A structure model of Al and K-containing tobermorite is proposed based on the results obtained by different methods-powder X-ray diffraction analysis, microdiffraction in an electron microscope, etc. The factors responsible for the stability of the structure modules typical of the specimens of this family are discussed. Most of the microcrystals were demonstrated to consist of two phases characterized by a high degree of siliconoxygen radical condensation. The examination of two-phase microcrystals in an electron microscope by the method of diffraction contrast allowed us to propose the mechanism of change of the degree of condensation of the tobermorite structures under an electron beam. Heating the starting crystals results in their transformation into an amorphous state with a simultaneous increase in the degree of condensation of the silicon-oxygen ribbons in the structure.
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