Phosphoric acid-activated metakaolin-based geopolymer: Optimizing P/A molar ratio to solidify Cs+ and Sr2+ in nuclear waste

Abstract

Phosphoric Acid-activated Metakaolin-based Geopolymer (PAMG) is an efficient solidified material. This paper reports the effect of the H3PO4 to Al2O3 (P/A) molar ratio on the hydration, phase assemblage, morphology, and compressive strength of PAMG and the solidification of simulated radionuclides Sr and Cs by PAMG. The hydration behavior and microstructure of PAMG were investigated using isothermal calorimetry, X-ray diffraction (XRD), and scanning electron microscope (SEM). The impacts of Cs and Sr on the leaching behavior and microstructure of PAMG were assessed using atomic absorption spectrum (AAS), SEM, and energy dispersion spectrum (EDS). The results indicate that the compressive strength of PAMG increases and then decreases as the P/A ratio increases. The highest compressive strength reaching 98.1 MPa after 28 days of curing, was observed at a P/A molar ratio of 1.8. The addition of Cs and Sr resulted in a decrease in the compressive strength of PAMG, with Sr showing a more significant reduction in PAMG's compressive strength. However, PAMG still exhibited a high efficiency in solidifying simulated nuclides Cs and Sr, with 42 days leaching rates of 3.7 × 10−5 cm/d and 5.0 × 10−6 cm/d, respectively. These results indicate the huge potential of PAMG in solidifying nuclides.