Lyashenko, T.; Kersh, V.; Kolesnikov, A. Heat-insulating gypsum based plaster compositions 1. Introduction When developing lightened heat-insulating materials on the base of calcium sulphate (increasingly important in construction) the possibilities to introduce various lightweight aggregates and the limits of their content have been studied [1, 2]. The purpose of this particular study has been to develop the compositions for interior plaster coverings, with fine perlite as basic filler. It is known that entering of perlite grains (of low thermal conductivity) in gypsum matrix can significantly improve heat and sound insulation properties of the composite, with not only its density being lowered, but, unfortunately, its strength as well. To strengthen the composite a part of perlite could be replaced with cenospheres. These hollow alumina-silica microspheres (formed as a part of fly ash) are known [3-6] as efficient filler due to their form, waterproofness, low density and thermal conductivity. The preliminary experiments showed that certain dosages of metakaolin, plasticiser and latex could improve gypsum matrix, structure and properties of the composite. 2. Preliminary trials To determine the upper limit of the content of heat-insulating component the method of electro-thermal analogy was used in preliminary model experiments. The method helps to establish the proportions in "conductor - insulator" systems, at which the electrical conductivity changes abruptly; percolation conductivity jump takes place (or percolation resistance jump at certain content of insulator). The conductive media was modelled with carbon powder, lightweight filler particles served as the insulator. The estimated value of percolation threshold for perlite grains (and for cenospheres as well), near 80 volume percents (Fig. 1), approximately corresponds to theoretical value Fig.1: Specific electric resistance and compression strength in dependence of the content of perlite in carbon powder (left) and in gypsum matrix (right)   Ω⋅                                