Immobilization of molten salt in alkali-activated and cement-based materials

In the context of PREDIS work package 6, the immobilization of a molten salt waste stream, received from CV Rez (Czechia) was investigated. The matrices investigated include alkali-activated (based on ground granulated blast furnace slag and metakaolin) and cementitious (blended system) binders. The molten salt was found to consist of hydrated sodium carbonate minerals. The direct immobilization of this salt produced materials which are especially sensitive to changes in humidity and temperature, which is undesirable for a waste form. Therefore, the sodium carbonate salts were reacted with Ca(OH)2 and Ca(NO3)2 to form CaCO3, which is insensitive to changes in temperature and humidity. The pretreated salt was then immobilized in the aforementioned matrices. In the case of alkali-activated binders, the Ca(NO3)2 pretreated salt did not produce a stable waste form due to nitrate ions interfering with the geopolymerization process. The Ca(OH)2 pretreatment did produce waste forms with acceptable mechanical strengths. In the case of the cementitious binder, both the Ca(OH)2 and Ca(NO3)2 pretreatments were found to produce materials with good mechanical strengths which are resistant to temperature and humidity changes. A remaining challenge involves the extent to which CaCO3 is formed in the pretreatment instead of the intermediary Na2Ca(CO3)2.5H2O phase which is potentially unstable on the long term.