Corrosion of borosilicate glasses subjected to aggressive test conditions: structural investigations

Sodium borosilicate (NBS) and barium sodium borosilicate (BBS) glasses, used for immobilization of high-level nuclear waste with compositions (SiO2)0.477(B2O3)0.239(Na2O)0.170(TiO2)0.023(CaO)0.068(Al2O3)0.023 and (SiO2)0.482(B2O3)0.244(Na2O)0.220(BaO)0.054 were subjected leaching experiments under hydrothermal conditions in an autoclave at 200°C for different time durations. Morphological and structural transformations associated with leaching, have been monitored with techniques like XRD, SEM, solid-state nuclear magnetic resonance. XRD and SEM along with NMR studies have confirmed that, upon leaching, formation of an aluminosilicate phase, Zeolite-P (Na6Al6Si10O32·12H2O), occurs with NBS glass. BBS glass upon subjecting to the same conditions leads to formation of multiple amorphous phases having Q4 (silica rich phase) and Q3 structural units of Silicon along with structurally modified residual glass. Upon leaching BO3 structural units preferentially get released from BBS glass. Comparison of results with international simple glass confirmed that, for the latter, mass loss rates are one order of magnitude lower

Role of sulfate in structural modifications of sodium barium borosilicate glasses developed for nuclear waste immobilization

A sodium barium borosilicate glass matrix with a higher solubility of sulfate has been developed recently at Bhabha Atomic Research Centre for vitrification of sulfate bearing high-level nuclear waste. We report here the studies carried out to understand the influence of sulfate ion on the three-dimensional borosilicate network. Experiments were carried out with sodium barium borosilicate base glass samples loaded with varying amounts of SO<SUB>4</SUB><SUP>2-</SUP> (0–5 mol%). Phase separation studies on the samples revealed that as much as 3 mol% of SO<SUB>4</SUB><SUP>2-</SUP> can be loaded within the base glass without any phase separation, however, beyond this limit BaSO<SUB>4</SUB> (barite) crystallizes within the matrix. Thermal analyses of the samples indicated a shift in glass transition temperature from 534° (0 mol% SO<SUB>4</SUB><SUP>2-</SUP>) to 495°C (3 mol% SO<SUB>4</SUB><SUP>2-</SUP>) and it remained more or less unaltered afterwards even with high SO<SUB>4</SUB><SUP>2-</SUP> loading. A similar observation of structure stabilization was obtained from <SUP>29</SUP>Si MAS–NMR studies also, which showed that with 2 mol% of SO<SUB>4</SUB><SUP>2-</SUP> loading, the Q<SUP>2</SUP>:Q<SUP>3</SUP> ratio changed from 59:41 (for samples with 0 mol% SO<SUB>4</SUB><SUP>2-</SUP> loading) to 62:38 and it remained almost the same afterwards even with higher SO<SUB>4</SUB><SUP>2-</SUP> loading. 11B MAS NMR patterns of the glass samples, however, remained unchanged with SO<SUB>4</SUB><SUP>2-</SUP> loading ([BO<SUB>4</SUB>]:[BO<SUB>3</SUB>]=38:62). Based on <SUP>29</SUP>Si and 11B MAS NMR studies, the authors propose two different ways of interaction of SO<SUB>4</SUB><SUP>2-</SUP> ions with the borosilicate network: (i) the network modifying action of SO<SUB>4</SUB><SUP>2-</SUP> ions with -Si–O–Si- linkages, at low SO<SUB>4</SUB><SUP>2-</SUP> ion concentration (&#60;2 mol%) and (ii) the preferential interaction of SO<SUB>4</SUB><SUP>2-</SUP> with the Ba<SUP>2+</SUP> ions at high SO<SUB>4</SUB><SUP>2-</SUP> concentration (>2 mol%)