Structure Formation in the SiO2–Na2O–H2O System during the Fabrication of Foam Glass Ceramics by Extrusion

Abstract

This paper presents a method for the fabrication of a foam glass ceramic, an inorganic heat-insulating material, by extruding a mixture of opal–cristobalite rock and a sodium hydroxide solution through calibrated holes. The method ensures excellent mixture homogenization and allows mixing of components and granulation of the mixture to be combined in a single step. As a result of active reactions of amorphous phases in the opal–cristobalite rock with sodium hydroxide in the SiO₂–Na₂O–H₂O system, the average density of the material decreases by a factor of 1.5. This ensures an economic benefit: a proportional reduction in the percentage of sodium hydroxide, an expensive component. Using electron microscopy, we have assessed the effect of temperature on structure formation in the material. The pore formation process begins at 200°C and is accompanied, up to 500°C, by the formation of a loose structure in the form of an inhomogeneous network. In the range 400–500°C, the material undergoes a transition. Above 600°C, a cellular structure begins to form. As a result, the materials synthesized at 850°C have an average density from 0.38 to 0.57 g/cm³ and compressive strength from 3.1 to 5.9 MPa.