Alkali-mediated Sr incorporation mechanism and binding capacity of alkali aluminosilicate hydrate in geopolymers

Geopolymers are promising materials for safe immobilisation and disposal of complex radioactive waste streams. This work investigates the effect of Sr incorporation and alkali-activator chemistry on 1) geopolymer chemistry, phase assemblage and nanostructure, 2) chemical binding mechanism of Sr2 + into the aluminosilicate framework of (N,K)-A-S-H gels in geopolymers, and 3) mass transport of Sr2+ during leaching, using high-field solid-state nuclear magnetic resonance spectroscopy and synchrotron-based X-ray absorption spectroscopy measurements. All geopolymers studied comprise a fully polymerised, X-ray amorphous Al-rich (N,K)-A-S-H type gel. Si exists predominantly in tetrahedral Q4(4Al) and Q4(3Al) sites and Al exists in tetrahedral sites, resulting in a net negative charge that is balanced by Na+ and/or K+ in extra-framework sites. Sr2+ was incorporated into extra-framework sites within (N,K)-A-S-H gels, without altering the local structure of the aluminosilicate framework by directly substituting for both Na+ and K+ in charge-balancing sites to form a (N,K,Sr)-A-S-H gel, at loadings equal to or below Sr/Na = 0.005. Above this limit, SrCO3 is formed, and the geopolymers simultaneously chemically bind Sr within a (N,Sr,K)-A-S-H gel, and physically encapsulate excess Sr as SrCO3. These findings have significant implications for use of geopolymers as materials for encapsulation and/or immobilisation of radioactive waste containing 90Sr.