process water Dow surfactant 190 silica fume (1/4 micron) chopped fibers (1/8 and down) fused cordierite (-200 mesh) calcined kyanite (-200 mesh) powdered aluminum metal (6-9 micron) Sodium Aluminate Slurry sodium meta-aluminate solution (46% solids) additional water Dow surfactant 190 silica fume (1/4 micron) chopped fibers (1/8 and down) fused cordierite (-200 mesh) calcined kyanite (-200 mesh) Using a high shear type mixer or blender apparatus, - equal weights (360 g) of the above slurries were combined and cast into a mold cavity with an 840 cc capacity to - produce a 10-inch diameter, plate-like configuration, 5/8 inches thick. Since the slurries had a specific gravity of 2.1 g/cc, only 41% of the mold cavity was filled. In approximately 30 seconds after the two slurries were com¬ bined at 70°F, the mix began to foam with a predominantly open-celled structure thereby filling the mold to yield a 0.86 g/cc wet density. Foaming stopped when the sodium aluminosilicate hydrogel binder phase "set" (approxi¬ mately 3-4 minutes), freezing the expanded structure in place. The hydrogel bond developed sufficient strength in 8-10 minutes to allow the cast part to be demolded. At this point the part contained 4.6% sodium oxide and 20.1% water at the above mentioned 0.86 g/cc density. In order to increase the refractoriness, the sodium oxide was then removed. This was accomplished by rinsing the part, in this case a 10 inch diameter plate, 5/8 inches thick, with 10 liters of purified water (preferably deionized water with a 50,000 ohm resistance or better). This rinse reduced the sodium oxide content to approxi¬ mately 2%, the stoichiometric amount. To remove the remaining sodium, the part was then subjected to 30-40 liters of a 1% ammonium chloride solution whereby sub¬ stantially all of the NH 4 + ions replaced the Na + ions. An additional 5 liter water rinse was then per¬ formed to remove excess Cl " ions after which the part was removed and allowed to drain and dry. An additional water rinse was then performed, after which the part was removed and allowed to drain and dry. After the initial draining and air drying period, the part was heated in a vented oven to 400-600°F to further dehydrate it. The length of time the part is in the oven varies with the particular type of oven (conven¬ tional or microwave) and the specific amount of water in the part as well as the porosity of the part. In this particular example, the 10 inch plate was heated to 600°F in 6 hours. The warm part was removed from the oven and placed directly in a kiln supported on suitable kiln furniture. The part was then slowly heated to the requir¬ ed firing temperature of 2425°F in 10-12 hours. Once at temperature, the part was held for 2 hours to complete the sintering operation before being allowed to furnace cool. Using the above formulation and process, " the 10 inch ceramic foamed plate was characterized as follows: Density 0.63 g/cc Sodium content less than 0.5% Permeability (in a ten- 4 inches of water inch plate) back-pressure at 100 scfm Predominant microstructure cordierite, mullite Coefficient of expansion 1.5 x 10 (-6) to 700°C 3.2 x 10 (-6) to 1000°C Modulus of rupture 400-450 psi Modulus of rupture after 100 380-410 psi cycles from RT to 1250°F Pore structure 30 pores per linear inch Apparent refractoriness 2500°F A series of 10 inch reticulated foamed plates pro¬ duced in this manner were placed in the exhaust stream from a 1982, 6.2 liter diesel engine to determine their effectiveness in filtering diesel particulates produced. The plates were arranged in a "stacked element" design. While the exact collection efficiency was not determined, a considerable amount of particulates were trapped within the cross-sectional area of the plates. Once filled with x. particulates, the plates were regenerated by placing them in a furnace and heating to the soot ignition temperature of 960°F at which time the plate increased in temperature over that of the surrounding furnace environment indicat- ing an exothermic reaction or burning of the particulates was taking place. After regeneration, the plates were subjected to the same air flow permeability test to determine how much of the particulates or soot had been removed. Since the same 4 inches . of water backpressure was reached, it was assumed that all particulates were burned. The plates were put back in the exhaust stream to collect more particulates, but this time regeneration was performed using a diesel fuel burner/blower arrangement that was able to heat the ceramic plates in 3-4 minutes to 1400°F thereby setting up a more thermal shock prone regeneration cycle that was more in tune with the expected service environment. ' t Again, - •the regenerated plates experienced the same backpressure resistance as new plates. After a number of such regeneration cycles were performed, the plates were sectioned into MOR test bars and broken to determine if any loss in strength occurred as the result of such thermal cycling. No significant loss in strength was recorded. EXAMPLE 2 An essentially all cordierite type ceramic foam was produced in a similar manner to that in Example 1 with fused cordierite aggregate being the primary aggregate in each of the two reactive slurries, i.e., silicate and aluminate slurries, as follows: Silicate Slurry sodium silicate grade 50 27.1 additional water 6.0 Dow 190 surfactant 0.6 silica fume 3.6 fused cordierite (-200 mesh) 60.8 powdered aluminum metal (6-9 microns) 0.1 chopped fibers 1•8 Aluminate Slurry sodium meta aluminate solution 24.7 additional water 6.9 Dow 190 surfactant 0.6 silica fume 3.2 fused cordierite (-200 mesh) 62.8 chopped fibers 1,8 The above slurries were blended together as in Example 1 with the same volume expansion due to the foaming action of the aluminum metal and sodium hydroxide portion of the binder. Rinsing, ion-exchange and drying were also performed as they were in Example 1. Firing however, was slightly lower with an upper holding tempera¬ ture of 2000 β F. The above formulation and procedures resulted in a reticulated ceramic foam with the following properties and characteristics: x. Density 0.8 g/cc Pores per linear inch -30 Coefficient of thermal expansion 1.8xl0" 6 in/in °C MOR 827 psi MOR after 100 thermal cycles (to 1250°F) 777 psi Permeability (backpressure at 100 scfm) 40 inches H2O Principal microstructural constituent Cordierite EXAMPLE 3 The same mix and procedures of Example 2 above were prepared with the exception that the final firing temperature was reduced to 1800 β F. The purpose in firing to a lower temperature was to increase the perme'ability (lowering the backpressure at the 100 scfm flow rate) of the final foamed ceramic. Using the lower firing temperature resulted in the following properties and characteristics: Density .74 g/cc Pores per linear inch ~ 30 Coefficient of thermal expansion 1.8xl0" 6 in/in °C MOR 505 psi MOR after 100 thermal cycles (at 1250°F) 469 psi Permeability (backpressure at 100 scfm) 18 inches of water Principal microstructural constituent Cordierite EXAMPLE 4 An essentially all calcined kyanite mix was also produced in a similar manner of mixing, leaching/rinsing, ion exchange and drying as that set forth in the above 5 examples using the following slurry formulations: Silicate Slurry sodium silicate grade 50 26.0 additional water 5.8 Dow 190 surfactant 0.6 10 silica fume 3.3 calcined kyanite (-200 mesh) 62.5 powdered aluminum metal (6-9 microns) 0.1 , chopped fibers 1.7 * Aluminate Slurry 15 sodium meta aluminate solution 23.7 additional water 6.6 Dow 190 surfactant 0.6 silica fume 3.0 calcined kyanite (-200 mesh) 64.5 20 chopped fibers 1.6 Once properly dried, the calcined kyanite foamed plates were fired at 2600°F with the following properties and characteristics obtained: x. Density 0.7 g/cc Pores per linear inch - 30 Coefficient of thermal expansion 4xl0 "6 in/in °C MOR 451 MOR after 100 thermal cycles (to 1250°F) Failed after 6 cycles Permeability (backpressure at 100 scfm) 12 inches of water Principal microstructural constituent Mullite EXAMPLE 5 A ceramic foam composition containing dispersed zirconium oxide as a toughening aid was also prepared using the sodium aluminosilicate hydrogel system and a fused zirconia-mullite aggregate using the following starting slurry formulations: Silicate Slurry sodium silicate grade 50 21.9 additional water 4.9 Dow 190 surfactant 0.5 silica fume 2.8 fused zirconia-mullite (-200 mesh) 68.4 powdered aluminum metal (6-9 microns) 0.1 chopped fibers 1.4 Aluminate Slurry sodium meta aluminate solution 19.9 additional water 5.5 Dow 190 surfactant 0.5 silica fume 2.5 fused zirconia-mullite kyanite (-200 mesh) 70.2 chopped fibers 1.4 Once properly dried, the foamed plates were fired at 2600°F with the following properties and characteris¬ tics obtained: Density .84 g/cc Pores per linear inch - 30 Coefficient of thermal expansion 5xl0" 6 in/in °C MOR 476 psi MOR after 100 thermal cycles (to 1250°F) 395 psi Permeability (backpressure at 100 scfm) 25 inches of water Principal microstructural constituent mullite/zirconia EXAMPLE 6 Another useful ceramic foamed system based on a cordierite-silicon carbide blend was also prepared using the sodium aluminosilicate hydrogel system and the same leaching/rinsing, ion exchange and drying procedures set forth in the above examples. The silicate and aluminate slurries used for this example consisted of the following materials: Silicate Slurry sodium silicate grade 50 27.4 additional water 6.2 Dow 190 surfactant .5 silica fume 2.8 fused cordierite (-200 mesh) 34.0 silicon carbide (-200 mesh) 27.4 powdered aluminum metal (6-9 microns) 0.1 chopped fibers 1.6 Aluminate Slurry sodium meta aluminate solution 23.6 additional water 6.8 Dow 190 surfactant " " 0.5 silica fume 2.4 fus,ed cordierite (-200 mesh) '65.1 chopped fibers 1.5 Once properly dried, the foamed plates were fired at 2200°F with the following properties and characteris¬ tics obtained: Density .75 g/cc Pores per linear inch - 30 Coefficient of thermal expansion 2.5xl0" 6 in/in β C MOR 380 psi MOR after 100 thermal cycles (to 1250 β F) 335 psi Permeability (backpressure at 100 scfm) 22 inches of water Principal microstructural constituent Cordierite, Sic EXAMPLE 7 The examples so far have been concerned with the formation of porous ceramic components with high perme¬ ability for use in filtration and other similar applica- tions requiring such permeability. One of the other major advantages of the present invention is the ability to change the pore morphology so that a mostly closed versus a mostly open cell structure results. Low density foams with mostly closed cell structures make, e.g., ideal insulation components with their refractoriness limited primarily by the refractory constituents of the final foamed product. In order to create such a ceramic foam, the same sodium aluminosilicate hydrogel bonding system was used but with a different type of surfactant that stabilizes a closed versus open cell structure. Since some permeability still occurs in the green or hydrogel state, the same leaching/rinsing, ion exchange and drying procedures used in the preceding examples was used to remove the sodium and make the final foamed product more refractory. The silicate and aluminate slurries used for one such example involving a calcined kyanite refractory consisted of the following compositions: Silicate Slurry sodium silicate grade 50 26.0 additional water 5.8 Dow 193 surfactant 0.6 silica fume 3.3 calcined kyanite (-200 mesh) 64.2 powdered aluminum metal (6-9 microns) 0.1 x. Aluminate Slurry sodium meta aluminate solution 23.7 additional water 6.6 Dow 193 surfactant .5 silica fume 3.0 calcined kyanite (-200 mesh) 66.2 Once properly dried, the calcined kyanite foamed plates were fired at 2600°F with the following properties and characteristics obtained: Density .8 g/cc Pores per linear inch - 35 Coefficient of thermal expansion 4x10 "6 in/in °C MOR 650 psi '. „ MOR after 100 thermal cycles (to 1250°F) N/A 1 Permeability (backpressure at To produce kiln furnifcure, two slurries were pre¬ pared, one containing sodium silicate and the other sodium aluminate. The slurries were prepared to a speci¬ fic gravity of 2.1 g/cc at a viscosity of 25,000 cps at 70°F. Sodium Silicate Slurry sodium silicate grade 50 (44.1% solids) 27.2% additional process water 5.4% Dow surfactant 190 0.6% silica fume (1/4 micron) 1.6% chopped fibers (1/8 and down) 2.0% fused cordierite (-200 mesh) 30.2% calcined kyanite (-200 mesh) 32.7% powdered aluminum metal (6-9 micron) 0.3% Sodium Aluminate Slurry sodium meta-aluminate solution (46* solids) 25.9% additional water 5.7% Dow surfactant 190 0.6% silica fume (1/4 micron) 1.5% chopped fibers (1/8 and down) 1.9% fused cordierite (-200 mesh) 33.9% calcined kyanite (-200 mesh) 31.0% Equal weights (360g) of the slurries were combined and cast into a mold cavity having an 840 cc capacity (since the slurries had a specific gravity of 2.1 g/cc, only 41% of the mold cavity was filled) . The mold was in the form suitable for preparing a 10-inch diameter plate, 5/8- inches thick. One side of the mold surface was coated with a release agent containing 17.5% polyethylene glycol 3350, 12.5% polyvinyl alcohol solution, 36.5% glycerine and 33.5% water. Approximately 30 seconds after the ceramic composition was cast into the mold, the mix began to foam to an open-cell porous structure having a wet density of 0.86 g/cc. Foaming stopped when the sodium aluminosilicate hydrogel binder phase set (approximately 3 to 4 minutes), freezing the expanded structure in place. Adjacent the release agent, the ceramic composi¬ tion gelled more rapidly, thereby preventing growth of any hydrogen gas bubbles formed near this surface. After x. 8 to 10 minutes in the mold, the hydrogel developed sufficient strength to be demolded, and the supportable cast shape displayed ' oh excellent smooth skin where it was in contact with the release agent composition. At this point the part contained 4.6% sodium oxide and 20.1% water at the above mentioned 0.86 g/cc density. In order to increase the refractoriness, the sodium oxide was then removed. This was accomplished by rinsing the part with 10 liters of purified water (deionized water with a 50,000 ohm resistance or better). This rinse reduced the sodium oxide content to approximately 2%, the stoichiometric amount. To remove the remaining sodium, the part was then subjected to 30-40 liters of a 1% ammonium chloride solution whereby all of the NH4 + ions replaced the Na + ions. An additional 5 liter water rinse was then performed to remove excess Cl" ions after which the part was removed and allowed to drain and dry. After the initial draining and air drying period, the part was heated to 600 β F in 6 hours. The warm part was removed from the oven and placed directly in a kiln. The part was then slowly heated to the required firing temperature of 2425 β F in 10-12 hours. Once at tempera¬ ture, the part was held for 2 hours to complete the sintering operation before being allowed to furnace cool. The ceramic article made in this manner had a predominant microstructure of cordierite/mullite, an apparent . refractoriness of about 2500°F, a coefficient of thermal expansion of about 1.5x10(-6) at 700°C and 3.2x10(-6) at 1000°C, and a room temperature modulus of rupture of about 400 - 450 psi. After exposure to 100 cycles of room temperature to 1250°C, no significant loss of strength was recorded. The apparent refractoriness of the article was about 2500°F, and its density was about 0.6 g/cc. The article so made is excellently suited for use as kiln furniture by reason of its low mass, porosity. refractoriness, resistance to thermal shock, and smooth- skinned surface. EXAMPLE 9 Utilizing the composition and process set forth in Example 8, but using instead a release agent consisting of 50% water and 50% glycerine, a porous refractory ceramic kiln furniture plate of low density was produced having a smooth skin on one surface thereof by -reason of rapid gelation of the aluminosilicate hydrogel in contact with the release agent prior to development of any large bubbles. EXAMPLE 10 Utilizing the composition and process set forth in Example 8, but coating one of the mold surfaces with a thin layer of woven mullite fibered.paper (instead of using the release agent composition of Example 1) , the part removed from the mold was found to have the woven paper significantly attached to its surface such that no separation occurred during subsequent sodium removal processing. Upon firing, there was produced a low density porous ceramic refractory kiln furniture article having a smooth skin on one of its surfaces. EXAMPLE 11 The same composition and process set forth in Example 8 was employed with the exception that a silicone release agent modified with a silicone defoaming surfac¬ tant was sprayed on one of the mold surfaces, and that mold surface was locally heated to about 140 - 150°F, while the other mold surface was kept at room tempera- ture. As a consequence of accelerated gelation of the hydrogel in contact with the heated surface insufficient time was available for the foam cells to grow to any appreciable size before setting occurred The result was ,a smooth surface on the kiln furniture article after firing. EXAMPLE 12 A particularly effective ceramic foam filter for the high temperature filtering of diesel particulates includes the casting of a sodium aluminosilicate hydrogel bonded system of Example 1 to create a reticulated porous plate in which one side of the mold surface was coated with a release agent based on polyethylene glycol 3350, polyvinyl alcohol, glycerine and water of the following composition: polyethylene glycol 3350 17.5% polyvinyl alcohol solution 12.5% glycerine 36.5% water 33.5% Once the silicate and aluminate containing slurries were combined and cast into the mold, the mix adjacent to the above release agent rapidly gelled, thereby prevent¬ ing the growth of any hydrogen gas bubble that may have formed near the mold surface as the result of the reac¬ tion of the aluminum metal powder and the sodium hydr- oxide in the mix. After demolding, the cast part dis¬ played an excellent "skin" or smooth membrane surface that upon further processing to remove sodium and water followed by firing at a suitable temperature to form ceramic bonds, remained porous even though by naked eye the surface appears dense. A series of 10 inch foamed plates produced in this manner with such excellent ceramic membrane surfaces on one side were fashioned into a stacked element filtering arrangement. The efficiency of the diesel particulate collection was measured using a suitable device at a major diesel engine manufacturer's test facility and found to be in the 65-70% efficient range. Such effici¬ encies would make the 1991 and 1994 diesel engine proto¬ types now being designed meet the EPA emission standards. Having described the invention with reference to particular compositions, processes, examples and embodi- ments, it is to be understood that these particulars are presented for purposes of illustration and description, and are not otherwise, intended as strict limitations upon the scope of the fundamental invention as defined in the appended claims.

Claims: WHAT IS CLAIMED IS: 1. A foamable ceramic composition which can be molded to a desired geometric 'configuration and which is self- settable in said configuration to achieve a self- supporting shaped structure of defined porosity, said composition comprising an aqueous admixture of effective amounts of an alkali metal silicate, an alkali metal aluminate, refractory ceramic materials, viscosity- modifying and gel strengthening agent, surfactant and metal powder, whereby said alkali metal silicate and said alkali metal aluminate react to form a self-setting alkali metal aluminosilicate hydrogel which serves as a binder to set the composition in the molded shape, and wherein said metal powder is present in an amount effec¬ tive to react with alkali materials in said composition so as to produce, as a by-product of such reaction, hydrogen - gas which, in conjunction with said surfactant, develops in said composition a significant degree of porosity prior to the setting of said hydrogel. 2. The foamable ceramic composition according to Claim 1 wherein said viscosity-modifying and gel strengthening agent consists of • finely-divided spherical particles of fumed metal-oxides. 3. The foamable ceramic composition according to Claim 2 wherein said viscosity-modifying and gel strengthening agent is selected from the group consisting of silica fume, fly ash, manganese oxide fume, ferro-silicon fume, and mixtures thereof. 4. The foamable ceramic composition according to Claim 3 wherein said gel strengthening agent consists of silica fume, and said metal powder is aluminum metal powder. 5. A process for preparing porous ceramic articles, comprising the steps of: (a) providing a foamable ceramic composition com¬ prised of an aqueous admixture of effective amounts an alkali silicate, an alkali aluminate, refractory ceramic materials, viscosity-modifying and gel strengthening agent, surfactant and metal powder; (b) forming said composition into a desired con¬ figuration utilizing a mold or extrusion chamber; (c) permitting said casting composition to reside in said mold or said chamber for a predetermined period of time during which components of said composition react to form an aluminosilicate hydrogel which binds.all com¬ ponents of said composition in a supportable shape con- forming to said mold or chamber and during which porosity is developed in said supportable shape by virtue of evolu¬ tion of hydrogen in said composition as a result of reaction between said metal powder and alkali materials in said composition; (d) removing said porous supportable shape from said mold or said chamber; and (e) drying said supportable shape. 6. The process according to claim 5 further comprising the step of treating the porous supportable shape to eliminate therein and therefrom substantially all alkali metal and then firing said porous supportable shape. 7. The process according to claim 5 or 6 wherein said porous supportable shape, either in its dried or fired condition, is provided on one or more surfaces thereof with a thin porous ceramic membrane layer, the pores of which have an average pore size less than that of the pores throughout the remainder of the porous supportable shape. 8. A porous ceramic article manufactured according to the processes of claims 5, 6 or 7. 9. A process for lowering the alkali metal content of a porous, unfired, supportable ceramic shape containing alkali metal so as to increase the refractoriness and thermal shock resistance of fired porous ceramic articles made therefrom, comprising the steps of first contacting said porous ceramic shape with water to remove therefrom leachable alkali compounds, and thereafter contacting said porous ceramic shape with a dilute aqueous solution of an ammonium salt selected from the group consisting of ammonium acetate, ammonium bicarbonate, ammonium carbo¬ nate, ammonium chloride, ammonium hydroxide, ammonium sulfate, and mixtures thereof, to effect substantially complete exchange of ammonium ion for remaining alkali metal ion in said porous ceramic shape. ,. 10. The process according to claim 9 wherein said porous ceramic shape comprises a porous aggregate of inorganic ceramic compounds bound by an alkali metal aluminosilicate hydrogel. 11. An element for filtering solid particulates from a diesel exhaust gas, comprising a shaped porous refractory ceramic body having a predominantly open pore structure and having an inlet and outlet surface, and wherein at least one of said inlet and outlet surfaces contains an integral thin porous ceramic membrane layer of open cell porosity, the pores of which have an average diameter smaller than that of the pores at the inlet surface and the pores throughout the interior of the body, and where- in said porous refractory ceramic body is comprised of a fired porous aluminosilicate hydrogel-bonded ceramic composition. 12. Kiln furniture for supporting ceramic ware during the firing thereof, comprised of a porous, low-density, refractory ceramic body having on one or more surfaces thereof an integral smooth skin in the form of a thin porous ceramic membrane layer, the pores of which are of an average size smaller than those of said body portion and remaining surfaces, said ceramic body being formed from the firing of a porous molded aluminosilicate-bound aggregate of refractory ceramic materials which has been treated prior to the firing to reduce the alkali metal content thereof. 13. Kiln furniture according to claim 12 wherein said porous molded aluminosilicate-bound aggregate of refrac¬ tory ceramic materials is formed by placing into a mold of suitable configuration a foamable aqueous composition comprised of alkali metal silicate, alkali metal alumi¬ nate, refractory ceramic materials, metal powder, viscosity-modifying and gel strengthening agent and surfactant, whereby in said mold, the alkali metal sili¬ cate and alkali metal aluminate react to form an alumino- silicate hydrogel binder while reaction occurs between and among ingredients in said composition to generate hydrogen gas so as to bring about formation of pores within said hydrogel-bound structure. 14. A process for producing porous ceramic articles comprising providing a foamable, moldable ceramic com¬ position capable of being molded to a desired configura¬ tion and set in said configuration to a self-supporting porous shape; providing on selected mold surfaces an agent for bringing about formation of a porous ceramic membrane layer on those surfaces of said porous shape in contact with said mold surfaces; casting or injecting said foamable, ceramic composition into said mold and permitting it to remain in said mold until such time as said self-supportable porous shape is formed; removing said self-supportable shape; and firing said self-, supportable porous shape to produce a porous ceramic article having on said selected surfaces a thin smooth, porous ceramic membrane layer, the pores of which have an average diameter less than that of the pores throughout the remainder of said article. 15. A process for manufacturing lightweight, hydrogel- bound aggregate shapes having a bulk density of less than about 40 pounds per cubic foot, comprising the steps of: (a) providing a foamable composition comprised of an aqueous admixture of effective amounts of an alkali metal silicate, an alkali metal aluminate, surfactant, metal powder, particulate aggregate material and viscosity-modifying and gel-strengthening agent, said foamable composition having a total solids content of at least about 70% by weight, a total aggregate content of at least about 60% by weight, said aggregate material having a particle size distribution such that greater than about 70% thereof is of a particle size of 200 microns or less, and wherein said viscosity-modifying and gel-strengthening agent comprises finely-divided spheri¬ cal particles of fumed metal oxide in an amount effective to reduce the starting viscosity of said foamable composi¬ tion to less than about 40,000 cps; (b) forming said foamable composition into a foamed, porous, self-supporting hydrogel-bound aggregate of desired configuration as a consequence of reaction between components of said composition to form a self- setting alkali metal aluminosilicate hydrogel which binds components of said composition in a self-supporting shape of desired configuration, and reaction between said metal powder and alkali materials in said composition to pro¬ duce therein hydrogen gas bubbles which, in conjunction with said surfactant, develop in said composition a significant degree of porosity prior to the setting of said hydrogel; and (c) drying said self-supporting foamed, porous, hydrogel-bound aggregate to remove moisture therefrom and produce a lightweight, hydrogel-bound aggregate shape having a ' bulk density of less than about 40 pounds per cubic foot, followed, if desired, by sintering or firing. 16. A lightweight hydrogel-bound aggregate, in the form of a building block, having a bulk density of about 40 pounds per cubic foot or less, made according to the process of claims 15. 17. A lightweight hydrogel-bound aggregate, in the form of a building block, having a bulk density of about 30 pounds per cubic foot or less, made according to the process of claims 15.

Inventors: HELFERICH RICHARD LEE, SCHENCK ROBERT C, HELFERICH, RICHARD, LEE, SCHENCK, ROBERT, C.
Applicants: DURIRON CO, THE DURIRON COMPANY, INC.
Application year: 1988
Main classification: C04B35
IPC codes: B01D39, C04B28, C04B35, C04B38, F01N3, F02B3, F27D1