Impact glass applied as a standard for long-term performance assessment of Na–Ba borosilicate glass forms in geological environment

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High-level nuclear waste is usually immobilized into high silica glass matrix for long-term and safe disposal in the geological repository. Performance of glass forms is uncertain for the lack of standardization against a natural analogue. Thus, alteration studies were performed on the impact glass as it is surviving in a natural environment since its inception (570 ka). Further, these results were compared to laboratory induced alteration in the impact glass and Na and Ba borosilicate glass from the literature. Alteration experiments were carried out in the de-ionized water under accelerated conditions at 100 °C temperature and 17.24 bar pressure in a variable time framework. Owing to rapid dissolution of synthetic glass, Al3+, Ca2+, K+, Mg2+, Na2+ and Si4+ were released into the solution and finally secondary silicates (SS) were formed in the residue. Comparing to impact glass, a three-fold higher alteration rate is noticed in the Na and Ba borosilicate glass forms. X-ray diffraction (XRD) and scanning electron microscopic (SEM) - energy dispersive X-ray spectrometric (EDS) analyses revealed formation of secondary mineral assemblages (analcime, aragonite, illite, montmorillonite, saponite and stilbite) for both naturally and experimentally altered impact glass types. Moreover, nontronite phase was formed as a major post-alteration product in Na and Ba borosilicate glass forms. The neo-formed SS were compared to naturally derived SS from the impact glass in three time domains of (1) 570 to 5.41 ka, (2) 5.41 to 0.53 ka and (3) 0.53 to Recent Period as determined by high resolution 14C Accelerator Mass Spectroscopy (AMS). Naturally altered impact glass presents paragenetic order of saponite > calcite > montmorillonite > montmorillonite/chlorite mixed layer > siderite > chabazite > aragonite > analcime. As compared to the impact glass, synthetic glass structure is very weak and friable towards alteration. Former attended as a more reliable standard glass.