Reaction mechanisms and kinetics in geopolymers incorporating strontium salts

Geopolymers are promising materials for immobilisation of ⁹⁰Sr, present in complex radioactive waste streams at sites such as Sellafield and Fukushima Daiichi. This study investigates the impact of strontium salts (Sr(OH)₂·8H₂O, SrCO₃, Sr(NO₃)₂, and SrSO₄) on geopolymer reaction mechanisms and kinetics. Isothermal conduction calorimetry, zeta potential, and X-ray diffraction data show that the introduction of the strontium salt leads to a reduction in the rate and extent of reaction. This is a result of (1) reduced dissolution of the metakaolin precursor due to (a) a reduction in pH of the aqueous phase and (b) sorption of Sr²⁺ cations to the surface of the metakaolin precursor, and (2) a 'filler effect' arising from the presence of insoluble solid particles of the strontium salts. The extent to which each mechanism occurs depends on the salt solubility in the high pH, high ionic strength fresh cement slurry. This insight is vital for designing geopolymer cements for the long-term immobilisation of radioactive waste streams containing ⁹⁰Sr.