Influence of leaching on the strontium immobilisation mechanism and nanostructural framework in geopolymer wasteforms

Geopolymer cements are highly promising materials for long-term immobilisation of Strontium-90 radioactive waste, offering superior durability and cation binding sites compared to conventional Portland cement matrices. This study investigates the influence of prolonged leaching on the Sr immobilisation mechanism and structural integrity of metakaolin-based geopolymers using the ANSI/ANS 16.1 semi-dynamic leaching test. All geopolymers demonstrated high Sr retention, with Leachability Indices at least 14.7 for all samples, significantly exceeding the industry guideline of 6.0, confirming their effectiveness. Importantly, potassium silicate–activated geopolymers exhibited reduced Sr release and substantially lower leaching rates than sodium silicate–activated geopolymers. Multiscale spectroscopic and diffractometric analysis, including synchrotron X-ray absorption spectroscopy and multinuclear high-field solid-state MAS NMR probing 39K, 23Na, 27Al, and 29Si, revealed that the alkali aluminosilicate gel framework remained structurally stable after leaching for 28 days, with no significant alterations to Si and Al bonding environments. Sr release is primarily controlled by diffusion, and the dominant immobilisation mechanism is the formation of insoluble SrCO3. Atomic-level Sr structural analysis using XANES/EXAFS revealed an increase in the average Sr coordination number in both systems after leaching, with a more pronounced rise in potassium-based geopolymers, consistent with enhanced SrCO3 formation. Overall, these findings demonstrate that geopolymers maintain structural integrity during leaching and show for the first time that using potassium rather than sodium as an alkali activator is definitively more advantageous for maximising the long-term effectiveness of geopolymer wasteforms. This demonstrates their strong suitability as wasteforms for the safe long-term immobilisation of Sr-containing radioactive wastes.