Reaction of vitreous sodium silicates and aluminosilicates with aqueous solutions. Communication 3. Effect of the addition of silica and alumina to sodium silicates on the instability of the latter to acids

Abstract

1. The chemical stability of vitreous sodium aluminosilicates can be characterized by the depth of the attacked layer of glass (8), which is a measure of the breakdown of electrovalent O-Na bonds, and by the degree of breakdown suffered by the aluminum-silicon-oxygen of the glass, expressed by the proportions of the Al2O3 and SiO2 originally present in the residual surface layer that have passed into the solution, 2. With increased additions of SiO2 to sodium silicates, the depth of the attacked layer of glass falls and the silica skeleton is strengthened, 3. When alumina is introduced into sodium silicates, on the one hand there is an increase in the number of silicon-oxygen groups that become united owing to the neutralization by sodium ions of the extra negative charge of the aluminum-oxygen tetrahedra, and on the other hand, owing to the formation of Si-O-Al linkages, depolymerization of silica in the glass increases, with the result that the instability of aluminosilicate glasses In acid solutions Increases greatly when the amount of aluminum oxide added to sodium disilicate exceeds 0.3 mole. 4. The mechanism of the interaction of sodium aluminosilicates with solutions of acids may be regarded as the combination of processes of ion exchange between sodium and aluminum in the glass and hydrogen in the solution, followed by the passage into the solution of small isolated aggregates of silicon-oxygen groups.