Gel Structures in Leached Alkali Silicate Glass
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GEL STRUCTURES IN LEACHED ALKALI SILICATE GLASS Bruce C. Bunker, Thomas J. Headley, and Sally C. Douglas Sandia National National Laboratories, P. 0. Box 5800, Albuquerque, New Mexico 87185 ABSTRACT Transmission electron microscopy of leached Na O" 3Si0 2 and K2 O'3SiO2 glasses showed that the hydrateg glass surface is a silica rich gel which phase separates during leaching. Phase separation is initiated when pockets of an aqueous phase nucleate and grow in the hydrated silica matrix. The aqueous pockets eventually become a network of interconnected pores with a pore diameter of v30 nm. In advanced stages of leaching, the remaining interconnected silica rich phase resembles an aggregation of colloidal The observed phase separation is silica particles. consistent with literature models concerning the chemistry of silica in aqueous solutions and in leached glass. INTRODUCTION When alkali silicate glasses react with water, an alteration layer which is depleted in alkali cations and enriched in water usually forms on the glass surface. The layer is referred to as the gel layer, since its composition and physical properties resemble those of silica gel. Recently, small angle x-ray scattering results[l] reported for leached Na O'3SiO (mole ratio) and 4BaO-28Na 2 O-68SiO2 (mole ratio) glass suggesteg that ihe gel layer is not homogeneous, but contains either voids or particles as large as 15 nm in diameter, indicative of phase separation. Raman spectroscopy analyses[2] on leached Na2 O23SiO2 glass suggest that the phases which form in the surface gel consist of a phase resembling fused silica and an aqueous phase containing silicic acid. Glass dissolution studies indicate that although the surface gel functions as a diffusion barrier in the early stages of leaching, the barrier is ineffective at longer times, A change in gel structure, such as leading to linear leaching kinetics[3]. phase separation, might account for the observed changes in leaching kinetics. In order to elucidate the structures of surface gels and to explore possible relationships between gel structures and alkali leaching kinetics, transmission electron microscopy (TEM) was performed on leached alkalisilicate glass as a function of solution pH, temperature, and leaching time. TEM confirmed that a silica-water phase separation occurs in leached surface gels. Observed structures are shown to be consistent with the aqueous chemistry of silica and silica gels. EXPERIMENTAL mole ratio) were prepared by Two glasses (Nag O'3SiO2 and K2 0"3SiO0, mixing desired amounts of reagent grade alkali carbonates and silica. These were melted at 1500 C, stirred for 18 hours, cast into slabs, and annealed at appropriate temperatures, depending on the glass composition. The slabs were cut into 1.0 cm x 0.5 cm x 0.15 cm samples, polished to a 1 jimfinish with diamond paste, and ultrasonically cleaned in methanol. Glass samples were leached for various times in 100 ml of deionized water, HC1, or NaOH, depending on the desired solution pH. For intermediate solution pH values,
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