Porous glass ceramic materials are widely used in industrial and civil engineering due to a number of redeeming features, such as high strength, low thermal conductivity, incombustibility, environmental friendliness, etc. A large number of researches are devoted to developing the compositions of foam glass ceramic materials based on siliceous rocks (diatomite, tripoli, opoka). Present article is devoted to studying the formation mechanism of the porous structure of glass ceramic materials as a result of heating a mechanically activated mixture (a mixture of siliceous rock and soda ash or thermonatrite). The experimental results were obtained using methods of gas permeability, scanning electron microscopy (SEM), infrared spectroscopy (IR), X-ray diffraction analysis (XRD), differential thermal analysis (DTA) and differential thermal gravimetric (DTG) analysis, physical-mechanical and thermophysical tests. It was determined that the minerals of the crystalline structure in the composition partially transfer to the amorphous phase with an increase in the charge activation time, and the amount of heulandite and sodium hydrosilicates increases. This contributes to an intensive increase in the amount of flux in the composition within the temperature range 700–800 °C. The water vapor generated during the condensation of free OH groups on the surface of silicate is formed in this temperature range. This is the formation source of the material’s porous structure. The developed porous glass ceramic materials have increased compressive strength (up to 5 MPa) at a relatively low average density (268.5 kg/m3). This is several times greater than the strength of foam glass from waste glass and from fly or coal ash. The minimum thermal conductivity of glass ceramics (0.0633 W/m·°С) was determined at a sample density of 220.7 kg/m3. The maximum operational temperature of the material was 850 °C, which allows using it as a thermal insulation of industrial equipment, such as melting furnaces, boiler equipment, etc.