Samples of natural zeolites of different structural type, genesis, and cation composition (Ca and Mg- or Na and K-rich zeolites) were tested for their foaming ability at 1200 oC. The degree of foaming was estimated by the density of solid foamed material obtained after calcination. Thermally-induced foaming ability of natural zeolites was shown to be a function of the chemical composition. Only Ca and Mg-rich zeolites foam at 1200 oC calcination. The FTIR method showed that upon dehydration of the Ca and Mg-rich zeolites original water molecules transform into thermostable framework OH groups that became the source of high-temperature water vapor for foaming zeolite. Foamed material is formed if the temperature of OH group recombination into H2O coincides with that of sintering, and the viscosity of the melt at 1200 oC is sufficient to enclose the H2O vapor within the pores. Both the sintering and viscosity of the melt are functions of the chemical composition of a zeolitic rock.

Keywords: foaming, natural zeolites, composition, cationes, sintering, melt viscosity, IR-spectroscopy