Silicate glass additive

Cerium(IV) oxidizes ferrous ion to ferric and the cerium ions are stable under the conditions of a molten silicate—glass bath. Furthermore, cerium itself has no absorption ia the visible region. Economical additions of cerium, as cerium concentrate, enable the efficient use of raw materials containing trace quantities of iron (26). 

Silicate glasses have amorphous structures produced by addition of salts that disrupt the crystalline structure. They can be attacked by strong base and hydrofluoric acid. 

The waste forms were a fully loaded, pressure sintered titanate waste containing zeolite and silicon additions, a copper boro-silicate glass prepared from Frit 199 supplied by Battelle Northwest Laboratories, and a copper borosUicate glass prepared from the same frit and containing 30% by weight of waste oxides. 

The viscosity of silicate glasses drops with decreasing O/Si ratios. The data from H. J. L. Trap and J. M. Stevels, Glastech Ber. 6 (1959) V131, are for equal molar additions of Na20, K20, CaO, SrO, and BaO. 

Various surface analysis techniques show that silicate glasses rapidly develop surface compositional profiles when exposed to water. When water is present as a vapor an alkali-rich layer (presumably a hydrated alkali carbonate) forms over the SiOj-rich layer. Water as a liquid dissolves the alkali and leaves the silica-rich film. As long as this SiC -rich film is stable the rate of corrosion due to diffusion is reduced with exposure time. Addition of multi-valent species to the glass or reactant results in formation of a complex protective surface layer in the glass which may be stable over a wide range of environmental conditions. 

Figure 2.7. Variations of contact angle with time for ternary silicate glasses containing Si02 and Na20 in a 4 1 mole ratio with additions of Ti02 on Pt substrates (Hocking and Rivers 1982). The R(t) curves are calculated from contact angles values assuming a spherical shape for the drop and taking a...

The alkah silicate glasses are easily corroded by aqneons acids and the main reaction is an exchange between H3O+ of the solntion with the alkah M+ of the glass the corrosion rate increases with the ionic raduofM+, that is, K+ > Na+ > Li+. The chemical durabihty is greatly increased by addition of divalent CaO or bivalent AI2O3 oxides, leading to conunercial soda-hme sihca compositions. 

The addition of B2O3 to a silicate glass markedly reduces the thermal expansion coefficient and consequently enhances the resistance to thermal shock. At the same time, the chemical durability, especially the resistance to acid attack, is greatly improved. 

Al ", Ca ", or Fe ", also significantly reduces glass solubility. A correlation exists between the ratio of free and total silanol groups and glass solubility (20). This ratio varies linearly as different metal ions are added to Na-, K-, or Li-based silicate glasses (4.0 mol ratio). The addition of water depresses this ratio significantiy. 

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