High-field multinuclear MAS NMR and synchrotron XANES reveal the influence of strontium salt chemistry on geopolymer nanostructure

Abstract

This study investigates the influence of strontium (Sr) salt chemistry (Sr(OH)₂•8H₂O, SrCO₃, Sr(NO₃)₂, and SrSO₄) on the nanostructural evolution of potassium silicate-activated geopolymers. High-field multinuclear (²⁷Al, ²⁹Si, ³⁹K, and ⁸⁷Sr) MAS NMR, synchrotron XANES, EPMA, XRD, and FTIR showed that while the primary binding phase in all samples is a disordered, highly cross-linked K-A-S-H gel, the Sr immobilisation mechanism is governed by salt solubility. Soluble nitrate and hydroxide salts release Sr²⁺ ions that are chemically incorporated into the K-A-S-H gel framework in brewsterite-type pseudo-zeolitic environments. In contrast, insoluble carbonate and sulfate salts act primarily as physical fillers, and are encapsulated as discrete particles within the geopolymer matrix, though sulfate additionally reacts to form secondary crystalline kalistrontite (K₂Sr(SO₄)₂). Sr 2+ adsorption on metakaolin surfaces is found to inhibit early-stage Al dissolution, resulting in a Si-rich K-A-S-H gel that transitions to an Al-rich K-A-S-H gel over 28 days. These results provide new insight into the mechanisms of immobilisation of Sr in geopolymers, and highlight their potential as wasteforms for long-term management of ⁹⁰Sr radioactive waste.