Formation of alteration products during dissolution of vitrified ILW in a high-pH calcium-rich solution

Abstract

To simulate the possible disposition of a vitrified intermediate-level waste (ILW) in a cementitious environment within a geological disposal facility (GDF), the durability of a laboratory simulant ILW vitrified in a borosilicate glass in a saturated Ca(OH)2 solution (pH ∼12.5) was measured. Both a low surface area to volume (SA/V) ratio (∼10m−1) Materials Characterisation Center test 1 (MCC-1) and a high SA/V ratio (∼10,000m−1) product consistency test type B (PCT-B) were used at 50°C for up to 170days. The formation of alteration layers and products was followed. The surfaces of the monoliths were analysed using SEM/EDX and showed the formation of magnesium-rich precipitates and distinct calcium silicate hydrate (CSH) precipitates. Cross sections showed the development of a calcium-rich alteration layer, which was observed from 14days. The altered layer was up to 5μm thick after 170days and showed accumulation of zirconium, iron and magnesium and to a lesser extent aluminium, along with calcium and silicon. Based on comparison of the rate data, it is suggested that the presence of this layer may offer some protection to the underlying glass. However, the high SA/V ratio experiments showed resumed alteration after 56days, indicating that the altered layer may not be protective in the long term (under accelerated conditions). The formation of a magnesium-containing smectite clay (likely saponite) in addition to CSH(II), a jennite-like CSH phase, were identified in the high SA/V experiment by X-ray diffraction after 170days. These results suggest that calcium and magnesium have important roles in both the long and shorter-term durability of vitrified wastes exposed to high pH.