Some technological laws and properties for highly porous ceramic concretes

Abstract

Highly porous, fired ceramic concretes, which are heterogeneous, polyfractionated compositions based on coarse porous refractory fillers and dispersed ceramic bond (cement) [i], are of interest in developing refractory, heat-insulating materials (in particular, in the production of large articles for lining furnaces). The authors of [2] present certain data on the production of specimens of zircon ceramic conc=ete with a total porosity of up to 70%. However, in contrast to dense ceramic concretes, the production method and properties of which have been studied in detail [i, 3, 4], special studies into the highly porous ceramic concretes have not been carried out, although data exist on the effective use of such materials [5].High porous ceramic concretes, compared with other highly porous materials (for example, foamed ceramics [2,6,7]), have some technical advantages (much lower water requirements for the molding system, and a reduction in shrinkage at the drying and firing stages), as well as improved exploitation properties, for example, thermal-shock resistance and volume stabi-This article, using ceramic concretes made of zirconia as an example, examines some general problems in obtaining highly porous ceramic concretes, and studies the effect of some technological factors on the material's properties.The original material consisted of highly dispersed (80% particles measuring not more than 5 ~m, maximum size i0 ~m) zirconia, stabilized with yttrium oxide and obtained by chemical precipitation from salt solutions [8]. The suspension (bond for ceramic concrete) was obtained by suspension in HCl-acidified water with a suspension pH of 2.0-2.5. The suspensions were subjected to mechanical mixing to deflocculate them, the effect of which for Zr02 was established previously [9]. This resulted in the removal of entrapped air. In view of the increased dispersion of the particles of solid phase, and the relatively low value of the ionic potential, determining the hydration capacity [ID], the Zr0= bond, after completion of the structure formation, possesses an enhanced (up to 3-4%) shrinkage during drying which, according to [i], complicates the technology for ceramic concretes in view of the creation of shrinkage tensions in the system. The highly porous (80-85%) filler was obtained by crushing briquets [Ii] sintered at 1580~ made of zircon foamed ceramics. In this case, the crushing product yielded both coarse (3-15 mm) and fine (0.06-0.63 mm) fractions of filler. It should be mentioned that with a reduction in the filler particle size, its actual porosity compared with that of the briquet was reduced as a result of the reduction in volume concentration in the grains of the coarse (0.1-0.4 mm) spherical pores. Moreover, while for grains measuring 5-10 mm (optimum size of coarse fraction) the porosity was about 80%, then for grains measuring 0.1-0.4 mm (optimum size of fine fraction) it was reduced to 30-35%. In accordance with this the porosity of the ceramic concrete to a large degree is determined by the volume ...

Sintering, structure, and properties of cast granular oxide refractories

“…We note that when a coarse grained filler is used, at low values of CVf the strength values can be lower that those obtained at high CVf because of the formation of longer cracks [18].…”

“…In contrast to the melted or densely sintered fillers (of constant volume), there are mildly fired fillets that make it possible to increase the density and strength of the material [2,3,18].…”

“…The principles of reducing this undesirable phenomenon have been previously dealt with [18] on the example of granular materials containing a highly dispersed zircon binder.…”

New population of refractory concretes: Grain size composition and volume factors

“…Moreover, in monomineral concrete (e.g., chamotte type) the filler grains (in contrast to those for VKVS) have a certain porosity -up to 10-20%. A special case is highly porous (heat-protective) concrete in which the filler has a porosity of up to 80-85%, [5,7].…”

“…In all these cases the grain-size composition should be evaluated in terms of the true specific volumes, as shown in [5,7] for highly porous zircon concretes. Using the selected specific volumes and the equations in [6] we can calculate the mass consumption of the components.…”