Rheological properties of hydrodispersions of silica with limiting concentration of R2O-containing electrolytes

The rheological properties of concentrated dispersions of silica were studied in the region of limiting concentrations of R2O-containing electrolytes (pH = 11.0-11.5). Quartz sand was used as the initial raw material, and water glasses and solutions of soda and polyphosphates with various moduli were used as electrolytes. Silica dispersions with a 0.68-0.70 volume fraction of solid phase were mechanically activated at various electrolyte concentrations in a ball mill. When the volume fraction of the solid phase is greater than 0.58, the casting system has a rather strong, typically solid-like structure, which fractures simultaneously with disruption of the continuity of the solid. When the R2O concentration in the dispersion medium increases above 0.15 n, reactions in the silicic suspension form hydrogel from kinetic units of the colloidal SiO2 by the sol-gel process. Mechanochemical activation of the dispersions leads to amorphisation of the surface of the quartz grains and formation of polysilicic hydrosilicates of alkali metals as an intermediate product. The dissolution of amorphous SiO2 intensifies substantially with the formation of associates of oligomers of polysilicic acid and hydrated R ions. In castings of the SiO2 dispersions studied, the R2O-containing components are not carried into the pores of the mould during casting but rather migrate with the colloidal phase to the surface of the castings. Gravitational stabilisation can lower the viscosity of the casting system and enhance its resistance to sedimentation, with a corresponding rise in the density and flexural strength of the castings. 6 refs.