Influence of heat treatment on the structure and properties of ceramic heat-insulating composites based on phosphate bound expanded perlite-expanded clay

The article is devoted to the production and research of composite heat-insulating porous ceramics based on common and available local raw materials in the form of perlite, expanded clay, kaolinite clays using phosphate binders. Experimental samples were obtained by semi-dry pressing and ramming, followed by heat treatment at temperatures from 600 degrees C to 1100 degrees C. The processes of structure formation occurring in the compositions during heating are characterized by X-ray diffraction and local X-ray microanalysis. The obtained results showed the formation of the crystal structure of alumophosphate AlPO4 in the form of tridimite and cristobalite in the filler grains, and calcium phosphate Ca3(PO4)2 in the binding matrix of the composite. Heating temperature increase to 1150 degrees C lead to an increase in the amount of melt in the areas with the largest number of impurity elements. The influence of formation methods and technological parameters on shrinkage processes, density and strength of phosphate-bonded specimens has been demonstrated. The analysis of experimental results showed that the samples containing filler (expanded perlite), clay (secondary Samarkand kaolin) and orthophosphoric acid (40 wt%) are optimal in combination of density, thermal conductivity and resistance to high temperatures.